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SYNTHETIC COMMUNICATIONSV
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with aryl ethers to obtain 3a–3m encouraged our further efforts toward the preparation
of other aromatic/heterocyclic aromatic sulfonic esters (results shown in Tables 3–10).
The reaction of benzene gave the desired product 3n in 44% yield. Next, the effects
of moderate electron donating groups (alkyl) on the aromatic ring such as methyl, ethyl,
isopropyl and tert-butyl were investigated. Alkyl derivatives of benzene such as toluene,
ethyl benzene, cumene and tert-butylbenzene under these conditions proceeded
smoothly to give the desired products (Table 3, 3o–3r) in good yields. In addition, the
dialkyl derivatives of benzene such as o, m and p-xylene’s were also transformed into
the desired products (Table 3, 3s–3u) in 42%, 50% and 45% yields, respectively.
Alkoxysulfonylation of biphenyl afforded 47% of mono (3v) and 8.0% of di-substi-
tuted (3w) sulfonic esters with DMS 3.0 equiv. and increasing the amount of DMS to 5
equiv. gave mono (3v) and diesters (3w) in 24% and 32% yields, respectively (Table 3).
Next, Phenol was examined in this system under the standard reaction conditions and
the desired sulfonic ester was obtained in 60% yield (Table 4, 3x) along with 6%
methoxy-sulfonic ester (Table 2, 3a). Halo and alkyl substituted phenols, such as o-
chloro, m-bromo and p-cresol all proceeded smoothly to afford the corresponding prod-
ucts (Table 4, 3y, 3ab and 3ae) in good yields. Whereas, m-fluorophenol under standard
conditions gave two isomers such as 2-fluoro-4-hydroxy benzenesulfonic acid methyl
ester and 4-fluoro-2-hydroxybenzenesulfonic acid methyl ester in 36% and 12% yields,
respectively (Table 4, 3z and 3aa). m-cresol also gave two isomers 3ac and 3ad in 33%
and 20% yields, respectively (Table 4).
We next studied the reaction with aromatic carboxylic acids such as benzoic acid
and o, p and m-toluic acids. Results show that, non-substituted benzoic acid failed to
produce sulfonic ester under the reaction conditions (Table 5, 3af), only carboxylic
ester was formed, which may be due to the electronic effects, whereas o and p-toluic
acids undergo carboxylic acid esterification and ring alkoxysulfonylation reactions
simultaneously (Table 5, 3ag and 3ah). In the case of m-toluic acid, the reaction
with DMS gave only methyl ester, tried at different temperatures (95 ꢁC, 120 ꢁC, 150ꢁ
and 175 ꢁC), no sulfonic ester formation was observed, (Table 5, 3ai). The reason for
this fact may be arisen from the steric effect and low activity of m-toluic acid. The
position of the substituted groups on the ring also showed an effect on the reaction.
When the two substituent’s on the ring are not the same, results become more com-
plex, in the above compound both are different directing groups (ortho and meta),
methyl is a week electron donor, whereas carboxylic group is strong withdrawing,
reduced electron density at o-and p positions to methyl group in m-toluic acid could
be influenced the reaction.
To examine whether the present protocol can useful to hydroxy-benzoic acids, we
took different phenolic acids (Table 5) and subjected them to the reaction conditions.
Interestingly, along with esterification of carboxylic group, sulfonic ester formation was
noticed for o, p and m-hydroxy benzoic acids (see Table 5). Reaction with p-hydroxy-
benzoic acid was studied at elevated temperatures to check the possibility of O-methyla-
tion of hydroxy group, along with carboxylic acid esterification and sulfonic ester
formation, methylation of the hydroxy group was noticed, however, a lower yield was
obtained (see Table 5, 3am).