Homolytic Cleavage of a B−B Bond by the Cooperative Catalysis of Two Lewis Bases: Computational Design and Experimental Verification

Abstract

Density functional theory (DFT) investigations revealed that 4-cyanopyridine was capable of homolytically cleaving the B-B σ 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-B activation mode. With this novel activation strategy, we have experimentally realized the catalytic reduction of azo-compounds to hydrazine derivatives, deoxygenation of sulfoxides to sulfides, and reduction of quinones with B2 (pin)2 at mild conditions.

The Phosphinoboration of N‐Heterocycles

“…However, a particularly insightful and perceptive suggestion by one of the reviewers of this manuscript highlighted the possibility that phosphinoboration may well occur via of a radical‐based process and a survey of the literature revealed several recent pertinent papers. For example, pyridine boryl radicals generated by a Lewis base‐assisted homolytic cleavage of the B−B σ‐bond in diborane are involved in the reduction of azo compounds to hydrazines, the catalytic borylation of aryl halides, the metal‐free synthesis of 4‐substituted pyridines via a novel radical addition/C−C coupling sequence and the borylative/cyclisation of 1,6‐enynes . Thus, this study will provide a platform to undertake a more comprehensive investigation into the phosphinoboration reaction and, to this end, further studies are currently underway to establish the viability of Ib as a key intermediate in the solvent dependent 1,2‐ and 1,4‐phosphinoboration and future computational investigations will focus on exploring the possibility of a radical based process involving base initiated homolytic cleavage of the P−B bond to generate an active pyridine boryl radical.…”

Recent Advances in Pyridine‐Based Organocatalysis and its Application towards Valuable Chemical Transformations

“…In 2016, research group of Li from China have witnessed 4‐cyanopyridine ( 4 c ) as a suitable organocatalyst for the successful cleavage of B−B bond. The witnessed protocol was able to form N−B bonds, which were subsequently used to reduce diazo compounds 16 , quinone 20 and deoxygenation of sulfoxides 18 into hydrazine derivatives 17 a – c , sulfides 19 a – c and hydroquinone 21 (Scheme and ) …”

“…The free‐radical trapping experiments were performed to ensure the formation of free‐radical during the cyclic pathway. The mechanistic pathway proceeds via successive boryl radical migration by utilizing intermediate A to reduce azobenzene (Scheme ) …”

Nucleophilic Neutral Diborane(4) Compounds with sp3–sp3‐Hybridized Boron Atoms

“…Homolytic cleavage of the B–B bond of B 2 (pin) 4 is achieved by the cooperative catalysis of two Lewis bases (4‐cyanopyridine) . In this case, the sp 3 –sp 3 ‐hybridized diborane(4) was not isolated, but was postulated as an intermediate.…”