Rhodium Enolate Complexes as Synthons and Catalysts in Organic Chemistry

Metal enolates are versatile nucleophilic reagents in organic synthesis, and are applicable to reactions with various electrophiles. The reactivity of transition metal enolates differs from that of enolates derived from main-group elements, although such reaction is rare other than for palladium and copper enolates. The focus in this chapter is formation and reaction of rhodium enolates, the following features of which are notable: (i) rhodium can assume various oxidation states and coordination modes; (ii) O-bound rhodium enolates have a low RhO bond polarity, because the difference in electronegativity between rhodium and oxygen is considerably smaller than that between lithium and oxygen, and the compounds show less basisity but modest nucleophilicity; (iii) oxidative addition and reductive elimination can be employed to form, react, and regenerate rhodium enolates; (iv) modest reactivity allows the use of various methods to control the reactivity and includes tolerance for various cosubstrates and metal reagents; (v) regeneration of rhodium enolates allows catalysis; (vi) use of chiral ligands allows applications to asymmetric reactions. Therefore, rhodium enolates react with various reagents, such as aldehydes, water, perfluoroalkyl iodides, perfluorobenzenes, arylboranes, aryltitaniums, disulfides, α-organothioketones, and ketones. Rhodium enolates can become important synthetic intermediates possessing diverse of reactivities in organic synthesis. Keywords: rhodium enolates; catalysis; modest nucleophilicity; low basisity; carbonyl compounds; 1,4-addition; transmetallation; retro-aldol reaction; CS bond cleavage and formation; α-deprotonation