The effect of solvent on the α-effect: C=O, P=O and SO2 centers

Article abstract of DOI:10.1039/b007000i

The α-effect for the reaction of a sulfonyl ester exhibits a bell-shaped dependence of the α-effect on solvent composition as do the corresponding reactions with a carbonyl and a phosphinyl ester, and the magnitude of the α-effect is found to be dependent

Full text of DOI:10.1039/b007000i

The effect of solvent on the a-effect: CNO, PNO and SO2 centers†
Ik-Hwan Um,*a Jin-Young Honga and Erwin Buncel*b
a Department of Chemistry, Ewha Womans University, Seoul 120-750, Korea
b Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada.
E-mail: ihum@mm.ewha.ac.kr; Fax: (+1)-(822)-3277-2844; Tel: (+1)-(822)-3277-2349
Received (in Cambridge, UK) 29th August 2000, Accepted 6th November 2000
First published as an Advance Article on the web 11th December 2000
The a-effect for the reaction of a sulfonyl ester exhibits a
bell-shaped dependence of the a-effect on solvent composi-
tion as do the corresponding reactions with a carbonyl and a
phosphinyl ester, and the magnitude of the a-effect is found
to be dependent on the magnitude of the bnuc value,
suggesting TS stabilization as the cause of the a-effect.
Since the a-effect term was given by Edwards and Pearson in
1962 to the abnormally enhanced reactivity shown by nucleo-
philes having a pair of unshared electrons adjacent to the
nucleophilic center,1 numerous studies have been performed to
account for this phenomenon.2–11 However, the cause of the a-
effect has not been clearly understood.2–11 One of the intriguing
aspects in a-effect studies has been the finding that the
magnitude of the a-effect is dependent on the nature of the
electrophilic center, generally increasing as sp3 < sp2 < sp for
carbon centers,2–4 though some exceptions exist.5 Equally
interesting, as well as controversial, has been the effect of
solvent on the a-effect.6–9
In order to shed light on the effect of solvent on the a-effect,
Fig. 1 Plots showing the effect of solvent on second-order rate constants
we initiated systematic studies and in 1986, we investigated the
reaction of p-nitrophenyl acetate (PNPA) with butane-2,3-dione
monoximate (Ox2) and p-chlorophenoxide (ClPhO2), as the a-
and corresponding normal-nucleophile, respectively, in
for the reaction of PNPBS with Ox2 and ClPhO2 in DMSO–H2O mixtures
at 25.0 ± 0.1 °C.
and ClPhO2 systems: the rate enhancement upon solvent
DMSO–H2O mixtures of varying compositions.6a We found,
change from 10 to 90 mol% of DMSO is 2190 and 3330 for Ox2
2
/
unexpectedly, a bell-shaped dependence of the a-effect (kOx
2
and ClPhO2, respectively. Interestingly, the plot of log kOx vs.
2
kClPhO ) on solvent composition.6a A similar bell-shaped trend
was observed for the corresponding reaction of p-nitrophenyl
diphenylphosphinate (PNPDPP).6b However, Moss reported
that the reaction of PNPA with o-iodosylbenzoate (IBO2) and
ClPhO2 in DMSO–H2O mixtures shows no maximum a-effect
but exhibits a decreasing a-effect trend.7 More surprisingly, a
contrasting solvent behaviour was found recently:8 the reaction
of PNPA with Ox2 and ClPhO2 in MeCN–H2O mixtures
exhibits an increasing a-effect trend as the mol% MeCN in the
medium is increased.8a Similarly, the a-effect for the reaction of
PNPA with benzohydroxamates and m-chlorophenoxide in
MeCN–H2O mixtures also resulted in an increasing a-effect
behaviour as the concentration of MeCN in the reaction medium
was increased.8b
mol% of DMSO shows downward curvature, while that of log
2
kClPhO vs. mol% DMSO exhibits upward curvature. As a result,
the difference in rate constant between Ox2 and ClPhO2
increases up to ca. 50 mol% DMSO but decreases beyond this
point. Such a differential solvent effect on rates leads to the
solvent dependent a-effect profile, shown in Fig. 2; i.e. the
sulfonyl system exhibits maximum a-effect at ca. 50 mol%
DMSO, as do the carbonyl and phosphinyl systems. Therefore,
It appeared to us as potentially highly informative, in
investigation of the a-effect, to vary the electrophilic center
systematically and to couple that with variation of solvent. We
report herein such a study for the reaction of a sulfur centered
substrate, p-nitrophenyl benzenesulfonate (PNPBS), with Ox2
and ClPhO2 in DMSO–H2O mixtures as shown in eqn. (1), and
compare the results with the data for the corresponding
reactions of PNPA and PNPDPP.
C6H5SO2-OC6H4NO2-p + Nu2
?
C6H5SO2Nu + 2OC6H4NO2-p
Nu2 = CH3C(O)C(CH3)NNO2 (Ox2), an a-nucleophile
p-ClC6H4O2 (ClPhO2), a normal nucleophile
(1)
As shown in Fig. 1, the second-order rate constant increases
as the mol% of DMSO in the medium increases for both Ox2
Fig. 2 Plots showing the effect of solvent on the a-effect for the reaction
of PNPA, PNPDPP and PNPBS with Ox2 and ClPhO2 in DMSO–H2O
mixtures at 25.0 ± 0.1 °C.
† Electronic supplementary information (ESI) available: Tables of rate
constants. See http://www.rsc.org/suppdata/cc/b0/b007000i/
DOI: 10.1039/b007000i
Chem. Commun., 2001, 27–28
This journal is © The Royal Society of Chemistry 2001
27

the bell-shaped a-effect behaviour has been found to be general
for the reactions of the three different electrophiles with Ox2
and ClPhO2 in DMSO–H2O mixtures.
substrate in the TS of the rate-determining step; hence the TS
structures of the carbonyl, phosphinyl and sulfonyl systems
would vary according to the different bnuc values. One can
expect that the TS stabilizing effect would be smaller for the
reaction system in which the degree of bond formation between
nucleophile and substrate in the TS is less advanced (reactant-
like TS), and vice versa. Accordingly, one can suggest that the
TS stabilizing effect would be developed to a lesser extent for
the phosphinyl system compared with the carbonyl and sulfonyl
systems, based on the smaller bnuc value obtained for the
former, which would explain the small a-effect observed for the
phosphinyl system.
More systematic studies are underway including theoretical
investigation for better understanding of solvent effect on the a-
effect.
The authors are grateful for the financial support from
KOSEF of Korea (1999-2-123-003-5 and 2000-123-02-2) and
E. B. also thanks NSERC of Canada for a research grant.
Moreover, interestingly, the magnitude of the a-effect is
strongly dependent on the electrophilic center; i.e. the a-effect
in 50 mol% is ca. 300, 40 and 200 for the carbonyl, phosphinyl
and sulfonyl systems, respectively. The small a-effect for the
phosphinyl system compared to the carbonyl and sulfonyl
systems is striking. Bruice showed that the magnitude of the a-
effect is dependent on the magnitude of the bnuc value for
reactions of a variety of substrates with hydrazine and
glycylglycine: the a-effect decreases with decreasing bnuc
value.10 Similarly, Bernasconi observed no a-effect for the
addition reaction of primary amines including hydrazine and o-
methylhydroxylamine to Meldrum’s acid, a system for which
bnuc = 0.22.11a
The bnuc values for the reactions of PNPA with substituted
phenoxides in various DMSO–H2O mixtures are available,12
but the ones for the reaction of the phosphinyl and sulfonyl
systems have not been reported. Therefore, we performed the
reaction of PNPDPP and PNPBS with a series of substituted
Notes and references
phenoxides in 50 mol% DMSO, in which the maximum a-effect
2
is observed. The plots of log kZC H O vs. pKa (ZC6H4OH)
exhibit good Brønsted type correlation: bnuc values are 0.64,
0.21 and 0.54 for the carbonyl, phosphinyl and sulfonyl
systems, respectively. Thus, the bnuc value for the phosphinyl
systems is much smaller than for the carbonyl and sulfonyl
systems, and, moreover, the bnuc value follows the same order
as the a-effect in magnitude. Therefore, one can suggest that the
small bnuc value is responsible for the small a-effect exhibited
by the phosphinyl system. This argument is consistent with our
recent report that the reaction for an sp hybridized carbon center
exhibited an unexpectedly small a-effect in which the bnuc
value was 0.32.5
As demonstrated in Fig. 2, the effect of solvent on reactivity
is significant. Such a solvent effect on rate can be achieved by
destabilizing the ground-state (GS) and/or stabilizing the
transition-state (TS). We recently found that the GS of Ox2 and
ClPhO2 becomes destabilized upon addition of DMSO to the
reaction medium,6c however, the GS energy difference between
Ox2 and ClPhO2 is constant for the three systems. Therefore,
if the GS energy difference between Ox2 and ClPhO2 were
mainly responsible for the a-effect, the magnitude of the a-
effect should be about the same, regardless of the nature of the
electrophilic center. However, our results show that this is not
the case. Therefore, the present results clearly suggest that the
difference in the GS energy is not solely responsible for the a-
effect.
1 J. O. Edwards and R. G. Pearson, J. Am. Chem. Soc., 1962, 84, 16.
2 Reviews: (a) E. Buncel and S. Hoz, Isr. J. Chem., 1985, 26, 313; (b)
A. P. Grekov and V. Y. Veselov, Usp. Khim., 1978, 47, 1200; (c) N. J.
Fina and J. O. Edwards, Int. J. Chem. Kinet., 1973, 5, 1.
3 (a) G. Moutiers, E. Guevel, L. Villien and F. Terrier, J. Chem. Soc.,
Perkin Trans. 2, 1997, 710; (b) F. Terrier, G. Moutiers, L. Xiao, E.
Guevel and F. Guir, J. Org. Chem., 1995, 60, 1748; (c) K. R. Fountain,
D. B. Tad-y, T. W. Paul and M. V. Golynskiy, J. Org. Chem., 1999, 64,
6547.
4 (a) E. Buncel, C. Chuaqui and H. Wilson, J. Am. Chem. Soc., 1982, 104,
4896; (b) S. Oae and Y. Kadoma, Phosphorus, Sulfur Silicon Relat.
Elem., 1997, 123, 293.
5 I. H. Um, J. S. Lee and S. M. Yuk, J. Org. Chem., 1998, 63, 9152.
6 (a) E. Buncel and I. H. Um, J. Chem. Soc., Chem. Commun., 1986, 595;
(b) R. M. Tarkka and E. Buncel, J. Am. Chem. Soc., 1995, 117, 1503; (c)
I. H. Um and E. Buncel, J. Org. Chem., 2000, 65, 577.
7 R. A. Moss, S. Swarup and S. Ganguli, J. Chem. Soc., Chem. Commun.,
1987, 860.
8 (a) I. H. Um and E. Buncel, J. Chem. Soc., Chem. Commun., 2000, 1917;
(b) I. H. Um, H. W. Yoon, J. S. Lee and H. J. Moon, J. Org. Chem.,
1997, 62, 5939.
9 (a) G. Moutiers, E. Guevel, L. Villien and F. Terrier, J. Chem. Soc.,
Perkin Trans. 2, 1997, 7; (b) C. A. Bunton, N. D. Gillitt and A. Kumar,
J. Phys. Org. Chem., 1997, 10, 221; (c) M. Laloi-Diard, J. F. Verchere,
P. Gosselin and F. Terrier, Tetrahedron Lett., 1984, 25, 1267.
10 H. J. Dixon and T. C. Bruice, J. Am. Chem. Soc., 1972, 94, 2052.
11 (a) C. F. Bernasconi and C. Murray, J. Am. Chem. Soc., 1986, 108, 5251;
(b) D. Herschlag and W. P. Jencks, J. Am. Chem. Soc., 1990, 112, 1951;
(c) L. E. Fikes, D. T. Winn, R. W. Sweger, M. P. Johnson and A. W.
Czarnik, J. Am. Chem. Soc., 1992, 114, 1493.
6
4
The magnitude of the bnuc value has been understood as a
measure of bond formation between the nucleophile and the
12 E. Buncel, I. H. Um and S. Hoz, J. Am. Chem. Soc., 1989, 111, 971.
28
Chem. Commun., 2001, 27–28