Tetrabutylammonium tribromide (TBATB) as an efficient generator of HBr for an efficient chemoselective reagent for acetalization of carbonyl compounds

Article abstract of DOI:10.1021/jo025701o

Acyclic and cyclic acetals of various carbonyl compounds were obtained in excellent yields under a mild reaction condition in the presence of trialkyl orthoformate and a catalytic amount of tetrabutylammonium tribromide (TBATB) in absolute alcohol. Chemoselective acetalization of an aldehyde in the presence of ketone, unsymmetrical acetal formation, shorter reaction times, mild reaction conditions, the stability of acid-sensitive protecting groups, high efficiencies, facile isolation of the desired products, and the catalytic nature of the reagent make the present methodology a practical alternative.

Full text of DOI:10.1021/jo025701o

TABLE 1. Aceta liza tion a of Ca r bon yl Com p ou n d s
(R1COR2)
Tetr a bu tyla m m on iu m Tr ibr om id e (TBATB)
a s An Efficien t Gen er a tor of HBr for a n
Efficien t Ch em oselective Rea gen t for
Aceta liza tion of Ca r bon yl Com p ou n d s†
time
(h)
b,c
b,c
entry
R1
R2
X1
X2
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Ph
H
H
H
H
H
H
H
H
H
H
H
H
0.16
0.33
2.50
0.50
0.16
24
2.0
2.0
0.50
3.5
0.16
0.33
0.33
0.33
0.41
2.00
97
80
95
95
94
00
62
25
60
85
96
90d
97
99
95
98
95
80
90
92
95
00
70
35
60
60
92
93d
89
90
87
89
p-(OMe)C6H4
o-(NO2)C6H4
p-(NO2)C6H4
p-(Cl)C6H4
o-(OH)C6H4
m-(OH)C6H4
p-(OH)C6H4
4-(OH)-3-(OMe)-C6H3
2-(Cl)-6-(NO2)-C6H3
furyl
Rangam Gopinath, Sk. J iaul Haque, and
Bhisma K. Patel*
Department of Chemistry, Indian Institute of Technology,
Guwahati-781 039, India
bkpatel@postmark.net
Received March 11, 2002
Abstr a ct: Acyclic and cyclic acetals of various carbonyl
compounds were obtained in excellent yields under a mild
reaction condition in the presence of trialkyl orthoformate
and a catalytic amount of tetrabutylammonium tribromide
(TBATB) in absolute alcohol. Chemoselective acetalization
of an aldehyde in the presence of ketone, unsymmetrical
acetal formation, shorter reaction times, mild reaction
conditions, the stability of acid-sensitive protecting groups,
high efficiencies, facile isolation of the desired products, and
the catalytic nature of the reagent make the present
methodology a practical alternative.
PhCHdCH
Ph
CH3
cyclohexanone
R-tetralone
2-cyclopentanone-methyl-
carboxylate
Ph
17
Ph
24
00
08
a
Reactions were monitored by TLC/GC. X1 ) dimethyl acetals;
X2 ) diethyl acetals. Confirmed by comparison with IR and 1H
b
NMR of the authentic sample. c Isolated yields. Trialkyl ortho-
d
formate (2.2 equiv) and 0.02 equiv of TBATB were used.
bromide (TBATB) has been used as a brominating
agent,3a-d for the cleavage of tert-butyldimethylsilyl
ethers4 and dithioacetals,5 and the pyranylation-depyra-
nylation of alcohols.6 The identification of tetrabutylam-
monium tribromide as a catalytic, mild, and chemose-
lective reagent for the acetalization of carbonyl compound
is the basis of this investigation.
During a multistep synthesis, a carbonyl group may
have to be protected against an attack by various
reagents such as nucleophiles, oxidants, basic, catalytic,
or hydride reducing agents, including organometallic
reagents.1a-d Acetals are generally formed under acidic
conditions, and water formed during a reaction is re-
moved either by physical or chemical methods.1c Ortho-
esters such as triethyl orthoformate are used as one of
the chemical methods for the removal of water, which
reacts with the water formed by the reaction of aldehyde
and ketone with an alcohol to form ethanol and ethyl-
formate, resulting in the equilibrium shifting to the
right.1c Methods are available for the conversion of
carbonyl groups in aldehydes and ketones to their cor-
responding acetals using trialkyl orthoformates in the
presence of acid catalysts such as HCl,2a FeCl3,2b Am-
berlyst-15,2c ZrCl4,2d DDQ,2e NBS,2f,g and Sc(NTf2)3.2h
Unfortunately, many of these procedures often require
a large excess of reagents, longer reaction times, drastic
reaction conditions, and moisture-sensitive and expensive
reagents. Also, some of these reagents do not always
prove to be satisfactory for the acetalization of cyclic and
aromatic ketones. Previously, tetrabutylammonium tri-
In this note, we report a mild, efficient, and environ-
mentally benign method for the acetalization of carbonyl
compounds using tetrabutylammonium tribromide (0.01
equiv) as a promoter in the presence of triethyl ortho-
formate (1.1 equiv) in absolute alcohol at room temper-
ature. Acetals of corresponding carbonyl compounds can
also be obtained using trimethyl orthoformate instead of
triethyl orthoformate. Open chain acetals have frequently
been subjected to special attention, adding to their
liability as compared with cyclic O,O-acetals.1 Under the
experimental conditions, various carbonyl compounds can
be acetalized to the corresponding O,O-acetals in excel-
lent yields, and the result is summarized in Table 1. HBr
generated in-situ from the reaction of TBATB with
alcohol,4,7 as shown in Scheme 1, may catalyze the
reaction. The solvent also plays a very important role in
this reaction. When benzaldehyde (1 equiv) was reacted
with triethyl orthoformate (1.1 equiv) and TBATB (0.01
† Dedicated to professor S. Ranganathan on the occasion of his 68th
birthday.
(1) (a) Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic
Synthesis, 3rd ed.; Wiley & Sons: New York, 1999. (b) Kocien˜ski, P.
J . Protecting Groups; Georg Thieme Verlag: New York, 1994. (c)
Meskens, F. A. J . Synthesis 1981, 501-522. (d) Schelhaas, M.;
Waldmann, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 2056-2083.
(2) (a) Fife, T. H.; J ao, L. K. J . Org. Chem. 1965, 30, 1492-1495.
(b) Bornstein, J .; Bedell, S. F.; Drummond, P. E.; Kosloski, C. L. J .
Am. Chem. Soc. 1956, 78, 83-86. (c) Patwardhan, S. A.; Dev, S.
Synthesis 1974, 348-349. (d) Firouzabadi, H.; Iranpoor, N.; Karimi,
B. Synlett 1999, 321-323. (e) Karimi, B.; Ashtiani, A. M. Chem. Lett.
1999, 1199-1200. (f) Karimi, B.; Seradj, H.; Ebrahimian, G. R. Synlett
1999, 1456-1458. (g) Karimi, B.; Ebrahimian, G. R.; Seradj, H. Org.
Lett. 1999, 1, 1737-1739. (h) Ishihara, K.; Karumi, Y.; Kubota, M.;
Yamamoto, H. Synlett 1996, 839-841.
(3) (a) Chaudhuri, M. K.; Khan, A. T.; Patel, B. K.; Dey, D.;
Kharmawophlang, W.; Lakshmiprabha, T. R.; Mandal, G. C. Tetrahe-
dron Lett. 1998, 39, 8163-8166. (b) Bora, U.; Bose, G.; Chaudhuri, M.
K.; Dhar, S. S.; Gopinath, R.; Khan, A. T.; Patel, B. K. Org. Lett. 2000,
2, 247-249. (c) Bose, G.; Bhujarbarua, P. M.; Chaudhuri, M. K.; Kalita,
D.; Khan, A. T. Chem. Lett. 2001, 290-291. (d) Bose, G.; Mondal, E.;
Khan, A. T.; Bordoloi, M. J . Tetrahedron Lett. 2001, 42, 8907-8909.
(4) Gopinath, R.; Patel, B. K. Org. Lett. 2000, 2, 4177-4180.
(5) Mondal, E.; Bose, G.; Khan, A. T. Synlett 2001, 785-786.
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7679-7681.
(7) Kajigaeshi, S.; Kakinami, T.; Hirakawa, T. Chemistry Lett. 1987,
627-630.
10.1021/jo025701o CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/27/2002
5842
J . Org. Chem. 2002, 67, 5842-5845

SCHEME 1. P r op osed Mech a n ism of Aceta liza tion
SCHEME 2. Electr on Den sity a t th e Ca r bon yl
Ca r bon
equiv) in CH2Cl2, instead of absolute alcohol, after 24 h,
only a small amount (<5%) of the corresponding diethyl
acetal was formed. This may be due to the fact that
alcohol accelerates the formation of a hemiacetal X, as
shown in Scheme 1, which is further facilitated by the
HBr generated in the medium. On the other hand, HBr
also protonates triethyl orthoformate to produce an
oxonium species Y, which in turn reacts with intermedi-
ate hemiacetal to give the desired acetal as shown in
Scheme 1. In a control experiment, treatment of the
reaction mixture with a catalytic amount of a saturated
HBr solution in absolute ethanol, instead of TBATB, led
to a quantitative conversion of benzaldehyde diethyl
acetal within 10 min when benzaldehyde was used as
the substrate.
By the present methodology, aromatic aldehyde 1 and
substituted aldehydes, with both electron-donating and
-withdrawing groups at the ortho and para positions such
as 2-5, produced the corresponding dialkyl acetals in
excellent yields. It may be mentioned here that the ortho-
substituted substrate reacts more slowly than the para-
substituted one, which could be due to steric factors. It
is interesting to note that the hydroxyl group substituted
at different positions in an aromatic ring greatly influ-
ences the reaction rates. For instance, the hydroxyl group
substituted at the meta and para positions, as in the case
of 7 and 8, in 2 h gave about 70 and 30% of the
corresponding dialkyl acetals, respectively. However,
when the same group is present at the ortho position (6),
no product could be detected even after 24 h, which could
be due to both steric and electronic factors. It is seen from
Table 1 that the electron-donating groups disfavor prod-
uct formation as demonstrated for 6 and 8, and the
electron-withdrawing group favors the formation of prod-
uct as shown for 3 and 4, respectively. In contrast, that
electron-withdrawing groups favor the formation of prod-
ucts is further supported with a sterically hindered
substrate containing electron-withdrawing groups (10).
Thus, under the reaction conditions, 2-chloro-6-nitrobenz-
aldehyde 10 gave dialkyl acetal in 60% yield. To further
support our explanation, the electron density at the
carbonyl carbon was calculated using semiempirical
molecular orbital calculations, the AM1 method as imple-
mented in the Hyperchem package (Hyperchem, Inc.;
Gainsville, FL), and the result is shown in Scheme 2. The
calculated electron density is in excellent agreement with
our explanation. Thus, due to both unfavorable steric and
electronic factors, o-hydroxybenzaldehyde 6 did not form
any trace of dialkyl acetal. For o-hydroxybenzaldehyde
6, due to a higher electron density around the carbonyl
carbon, it is less susceptible to nucleophilic attack by
alcohols. Moreover, a higher activation energy is required
due to steric crowding by the o-hydroxyl group. For the
substrate p-hydroxybenzaldehyde 8, where steric crowd-
ing is absent, due to a relatively higher electron density
around the carbonyl carbon, it is less susceptible to
nucleophilic attack by alcohols for the acetal formation.
However, due to a relatively favorable electronic factor
and the absence of steric crowding, m-hydroxybenzalde-
hyde 7 gave a better yield than ortho and para hydroxy-
benzaldehydes. Thus, for acetalization, electronic factors
predominate over the steric factors, which is clearly
demonstrated in the case of 2-chloro-6-nitrobenzaldehyde
10. This methodology can also be extended to the
heterocyclic aldehyde 2-furaldehyde 11 and unsaturated
aldehydes such as cinnamaldehyde 12.
The formations of dialkyl ketals from the corresponding
cyclic and aromatic ketones using conventional acid
catalysis were not generally satisfactory due to stereo-
electronic factors. It is well-known that aldehydes (RCHO)
are more reactive than ketones (RCOR) for two reasons.
First, ketones are more stable in the ground state than
aldehydes; this is primarily due to the appended alkyl
groups providing electron density through the sigma bond
framework to stabilize the carbonyl dipole. Second, the
transition state for an aldehyde addition reaction is lower
in energy than that of the ketone. This is due to steric
crowding of the transition state by the ketonic R groups.
Since the ketone starts at a lower energy in the ground
state, it has a lower reactivity than an aldehyde. How-
ever, by the present method, dialkyl ketals were also
obtained in high yields when acetophenone 13 and
cyclohexanone 14 were used as the models for acyclic and
cyclic saturated ketone. The efficacy of the methodology
was successfully applied to other ketones such as 15 and
16. However, the dialkyl ketalization of hindered ketones
such as benzophenone 17 was not successful, probably
due to the higher electron density at the carbonyl carbon
(Scheme 2), thereby making it less susceptible to attack
by hydroxyl nucleophiles for ketalization.
J . Org. Chem, Vol. 67, No. 16, 2002 5843

SCHEME 3. Tr a n sa ceta liza tion Rea ction
SCHEME 4. Aceta l Exch a n ge Rea ction w ith a n
Alcoh ol
Formation of the mixed acetal was not observed in the
above cases since the presence of absolute alcohol was
in excess. However, significant amounts of the mixed
acetal, ethyl-methyl acetal C, could be obtained when
alcohol was not used in excess. When benzaldehyde was
reacted with triethyl orthoformate (1.1 equiv) in absolute
MeOH (0.1 mL, 2.47 equiv), the ratio of dimethyl, ethyl-
methyl, and diethyl acetals formed was 70:23:1.3 after 1
h. When the same reaction was performed with trimethyl
orthoformate (1.1 equiv) in absolute ethanol (0.1 mL, 1.77
equiv), the product distribution after 1 h was the op-
posite: the ratio of dimethyl, ethyl-methyl, and diethyl
acetals formed was 4:32:60. Unsymmetrical acetals are
normally formed from the symmetric diacetals via
transacetalization.8a,b Treatment of an equimolar mixture
of dimethyl acetal A and diethyl acetal B of p-nitrobenz-
aldehyde with TBATB (0.01 equiv) in acetonitrile after
1 h gave 42% of the unsymmetrical acetal, p-nitroben-
zaldehyde ethyl-methyl acetal C, along with 19% A, 22%
B, and 15% p-nitrobenzaldehyde as shown in Scheme 3.
Similar acetal exchange has been observed with other
acid-catalyzed reactions.8a,b The formation of a small
amount of p-nitrobenzaldehyde could be due to the
generation of HBr by the reaction of TBATB with
acetonitrile containing a trace amount of water. Thus,
our method will be useful for the preparation of unsym-
metrical acetals if desired.
TABLE 2. Aceta liza tion a of Ca r bon yl Com p ou n d s
(R1COR2)
time
(h)
R1
R2
X3
X4
b,c
b,c
entry
1
2
4
6
8
10
11
12
13
15
16
Ph
H
H
H
H
H
H
H
H
0.08
0.08
0.5
1.5
0.08
92
75
93
p-(OMe)C6H4
p-(NO2)C6H4
o-(OH)C6H4
p-(OH)C6H4
2-(Cl)-6-(NO2)-C6H3
furyl
PhCHdCH
Ph
R-tetralone
2-cyclopentanone-methyl-
carboxylate
Ph
85
97
95
93
21
85
49
80
94
97
97
88d
93d
00
24
0.08
0.16
0.08
0.08
0.08
80
65
CH3
90d
94
97
17
18
Ph
H
24
0.08
00
92
64
93
p-(OTBDMS)-C6H4
a
Reactions were monitored by TLC/GC. X3 ) 1,3-dioxolanes;
X4 ) 1,3-dioxanes. Confirmed by comparison with IR and 1H
b
NMR of the authentic sample. c Isolated yields. Trialkyl ortho-
d
Alkoxy exchange also occurs between an acetal and an
alcohol through the formation of a mixed acetal. The
equilibrium can be shifted to the right by using a large
excess of alcohol. Thus, when dimethy acetal A (1 equiv)
was treated with ethanol (0.1 mL, 1.77 equiv) and TBATB
(0.01equiv), significant amounts of ethyl-methyl C and
diethyl acetals B were obtained as shown in Scheme 4.
In another experiment, when diethyl acetal B was
treated with absolute methanol (0.1 mL, 2.47 eqiuv) in
TBATB, dimethyl acetal A, along with a small amount
of mixed acetal C, was obtained as shown in Scheme 4.
Thus, mixed acetals and other acetals can be prepared
by this methodology using appropriate quantities of
alcohol.
Cyclic Aceta l F or m a tion . Cyclic acetals such as 1,3-
dioxolanes and 1,3-dioxanes are also important protecting
groups for carbonyl compounds. They are generally
prepared by the reaction of aldehydes and ketones with
1,2-ethanediol or 1,3-propanediol in the presence of acid
catalysts1 under homogeneous conditions. They are also
prepared under heterogeneous media using inorganic
solids such as [Zr(O3PCH3)1.2(O3PC6H4SO3H)0.8],9a sup-
formate (2.2 equiv) and 0.02 equiv of TBATB were used.
ported silica gel,9b Al2O3,9c clay,9d,e zeolites,9f,g and kaoline.9h
Cyclic acetals are generally more easily formed than open
chain acetals.1c Various aldehydes and ketones gave
corresponding 1,3-dioxolanes and 1,3-dioxanes in excel-
lent yields upon treatment with (EtO)3CH (1.1 equiv),
1,2-ethanediol, or 1,3-propanediol (4 equiv) and a cata-
lytic amount of TBATB (0.01 equiv) as shown in Table
2. Carbonyl compounds such as 1, 2, 4, 6, and 8 gave the
corresponding cyclic acetals in good to excellent yields.
It may be mentioned here that acetalization of electron-
rich aromatic aldehydes 6 and 8 are generally unsuc-
cessful by conventional methods.1,2g Nevertheless, this
acetal has been prepared using an expensive ruthenium
catalyst and longer reaction times.10
(9) (a) Curini, M.; Epifano, F.; Marcotullio, M. C.; Rosati, O. Synlett
2001, 1182-1184. (b) Yadav, J . S.; Reddy, B. V. S.; Srinivas, R.;
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dron Lett. 1997, 38, 7867-7870. (f) Tanaka, Y.; Sawamura, N.;
Iwamoto, M. Tetrahedron Lett. 1998, 39, 9457-9460. (g) Climent, M.
J .; Corma, A.; Velty, A.; Susarte, M. J . Catal. 2000, 196, 345-351. (h)
Ponde, D.; Borate, H. B.; Sudalai, A.; Ravindranathan, T.; Deshpande,
V. H. Tetrahedron Lett. 1996, 26, 4605-4608.
(8) (a) Moedritzer, K.; van Wazer, J . R. J . Org. Chem. 1965, 30,
3925-3926. (b) Fujiwara, J .; Fukutani, Y.; Hasegawa, M.; Maruoka,
K.; Yamamoto, H. J . Am. Chem. Soc. 1984, 106, 5004-5005.
5844 J . Org. Chem., Vol. 67, No. 16, 2002

SCHEME 5. Ch em oselective Aceta liza tion of
Ald eh yd es
SCHEME 6. Ch em oselective Aceta liza tion of
Ald eh yd es
In conclusion, we have shown that acetalization of
various carbonyl compounds can be achieved by this
methodology. Chemoselective acetalization of aldehydes
in the presence of ketones can be accomplished by this
method. Cyclic acetals of activated substrates can be
achieved in high yields. Acid-sensitive groups such as the
phenolic OTBDMS are stable under the reaction condi-
tions. This method is high yielding, safe, operationally
simple under mild reaction conditions, and cost effective.
The catalytic nature of this methodology makes it more
suitable for practical organic synthesis.
As shown in Table 2, electron-withdrawing groups
favored acetal formation as demonstrated with 4. Steri-
cally hindered aldehyde 10 did not yield any trace of 1,3-
dioxolanes but gave a 21% yield of 1,3-dioxanes. This
shows the preferential formation of 1,3-dioxanes over 1,3-
dioxolanes for the hindered aldehyde. Acid-sensitive
substrate 11 and the conjugated carbonyl compound 12
were converted to corresponding cyclic acetals in good
yields. Aromatic ketones also formed respective cyclic
ketals under these conditions. Substrates 13, 15, and 16
could be converted to the corresponding cyclic ketals in
satisfactory yields. Hindered ketone 17 gave very a poor
yield of 1,3-dioxolanes even after 24 h but gave a
moderate yield of 1,3-dioxanes. Thus, from Table 2 it is
evident that the unhindered ketone prefers 1,3-dioxolane,
while a hindered ketone and an aldehyde prefer 1,3-
dioxanes, as demonstrated for substrates 10 and 17. It
is also worthy to note that the aldehyde containing acid-
sensitive groups (18) such as phenolic OTBDMS gave an
excellent yield of cyclic acetals without affecting the
phenolic OTBDMS group.
Exp er im en ta l Section
See Supporting Information for details of the instrumentation
employed.
Aldehyde containing tert-butyldimethylsilyl ether 18 was
prepared by silylation of p-hydroxybenzaldehyde according to
the literature procedures.14 The following acetals derived from
parent aldehydes have been reported in the literature: dimethyl
11
acetals1c, 1, 2, 5, 12, and 14; diethylacetals2,12 1, 2, 4, 5, 12-
14, and 17; 1,3-dioxolanes 1,13e 2,2g 4,9a 6,10 8,10 11,13d 12,13e and
13;13e and 1,3-dioxanes 1,2g,13a 4,9a 6,10 8,10 12,2g and 13.2g,13a
Ack n ow led gm en t. R.G. acknowledges the financial
support of the institute, and B.K.P. acknowledges the
support of this research from DST New Delhi and CSIR
01(1688)/00/EMR-II. We thank Professor M. K. Chaudhu-
ri for his encouragement. Thanks are due to RSIC,
Lucknow, and IACS, Kolkata, for providing NMR spec-
tra. We are thankful to all three of the referees for their
invaluable comments and suggestions.
It is well-known that aldehydes react faster than the
ketones, which is also the case with our methodology.
When an equimolar mixture of benzaldehyde 1 and
acetophenone 13 was allowed to react with 1,2-ethanediol,
benzaldehyde was chemoselectively acetalyzed over ac-
etophenone. A greater degree of selectivity was found
when 1,3-propanediol was used as shown in Scheme 5.
Thus, this methodology will be useful for chemoselective
acetalization of aldehydes in the presence of ketones.
Su p p or tin g In for m a tion Ava ila ble: General procedures
for preparation of acyclic and cyclic acetals and chemoselective
acetalization of aldehydes in the presence of ketones, and
characterization data for the following compounds: dimethyl
acetals 3, 4, 7-11, 13, 15, and 16; diethylacetals 3, 7-11, 15,
and 16; 1,3-dioxolanes 2, 11, 15, 16, and 18; and 1,3-dioxanes
2, 4, 10, 11, and 15-18. This material is available free of
charge via the Internet at http://pubs.acs.org.
The preferential formation of 1,3-dioxanes over 1,3-
dioxolanes for aldehydes was also supported by the
following experiment. When benzaldehyde 1 was reacted
in the presence of both 1,2-ethanediol and 1,3-propanediol
in equimolar amounts, the ratio of 1,3-dioxolane and 1,3-
dioxane was found to be 1:2. It is interesting to note that,
when a similar competitive reaction was done with
acetophenone 13, exactly opposite selectivity was ob-
served as shown in Scheme 6, supporting a preferential
formation of 1,3-dioxolanes for ketones. From the present
study, the apparent order of acetal formation for different
carbonyl groups is aldehyde-1,3-dioxanes > aldehyde-1,3-
dioxolanes > ketone-1,3-dioxolanes > ketone-1,3-diox-
anes.
J O025701O
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