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Lewis base-catalyzed reactions of SiX 3-based reagents with C=O, C=N (n → σ*)
chapterposted on 20.09.2016 by Andrei Malkov, Pavel Kocovsky
Division of a book, which in a scholarly context usually treats a part of a larger subject in a stand-alone manner.
Nucleophilic addition to carbonyl and heterocarbonyl compounds is the cornerstone of organic synthesis, highlighted by the classical examples of Grignard reactions, aldol condensation, and H4 reductions. However, when less reactive nucleophiles are employed, such as allyl-silane, activation is required. This chapter focuses on the activation of silicon reagents by chiral Lewis bases. The field of nucleophilic activation of the reagents 3/4 was pioneered by Sakurai and coworkers, who, in the late 1980s, reported on a highly diastereoselective allylation of aldehydes with allyltriha-losilanes 3 and 4 in the presence of stoichiometric amounts of CsF. If two molecules of the Lewis base are involved in the formation of the catalytically active species, or if a biden-tate catalyst is used, the reaction would proceed through the cationic hexacoordinate transition complex. The early phosphoramides exhibited modest enantioselectivity in the allylation reaction but played an important role in the mechanistic elucidation and development of the second generation of catalysts.