1.4 Cooperative Asymmetric Catalysis
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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