Oxidation of Substituted Benzaldehydes by BBCP
J . Org. Chem., Vol. 61, No. 4, 1996 1313
Ta ble 7. Tem p er a tu r e Dep en d en ce for th e Rea ction Con sta n ts for th e Oxid a tion of Su bstitu ted Ben za ld eh yd es by
BBCP
T (K)
L
D
R
S
η
R2
sd
PD
PS
Para-Substituted
288
298
308
318
-1.40 ( 0.05
-1.39 ( 0.02
-1.30 ( 0.05
-1.25 ( 0.01
-1.61 ( 0.04
-1.57 ( 0.01
-1.52 ( 0.04
-1.45 ( 0.01
-0.67 ( 0.16
-0.57 ( 0.05
-0.59 ( 0.16
-0.55 ( 0.03
0.42
0.36
0.39
0.38
0.9975
0.9997
0.9998
0.9998
0.04
0.01
0.01
0.01
0.04
0.01
0.01
0.01
53.5
53.1
53.9
53.7
Meta-Substituted
288
298
308
318
-1.64 ( 0.03
-1.60 ( 0.01
-1.51 ( 0.02
-1.46 ( 0.02
-1.09 ( 0.03
-1.05 ( 0.01
-0.96 ( 0.02
-0.91 ( 0.01
-0.32 ( 0.13
-0.31 ( 0.06
-0.32 ( 0.10
-0.28 ( 0.01
0.29
0.30
0.31
0.31
0.9988
0.9998
0.9992
0.9998
0.02
0.01
0.02
0.01
0.03
0.01
0.02
0.01
40.7
39.6
38.9
37.6
Ortho-Substituted
288
298
308
318
-1.57 ( 0.02
-1.55 ( 0.01
-1.46 ( 0.01
-1.38 ( 0.03
-1.38 ( 0.01
-1.31 ( 0.01
-1.28 ( 0.01
-1.22 ( 0.02
-0.48 ( 0.08
-0.48 ( 0.04
-0.47 ( 0.06
-0.53 ( 0.13
1.04 ( 0.01
1.02 ( 0.01
0.95 ( 0.01
0.88 ( 0.02
0.35
0.37
0.37
0.43
0.9997
0.9999
0.9998
0.9989
0.01
0.01
0.01
0.02
0.02
0.01
0.01
0.01
35.3
35.7
34.7
33.8
26.1
26.3
25.7
25.3
Cor r ela tion An a lysis of Rea ctivity. A perusal of
the data recorded in Tables 2 and 3 reveals that the
formation constants, K, for the ArCHO-BBCP complexes
are not very sensitive to the nature of the substituent in
the aldehyde substrate. This may be because of the fact
that the formation constant, K, is a composite value of
two preequilibria (the first two steps of Scheme 1 and 2)
and that the electronic requirements of the two steps are
opposite and almost equal. The rate constant for decom-
position of the complex, k2, however, showed considerable
variation as a function of substituent in the substrate.
Similar observations have been previously recorded in
the oxidation of benzyl alcohols14 and mandelic acids15
by ceric ammonium nitrate and of aliphatic primary
alcohols by BBCP,5 pyridinium fluorochromate,16 and
pyridinium hydrobromide perbromide.17
Since the discovery of significant effects of substituents
on reactivity, many workers have attempted correlations
with the Hammett equation18 or with dual substituent-
parameter equations.19,20 In the 1980’s, Charton21 intro-
duced a triparametric LDR equation for the quantitative
description of structural effects on chemical reactivities.
This triparametric equation results from the fact that
substituent types differ in their mode of electron delo-
calization. This difference is reflected in a different
sensitivity to the electronic demand for the phenomenon
being studied. It has the advantage of not requiring a
choice of parameters, as the same three substituent
constants are reported to cover the entire range of
electrical effects of substituents. We have, therefore,
begun a study of structural effects on reactivity by means
of the LDR equation. In this work, we have applied the
LDR equation (eq 5) to the rate constants, k2. Here, σl
For ortho-substituted compounds, it is necessary to
account for the possibility of steric effects, and Charton,
therefore, modified the LDR equation to generate the
LDRS equation (7)21
log k2 ) Lσ1 + Dσd + Rσe + SV + h
(7)
where V is the well known Charton’s steric parameter
based on van der Waals radii.22
The rates of oxidation of ortho-, meta-, and para-
substituted benzaldehydes show excellent correlations
with structure via the LDR/LDRS equations (Table 7).
All three series of substituted benzaldehydes meet the
requirement of a minimum number of substituents for
analysis by LDR and LDRS equations.21 We have used
the standard deviation (sd), the coefficient of multiple
determination (R2), and Exner’s23 parameter, ψ, as
measures of goodness of fit.
The comparison of the L and D values for the substi-
tuted benzaldehydes showed that the oxidation of para-
substituted benzaldehydes is more susceptible to the
delocalization effect than to the localized effect. However,
the oxidation of ortho- and meta-substituted compounds
exhibited a greater dependence on the field effect. In all
cases, the magnitude of the reaction constants decreases
with an increase in the temperature, pointing to a
decrease in selectivity with an increase in temperature.
All three regression coefficients, L, D, and R, are
negative, indicating an electron-deficient carbon center
in the activated complex for the rate-determining step.
The positive value of η adds a negative increment to σd
(eq 6), reflecting the electron-donating power of the
substituent and its capacity to stablize a cationic species.
The positive value of S indicates that the reaction is
subject to steric acceleration by an ortho-substituent.
To test the significance of localized, delocalized, and
steric effects in the ortho-substituted benzaldehydes,
log k2 ) Lσ1 + Dσd + Rσe + h
(5)
is a localized (field and/or inductive) effect parameter,
σd is the intrinsic delocalized (resonance) electrical effect
parameter when active site electronic demand is mini-
mal, and σe represents the sensitivity of the substituent
to changes in electronic demand by the active site. The
latter two substituent parameters are related by eq 6.
(14) Young, Y. B.; Trahanovsky, W. S. J . Am. Chem. Soc. 1969, 91,
5060.
(15) Banerji, K. K. J . Indian Chem. Soc. 1975, 52, 573.
(16) Banerji, K. K. J . Chem. Soc., Perkin Trans. 2 1988, 1547.
(17) Mathur, D.; Sharma, P. K.; Banerji, K. K. J . Chem. Soc., Perkin
Trans. 2 1993, 205.
(18) J ohnson, C. D. The Hammett Equation; Cambridge University
Press: Cambridge 1973; p 78.
(19) Dayal, S. K.; Ehrenson, S.; Taft, R. W. J . Am. Chem. Soc. 1974,
94, 9113.
σD ) ησe + σd
(6)
(20) Swain, C. G.; Unger, S. H.; Rosenquest, N. R.; Swain, M. S. J .
Am. Chem. Soc. 1983, 105, 492.
(21) Charton, M.; Charton, B. Bull. Soc. Chim. Fr. 1988, 199 and
references cited therein.
(22) Charton, M. J . Org. Chem. 1975, 40, 407.
(23) Exner, O. Collect. Czech. Chem. Commun. 1966, 31, 3222.
where η represents the electronic demand of the reaction
site which is given by η ) R/D and σD represents the
delocalized electrical parameter of the diparametric LD
equation.