131180-63-7Relevant articles and documents
London Dispersion Interactions Rather than Steric Hindrance Determine the Enantioselectivity of the Corey–Bakshi–Shibata Reduction
Eschmann, Christian,Song, Lijuan,Schreiner, Peter R.
supporting information, p. 4823 - 4832 (2021/02/01)
The well-known Corey–Bakshi–Shibata (CBS) reduction is a powerful method for the asymmetric synthesis of alcohols from prochiral ketones, often featuring high yields and excellent selectivities. While steric repulsion has been regarded as the key director of the observed high enantioselectivity for many years, we show that London dispersion (LD) interactions are at least as important for enantiodiscrimination. We exemplify this through a combination of detailed computational and experimental studies for a series of modified CBS catalysts equipped with dispersion energy donors (DEDs) in the catalysts and the substrates. Our results demonstrate that attractive LD interactions between the catalyst and the substrate, rather than steric repulsion, determine the selectivity. As a key outcome of our study, we were able to improve the catalyst design for some challenging CBS reductions.
Intramolecular [2+2] Photocycloaddition of Cyclic Enones: Selectivity Control by Lewis Acids and Mechanistic Implications
Poplata, Saner,Bauer, Andreas,Storch, Golo,Bach, Thorsten
supporting information, p. 8135 - 8148 (2019/05/29)
The intramolecular [2+2] photocycloaddition of 3-alkenyl-2-cycloalkenones was performed in an enantioselective fashion (nine representative examples, 54–86 % yield, 76–96 % ee) upon irradiation at λ=366 nm in the presence of an AlBr3-activated oxazaborolidine as the Lewis acid. An extensive screening of proline-derived oxazaborolidines showed that the enantioface differentiation depends strongly on the nature of the aryl group at the 3-position of the heterocycle. DFT calculations of the Lewis acid–substrate complex indicate that attractive dispersion forces may be responsible for a change of the binding mode. The catalytic [2+2] photocycloaddition was shown to proceed on the triplet hypersurface with a quantum yield of 0.05. The positive effect of Lewis acids on the outcome of a given intramolecular [2+2] photocycloaddition was illustrated by optimizing the key step in a concise total synthesis of the sesquiterpene (±)-italicene.
Enantioselective Intermolecular [2+2] Photocycloaddition Reaction of Cyclic Enones and Its Application in a Synthesis of (-)-Grandisol
Poplata, Saner,Bach, Thorsten
supporting information, p. 3228 - 3231 (2018/03/13)
The intermolecular [2+2] photocycloaddition of typical cyclic α,β-unsaturated enones, such as 2-cyclohexenone, with olefins was performed in moderate to good yields (42-82%) and with high enantioselectivity (82%-96% ee). An unusual substitution pattern at the chiral oxazaborolidine-AlBr3 Lewis acid complex that promotes the reaction was found to be crucial for the success of the reaction. The method was applied to the enantioselective synthesis of the monoterpene (-)-grandisol, which could be accomplished in six steps and with an overall yield of 13% starting from 3-methyl-2-cyclohexenone.
Organocatalytic enantioselective α-hydroxymethylation of aldehydes: Mechanistic aspects and optimization
Boeckman, Robert K.,Biegasiewicz, Kyle F.,Tusch, Douglas J.,Miller, John R.
, p. 4030 - 4045 (2015/05/05)
Further studies of the direct enantioselective α-hydroxymethylation of aldehydes employing the α,α-diarylprolinol trimethylsilyl ether class of organocatalysts are described. This process has proven efficient for access to β-hydroxycarboxylic acids and δ-hydroxy-α,β-unsaturated esters from aldehydes in generally good yields, excellent enantioselectivity, and compatibility with a broad range of functional groups in the aldehyde. The goal of these studies was to identify the critical reaction variables that influence the yield and enantioselectivity of the α-hydroxymethylation process such as catalyst structure, pH of the medium, purity of the reactants and reagents particularly with respect to the presence of acidic impurities, and the nature of the buffer, along with the standard variables including solvent, time, temperature and mixing efficiency. The previously identified intermediate lactol has been further characterized and its reactivity examined. These studies have led to identification of the most critical variables translating directly into improved substrate scope, reproducibility, enantioselectivity, and yields.
Nonenzymatic acylative kinetic resolution of Baylis-Hillman adducts
O Dalaigh, Ciaran,Connon, Stephen J.
, p. 7066 - 7069 (2008/02/11)
(Chemical Equation Presented) The first efficient nonenzymatic acylative kinetic resolution of Baylis-Hillman adducts is reported. Chiral pyridine catalyst 1a and an optimized analogue 1e are capable of promoting the synthetically useful enantioselective
Asymmetric alternating copolymerization of cyclohexene oxide and CO2 with dimeric zinc complexes
Nakano, Koji,Nozaki, Kyoko,Hiyama, Tamejiro
, p. 5501 - 5510 (2007/10/03)
Dimeric zinc complex 2a [= Et2Zn2(1a)2] has been synthesized by the reaction of Et2Zn and (S)-diphenyl(pyrrolidin-2-yl)methanol (1a-H). X-ray crystallography revealed that the alkoxide ligand replaced one of the two ethyl groups of Et2Zn and formed a five-membered chelate ring through a Zn - N dative bond. Two zinc centers were bridged by oxygen atoms to form a Zn2O2 four-membered ring with a syn relationship between the two ethyl groups on the zinc centers. Dimeric zinc complex 2a was an active catalyst for asymmetric alternating copolymerization of cyclohexene oxide and CO2. An MALDI-TOF mass spectrum of the obtained copolymer showed that the copolymerization was initiated by the insertion of CO2 into Zn - alkoxide to give [(S)-diphenyl(pyrroridin-2-ly)methoxy] - [C(=O)O - (1,2-cyclohexylene) - O]n -H (copolymer I), including chiral ligand 1a as an initiating group. Complex 3a-OEt (= EtZn(1a)2ZnOEt), in which an ethoxy group replaced one of the two ethyl groups in 2a, also polymerized cyclohexene oxide and CO2 with higher catalytic activity and enantioselectivity than 2a and afforded EtO - [C(=O)O - (1,2- cyclohexylene) - O]n - H (= copolymer III), including an ethoxy group as an initiating group. Throughout the studies, dimeric zinc species are indicated to be the active species for the copolymerization. It is also depicted that the substituent on the aryl moiety in diaryl(pyrrolidin-2-yl)methanol 2b - e influenced the polymerization activity.
A Practical Enantioselective Synthesis of α,α-Diaryl-2-pyrrolidinemethanol. Preparation and Chemistry of the Corresponding Oxazaborolidines
Mathre, David J.,Jones, Todd K.,Xavier, Lyndon C.,Blacklock, Thomas J.,Reamer, Robert A.,et al.
, p. 751 - 762 (2007/10/02)
A practical two-step enantioselective synthesis of α,α-diaryl-2-pyrrolidinemethanols (1) from proline, based on the addition of aryl Grignard reagents to proline-N-carboxanhydride (3), is reported.An investigation into the chemistry of the corresponding B-alkyl- and B-aryloxazaborolidines (2) led to the development of a reliable procedure for their preparation.
