Organozinc compounds carbenoids

The study of the Schlenk equilibrium for organozinc compounds represents a major chapter in the understanding of these reagents in general [26]. Before elaborating the studies on zinc carbenoids, it is appropriate to briefly review the definitive investigations on organozinc halides themselves. 

H. E. Simmons and R. D. Smith of the du Pont Company) the carbenoid is an organozinc compound which delivers methylene stereospecifically (and without competing insertion) to the double bond. 

The bis-diazoalkane compound, l,3-bis(diazomethyl)benzene, reacts with Znl2 in tetrahydrofuran (THF) or Et20 to give the bis(organozinc) compound (I). The carbenoid (I) is more stable in THF than in EtjO ... 

The rhodium chemistry just presented actually involves C=Rh species rather than a free carbene. In this section, we will look at a general class of compounds called carbenoids. A carbenoid is a reactive intermediate that reacts similarly to a carbene but does not actually involve formation of a carbene. The most commonly used carbenoid is generated by reaction of diiodomethane and a Zn/Cu couple this reaction, when it adds to alkenes, is called the Simmons-Smith reaction.316 A simple example is the conversion of cyclohexene to bicyclo[4.1.0]heptane (norcarane, 336). Initial reaction of diiodomethane with zinc gave an iodozinc compound (399-see sec. 9.8.B for a brief discussion of organozinc compounds), which added to the alkene to give 400. Loss of zinc iodide (Znl2) gave the cyclopropane derivative (343 in this case).3ll A one-step... 

The first reaction is a variation on Simmons-Smith cyclopropanation. Though strictly a carbenoid rather than a carbene, it delivers a CH2 group from an organozinc compound bound to an oxygen atom, in this case the OMe group. Only that alkene reacts. 

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 a variety of a,/i-unsaturated carbonyl compounds with zinc and 1,2-bis(chlorodimethylsilyl)ethane leads to the formation of organozinc carbenoids which may be trapped with olefins to give alkenyl- and arylcyclopropanes (equation 78)131. With... 

The organozinc carbenoids generated from aryl and a, -unsaturated carbonyl compounds with zinc and l,2-bis(chlorodimethylsilyl)ethane can be trapped intramolecularly to give cyclo-... 

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. 

Formation of Organozinc Carbenoids. When aromatic aldehydes and certain , -unsaturated carbonyl compounds are treated with zinc and l,2-bis(chlorodimethylsilyl)ethane, an organozinc carbenoid results. This two-electron process is postulated to occur as depicted in eq 8. Sequential reaction of the two silicon atoms with the zinc carbonyl complex and subsequent extrasion of the cyclic siloxane (23) gives rise to the putative organozinc carbenoid... 

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