Organozinc carbenoids

The preparation of an unusual organozinc carbenoid 285 from amide 284 has been recently reported.390 The carbenoid 285 was used in amidocyclopropanation reactions of diverse alkenes (Scheme 148).390 Unfortunately, experiments employing other amides yielded the corresponding products in low yields. 

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... 

This methodology can be extended to encompass the synthesis of 1,1,3-trisubstituted allenes 51 if substituted organozinc carbenoids are employed as outlined in equation 2125. 

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... 

Motherwell and Roberts dicarbonyl coupling reaction19 can be improved by substituting l,2-bis(chlorodimethylsilyl)ethane for TMSC1. The intermediate organozinc carbenoids can be trapped with alkenes to produce cyclopropanes, as exemplified in the intramolecular case in equation 3. 

For a discussion of organozinc carbenoid homologation reactions, see Marek, I. Tetrahedron 2002,58, 9463. 

Alkoxycyclopropanes. In the presence of MejSiCl the zinc amalgam reacts with orthoformic esters to generate organozinc carbenoids which are captured by alkenes. 

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. 

Organozinc carbenoids, C=Zn.1 These reagents can be generated by rcac-R2... 

Conjugated trienes are amongst the products accessible via the Zn/TMSCl mediated dimerisation of carbonyl substrates, including a, 0-unsaturated ketones. The reaction is thought to proceed via the formation of an organozinc carbenoid which combines with a second equivalent of carbonyl substrate to produce an intermediate epoxide, which then undergoes deoxygenation, (Scheme 67). 

In the presence of TMSCl and ZnCl2, Zn(Hg) can convert ortho formates, or acetals into organozinc carbenoid species, which then undergo a variety of reactions, such as cyclopropanation. N-Diethoxymethyl amides function similarly and give amidocyclo-propanation reactions, or diastereoselective amidocyclopropa-nations if a chiral auxiliary is incorporated into the original amide... 

Insertion of zinc into the carbon-halogen bond of organozinc carbenoids is accelerated by the presence ofa catalytic amount of Pb(II) salts [40]. By this means, several l,l-(biszincio)alkane reagents such as 14,15 [41], or 16 [42] can be prepared (Scheme 4.4). The commercially available Nysted reagent 17 is also prepared through zinc insertion with zinc-lead couple [43]. In the case of aUcyl-substituted l,l-(biszincio)aIkanes, P-hydride elimination is prevented with the adjunction of TMEDA (N,N,N, N -tetramethylethylenediamine) [44]. 

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... 

---