REACTIONS OF BINUCLEAR RHODIUM HYDRIDES WITH IMINES - FACTORS INFLUENCING THE INSERTION OF CARBON-NITROGEN DOUBLE-BONDS INTO RHODIUM-HYDRIDE BONDS

The reaction of the binuclear rhodium hydride complexes [(R2PCH2CH2PR2)Rh]2(µ-H)2 (R = Pri, 1a, [(dippe)Rh]2(µ-H)2; R = OPri, 1b, [(dipope)Rh]2(µ-H)2) with simple aldimines (RCH═NR’), the ketimine benzophenone imine (Ph2C═NH), and the cyclic imine isoquinoline are described. Amido–hydride products of the general formula [(R2PCH2CH2PR2)Rh]2(µ-NR’CH2R)(µ-H) are produced. The mechanism of this reaction has been investigated by labeling studies and kinetic analysis. Intermediates have been detected and characterized spectroscopically that are consistent with the imine binding through the lone pair on nitrogen and through the π-system of the carbon–nitrogen double bond. On the basis of these studies it is proposed that a necessary condition for migratory insertion of a C═N double bond into a metal hydride is prior coordination through the C═N π-system. Iminium salts of the formula [PhCH═NHR’]+BF4- react with the hydride dimer 1a to generate cationic complexes of the formula |[(Pri2PCH2CH2PPri2)RH]2(μ-NR’═CHPh)(µ-H)}+BF4- having the imine bound in the μ-η2-σ-mode. Further reaction of this material with LiAl(OBut)3H generates the corresponding amido hydride. Attempts at using the hydride dimers 1a and 1b as catalyst precursors for the homogeneous hydrogenation of imines were only partially successful, resulting in at best seven turnovers (1 atm, 16 h). © 1990, American Chemical Society. All rights reserved.