Hydrometallation and Dimetallation

Facile metallation of allenes offers convenient synthetic methods of functionalized alkenes. Pd-catalyzed cis hydrostaimation of allenes proceeds smoothly via either hydropalladation or staimylpalladation of 1,2-dienes with H-Pd-SnBu3 to produce allylstaimanes 122 or vinylstaimane 123 [38]. 

Hydrostaimation of arylallenes such as 124 proceeded regio- and stereoselec-tively to provide allylstannane 125 [39]. Reaction of allenyl silyl ethers such as 126 also proceeded regioselectively to produce allylstannanes. A mixture of the stereoisomers 127 and 128 was obtained, but in this case the Z isomer 128 was obtained predominantly. As shown above, allylstannanes are obtained by metallation at the terminal carbon [40]. However, Lautens et al. reported that the vinylstaimane 130 was obtained from alkylallene 129 when ligandless heterogeneous Pd(OH)2 on carbon was used as a catalyst. It was confirmed that the allylstannane 131 was obtained when Pd(PPh3)4 was used. The catalysts changed the selectivity [41]. 

As a synthetic application, Grigg applied the regioselective hydrostannation of alkoxyallene to the hydrostarmation-cyclization sequence, and obtained the ben-zodihydrofuran 134 from the allenyl ether 132 via the allylstannane 133 in one pot [42]. 

Pd-catalyzed dimetallation of allenes with dimetallic reagents has been studied extensively. For example, regioselective 1,2-disilylation of alkylallene gave 139 [44]. 

Regio- and stereoselectivities in Pd-catalyzed silylstaimation (silastannation) of allenes are not always satisfactory. They are markedly improved by the use of phosphine-free Pd catalyst, particularly Pd2(dba)3. The allylstannane 141 was 

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