text: The rhodium-catalyzed hydroboration reaction is thought to be initiated with the dissociation of a triphenylphosphine from the Rh(I) centre. Oxidative addition of the B-H bond of the borane reagent to this 14 e species is then followed by coordination of the alkene to the 16e Rh(III) hydride complex. Subsequent migratory insertion of the alkene into the rhodium-hydride bond can give two regioisomeric alkyl rhodium(III) boride complexes.  Reductive elimination of the boronate ester regenerates the catalyst. Catalyst prepared and handled under anaerobic condition reverses the selectivity to favor the secondary boronate ester.  What has been debated is the coordination of the alkene. In the dissociative mechanism, proposed by Männig and Nöth, and supported by Evans and Fu the coordination is accompanied by the loss of one triphenylphosphine ligand.
  In the associative mechanism (see below), proposed by Burgess et al., the alkene binds trans to the chloride without dissociation of a triphenylphosphine ligand. The mechanism has been studied by computational methods. Dorigo and Schleyer excluded the associative mechanism by an ab initio study on the dissociative mechanism, whereas Musaev and co-workers support the associative mechanism.