5154
J. R. Hwu et al. / Tetrahedron Letters 45 (2004) 5151–5154
The second stepwould be
ÅCl + RR0CHOH fi
References and notes
HCl + RR0CÅOH.10;11 To provide evidence in support of
our assumption, we added phenyl N-tert-butylnit-
rone12–14 and 2,20-azobis(isobutyronitrile),15 individu-
ally, as a radical quencher in 2.0 equivalents. The
photolytic esterification of phenylacetic acid with
methanol was retarded completely. Therefore we believe
that the reaction mechanism involves radical interme-
diates.
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Furthermore, we found that photolytic esterification of
phenylacetic acid did not proceed with tert-butanol or
phenol. Thus one can envisage two legitimate reasons.
First, tert-butanol and phenol lack an a-H to be
2. For recent works, see: (a) Hao, X.; Yoshida, A.; Nishi-
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Å
abstracted by Cl. As a consequence, the catalyst HCl
could be not generated. Second, the steric effect
imparted by a tertiary butyl group may slow down the
esterification with tert-butanol.
We bore out the feasibility in the conversion of phenyl-
acetic acid to the corresponding methyl ester with HCl
in different forms, including HCl , HCl
HCl
, and
. Furthermore, performance of the
ðgÞ
ðfumingÞ
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ðaqueous; 0:50–2:0 NÞ
photolytic transformation in the media containing
NaHCO3ðsÞ did not produce any ester. These data cor-
roborate an acid-catalyzed mechanism for the photolytic
esterification. An intriguing point is that the acid was
generated photochemically from CCl4 and alcohols in
situ under controlled conditions.
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In conclusion, esterification of carboxylic acids with
various alcohols in carbon tetrachloride can be accom-
plished efficiently by exposure of the solution to UV
light. The acids include alkyl, aromatic, and amino
acids. The photolytic esterification showed selectivity
between primary and secondary alcohols, yet no reac-
tion occurred to a tertiary alcohol. Performance of these
esterifications involves simple manipulation; mineral or
Lewis acids/bases are not required as the external cata-
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For financial support, we thank Ministry of Education
and Academia Sinica of Republic of China.