Facile Preparation of Allylzinc Species from Allyl Bromides and Unactivated Zinc Induced by a Catalytic Amount of Aluminum Chloride and Their Reactions with Carbonyl Compounds and Acetals.
作者:Hatsuo MAEDA、Kazushige SHONO、Hidenobu OHMORI
DOI:10.1248/cpb.42.1808
日期:——
The effects of Lewis acids (ZnCl2, AlCl3, BF3, and TiCl4) on the generation of allylzinc species from allyl bromide and unactivated zinc powder in dry tetrahydrofuran (THF) were examined by trapping the organozinc compound with benzaldehyde, that is, Grignard-type allylation of the aldehyde. Among the Lewis acids employed, AlCl3, was found to be the promoter of choice. The allylzinc species preformed in the presence of a catalytic amount of AlCl3 effectively allylated carbonyl compounds. Various aromatic and aliphatic aldehydes as well as ketones were converted into homoallylic alcohols in good to excellent yields. Under the reaction conditions employed, ester, hydroxy, acetal, and aromatic nitro and halide groups were tolerated. In the case of α, β-unsaturated carbonyl compounds, selective 1, 2-addition was observed. Substituted allyl bromides such as prenyl, crotyl, cinnamyl, and 2-cyclohexenyl bromides were smoothly converted to the corresponding allylzinc compounds, which reacted with carbonyl compounds to give substituted homoallylic alcohols in excellent yields. The diastereoselectivity in crotylation, cinnamylation, and 2-cyclohexenylation depended upon the structures of both the organic metals and the electrophiles. The origin of the observed selectivity is discussed. The allylation of dimethyl and cyclic acetals accompanied with carbon-oxygen bond cleavege also proceeded in excellent yields provided that two equivalents of AlCl3 was present.
在干燥的四氢呋喃(THF)中,研究了路易斯酸(ZnCl2、AlCl3、BF3和TiCl4)对烯丙基溴和未活化的锌粉生成烯丙基锌物种的影响,并通过苯甲醛捕获有机锌化合物,即醛的格氏型烯丙基化反应。在所用的路易斯酸中,AlCl3被发现是最优的促进剂。在催化量的AlCl3存在下预形成的烯丙基锌物种能有效地烯丙基化羰基化合物。各种芳香族和脂肪族醛以及酮被转化为同烯丙基醇,产率良好至优秀。在所采用的反应条件下,酯、羟基、缩醛以及芳香族硝基和卤素基团均被容忍。对于α,β-不饱和羰基化合物,观察到了选择性的1,2-加成。取代的烯丙基溴如香叶基、巴豆基、肉桂基和2-环己烯基溴均能顺利转化为相应的烯丙基锌化合物,这些化合物与羰基化合物反应生成取代的同烯丙基醇,产率极佳。在巴豆基化、肉桂基化和2-环己烯基化中的立体选择性取决于有机金属和亲电试剂的结构。讨论了观察到的选择性的起源。在AlCl3的两当量存在下,伴随着碳-氧键断裂的二甲基和环状缩醛的烯丙基化也以极佳的产率进行。