Heck reactions of ortho-substituted arenediazonium salts: Critical observations on electronic effects

Article abstract of DOI:10.1016/S0040-4039(01)00067-3

An attempted Heck reaction of o-diethylamido phenyldiazonium tetrafluoroborate follows a unique radical-chain protodediazoniation pathway that is triggered by a facile 1,5-[H] shift of the derived aryl radical. Other ortho-substituted salts, irrespective of their redox potentials and structure, behave in the normal way.

Full text of DOI:10.1016/S0040-4039(01)00067-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2035–2037
Heck reactions of ortho-substituted arenediazonium salts: critical
observations on electronic effects
Saumitra Sengupta* and Sanchita Bhattacharyya
Department of Chemistry, Jadavpur University, Calcutta 700 032, India
Received 10 November 2000; revised 4 January 2001; accepted 10 January 2001
Abstract—An attempted Heck reaction of o-diethylamido phenyldiazonium tetrafluoroborate follows a unique radical-chain
protodediazoniation pathway that is triggered by a facile 1,5-[H] shift of the derived aryl radical. Other ortho-substituted salts,
irrespective of their redox potentials and structure, behave in the normal way. © 2001 Published by Elsevier Science Ltd.
Recently, much attention has been paid to arenediazo-
nium salts as alternative substrates to aryl halides and
triflates for palladium catalyzed reactions. We and oth-
ers have shown that these salts are much superior to
aryl bromides, triflates and even, iodides in Heck1 and
boronic acid cross-coupling reactions.2 Heck reactions
of arenediazonium salts are a facile process, e.g. pro-
ducing high yields of products under mild conditions
(Pd(OAc)2, EtOH or MeOH, with or without CaCO3,
60–80°C) and within short reaction times (30 min to 1
h). With a view to expanding the scope of these reac-
tions, we became interested in Heck reactions of the
ortho-diethylamido phenyldiazonium salt 1 so that the
immensely useful ortho-metallation chemistry of aryl
diethylamides3 could be synthetically linked to the Pd-
catalyzed reactions of arenediazonium salts.
tion. We initially rationalized this to be a case of
radical-chain protodediazoniation caused by the high
redox potential of the electron deficient diazonium salt
1.4 Such an argument drew support from our previous
observations that highly electron-poor nitro-substituted
diazonium salts also failed in their Heck and cross-cou-
pling reactions.5 However, later on we were confronted
with results that are contrary to this argument. Thus,
the ester-substituted diazonium salts 4a and b also
having high redox potentials, successfully underwent
Heck reactions with methyl acrylate to give 5a and b in
good yields (Scheme 2). Moreover, the p-diethylamido
salt 4c also reacted smoothly to produce the cinnamate
product 5c in 67% yield. Since aryl radical formation is
equally favored from 1 and 4a–c, the fact that protode-
diazoniation occurs only for 1 could no longer be
explained through simple trapping of the derived aryl
radical with the solvent methanol. This led us to pro-
pose an alternative mechanism for the protodediazonia-
tion of 1 taking into account the unique structural
features of this salt and the derived aryl radical. Thus,
Much to our surprise, the Heck reaction of 1 with
methyl acrylate in MeOH gave N,N-diethyl benzamide
(3) in 78% yield (Scheme 1). None of the expected
cinnamate product 2 could be isolated from this reac-
Scheme 1.
Keywords: Heck reaction; arenediazonium salts; aryl radicals; radical translocation.
* Corresponding author. Fax: 91-033-4734266; e-mail: jusaumitra@yahoo.co.uk
0040-4039/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0040-4039(01)00067-3

2036
S. Sengupta, S. Bhattacharyya / Tetrahedron Letters 42 (2001) 2035–2037
The lack of protodediazoniation in 4a–c can also be
explained in light of the above mechanism. Thus, a
1,5-[H] shift is geometrically precluded for aryl radicals
from 4b and c and hence the latter prefer the Pd-medi-
ated pathway. On the other hand, a 1,5-[H] shift
although feasible for the ortho-ester radical 8 derived
from 4a is unlikely since this latter radical should exist
almost entirely in the more stable anti-conformations
8a and b10 where, again, a 1,5-[H] shift is geometrically
precluded (Scheme 4).
However, it could still be argued that Heck reactions of
arenediazonium salts have an inherent radical compo-
nent and in those cases (viz. 1) where the aryl radical
can find a thermodynamic sink, e.g. via a 1,5-[H] shift,
the radical pathway should take preference over the
Pd-mediated process. Hence, the protodediazoniation
reaction observed for 1 may not be a unique case and
should occur for other diazonium salts also where,
irrespective of their redox potentials, geometric require-
ments for a 1,5-[H] shift are properly met. In fact,
Beckwith et al. have shown that, if geometry permits,
even electron rich diazonium salts, e.g. the ortho-
ethoxyphenyl diazonium salt, lead to rapid 1,5-[H]
shifts under radical generating conditions.7 To answer
this, we carried out the Heck reaction of the ortho-n-
propyloxyphenyl diazonium salt 4d with methyl acry-
late which produced the normal Heck-product 5d in
81% yield (Scheme 2). Hence, in Heck reactions of
arenediazonium salts, meeting the geometric require-
ments for a 1,5-[H] shift cannot be the sole reason for
a switch over to radical pathways. The protodediazoni-
ation of 1 must be a unique case where the high redox
potential of the salt and a 1,5-[H] shift of the aryl
radical worked in unison.
Scheme 2.
Scheme 3.
the aryl radical 6, generated from 1, is ideally poised for
a facile 1,5-[H] shift to give the more stable a-amido
radical 7 and it is this translocated radical (and not 6)
which abstracts a hydrogen from methanol to produce
While our investigations continue, we believe that the
above results provide important structural and elec-
tronic guidelines for the choice of ortho-substituted
arenediazonium salts in Pd-catalyzed reactions.
’
3 and CH2OH (Scheme 3). Although we have no
conclusive proof for this mechanism at this moment
(labeling studies are in progress), it may be noted that
1,5-[H] shifts in ortho-substituted aryl radicals have
wide precedence in the literature6–9 and are especially
common for ortho-dialkylamido aryl radicals.6,9b Trap-
ping of the translocated radical 7 with MeOH must be
an extremely fast process since products arising out of
reaction of this radical with methyl acrylate9b were not
observed in our case.
Acknowledgements
Financial support of this work from DST, New Delhi
(SP/S1/G-14/97) is gratefully acknowledged.
Scheme 4.

S. Sengupta, S. Bhattacharyya / Tetrahedron Letters 42 (2001) 2035–2037
2037
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