Internal alkyne-to-vinylidene isomerization in the Ru complexes ([CpRu(η(2)-PhC≡CC(6)H(4)R-p)(dppe)](+) (Cp = η(5)-C(5)H(5); dppe = Ph(2)PCH(2)CH(2)PPh(2); R = OMe, Cl, CO(2)Et)) has been investigated using a combination of quantum mechanics and molecular mechanics methods (QM/MM), such as ONIOM(B3PW91:UFF), and density functional theory (DFT) calculations. Three kinds of model systems (I-III), each having a different QM region for the ONIOM method, revealed that considering both the quantum effect of the substituent of the aryl group in the η(2)-alkyne ligand and that of the phenyl groups in the dppe ligand is essential for a correct understanding of this reaction. Several plausible mechanisms have been analyzed by using DFT calculations with the B3PW91 functional. It was found that the isomerization of three complexes (R = OMe, CO(2)Et, and Cl) proceeds via a direct 1,2-shift in all cases. The most favorable process in energy was path 3, which involves the orientation change of the alkyne ligand in the transition state. The activation energies were calculated to be 13.7, 15.0, and 16.4 kcal/mol, respectively, for the three complexes. Donor-acceptor analysis demonstrated that the aryl 1,2-shift is a nucleophilic reaction. Furthermore, our calculation results indicated that an electron-donating substituent on the aryl group stabilizes the positive charge on the accepting carbon rather than that on the migrating aryl group itself at the transition state. Therefore, unlike the general nucleophilic reaction, the less-electron-donating aryl group has an advantage in the migration.
Cationic ruthenium and iron complexes [CpM(PP)]+ (Cp = η5-C5H5; M = Ru and Fe; PP = Ph2PCH2CH2PPh2, 2PPh3) can affect vinylidene rearrangement of general internal alkynes via the 1,2-migration of aryl and alkyl groups. Judging from the migratory aptitude of substituted aryl groups, the present reaction is viewed as an uncommon electrophilic rearrangement.
The cationic disubstituted vinylideneruthenium complexes [CpRu{CC(Ph)R}(dppe)][BArF
4], which are prepared directly from internal alkynes via 1,2-migration of carbon substituents, are shown to undergo disubstituted vinylidene-to-internal alkyne isomerization on reaction with a monophosphine. The observation provides the first examples in which the reversible conversion between internal alkynes and disubstituted vinylidenes is experimentally confirmed, and the kinetic and mechnistic investigations indicated that an uncommon electrophilic 1,2-migration of the carbon substituents takes place in the internal alkyne−disubstituted vinylidene interconversion.
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