Determining the Enantioselectivity of Chiral Catalysts – Mass Spectrometry as a Mechanistic and Screening Tool
Inauguraldissertation
zur
Erlangung der Würde eines Doktors der Philosophie vorgelegt der
Philosophisch-Naturwissenschaftlichen Fakultät der Universität Basel
von Florian Bächle
aus Bad Säckingen, Deutschland
Basel, 2014
Genehmigt von der Philosophisch-Naturwissenschaftlichen Fakultät auf Antrag von
Prof. Dr. Andreas Pfaltz Prof. Dr. Dennis Gillingham
Basel, den 20.05.2014
Prof. Dr. Jörg Schibler
Dekan
für Anja
“Our greatest glory is not in never failing,
but in raising up every time we fail.”
Ralph Waldo Emerson
This thesis was supervised by Prof. Dr. Andreas Pfaltz from April 2010 to May 2014 at the University of Basel, Department of Chemistry.
Parts of this work have been previously published:
“Organocatalytic Asymmetric Conjugate Addition of Aldehydes to Nitroolefins: Identification of Catalytic Intermediates and the Stereoselectivity-Determining Step by ESI-MS“
F. Bächle, J. Duschmalé, C. Ebner, A. Pfaltz, H. Wennemers, Angew. Chem. Int. Ed. 2013, 52, 12619-12623.
Acknowledgement
First and foremost my thanks goes to my mentor Prof. Dr. Andreas Pfaltz for the opportunity to work in his research group and for his constant support and confidence.
Special thanks goes to Prof. Dr. Dennis Gillingham, who agreed to co-examine this thesis and to Prof. Dr. Oliver Wenger for charring the examination.
I am very thankful to Prof. Dr. Helma Wennemers, Dr. Jörg Duschmalé and Dr. Christian Ebner for the fruitful collaboration we had.
A very big thank you goes to Dr. Denise Rageot, Dr. Jürgen Rotzler, Dr. Tom Eaton and Dr.
Marc Müller for the time they invested for critically proofreading and improving this manuscript.
I want to thank all my colleagues I was working with over the last four years for the enthusiastic chemical discussions and fruitful working atmosphere in the department and our group. I want to thank Johanna Auth and Jaroslav Padevet for recording 2D NMR spectra.
Specially acknowledged is Marc Müller for the pleasant years as my lab mate and friend and for his support in every chemical issue. A big thanks goes also to Mr. Frenchy for sharing his knowledge and the stimulating discussions we had.
I want to thank Denise Rageot, Adnan Ganic, Christian Ebner, Lars Tröndlin, Dominik Frank, Johannes Hoecker, José Gomes and Jürgen Rotzler for the great time we spent together in the institute and elsewhere.
My students Patrick Isenegger, Cedric Stress, Joel Rösslein, Mirjam Schreier, Cedric Hugelshofer, Daniel Ris and Florian Lüttin are gratefully acknowledged for their contribution to this research and their enthusiastic lab work during their practical courses.
A very, very big thank you goes to Marina Mambelli for all her organizational work, her efforts to make our daily life more pleasant and for being a very good friend.
I want to thank all the staff of the University of Basel, especially the Werkstatt Team for their technical support.
Financial support by the Swiss National Science Foundation is gratefully acknowledged.
Ich möchte mich bei meiner ganzen Familie und allen meinen Freunden bedanken. Ganz besonders möchte ich mich bei meiner Oma, meiner Mutter und meinem Vater bedanken die immer für mich da waren und mich in allen Lebensbereichen so unglaublich unterstützt haben und mich zu der Person gemacht haben die ich heute bin. Ohne euch wäre all dies nicht möglich gewesen.
Zu guter Letzt möchte ich mich bei der wichtigsten Person in meinem Leben bedanken.
Danke Schatz für deine Liebe, deine Geduld, dein Rückhalt und deine uneigennützige Unterstützung über all die Jahre.
T
ABLE OFC
ONTENTS1" INTRODUCTION" 3"
1.1! MASS"SPECTROMETRY"&"HIGH6THROUGHPUT"SCREENING"IN"ASYMMETRIC"CATALYSIS" 3!
1.2! THESIS"OUTLINE" 8!
2" ESI6MS"SCREENING"OF"ORGANOCATALYZED"ALDOL"REACTIONS" 11"
2.1! INTRODUCTION" 11!
2.1.1! HISTORICAL!OVERVIEW! 11!
2.1.2! MECHANISM!OF!THE!PROLINE5CATALYZED!ALDOL!REACTION! 13! 2.1.3! PROLINE!AMIDE5BASED!ORGANOCATALYSTS!FOR!THE!ALDOL!REACTION! 15! 2.2! PRINCIPLE"OF"THE"ESI6MS"SCREENING"OF"ALDOL"REACTIONS" 21!
2.3! PRELIMINARY"EXPERIMENTS" 22!
2.3.1! IDENTIFICATION!OF!ALDOL!PRODUCTS!SUITABLE!FOR!THE!BACK!REACTION!SCREENING! 22! 2.4! SYNTHESIS"OF"ISOTOPE6LABELED"QUASIENANTIOMERIC"ALDOL"PRODUCTS" 26! 2.4.1! SYNTHESIS!OF!THE!UNLABELED!ALDOL!PRODUCT! 26! 2.4.2! SYNTHESIS!AND!APPLICATION!OF!THE!DEUTERATED!ALDOL!PRODUCT! 28! 2.4.3! STUDIES!TOWARDS!THE!SYNTHESIS!OF!13C5LABELED!ALDOL!PRODUCTS!WITH!UNLABELED!ACETONE!AS!TEST!
SUBSTRATE! 30!
2.4.4! PREPARATION!OF!13C5LABELED!ALDOL!PRODUCTS! 34! 2.5! ELABORATION"OF"A"GENERAL"ESI6MS"SCREENING"PROTOCOL" 35!
2.5.1! INFLUENCE!OF!THE!MASS5LABEL! 35!
2.5.2! FIRST!RESULTS,!INFLUENCE!OF!DILUTION!CONDITIONS!AND!AN!UNEXPECTED!ADDITIVE!EFFECT! 36! 2.5.3! CH3CN!AS!SOLVENT!FOR!THE!ESI5MS!SCREENING! 38! 2.5.4! ACIDS!AS!REACTION!ADDITIVES!FOR!THE!ESI5MS!BACK!REACTION!SCREENING!IN!CH3CN! 43!
2.6! CATALYST"SYNTHESIS" 48!
2.6.1! SYNTHESIS!OF!AMINOALCOHOLS! 49!
2.6.2! SYNTHESIS!OF!PYRROLIDINE5BASED!ORGANOCATALYSTS! 51! 2.6.3! SYNTHESIS!OF!PRIMARY!AMINES!AS!ORGANOCATALYSTS! 55!
2.7! ESI6MS"SCREENING"OF"SINGLE"CATALYSTS" 57!
2.7.1! ESI5MS!SCREENING!OF!SINGLE!CATALYSTS!IN!MEOH! 57! 2.7.2! ESI5MS!SCREENING!OF!SINGLE!CATALYSTS!IN!CH3CN!WITH!TERT5BNP!AS!ADDITIVE! 60! 2.7.3! ESI5MS!MONITORING!OF!TEMPERATURE!EFFECTS!FOR!ALDOL!REACTIONS!IN!CH3CN! 65! 2.7.4! TERT5BNP!AS!DILUTION!ADDITIVE!FOR!THE!ESI5MS!BACK!REACTION!SCREENING! 66! 2.7.5! ESI5MS!SCREENING!OF!PRIMARY!AMINES!AS!ORGANOCATALYSTS!FOR!ALDOL!REACTIONS! 68!
2.8! MULTI6CATALYST"SCREENING" 71!
2.8.1! MULTI5CATALYST!SCREENING!OF!EQUIMOLAR!MIXTURES!OF!PURIFIED!CATALYSTS! 71! 2.8.2! SYNTHESIS!OF!CATALYST!MIXTURES!USING!HATU!AS!COUPLING!REAGENT! 74! 2.8.3! SYNTHESIS!OF!CATALYSTS!MIXTURES!APPLYING!BERKESSEL’S!PROTOCOL! 76!
2.9! OPTIMIZATION"OF"PREPARATIVE"ALDOL"REACTIONS" 79!
2.10! CONCLUSION" 82!
3" ESI6MS"SCREENING"OF"PHOSPHINES"AS"CATALYSTS"FOR"MORITA6BAYLIS6HILLMAN""
REACTIONS" 85"
3.1! INTRODUCTION" 85!
3.1.1! MORITA5BAYLIS5HILLMAN!REACTION! 85!
3.1.2! BIFUNCTIONAL!PHOSPHINE!CATALYSTS! 86!
3.1.3! MECHANISM!OF!THE!MORITA5BAYLIS5HILLMAN!REACTION! 87!
3.2! ESI6MS"SCREENING"OF"PHOSPHINE"CATALYSTS" 89!
3.2.1! SCREENING!METHODOLOGY! 89!
3.2.2! SYNTHESIS!OF!MBH!PRODUCTS! 91!
3.2.3! PRELIMINARY!ESI5MS!RESULTS! 92!
3.3! CONCLUSION"AND"CURRENT"RESEARCH"PROGRESS" 97!
4" ORGANOCATALYTIC"ASYMMETRIC"CONJUGATE"ADDITION"OF"ALDEHYDES"TO"NITROOLEFINS:"
MECHANISTIC"INVESTIGATIONS"BASED"ON"ESI6MS"STUDIES"OF"THE"BACK"REACTION" 101"
4.1! INTRODUCTION" 101!
4.1.1! ORGANOCATALYTIC!ASYMMETRIC!CONJUGATE!ADDITION!OF!ALDEHYDES!TO!NITROOLEFINS! 101! 4.1.2! CURRENT!MECHANISTIC!CONSIDERATIONS!OF!CONJUGATE!ADDITION!REACTIONS!–!ESI5MS!BACK!REACTION!
ANALYSIS!AS!MECHANISTIC!TOOL! 103!
4.2! ESI6MS"BACK"REACTION"EXPERIMENTS"ON"THE"CONJUGATE"ADDITION"REACTION" 108!
4.2.1! PREPARATIVE!REACTIONS! 108!
4.2.2! INITIAL!ESI5MS!EXPERIMENTS! 109!
4.2.3! ESI5MS!BACK!REACTION!SCREENING!OF!TRIPEPTIDES!BEARING!AN!ACIDIC!SIDE!CHAIN! 110! 4.2.3! ESI5MS!BACK!REACTION!SCREENING!OF!CATALYSTS!WITHOUT!AN!INTRAMOLECULAR!PROTON!DONOR! 114!
4.3! SUMMARY" 120!
5" ESI6MS"SCREENING"OF"RACEMIC"ORGANOCATALYSTS"FOR"THE"MICHAEL"ADDITION" 123"
5.1! INTRODUCTION" 123!
5.1.1! APPROACHES!TO!DETERMINE!OR!INDUCE!ENANTIOSELECTIVITY!EMPLOYING!RACEMIC!CHIRAL!CATALYSTS! 124! 5.1.2! ESI5MS!SCREENING!OF!RACEMIC!PD5CATALYSTS!FOR!THE!ALLYLIC!SUBSTITUTION!REACTION! 128! 5.2! ESI6MS"SCREENING"OF"ORGANOCATALYSTS"FOR"THE"MICHAEL"ADDITION" 130!
5.2.2! PRINCIPLE!OF!THE!ESI5MS!SCREENING!OF!RACEMIC!ORGANOCATALYSTS! 131!
5.2.3! PRELIMINARY!EXPERIMENTS! 132!
5.3! CATALYST"SYNTHESIS" 134!
5.3.1! 1ST!GENERATION!CATALYST!SYNTHESIS! 134!
5.3.2! 2ND!GENERATION!CATALYST!SYNTHESIS! 135!
5.3.3! SYNTHESIS!OF!FURTHER!PYRROLIDINE5BASED!CATALYSTS! 144! 5.3.4!! SYNTHESIS!OF!ISOINDOLINE5DERIVED!CATALYSTS! 146!
5.4! ESI6MS"SCREENING" 152!
5.4.1! DEVELOPMENT!AND!VALIDATION!OF!A!SCREENING!PROTOCOL! 152!
5.4.2! ESI5MS!SCREENING!OF!NEW!CATALYSTS! 157!
5.5! SUMMARY" 164!
6" EXPERIMENTAL"PART" 167"
6.1! GENERAL"REMARKS" 167!
6.1.1! ANALYTICAL!METHODS! 167!
6.1.2! REAGENTS!AND!WORKING!TECHNIQUES! 169!
6.2! ESI6MS"SCREENING"OF"ORGANOCATALYZED"ALDOL"REACTIONS" 170! 6.2.1! SYNTHESIS!OF!THE!MASS5LABELED!ALDOL!PRODUCTS! 170! 6.2.2! SYNTHESIS!OF!TEST!SUBSTRATES!FOR!THE!ESI5MS!BACK!REACTION!SCREENING! 175!
6.2.3! SYNTHESIS!OF!AMINOALCOHOLS! 188!
6.2.4! CATALYST!SYNTHESIS! 195!
6.2.5! SYNTHESIS!OF!CATALYST!MIXTURES! 212!
6.2.6! PREPARATIVE!ORGANOCATALYZED!ALDOL!REACTIONS! 217!
6.2.7! ESI5MS!BACK!REACTION!SCREENING! 219!
6.3! MORITA6BAYLIS6HILLMAN"REACTION" 222!
6.3.1! SYNTHESIS!OF!ACRYLOYL!ESTERS! 222!
6.3.2! SYNTHESIS!OF!MORITA5BAYLIS5HILLMAN!PRODUCTS! 225! 6.4! ESI6MS"BACK"REACTION"SCREENING"AS"TOOL"FOR"MECHANISTIC"INVESTIGATIONS" 230!
6.5! ESI6MS"SCREENING"OF"RACEMIC"CATALYSTS" 231!
6.5.1! 1ST!GENERATION!CATALYST!SYNTHESIS! 231!
6.5.2! 2ND!GENERATION!CATALYST!SYNTHESIS! 234!
6.5.3! SYNTHESIS!OF!FURTHER!PYRROLIDINE5BASED!ORGANOCATALYSTS! 250! 6.5.4! SYNTHESIS!OF!ISOINDOLINE5DERIVED!ORGANOCATALYSTS! 252! 6.5.5! ESI5MS!SCREENING!OF!RACEMIC!CATALYSTS! 264!
!
7" APPENDIX" 267"
7.1! ESI6MS"SIGNAL"RATIOS"DETERMINED"IN"THE"ALDOL"REACTION" 267! 7.2! ESI6MS"RESULTS"DETERMINED"IN"THE"MICHAEL"ADDITION" 268!
7.3! CRYSTALLOGRAPHIC"DATA" 270!
7.4! LIST"OF"ABBREVIATIONS" 271!
8" REFERENCES" 277"
SUMMARY" 283"
!
CHAPTER 1
I
NTRODUCTION1.1 Mass Spectrometry & High-Throughput Screening in Asymmetric Catalysis
Asymmetric catalysis is an important tool to selectively create new stereogenic centers starting from prochiral molecules.[1] However, the identification of effective, highly selective catalysts is often a costly, time intensive and material demanding process. Each new catalyst, even with small structural modifications, and every optimization of the reaction parameters (solvents, temperatures, additives) usually needs to be applied repeatedly to a benchmark reaction. This complex optimization process is illustrated in Figure 1 and is associated with multiple reaction set-up, long reaction times, product purification steps and product analyses to determine the enantioselectivity.
Figure 1: Typical screening process of chiral catalysts for an asymmetric reaction.
With the introduction of combinatorial methods and automated systems in modern synthesis,[2] the development of new high-throughput screening methods has received increased attention in recent years, as they accelerate the process of product analysis, which usually remains the bottleneck of high-throughput catalyst screenings.[3] Routinely, determination of the enantioselectivity relies on chromatographic methods such as high performance liquid chromatography (HPLC) or gas chromatography (GC) analysis with a chiral stationary phase. However, common drawbacks of these analytical methods are time consuming identification of suitable separation conditions and the associated elution times, as well as the required pre-treatment of the sample and the waste amount of solvent.
Catalyst Design & Syntheses cat-1 cat-2 cat-3 cat-4 cat-5 cat-6 … cat-x
Benchmark Reaction
A + B Catalyst* C*
Solvent, Temperature, Additive
Applying to the Benchmark Reaction Reaction set up and reaction time
Workup and product purification Product analysis (e.g. HPLC, GC)
Reaction scale
(starting material and catalyst loading)
Racemization Racemic background reactions Optimization of the Reaction Solvents Temperature Additives
Time Consuming Material Consuming Erosion of ee
Additionally, for GC analysis the analyte needs to be volatile and thermal stable, thus limiting the scope of this method. In recent years, new optical methods for a rapid determination of the enantioselectivity have been reported.[3a] For example, REETZ applied infrared thermography to monitor the different levels of heat output from lipase-catalyzed acylations of (R)- and (S)- alcohols.[4] MAHMOUDIAN used the same reaction to generate calibration curves for the subsequent estimation of the enantioselectivity of catalysts for the Corey-Bakshi-Shibata reduction of ketones.[5] Other examples are found in the application of supramolecular sensors (host) interacting with an analyte (guest) in combination with UV-vis, fluorescence or circular dichroism spectroscopy.[3a] However, thus far these methods are seldom applicable for a quantitative determination of the enantiomeric excess (ee) and are usually considered as preselective screening methods to identify promising reactions, which are then analyzed more thoroughly by standard chromatography techniques.
In addition to the optical methods mentioned above, mass spectrometry has also become an important tool for the evaluation of enantiomer discriminating systems.[6] However, mass spectrometric methods suffer from their inability to distinguish between enantiomers and diastereoisomers, due to the identical mass of the compounds. In 1990, HOREAU introduced an elegant and simple method to overcome this limitation (Figure 2).[7] An equimolar mixture of enantiomeric anhydrides 1, where enantiomer 1b was mass-labeled by deuteration (so called quasienantiomers[8]), were reacted in a kinetic resolution with an optically active alcohol of unknown configuration. The diastereomeric products 2 differ in the mass-label and therefore become distinguishable by mass spectrometry. The absolute configuration of various alcohols were assigned according to the relative peak heights of the two fragmentation products determined by electron ionization mass spectrometry (EI-MS). Later, FINN further elaborated HOREAU´S method allowing for a quantitative determination of the enantioselectivity of optically active alcohols by electrospray ionization mass spectrometry (ESI-MS).[9]
Ph O
Ph O O
CH3 CH3
Ph O
Ph O O
CH2D CH2D
(–)-1b (+)-1a
fast
slow OH
Ph O
O CH3
Ph O
O CH2D
2b 2a 50 : 50
EI MS analysis
Ph CH+ C2H5
Ph CH+ H2C CH2D
100 : 68
L S OH H
REETZ et al. applied a similar concept of mass-labeled quasienantiomers for lipase-catalyzed kinetic resolutions and asymmetric transformations of prochiral substrates bearing enantiotopic groups.[3f, 10] Recently, SPERANZA reported an alternative procedure to determine the enantioselectivity based on ESI-MS.[11] The method relies on analysis of the reaction kinetic of a diastereomeric proton bound complex with a reactant in the gas phase. This approach avoids any “wet” chemical transformationof theenantiomerically pure compound in solution prior to ESI-MS analysis and eliminates conceivable sources of error (such as formation of supramolecular aggregates with an excess of the chiral reagent) in the ESI nanodrops.
Thus far, the determination of the enantioselectivity by mass spectrometry for an investigated reaction relied on analysis of the composition of reaction products, residual starting materials or further derivatized products. In 1999, CHEN introduced a straightforward method to identify the activity of Brookhard-type polymerization catalysts out of catalysts mixtures by means of ESI-MS/MS studies.[3d, 12] Therein, charged reaction intermediates derived from eight catalysts were directly monitored by ESI-MS, whereupon the most active catalyst was represented by the intermediate bearing the longest chain (i.e. highest mass). To simplify the complex spectra, the signal patterns with highest mass were subjected to Xe-collision induced β-hydride elimination in the gas phase to afford the signal of the most active catalyst as single peak (Figure 3).
Figure 3: ESI-MS spectrum of the quenched reaction mixture of 8 catalysts (left) and the spectrum after β-hydride elimination of the polymeric ions m/z >2200 (right).[3d]
Another simple approach to estimate the activity of a catalyst by monitoring reaction intermediates was reported by BICKELHAUPT and REEK in 2010.[13] In their principle of the
“survival of the weakest” the least abundant Pd-allyl intermediate represents the most active catalyst. The proposed trend was confirmed by ESI-MS screening of the Pd-catalyzed allylic
with parallel screening. Moreover, while this library was constructed from known components, the experiment is a prototype for screening libraries of unknown substances in a catalytic formulation.
The final experiment combines the quantitative kinetics of the one-catalyst example with the pooled library of the
screening case.[16]This experiment shows the power of two- dimensional, or double-selection, mass-spectrometric meth- ods in screening pooled libraries of catalysts.
The experiment uses a so-called “parent-ion scan” in which the second quadrupole in a tandem mass spectrometer is set to detect a product of a reaction—a daughter ion—while Figure 10.ESI-MS of the quenched mixture of eight catalysts (left) and the result when the ions with m/z>2200 are chosen and then subjected tob-hydride elimination (right). The clean signal arising from9 cindicates that this catalyst was responsible for the highest molecular-weight component of the oligomers in the mixture.
Scheme 6.Preparation and activation of a pool library of eight catalysts in the presence of ethylene for simultaneous tests; cod=cyclooctadiene, DMSO=dimethylsulfoxide, CID=collision induced dissociation.
P. Chen
Reviews
2844 !2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org Angew. Chem. Int. Ed.2003,42, 2832 – 2847
[Cat-Pd]
CID (collision induced dissociation) Reaction mixture with 8 different catalysts
CHAPTER 1
alkylation reaction with a set of various ligands present in the reaction mixture and was validated by measuring the turnover frequencies (TOF) of the corresponding reactions with the single ligand (Figure 4). However, this application is very restricted to special kinetic scenarios.
Figure 4: Inverse correlation between the ESI-MS intensity of the Pd-allyl intermediates and the turnover frequency (TOF) for the Pd-catalyzed allylic alkylation reaction.[13]
Inspired by the work of CHEN,[3d] the PFALTZ research group approached the demand for a more efficient analysis of enantioselective reactions by the development of a high-throughput screening method to determine the enantioselectivity of chiral catalysts for the Pd-catalyzed allylic substitution reaction.[6a, 14] This methodology relies on quantification of intensity ratios of charged reaction intermediates monitored by ESI-MS. These intermediates become distinguishable by mass spectrometry due to the installation of a mass label on a remote position on the individual enantiomers of the substrate (quasienantiomers). The mass labels used are not necessarily based on an isotope-labeling strategy as initially introduced by HOREAU (Figure 2).For example,in all previous studies examined in the PFALTZ group, alkyl- derived mass labels in the para-position of an aryl substituent far away from the reaction center were identified to be suitable for the ESI-MS screening (Figure 5). Molecules that differ in such mass labels are usually more easily accessible in perfect enantiopurity and were found to be chemically similar enough to have no influence on the selectivity outcome of the catalyst and therefore behave identically in terms of their electrospray ionization efficiency.
Initially, the kinetic resolution of allylic esters was investigated by ESI-MS leading to the identification of new, highly selective ligands for the Pd-catalyst.[14b, 14c] The subsequent extension of this ESI-MS methodology to a back reaction screening gave access to selectivity determination of catalysts for enantioselective transformations of prochiral substrates, which considerably broadened the scope of this method.[14a] In accordance with the principle of microscopic reversibility, the analysis of the ratio of the intermediates of the back reaction reflects the enantioselectivity induced by the catalyst in the corresponding forward reaction
It seems that, with the exception of ligand7b, the arylphosphines (R¼Ph, o-tol) give the least-stable complexes, which can be rationalized in terms of electronic effects as the Pd(II) oxidation state is stabilized by more electron-rich ligands. Importantly, for this series of ligands the relative stability determined by MS also cor- relates inversely with the turnover frequencies of the corresponding complexes, even though not as quantitatively as for the above series in which only the bite angle changed. As the IndolPhos library is more diverse in terms of steric and electronic properties, the position of the transition state may move from late to early within the series (see above), and thus lead to a correlation that is less quantitative. However, the method is capable of qualitatively
predicting the most-active catalyst and the order of activity within the IndolPhos ligand library. For larger ligand libraries, the mor- e-active species may not be visible in the first screening. However, screening rounds in which the stable complexes are eliminated should lead to the discovery of the fastest catalyst.
In conclusion, we introduced a new technique to select the most- active catalyst from a dynamic mixture of palladium complexes for allylic alkylations, as demonstrated using three different ligand libraries. According to our hypothesis, the least-stable intermediate gives the most-active catalyst, and hence the ‘survival of the weakest’.
High-level DFT calculations show that destabilization of the inter- mediate Pd-allyl complexes is caused by steric repulsion between the ligand and the substrate for the diphosphine ligands, which overrules the counterbalancing electronic effect. Importantly, no specialized equipment is necessary for this selection methodology.
A liquid chromatography mass spectrometer that contains an ESI probe and a direct injection valve is all that is required. After this proof-of-concept we aim to expand this methodology to larger ligand libraries and other reactions that involve charged intermedi- ates, and that precede the rate-limiting step.
Methods
General procedure for ESI-MS selection experiments using diphosphine ligands.
One equivalent of preformed [Pd(crotyl)(diphosphine)]OTf (0.020 mmol) in CH2Cl2(1 ml) was added to a library with equimolar amounts of diphosphine ligands (0.020 mmol) in CH2Cl2(2 ml). The solution was stirred for one hour and the equilibrium mixture injected after dilution with CH2Cl2. Peak heights were determined by taking an average over the four most-intense isotope signals.
The ESI-MS signal was calibrated by injecting mixtures of preformed [Pd(allyl)(diphosphine)]OTf complexes in CH2Cl2at different ratios. As depicted in Supplementary Fig. S2, the peak intensities of the Pd complexes in the mass spectrum are proportional to the amount of Pdþcomplex present in the mixture.
General procedure for ESI-MS selection experiments using IndolPhos ligands.
A solution of equimolar amounts of ligands7a–7e(1.77mmol) and one equivalent of [Pd(1,3-diphenylallyl)(MeCN)2]PF6(1.90 mg, 1.77mmol) in MeCN (1.77 ml) was stirred for three days at room temperature. After dilution with MeCN, the mixture was injected into the ESI mass spectrometer (Supplementary Fig. S3). Peak heights were determined by taking an average over the four most-intense isotope signals. The ESI-MS signal was calibrated by injecting mixtures of preformed [Pd(1,3- diphenylallyl)(3)]PF6complexes in MeCN at different ratios. Supplementary Fig. S4 shows the ratio of peak intensities versus the molar ratio referenced to [Pd(1,3- diphenylallyl)(7a)]PF6. The slopes of the corresponding calibration curves give the correction factor for the spraying efficiencies.
Crystallographic details for [Pd(crotyl)(dtpp)]OTf (2b).C36H41F3O3P2PdS, formula weight¼779.09, pale-yellow thick plate, 0.38×0.17×0.08 mm3, triclinic, P¯1 (no. 2),a¼10.9546(7) Å,b¼11.5774(8) Å,c¼30.591(2) Å,a¼85.323(3)8, b¼81.590(3)8, g¼63.569(4)8,V¼3,436.2(4) Å3,Z¼4, Dx¼1.51 g cm23, m¼0.75 mm21; reflections were measured up to a resolution of (sinu/l)max¼ 0.62 Å21; 13,423 reflections were unique (Rint¼0.054), of which 11,929 were observed (I.2s(I)); 1,085 parameters were refined and 1,048 restraints were used for the refinement;R1/wR2 (I.2s(I))¼0.0525/0.0999;R1/wR2 (all reflections)¼0.0651/0.1044;S¼1.255; residual electron density found between 20.63 and 0.68 eÅ23; Cambridge Crystallographic Data Centre 760831.
Table 3|Bonding analysis of [Pd(ligand)(crotyl)]1.
Ph2P PPh2 Pd(II)
Me (CH2)n + Ph2P PPh2
(CH2)n
+ Pd(II) Ph2P PPh2 Pd(II)
(CH2)n + Me
∆E1 ∆E2
2+
DE1†,‡ DE2‡ Strain 2§
Ligand Bite angle* DE Total Interaction Strain Total Interaction Strain§ [Pd(ligand)]21 Crotyl
dppe 87.5 2699.5 2446.1 2470.3 24.2 2253.4 2271.1 17.7 10.4 7.3 dppp 99.3 2699.1 2451.2 2475.4 24.2 2247.9 2267.9 20.0 13.2 6.8
dppb 103.4 2695.6 2449.5 2474.6 25.2 2246.1 2268.0 21.9 15.1 6.8
Bonding analysis of [Pd(ligand)(crotyl)]+resulting from the formation of diphenylphosphinoalkane–palladium–crotyl complexes in two steps (R¼Ph,n¼2, 3, 4). Computed at ZORA-BLYP/TZ2P according to the scheme shown:DE¼DE1þDE2. *P–Pd–P angle in [Pd(ligand)(crotyl)]þ.†DE1is computed relative to Pd(II) in its closed-shell valenced8s0state.‡See equation (1).§DE2,strain¼DE2,strain[Pd(ligand)]2þþ DE2,strain(crotyl).
Ph Ph
OAc
0.5 mol%
[Pd(Ph2-allyl)(MeCN)2l]PF6 0.55 mol% 7a–7e
CH2(CO2Me)2 / BSA Ph Ph CH(CO2Me)2
O OP N
R2P Me R'
R'
7a R = Ph; R' = H 7b R = iPr; R' = H 7c R = o-tol; R' = H 7d R = iPr; R' = Me 7e R = Cy; R' = H 6
i a
b ii
8
Abundance in MS selection Relative TOF
7a (R=Ph) 7b (R=iPr) 7c (R=o-tol) 7d (R=iPr, 7e (R=Cy) R' = Me)
Figure 2|‘Survival of the weakest’ method applied in the IndolPhos-Pd catalysed allylic alkylation ofrac-diphenylpropenyl acetate. a, Reaction scheme (i) and the structure of IndolPhos ligands7a–7e(ii). BSA,N,O- bis(trimethylsilyl) acetamide.b, Relative abundance of Pd-allyl complexes in MS selection experiments (dark grey) and relative turnover frequencies (TOF, light grey) of IndolPhos ligands7a–7e. An inverse correlation is obtained between these parameters, which confirms the prediction based on the theoretical model. Cy, cyclohexyl.
ARTICLES
NATURE CHEMISTRY DOI: 10.1038/NCHEM.614NATURE CHEMISTRY| VOL 2 | MAY 2010 | www.nature.com/naturechemistry 420
It seems that, with the exception of ligand7b, the arylphosphines (R¼Ph, o-tol) give the least-stable complexes, which can be rationalized in terms of electronic effects as the Pd(II) oxidation state is stabilized by more electron-rich ligands. Importantly, for this series of ligands the relative stability determined by MS also cor- relates inversely with the turnover frequencies of the corresponding complexes, even though not as quantitatively as for the above series in which only the bite angle changed. As the IndolPhos library is more diverse in terms of steric and electronic properties, the position of the transition state may move from late to early within the series (see above), and thus lead to a correlation that is less quantitative. However, the method is capable of qualitatively
predicting the most-active catalyst and the order of activity within the IndolPhos ligand library. For larger ligand libraries, the mor- e-active species may not be visible in the first screening. However, screening rounds in which the stable complexes are eliminated should lead to the discovery of the fastest catalyst.
In conclusion, we introduced a new technique to select the most- active catalyst from a dynamic mixture of palladium complexes for allylic alkylations, as demonstrated using three different ligand libraries. According to our hypothesis, the least-stable intermediate gives the most-active catalyst, and hence the ‘survival of the weakest’.
High-level DFT calculations show that destabilization of the inter- mediate Pd-allyl complexes is caused by steric repulsion between the ligand and the substrate for the diphosphine ligands, which overrules the counterbalancing electronic effect. Importantly, no specialized equipment is necessary for this selection methodology.
A liquid chromatography mass spectrometer that contains an ESI probe and a direct injection valve is all that is required. After this proof-of-concept we aim to expand this methodology to larger ligand libraries and other reactions that involve charged intermedi- ates, and that precede the rate-limiting step.
Methods
General procedure for ESI-MS selection experiments using diphosphine ligands.
One equivalent of preformed [Pd(crotyl)(diphosphine)]OTf (0.020 mmol) in CH2Cl2(1 ml) was added to a library with equimolar amounts of diphosphine ligands (0.020 mmol) in CH2Cl2(2 ml). The solution was stirred for one hour and the equilibrium mixture injected after dilution with CH2Cl2. Peak heights were determined by taking an average over the four most-intense isotope signals.
The ESI-MS signal was calibrated by injecting mixtures of preformed [Pd(allyl)(diphosphine)]OTf complexes in CH2Cl2at different ratios. As depicted in Supplementary Fig. S2, the peak intensities of the Pd complexes in the mass spectrum are proportional to the amount of Pdþcomplex present in the mixture.
General procedure for ESI-MS selection experiments using IndolPhos ligands.
A solution of equimolar amounts of ligands7a–7e(1.77mmol) and one equivalent of [Pd(1,3-diphenylallyl)(MeCN)2]PF6(1.90 mg, 1.77mmol) in MeCN (1.77 ml) was stirred for three days at room temperature. After dilution with MeCN, the mixture was injected into the ESI mass spectrometer (Supplementary Fig. S3). Peak heights were determined by taking an average over the four most-intense isotope signals. The ESI-MS signal was calibrated by injecting mixtures of preformed [Pd(1,3- diphenylallyl)(3)]PF6complexes in MeCN at different ratios. Supplementary Fig. S4 shows the ratio of peak intensities versus the molar ratio referenced to [Pd(1,3- diphenylallyl)(7a)]PF6. The slopes of the corresponding calibration curves give the correction factor for the spraying efficiencies.
Crystallographic details for [Pd(crotyl)(dtpp)]OTf (2b).C36H41F3O3P2PdS, formula weight¼779.09, pale-yellow thick plate, 0.38×0.17×0.08 mm3, triclinic, P¯1 (no. 2),a¼10.9546(7) Å,b¼11.5774(8) Å,c¼30.591(2) Å,a¼85.323(3)8, b¼81.590(3)8,g¼63.569(4)8,V¼3,436.2(4) Å3, Z¼4, Dx¼1.51 g cm23, m¼0.75 mm21; reflections were measured up to a resolution of (sinu/l)max¼ 0.62 Å21; 13,423 reflections were unique (Rint¼0.054), of which 11,929 were observed (I.2s(I)); 1,085 parameters were refined and 1,048 restraints were used for the refinement;R1/wR2 (I.2s(I))¼0.0525/0.0999;R1/wR2 (all reflections)¼0.0651/0.1044;S¼1.255; residual electron density found between 20.63 and 0.68 eÅ23; Cambridge Crystallographic Data Centre 760831.
Table 3|Bonding analysis of [Pd(ligand)(crotyl)]1.
Ph2P PPh2 Pd(II)
Me (CH2)n + Ph2P PPh2
(CH2)n
+ Pd(II) Ph2P PPh2 Pd(II) (CH2)n
+ Me
∆E1 ∆E2
2+
DE1†,‡ DE2‡ Strain 2§
Ligand Bite angle* DE Total Interaction Strain Total Interaction Strain§ [Pd(ligand)]21 Crotyl
dppe 87.5 2699.5 2446.1 2470.3 24.2 2253.4 2271.1 17.7 10.4 7.3 dppp 99.3 2699.1 2451.2 2475.4 24.2 2247.9 2267.9 20.0 13.2 6.8
dppb 103.4 2695.6 2449.5 2474.6 25.2 2246.1 2268.0 21.9 15.1 6.8
Bonding analysis of [Pd(ligand)(crotyl)]+resulting from the formation of diphenylphosphinoalkane–palladium–crotyl complexes in two steps (R¼Ph,n¼2, 3, 4). Computed at ZORA-BLYP/TZ2P according to the scheme shown:DE¼DE1þDE2. *P–Pd–P angle in [Pd(ligand)(crotyl)]þ.†DE1is computed relative to Pd(II) in its closed-shell valenced8s0state.‡See equation (1).§DE2,strain¼DE2,strain[Pd(ligand)]2þþ DE2,strain(crotyl).
Ph Ph
OAc
0.5 mol%
[Pd(Ph2-allyl)(MeCN)2l]PF6 0.55 mol% 7a–7e
CH2(CO2Me)2 / BSA Ph Ph CH(CO2Me)2
O OP N
R2P Me R'
R'
7a R = Ph; R' = H 7b R = iPr; R' = H 7c R = o-tol; R' = H 7d R = iPr; R' = Me 7e R = Cy; R' = H 6
a i
b ii
8
Abundance in MS selection Relative TOF
7a (R=Ph) 7b (R=iPr) 7c (R=o-tol) 7d (R=iPr, 7e (R=Cy) R' = Me)
Figure 2|‘Survival of the weakest’ method applied in the IndolPhos-Pd catalysed allylic alkylation ofrac-diphenylpropenyl acetate. a, Reaction scheme (i) and the structure of IndolPhos ligands7a–7e(ii). BSA,N,O- bis(trimethylsilyl) acetamide.b, Relative abundance of Pd-allyl complexes in MS selection experiments (dark grey) and relative turnover frequencies (TOF, light grey) of IndolPhos ligands7a–7e. An inverse correlation is obtained between these parameters, which confirms the prediction based on the theoretical model. Cy, cyclohexyl.
NATURE CHEMISTRY| VOL 2 | MAY 2010 | www.nature.com/naturechemistry 420
La-e La-e
R Pd+
L L
monitored by ESI-MS
