Baciocchi et al.
JOCFeatured Article
PINO radical is the active oxidant in several synthetically
useful metal-catalyzed aerobic oxidations of organic com-
pounds.1 Moreover, PINO and BTNO play a key role in the
oxidative degradation of lignin promoted by the laccase/O2
system mediated by N-hydroxyphthalimide (HPI) and N-
hydroxybenzotriazole (HBT),2 a process which has a poten-
tial application in the pulp and paper industry.3
SCHEME 1
Thus, the reactivity of these short-lived aminoxyl radicals
has been quite intensively investigated, but mainly in reac-
tions where they behave as hydrogen atom abstractors from
C-H4 and O-H5,6 bonds (HAT, Scheme 1, path a). Much
less information is instead available about their involvement
in electron transfer (ET) processes (Scheme 1, path b).7
Recently, however, we found evidence that PINO can
N-dealkylate N,N-dimethylanilines by an ET mechanism8
and that both N-oxyl radicals are very effective in the
oxidation of ferrocenes to ferricinium cations.9,10 From the
latter study, it was also possible to determine that the
intrinsic reactivity of BTNO in ET reactions is larger than
that of PINO, which led to the prediction that in ET
processes BTNO should be more reactive than PINO in spite
of the lower (ca. 60 mV) reduction potential of the former
radical.
In this context, it appeared of interest to investigate also
the reaction of BTNO with N,N-dimethylanilines. On the
basis of previous predictions and in consideration of the fact
that the NO-H BDE in HBT is ca. 3 kcal/mol lower than that
of HPI,7 the ET mechanism should be favored in the possible
competition with the hydrogen atom transfer (HAT) process
and the reactivity of BTNO should be greater than that of
CHART 1
PINO. The oxidative N-dealkylation of tertiary amines is
attracting a continuous attention in recent years11,12 for its
great chemical and biological importance. This reaction has
also the advantage that a number of reliable mechanistic
criteria are available to assess the mechanistic dichotomy
between HAT and ET mechanisms.13,14
In this paper, we report on a product and kinetic study of
the reaction of BTNO with a number of 4-X-substituted N,
N-dimethylanilines 1-6 (X = CN, CF3, CO2Et, Me, PhO,
MeO) (Chart 1). Substituent effects on the reaction rates
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