Reaction of Atomic Carbon with Isomeric Cresols
Letters in Organic Chemistry, 2009, Vol. 6, No. 8
651
Table 1. Relative Product Ratios of Carbon Atom Reactions
of 5a-c
filtered and the solvent was evaporated by rotary
evaporation. The resulting product mixture was analyzed by
GC/MS. For the purification of the products, thin layer
chromatographic methods were used using aluminum backed
silica gel 60 F254 plates, silica gel precoated 60 F254 TLC
glass plates and 3:2 ethyl acetate/methylene chloride solvent
mixture as solvents. The final mixtures were analyzed by
GC/MS and 13C, 1H NMR spectrometers.
Reactants
Products
Methyltropone*
xylenols
5a
5b
5c
1 (6a)
1 (6b+6c)
1 (6c)
1.05 (7a+7b+7c+7d)
0.67 (7a+7e+7f)
1.73 (7b+7e)
3. RESULTS AND DISCUSSION
*For simplicity, ratios are given relative to the total amount of corresponding
methyltropones that are formed in each case.
After the reaction of arc generated carbon atoms with 5a-
c
at
77K
individually,
methyltropones
(6a-c),
supporting material). An insertion into ortho and meta C-H
bonds of 5c generates cresolcarbenes 8 and 9 which produces
3-methytropone (20) after a series of rearrangements.
Formation of 20 is also plausible via addition of C atom to
double bonds of 5c followed by ring expansion processes.
The singlet carbenes 8 and 9 can undergo intersystem
crossing (ISC) to their corresponding triplet states which can
then abstract hydrogen from unreacted p-cresol to give the
xylenols 7b and 7e respectively.
hydroxyxylenes (7a-f) and side products were obtained (Fig.
(1)) and the product ratios are shown in Table 1. Since the
carbon arc operates at extremely high temperatures (>2000
ꢀC) pyrolysis and/or photolysis of both substrate and
products is a severe problem in the reactions of arc generated
carbon atoms in the gas phase [4,11], we believe that side
products, which obviously do not belong to the well known
carbon atom reactions, are formed from pyrolysis and will
not be discussed here.
Gaspar and coworkers studied with toluene as the
substrate to react with nucleogenic 11C atoms [12]. As a
result of C=C ꢁ bond addition (DBA) and C-H bond
insertion on the ring finally led to styrene and
benzocyclobutene. C-H insertion on the methyl group
exclusively led to styrene. And similar results were obtained
in the reaction of arc-generated carbon atoms with ter-butyl
benzene [2]. Products from methyl C-H bond insertion and
from aromatic ring C-H bond insertion were observed. But,
in the case of cresols, a product from methyl C-H bond
insertion reaction was not observed. A preferential attack of
carbon atom on aromatic ring rather than on substituents was
observed. Since benzene and substituted benzenes are
postulated to react by C-H insertion [4], therefore we believe
that DBA reactions will not have a major effect on product
ratios.
O
OH
OH
OH
CH3
CH3
CH3
CH3
CH3
C
+
+
77 K
5a
6a
7a
7b
CH3
OH
OH
CH3
H3C
CH3
+
+
7c
7d
H3C
O
OH
O
OH
CH3
C
77 K
CH3
+
+
CH3
The product ratios shown in Table 1, demonstrate that the
reaction of C with 5a and 5b produces methyltropone with
smaller amounts of xylenols. However, reaction of 5c
produces mainly xylenol with smaller amount of
methyltropone. We know that methyltropones are formed
through a familiar phenyl carbene type rearrangement of
singlet carbenes and xylenols are formed after hydrogen
abstraction of triplet carbenes that are formed by the
intersystem crossing of singlet carbenes. We can expect that
any effect that stabilizes the singlet carbenes should give a
raise to the singlet carbene products over triplet products.
Geise et al. [10] found in their computational study that
singlet phenyl carbenes can be stabilized more when they are
substituted at ortho and para position (especially on para
position if the substituent is -OH) by an electron donating
group. Thus, the most stable carbenes should be taken into
account in determining product ratios. In case of 5a, two
carbenes can form after C-H insertion of atomic carbon: one
is at ortho and the other is at para position with respect to
hydroxyl group. In 5b we have three possibilities: two are at
ortho and one is at para position and finally in 5c even there
are two ortho positions available for attack, para position is
occupied by a methyl substituent. So we shall expect that
CH3
CH3
7a
5b
6c
6b
OH
OH
7c
+
+
+
CH3
H3C
CH3
7e
7f
CH3
OH
C
6c
+
7b
+
7e
77 K
CH3
5c
Fig. (1). Observed products after the reaction of C + 5a-c at 77K.
In Fig. (2), we have outlined the most reasonable routes
to products observed in the case of 5c (The possible
pathways to the products for 5a and 5b and spectra
corresponding to methyltropones are all given in the