68
M. R. CRAMPTON ET AL.
K1k2 þ K1kAn½Anꢀ
ever, major steric interactions between the ortho substi-
kA ¼
ð5Þ
kAn½Anꢀ
k2
tuents in the two rings are avoided by twisting of the
planes of the two aromatic rings by an angle which
increases from 66 ꢂ in 3a to 76 ꢂ in 4f.
Although it is not, of course, possible to obtain x-ray
structures, it is expected that steric crowding around C(1)
will be reduced in the zwitterionic intermediates 5, as the
hybridization changes from sp2 to sp3.
1 þ
þ
k
k
ꢁ1
ꢁ1
The values in Table 2 correspond to Eqn (5) with
the values K1kAn ¼ 2.2 dm6 molꢁ2 sꢁ1 and kAn/k
¼
ꢁ1
value for k1 of
2.75 dm3 molꢁ1
,
leading to
a
0.8 dm3 molꢁ1 sꢁ1, and with K1k2 ¼ 0.08 dm3 molꢁ1 sꢁ1
and k2/k ¼ 0.1. Calculated values for kAn are given in
ꢁ1
Table 2. DABCO catalysis was observed.
For 4f, the 2,6-dinitro derivative, the values of kA were
independent of aniline concentration, indicating that
Comparisons
either the condition k2 þ kAn[An] >> k applies so that
ꢁ1
In general, the values of K1kAn, in Table 3, increase
regularly as the X substituents become more electron-
withdrawing. Exceptions are 3f, the 2,6-dimethyl-substi-
tuted derivative, where the value is ꢄ100 times smaller
than for 3e, its 2,4-disubstituted isomer, and 4f, the 2,6-
dinitro-substituted compound, where base catalysis is not
observed. Similarly, values of k1 increase slightly, by a
factor of ꢄ6, from 3e to 4e. Significantly, the minimum
possible value for k1 for 4f is higher than for 4e, its 2,4-
disubstituted isomer. Also, the value of k1, in Table 3, for
3f is a minimum so that the value may be no lower than
for 3e. These results show that there can be little steric
hindrance to attack by aniline at the 1-position of the
parent molecules, even in the presence of di-ortho sub-
stitution. In fact, the crystal structure in Fig. 1(c) indi-
cates that a relatively unhindered trajectory exists for
approach of the aniline molecule. Similarly, Nudelman
and co-workers16,17 have noted the absence of a primary
steric effect in their work on the reactions of amines
with 4- and 6-substituted-2-nitroanisoles. The small in-
creases in k1 observed in the present work are consis-
tent with increasing electron withdrawal by the X
substituents at the reaction centre and, as argued pre-
viously,15 with an ‘early’ transition state for nucleophilic
attack.
kA ¼ k1, and/or that the condition k2 >> kAn[An] applies
so that kA ¼ k1k2/(kꢁ1 þ k2). Hence the minimum value
that can be assigned to k1 is 1.5 dm3 molꢁ1 sꢁ1
.
Before discussing these results, it is useful to consider
the x-ray crystal structures which have been determined
for 3a, 3f and 4f.
X-ray crystal structures
Selected bond lengths and angles for 3a, 3f and 4f are
given in Table 4 and perspectives of 3f and 4f are shown
in Fig. 1. In all of the three structures there is evidence of
steric crowding around the C(1) position. The shortened
O(1)—C(1) bond lengths indicate p–ꢀ conjugation with
the strongly electron-withdrawing trinitro-substituted
ring. There is also evidence of some conjugation with
the dinitro-substituted ring of 4f. The C(1)—O(1)—C(7)
bond angle is close to 120 ꢂ except for 4f, where it opens
to 130 ꢂ. The steric interaction of the two rings is
accommodated partly by rotation, by around 45 ꢂ, of the
o-nitro groups in the trinitro-substituted ring. There are
also small deviations of O(1) from the ring plane. How-
Base catalysis will involve rate-limiting proton transfer
from the zwitterions, 5, to amine.14,15 In previous work, it
has been shown14,15,18,19 that in reactions involving
trinitro-activated compounds, this proton-transfer process
is thermodynamically favoured. In the present systems,
values of kAn/kDABCO are 0.4 ꢃ 0.1, showing that despite
the differences in basicity (Coetzee and Padmanabhan20
give pKa values of DABCO-Hþ 18.29 and aniline-Hþ
10.56), the abilities of aniline and DABCO to catalyse the
reaction are similar. Hence steric effects are dominant in
determining the values of the rate constant kAn; for the
range of compounds 3e to 4e, the results in Table 3
provide no evidence for increases in steric hindrance to
proton transfer with increasing substitution. Thus, the
increases in K1kAn are reasonably attributed to increases
in K1 as the electron withdrawal at the 1-position in-
Table 4. Selected bond lengths and angles
3a
3f
4f
˚
˚
O(1)—C(1) (A)
1.350
1.406
120.3
45
46
14
1.343
1.423
119.4
53
58
10
1.359
1.373
130.4
45
36
2
12
O(1)—C(7) (A)
C(1)—O(1)—C(7) ( ꢂ)
2-Nitro (twist) ( ꢂ)
6-Nitro (twist) ( ꢂ)
4-Nitro (twist) ( ꢂ)
Deviation of O(1) from
trinitro ring plane ( ꢂ)
Angle between aromatic
rings ( ꢂ)
creases. Similarly, increases in kAn/k will reflect de-
7
6
ꢁ1
creases in kꢁ1. The low value observed for K1kAn for 3f
and the absence of base catalysis in 4f may then be
attributed to decreases in kAn. Thus 2,6-disubstitution
66
74
76
Copyright # 2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2004; 17: 65–70