and Their Application in Asymmetric Transition-Metal Catalysis
Dissertation
zur Erlangung des mathematisch-naturwissenschaftlichen Doktorgrades
“Doctor rerum naturalium“
der Georg-August-Universität Göttingen
im Promotionsprogramm
“Catalysis for Sustainable Synthesis”
vorgelegt von Torben Böhnisch
aus Hannover
Göttingen, 2015
C
2-Symmetric Pyrazole-Bridged Ligands and Their Application in Asymmetric Transition-Metal Catalysis
Dissertation
zur Erlangung des mathematisch-naturwissenschaftlichen Doktorgrades
"Doctor rerum naturalium"
der Georg-August-Universität Göttingen
im Promotionsprogramm CaSuS “Catalysis for Sustainable Synthesis”
der Georg-August University School of Science (GAUSS)
vorgelegt von Torben Böhnisch
aus Hannover Göttingen, 2015
Prof. Dr. Franc Meyer, Institut für Anorganische Chemie
Prof. Dr. Guido Clever, Institut für Anorganische Chemie
Prof. Dr. Ulf Diederichsen, Institut für Organische und Biomolekulare Chemie
Mitglieder der Prüfungskommission
Referent/in: Prof. Dr. Franc Meyer, Institut für Anorganische Chemie
Korreferent/in: Prof. Dr. Guido Clever, Institut für Anorganische Chemie
ggf. 2. Korreferent/in: Prof. Dr. Ulf Diederichsen, Institut für Organische und Biomolekulare Chemie
Weitere Mitglieder der Prüfungskommission:
Prof. Dr. Dietmar Stalke, Institut für Anorganische Chemie
Prof. Dr. Konrad Koszinowski, Institut für Organische und Biomolekulare Chemie
Prof. Dr. Thomas Waitz, Institut für Anorganische Chemie
Tag der mündlichen Prüfung: 23. Juli 2015
T ABLE OF C ONTENTS
1 General Introduction 1
1.1 Two-Center Catalysis 2
1.2 Pyrazole Bridged Ligands 4
1.3 Asymmetric Catalysis 7
1.3.1 Privileged Ligands and C2-Symmetry 9
1.3.2 Bis(oxazoline) (BOX) Ligands 11
1.3.3 Bis(oxazolinyl)pyridine (PyBOX) Ligands 13
1.4 Cooperative Asymmetric Catalysis 14
1.4.1 ProPhenol Catalysts 14
1.4.2 Salen Catalysts 18
2 State of Knowledge 29
3 Objectives 30
4 Ligand Synthesis 31
4.1 Pyrazole-Bisoxazoline (PyrBOX) Ligands 32
4.1.1 Synthesis of PyrBOX Ligands HL1-HL4 32
4.1.2 Attempted Synthesis of PyrBOX Ligands 6a-d 36
4.2 Pyrazole-Bispyridyl-Bisoxazoline (PyrBPyBOX) Ligands 38
4.2.1 General Synthetic Strategy 38
4.2.2 Synthesis of Pyrazole Building Block 11 39
4.2.3 Synthesis of Dicarboxylic Acid 12 40
4.2.4 Synthesis of PyrBPyBOX ligands HL5-HL6 42
4.2.5 Synthesis of PyrBPyBOX ligands HL7-HL8 45
4.3 Summary for Pyrazole Bridged Bis(oxazoline) Ligands 47
4.4 Pyrazole-Bisprolinol (ProPyrazole) Ligands 48
4.4.1 General Synthetic Strategy and Initial Results (H3L9-H3L10) 48
4.4.2 Modification of the Hydroxy Group (H3L11) 50
4.4.3 Modification of the Aryl-Substituents (H3L12-H3L17) 51
4.4.4 Modification of the Pyrrolidine Ring (H3L18-H3L24) 56
4.4.5 Modification of the Pyrazole-Bridging Unit (H3L25-H3L26) 60
4.5 Summary for Prolinol Pyrazole Bridged Ligands 62
5 Allyl Palladium Chemistry 64
5.1 General Introduction into the Tsuji-Trost Reaction 65
5.2 Dynamic Processes for Allylic Compounds 68
5.2.1 Stereochemical Notations for Allylic Moieties 68
5.2.2 Isomerization through a η3-η1-η3-Pathway 69
5.2.3 Isomerization via Apparent Allyl Rotation 70
5.2.4 Isomerization via other Processes 71
5.3 Asymmetric Allylic Substitution 72
5.3.1 General Mechanisms of Enantiodiscrimination 72
5.3.2 Symmetrical 1,3-Disubstituted Substrates 74
5.3.3 Unsymmetrical Tri- or Monosubstituted Substrates 78
6 State of Knowledge on PyrBOX Allyl Palladium Chemistry 83 7 Dinuclear PyrBOX Allyl Palladium Complexes 86
7.1 Cyclohexenyl Palladium Complexes 87
7.1.1 Synthesis and Characterization of Complexes 48a-d 87
7.1.2 Solid-State Structure of 48a = [L1Pd2(C6H9)2]BF4 89
7.1.3 NMR-Investigations of the Dynamic Behavior of 48a-d 90
7.1.4 Application of PyrBOX Cyclohexenyl Complexes 48a-d in Asymmetric Allylic Substitutions 101
7.2 1,3-Diphenyl Allyl Palladium Complexes 110
7.2.1 Synthesis and Characterization of Complexes 54a-d 110
7.2.2 Solid-State Structure of 54d = [L4Pd2(1,3-Ph2C3H3)2]BF4 113
7.2.3 NMR-Investigations of the Dynamic Behavior of 54a-d 117
7.2.4 Application of PyrBOX Diphenyl Allyl Complexes 54a-d in Asymmetric Allylic Substitutions 140
7.3 1,1,3-Triphenyl Allyl Palladium Complexes 151
7.3.1 Synthesis and Characterization of Complexes 62a-d 151
7.3.2 Solid-State Structure of 62a’ = [L1Pd2(Ph3C3H2)2]BArF24 154
7.3.3 NMR-Investigations of the Dynamic Behavior of 62a-d 156
7.3.4 Application of PyrBOX Triphenyl Allyl Complexes 62a-d in Asymmetric Allylic Substitutions 159
7.4 Summary for PyrBOX Allyl Palladium Chemistry 163
8 Introduction to Chirality in Coordination Compounds 167
8.1 Initial Results 168
8.2 Introduction to Chirality of Transition Metal Complexes 170
9 Supramolecular Coordination Chemistry of PyrBOX Ligands 180
9.1 PyrBOX Copper(II) Complexes 181
9.1.1 Solid State Structure of 68a = [(L1)6Cu5O](BF4)2 182
9.1.2 Magnetic Properties of 68a = [(L1)6Cu5O](BF4)2 186
9.1.3 Behavior in Solution of 68a = [(L1)6Cu5O](BF4)2 188
9.1.4 ESI-MS Investigations 191
9.1.5 Copper(II) Complexes 69d and 70d with Hydrolyzed HL4 197
9.2 PyrBOX Zinc(II) Complexes 202
9.3 PyrBOX Manganese(II) Complexes 205
9.3.1 General Remarks and Initial Experiments 205
9.3.2 Solid State Structure of 72a = [(L1)3Mn2]ClO4 209 9.3.3 Underlying Forces for the Trigonal-Prismatic Distortion in 72a 218
9.3.4 Magnetic Properties of 72a = [(L1)3Mn2]ClO4 221
9.3.5 Behavior in Solution of 72a = [(L1)3Mn2]ClO4 222
9.4 PyrBOX Cadmium(II) Complexes 224
9.4.1 General Remarks and Initial Experiments 224
9.4.2 Evaluation of Ligand Substituent Effects 226
9.4.3 Solid State Structure of 73a = [(L1)3Cd2]BF4 233
9.4.4 Behavior in Solution of 73a = [(L1)3Cd2]BF4 241
9.5 Preliminary Investigations with other Metal Ions 250 9.6 Summary and Outlook for Supramolecular Coordination Chemistry of PyrBOX
Ligands 254
10 Introduction into Oxidative Dehydrogenative Couplings 258
10.1 General Remarks 259
10.2 Atropisomerism 260
10.2.1 Axially Chiral Biaryl Natural Products 261
10.2.2 Axially Chiral Biaryl Ligands for Asymmetric Catalysis 262
10.3 Methods for the Atropselective Formation of Biaryls 263
10.3.1 Enantioselective Copper-Mediated Oxidative Coupling 264
10.3.2 Enantioselective Vanadium-Mediated Oxidative Coupling 268
10.3.3 Oxidative Coupling Mediated by other Transition Metals 269
10.3.4 Summary for Enantioselective Oxidative Coupling 271
11 Oxidative Coupling Reactions with Pyrazole Bridged Ligands 275
11.1 Preliminary Experiments 276
11.2 Basic Kinetic Investigations 282
11.2.1 General Remarks 282
11.2.2 Derivation of Rate Laws and Reaction Orders 283
11.2.3 Rate Behavior of the Catalytic Process 287
11.2.4 Influencing Factors on Catalytically Active Intermediates 292
11.2.5 Summary 294
11.3 Catalyst Preparation 295
11.3.1 General Synthetic Approach 295
11.3.2 Effect of the Copper- Ligand Stoichiometry 298
11.4 Identification of Active Species and Relevant Intermediates 301
11.4.1 ESI-MS Investigations 301
11.4.2 Role of the Halide Ions 308
11.4.3 Crystallization Experiments 312
11.4.4 Catalytic Experiments using 76 = [(L9)2Cu2H2] 318
11.5 Ligand Modifications and Mechanism for Enantiocontrol 321
11.5.1 Modification of the Aryl-Substituents 321
11.5.2 Mechanism of Enantiocontrol 323
11.5.3 Studies of the Copper(II) Complexes of Ligand H3L19 325
11.5.4 Modification of the Pyrrolidine Ring 335
11.5.5 Non-Linear Effects and the Role of Tetranuclear Structure 336
11.6 Related Ligand Systems and Essential Features for Reactivity 343
11.6.1 Copper(II) Complexes of Diaryl-Prolinol Ligands H2L27-29 344
11.6.2 Copper(II) Complexes of ProPhenol Ligand H3L30 348
11.6.3 Copper(II) Complexes of ProPyrazole Ligand H3L25 351
11.6.4 Copper(II) Complexes of ProPyrazole HL11 and H3L26 352
11.6.5 Summary 354
11.7 Effects of Additives on the Reaction Progress 355
11.7.1 Addition of [PPh4]Cl to Complexes derived from H3L10 355
11.7.2 Addition of Na[B(ArF)4]to Complexes derived from H3L10 357
11.7.3 Addition of Na[B(ArF)4]to other ProPyrazole Systems 363
11.8 Further Mechanistic Investigations 365
11.8.1 Temperature Dependence of the Rate Constants 365
11.8.2 Kinetic Isotope Effects 368
11.8.3 Radical Scavengers 372
11.8.4 Stoichiometric Reaction and Exclusion of Dioxygen 373
11.9 Application of other 2-Naphthols 376
11.9.1 Substrate Synthesis 376
11.9.2 Reactivity Studies 379
11.9.3 Investigations of the Homo-Coupling Reaction 383
11.10 Preliminary Results for the Cross-Coupling of 2-Napthols 386
11.11 Summary and Outlook 390
12 Concluding Remarks 394
13 Experimental Section 396
13.1 General Considerations 396
13.2 Chemical Analysis 396
13.2.1 Nuclear Magnetic Resonance Spectroscopy 396
13.2.2 Other Spectroscopy 397
13.2.3 Mass Spectrometry 397
13.2.4 Other Applied Methods 397
13.3 Ligand Synthesis 399
13.3.1 General Remarks 399
13.3.2 Applied Reagents or Starting Materials 400
13.3.3 Pyrazole-Bisoxazoline (PyrBOX) Ligands 401
13.3.4 Pyrazole-Bispyridyl-Bisoxazoline (PyrBOX) Ligands 410
13.3.5 Pyrazole-Bisprolinol (ProPyrazole) Ligands 422
13.4 Allyl Palladium Chemistry 456
13.4.1 General Remarks 456
13.4.2 Applied Reagents or Starting Materials 457
13.4.3 Cyclohexenyl Palladium Complexes 458
13.4.4 1,3-Diphenyl Allyl Palladium Complexes 463
13.4.5 1,1,3-Triphenyl Allyl Palladium Complexes 468
13.4.6 Asymmetric Allylic Substitution Reactions 473
13.5 Supramolecular Coordination Chemistry of PyrBOX Ligands 477
13.5.1 General Remarks 477
13.5.2 Copper(II) Complexes 477
13.5.3 Manganese(II) and Cadmium (II) Complexes 480
13.6 Oxidative Coupling Reactions with Pyrazole Bridged Ligands 483
13.6.1 General Remarks 483
13.6.2 Applied Reagents or Starting Materials 483
13.6.3 Substrate Synthesis 483
13.6.4 Catalyst Preparation and Product Characterization 489
Appendix 497
Appendix A: Chiroptical Methods 497
Appendix B: Basic Principles of Chemical Kinetics 499 Appendix C: Derivation of Rate Laws for Oxidative Couplings 501
Preequilibrium Assumption 501
Steady-State Approximation 502
Appendix D: Non-linear Effects (NLE) 504
Appendix E: List of Abbreviations 506
Appendix F: Crystallographic Data 510
Acknowledgement 517
1 General Introduction
In the wake of depleting natural resources and increased environmental awareness, a growing demand for a more economic and ecological production of chemical compounds has recently emerged, which is often summarized under the expression “Green Chemistry”. One key principle of this philosophy is the preferential usage of catalytic systems over stoichiometric reagents, from which the relevance of transition metal complexes equipped with an appropriate ligand scaffold arises. Hereby, the reaction of the substrate takes place at the metal center, which effectively increases reaction rates by lowering activation barriers. Furthermore, by modulating the catalyst design, chemo- and stereoselectivity can be achieved.
If, for instance, a chiral organic ligand is combined with a suitable transition metal, asymmetric induction is possible. Herein, small amounts of a chiral, enantiomerically pure (or enriched) catalyst, transfers its chiral information to prochiral or chiral racemic substrates, yielding large amounts of optically active products. The relevance of these enantiomerically pure compounds stems from the ubiquitous role of enantiomer recognition for biological activity. Controlling stereochemical outcome of a catalytic reaction therefore represents an eminently important undertaking and is still considered one of the most challenging tasks in synthetic chemistry.
1.1 Two-Center Catalysis
Over the past three decades the interest in two-center catalysis has rapidly grown. During this time, several active sites of dinuclear metalloenzymes have been characterized and many model complexes with analogous catalytic activity have been synthesized. These results provide a deeper insight into possible cooperative effects between two metal centers, indicating their considerable potential in catalysis and distinct advantages over single metal centers.[1]
A well-understood example of cooperative effects in enzymatic catalysis is the hydrolysis of urea by Urease.[2] The active site contains two Lewis acidic nickel ions (Ni‒Ni distance: 3.5 Å) which fulfill different functions crucial for the efficiency of the process (Scheme 1.1).
Scheme 1.1: Proposed mechanism at the active site of Urease.[3]
Due to the coordination of both substrates, a better nucleophile is created and the reactivity of the electrophile is further enhanced. This principle is often referred to as “dual activation”.[4] Apartfrom these enthalpic contributions, rate enhancements of up to a factor 108 for 1 M reactants can be achieved on entropic grounds. The bimetallic binding mode does not only provide a higher degree of activation but also achieves to orientate the urea and hydroxide ions in a favorable geometry for this reaction. Due to released binding energy prior to the reaction, the entropic loss for arranging both reactants can be compensated.
This effect often referred to as temporary or induced intramolecularity.[5] Over all, these cooperative effects result in an enhancement of the reaction rates up to a factor of 1014.[6]
Another important cooperative effect in two-center catalysis is the mediation of multi- electron transfers, which becomes apparent in the case of the Catechol Oxidase. In this type-3 copper enzyme the distance between the two metal centers varies between 2.9 Å and 4.4 Å.[7] In its oxy state, oxygen is dual side-on coordinated by two copper ions yielding a (μ-η2:η2-peroxo)dicopper(II) complex) I (Scheme 1.2). By means of this coordination, the normally kinetically inert oxygen is activated for the four-electron reduction to water, while the catechol substrate is oxidized to the corresponding 1,2-benzoquinone.
Scheme 1.2: Catechol oxidase: Oxy-state and catalytic oxidation of 1,2-dihydroxybenzene to 1,2-benzoquinone.
Moreover, other cooperative effects for synthetic catalysts have been documented. For instance, Stanley et al. reported an improved regioselectivity and reactivity in the hydroformylation reaction by using a dinuclear rhodium catalyst II (Rh‒Rh-distance: 5.5 Å) as opposed to a mononuclear complex III (see Figure 1.1).[8]
Figure 1.1: Hydroformylation-catalysts: active catalyst II, less active analogs III, IV.
Furthermore, low or no activities were observed for another bimetallic catalyst IV. As the steric and electronic properties of these ligands are similar to II, the proposed mechanism involves intramolecular hydride transfer between the rhodium centers. As no beneficial effects of the two metal centers could be observed for IV, it was deduced that a specific Rh‒
Rh-distance is crucial for the cooperativity.
In addition to these selected examples, cooperative effects have been observed for several types of artificial catalysts[9] and considerable efforts are made to understand and control these interactions.
1.2 Pyrazole Bridged Ligands
The importance of the metal-metal separation for the facilitation of cooperative effects is a well-established fact. For that purpose, a large variety of different bridging units have been reported.[1,10]
Out of this group, the pyrazole unit is a frequently used example. After deprotonation it is able to span between two metals in the range of 3.4‒4.5 Å and accordingly facilitate cooperative interactions.[11] The pyrazolate anion acts as a strong σ-donor and is regarded as close to π-neutral.[12] In general, three different coordination modes for the pyrazolate unit have most commonly been observed (see Figure 1.2: A,B,C).[13]
In order to achieve stabilized bimetallic complexes, additional sidearms bearing donor-atoms can be introduced creating a chelating polydentate binding site (see Figure 1.2: D). By modulation of these ligand sidearms (chelate ring size, nature and quantity of the donor- atoms), fine tuning of the properties (metal-metal distance or angles) can be achieved.[14]
Figure 1.2: Coordination behavior of the pyrazolate anion: (A) monodentate; (B) endo- bidentate; (C) exo-bidentate; (D) exo-bidentate with additional coordinating sidearms (L).
A large number of 3,5-disubstituted pyrazoles with mono-, bi- and tridentate sidearms and their di-, oligo- and polynuclear transition metal complexes have been documented.[15] One of the most efficient and commonly used synthetic approaches towards 3,5-disubstituted pyrazoles is the ring-closure reaction of 1,3-diketones with hydrazine (Scheme 1.3).
Scheme 1.3: Ring-closure condensation reaction of 1,3-diketones with hydrazine to 3,5- disubstituted pyrazoles.
Apart from this, synthetic pathways to other pyrazole building blocks are well established and can be performed on up to 100 g scales, starting from cheap, commercially available
compounds. The multi-step syntheses of the pyrazole starting materials, which are relevant for this thesis, are briefly recapitulated.
Pyrazole precursors with a hydrogen atom in the 4-position can be synthesized starting from 3,5-dimethylpyrazole V which is oxidized with potassium permanganate, followed by an acid catalyzed esterfication yielding the corresponding diester VIIa.[16]
Scheme 1.4: Synthesis of pyrazole building blocks 1.
Phenyl substitution in the 4-position can be easily achieved by the following synthetic approach. Methyl 2-aminoacetate is first converted to the diazo compound VIII with sodium nitrite under acidic conditions (see Scheme 1.5).[17] Pyrazoline X is then formed by a dipolar cycloaddition of VIII with methyl cinnamate IX. Subsequent oxidation with bromine produces pyrazole VIIb.[18]
Scheme 1.5: Synthesis of pyrazole building blocks 2.
Both pyrazole diesters VIIa and VIIb can be further modified (Scheme 1.6). After reduction with lithium aluminium hydride the according diols are obtained,[17,18] which are consecutively chlorinated with thionyl chloride and protected at the pyrazole NH with 2,3-dihydropyrane.[17,19,20] Typically, the final ligand is obtained after coupling of the desired sidearm with one of these or similar pyrazole building blocks.
Scheme 1.6: Synthesis of pyrazole building blocks 3.
Recently, Akita and coworkers published the synthesis of a new pyrazole building block which allows for shorter metal-metal separations and a higher degree of flexibility for the resulting complexes (Scheme 1.7).[21] After acylation of XIV with XIII, 1,3-diketone XV is obtained which can be readily converted to the corresponding pyrazole XVI by treatment with hydrazine. After reduction with lithium aluminium hydride and chlorination of alcohol XVII, the final building block XVIII is obtained.
Scheme 1.7: Synthesis of pyrazole building blocks 4.
On the basis of these or other precursors, many pyrazolate containing complexes have been synthesized and widely adapted in different branches of inorganic chemistry.
For instance, pyrazolate-based model complexes for Catechol Oxidase have been intensively studied[22] and applied in bio-inspired selective benzylic C‒C coupling.[23] Furthermore, model complexes which mimic the active sites of dinuclear hydrolase enzymes such as Urease[24]
and Phosphodiesterase[25] have been reported.
Additionally, dinuclear pyrazolate-bridged palladium(II) and nickel(II) complexes have been employed as catalysts in the C‒C-coupling of simple olefins[26] and in the polymerization of norbornene.[27] Various dinuclear ruthenium polypyridine complexes containing a central pyrazole ring have demonstrated their unique potential in water-oxidation catalysis.[28]
Beyond their manifold implementation in catalysis, other pyrazolate complexes have been isolated showing unique electronic[29] and magnetic properties.[30]
1.3 Asymmetric Catalysis
As aforementioned, chiral compounds and their synthesis are of outmost importance due to their biological activities, thus, various different approaches have been developed (Figure 1.3).
Figure 1.3: Approaches towards asymmetric synthesis: * represents chiral molecule, A = starting material, P = product, X = auxiliary, Y = resolution reagent, cat = catalyst, for chiral resolution absolute configurations were assigned at random.
The oldest synthetic concept uses nature’s readily available chiral pool, which is often limited to the availability of the starting materials. Further progress could be made using chiral auxiliaries, however still stoichiometric amounts of chiral reagents are necessary, while their de- and attachment yields two additional synthetic steps. Chiral resolution represents an efficient methodology for the intermolecular chirality transfer, but even in the event of complete enantiomer selection, the maximum yield cannot exceed 50%, unless a racemization of the substrate occurs. Consequently, catalytic chirality-creating processes are in the majority of cases the preferred method in asymmetric synthesis if available.
Although enantioselective catalysis has been known for more than 40 years,[31] this field has recently received greater attention, since in 2001 Knowles, Noyori and Sharpless won the nobel prize in chemistry for their contributions in the field of chirally catalyzed hydrogenation and oxidation reactions.[32]
The general concept of asymmetric catalysis can be defined as follows: By reaction of the chiral catalyst with the prochiral substrate, several diastereomeric transition states are formed (Figure 1.4). Due to electronic and steric interaction between the substrate and the catalyst, these transition states differ in their relative energies (∆G1‡ and ∆G2‡) resulting in
different rates for the product formation. The outcome of the energetic differentiation is the kinetically preferred formation of one of the two enantiomeric products, which is described by the following term for any given temperature T (R = gas constant).
[enantiomer 1]
[enantiomer 2]= exp(−(∆G1‡−∆G2‡)
RT )= exp(−∆∆G‡ RT )
Figure 1.4: Energy diagram for asymmetric catalysis; TS1/TS2 = Diastereotopic transition states; production of enantiomer 1 is favored, due to the lower relative energy of TS1 / lower free activation enthalpy ∆G2‡ to enantiomer 1.
Since the ground-breaking work of the previously mentioned nobel laureates, significant efforts have been made in establishing synthetic methods to effectively control the absolute stereochemistry. Often the complexity of most catalytic processes precludes an approach purely based on structural and mechanistic criteria. Therefore the effectivity of a catalyst is mostly found empirically by chance and/or after screening all possible important factors.[33]
In general, the catalysts in asymmetric synthesis can be divided into chiral organocatalysts, biocatalysts and metal complexes with chiral ligands. Even though promising results have been achieved with organocatalysts[34] and many enzyme catalyzed transformations have proven to be very efficient, the following discussion will be focused on chiral metal ligand complexes only.
1.3.1 Privileged Ligands and C
2-Symmetry
Among the numerous reported chiral ligands, only a few have shown a widespread applicability. These so-called privileged ligands,[35] have proven to reliably introduce chiral information to a wide range of substrates in a large number of different metal catalyzed reactions. This generality of scope can only be observed in a few cases in enzymatic catalysis, which underlines the significance of these privileged ligand classes.
Figure 1.5: C2-symmetric privileged ligand structures.[35]
One common structural feature of these molecules is their rigid framework, with multiple oxygen, nitrogen or phosphorous donor functionalities, which allows them to strongly bind the reactive metal centers. Furthermore, the presence of a two-fold axis is a shared feature of most of these ligands (see Figure 1.5).
The origin of the two-fold symmetry is different in each case. While BINOL and BINAP are purely synthetic molecules, BOX and TADDOL are derived from readily available natural sources, primary amino acids and tartaric acid, respectively.[35]
Historically, the first contributions to C2-symmetric catalysts were made by Dang and Kagan[36] in 1971. In comparison to nonsymmetric ligands, their diphosphine ligand (DIOP) significantly improved the enantioselectivity in the rhodium catalyzed homogeneous
asymmetric hydrogenation of dehydroamino acid derivatives (Figure 1.6). Based on this work, other C2-symmetric ligands, such as P-chiral DiPAMP, were synthesized,[37] which then again turned out to be superior to their nonsymmetric counterparts.[33]
Figure 1.6: Kagan's DIOP and Knowles' DiPAMP ligand.
This beneficial effect of C2-symmetry on enantioselectivity is often attributed to halving of the number of possible diastereomeric transition states (Figure 1.7).[38] Therefore, less competing reaction pathways are possible, leading to higher enantioselectivity. In addition to this, the mechanistic analysis is often facilitated by the two-fold symmetry, as less transition state geometries have to be taken into account.[33]
Figure 1.7: Beneficial effect of the C2-symmetry for metal-bis(oxazoline) complexes; blue represents favored and red disfavored trajectories, due to steric repulsive forces.[39]
Overall, C2-symmetric ligands have demonstrated their relevance in asymmetric synthesis during recent years and still play a crucial role in enantioselective transition-metal catalysis.
Using the example of bis(oxazoline) or BOX ligands (as well as the derived PyBOX) one class of the privileged ligands will be discussed further in the following section.
1.3.2 Bis(oxazoline) (BOX) Ligands
Despite the fact that the synthesis of 4,5-dihydro-1,3-oxazoles (commonly known as oxazolines) has been known since 1884[40] and their versatility in many areas of chemistry has been shown before,[41] widespread application of these ligands in asymmetric catalysis did not set in until the early 1990s.
Figure 1.8: C2-symmetric bis(oxazolinyl)pyridine (PyBOX) ligand (XIX) and bis(oxazoline) ligands (XX, XXI).
The first tridentate C2-symmetric bis(oxazolinyl)pyridine-type ligand XIX (PyBOX) was reported by Nishiyama[42] and will be discussed later (Figure 1.8).
Subsequently, Masamune[43] and Evans[44] reported the preparation of bidentate bis(oxazolines) (BOXes) XX and XXI, and their application in asymmetric cyclopropanations. These results and the communication by Corey et al. about enantioselective Diels-Alder reactions[45] marked the beginning of the widespread success of BOX ligands. Since then, several different BOX ligands and complexes have been synthesized and applied in various catalytic reactions.[46]
By 2006, more than 140 different BOX ligands derived from malonic acid had been reported, largely due to their easy and flexible synthesis. Most of those synthetic approaches follow the original method carried out by Evans[44] and Corey[45] (A1) (see Scheme 1.8). This approach starts from disubstituted malonyl dichloride which is reacted with 2 equivalents of a β-amino alcohol (available after reduction of the corresponding amino acid). The resulting bis-amide is chlorinated with thionyl chloride and then cyclized under basic conditions. The hydroxy group can alternatively be transformed into other good leaving groups with mesyl or tosyl chloride (A2).[47] The original Masamune protocol uses Bu2SnCl2 in refluxing xylene (A3),[43,48] while Evans established the one-pot cyclization including an initial Appel-reaction using a mixture of Ph3P/CCl4/Et3N as reagents (A4).[49]
Scheme 1.8: Most common routes to 4-substituted bis(oxazolines).[46]
A variety of other methods have been developed applying other starting materials. For instance, the oxazoline ring can be created by reaction of an appropriate nitrile with the desired amino alcohol, using either catalytic amounts of zinc salts[50] or prior activation to the corresponding imidate[51] under acidic or basic conditions (Scheme 1.9).
Scheme 1.9: Synthetic route towards 4,5-disubstituted bis(oxazolines).[46]
Due to this broad variety of different BOX ligands and complexes, their coordination chemistry is well understood.[39,52] In general, BOX ligands coordinate metal centers in a bidentate fashion and can stabilize chelate ring sizes from five- to nine-membered rings. In these complexes the coordination number is mostly four or six, and the coordination takes place exclusively through the nitrogen atoms. This nitrogen coordination mostly stabilizes relatively low metal oxidation states due to its ability to act as a π-acceptor.[53] Due to the Brønsted acidity of the methylene bridge, bis(oxazolines) derived from malonic acid (Figure 1.8, see XXI, R1 = H) can be deprotonated. The resulting anionic ligands are, owing to their σ- and π-donor properties, able to stabilize high-metal oxidation states.[54]
1.3.3 Bis(oxazolinyl)pyridine (PyBOX) Ligands
Although PyBOXs (Figure 1.8: XIII) are often integrated in the group of BOX ligands, this ligand design occupies a firm place on its own in asymmetric catalysis.
As already mentioned, the first PyBOX ligand was synthesized by Nishiyama in 1989 (Scheme 1.10).[42] The pyridine ring herein acts as a spacer between both oxazoline moieties, resulting in a tridentate ligand which forms mostly hexacoordinated (Ru, Rh, Mo, W, Re) or tetracoordinated (Pd, Cu) metal complexes.[52]
Generally, the synthetic approaches to this type of ligands are similar to the ones discussed for the bis(oxazolines) (Scheme 1.8). Instead of malonic or oxalic acid, dipicolinic acid derivates such as XXII are used as precursors.
Scheme 1.10: Original synthetic protocol by Nishiyama.[42]
PyBOXs are regarded as a direct evolution of the BOX ligands. They combine all the advantages of BOX ligands with the introduction of a new pyridine donor functionality to yield a more rigid tridentate ligand scaffold, which has demonstrated its versatility in numerous catalytic asymmetric transformations.[52,55]
Generally, it can be said that oxazolines feature several interesting properties, a large variety of applications and a straightforward synthesis from readily available chiral precursors, which allows control of the chiral center next to the coordinating nitrogen atom.
1.4 Cooperative Asymmetric Catalysis
In the previous subchapters, the advantages of two-center catalysis were outlined and basic concepts of asymmetric catalysis were discussed. Next, examples will be given that beneficially unite both of these fields, which is often referred to as cooperative asymmetric catalysis.
1.4.1 ProPhenol Catalysts
The first striking example for the beneficial effect of two metals in asymmetric catalysis was reported by Trost et al. with the introduction of the ProPhenol ligand XXIII almost 15 years ago.[56] After addition of two equivalents of diethylzinc (or dimethylzinc) to the chiral ligand XXIII, the active catalyst is assumed to be a dinuclear complex, which was derived from ESI-MS experiments.[57] With this in situ formed catalyst system, aldol reactions of unactivated ketones could be performed, hence with perfect atom-economy, in a very efficient and selective manner (Scheme 1.11).
Scheme 1.11: ProPhenol ligand XXIII and its application in the direct catalytic enantioselective aldol reaction.[56]
One great asset of this ligand system is its straightforward synthesis and the facile introduction of chirality by starting from the commercially available enantiopure amino acid
L-proline. The different synthetic routes to the ProPhenol ligand are shown in Scheme 1.12.
The first approach starts from the dibromide XXIV. This is coupled with L-proline methyl ester XXV to XXVI which further reacts under addition of phenylmagnesium chloride to yield XXIII.[56]
A second route was subsequently published in which the dibromide XXIV was directly coupled with the (S)-prolinol XXVa (synthesized from L-proline). In order to modify the catalyst design, it was shown that the phenyl groups can be easily replaced by other aryl groups, such as 4-biphenyl or 2-naphthyl. These ligands bear more sterically demanding groups, often resulting in an enhancement of the enantiodiscrimination.[58] Furthermore, significant effects or the electronic properties of the diarylcarbinol substructure,[59] as well as the bridging unit[58] have been reported.
Scheme 1.12: Synthetic route to ProPhenol XXIII.[58]
The basic concept of this self-assembled catalyst systems will be discussed in the following section. Initially, the active dinuclear zinc catalyst is expected to form after addition of two equivalents of dialkylzinc reagents to XXIIIa which results in evolution of three equivalents of the corresponding hydrocarbon gas, as depicted in Scheme 1.13.[60]
Scheme 1.13: Bifunctionality of Trost’s ProPhenol catalyst.[60]
It was furthermore proposed that the dinuclear complex XXIIIa exhibits certain bifunctionality as a result the presence of a Lewis acidic zinc atom, as well as a Brønsted base in close proximity. Due to this feature the complex is capable of dual activation of both
nucleophiles and electrophiles within the chiral space created by the ligand. For intermediate XXIIIb, according to this model, the stereoselective attack of the nucleophile sets the stereochemistry and the resulting zinc alkoxide restarts the catalytic cycle as the new Brønsted base.[61]
After the introduction of the ProPhenol system, other ligands (XXVII, XXVIII, XXIX) were reported which incorporate a diphenylprolinol group and are also potentially capable of forming dinuclear complexes (Figure 1.9).[62] However, only BINOL-derived XXIX was employed for the direct asymmetric direct aldol addition and a dinuclear zinc catalyst was proposed.[63] The reactivity and selectivity of this system were significantly lower, and further activators had to be added. Thus, the ProPhenol catalyst design might only be matched in the field of transition metal catalyzed aldol reaction by heterobimetallic systems XXX[64a-b,65]
or Zn3/linked BINOL catalysts XXXI[64b-c,66] introduced by Shibasaki. These latter catalyst systems further underline the beneficial role of two metal centers in close proximity for this type of transformations.
Figure 1.9: C2-symmetric diphenylprolinol incorporating ligands XXVII-XXIX and other efficient multimetallic catalysts XXX and XXXI for the direct aldol reaction.
Apart from its initial application in direct aldol reactions, the ProPhenol catalyst design has been successfully adopted in asymmetric catalysis for many other reactions such as the Henry-reaction,[67] the Aza-Henry-reaction,[68] Mannich-type transformations[69] and the Michael Addition.[70] Furthermore, it was successfully employed in total synthesis several times.[71] When compared, the ProPhenol ligand generally outcompetes its mononucleating analogs or related compounds in both reactivity and selectivity.[72]
One reasonable explanation for its unique catalytic behavior might be derived from the structural properties of the related dinuclear zinc complex reported by Ding et al. They succeeded in isolating the first dinuclear p-nitro-phenoxy-brigded ProPhenol complex XXIIIc as single crystals (Scheme 1.14).[73]
Scheme 1.14: Synthesis of dinuclear p-nitro-phenoxy-brigded ProPhenol complex XXIIIc.[73]
In this complex, a rather short Zn-Zn distance of 3.17 Ǻ is found, which might be essential for enabling bifunctionality for such dinuclear zinc complexes mimicking class II aldolase enzymes.[74] Interestingly, within this study about the copolymerization of CO2 with cyclohexene-oxide, the crystalline material of XXIIIc did not produce any polymers in contrast to the in situ formed catalyst.[73,75a] The authors attributed this lack of reactivity to the inhibiting binding of THF molecules to the Lewis acidic zinc atom.
Besides the zinc complexes of XXIII, a few examples exist in which the magnesium,[75]
gallium[72a,b] or aluminium[76] complexes of ProPhenol were applied instead, indicating the ability for coordination of various metals.
Overall, the ProPhenol ligand has clearly proven to be a versatile tool for asymmetric synthesis.
1.4.2 Salen Catalysts
Another persuasive classic example for the beneficial effects of two proximate metal centers in asymmetric catalysis are the metal salen complex catalyzed reactions reported by Eric N.
Jacobsen and others. These systems were first applied in the ring-opening of meso-epoxides,[77] leading to a variety of biologically interesting compounds (Scheme 1.14).[78]
Scheme 1.14: Asymmetric ring opening (ARO) of meso-epoxides catalyzed by chromium salen complexes; XXXIIa acts as precatalyst.[78]
The group further succeeded in isolating crystalline material suitable for X-ray crystallography for the THF adduct of the active catalyst XXXIIb. This coordination might indicate a potential Lewis acidic role of the chromium center, apart from the initial activation of TMSN3 (or rather the hydrolysis product HN3). This hypothesis was supported by the observed second-order rate dependence on the catalyst concentration, as well as the significant non-linear effects observed for the enantiopurity of the catalyst on the enantioselectivity of the reaction.[77a,79] Thus, despite its mononuclear nature, two molecules of XXXIIb are presumed to be involved in the rate- and selectivity determining step. Hence, a dual activation is assumed, in which one metal center provides the nucleophile while the other enhances the electrophilicity of the epoxides (Figure 1.10).
Figure 1.10: Model for cooperative effects for metal salen complex.
Jacobsen further assessed the potential of the related salen cobalt complexes in the hydrolytic kinetic resolution (HKR) of terminal epoxides, which gives rise to a large variety of
different enantiomerically enriched 1,2-diols, as well as otherwise hardly accessible epoxides (Scheme 1.15).[80]
Scheme 1.15: Hydrolytic kinetic resolution (HKR) catalyzed by cobalt salen complexes.[80]
Detailed mechanistic investigations (Scheme 1.16) on catalyst system XXXIIIa revealed, that although both enantiomers possess similar binding affinities (see KE,mat and KE,mis), selectivity is determined by the complex providing the nucleophile. Further, both reactants (H2O and epoxides) were found to bind similarly strong to the cobalt centers.
Scheme 1.16: Kinetic parameters for the HKR of 1-hexene oxide (R = C4H9) catalyzed by XXXIIIa.[81a]
Additionally, the effect of the counterion Y was studied (Scheme 1.17), revealing the highest activities for equimolar mixtures of two different complexes (Y = OH and Y ≠ OH). For these investigations, varying the latter complex revealed a strong dependence on the nature of counterions (Y = Cl < OAc < OTos < SbF6) for achieving an efficient transformation. This was attributed to the unequal nucleophilicity of the counterions for the undesired side-reaction with the epoxide substrate (see Scheme 1.17, left), which changes the ratios of the active catalytic species present in solution.
Scheme 1.17: Dominant catalytic cycle in HKR reactions catalyzed by Co-Y (Y ≠ OH), where addition of Y to epoxide is incomplete.[81a]
For Y = Cl this addition is fast, thus, Y = OH becomes the predominant but less Lewis acidic species. On the other hand, for less-nucleophilic counterions (Y = OTos), the ratio of the Lewis acidic and the nucleophilic bearing complex (Y = OH) is in principal constant over the course of the reaction, thus, optimal for an efficient conversion.[81] This counterion effect further stresses the opposite roles of the two metal centers for their cooperative working mode.
Various dinuclear analogs of the chromium complexes with variable covalent linkages XXXIVa-XXXIVg were subsequently synthesized (see Figure 1.11, left), in order to enforce cooperativity for the aforementioned desymmetrization reaction.[82]
Figure 1.11: Schematic representation of the various synthesized chromium salen complexes.[82]
Some of these systems showed displayed reactivities 1-2 orders of magnitudes greater than of the mononuclear catalyst XXXV, without loss of enantioselectivity (see Figure 1.12).
Figure 1.12: Initial rate kinetics for the asymmetric ring opening (ARO) of cyclopentene oxide catalyzed by various chromium salen complexes.[82]
Kinetic investigations further revealed the presence of competing intra- and intermolecular pathways. At high catalyst concentrations a second order dependence and non-linear behavior is observed, while at low concentration a strictly linear effect (see Figure 1.13) and a first order dependence on the catalyst concentration has been reported.
Figure 1.13: Plot of the enantiopurity of the catalyst versus the enantioselectivity of the ARO reaction of cyclopentene oxide and TMSN3 catalyzed by chromium salen complexes (left).[82]
By this approach the ratio for the rates for intra- to intermolecular reaction could derived, which is often quantified by the effective molarity EM (= kintra/kinter).[83] This value in concentration units describes the catalyst concentration necessary in order for the intermolecular reaction to kinetically compete with the intramolecular pathway.
The following introduction of oligomeric salen variants can be seen as the next step in the evolution of the original mononuclear catalyst.[84] Further, polymer supported[85] and dendrimeric catalysts[86] were introduced, which also provided remarkable rate enhancements.
For the previously discussed examples reactivity enhancement was achieved by linkage in the ligand backbone, however, anionic co-ligands such as oxides and halides are as well capable of stabilizing similar dinuclear aggregates. One prominent example is the oxo-bridged titanium salen complex XXXVI introduced by North,[87] which allowed the isolation of various cyanohydrine derivates (Scheme 1.18).[88]
Scheme 1.18: Cyanation of aldehydes catalyzed by dimeric titanium salen complex XXXVI.[87]
While in solid state, various isomers can be adopted,[87,89] in solution 17O- as well as 1H-NMR spectroscopy indicated an additional equilibrium with the 5-fold coordinated mononuclear oxo titanium complex.[89,90]
Subsequent kinetic investigations stressed the importance of this equilibrium, as for the catalyst concentration an order in between 1 and 2 was determined, depending on the type of ligand used. Based on the performed experiments, the authors ultimately concluded a dimeric nature of the active catalyst, which could be supported by the following competition experiment (see Table 1.1). By mixing XXXVI with an achiral catalytically less active analog XXXVII, a lower activity and selectivity was observed, thus, a new mixed bimetallic complex
can be presumed.[89] In case of a monomeric active catalytic species, this should still be present in solution, therefore dominate the catalytic process giving rise to high conversions and selectivities.
entry catalyst conv. (%) ee (%)
1 XXXVI 100 82
2 XXXVII 34 0
3 XXXVI + XXXVII 20 34
Table 1.1: Evaluation of various dimeric titanium complexes for the cyanation of aldehydes.[89]
The relevance of the dimeric structure was later underlined by analogous catalysts possessing a linkage in the ligand backbone reported by Ding.[91]
Another related outstanding demonstration for the opportunities cooperative asymmetric catalysis offers, is the enantioselective conjugate addition of cyanide (see Table 1.2). Initially, the Jacobsen group had to apply high catalyst loadings of otherwise highly active aluminium salen complexes XXXVIII[92] for this particular transformation.[93]
entry catalyst conv.(%) ee (%)
1 (S,S)-XXXVIII < 3 -
2 (S,S)-XXXIX < 3 -
3 (S,S)-XXXVIII
+ (S,S)-XXXIX 99 96
Table 1.2: Conjugate addition reaction for various catalysts.
Optimization could subsequently be achieved by combination of two catalyst systems. It was assumed, that the activation of TMSCN by the aluminium complex was not sufficient, thus, a previously reported lanthanide PyBOX complex XXXIX was chosen that showed no activity in the conjugate addition reaction. By combining each catalyst’s independent activation, significant improvements could be achieved.[94] The authors proposed an involvement of both complexes in the rate-determining step, while both are cooperatively inducing chirality.
Interestingly, a later introduced dinuclear aluminium analog further improved the catalyst system and additionally expanded the scope of the reaction.[95]
In a nutshell, the discussed examples may provide basic understanding and guidelines for cooperative asymmetric catalysis. On one hand, Trost’s compartmental ProPhenol ligand represents a bio-inspired approach, in which the central bridging unit is crucial for providing reactivity and selectivity within the chiral pocket. The salen catalyzed reactions, pioneered by Jacobsen on the other hand, represent a mechanism guided approach. These investigations ultimately lead to the development of bimetallic and polymetallic complexes, which outcompeted their corresponding mononuclear analogs. By linkage of two (or more) isolated donor domains with an appropriate spacer and/or the presence of bridging co-ligands, the dinuclear nature of these catalysts can be enforced. Finally, heterobimetallic combinations with their adverse reactivities can provide fascinating opportunities.
Apart from the herein discussed systems, various other binucleating systems such as Shibasaki’s chiral Schiff-base catalysts,[96] as well as their success in CO2/epoxide copolymerizations[97] or other transformations,[4,98,99] further emphasize the advantages that can arise from cooperative asymmetric catalysis.
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