On the use of mixtures of organotin species for catalytic enantioselective ketone allylation—a detective story
作者:Anthony Cunningham、Vijaya Mokal-Parekh、Claire Wilson、Simon Woodward
DOI:10.1039/b313384b
日期:——
In the presence of enantiopure MTBH2
(monothiobinaphthol, 2-hydroxy-2â²mercapto-1,1â²-binaphthyl; 0.2 eq.) quantitative allylation of ArC(O)Me takes place with impure Sn(CH2CHCH2)4
(prepared from allyl chloride, air-oxidised magnesium and SnCl4) to yield tert-homoallylic alcohols in 85â92% ee. In the same process highly purified, or commercial, Sn(CH2CHCH2)4 yields material of only 35â50% ee. The origin of these effects is the presence of small amounts of the compounds, EtSn(CH2CHCH2)3, ClSn(CH2CHCH2)3 ClSnEt(CH2CHCH2)2 in the tetraallyltin sample and the presence of traces of water (which inhibits achiral background reactions). All the triallyl and diallyl species enhance the stereoselectivity in the catalytic allylation reaction, the chlorides more so than the ethyl compound. Hydrolysis of ClSnEt(CH2CHCH2)2 affords crystallographically characterised Sn4(μ3-O)(μ2-Cl)2Cl2Et4(CH2CHCH2)4. Reaction of this latter compound with MTBH2 leads to the most potent catalyst.
在手性纯MTBH2(单硫代苯二醇,2-羟基-2′-巯基-1,1′-联萘;0.2当量)的存在下,与不纯的Sn(CH2CHCH2)4(由丙烯氯、空气氧化镁和SnCl4制备)进行定量的烯丙基化反应,能够生成含有85–92%光学纯度的叔同烯丙醇。在同一过程中,高度纯化的或商业化的Sn(CH2CHCH2)4仅产生35–50%的光学纯度。这些效果的来源是四丙基锡样品中存在少量化合物EtSn(CH2CHCH2)3、ClSn(CH2CHCH2)3以及ClSnEt(CH2CHCH2)2,以及水的微量存在(抑制了非手性背景反应)。所有的三烯丙基和二烯丙基物种都增强了催化烯丙基化反应中的立体选择性,其中氯化物的效果优于乙基化合物。ClSnEt(CH2CHCH2)2的水解反应生成了具有晶体结构特征的Sn4(μ3-O)(μ2-Cl)2Cl2Et4(CH2CHCH2)4。后者与MTBH2的反应生成了最强的催化剂。