The Role of Metals in Carbene Synthon Introduction

A variety of organometallic reagents have been introduced as synthetic equivalents of carbene synthons, or methylene dipoles. First, the author recalls some highlights of research activities at Kyoto University during the period of 1963 - 1985. The topics are (1) asymmetric cyclopropanation via copper-carbene complexes, (2) one-carbon ring-expansion utilizing a ß-oxido lithium-carbenoid intermediate, (3) double alkylation of gem-dibromocyclopropane carbon by means of dialkylcuprate leading to stereospecific preparation of a sirenin intermediate, (4) Skattebøl cyclic allene synthesis mediated by chromium(II) salts in protic and aprotic solvents, (5) carbadianion species as "nonbasic" Wittig reagents. Recent topics after 1985 at Okayama are concerned with the use of a monothioacetal reagent, LiCH(SPh)OMe, as a carbene synthon and particularly synthetic reactions via 2-phenylthioalkanal intermediates arising therefrom. The final products are either RCOCH₂CHYCOZ or RCH₂COCHYCOZ, where ^-CHYCOZ synthons come from the Wittig-Horner reagents or silyl enolates CHY =C(OSiMe₃)Z. 1. Copper Chelate Catalyzed Asymmetric Cyclopropanation 2. Lithium Carbenoids in Regioselective Ring Expansion 3. Stereoselective Dialkylation of Dibromocyclopropenes 4. Organochromium Species 5. Methylene Dianions in Wittig-Type Olefination 6. Organotin Studies at Okayama 7. Methoxymethylene and Phenylthiomethylene Synthons 8. 2-Phenylthioalkanals: Transformation into 1,3- and 1,4-Dicarbonyl Compounds