206
LETTER
Yttria-Zirconia Based Lewis Acid: An Efficient and Chemoselective Catalyst
for Acylation Reactions
Pradeep Kumar,a* Rajesh Kumar Pandey,a Mandar S. Bodas,a Mohan K. Dongareb
aDivision of Organic Chemistry: Technology, bCatalysis Division, National Chemical Laboratory, Pune-411008, India
Fax 0091-20-5893614; E-mail: tripathi@dalton.ncl.res.in
Received 25 November 2000
alcohol, amine, or thiol (10 mmol) in dry acetonitrile
Abstract: Yttria-zirconia based strong Lewis acid efficiently
(5ml) containing catalyst (20% by weight), and the mix-
catalyzes acylation of alcohols, amines and thiols under environ-
ture was refluxed for the indicated length of time (Table
mentally safe, heterogeneous reaction conditions with high selectiv-
1–3). The reaction was monitored by TLC. After comple-
tion of reaction, the catalyst was filtered and the filtrate
was concentrated, diluted with water (15 ml) and extract-
ed with CH2Cl2 (2 20 ml). The organic layer was sepa-
rated, washed with 10% aq. NaHCO3, brine, water and
dried over Na2SO4. The solvent was removed and the
crude product was chromatographed on a silica gel col-
umn to afford the pure product. After the reaction, the cat-
alyst is recovered with retention of its catalytic activity. It
can be further reactivated for reuse by heating it at 500 °C
in the presence of air. The substrates examined in our
studies and the results obtained are summarized in Table
1–3.
ity and in excellent yields.
Key words: yttria-zirconia based Lewis acid, heterogeneous
catalysis, acylation reaction, chemoselectivity
The acylation of alcohols, amines, and thiols by acyl chlo-
ride or acid anhydride under basic conditions is a well es-
tablished reaction in organic synthesis.1 The most
commonly employed basic catalysts for this purpose are
4-(dimethylamino)pyridine and 4-pyrrolidinopyridine
(PPY).2 The Lewis acid catalyzed acylation of alcohols
and amines with acid anhydride is a mild, strategic alter-
native to basic and nucleophilic catalysts. Some proce-
dures have been developed wherein Lewis acid catalysts
Thus, the present procedure for acylation is quite general
as a wide range of structurally varied alcohols such as
open chain, cyclic and aromatic ones underwent acylation
with acid anhydrides. However, the reaction of alcohols
with benzoic anhydride is found to be sluggish and hence
a little longer time is required to complete the reaction af-
fording relatively low yield of the products (Table 1, en-
tries 2,7). The efficacy of the yttria-zirconia based catalyst
can be clearly visualized in the acetylation of polyhydroxy
compounds under similar conditions. For example, both
aliphatic and aromatic polyols were acetylated in very
high yield (Table 1, entries 9–12). Another noteworthy
feature of this methodology is that polyol such as D-man-
nitol underwent exhaustive acetylation smoothly demon-
strating the practical utility of this method (Table 1, entry
12).
6
such as Cu(OTf)2,3 TaCl5,4 TMSOTf,5 Sc(OTf)3,
In(OTf)3,7 CoCl2,8 Bu3P9 have been used for acyl transfer
reactions in alcohols. More recently, the use of montmo-
rillonite K-10 and KSF10 as heterogeneous catalysts has
been reported to effect acylation reactions. As part of a re-
search program aimed at developing a new solid catalyst
and its subsequent application for various organic trans-
formations, the yttria-zirconia based Lewis acid was
found to be an extremely efficient catalyst for the Diels-
Alder reaction11 and transesterification of -keto esters.12
This prompted us to use this catalyst for acylation reac-
tions, and here we report that an yttria-zirconia based
Lewis acid serves as an excellent catalyst for the acylation
of alcohols, thiols and amines (Scheme).
In order to explore the generality and scope of the yttria-
zirconia based Lewis acid catalyzed reaction, the proce-
dure has been extended to a variety of other substrates
such as amines and thiols. Thus, the aromatic and aliphat-
ic amines were successfully acylated in the presence of a
yttria-zirconia based Lewis acid catalyst (Table 2).
Another notable feature of the reaction is that even a hin-
dered amine was acylated in very high yield (Table 2, en-
try 2). In a similar manner acylation of thiols was also
achieved by this procedure in excellent yields (Table 2,
entries 6-8). Encouraged by this finding, it was felt worth-
while to study the reactivity pattern of different kinds of
amino alcohols, mercapto alcohols and diols for acylation
reaction over yttria-zirconia Lewis acid catalyst. The re-
sults of this study are presented in Table 3.
Scheme
Thus, when a variety of alcohols were treated with acid
anhydrides in the presence of a catalytic amount of the
new yttria-zirconia based catalyst, the corresponding ace-
tates were obtained in excellent yields. In a typical exper-
imental procedure, acid anhydride (11 mmol; 1.1 equiv.
per OH, NH2 or SH) was added dropwise to a solution of
Synlett 2001, No. 2, 206–209 ISSN 0936-5214 © Thieme Stuttgart · New York