DOI: 10.1002/chem.201400197
Full Paper
&
Radical Chemistry
On the Synthesis, Characterization and Reactivity of
N-Heteroaryl–Boryl Radicals, a New Radical Class Based on
Five-Membered Ring Ligands
Mohamad-Ali Tehfe,[a] Stꢀphane Schweizer,[b] Anne-Caroline Chany,[b] Cꢀdric Ysacco,[c]
Jean-Louis Clꢀment,[c] Didier Gigmes,[c] Fabrice Morlet-Savary,[a] Jean-Pierre Fouassier,[d]
Markus Neuburger,[e] Thꢀophile Tschamber,[f] Nicolas Blanchard,*[f] and Jacques Lalevꢀe*[a]
Abstract: The synthesis and physical characterization of
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-heterocy-
cle–boryl radicals are also proposed herein, for radical poly-
merization and for radical dehalogenation reactions.
a new class of N-heterocycle–boryl radicals is presented,
based on five membered ring ligands with
a
N(sp2)
complexation site. These pyrazole–boranes and pyrazaboles
exhibit a low bond dissociation energy (BDE; BꢀH) and ac-
cordingly excellent hydrogen transfer properties. Most im-
portantly, a high modulation of the BDE(BꢀH) by the fine
tuning of the N-heterocyclic ligand was obtained in this
series and could be correlated with the spin density on the
Introduction
phine-boranes could be used in organic synthesis although
their high bond dissociation energy (BDE; BꢀH) was a serious
concern for applications in radical chain reactions. It should
also be noted that their behavior as polarity reversal catalysts
has been evidenced, thus clearly expanding their synthetic in-
terest.[3] Interestingly, it was demonstrated through theoretical
approaches that the BDE(BꢀH) can be tuned by an appropriate
design of the Ligand (L)[4a,b] and that synergistic effects be-
tween the ligand and the substituents on the boron atom had
a pronounced effect on the electronic structure of the boryl
and also on its reactivity.[4c]
Boryl radicals are versatile nucleophilic species that have re-
cently triggered intense studies in organic synthesis.[1] These
radicals are classically produced through a hydrogen abstrac-
tion reaction between a borane and a suitable hydrogen ac-
ceptor. Historically,[2] Roberts demonstrated in a series of ele-
gant reports that boryl radicals derived from amine- or phos-
[a] Dr. M.-A. Tehfe, Dr. F. Morlet-Savary, Prof. J. Lalevꢀe
Institut de Science des Matꢀriaux de Mulhouse IS2M, UMR CNRS 7361
UHA, 15, rue Jean Starcky, 68057 Mulhouse Cedex (France)
A recent breakthrough has led to the development of a new
class of boryl radicals based on N-heterocyclic carbene–bor-
anes (NHC–boranes) that can efficiently overcome this BDE(Bꢀ
H) limitation, the BDE(BꢀH) being brought down to about 80–
90 kcalmolꢀ1 compared to 92–105 kcalmolꢀ1 for amine- or
phosphine-boranes.[1] NHC–boranes are now used in elegant
chemical reactions such as radical reductions of xanthates and
related derivatives where they serve as radical hydrogen atom
donors in Barton–McCombie reactions.[1] Based on quantum
mechanical calculations, the 80–90 kcalmolꢀ1 range of BDE
values can also be covered with some borane complexes con-
taining five or six membered ring heterocycles with a N(sp2)
complexation site for the borane moiety.[5] Recently, N-hetero-
aryl-boranes such as 1a (Figure 1), characterized by such
a BDE(BꢀH) feature, have been reported by our laboratories
and the formation as well as the reactivity of the associated
boryl radicals were investigated.[6,7] The kinetic data on boryl
radicals remain, however, rather scarce.[8]
[b] Dr. S. Schweizer, Dr. A.-C. Chany
Universitꢀ de Haute-Alsace, ENSCMu
Laboratoire de Chimie Organique et Bioorganique, EA 4566
3 rue Alfred Werner, 68093 Mulhouse Cedex (France)
[c] Dr. C. Ysacco, Dr. J.-L. Clꢀment, Dr. D. Gigmes
Aix-Marseille Universitꢀ, CNRS, Institut de Chimie Radicalaire
UMR CNRS 7273, 13397 Marseille, Cedex 20 (France)
[d] Prof. J.-P. Fouassier
Formerly, ENSCMu, 3 rue Alfred Werner
68093 Mulhouse Cedex (France)
[e] Dr. M. Neuburger
Department of Chemistry, University of Basel
Spitalstrasse 51, 4056 Basel (Switzerland)
[f] Dr. T. Tschamber, Dr. N. Blanchard
Universitꢀ de Strasbourg, ECPM
Laboratoire de Chimie Molꢀculaire UMR CNRS 7509
25 rue Becquerel, 67087 Strasbourg (France)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400197.
Chem. Eur. J. 2014, 20, 5054 – 5063
5054
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim