Density functional theory investigation into the mechanism for 2-alkyne to vinylidene isomerization by the addition of phenylacetylene to [(3-C3H5)Rh(PiPr3)2]
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The mechanism for the formation of the alkynyl, vinylidene complex [(PiPr3)2Rh(CCPh)(CC(H)(Ph))] (2) by the addition of 2 equiv of phenylacetylene to [(3-C3H 5)Rh(PiPr3)2] (1) was studied through DFT calculations. At the B3LYP/DZP level of theory, the formation of 2 is exothermic and exergonic by 25.6 and 18.1 kcalmol-1, respectively. The experimentally observed 2-phenyacetylene, alkynyl complex [(PiPr3)2Rh(2-HCCPh)(CCPh)] (Ia), is exothermic and exergonic by 22.4 and 12.5 kcalmol-1, respectively. In the lowest energy pathway from Ia to 2, the 2-phenylacetylene of Ia slips to bind through the -C-H bond to form the intermediate [(PiPr3)2Rh(-HCCPh) (CCPri)]. Then, a single transition state (TS) (H = 24.6 kcalmol-1) connects this intermediate to 2. In an alternative route, oxidative splitting of the -C-H bond results in the experimentally observed bis-alkynyl rhodium hydride complex [(PiPr 3)2Rh(H)(CCPh)2] (Ib), which was observed experimentally to be in equilibrium with Ia. Species Ib is isoenthalpic with respect to Ia (HIb-Ia = -0.36 kcalmol -1). A pathway was located for formation of 2 from Ib; however, the calculated barrier of 35.4 kcalmol-1 is too high for this pathway to be viable. This alternative pathway seems to be the lowest energy pathway for the chloro analogue [ClRh(PiPr3)2( 2-HCCPh)] (Ia-Cl). Here, the -bound phenylacetylene complex was not located; instead, the first transition state results in oxidative cleavage of the -C-H bond to form the analogue of Ib, [ClRh(PiPr 3)2(H)(CCPh)] (Ib-Cl). A single transition state connects this species with the vinylidene product [ClRh(PiPr3) 2(CC(H)(Ph))] (2-Cl). The barrier for hydrogen migration in this pathway is 25.4 kcalmol-1. The phenylacetylide ligand stabilizes the -binding mode of the second phenylacetylene; as a result, the mechanism for vinylidene formation from Ia is altered slightly from the common 1-3 shift mechanism operating for the analogue Ia-Cl. 2008 American Chemical Society.
