Eight N-heterocyclic carbene-boranes (NHC-boranes) are proposed as new efficient co-initiators for acrylate photopolymerization reactions. They are particularly interesting in aerated conditions, where they help overcome the classical oxygen inhibition. The carbene boryl radicals that are the initiating species have been characterized by their transient absorption spectra obtained in laser flash photolysis (LFP) experiments. Rate constants for the generation of the carbene boryl radicals by hydrogen abstraction with t-butoxyl radical and triplet benzophenone as well as the reactions with oxygen, electron rich and electron poor alkenes, two alkyl halides (CHCl 3 and iodopropane) and diphenyliodonium hexafluorophosphate have been measured. The reactivity of N-heterocyclic carbene borane radicals is clearly affected by the NHC substituent.
Complexation of a boron atom by an N-heterocyclic carbene has been enlisted to make an assortment of unusual lowvalent boron compounds [1] and rare boron-containing reactive intermediates including boryl radicals [2, 3a] and borenium ions.[3] Such species have interesting fundamental properties and are potentially useful reagents in organic synthesis, among other applications.Nucleophilic boron reagents are extremely rare.[4] Recent reviews list only two characterized boryl anions.[5] The tricyclohexylphosphine boryl anion A reported by Imamoto and Hikasaka [6] can be considered an analogue of the unknown parent dianion (:BH 3 2À ). Very recently, Braunschweig and co-workers described the generation and characterization of the unusual N-heterocyclic carbene (NHC) borole anion B.[7] Theoretical studies suggested that this anion was stabilized by aromaticity; in other words, it is a boron analogue of the tetraphenylcyclopentadienyl anion. Both A and B were generated by reductive metalation.We became interested in the generation and reactions of unsubstituted NHC boryl anions in the context of a program of studying applications of new NHC boranes (NHC-BH 2 R) in both small-molecule and polymer synthesis.[8] The usual synthesis of such complexes by complexation of substituted boranes with free NHCs [9] is limited because boranes are reactive towards nucleophiles (whereas NHC boranes are usually stable) and because certain classes of boranes are not easily available by hydroboration or other means. Herein we report that the lithium derivative of the prototypical NHC boryl anion C can be generated by reductive lithiation, and trapped by assorted electrophiles to provide new substituted NHC boranes.Several attempts to deprotonate 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene borane (1; see Scheme 1) with various strong bases did not lead to the desired boryl anion, so we moved quickly to reductive approaches to anion formation. However, precedent from Robinson and co-workers was not necessarily encouraging. They reduced an NHC-BBr 3 complex with potassium graphite to produce novel dimers with boron-boron bonds.[1a] If boryl anions are involved in this process, then they must have reacted rapidly with the starting material or other reaction intermediates.Boryl iodide 2 can be made rapidly in essentially quantitative yield in situ by reaction of 1 (2 equiv) with iodine (1 equiv) in benzene (Scheme 1). As a prelude to reductive metalations, we studied electrochemical reduction Scheme 1. Reductive metalation and quenching with diethyl carbonate.
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