Selective B-B Bond Activation in an Unsymmetrical Diborane(4) by [(Me₃p)₄rh-X] (X = Me, OtBu): a Switch of Mechanism?

The unsymmetrical diborane(4) pinB-B((RN)2(C6H4)) (R = Me, Bn) reacts with [(Me3P)4Rh-X] (X = Me, OtBu) giving predominantly either [(Me3P)4Rh-Bpin] or [(Me3P)3Rh-B((RN)2(C6H4))] depending on X. At low temperatures in the presence of excess PMe3 the unprecedented equatorial boryl complex [(Me3P)4Rh-B((MeN)2(C6H4))] is formed.