Recent advances in selective organic synthesis mediated by transition metal complexes

This short review covers some useful synthetic transformations mediated by group 6 Fischer alkoxy carbene complexes. Alkenyl carbene complexes derived from (-)-8-phenyl- menthol are suitable Michael acceptors towards organolithium reagents -alkyllithium and lithium enolate derivatives-yielding ultimately b-alkyl substituted aldehydes with 80-98% enantiomeric excess. The reaction of (menthyloxy)aryl carbene complexes with sBuLi and tBuLi resulted in clean dearomatization leading to 1,4-cyclohexadiene derivatives. If alkynyl lithium, instead of alkyllithium, compounds are reacted with aryl carbene complexes different products -furans, propargyl ethers, alkoxyindenes are selectively produced. Enamines derived from (S)-methoxymethylpyrrolidine and both aldehydes and ketones react with alkenyl carbenes to afford with very high selectivity (3a2) cycloadducts which are elaborated into useful, enantiopure 3,4-disubstituted cyclopentanones and 2,3-disubstituted 2-cyclopente- nones, respectively. The (4a3) carbocyclization occurs from alkenyl carbene complexes and 2-aminobutadiene derivatives, while the analogous heterocyclization can be accomplished from various types of azabutadienes and both alkenyl and alkynyl derivatives. Efficient structural modifications of terpenes can be done using boroxycarbene complexes via C-H insertion reactions. Finally, representative examples of cyclopropanation of electronically neutral alkenes with Fischer carbene complexes are provided.