Carbometallation allenes

All of the reactions discussed above are cyclic carbometallation reactions of metallacycles. Very recently, an interesting Cr-catalyzed carboalumination of propargyl derivatives producing allenes via a carbometallation-elimina-tion sequence has been studied. This reaction provides an asymmetric synthesis of chiral allenes (Scheme 57). 

Two catalytic cycles are proposed to explain the difference in selectivity. In both cases, catalytic cycle is initiated by the oxidative addition of an alkynylstannane to nickel(O) species, leading to the formation of alkynylnickel(ll) complex 77 (Scheme 24).92 Then, an allene is inserted into the nickel(ll) complex in a manner which avoids steric repulsion with the butyl group to afford the anti-ir-a y complex 80. The carbometallation of the terminal alkyne can take place at the non-substituted allylic carbon of the corresponding syn-Ti-a y complex 78. The stereoselectivity is determined by the relative rate of the two possible insertion modes which depend on the ligand used. A bidentate... 

Carbometallation of allenes with alkenylruthenium complexes has been reported in a stoichiometric process for the preparation of 2-alkenylallylruthenium complexes.97 Such types of organometallic species have not been well explored, and could give rise to new applications in transition metal catalysis. 

Very recently, Ma has reported a rhodium-catalyzed route to 18,19-norsteroid skeletons from bis-allenes, involving a cyclometallation-carbometallation-reductive elimination-Diels-Alder reaction cascade process.410... 

Although detailed mechanistic studies are not reported, the postulated mechanism for the reductive cyclization of allenic carbonyl compounds involves entry into the catalytic cycle via silane oxidative addition. Allene silylrhodation then provides the cr-allylrhodium hydride A-18, which upon carbometallation of the appendant aldehyde gives rise to rhodium alkoxide B-14. Oxygen-hydrogen reductive elimination furnishes the hydrosilylation-cyclization product... 

The utilization of the allene unit as a site for intramolecular carbometallation was reinvestigated recently. The iodide 31 (R = H) was readily converted to the 3,4-pentadien-l-yllithium 32, and the latter undergoes a carbocyclization reaction to its cyclobutyl isomer [25] 33, although the corresponding organomagnesium compound leads to the cyclopropane derivatives [26],(Scheme 7-26). 

Gouin [24] also reported that alklymagnesium halides, after addition to heptyn-2-ol 33, can to some extent give rise to the Sn2 products 35 namely, the allenic derivatives, as by-products of the carbometallation reaction [Eq. (12)]. 

The carbometallation of allylboranes is known . Alkenes are less reactive than alkynes or allenes in reaction with allylboranes. Only highly reactive alkenes, such as vinyl ethers and cyclopropenes , are used. 

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