Dichlorobis[1,4-diazabicyclo(2,2,2)octane]-(tetrahydroborato)zirconium, [Zr(BH4)2Cl2(dabco)2] (ZrBDC), a new stable, efficient, and selective reducing agent

Article abstract of DOI:10.1081/SCC-120014968

High yield preparation of a new stable zirconium borohydride, [Zr(BH₄)₂Cl₂(dabco)₂] is described and its application for the selective and efficient reduction of aldehydes and ketones is presented.

Full text of DOI:10.1081/SCC-120014968

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DICHLOROBIS[1,4-DIAZABICYCLO(2,2,2)OCTANE]-
(TETRAHYDROBORATO)ZIRCONIUM,
[Zr(BH4)2CL2(dabco)2] (ZrBDC), A NEW STABLE,
EFFICIENT, AND SELECTIVE REDUCING AGENT
H. Firouzabadi a , N. Iranpoor b & H. Alinezhad a
a Chemistry Department , College of Sciences , Shiraz University , Shiraz, 71454, Iran
b Chemistry Department , College of Sciences , Shiraz University , Shiraz, 71454, Iran
Published online: 16 Aug 2006.
To cite this article: H. Firouzabadi , N. Iranpoor & H. Alinezhad (2002) DICHLOROBIS[1,4-DIAZABICYCLO(2,2,2)OCTANE]-
(TETRAHYDROBORATO)ZIRCONIUM, [Zr(BH4)2CL2(dabco)2] (ZrBDC), A NEW STABLE, EFFICIENT, AND SELECTIVE REDUCING
AGENT, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 32:23,
3575-3586, DOI: 10.1081/SCC-120014968
To link to this article: http://dx.doi.org/10.1081/SCC-120014968
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SYNTHETIC COMMUNICATIONS
Vol. 32, No. 23, pp. 3575–3586, 2002
DICHLOROBIS[1,4-
DIAZABICYCLO(2,2,2)OCTANE]-
(TETRAHYDROBORATO)ZIRCONIUM,
[Zr(BH4)2Cl2(dabco)2] (ZrBDC), A NEW
STABLE, EFFICIENT, AND SELECTIVE
REDUCING AGENT
*
H. Firouzabadi, N. Iranpoor, and H. Alinezhad
*
Chemistry Department, College of Sciences,
Shiraz University, Shiraz 71454, Iran
ABSTRACT
High yield preparation of a newstable zirconium boro-
hydride, [Zr(BH4)2Cl2(dabco)2] is described and its application
for the selective and efficient reduction of aldehydes and
ketones is presented.
After the discovery of NaBH4, many research groups have put their
efforts into preparation of modified borohydride agents in order to decrease
the problems encountered with this reagent. Some of the drawbacks of
NaBH4 for the reduction of functional groups may be mentioned as follows;
poor chemo- and regio-selective reduction, high rate of decomposition in
*Corresponding authors. E-mail: firouzabadi@chem.susc.ac.ir
3575
DOI: 10.1081/SCC-120014968
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3576
FIROUZABADI, IRANPOOR, AND ALINEZHAD
solvents like H2O and CH3OH, inability to work in aprotic solvents and also
a limited number of the functional groups that can be reduced by the
reagent.[1]
Transition metal borohydrides are mostly very reactive, volatile,
unstable, difficult to handle and as such can not be used in organic synthesis
as reducing agents without modification.[2] Ligand–metal borohydride
complexes [LXM(BH4)n] form a category of modified borohydride
agents that some of them have been used effectively for the reductive trans-
formation of functional groups in organic synthesis.[2–6] The typical
examples are; Cu(BH4)(PPh3)2,[3] Cu2(BH3CN)2(PPh3)4,[4] Zn(BH4)2
dabco,[5] Zn(BH4)2PPh3 and Zn(BH4)2.crown ethers.[7]
[6]
Zirconium borohydride, Zr(BH4)4, is a highly volatile colorless solid,
inflames in air, hydrolyzes rapidly with explosion and not easy to
handle.[2,8,9] A practical, easy to handle reducing agent of borohydride bear-
ing zirconium cation is rare. To the best of our knowledge, there are onyl
two reagents, [Cp2Zr(Cl)BH4][10] and [crosslinked PVP4-Zr(BH4)4][11] are
reported as hydride tranfering agents for the reduction of a limited
number of carbonyl compounds.
We here report the high yield (97%) easy preparation of a newstable
ligand-metal zirconium borohydride [Zr(BH4)2Cl2(dabco)2] (ZrBDC) as a
white, stable, non-hygroscopic powder. This compound can be stored at
room temperature for months without significant change in its reducing
ability. This zirconium borohydride is much more stable than NaBH4 in
H2O, CH3OH and also in mild aqueous acidic conditions. Chemical formula
of the compound has been established by quantitative chemical analysis and
1
also by IR, H NMR and 13C NMR spectroscopy.[12]
The results of the reduction of carbonyl compounds in our study with
[Zr(BH4)2Cl2(dabco)2] (ZrBDC) are summarized in Table 1. High yield
reduction of aldehydes and ketones were conducted by this reagent in iso-
propyl alcohol under reflux conditions. We have observed that functional
groups like -NO2, -CN and -CO2R remained intact during the progress of
the reaction by this reagent. The effect of the nature of the substituted
groups on the aromatic rings of the carbonyl compounds is quite clear. In
comparison with benzaldehyde, electron-withdrawing groups facilitate the
reduction reactions and electrons-donating groups retard the rate of the
reactions. Selective and high yield reduction of benzaldehydes with different
substituted groups was performed smoothly in the presence of this reducing
agent. We have shown the selectivity between benzaldehyde derivatives in
Table 2.
Selective reduction of aldehydes in the presence of ketones is of
synthetic value. Therefore, we have presented the ability of the reagent for
this kind of reaction. We have observed that ZrBDC reduces aldehydes in

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PREPARATION OF ZIRCONIUM BOROHYDRIDE
3577

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3578
FIROUZABADI, IRANPOOR, AND ALINEZHAD

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PREPARATION OF ZIRCONIUM BOROHYDRIDE
3579
Table 2. Selective Reduction of Benzaldehyde Derivatives with (ZrBDC) in
Isopropyl Alcohol Under Reflux Conditionsa
Entry
1
Substrate (1, 2)
Product (1, 2)
Time (h)
1.0
% Yieldb
100
0
2
3
100
12
1.0
0.8
94
7
4
100
15
1.2
1.2
1.2
5
6
100
20
100
25
aThe molar ratio of the reagents/subs.1/subs.2 is 0.5/1.0/1.0; bYields based on NMR
analysis.
the presence of ketones with absolute chemoselectivity that has been
1
determined by H NMR spectroscopy (Sch. 1).
High yield and selective production of allyl alcohols via 1,2-reduction
of a,b-unsaturated carbonyl compounds is an important reaction.
Reduction of a,b-unsaturated carbonyl compounds by ZrBDC was
performed smoothly and the corresponding allyl alcohols were produced
in high yields (Sch. 2).

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3580
FIROUZABADI, IRANPOOR, AND ALINEZHAD
Scheme 1.
Scheme 2.
Recently, much attention has been paid to the reactions conducted in
water in which the vast majority of biochemical processes take place.[13] We
have found that ZrBDC is quite stable in aqueous media therefore, we have
presented the potential of this reagent in aqueous media for the reduction of
carbonyl compounds. Our studies showed that the rate of the reactions in
water were faster than those performed in isopropyl alcohol under similar
reaction conditions. The yields of the products were more or less similar to
that observed for the reductions in isopropyl alcohol (Table 3).
In order to showthe advantages and the drawbacks of ZrBDC for
the reduction of carbonyl groups, we have compared the results with some
of those reported with other modified borohydride reagents in Table 4.
In conclusion, (ZrBDC) as a white powder, unlike its mother
compound Zr(BH4)4, is thermally stable and could be stored for months
without appreciable loss of its reducing ability at room temperature.

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PREPARATION OF ZIRCONIUM BOROHYDRIDE
3581

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PREPARATION OF ZIRCONIUM BOROHYDRIDE
3583

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3584
FIROUZABADI, IRANPOOR, AND ALINEZHAD
The preparation of the reagent is easy (in 97% yield) and unlike its polymer
supported analogue [cross-linked PVP4-Zr(BH4)4] is quite reproducible by
different hands.[18] This reagent has been applied successfully for the reduc-
tion of structurally different carbonyl compounds in organic media and
water as well. Moreover, the mildness, easy reaction work up, efficiency,
selectivity, high reaction yields, lack of requirement of an inert atmosphere
make this newstabilized transition-metal borohydride a practical bench top
modified zirconium borohydride reagent and a useful addition to borohy-
dride chemistry.
Reduction of imines, enamines, acid chlorides, azides, reductive
amination of aldehydes and ketones, and also methylation of amines are
under investigation in our laboratories.
EXPERIMENTAL
All yields refer to isolated products. The products were purified by
column chromatography and the purity determination of the products were
accomplished by GLC on a Shimadzu model GC-8A instrument or by TLC
on Silica-gel polygram SIL G/UV254 plates, mass spectra were run on a
Shimadzu GC MS-QP 1000EX at 20 eV. IR spectra were recorded on a
Perkin-Elmer 781 spectrophotometer. The NMR spectra were recorded on
a Bruker Avance DPX 250 MHz instrument.
Reduction of cinnamaldehyde, a typical procedure for the reduction of
a,b-unsaturated carbonyl compounds in iso-PrOH or in H2O: To a solution
of cinnamaldehyde (2.0 mmol) in iso-PrOH (5–10 mL), the reducing agent
(1.0 mmol) was added and refluxed with stirring (2.7 h). The mixture was
cooled to room temperature, silica gel (2 g) was added and the solvent was
evaporated. The resulting solid was applied on a silica gel column and eluted
with petroleum benzene (60–80ꢀC)/Et2O (2/1). Evaporation of the solvent
afforded pure cinnamyl alcohol in 92% yield (Sch. 2). In the case of the
reaction in H2O, after 1.7 h, the reaction was quenched by the addition of
10% HCl (aq.) and extracted with EtOAc or Et2O (3 Â 30 mL). The organic
layer was washed with H2O, brine, dired over anhydrous MgSO4, and was
evaporated to afford cinnamyl alcohol in 90% yield (Table 3).
ACKNOWLEDGMENTS
We are thankful to professor M. Rashidi for useful discussions and
also to the Shiraz University Research Council and The Ministry of Science,
Research and Technology of Iran for a grant in support of this work.

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PREPARATION OF ZIRCONIUM BOROHYDRIDE
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3585
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phere and the resulting mixture[19] was stirred for 12 h. Then an ethe-
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was left under vacuum overnight. ZrBDC was isolated as a white,
stable, non-hygroscopic powder in 97% yield. This reducing agent
could be stored at room temperature for months without significant
change in its reducing ability. The chemical formula of the reagent
was established by IR and NMR studies.[9] IR (KBr, dabco species),
ꢀ (cmÀ1): 2974s, 296À3s, 2897s, 1623m, 1458s, 1324s, 1056s, 993s, 846s,
702b, and for BH4 species, ꢀ (cmÀ1): 2383s, 2373s, 2306s, 2268s,
1
1155s. H NMR (CDCl3, 250 MHz): ꢁ (ppm) 1.42 (broad band, 8H,
two BHÀ4 ), that its coupling constant and pattern are similar to
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(CDCl3, 63 MHz): ꢁ (ppm) 51.7, 52.1 (two different CH2 groups).
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method,[21] quantitative measurement of chloride content was
proceeded by potentiometric titration technique,[22] and quantitative

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3586
FIROUZABADI, IRANPOOR, AND ALINEZHAD
measurement of Zr was conducted by UV/Vis. spectrophotometric
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Received in the UK October 8, 2001