Ketones zinc carbenoids

The Clemmensen reduction can be formulated to proceed by a sequence of one-electron and proton transfer reactions. It is a heterogenous reaction, taking place at the zinc surface. Initially an electron is transferred from zinc to the carbonyl group of ketone 1, leading to a radical species 3, which is presumed to react further to a zinc-carbenoid species 4 ... 

Functionalized zinc carbenoids have been prepared from carbonyl compounds by an indirect strategy. The deoxygenation of a carbonyl compound to an organozinc carbenoid can be induced by a reaction with zinc and TMSCl. Therefore, the aldehyde or ketone, when treated with TMSCl or l,2-bis(chlorodimethylsilyl)ethane in the presence of an alkene, generates the cyclopropanation product. This method is quite effective for the production of alkoxy-substituted cyclopropane derivatives. A 55% yield of the... 

Treatment of ketones wifh a mixture of CH2I2, TiCl4, and zinc in THF at 0°C for 90 min produced fhe mefhylenation product in 5-8% yield whereas when lead dichloride was added the reaction proceeded smoothly at 0 °C to give the product in 81% yield wifhin 30 min (Scheme 13.68) [85]. A catalytic amount of lead dichloride probably promotes furfher reduction of zinc carbenoid (ICH2ZnI) by zinc in THF to give a geminal dizinc compound (CH2(ZnI)2) which is a key intermediate for the methylenation of carbonyl compounds. 

The zinc carbenoid derived from CFBrj and Et Zn adds to aldehydes readily to form RCH(OH)CFBr2. The remarkable chemoselectivity favoring aldehydes to ketones (> 99 1) is in stark contrast to that of LiCEBr, which reacts almost indiscriminately. 

Although the Simmons-Smith reaction has found considerable use in organic synthesis, it is not readily applicable to the formation of highly substituted cyclopropanes, since 1,1 -diiodoalkanes (other than diiodomethane) are not readily available. Substituted zinc carbenoids can be prepared from aryl or a,p-unsaturated aldehydes (or ketones) with zinc metal, and these species can be trapped with an alkene to give substituted cyclopropanes.The addition of chromium carbenes (see Section 1.2.2) to alkenes can be used to effect cyclopropanation to give substituted cyclopropanes. Thus, addition of excess 1-hexene to the chromium carbene 113 gave the cyclopropane 114 as a mixture of diastereomers, with the isomer 114 predominating (4.92). ... 

This methodology was subsequently extended to the reaction with ketones, which allowed the diastereoselective preparation of adjacent quaternary aU-carbon stereogenic centers in an acyclic system [105]. In the case of ethoxyacetylene 360, the alkenylcopper derivatives 361 were formed quantitatively, followed by homologation with a zinc carbenoid and reaction of the resulting aUenylzinc intermediates with ketones (Scheme 10.124). 

Recently, even organozinc carbenoids have been used for the preparation of vinylcyclo-propanes. These species are formed upon reaction of zinc with an aldehyde or ketone in the presence of 1,2-bis(dichlorodimethylsilyl)ethane. A synthetic procedure that avoids competitive carbonyl coupling has been developed the carbonyl compound is added to a refluxing mixture of the reagents, namely amalgamated zinc, the silyl chloride and two equivalents of the alkene. 

---