Enantioselective iminium-catalyzed epoxidation of hindered trisubstituted allylic alcohols
摘要:
The reactivity of diastereomeric biaryl iminium cations made of a (Ra)-5,5',6,6',7,7',8,8'-octahydrobinaphthyl core and exocyclic appendages derived from (S)- or (R)-3,3-dimethylbutyl-2-amine was investigated with hindered trisubstituted allylic alcohols a class of alkenes which had not been previously studied in detail in epoxidation reactions with cyclic iminium catalysts (ee up to 98%). Surprisingly, generally strong matched/mismatched effects are observed not only in terms of reactivity but also on the enantioselectivity of the reaction (Ace up to 16%). Also, for the most hindered substrates, two sets of reaction conditions were tested in a preliminary study and little advantage was found in running reactions in MeCN/water instead of CH(2)Cl(2)/water/18-C-6. In any case, the presence of the hydroxyl group did not reveal any anchimeric effect. (C) 2010 Elsevier Ltd. All rights reserved.
Not only symmetrical, but also unsymmetrical α,α‐diaryl allylic alcohols are employed as substrates in the title reaction. A number of arenes and even heteroarenes underwent radical 1,2‐arylmigration (“neophyl rearrangement”) to produce α‐aryl β‐trifluoromethyl ketones. The preferential migration of electron‐deficient arylgroups over electron‐rich ones in unsymmetrical substrates supports the radical