Organozinc Compounds in Synthesis

Abstract

This chapter describes the developments mainly after 1960 in synthetic reactions using organozinc compounds. The reaction of the triplet carbene with olefins gives cyclopropane derivatives in a nonstereospecific way and is accompanied by hydrogen abstraction, because the triplet carbene is a biradical. The presence of ether functions in olefins frequently facilitates the Simmons–Smith reaction. The reaction of phenanthrene with zinc dust and methylene iodide in 1, 2-dimethoxyethane gives 9,10 dihydro–9,10–methanophenanthrene (XIX) in 25% yield. Ring expansion of aromatic compounds by carbene, carbethoxycarbene, chlorocarbene, and carbenoid is well known. The reaction of aldehydes with the Simmons–Smith reagent in the presence of an excess amount of zinc dust gives the corresponding olefins in 29–63% yields. The reaction of the Simmons–Smith reagent with a carbon–carbon triple bond gives cyclopropene and its isomers along with small amounts of allene derivatives. Zinc carbenoids do not insert into olefinic C–H bonds, although the Simmons–Smith reagent was reported to attack ether to give products resulting from the insertion of a methylene group into the α–C–H bond. Organozinc reagents are generally much less reactive than Grignard reagents and organolithium reagents in addition reactions with carbonyl and other unsaturated compounds. The reaction of a carbonyl compound with an α–haloester in the presence of zinc is commonly known as the Reformatsky reaction. Allylic, benzylic, propargylic, and related organic halides react with many aldehydes and ketones in the presence of zinc to form alcohols.