Enamine intermediates SOMO activation

In 2007, the groups of MacMillan and Sibi almost simultaneously introduced a new mode of organocatalytic activation, termed SOMO (singly occupied molecular orbital) catalysis, which was founded upon the transient production of a 37r-electron radical cation species that could function as a generic platform of induction and reactivity. This new mode of organocatalytic activation, was founded upon the mechanistic hypothesis that one-electron oxidation of a transient enamine intermediate, derived from the aldehyde and the chiral amine catalyst, rendered a 37i -electron SOMO-activated species, which could readily participate in asymmetric bond construction. 

SOMO Activation Within the field of aminocatalysis, asymmetric organo-SOMO (singly occupied molecular orbital) catalysis has recently emerged as a powerful technique for the preparation of optically active compounds. In this context, MacMillan and coworkers described in 2008 the formation of y-oxyaldehydes from aldehydes and styrenes by organo-SOMO catalysis [25]. The condensation between the amine catalyst 46 and an aldehyde gave rise to an enamine intermediate, which was then oxidized by ceric ammonium nitrate (CAN) to give a radical cation. Reaction of this radical cation with a nonactivated olefin, namely styrene, led to the... 

The use of chiral primary or secondary amines as covalent catalysts allows for the activation of carbonyl componnds for different reactions. Either the initially formed imininm species are the reactive intermediate (LUMO lowering), which is mainly the case when using a,p-unsatnrated carbonyl compounds, or the derived enamine can be ntilized for enolate-type reactions (HOMO activation), or, after a single electron oxidation of the enamine, a singly occnpied molecular orbital (SOMO) activation is possible (Scheme 6.18) [14, 31, 32], In addition, by combining these complementary activation modes, it has been possible to carry out organocascade reactions with excellent control of... 

Scheme 39.1 Formation of SOMO activated chiral intermediate upon preferential oxidation of enamine.

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