On the Synthesis, Characterization and Reactivity of N‐Heteroaryl–Boryl Radicals, a New Radical Class Based on Five‐Membered Ring Ligands

Abstract

The synthesis and physical characterization of a new class of N-heterocycle-boryl radicals is presented, based on five membered ring ligands with a N(sp(2) ) complexation site. These pyrazole-boranes and pyrazaboles exhibit a low bond dissociation energy (BDE; BH) and accordingly excellent hydrogen transfer properties. Most importantly, a high modulation of the BDE(BH) by the fine tuning of the N-heterocyclic ligand was obtained in this series and could be correlated with the spin density on the boron atom of the corresponding radical. The reactivity of the latter for small molecule chemistry has been studied through the determination of several reaction rate constants corresponding to addition to alkenes and alkynes, addition to O2 , oxidation by iodonium salts and halogen abstraction from alkyl halides. Two selected applications of N-heterocycle-boryl radicals are also proposed herein, for radical polymerization and for radical dehalogenation reactions.

Boron chemistry in a new light

“…By selecting the easily prepared 3,5-dimethylpyrazole-borane, Blanchard and Lalevée reported the first generation of a boryl radical under visible light irradiation (Scheme 16). 33 The experiment run at room temperature in acetonitrile as the solvent yielded the typical blue H + W 10 O 32 5– reduced form of the polytungstate, accounting for the formation of the pyrazolyl-boryl radical. The formation of this radical was also confirmed through EPR experiments.…”

“…33 Tetrabutylammonium decatungstate ( n Bu 4 N + ) 4 W 10 O 32 4– in combination with silane or ligated borane derivatives were selected as a source of respectively silyl and boryl radicals, that could be further oxidised to the corresponding cationic species. By selecting the easily prepared 3,5-dimethylpyrazole-borane, Blanchard and Lalevée reported the first generation of a boryl radical under visible light irradiation (Scheme 16).…”

Cyclic Group 15 Radical Cations

“…However, the choice of anion with respect to size and basicity was crucial for the generation of radical cations.I ft he anion does not feature enough steric bulk and is too basic, it simply will attach to the phosphorus atom introducing polar covalent bonds (Scheme 3). It is interesting to note that attempts to generate ap ossible dication 4 2+ by two equivalents of the silver salt were not successful, but resulted always in the formation of ac rystalline double salt 4 ] À rapidly become yellow when traces of water or oxygen are present. Analogously,i nt he reaction of 2P 2 with Ag[BF 4 ], the anion decomposed into BF 3 and F À ,w hich in turn attacked the transient radical cation 3P 2 + ,f inally yielding the difluorodiphospha-diazane (6,s ee the Supporting Information).…”

“…Stable and transient radicals of main-group elements attracted great interest in recent years.S uch species were reviewed by Power [1] and, more recently with focus on carbene-substituted radicals,b yB ertrand et al [2] Selected examples for radicals centered on boron, [3][4][5][6] carbon, [7,8] nitrogen, [9] phosphorus, [10,11] and silicon [12,13] can be found. Most recently,Beckmann et al succeeded in the oxidation of dichalcogenides and the subsequent isolation of radical cations.…”

Aminopyridine‐Borane Complexes as Hydrogen Atom Donor Reagents: Reaction Mechanism and Substrate Selectivity

“…This is also true for DMAP‐borane 7 [BDE( 7 )= +324.8 kJ mol −1 ], the utility of which in actual reduction reactions has not been significantly explored. In one of the few studies, only activated α‐iodo esters could be reduced in a radical chain process to the dehalogenated esters in moderate yields . In the following, we explore the utility of pyridine‐borane complexes such as 7 in radical reduction reactions and show that the success of these types of transformations is tightly linked to the type of radical precursor and initiator system used.…”