Electrophilicity and Nucleophilicity of Boryl Radicals

Abstract

We carried out a survey of the relative reactivity of a collection of 91 neutral boryl radicals using density functional calculations. Their reactivities were characterized by four indices, i.e., the global electrophilicity, global nucleophilicity, local electrophilicity, and local nucleophilicity. Particularly, the global electrophilicity and nucleophilicity indices span over a moderately wider range than those of carbon radicals, indicating their potentially broader reactivity. Thus, boryl radicals may be utilized in electrophilic radical reactions, while traditionally they are only considered for nucleophilic radical reactions. In contrast, the local electrophilicity and nucleophilicity indices at the boron center show a different reactivity picture than their global counterparts. The inconsistency is rooted in the low and varying spin density on boron (for most radicals, less than 50%) in different boryl radicals, which is a combinative result of radical stabilization via electron delocalization and the low electronegativity of boron (compared to carbon). In short, the boron character in boryl radicals may be weak and their reactivity is not reflected by the local indices based on boron but by the global ones.

Boryl Radical Addition to Multiple Bonds in Organic Synthesis

“…The nucleophilicity of ligated boryl radicals seems to be general . Even if they are boranes ligated by NHCs having electron‐withdrawing groups, the corresponding NHC‐boryl radicals are still nucleophilic …”

Visible‐Light‐Induced Selective Defluoroborylation of Polyfluoroarenes, gem‐Difluoroalkenes, and Trifluoromethylalkenes

“…gem-Difluoroallylboranes are important synthons for preparation of bioactive fluorinated compounds. [13] We speculated that the HAT-induced nucleophilic NHC-boryl radical [33] could add to an a-trifluoromethyl alkene,and asubsequent single electron reduction [20] would promote an E1cB-type fluoride elimination [34] to access such boranes.I ndeed, after slight modifications of the photo-induced defluoroborylation protocol, this transformation was achieved efficiently under the transition-metal free conditions,w ith 4-CzIPN as the photocatalyst. Thes cope of trifluoromethylalkenes is illustrated in Scheme 4.…”

Difluorination at Boron Leads to the First Electrophilic Ligated Boryl Radical (NHC‐BF2.)

“…This is sensible given that the boron atom in ligated‐boryl radicals is naturally electron rich and that NHCs are good σ‐donor ligands . Indeed, calculations suggest that most classes of ligated boryl radicals are nucleophilic …”