Mulder et al.
TABLE 4. Desulfurization (in %) after 48 h of Mixtures of
-YC6H4SH in 9,10-Dihydroanthracene at 523 Ka
low yield. (ii) Sulfur is extruded from 4-N(CH3)2C6H4SH,
although tautomerization cannot occur.
4
4
-CH3OC6H4SH
3-CF3C6H4SH
4-NH2C6H4SH
Surprisingly, desulfurization does not require an external
source of hydrogen (atoms) because the reaction decreased in
efficiency along the solvent series: anthracene, hexadecane, 9,-
10-dihydroanthracene (see Table 2). Since the loss of sulfur from
b
c
mix 1
mix 2
0.92
0.13
nd
nd
32.8
a
Results are the average of two runs using toluene as an external standard
4
-NH2C6H4SH follows first-order kinetics, the desulfurization
of this compound could, most simply, be represented by reaction
. However, a unimolecular elimination of atomic sulfur cannot
for the GC analyses; nd ) not detected (less than 0.01%). Desulfurization
b
defined as 100 × [C6H5Y]t)τ/{[YC6H4SH]t)τ + [C6H5Y]t)τ}. Mix 1:
4
5
3
5
-CH3OC6H4SH (146 µmol) and 3-CF3C6H4SH (29 µmol) dissolved in ca.
9
c
00 mg of 9,10-dihydroanthracene. Mix 2: 4-CH3OC6H4SH (149 µmol),
-CF3C6H4SH (30 µmol), and 4-NH2C6H4SH (162 µmol) dissolved in ca.
4
-NH C H SH f C H NH + S (9)
2
6
4
6
5
2
00 mg of 9,10-dihydroanthracene.
be the mechanism of desulfurization because reaction 9 has an
-
1 8,23
enthalpy, ∆H9, of about 60 kcal mol .
If the activation
demethylation can be represented by eqs 7 and 8. If, as would
be expected, the rate constants for methylation of the two
-1
enthalpy is assumed to have its minimum value of 60 kcal mol
and A9 is assumed to have the very high value of 10 s , then
at 523 K, a value that is 5 orders of
magnitude smaller than the experimental rate constant for this
15 -1
-
10 -1
k9 will be 10
s
4
-N(CH ) C H SH + 4-N(CH ) C H SH f
3
2
6
4
3 2
6
4
4
-N(CH )(H)C H SH + 4-N(CH ) C H SCH (7)
-6 -1
3
6
4
3 2
6
4
3
desulfurization at this temperature (i.e., k3 ) 7.0 × 10 s ;
see Scheme 1). To fit the experimental rate constant using the
4
-N(CH ) C H SH + 4-N(CH )(H)C H SH f
3
2
6
4
3
6
4
same A factor would require an activation enthalpy of about 48
kcal mol , which is at least 12 kcal mol lower than the
minimum value of E9.
-
1
-1
4
-N(CH )(H)C H SH + 4-N(CH )(H)C H SCH (8)
3
6
4
3
6
4
3
A similar problem exists for the thermal elimination of sulfur
from thiirane, reaction 10.
thiols are approximately equal (i.e., k7 ≈ k8), the ratio
4-N(CH3)(H)C6H4SCH3]/[4-N(CH3)2C6H4SCH3] should be simi-
[
lar to the product ratio {([C6H5N(CH3)(H)] + [4-N(CH3)(H)C6H4-
SH])/([C6H5N(CH3)2] + [4-N(CH3)2C6H4SH])}, and this is
indeed the case (0.25 versus 0.22).
Mixtures of 4-YC6H4SH. Because desulfurization might be
due to the formation of some specific reactive species from the
-1 8,20
That is, ∆H10 ) 59.1 kcal mol , but if the sulfur atom was
formed in its excited singlet state for reasons of spin conserva-
tion, the required enthalpy would be even higher. However, the
measured activation enthalpy in the gas phase is 40-42 kcal
mol , ruling out desulfurization by a simple unimolecular
extrusion of atomic sulfur, despite the fact that the reaction is
Attempts to explain this anomaly
have been inconsistent with experimental data. For example,
Benson suggested a complex chain reaction initiated by
4
-aminothiophenols that would desulfurize other thiophenols,
two experiments were conducted using mixtures of thiophenols
in 9,10-dihydroanthracene at 523 K. The results are compiled
in Table 4.
In both mixtures, there was no desulfurization of 3-CF3C6H4-
SH (just as there was no desulfurization of 4-CNC6H4SH and
C6H5SH used alone under these conditions). The conversion of
-1 24
24-27
first order in thiirane.
20
4
-CH3OC6H4SH to C6H4OCH3 was very slow, but it could be
reaction 10, the atomic sulfur produced then attacks the thiirane
to form S2 which is the chain carrier and reacts with thiirane in
the rate-determining step (rds), eq 11. The S2 is
detected in both mixtures. The desulfurization of 4-NH2C6H4-
SH in the presence of 4-CH3OC6H4SH and 3-CF3C6H4SH
(32.8%) occurred to the same extent as in the absence of these
C H S
two thiophenols (33%, see Table 2). From Table 4, it can be
estimated that the rate of desulfurization of 4-NH2C6H4SH is
at least 300 times greater than the rate for 4-CH3OC6H4SH,
while the rate for 3-CF3C6H4SH must be at least 3 orders of
magnitude smaller.
2 4
S + C H S f C H + S
8 C H + S f f
2 4 3
2
4
2
4
2
rds
S H 4 S (11)
8
2
in equilibrium with S8. Such a mechanism can be ruled out on
several grounds, most tellingly by the fact that Gunning and
Discussion
(23) The heat of formation, ∆Hf, of 4-NH2C6H4SH can be calculated to
-
1
be 26.95 kcal mol using the isodesmic reaction: C6H5NH2 + C6H5SH
-
1 12
There is a remarkable difference in the thermal stability of
ring-substituted thiophenols, YC6H4SH. For Y ) 4-H, 4-CN,
and 3-CF3, the thiophenols are inert under the standard condi-
tions of 523 K in 9,10-dihydroanthracene and a reaction time
of 48 h. In contrast, for Y ) 4-NH2, 4-N(CH3)(H), and
f 4-NH2C6H4SH + C6H6 (computed reaction enthalpy, -0.91 kcal mol
)
in conjunction with auxiliary thermodynamic data, cf. ∆Hf(C6H5SH) ) 26.9
kcal mol . With a 4-NH2C6H4S-H BDE of 76.5 kcal mol , the ∆Hf-
-
1 8
-1
•
-1
(
4-NH2C6H4S ) ) 51.4 kcal mol .
(24) There have been three reports on the thermolysis of thiirane in the
All are agreed that this reaction follows first-order kinetics,
and the following Arrhenius parameters have been reported: log A/s
gas phase.2
6-28
-
1
,
4-N(CH3)2, desulfurization to the responding anilines takes place
-1
25
26
27
Ea/kcal mol ; 13.2, 40.2; 15.8, 42.4; and 13.2, 40.9.
at appreciable and identical rates. Desulfurization also occurs
with 4-CH3OC6H4SH, albeit at a rate which is at least 300 times
slower than the rate for 4-NH2C6H4SH. The possibility that the
desulfurization of 4-NH2C6H4SH follows a tautomerization/RRD
mechanism (Scheme 2B) can be excluded for two simple
reasons: (i) The thermolysis of 4-NH2C6H4SCH2C6H5 (1)
actually does yield the product of a tautomerization/RRD
mechanism, benzyl sulfide, 8 (Scheme 3), but only in a very
(25) Lown, E. M.; Sandhu, H. S.; Gunning, H. E.; Strausz, O. P. J. Am.
Chem. Soc. 1968, 90, 7164-7165. The reported activation entropy of -2.5
-
5
-1
eu is in error. Recalculation using the rate constant of 3.70 × 10
s
at
4
98 K and Ea ) 40.2 leads to a reaction entropy of -1.1 eu. In subsequent
26-28
studies
on the thermolysis of thiirane, the erroneous value (-2.5 eu)
has been quoted without comment.
26) Amano, A.; Yamada, M.; Mizuuchi, K.; Kamo, T. Kenkyu
Hokoku: Asahi Garasu Kogyo Gijutsu Shoreikai 1982, 41, 151-157.
27) Yamada, M.; Kamo, T.; Tang, J.; Amano, A. Nippon Kagaku Kaishi
1987, 60, 1377-1384.
(
(
2384 J. Org. Chem., Vol. 72, No. 7, 2007