Transition-State Imbalance in Carbene Complexes?
A R T I C L E S
the (CO)5Cr moiety toward its structure and charge in the anion.
Since this preorganization is likely to be maintained at the
transition state, the difference in the distribution of the negative
charge between transition state and anion is less dramatic,
implying a smaller imbalance.
The notion that the π-donor effect of the methoxy group is
responsible for the reduced difference between RCH and âB is
supported by data on the reaction of 16H-Z with amines, which
indicate a much smaller difference between RCH and âB than
for the corresponding reactions of 6H-Z.27
Conclusions
The proton-transfer reactions of 3H-Z are characterized by
q
high intrinsic barriers (∆G0 ) or low intrinsic rate constants (k0),
Figure 5. Schematic representation of how the Brønsted slope (RCH) is
lowered as the result of the π-donor effect of the MeO group; see text.
as is typical for carbon acids activated by strong π-acceptors.
However, the transition-state imbalance that has been shown
to be responsible for the high intrinsic barriers with purely
organic carbon acids does not manifest itself in the Brønsted
coefficients of the deprotonation of 3H-Z; specifically, RCH does
not significantly exceed âB. This result is attributed to the
π-donor effect of the methoxy group, which contributes to the
intrinsic barrier because the loss of the stabilizing influence of
the methoxy group on the neutral carbene complex runs ahead
of the proton transfer at the transition state. Since the π-donor
effect increases with increasing electron-withdrawing strength
of Z, k0 decreases and partially offsets the substituent effect on
the deprotonation rate constant; the result is a lower RCH value.
A more reasonable hypothesis is that the transition state 12-Z
is imbalanced, as shown in eq 1, but that there is a structural
feature characteristic of carbene complexes, which is absent from
other carbon acids, that masks the imbalance by reducing RCH
.
The most likely candidate is the methoxy group attached to the
CrdC system. It is well-known that because the carbene carbon
is highly electron-deficient,25,26 the methoxy group exerts a
strong π-donor effect (3H-Z() that stabilizes the carbene
complex.4,25,26 Inasmuch as the contribution of 3H-Z( leads to
Experimental Section
Synthesis of 3H-Z (Z ) 4-Me, 4-OMe, 3-F). The synthesis of the
carbene complexes involved the transformations shown in Scheme 1.
resonance stabilization of the carbene complexes, this resonance
is expected to add to the intrinsic barrier of proton transfer.
This is because, as is true for resonance effects in general, its
loss at the transition state should be ahead of the proton
transfer.10 As Z becomes more electron-withdrawing, the greater
electron deficiency of the carbene carbon induces a stronger
π-donor effect by the methoxy group. There are two conse-
quences. One is that the increased contribution of the resonance
structure 3H-Z( partially compensates for the destabilization
of the electron-deficient carbene carbon by the electron-
withdrawing inductive effect of Z. The second is that the
increased resonance stabilization of the carbene complex
Scheme 1
4-Methylbenzyl Methyl Ether.28 4-Methylbenzyl bromide (15 g,
0.08 mol) in 25 mL of diethyl ether was added to a NaOMe solution
prepared from 96 g (0.4 mol) of sodium and 100 mL of methanol at
reflux within 40 min. Refluxing and stirring continued for 15 h. Water
(40 mL) was added to the cooled solution and methanol was removed
by distillation. The residue was extracted 3 times, each with 20 mL of
diethyl ether. The ether solution was washed with water and dried over
CaCl2. The product was obtained by vacuum distillation with a yield
B
increases the intrinsic barrier and hence the rate (k1 ) enhance-
ment caused by the inductive effect of Z will be attenuated.
This attenuation is proportional to the electron-withdrawing
strength of Z as illustrated schematically in Figure 5 and hence
the slope of the Brønsted plot (RCH) is reduced.
1
of 65%. H NMR (250 MHz, CDCl3) δ 2.30 (s, 3H, ArCH3), 3.31 (s,
3H, OCH3), 4.36 (s, 2H, CH2), 7.01-7.14 (m, 4H, C6H4).
4-Methoxybenzyl Methyl Ether and 3-Fluorobenzyl Methyl
Ether. The procedures were identical to that for the synthesis of
4-methylbenzyl methyl ether. Methoxy derivative: yield 80%; 1H NMR
(250 MHz, CDCl3) δ 3.40 (s, 3H, CH2OCH3), 3.81 (s, 3H, ArOCH3),
4.41 (s, 2H, CH2), 7.10 (q, 4H, C6H4). Fluoro derivative: yield 34%;
1H NMR (250 MHz, CDCl3) δ 3.39 (s, 3H, OCH3), 4.44 (s, 2H, CH2),
7.08-7.31 (m, 4H, C6H4).
An alternative view is that the contribution of 3H-Z( to the
structure of the carbene complex is tantamount to preorganizing
(23) These results are reminiscent of the “nitroalkane anomaly”, where the
acidities of methyl substituted nitroalkanes follows the order CH3NO2
<
CH3CH2NO2 < (CH3)2CHNO2 and the rate constants for deprotonation by
OH- decrease in the order CH3NO2 > CH3CH2NO2 > (CH3)2CHNO2.24
(24) Kresge, A. J. Can. J. Chem. 1974, 52, 1897.
(25) Do¨tz, K. H.; Fischer, H.; Hofmann, P.; Kreissl, F. R.; Schubert, U.; Weiss,
K. Transition Metal Carbene Complexes; Verlag Chemie: Deerfield Beach,
FL, 1983.
(26) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals;
Wiley-Interscience: New York, 1988; p 244.
(27) Bernasconi, C. F.; Ali, M. Unpublished observations.
(28) Gilman, H.; McNinch, H. A. J. Org. Chem. 1961, 26, 3723.
(29) Eigen, M. Angew. Chem., Int. Ed. Engl. 1964, 3, 1.
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J. AM. CHEM. SOC. VOL. 124, NO. 10, 2002 2303