A complex situation: Asymmetricepoxidation of conjugatedolefins was achieved at room temperature using ruthenium complex 1 as the catalyst and air as the oxidant to give epoxides in up to 95 % ee (see scheme). When the product was acid sensitive, the reaction was carried out at 0 °C under oxygen.
clarified, some experimental results support the notion that an aqua ligand coordinated with the ruthenium ion serves as a protontransfer agent for the oxygen activation process, and it is recycled and used as the protontransfermediator during the process. Thus, we have achieved catalytic asymmetric oxygen atom transfer reaction using molecular oxygen that can be carried out under ambient conditions.
Enzyme‐Catalyzed Meinwald Rearrangement with an Unusual Regioselective and Stereospecific 1,2‐Methyl Shift
作者:Ruipu Xin、Willy W. L. See、Hui Yun、Xirui Li、Zhi Li
DOI:10.1002/anie.202204889
日期:2022.7.11
A highly regioselective and stereospecific enzyme-catalyzed Meinwald rearrangement of internal and cyclic epoxides through a 1,2-methyl shift gave aldehydes or ketones as the sole product with enantio-retention. The unique isomerization was incorporated into enantioselective cascades, providing novel access to chiral alcohols, acids and amines from readily available trans-β-methylstyrenes.
Palladium-Catalyzed Isomerization of Aryl-Substituted Epoxides: A Selective Synthesis of Substituted Benzylic Aldehydes and Ketones
作者:Sanjitha Kulasegaram、Robert J. Kulawiec
DOI:10.1021/jo970743b
日期:1997.9.1
Aryl-substituted epoxides bearing multiple methyl substituents on the epoxide ring isomerize in the presence of 5 mol % Pd(OAc)(2)PR3 (R = n-Bu, Ph) to form the corresponding benzylic aldehyde or ketone, with complete regioselectivity for the carbonyl compound formed via cleavage of the benzylic C-O bond. No allylic alcohols or products arising from alkyl migration are observed. Rapid reaction rates and nearly quantitative yields are obtained, even with highly sterically hindered epoxides, using tri-n-butylphosphine as ligand and tert-butyl alcohol as solvent. 2-Aryl-substituted epoxides with two methyl substituents on C3 are completely unreactive, consistent with an oxidative addition/beta-hydride elimination mechanism. Catalyst variation studies show that both Pd(OAc)(2) and PR3 are essential for optimal activity and that palladium catalysts formed in this manner are superior to other Pd(O) catalysts (e.g., Pd(PPh3)(4)). The reactivity of catalytic Pd(OAc)(2)/PR3 toward multiply-substituted epoxides is compared to traditional Lewis acid catalysts; the former is found to be much more selective for isomerization without skeletal rearrangement. A mechanistic rationale involving turnover-limiting S(N)2-like attack of Pd(O) at the benzylic carbon is proposed.
Chemo- and Regioselective Isomerization of Epoxides to Carbonyl Compounds via Palladium Catalysis
作者:Sanjitha Kulasegaram、Robert J. Kulawiec
DOI:10.1021/jo00103a005
日期:1994.12
Palladium(O)-tertiary phosphine complexes catalyze the chemo- and regioselective isomerization of epoxides to carbonyl compounds in good to excellent yields; alkyl-substituted epoxides afford methyl ketones, and aryl-substituted epoxides form aldehydes or ketones via cleavage of the benzylic C-O bond.