Variation on the π-Acceptor Ligand within a RhI−N-Heterocyclic Carbene Framework: Divergent Catalytic Outcomes for Phenylacetylene-Methanol Transformations

A series of neutral and cationic rhodium complexes bearing IPr {IPr=1,3-bis-(2,6-diisopropylphenyl)imidazolin-2-carbene} and π-acceptor ligands are reported. Cationic species [Rh(η4-cod)(IPr)(NCCH3)]+ and [Rh(CO)(IPr)(L)2]+ (L=pyridine, CH3CN) were obtained by chlorido abstraction in suitable complexes, whereas the cod-CO derivative [Rh(η4-cod)(IPr)(CO)]+ was formed by the carbonylation of [Rh(η4-cod)(IPr)(NCCH3)]+. Alternatively, neutral derivatives of type RhCl(IPr)(L)2 {L=tBuNC or P(OMe)3} can be accessed from [Rh(μ-Cl)(η2-coe)(IPr)]2. In addition, the mononuclear species Rh(CN)(η4-cod)(IPr) was prepared by cyanide-chlorido anion exchange, which after carbonylation afforded the unusual trinuclear compound [Rh{1κC,2κN-(CN)}(CO)(IPr)]3. Divergent catalytic outcomes in the phenylacetylene-methanol transformations have been observed. Thus, enol ethers, arisen from hydroalkoxylation of the alkyne, were obtained with neutral Rh−CO catalyst precursors whereas dienol ethers were formed with cationic catalysts. Variable amounts of alkyne dimerization, cyclotrimerization or polymerization products were obtained in the absence of a strong π-acceptor ligand on the catalyst.