Carbometallation of allenes

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. 

Allylboration of terminal alkynes generates vinylboron dihalides. Recently, this chemistry has been snccessfnlly applied to the synthesis of bicyclic and cage boron compounds (Scheme 23.35). The carbometallation of allenes is also known." ... 

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... 

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 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. 

A new Ni-catalysed carbocyanative cyclization of allene-ynes and bis-allenes has been reported. The reaction starts with the regioselective hydronickelation of the allene moiety, which is followed by cyclization via carbometalation. ... 

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). 

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)]. 

Thus, the optimized carbometallation reaction of propargylic alcohols with aromatic Grignard reagents proceeds without an additional metal catalyst and leads to the formation of an alkenyl fluoride. The stereoselective defluorometallation proceeds to form predominantly the fluorinated (Z)-carbon-carbon double bond in the final allene 276. Two reasons were proposed by the authors for such a high level of stereoselectivity the defluorometallation is favored from the intermediate... 

DFT computations showed that the intramolecular addition of the aryllithium generated from 425 occurs on the central carbon atom of the allenic moiety to yield the intermediate 427 from which lithium ethoxide is eliminated to furnish the benzofuran product 426. Both cyclization processes were found to pass through low-lying transition states, as it would be expected for fast reactions at low temperatures [123]. Further explorations on the anti selectivity in the intramolecular carbolithi-ation by DFT computations revealed additional details regarding the mechanism of this carbocyclization and led the authors to conclude that such a transformation is controlled by the appropriate molecular editing [123]. The synthesis of functionalized heterocycles can be relatively easy to achieve by intramolecular carbometallation reactions. For example, Kunz and Knochel [124] recently reported the preparation of benzothiophene scaffolds 429 by copper-catalyzed carbomagnesiation ofalkynyl thioethers 428 (Scheme 10.148). 

---