On the Dual Reactivity of a Nucleophilic Dihydrido‐Diborane: Reaction at the B−B Bond and/or the B−H Bond

The electron‐rich, double‐base stabilized dihydrido‐diborane(4) [HB(hpp)] 2 (hpp=1,3,4,6,7,8‐hexahydro‐2 H ‐pyrimido[1,2‐a]pyrimidinate) combines two different, easily accessible reactive sites: the B−B and B−H bond. Herein, we report two basically similar reactions of [HB(hpp)] 2 with catecholborane and B ‐chlorocatecholborane unsuspectedly resulting in two different reactivities. While reaction of the diborane with two equivalents of HB(cat) proceeded with formation of a cationic triborane with a (3c2e) bond, [{HB(μ‐hpp)} 2 (μ‐BH 2 )] + , reaction with ClB(cat) led to hydride–chloride substitution yielding [ClB(hpp)] 2 . The cationic triborane was also obtained by the addition of BH 3 to the in situ generated [HB(hpp) 2 B] + . Proceeding from the new compound [ClB(hpp)] 2 , we investigated its reactivity towards chloride abstraction with AlCl 3 and GaCl 3 . Irrespectively of the salt chosen, the reaction led to formation of a radical tricationic tetraborane [{B(hpp)} 4 ] .3+ with a (4c,5e) bond. Moreover, the dichloro‐diborane proves as a versatile reagent for further substitution cleanly reacting with the strong nucleophile n BuLi to a new dibutyl‐diborane [ n BuB(hpp)] 2 .