Palladium-Catalyzed Isomerization of Aryl-Substituted Epoxides: A Selective Synthesis of Substituted Benzylic Aldehydes and Ketones
摘要:
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.
Highly Efficient Alkylation to Ketones and Aldimines with Grignard Reagents Catalyzed by Zinc(II) Chloride
作者:Manabu Hatano、Shinji Suzuki、Kazuaki Ishihara
DOI:10.1021/ja0628405
日期:2006.8.1
efficient alkylation to ketones and aldimines with Grignardreagents in the presence of catalytic trialkylzinc(II) ate complexes derived from ZnCl2 (10 mol %) in situ was developed. This simple Zn(II)-catalyzed alkylation could minimize the well-known but serious problems with the use of only Grignardreagents, which leads to reduction and aldol side products, and the yield of desired alkylation products
在由 ZnCl2 (10 mol%) 衍生的催化三烷基锌 (II) 酸酯配合物的存在下,使用格氏试剂高效地烷基化为酮和醛亚胺。这种简单的 Zn(II) 催化烷基化可以最大限度地减少仅使用格氏试剂而导致还原和羟醛副产物的众所周知但严重的问题,并且可以提高所需烷基化产物的产率。