Alkaline Earth Catalysis of Alkynyl Alcohol Hydroalkoxylation/Cyclization
Alkaline Earth Catalysis of Alkynyl Alcohol Hydroalkoxylation/Cyclization
复制标题
DOI：
10.1021/om3008663
复制
发表时间：
2012-10-22
期刊：
影响因子：
2.8
通讯作者：
Reidt, Stephanie
中科院分区：
文献类型：
--
作者：
Brinkmann, Christine;Barrett, Anthony G. M.;Reidt, Stephanie
较重的碱土双(三甲基硅基)酰胺[Ae{N(SiMe3)(2)}(2)](2)(Ae=Ca，Sr，Ba)对多种烯醇和烯醇的区域选择性分子内氢烷氧基化/环化反应起到了有效的预催化剂作用。在大多数情况下，炔醇的环化反应产生可能的内环烯醇醚产物和外环烯醇醚产物的混合物，由于炔基醇氧化物异构化为相应的烯丙烯衍生物而使其合理化。研究发现，末端炔醇的环化速率明显高于具有内部炔基取代基的底物，而环化的效率一般由立体电子影响和特定碱土金属催化的热化学活性共同决定，我们认为这是由个别M-O键强度决定的。动力学研究提供了与催化剂随[底物]增加而显著抑制有关的速率定律，表明限制C-C不饱和进入M-O键的周转限制插入需要底物分子从Lewis酸性碱土中心解离。
Heavier alkaline earth bis(trimethylsilyl)amides [Ae{N(SiMe3)(2)}(2)](2) (Ae = Ca, Sr, Ba) are shown to act as effective precatalysts for the regioselective intramolecular hydroalkoxylation/cyclization of a wide range of alkynyl and allenyl alcohols. In the majority of cases, cyclization of alkynyl alcohols produces mixtures of the possible endo- and exocyclic enol ether products, rationalized as a consequence of alkynylalkoxide isomerization to the corresponding allene derivatives. Cyclization rates for terminal alkynyl alcohols were found to be significantly higher than for substrates bearing internal alkynyl substituents, while the efficacy of cyclization was in general found to be determined by a combination of stereoelectronic influences and the thermochemical viability of the specific alkaline earth metal catalysis, which we suggest is determined by the individual M-O bond strengths. Kinetic studies have provided a rate law pertaining to a pronounced catalyst inhibition with increasing [substrate], indicating that turnover-limiting insertion of C-C unsaturation into the M-O bond requires the dissociation of substrate molecules away from the Lewis acidic alkaline earth center.