Regioselective Addition of Dienes to the Cβ−Cγ Double Bond of the Allenylidene Ligand of [Ru(η5-C5H5)(CCCPh2)(CO)(PiPr3)]BF4

The C β −C γ bond of the allenylidene ligand of [Ru(η 5 -C 5 H 5 )( C C CPh 2 )(CO)(P i Pr 3 )]BF 4 ( 1 ) adds dienes such as isoprene, cyclopentadiene, and 1,3-cyclohexadiene. The reaction with isoprene selectively affords the disubstituted vinylidene derivative ( 2 ), whereas in the presence of cyclopentadiene a 1:1 mixture of the diastereomers ( 3 ) and ( 4 ) is obtained. In contrast to the addition of cyclopentadiene, the reaction with 1,3-cyclohexadiene is diastereoselective and gives only the racemic mixture of one diastereomer: ( 5 ). The structures of 2, 3, and 5 have been determined by X-ray diffraction analysis. In the three cases, the geometry around the ruthenium center is close to octahedral, with the cyclopentadienyl ligand occupying one face.