J . Org. Chem. 2001, 66, 7527-7529
7527
Sch em e 1
Iod in e Ca ta lyzes Efficien t a n d
Ch em oselective Th ioa ceta liza tion of
Ca r bon yl F u n ction s,
Tr a n sth ioa ceta liza tion of O,O- a n d
S,O-Aceta ls a n d Acyla ls
Habib Firouzabadi,* Nasser Iranpoor,* and
Hassan Hazarkhani
Department of Chemistry, College of Sciences,
Shiraz University, Shiraz, 71454, Iran
Very recently we have introduced silica chloride as an
efficient catalyst for this purpose.8
firouzabadi@chem.susc.ac.ir
Received May 27, 2001
However, some of the reported methods require long
reaction times, stoichiometric use of expensive reagents,
and harsh reaction conditions and sometimes give poor
selectivity when applied to the mixture of aldehydes and
ketones.
In tr od u ction
A literature survey shows that iodine (I2) has been used
for transformation of aldehydes into acylals,9a alcoholysis,
hydrolysis and acetolysis of epoxides and thiiranes,9b
deprotection of thioacetals,9c and thioketalization of car-
bonyl compounds.9d In this paper, we report that I2
catalyzes transformation of aromatic and aliphatic alde-
hydes and ketones, O,O-acetals, O,O-ketals, O,S-acetals,
and acylals to their thioacetals in excellent yields at room
temperature (Scheme 1, Table 1). The reactions could be
performed in different organic solvents such as CHCl3,
CH2Cl2, benzene, and n-hexane efficiently. Due to the
more solubility of the substrates and higher rates of the
reactions, CHCl3 was chosen as the solvent. It should be
pointed out that in the absence of iodine the reactions
did not proceed in CHCl3 even after prolonged reaction
times.
Protections of carbonyl groups as thioacetals are quite
often a necessary requirement in the synthesis of mul-
tifunctional organic molecules.1 Thioacetals are quite
stable toward a wide variety of reagents2 and are also
useful in organic synthesis as acyl carbanion equivalents2
in C-C bond-forming reactions. Moreover, S,S-acetals
could be used as intermediates for the conversion of the
carbonyl function to the parent hydrocarbons.3 In general,
S,S-acetals are prepared by protic or Lewis acid catalyzed
condensation of carbonyl compounds with thiols and
dithiols.1,4 Dithioacetalization has also been reported in
a 5 M ethereal solution of LiClO4 (LPDE)5 and some other
supported reagents.6 Transthioacetalization of acetals is
a useful transformation for the preparation of S,S-
acetals7 and, in comparison with thioacetalization of
carbonyl compounds, is a faster and cleaner reaction.
As shown in Table 1, various types of aromatic alde-
hydes with electron-donating and electron-withdrawing
groups were cleanly and rapidly converted to the corre-
sponding dithianes and dithiolanes in the presence of 10
mol % of I2 (entries 1a-m). Aliphatic and R,â-unsaturated
aldehydes were also thioacetalized in good yields (entries
1n-q). However, aromatic and aliphatic ketones were
slowly converted to their corresponding S,S-acetals (en-
tries 1hh-ll). Transthioacetalization of O,O-acetals, O,O-
ketals, O,S-acetals and acylals was also achieved effi-
ciently with a catalytic amount of I2 to afford the
corresponding S,S-acetals in high yields (entries 1r-bb,-
ff,gg). Diacetals of 2,2-bis(hydroxymethyl)-1,3-propanediol
were also efficiently converted to their dithianes and
dithiolanes (entries 1cc-ee). Side product formation was
not observed in the reactions we have studied.We have
also used supported iodine on polyvinylpyridine (PVP)
for this purpose.9b Our results showed that the supported
iodine works also efficiently as a catalyst for the above
functional group interconversions. High chemoselectivity
of the method is demonstrated by competetive reactions
between an aldehyde and a ketone, a ketal and a ketone,
* To whom correspondence should be addressed. Fax: +98(0711)-
2280926.
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(10) Some spectral data of dithianes and dithiolones.
10.1021/jo015798z CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/11/2001