Allyl carbenoids

The synthesis of vinylaziridines through reactions between allylic carbenoid reagents and imines (i.e., Darzen-type reactions) was first reported by Mauze in 1980 [13]. Treatment of aldimines or ketimines 16 with gem-chloro(methyl)allyllithium (17) afforded N-substituted vinylaziridines 18 (Scheme 2.6). Similarly, 2,3-trans-N-diphenylphosphinyl-2-vinylaziridines 21 were prepared with good stereoselectivities (trans cis= 10 1 Scheme 2.7) by treatment of a-bromoallyllithium (20) with N-diphenylphosphinyl aldimines 19 in the presence of zinc chloride [14]. 

Insertion of Allyl Carbenoids into Organozirconium Species... 

Allyl chlorides are readily deprotonated a to the halide by strong bases to give allyl carbenoids, which insert into organozirconocene chlorides to afford allyl- or pentadienyl-zirco-nocene chlorides (Scheme 3.23). These allylmetallic species are reactive towards carbonyl compounds, and so an efficient three-component coupling results [50]. 

Scheme 3.23. Tandem insertion of allyl carbenoids and aldehydes into organozirconocene chlorides.

The most extensively studied application of carbenoid insertion into organozirconocene species is the insertion of allyl carbenoids into zirconacycles and subsequent elaboration of the thus formed allylzirconocenes with electrophiles (Schemes 3.24—3.26) [48,53,55-60],... 

The tandem zirconocene-induced co-cyclization of dienes or enynes/insertion of allyl carbenoid/addition of electrophile is a powerful method for assembling organic structures. Two illustrations of its application are the synthesis of the dollabelane natural product acetoxyodontoschismenol 99 [57,62,63] and the one-pot construction of linear terpenoids 100 (Scheme 3.25) [59,64],... 

Closely related to both allyl carbenoids and the allenyl carbenoids discussed above, propargyl carbenoids 101 are readily generated in situ and insert into zirconacycles to afford species 102 (Scheme 3.27), which are closely related to species 84 derived from allenyl carbenoids [65], Protonation affords a mixture of allene and alkyne products, but the Lewis acid assisted addition of aldehydes is regioselective and affords the homopropargylic alcohol products 103 in high yield. Bicydic zirconacyclopentenes react similarly, but there is little diastereocontrol from the ring junction to the newly formed stereocenters. The r 3-propargyl complexes derived from saturated zirconacycles are inert towards aldehyde addition. 

Thermolysis (200°) of bicyclo[1.1.0]butane (3) severs two bonds to produce 1,3-butadiene, but in the presence of 1, 3 undergoes cleavage of the central peripheral bond to generate an allyl carbenoid intermediate, which reacts with an electron-deficient alkene to give an allylcyclopropane (equation II).3... 

The regioselectivity of insertion of lithium chloroallylides (allyl carbenoids) into a wide variety of unsymmetrical zirconacycles has been determined. In all but one case a single regioisomer was obtained. A combination of steric and electronic effects was needed to explain the results and imply that the carbenoid was acting predominantly as an electrophilic species. The first carbenoid insertion into a zirconacyclopentadiene has been noted. " ... 

Compared with five-membered metallacycles, relatively fewer reports are known on the preparative methods and reaction chemistry of six-membered metallacycles. Whitby and coworkers have systematically investigated insertion of carbenoids into five-membered zirconacycles and developed a number of interesting six-membered zirconacycles [17, 26]. Isonitriles, 1-halo-l-lithioalkenes, allenyl carbenoids, allyl carbenoids, propargy carbenoids, benzyl carbenoids, and 1-nitrile-1-lithio epoxides can all insert into zirconacyclopentanes and zirconacyclopentenes to afford various six-membered zirconacycles (Eqs. 23,24). 

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