Titanyl acetylacetonate as an efficient catalyst for regioselective 1,2-reduction of α,β-unsaturated carbonyl compounds and conversion of α-diketones & acyloins to vicinal diols with sodium borohydride

Article abstract of DOI:10.1002/jccs.200500076

α,β-Unsaturated aldehydes and ketones were reduced readily and exclusively to their corresponding allylic alcohols with NaBH₄ and catalytic amounts of titanyl acetylacetonate at room temperature. Reduction reactions were carried out in CH₃CN or THF. This reducing system was also efficient for the reduction of α-diketones and acyloins to their corresponding vicinal diols in CH₃CN.

Full text of DOI:10.1002/jccs.200500076

Journal of the Chinese Chemical Society, 2005, 52, 525-530
525
Titanyl Acetylacetonate as an Efficient Catalyst for Regioselective
,2-Reduction of a,b-Unsaturated Carbonyl Compounds and Conversion
of a-Diketones & Acyloins to Vicinal Diols with Sodium Borohydride
1
Behzad Zeynizadeh
Department of Chemistry, Faculty of Sciences, Urmia University, Urmia 57159-165, Iran
a,b-Unsaturated aldehydes and ketones were reduced readily and exclusively to their corresponding al-
lylic alcohols with NaBH
actions were carried out in CH
a-diketones and acyloins to their corresponding vicinal diols in CH
4
and catalytic amounts of titanyl acetylacetonate at room temperature. Reduction re-
CN or THF. This reducing system was also efficient for the reduction of
CN.
3
3
Keywords: Titanyl acetylacetonate; Sodium borohydride; a,b-Unsaturated carbonyl compounds;
a-Diketones; Acyloins.
INTRODUCTION
Regioselective 1,2-reduction of a,b-unsaturated alde-
In this context, very recently, we reported an efficient
and convenient method for regioselective 1,2-reduction of
conjugated carbonyl compounds with sodium borohydride in
9
hydes and ketones to allyl alcohols is one of the most widely
utilized methods in organic synthesis and therefore numerous
hydride-transferring agents have been made for this achieve-
the presence of Dowex1-x8 as an anionic exchange resin.
Herein, we describe a new and alternative method for regio-
selective 1,2-reduction of a,b-unsaturated aldehydes and ke-
tones to their corresponding allylic alcohols under mild con-
ditions.
1
ment. Reduction of conjugated enals and enones to the corre-
sponding allylic alcohols with metal hydrides often occurs
with varying amounts of concomitant double bond reduction
leading to saturated alcohols and/or saturated carbonyl com-
pounds due to competing 1,2- vs. 1,4-attack by hydride. It has
been shown that reduction of a,b-unsaturated carbonyl com-
pounds with sodium borohydride yields mixtures of predomi-
nating 1,2- or 1,4-reduction products which depend on the
RESULTS AND DISCUSSION
Regioselective 1,2-Reduction of a,b-Unsaturated Alde-
hydes and Ketones
2
substrates and kinds of solvents used. Therefore, the regio-
The requirement to achieve a useful regioselectivity in
the reduction of a,b-unsaturated aldehydes and ketones is
frequently encountered in organic synthesis. Reduction of
unsaturated carbonyl compounds with sodium borohydride,
one of the most widely utilized reducing agents, is highly sol-
vent-dependent and generally does not show a useful regio-
4
selectivity achieved by NaBH is not practically useful.
In the past decades, the frequent requirements for re-
gioselective reduction of a,b-unsaturated aldehydes and ke-
tones have been stimulated by considerable progress and
have led to the development of new reducing systems for se-
3
2,4-8
2
lective 1,4- or 1,2-reductions.
Among the various reduc-
selectivity. To control the reducing power and selectivity of
ing systems which have been developed for 1,2-reduction of
conjugated carbonyl compounds, the reagents such as diiso-
4
NaBH , various modifications have been realized on this re-
ducing agent in the following ways: a) by the replacement of
hydride(s) with sterically bulky substituents or electron-
withdrawing/releasing groups in order to discriminate be-
tween the structural and electronic environments of the car-
5
butylaluminum hydride (DIBAL-H), triisobutylaluminum
6
2a
(
[
TIBAL), lithium aluminum hydride, 9-borabicyclo-
7
4c
3.3.1]nonane (9-BBN), lithium n-butylborohydride, and
2
c,4c,7,10-12
4b,8
sodium borohydride in the presence of rare earth metal
bonyl groups,
b) combination with Lewis acids
8
4b
2a-c,12
halides or calcium chloride are generally the most efficient
and convenient.
and mixed solvent systems,
c) using of transition metal
1
3
hydroborates and their new modifications, d) using of qua-
*
Corresponding author. E-mail: b.zeynizadeh@mail.urmia.ac.ir

5
26 J. Chin. Chem. Soc., Vol. 52, No. 3, 2005
Zeynizadeh
1
4
15
i
ternary ammonium and phosphonium tetrahydroborates,
air-moisture sensitivity of TiCl
4
or Ti(O- Pr)
4
, their difficul-
1
6
and finally e) immobilization on an ion exchange resin.
In connection to our continuous efforts for the devel-
ties in handling and storage and their consumption of equiva-
lent amounts of Ti(IV), put some restrictions on the use of
these systems as general procedures in organic synthesis.
Our observations reveal that the combination system of
1
3a-c,17
opment of new modified hydroborate agents
and syn-
1
8
thetic utilities of Ti(IV) in organic synthesis, very recently
we applied the combination system of NaBH
amounts of titanyl acetylacetonate, TiO(acac)
4
and catalytic
4
NaBH and catalytic amounts of titanyl acetylacetonate
2
, in the reduc-
readily reduces a,b-unsaturated aldehydes and ketones to
their corresponding allylic alcohols at room temperature. Re-
duction reactions were carried out under aprotic conditions in
1
7e
tion of aldehydes and ketones. Due to the excellent effi-
ciency which was achieved by this system, we decided to in-
vestigate the reducing potential of NaBH
4
/TiO(acac)
2
for
3
CH CN or THF. The regioselectivity achieved in these reac-
selective 1,2-reduction of a,b-unsaturated carbonyl com-
pounds under aprotic conditions.
tions was essentially perfect and the product allyl alcohols
were obtained in high to excellent yields (90-98%) (Table 1).
Unsaturated aldehydes were reduced with one molar equiva-
Our literature review shows that the combination sys-
tem of NaBH
4
/Ti(IV) has been rarely studied and in the form
lent of NaBH
4
and 5 mol% of Ti(IV) catalyst in CH
3
CN, but
re-
of NaBH /TiCl
4
4
has been used for the reduction of nitros-
reduction of ketones with one molar equivalent of NaBH
quired 10 mol% of the catalyst and THF as a solvent.
4
1
9
20
amines, carboxylic acids, amides, oximes, sulfoxides,
2
1
deoxygenation of heteroaromatic amine oxides and prepa-
To highlight the efficiency and selectivity of the NaBH
4
/
2
2
ration of alcohols from alkenes. All of the reduction reac-
tions were carried out in dimethoxyethane via low-valent ti-
TiO(acac) system for regioselective 1,2-reduction of a,b-
2
unsaturated carbonyl compounds, we compared our results
with those achieved by other reducing systems such as
tanium-borane complex as an active species. The combina-
i
9
9
2c
12
tion system of NaBH
4
/Ti(O- Pr)
4
has also been used for the
NaBH
4
, NaBH
4
/Dowex1-x8, NaBH
3
(OAc), NaBH
3
15
CN,
2
3
4c
13d
synthesis of disubstituted ureas from aldehydes. Although
success has been achieved by these systems, performing the
reductions in DME, an expensive solvent, and with the highly
Li-n-BuBH
3
,
(i-PrO)
2
TiBH
4
,
and Ph
3
PMeBH
4
.
It is
clear that in most cases, our combination system showed
more or comparable efficiency and selectivity (Table 2).
a
Table 1. Reductions of a,b-unsaturated carbonyl compounds with the NaBH /TiO(acac) , system
4
2
Molar Ratio
Yield
(%)
Entry
Substrate
Product
Solvent Ratio 1,2:1,4 Time (min)
b
NaBH /Ti(IV)/Subs.
4
1
2
3
4
1:0.05:1
1:0.1:1
1:0.1:1
1:0.1:1
CH CN
100:0
100:0
100:0
100:0
10
55
35
12
96
97
98
92
3
THF
THF
CH CN
3
5
6
1:0.1:1
CH CN
100:0
100:0
10
50
90
95
3
1:0.05:1
CH CN
3
7
1:0.1:1
THF
100:0
40
96
a
All reactions were performed at room temperature.
Yields refer to isolated pure products.
b

Reduction with NaBH4 and Titanyl Acetylacetonate
J. Chin. Chem. Soc., Vol. 52, No. 3, 2005 527
Table 2. Comparison of regioselective 1,2-reduction of a,b-unsaturated carbonyl compounds with the NaBH /TiO(acac) and other
4
2
reducing systems
a
Molar Ratio, Time (h), Yield (%) and Ratio of 1,2:1,4
Entry
Substrate
9
9
2c
12
4c
13d
15
I
II
III
IV
V
VI
VII
VIII
b
c
(
1:0.05) (0.17)(96) 1(0.7)(96) 1(3)(93) 1.67(20)(70) 2(1.5)(80)
1(0.08)(90)
(> 99:< 1)
1( )(95)
(100:0)
1
2
3
4
----
(
100:0)
(100:0)
(95:5)
(99:1)
(100:0)
b
(
1:0.1) (0.92)(97) 1(1.4)(98) 1(2.8)(96) 1.67(20)(70) 2(1.5)(77) 1(2)(98) 1(0.08)(97) 1(3.5)(90)
(
100:0)
b
(100:0)
(95:5)
(96:4)
(100:0)
(100:0) (> 99:< 1)
(100:0)
(
1:0.1) (0.6)(98) 1(0.7)(95) 1(1.7)(95)
1(2)(99)
----
1.2(6)(90)
(100:0)
----
3(2.5)(0)
(
100:0)
b
(100:0)
(90:10)
(100:0)
(
1:0.05) (0.83)(95) 1(1.3)(94) 1(2.8)(90) 1.67(20)(86)
1(0.08)(95)
----
----
----
(
100:0)
(100:0)
(99:1)
(99:1)
(> 99:< 1)
]
b
(
1:0.1) (0.67)(96) 1(2.2)(91) 1(3)(80)
2(2)(88) 1(2)(98)
1(6)(71)
(100:0)
5
6
----
----
----
(
100:0)
(100:0)
(95:5)
(100:0)
(100:0)
b
(
1:0.1) (0.2)(92) 1(0.8)(89) 1(1)(85) 1.67(20)(32)
1(2)(84)
(92:8)
----
----
(
100:0)
(100:0)
(80:20)
(99:1)
I
II
III
IV
V
VI
VII
NaBH /TiO(acac) ; NaBH /Dowex1-x8; NaBH4; NaBH (OAc); NaBH CN; Li-n-BuBH3; (i-PrO) TiBH ;
4
2
4
3
3
c
2
4
VIII
a
b
Ph PMeBH ; Reducing agent/Subs.; Molar ratio as NaBH /Ti(IV)/Subs.; Immediate reaction.
3
4
4
9
Reduction of Acyloins and a-Diketones
reported to be efficient for this achievement. The further
utility of NaBH /TiO(acac) was examined by the reduction
Reduction of a-hydroxy ketones and a-diketones to
vicinal diols and/or acyloins is synthetically important, and
many developments have been realized for this achievement
4
2
of acyloin compounds. In this case, the reductions took place
efficiently under mild conditions, and the corresponding vici-
nal diols were obtained in high to excellent yields (94-98%)
(Table 3).
1
24
25
in organic synthesis. Zn/aq.DMF, Zn/H
2
SO
4
,
TiCl
28
3
or
2
6
27
VCl
2
/THF/H
2
O, (C
2
H
5
O)
3
P,
H
2
S/piperidine/DMF and
2
9
heating with benzpinacol are the methods which have been
reported for the conversion of a-diketones to acyloin com-
3
0
pounds, whereas Cryptococcus macerans converts a-di-
ketones to vicinal diols. Reduction of a-diketones to vicinal
diols with modified hydroborate agents is also a subject of in-
terest, and the application of some ligand-zinc tetrahydro-
CONCLUSION
We have shown that sodium borohydride with catalytic
2
amounts of TiO(acac) readily reduces conjugated carbonyl
1
3a-c
borate complexes have been reported for this purpose.
compounds to their corresponding allylic alcohols at room
temperature with perfect regioselectivity. This combination
system was also efficient for the reduction of a-diketones and
acyloins to their corresponding vicinal diols under mild con-
ditions. Therefore, we think that in addition to the above ad-
vantages, availability of the reagents and a simple work-up
procedure could make this protocol an alternative and useful
addition to the present methodologies.
Recently, we reported that sodium borohydride in the pres-
ence of Dowex1-x8 performs this transformation with high
9
efficiency. We observed that the combination system of
4 2
NaBH (1.5-2 molar equivalents) and 10% mol of TiO(acac)
can reduce readily and efficiently a-diketones to their corre-
sponding vicinal diols at room temperature within 3-15 min-
utes (Table 3). It is notable that under different conditions,
our attempts to perform this transformation into acyloins
were unsatisfactory.
Transformation of a-hydroxy ketones to vicinal diols is
EXPERIMENTAL SECTION
3
1
also the subject of interest and application of H
2
/CuCr
2 4
O ,
3
2
Saccharomyces cerevisiae (baker’s yeast) and modified
All products were characterized by a comparison with
1
3a-c
1
hydroborate agents
have been reported for this goal. In
/Dowex1-x8 was
authentic samples (melting or boiling points) and their H-
addition, the combination system of NaBH
4
NMR and IR spectra. All yields refer to isolated pure prod-

5
28 J. Chin. Chem. Soc., Vol. 52, No. 3, 2005
Zeynizadeh
a
Table 3. Reductions of a-diketones and acyloins with the NaBH
4
/TiO(acac)
Molar Ratio
NaBH /Ti(IV)/Subs.
2
system
b
Entry
1
Substrate
Product
Time (min) Yield (%)
4
1.5:0.1:1
1:0.1:1
15
7
95
98
2
3
1.5:0.1:1
1.5:0.1:1
10
8
96
96
4
5
6
2:0.1:1
1:0.1:1
3
5
98
94
7
8
1.5:0.1:1
1:0.1:1
8
4
92
95
a
All reactions were performed in CH CN at room temperature.
Yields refer to isolated pure products.
3
b
ucts. TLC was applied for the purity determination of sub-
strates or products and reaction monitoring on silica gel Poly-
Gram SILG/UV-254 plates.
magnetically stirred for an additional 10 min. Then, the mix-
ture was extracted with CH Cl (3 ´ 10 mL) and dried over
2
2
anhydrous sodium sulfate. Evaporation of the solvent and
short column chromatography of the resulting crude material
A Typical Procedure for Regioselective 1,2-Reduction of
a,b-Unsaturated Aldehydes and Ketones with the
NaBH4/TiO(acac)2 System
4 2
over silica gel (eluent; CCl /Et O: 5/2) afforded the pure liq-
uid cinnamyl alcohol (0.l28 g, 96% yield, Table 1).
In a round-bottomed flask (15 mL) equipped with a
magnetic stirrer, a solution of cinnamaldehyde (0.132 g, 1
A Typical Procedure for Reduction of Acyloins and
a-Diketones with the NaBH4/TiO(acac)2 System
In a round-bottomed flask (15 mL) equipped with mag-
netic stirrer, to a solution of benzil (0.21 g, l mmol) and
2 3
mmol) and TiO(acac) (0.013 g, 0.05 mmol) in CH CN (5
mL) was prepared. Sodium borohydride (0.037 g, 1 mmol)
was then added and the mixture was stirred magnetically at
room temperature for 10 min. TLC monitored the progress of
2 3
TiO(acac) (0.026 g, 0.1 mmol) in CH CN (5 mL), sodium
borohydride (0.056 g, 1.5 mmol) was added and the mixture
was stirred magnetically at room temperature for 15 min.
reaction (eluent; CCl
4 2
/Et O: 5/2). After completion of the re-
action, water (7 mL) was added and the reaction mixture was
4 2
TLC monitored the progress of reaction (eluent; CCl /Et O:

Reduction with NaBH4 and Titanyl Acetylacetonate
J. Chin. Chem. Soc., Vol. 52, No. 3, 2005 529
5
/3). After completion of the reaction, water (7 mL) was
added and the mixture was stirred magnetically for an addi-
tional 10 min. Then, the mixture was extracted with CH Cl
3 ´ 10 mL) and dried over anhydrous sodium sulfate. Evapo-
ration of the solvent and short column chromatography of the
resulting crude material over silica gel (eluent; CCl /Et O:
/3) afforded the pure crystals of hydrobenzoin (0.203 g, 95%
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