Metathesis Reactions of Tris(adamantylimido)methylrhenium and Aldehydes and Imines

The tris(imido)methylrhenium compound CH₃Re(NAd)₃ (Ad = 1-adamantyl) was prepared and characterized. It reacts with aromatic aldehydes ArCHO forming the imines ArCHNAd. The reaction occurs in three stages, during which CH₃Re(NAd)₂O and CH₃Re(NAd)O₂ could be detected. In the third and slowest stage CH₃ReO₃ (MTO) was formed, eventually in quantitative yield. The second-order rate constant for PhCHO in C₆D₆ at 298 K is 1.4 × 10^-4 L mol^-1 s^-1. Electron-donating substituents at the para-position of ArCHO cause a significant diminution in rate. Treated by the Hammett equation, the reaction constant is ρ = +0.90. The reactions between CH₃Re(NAd)₃ and linear aliphatic aldehydes occur much faster than do reactions of nonlinear aliphatic or aromatic aldehydes, indicating an important steric effect. Ketones do not react. The imidorhenium complex evidently undergoes a metathesis reaction with the aldehyde. Analogously, CH₃Re(NAd)₃ reacts with imines. Imine−imine metathesis is catalyzed by MTO homogeneously and by MTO supported on Nb₂O₅.