1556
J . Org. Chem. 1996, 61, 1556-1557
Communications
An Un p r eced en ted Ra d ica l Rea ction of
Ben zotr ia zole Der iva tives. A New Efficien t
Meth od for th e Gen er a tion of Im in yl
Ra d ica ls
Sch em e 1
L. El Kaim* and C. Meyer
Laboratoire Re´acteurs et Processus, Ecole Nationale
Supe´rieure de Techniques Avance´es, 32 Boulevard Victor,
75015 Paris, France
Received December 8, 1995
Iminyl radicals represent a new class of interesting
intermediates in synthetic radical chemistry. Indeed,
their cyclization on double bonds has been shown to yield
∆1-pyrrolines, whereas opening of cyclobutylimino radi-
cals offers a route to different substituted nitriles.1 These
species have been generated using various N-substituted
imine derivatives incorporating a rather labile nitrogen-
heteroatom bond under radical conditions. We report
herein that N-benzotriazolylimines are suitable precur-
sors for the generation of iminyl radicals and disclose a
new radical chain process involving attack of a stannyl
radical on a nitrogen atom.
At first sight indeed, N-benzotriazolylimines could have
several attractive features: first, their preparation from
the corresponding carbonyl derivatives and the readily
available N-aminobenzotriazole2 is an easy process, and
moreover, these compounds should incorporate a rela-
tively weak N-N bond. However, literature data in this
field indicate that their use for the generation of iminyl
radicals might be troublesome. Barton reported that the
reaction of Bu3SnH with an acyl derivative of N-hydroxy-
benzotriazole did not result in fragmentation to the cor-
responding acyloxy radical and then to an alkyl radical
by decarboxylation.3 To our knowledge, the only ex-
amples involving benzotriazole heterocycle in an appar-
ently radical process are the oxydation of alcohols or
ethers with N-chlorobenzotriazole4 and the samarium
diiodide reduction of R-(aminoalkyl)benzotriazole deriva-
tives.5
Nevertheless, we have discovered that treatment of
N-benzotriazolylimines 1 with Bu3SnH in benzene con-
taining a catalytic amount of AIBN leads to the formation
of the corresponding iminyl radical 2 and N-(tributyl-
stannyl)benzotriazole 7d . Further evolution of 2 to a new
radical 3, either by cyclization or fragmentation, is
followed by reduction of the latter to the expected product
4, thus enabling propagation of the chain process (Scheme
1).
Clearly, in this process, we have thus established a
radical chain reaction involving in its propagation step
an uncommon attack on a heterocyclic nitrogen tether
by a stannyl radical. While halides, sulfides, and se-
lenides have been widely used to generate radicals in the
tin hydride method,6 the literature offers few examples
of nitrogen precursors in reactions of synthetic interest.7
Diazirines have been used by Barton to achieve amina-
tion of carbon radicals in a nonchain process.8
The results obtained in the course of our study reflect
the general pattern of reactivity for iminyl radicals.1
Cyclization (for compounds 2a and 3a ) and fragmentation
reactions proceed in good to excellent yield (Table 1).
This new method for the generation of iminyl radicals
has several attractive features. First, no slow addition
of the tin hydride is required. The reaction generally
starts after a short period of induction and is generally
complete within 2 h. Even under these conditions, not a
trace if any of the corresponding ketone that would have
resulted from the premature reduction of the iminyl
radical is detected in the crude reaction mixtures.
Worthy of note is also the fact that in the case of
compound 6a derived from (-)-â-pinene an equimolar
mixture of 6c and 6d resulting from the fragmentation,
followed by a 1,5-hydrogen shift for the latter, is obtained
in spite of our experimental conditions. Moreover, no
difficulties were encountered in the separation of tin
residues from the reaction mixture. N-(Tributylstannyl)-
benzotriazole (7d ) undergoes a smooth protodestannyla-
tion on silica leading to benzotriazole as the only byprod-
uct, the latter being easily separated by standard
chromatography techniques and essentially quantita-
tively recovered.
Two mechanisms may be considered, depending on
whether the tin radical attacks the N(1) or N(2) nitrogen
atom (Scheme 2). Both imply the generation of a
transient nitrogen-centered radical which upon subse-
quent rearomatization could liberate either a tin radical
in a degenerate reaction or an iminyl radical 2 capable
of further interesting synthetic evolution. The spectro-
(6) (a) Giese, B. Radicals in Organic Synthesis: Formation of
Carbon-Carbon Bonds; Pergamon Press: Oxford, 1986. (b) Curran,
D. P. Synthesis 1988, 417, 489.
(7) Addition of stannyl radicals to aryl triazenes to form stannyl-
amines and aromatic radicals by loss of nitrogen has been reported
but has received no synthetic application in radical chemistry: Hol-
laender, J .; Neumann, W. P.; Alester G. Chem. Ber. 1972, 105, 1540.
Radical addition to azide have been shown to proceed by a radical
attack on nitrogen: Kim, S.; J oe G. H.; Do T. Y. J . Am. Chem. Soc.
1994, 116, 5521.
(1) See, for instance: (a) Boivin, J .; Fouquet, E.; Zard, S. Z.
Tetrahedron 1994, 50, 1745. (b) Atmaran A.; Forrester A. E.; Gill, M.;
Fhomson, R. H. J . Chem. Soc., Perkin Trans. 1 1981, 1721.
(2) Campbell, C. D.; Rees, C. W. J . Chem. Soc. C 1969, 742.
(3) Barton, D. H. R.; Blundell, P.; J aszberenyi, J . Cs. Tetrahedron
Lett. 1989, 30, 2341.
(4) (a) Rees, C. W.; Storr, R. C. J . Chem. Soc. C 1969, 1474. (b) Pojer,
P. M. Aust. J . Chem. 1979, 32, 2787.
(5) Aurrecoechea, J . M.; Fernandez-Acebes, A. Tetrehedron Lett.
1993, 34, 549.
(8) Barton, D. H. R.; J aszberenyi, J . Cs.; Theodorakis, E. A. J . Am.
Chem. Soc. 1992, 114, 5904.
0022-3263/96/1961-1556$12.00/0 © 1996 American Chemical Society