[figure: see text] The asymmetric hydrogenation of trifluoromethyl ketones to yield chiral alpha-trifluoromethyl alcohols with enantiomeric excesses up to 98% was achieved in the presence of chiral rhodium-(amidephosphine-phosphinite) complexes.

Neighboring group participation is one of the fundamental interactions in organic reactions and can influence the reaction rate, stereoselectivity, and reaction pathway through transient carboncarbon or carbon-heteroatom bond formation. The latter category includes cyclic three-and five-membered bromonium ions, wherein lone-pair electrons of the monovalent bromine atom stabilize a trigonal carbocation. Although similar nucleophilic interactions of monovalent halogen atoms with non-carbon atom-centered cations have long been predicted, we know of no experimental evidence of such an interaction. Here, we demonstrate a nucleophilic interaction of neighboring monovalent halogen to stabilize an imino sp 2 nitrogen cation. This interaction has an overwhelming impact on the reaction pathway, completely altering the migratory preference under acid-catalyzed Beckmann rearrangement conditions. In sharp contrast to the general case of anti-migration, peri-chloroand peri-bromo-substituted O-tosyl oximes of 1-tetralone substructures and their derivatives undergo syn-migration under Beckmann rearrangement conditions (i.e., migration of the group on the syn side of the leaving group). The peri-chloro or peribromo neighboring group turned out to provide strong anchimeric assistance for syn-migration via transient formation of a cyclic five-membered imino-halonium cation with dissociation of tosylic acid. Thus, formation of the syn-migration products can be attributed to a reaction mechanism that is different from the conventional Beckmann rearrangement mechanism. That is, the positively charged imino nitrogen atom can be stabilized by, or interact with, a chloro or bromo group in close spatial proximity, and this interaction dramatically changes the reaction pathway, selectively affording regioisomeric lactams from closely related starting materials.halogen-nitrogen bonding | organic reaction mechanisms | stereochemistry

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-034Enantioselective Rhodium(I)-Catalyzed Hydrogenation of Trifluoromethyl Ketones.

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