Density functional theory (DFT) investigations revealed that 4-cyanopyridine was capable of homolytically cleaving the BÀB s bond of diborane via the cooperative coordination to the two boron atoms of the diborane to generate pyridine boryl radicals.Our experimental verification provides supportive evidence for this new B À Ba ctivation mode.Witht his novel activation strategy,wehave experimentally realized the catalytic reduction of azo-compounds to hydrazine derivatives,d eoxygenation of sulfoxides to sulfides, and reduction of quinones with B 2 (pin) 2 at mild conditions. Diborane compounds,R 2 B À BR 2 ,a re highly useful synthetic modules for concise synthesis of organoboronates, [1] which are versatile building blocks for the generation of new chemical bonds using the Suzuki-Miyaura crossing-coupling reactions. [2] Many transition-metal complexes are known to be able to effectively cleave the B À Bb ond of diborane and catalyze the borylation or diboration of unsaturated organic substrates. [3,4] On the other hand, transitionmetal-free catalytic systems for activating the BÀB bond of diborane compounds have also attracted much attention. [5] Recently,t he Lewis adducts of sp 2 -sp 3 diborane compounds have been reported for the borylation or diboration of unsaturated substrates. [5][6][7][8][9] It has been shown that diborane compounds became asource of nucleophilic boryl moiety,w hen one of the boron atoms of the diborane was tetracoordinated to as trong base (Scheme 1a). In such cases,t he B-(sp 2 ) À B(sp 3 )b onds are always cleaved heterolytically. [6c, 7b] In addition, reductive addition of diboranes to sterically hindered pyrazines in the presence of bipyridine derivatives has also been reported. [10] Nevertheless,t he mechanistic details of this reaction are unclear yet. Owing to the high BÀBbond dissociation energy (BDE) of diborane derivatives, [11] strong bases such as N-heterocyclic carbenes (NHC), [6] Cs 2 CO 3 , t-BuOK [7b] are usually used in the activation of diborane compounds.Given the considerable utility of diboranes in synthetic chemistry,i ti sh ighly desirable to develop new strategies for the activation of B À Bb ond.Note that in the Lewis adducts of sp 2 -sp 3 diborane compounds,t here is another sp 2 hybridized boron atom. We envisioned that if the empty porbital of the uncoordinated boron atom is associated with as econd Lewis base,t he BÀB bond would be further weakened, and might be cleaved in ahomolytic manner.With appropriate Lewis bases,the boryl radical generated from the homolytic cleavage of diborane BÀBb ond could be sufficiently stable,t hrough the possible captodative effect. [12] Pyridines with electron-withdrawing substituents may act as appropriate Lewis bases to activate the B À Bbond of diborane compounds as aresult of their high boryl radical stabilization energies (see Scheme S1 in the Supporting Information). Herein, we report ac omputationally designed and experimentally verified catalyst which can homolytically cleave the BÀBb ond of B 2 (pin) 2 via the coo...
