Gas-Phase Hydroboration of Allene, Other Olefins, and Acetylenes

The gas phase hydroboration of simple mono-olefins is shown to be a general method for the synthesis of mono- and 1,1-di-alkyldiboranes in yields of 55-65% and 30-40%, respectively, when carried out at 60-65 C using a 5:1 diborane olefin molar ratio. Terminal addition of diborane to allene results in the formation of the cyclic 1,2-trimethylene diborane. Polymerization of this compound, which takes place rapidly in liquid phase, is probably due to release of ring strain. Poly-mu-trimethylene- diborane is observed to depolymerize quantitatively at 60 C, thus establishing the reversibility of the reaction, which probably proceeds via mobile hydrogen bridges. Nonvolatile organoboron polymers, the structure of which is discussed in the case of 2-butyne, are formed nearly quantitatively by gas phase hydroboration of acetylenes with excess diborane; in addition, small fractions of alkyldi boranes are isolated. (Author)