Mechanism of Rhodium-Mediated Alkyne to Vinylidene Transformation

Mechanism of Rhodium-Mediated Alkyne to Vinylidene Transformation 

One of the first theoretical studies used to investigate this problem employed the MP2 approach (later found to be less reliable for transition-metal complexes than most DFT approaches) with simplified phosphine ligands, PHg, and an unsubstituted alkyne (entry 1, Table 3.1) [47]. This study demonstrated that conversion of the alkyne complex [RhCl(ti -HC=CH)(PH3)2] to the alkynyl hydride [Rh(-C=CH)C1H(PH3)2] may proceed via a transition state in which the ligand is bound as a C-H c-complex. The vinylidene complex, which is the global minimum on the PES, may be accessed by hydride migration from the alkynyl hydride, but not directly from the alkyne. An alternative bimolecular pathway (proceeding via transition state TS2 3D) for the conversion of the alkyl hydride to the vinylidene complex was also considered. The calculations 

With a view to resolving the role of either uni- or bimolecular pathways from 

Later DFT studies (Table 3.1) present a consistent picture the vinyiidene complex is always the global minimum and the unimolecular conversion ofthe alkynyl hydride complex to the vinyiidene is the rate-controlling step, although the energy difference between the two competing transition states (TSj 2 and TS2 3) is quite small. The largest outlier is the study by Hall, entry 3 [50], which puts TSj 2 at just 3 kcal mol , but it appears that what is reported as TSj 2 maybe the C-H o-complex that precedes formal oxidative addition and not the transition state itself. 

In principle, calculations such as these may be benchmarked against experimental studies where the kinetics of the conversion between the alkyne, alkynyl 

---