Reduction of α,β unsaturated carbonyl compounds by Ni2+-BH-4

Article abstract of DOI:10.1016/S0022-328X(00)00741-5

The reduction of α,β unsaturated carbonyl compounds by sodiumborohydride is catalysed by Ni(bpy)Cl₂ (bpy = 2,2′-bipyridine). Various carbonyl compounds having the general formula R₁CH=CHCRO [where R₁, R = C₆H₅, H; p-MeO-C₆H₄-,C₆H₄; p-CH₃-C₆H₄, C₆H₅; (m-OMe-)(p-OMe-)C₆H₃, C₆H₅; C₆H₅, (CH₃)₂CH-; CH₃, H; W-Br-C₆H₄-, C₆H₅] are reduced to corresponding allylicalcohol [R₁CH=CHCRHOH] at 25°C within half an hour. During these reductions the double bond is partially reduced to give saturated alcohols as minor products having the molecular formula R₁CH₂CH₂CRHOH. The reduction of trans-3-phenyl-2-propenal with NaBH₄ and catalytic amounts of Ni(bpy)Cl₂ in solvents containing active deuterium (D₂O, CD₃OD), leads to the partial incorporation of deuterium at the α and γ positions to give C-D bonded alcohols.

Full text of DOI:10.1016/S0022-328X(00)00741-5

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Journal of Organometallic Chemistry 620 (2001) 90–93
Reduction of a,b unsaturated carbonyl compounds by Ni2+ –BH4−
Jyotirekha G. Handique, Arup Purkayashtha, Jubaraj B. Baruah *
Department of Chemistry, Indian Institute of Technology, North Guwahati 781031, India
Received 6 June 2000; accepted 8 September 2000
Abstract
The reduction of a,b unsaturated carbonyl compounds by sodiumborohydride is catalysed by Ni(bpy)Cl2 (bpy=2,2%-
bipyridine). Various carbonyl compounds having the general formula R1CHꢀCHCRO [where R1, R=C6H5, H; p-
MeOꢁC6H4ꢁ,C6H4; p-CH3ꢁC6H4, C6H5; (m-OMeꢁ)(p-OMeꢁ)C6H3, C6H5; C6H5, (CH3)2CHꢁ; CH3, H; m-BrꢁC6H4ꢁ, C6H5] are
reduced to corresponding allylicalcohol [R1CHꢀCHCRHOH] at 25°C within half an hour. During these reductions the double
bond is partially reduced to give saturated alcohols as minor products having the molecular formula R1CH2CH2CRHOH. The
reduction of trans-3-phenyl-2-propenal with NaBH4 and catalytic amounts of Ni(bpy)Cl2 in solvents containing active deuterium
(D2O, CD3OD), leads to the partial incorporation of deuterium at the a and g positions to give CꢁD bonded alcohols. © 2001
Elsevier Science B.V. All rights reserved.
Keywords: Sodiumborohydride; Nickel catalyst; Reduction; Deuteration; a,b Unsaturated carbonyl compounds
1. Introduction
pounds can efficiently be carried out by the reaction of
hydrated nickel chloride in the presence of radical
anions [5a]. However, such reactions require the genera-
tion of anion radicals by reactive alkali metals and they
require extensive care in handling. The incorporation of
deuterium from performed deuterohydrated nickel
chloride to unsaturated organic compounds makes
these reactions attractive, and exploration of new
reagents have increasing demand [5b]. Reports are
available on partial incorporation of deuterium on car-
bonyl compounds by borohydride anions without a
catalyst as well as in the presence of late transition
metal complexes [6]. This article describes the use of
nickel catalysts to modify the course of the reduction
reaction of a,b unsaturated carbonyl compounds by
sodiumborohydride under neutral condition, and also
on the incorporation of deuterium from solvent-con-
taining active deuterium into the products of such
reactions.
The reduction of organic compounds is a common
reaction extensively used in synthetic chemistry and in
industry [1]. New reagents are developed for optical
induction and selective reduction of unsaturated and
carbonyl compounds [2]. The reduction of a,b unsatu-
rated carbonyl compounds has been thoroughly ex-
ploited and continues to attract attention [3].
Sodiumborohydride can cause selective reduction of a,b
unsaturated carbonyl compounds and sodiumborohy-
dride, and in combination with various metal catalysts
can cause hydrogenation and hydroboration of unsatu-
rated compounds [4]. The enhancement of selectivity
and efficacy of these reactions continues to be a nodal
point to diverse [2]. The reduction of unsaturated com-
2. Results and discussion
Scheme 1.
The reaction of trans-3-phenyl-2-propenal with
sodium borohydride and
a catalytic amount of
* Corresponding author.
E-mail address: juba@iitg.ernet.in (J.B. Baruah).
Ni(bpy)Cl2 (bpy=2,2% bipyridine) leads to trans-3-
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00741-5

J.G. Handique et al. / Journal of Organometallic Chemistry 620 (2001) 90–93
91
Table 1
yielded the deuterated saturated and unsaturated alco-
hols. Fig. 1(a) and (b) show the NMR spectra of the
reaction mixture in which the reduction of trans-3-
phenyl-2-propenal with sodium borohydride carried out
in methanol-d4 with and without Ni(bpy)Cl2. The mul-
Results on Ni(bpy)Cl2 catalysed reduction
1
tiplicity of a and g protons in H-NMR clearly discerns
the incorporation of deuterium at these positions when
the Ni(bpy)Cl2 was used as a catalyst. In the undeuter-
ated trans-3-phenyl-2-propen-1-ol the a and g protons
appear at l 6.56 as a doublet (J=18 Hz, trans cou-
pling) and at l 6.34 as a doublet of a triplet (18 and 9
Hz, respectively). However, the product obtained from
the reaction in CD3OD by the Ni2+ꢁBH4− system and
the coupling pattern of the signal at l 6.56 is not
affected but becomes slightly broadened due to the
presence of deuterium with I=1 and the signal at l
6.34 appears as a multiplet due to the presence of
deuterium at the h and g positions. The integrations
confirm the ratio of deuterium at the a and g position
as 46:54. The proton signal at l 4.21 appearing as a
doublet of a multiplet (J=9 Hz) from saturated carbon
(g position) also indicates the deuterium being incorpo-
rated at the g position. Only the trans unsaturated
alcohol was obtained in these reactions. About 7%
3-phenyl propan-1-ol was also obtained on reduction
by the Ni2+ꢁBH−4 system.
A plausible path leading to the incorporation of
deuterium during the reduction of the a,b unsaturated
carbonyl compound is shown in Scheme 3. The genera-
tion of reduced nickel(0) complexes similar to (A)
(Scheme 3) by reducing agents is established [7]. A
methanolic solution of Ni(bpy)Cl2 has its absorption
maximum at 296 and 306 nm due to the metal to ligand
charge transfer [11]. These absorptions disappear after
the addition of sodium borohydride (Fig. 2) and a new
absorption at 282 nm develops. Vigorous hydrogen gas
evolution occurs initially and once the borohydride is
consumed, the two absorption maxima at 296 and 306
nm reappear and the absorption at 282 nm disappears.
This suggests that an intermediate [7c] of Ni(0) of the
type (A) having absorption at 282 nm gets converted to
Ni(II) species during the dehydrogenative coupling re-
action [7]. A similar pattern on the change of ab-
sorbance was also observed during the reduction
reaction of trans-3-phenyl-2-propenal by sodiumboro-
hydride in the presence of Ni(bpy)Cl2 in methanol. An
oxidative addition and reductive elimination process
may be favourable to explain the equi-distribution of
deuterium at the a and g positions. Since the exchange
of deuterium at a and g positions were approximately
equal, it suggests intermediacy of the p-allylic type of
complex in the reaction. There are several structural
possibilities of such intermediates and in view of the
inability to isolate such intermediates the discussion is
minimised. The existing literature suggests that the
Scheme 2.
phenyl-2-propen-1-ol along with 3-phenylpropan-1-ol
within 30 min. The reaction is applicable to other a,b
unsaturated carbonyl compounds (Scheme 1) and is
substrate dependent. The results on product ratios of
unsaturated (U) and saturated (S) alcohols are shown
in Table 1. The catalytic activity of different nickel salts
was tested by comparing their percentage conversion of
the reduction of trans-3-phenyl-2-propenal after a 15
min time interval of independent experiments and reac-
tivity are in the order of Ni(bpy)Cl2\bis-
triphenylphosphinenickel(II)chloride\nickel(II)acetate
\nickel(II)chloridehexahydrate. The product ratios of
saturated and unsaturated (U:S as in Scheme 1) are
effected by the nickel catalyst and in each case the
saturated (S) and unsaturated products were formed in
approximately equal ratios except in the case of
Ni(bpy)Cl2. In this case, the maximum selectivity on the
formation of unsaturated alcohol was observed (\
90%).
The significant feature of these reactions is the trans-
fer of active deuterium from the solvent containing
active deuterium during reduction to form CꢁD bond.
It was observed that the reaction of trans-3-phenyl-2-
propenal in deuteriumoxide with Ni(bpy)Cl2 as the
catalyst gave trans-3-phenyl-2-propen-1-ol with the in-
corporation of deuterium at a and g positions in a
43:57 ratio. Deuterium incorporation from D2O does
not occur without the use of the nickel catalyst (Scheme
2). A similar observation was also observed from the
reaction of sodiumborohydride in methanol-d4. The
reaction of trans-3-phenyl-2-propenal with sodium-
borohydride in the presence of Ni(bpy)Cl2 catalyst has

92
J.G. Handique et al. / Journal of Organometallic Chemistry 620 (2001) 90–93
1
Fig. 1. The 400 MHz H-NMR of the reaction mixture of trans-3-phenyl-2-propenal with sodium borohydride in CD3OD (a) without Ni (bpy)Cl2
catalyst; (b) with Ni (bpy)Cl2 catalyst (* are signals from the saturated alcohol and are from the solvent). Inset is the expansion of a and
b-proton signals of (b).
tive deuterium to an a,b unsaturated carbonyl
compound.
carbonyl compounds can be partially deuterated by
sodium borohydride [6a] and the partial deuteration of
the carbonyl compound by sodiumborohydride in
deuterated solvents is enhanced by heavy metal salts
such as that of platinum [6b]. However, our reactions
are performed under neutral conditions and the selec-
tive carbonyl reductions of a b unsaturated carbonyl
compounds are observed. The dehydrogenative cou-
pling reactions by other hydrides such as deuterosilanes
in the presence of transition metal catalysts also allows
hydrogenation of olefins through deuterium scrambling
on the products [8].
In conclusion this study showed the catalytic effect of
Ni(bpy)Cl2 during the sodiumborohydride reduction
and also has shown that the deuterium can be partially
incorporated from a deuterated solvent containing ac-
Scheme 3.

J.G. Handique et al. / Journal of Organometallic Chemistry 620 (2001) 90–93
93
added, followed by the addition of HCl (1% in 5 cm3
water). The CH2Cl2 layer was separated and dried over
anhydrous sodium sulphate and filtered. Removal of
the solvent under reduced pressure gave the corre-
sponding alcohols in near quantitative yield (refer to
Table 1). The products were purified by column chro-
matography except in the case of the reduced product
of crotonaldehyde where purification was done by dis-
tillation. The saturated and unsaturated alcohols were
obtained as a mixture. The purified alcoholic com-
pounds were characterised by comparing their 400
1
MHz H-NMR and IR with an authentic sample.
Acknowledgements
Fig. 2. Change of absorbance spectra of a methanolic solution
containing Ni(bpy)Cl2 (0.023 mmol) with sodium borohydride (0.05
mmol) after 2 min time intervals.
The authors thank the Sophisticated Instrumentation
Facility, Indian Institute of Science, Bangalore, for
NMR facilities and the Council of Scientific and Indus-
trial Research, New Delhi for financial support.
Table 2
Results on deuterium incorporation a
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3. Experimental
The a,b unsaturated carbonyl compounds used in
this study were either prepared by standard procedure
[9] or obtained from E. Merck and used as procured.
The Ni(PPh3)2Cl2 [10], Ni(bpy)Cl2 [11] were prepared
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To a well stirred solution containing a,b unsaturated
carbonyl compound (1 mmol), Ni(bpy)Cl2 (0.01 mmol)
in methanol (2 cm3), the NaBH4 (2 mmol) was added in
portions. A rapid reaction took place with vigorous gas
evolution and the reaction mixture was stirred for 30
min at room temperature (25°C). The solution turned
black. To the reaction mixture, CH2Cl2 (20 cm3) was