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 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...

Synthesis of Functionalized Hydrazines: Facile Homogeneous (N‐Heterocyclic Carbene)‐Palladium(0)‐Catalyzed Diboration and Silaboration of Azobenzenes

“…During our investigations, we found that 2 can undergo hydrolysis upon stirring in degassed deionized H 2 O overnight, affording the corresponding 1,2‐diphenylhydrazine ( 13 , Scheme , a ). This result, albeit accessed through palladium catalysis, supports the proposed mechanism by Li and co‐workers, whereby 2 was computationally calculated as an intermediate in the organocatalytic formation of hydrazines from their corresponding azobenzenes . Interestingly, theirreaction conditions proved to be ineffective for the hydrolysis of 9 .…”

“…These require the use of either an extremely reactive B–B bond in the form of azadiboriridenes, dichlorodiboranes, or a highly strained B–B bond as in [2]borametallarenophanes (Scheme ) . Recently, however, a combined computational and experimental article from Li and co‐workers showed that the diboration of N=N bonds using a commercially available and air‐stable tetraalkoxydiboron reagent such as bis(pinacolato)diboron is feasible . To the best of our knowledge there are no examples in the literature of N=N silaborations.…”

Modification of TiO2 Nanoparticles with Organodiboron Molecules Inducing Stable Surface Ti3+ Complex

“…We were particularly interested in the use of organic diboron reagents because of their unique Lewis acidity and reducing ability ( vide supra ). Previous work in the area of synthetic organic chemistry showed that, upon binding to a Lewis basic oxygen atom, these organic diboron species can function as single electron reducing agents, thus allowing for various important transformations (Liu et al., 2019, Mo et al., 2010, Mo et al., 2018, Pietsch et al., 2015, Wang et al., 2016, Zhang and Jiao, 2017). Based on these reasons, we envisioned that, upon the coordination of such diboron compounds with the surface oxygen atom in metal oxide materials, the formation of surface diboron-oxygen Lewis pair may induce single electron transfer from the ipso -O 2c site to the adjacent Ti site.…”

Recent Advances in C–B Bond Formation through a Free Radical Pathway

“…Finally, radical coupling of the aryl radical and boryl radical C produces the borylation product, along with the pyridine catalyst. Very recently, Li and co‐workers reported the application of a pyridine‐stabilized boryl radical to the catalytic reduction of azobenzene compounds and radical addition/carbon‐carbon coupling of α,β‐unsaturated ketones . Therefore, this in situ generated base‐stabilized boryl radical can be used as a bifunctional “reagent”, which acts not only as a boryl radical, but also as a pyridine precursor.…”