Intermediates in asymmetric hydrogenation

Mechanistic Investigations of the Asymmetric Hydrogenation of Enamides with Neutral Bis(phosphine) Cobalt Precatalysts

The mechanism of the asymmetric hydrogenation of prochiral enamides by well-defined, neutral bis(phosphine) cobalt(0) and cobalt(II) precatalysts has been explored using(R,R)-^iPrDuPhos ((R,R)-^iPrDuPhos = (+)-1,2-bis[(2R,5R)-2,5-diisopropylphospholano]benzene) as a representative chiral bis(phosphine) ligand. A series of (R,R)-(^iPrDuPhos)Co(enamide) (enamide = methyl-2-acetamidoacrylate (MAA), methyl(Z)-α-acetamidocinnamate (MAC), and methyl(Z)-acetamido(4-fluorophenyl)acrylate (^4FMAC)) complexes (1-MAA, 1-MAC, and 1-^4FMAC), as well as a dinuclear cobalt tetrahydride, [(R,R)-(^iPrDuPhos)Co]₂(μ₂-H)₃(H) (2), were independently synthesized, characterized, and evaluated in both stoichiometric and catalytic hydrogenation reactions. Characterization of (R,R)-(^iPrDuPhos)Co(enamide) complexes by X-ray diffraction established the formation of the pro-(R) diastereomers in contrast to the (S)-alkane products obtained from the catalytic reaction. In situ monitoring of the cobalt-catalyzed hydrogenation reactions by UV-visible and freeze-quench electron paramagnetic resonance spectroscopies revealed (R,R)-(^iPrDuPhos)Co(enamide) complexes as the catalyst resting state for all the three enamides studied. Variable time normalization analysis kinetic studies of the cobalt-catalyzed hydrogenation reactions in methanol established a rate law that is first order in (R,R)-(^iPrDuPhos)Co(enamide) and H₂ but independent of the enamide concentration. Deuterium-labeling studies, including measurement of an H₂/D₂ kinetic isotope effect and catalytic hydrogenations with HD, established an irreversible H₂ addition step to the bound enamide. Density functional theory calculations support that this step is both rate and selectivity determining. Calculations, as well as HD-labeling studies, provide evidence for two-electron redox cycling involving cobalt(0) and cobalt(II) intermediates during the catalytic cycle. Taken together, these experiments support an unsaturated pathway for the [(R,R)-(^iPrDuPhos)Co]-catalyzed hydrogenation of prochiral enamides.

Rational Optimization of Supramolecular Catalysts for the Rhodium-Catalyzed Asymmetric Hydrogenation Reaction

Rational design of catalysts for asymmetric transformations is a longstanding challenge in the field of catalysis. In the current contribution we report a catalyst in which a hydrogen bond between the substrate and the catalyst plays a crucial role in determining the selectivity and the rate of the catalytic hydrogenation reaction, as is evident from a combination of experiments and DFT calculations. Detailed insight allowed in silico mutation of the catalyst such that only this hydrogen bond interaction is stronger, predicting that the new catalyst is faster. Indeed, we experimentally confirmed that optimization of the catalyst can be realized by increasing the hydrogen bond strength of this interaction by going from a urea to phosphine oxide H-bond acceptor on the ligand.

Investigations of the Asymmetric Intramolecular [2 + 2] Photocycloaddition and Its Application as a Simple Access to Novel C(2)-Symmetric Chelating Bisphosphanes Bearing a Cyclobutane Backbone

The asymmetric intramolecular [2 + 2] photocycloaddition of alpha,beta-enoates was evaluated as a simple access to the novel C(2)-symmetric bisphosphanes 22 and 27 possessing a cyclobutane backbone. A source of different chiral auxiliaries for investigations of the photochemical key step was provided by the transacetalization of dialkyl tartrates 3 with the corresponding 3,3-dialkoxybutan-2-ones 4. An insight into the selection mechanism was gained by temperature dependent measurements on the irradiation of the dicinnamates 10a-d, since the corresponding Eyring diagram discloses strictly linear functions as well as an isoselective relationship. Diol 8a turned out to be a structurally optimized auxiliary in terms of chiral induction and product crystallization and was also successfully applied in the first asymmetric photodimerization of 2-indenecarboxylic acid esters. Indeed, in this case excellent diastereoselectivities were achieved, too, but head-to-tail dimers 16a and 16b were formed predominantly. Diesters 11a and 16a were converted by standard procedures into the desired enantiopure 1,4-diphosphane 22 and 1,5-diphosphane 27. Furthermore, the hitherto unknown absolute configuration of delta-truxinic acid was elucidated from a single crystal X-ray structure analysis of 11a.