Synthesis of Phosphonium-Substituted Vinylidene Complexes from [HCCCH<sub>2</sub>PPh<sub>3</sub>]<sup>+</sup>: Exploring the Competition between Allene and Vinylidene Formation.
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Reaction of the alkenyl-substituted carbene complex [Ru(κ2-O2CMe)(OC{Me}O(CCHCH2)(PPh3)2]+ with PPh3 results in the formation of the vinylidene-containing species [Ru(κ1-O2CMe)(κ2-O2CMe)(CCHCH2PPh3)(PPh3)2]+, among other products. This complex could also be obtained by reaction of [Ru(κ2-O2CR)(PPh3)2] (R = Me, Ph) with [HCCCH2PPh3]+ although in this instance competitive formation of allene-containing [Ru(κ1-O2CR)(κ2-O2CR)(η2-H2CCCHPPh3)(PPh3)2] occurred. Reaction of [Ru(κ2-O2CMe)2(PPh3)2] with [H2CCCHPPh3]+ gives this allene complex in a selective fashion. Similar results were obtained when [RhCl(PiPr3)2]2 was used as the metal precursor. The reaction with [H2CCCHPPh3]+ gave [RhCl(η2-H2CCCHPPh3)(PiPr3)2]+, whereas treatment with [HCCCH2PPh3]+ afforded a mixture of the same complex and [RhCl(η2-HCCCH2PPh3)(PiPr3)2]+, which subsequently isomerized to vinylidene-containing [RhCl(CCHCH2PPh3)(PiPr3)2]+ via the alkynyl hydride complex [RhCl(H)(CCCH2PPh3)(PiPr3)2]+. Consistent with previous findings [HCCCH2PPh3]+ isomerizes to [H2CCCHPPh3]+ in the absence of a metal and the balance between the vinylidene and allene complexes formed in these reactions appears to relate to the efficiency of the reaction with each substrate.
