To study substituent effects on the reactivity of the
phenol, the reactions were performed on a variety of
phenols. The reactions worked well and the results are
illustrated in Table 1. For most of the substrates, the
reaction time is reduced drastically even at ambient
conditions in contrast to reported procedures,7,8 with
an excellent yield of coumarins. Substrates (entries 1–6)
having electron-donating groups in para to the site of
electrophilic substitution gave maximum yields at room
temperature in the minimum time. 3-Methoxyphenol
(entry 2) showed no detectable demethylation under the
given conditions. However, control experiments20 by
conventional method using AlCl3 in nitrobenzene
showed significant demethylation at high temperature
which is necessary to drive the reaction to completion.
1-Naphthol (entry 7) requires a slightly higher temperature
and longer reaction time, due to the presence of
another phenyl ring. Similarly, phenol (entry 8)
required a higher reaction temperature and longer reaction
duration, as no electron-donating group is present.
A literature survey revealed that, resacetophenone
(entry 9) failed to react to give a coumarin derivative in
the presence of sulfuric acid as the catalyst. However,
the reaction was observed under aluminium chloride
catalysis,10 but required a temperature of 130°C in
nitrobenzene as the solvent. In contrast, the ionic liquid
furnished comparable yields of the product at 60°C.
Furthermore, the method succeeds in exploiting this
novel system as a Lewis acid catalyst and solvent which
is needed in the reactions in only stoichiometric
amounts.