AN AB-INITIO MOLECULAR-ORBITAL STUDY OF THE MECHANISM OF THE RHODIUM(I)-CATALYZED OLEFIN HYDROBORATION REACTION

Potential energy surfaces of the rhodium(I)-catalyzed olefin hydrobora tion reactions, RhCl(P(H)3)(2) + HB(OH)(2) + C2H4 --> RhCl(PH3)(2) + C 2H5B(OH)(2) (1) and RhCl(PH3)(2) + HBO2(CH2)(3) + C2H4 --> RhCl(PH3)(2 ) + C2H5BO2(CH2)(3) (2), have been studied by using ab initio molecula r orbital method at the MP2/ECP+DZ level. The following mechanisms hav e been considered: (I) oxidative addition of a B-K bond to the metal c enter, followed by olefin coordination to the complex in various posit ions without dissociation of PH3 group, further followed by insertion of olefin into either M-H or M-B bond and reductive elimination of B-C or B-H bond, respectively and (II) coordination of olefin to the meta l center, followed by ''sigma-bond metathesis'' involving coordination of borane and simultaneous cleavage of the M-C and B-H bonds with for mation of the M-B and H-C or M-H and B-C bonds. For both reactions, th e most favorable mechanism is shown to involve oxidative addition of b orane to the catalyst and coordination of C2H4 to the complex between B and H ligands trans to Cl, followed by insertion of C=C into the Rh- B bond. The reactions are completed by dehydrogenative reductive elimi nation of C(2)H(5)BR which is calculated to be the rate determining st ep and to have the barriers of 22.4 and 20.8 kcal/mol for eqs 1 and 2, respectively. Other competitive mechanisms involve as the rate-contro lling step the ''sigma-bond metathesis'' to break B-H and to form M-H and B-C bonds after formation of the RhCl(PH3)(2)(C2H4) complex, with the barrier of 23.9 kcal/mol for reaction 1.