Titanium promoted reduction of imines with Grignards, silanes, and zinc: Identification of a new mechanism with silanes

Article abstract of DOI:10.1016/j.tet.2014.03.035

Aldimines react with reducing agents, such as Grignards, phenylsilane or zinc in the presence of titanium(IV) isopropoxide to form amines and reductively coupled imines (diamines). Using deuterium labeled reagents, the mechanism of reduction to form amine

Full text of DOI:10.1016/j.tet.2014.03.035

Tetrahedron 70 (2014) 3185e3190
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Titanium promoted reduction of imines with Grignards, silanes,
and zinc: identification of a new mechanism with silanes
*
Akshai Kumar, Arun Kumar Pandiakumar, A.G. Samuelson
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Aldimines react with reducing agents, such as Grignards, phenylsilane or zinc in the presence of tita-
nium(IV) isopropoxide to form amines and reductively coupled imines (diamines). Using deuterium
labeled reagents, the mechanism of reduction to form amines is described. Reducing agents, such as the
Grignard and zinc result in the formation of low valent titanium (LVT), which in turn reduces the imine.
On the other hand, phenylsilane reacts by a distinctly different mechanism and where a hydrogen atom
from silicon is directly transferred to the titanium coordinated imine.
Received 24 January 2014
Received in revised form 7 March 2014
Accepted 11 March 2014
Available online 26 March 2014
Keywords:
Ó 2014 Elsevier Ltd. All rights reserved.
Deuterium labeling
Low valent titanium
Grignard
Phenylsilane
Zinc
1. Introduction
intermediacy of a dianion. The intermediacy of the dianion or
a three membered azametallacycle is well established in the case
Amines are key functional groups, extensively present in nat-
ural products, in catalysts, and in radiopharmaceuticals.1 A con-
venient route to synthesize amines is through reduction of imines.
Unfortunately, this reaction is often complicated by a reductive
coupling reaction, leading to the formation of diamines. Thus, in
the reduction of N-benzylideneaniline mediated by alkali metals,
only minor amounts of the reduced product are observed, which is
attributed to the dimetallation of the imine.2 However, the major
product is a diamine, which is reminiscent of the pinacol type
coupling promoted by reduced titanium species.3 The formation of
dimetallated species in such reactions is confirmed by quenching
the reaction with deuterium oxide, which results in the formation
of titanium.3 In these reactions, Grignard reagents or alkyl lithium
reagents are used to generate the low valent titanium(II)
species.3h,i
In the case of titanium, a second path has also been established
where the low valent titanium reagent generated from TiCl4
transfers a single electron (SET) to the imine.5 This generates
a radical anion, which then abstracts a hydrogen from the solvent
to give the reduced product. The facile delivery of a hydrogen
radical by THF is said to be responsible for the formation of the
amine.
We have recently reported6 that the reaction of Grignard re-
agents with imines at low temperatures (ꢀ78 ꢁC), in the presence of
stoichiometric amounts of titanium isopropoxide (Ti(OiPr)4) is
significantly different from that carried out at ꢀ40 ꢁC.3c It leads to
formation of the coupled imine 3 (major) rather than the reduced
of N-(a
-deuterobenzyl)aniline with >85% deuteration.3d Fujiwara
and co-workers have reported that the reaction of ketimines with
Yb metal in the presence of a catalytic amount of MeI gave re-
duced imines in good yield via hydrolysis of the Yb(II) N-diary-
lmethyleneaniline dianion.4 In order to get a better understanding
of the La promoted reactions, Nishiyama and co-workers have
performed similar reactions and hydrolyzed the intermediate with
D2O instead of H2O.4c About 79% deuterium incorporation was
observed in the resulting reduced imine, suggesting the
amine 4, with minor amounts of the alkylation product
2
(Scheme 1). Deuterium labeled Grignard reagents were used to
understand the mechanism of alkylation leading to 2 and the for-
mation of the coupled imine.6
* Corresponding author. E-mail addresses: ashoka@ipc.iisc.ernet.in, ashoka.
samuelson@gmail.com (A.G. Samuelson).
Scheme 1. Titanium isopropoxide mediated reaction of imine 1 with EtMgBr.
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.03.035

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A. Kumar et al. / Tetrahedron 70 (2014) 3185e3190
In the present study we have investigated the mechanism of
In order to address the possibility of intermediates like 11
shown in path B of Scheme 2, the reaction of imine 1 was carried
out with 2 equiv of ethylmagnesium bromide (CH3CH2MgBr) in
the presence of 1 equiv of titanium isopropoxide. The reaction
mixture was quenched with deuterium oxide (D2O). Not surpris-
ingly 2H and 13C{1H} NMR spectroscopy analysis of the reduced
product formed showed significant amount of deuterium in-
formation of amines from imines using titanium(IV) isopropoxide
and three different reducing agents (Grignard reagents at ꢀ78 ꢁC,
phenylsilane, and zinc metal). Isotopic labeling studies and
quenching experiments with deuterium oxide indicate that the
mechanisms involved in these reactions are significantly different
from one another and in the case of zinc, dependent on the
solvent.
corporation. The ratio of the triplet at
d 47.95 to the singlet at
d
48.28 was 3.11:1.00. The reduced product 4 formed was found to
be about 76% deuterated. This clearly indicates that the three
membered titanacycle intermediate 11, which is responsible for
the formation of the coupled product 3 as reported earlier,3,6 is
also responsible for the formation of the reduced product from
a hydrolysis reaction. Although the product distribution is differ-
ent from that reported by Sato from a reaction at ꢀ40 ꢁC, the
intermediate involved in the reaction appears to be the same. Sato
and co-workers have previously reported 90e98% deuterium in-
corporation due to a three membered azatitanacycle in similar
reactions.3c
2. Results and discussion
2.1. Reduction of N-benzylideneaniline with Grignards
The reduced product 4 occurs via a parallel reaction and can be
formed either through path A or through path B (Scheme 2). Path A
involves transfer of
b hydrogen from the alkyl group to the aza
carbonyl carbon of the imine via Ti. Hence if path A is involved in
the reduction of imine, then on use of deuterium labeled substrate
ethyl-2,2,2-d3-magnesium bromide (CD3CH2MgBr), which contains
A comparison of the peaks corresponding to reduced product 4
a
b
deuterium, one should observe deuterium incorporation in the
in 13C{1H} NMR for CD3CH2MgBr on hydrolysis and CH3CH2MgBr on
reduced product 4.
Scheme 2. Plausible mechanism for the formation of reduced imine 4 in the titanium isopropoxide mediated reaction of imine 1 with CX3CH2MgBr through path A and through
path B where X¼H/D.
The reaction of imine 1 with 2 equiv of CD3CH2MgBr in the
presence of 1 equiv of titanium isopropoxide was carried out. After
completion of the reaction it was quenched with water. The re-
duced product formed was analyzed by 2H and 13C{1H} NMR
spectroscopy. The 2H NMR spectrum showed a very small peak at
deuterolysis is given in Fig.1. Only a minor amount (12%) of reduced
product is formed via path A due to the transfer of hydride to imine
1 from a TieH species 8 formed as a result of
b hydride elimination.
2.2. Reduction of N-benzylideneaniline with phenylsilane
d
4.34 and the 13C{1H} NMR spectrum showed a small triplet cen-
tered at
d
47.95 and a more intense singlet at
d
48.28 in the ratio
It would be interesting to see if the same type of intermediate 11
is involved, when reducing agents other than Grignards are used. To
probe this, we examined the reaction of phenylsilane with imine 1
mediated by titanium isopropoxide where minor amounts of
0.14:1.00. The deuterium incorporation observed in the product
was about 12% indicating that only a small amount of the product is
formed through path A.

A. Kumar et al. / Tetrahedron 70 (2014) 3185e3190
3187
Fig. 1. Comparison of the 13C{1H} NMR spectrum (benzylic carbon of 4) in the crude product obtained from (a) Reaction of 1 with Ti(OiPr)4 and CD3CH2MgBr on hydrolysis (b)
Reaction of 1 with Ti(OiPr)4 and CH3CH2MgBr on deuterolysis.
reduced imine 4 is formed in addition to imine coupled product 3.7
When this reaction was quenched with D2O, no peak was seen in
the 2H NMR spectrum and only a singlet was observed in the 13C
{1H} NMR spectrum at
d 48.28 corresponding to undeuterated
benzylic carbon of the reduced product 4. Coupled product 3 and
reduced product 4 were formed in the ratio 70:30. The absence of
deuterium in the reduced product excludes the involvement of
titanacycle intermediate 11, such as the one shown in path B when
a Grignard is used as the reducing agent (Scheme 2).
The reduced product is likely to form either by transfer of Hꢂ
from THF or by a transfer of H from PhSiH3. Conclusive evidence of
the mechanism involved in this case can only be obtained if the
reaction is carried out with deuterium labeled phenylsilane.
Transfer from PhSiD3 can occur through the Ti to the imine as il-
lustrated in Scheme 3 or directly to imine 1 coordinated to Ti as
shown in Scheme 4. Deuterated phenylsilane was prepared by the
reaction of trichlorophenylsilane with LiAlD4 and its formation was
confirmed by 1H, 2H, and 13C NMR spectroscopy. The 2H NMR
spectrum showed a singlet at 4.25 ppm corresponding to the
deuterium bound to the silicon atom.
Scheme 4. Proposed mechanism for formation of reduced imine 4 in the titanium
isopropoxide mediated reaction of imine 1 with PhSiD3.
in the ratio 72:28. A triplet was observed in the 13C{1H} NMR
spectrum centered at
corporation on the benzylic carbon of the reduced product 4. A
comparison of the peaks corresponding to reduced product 4 in 13
d 47.95 indicating complete deuterium in-
C
{1H} NMR for PhSiH3 on deuterolysis and PhSiD3 on hydrolysis is
given in Fig. 2. These studies clearly indicate that when phenyl-
silane is used, the reduced product 4 is formed via Path C by the
transfer of H from PhSiH3 to the Ti as in Scheme 3 or directly to the
imine 1 coordinated to titanium as shown in Scheme 4. Path D
shown in Scheme 4 appears more plausible because Ti-D generated
from deuterated Grignards do not transfer D to the imine (vide
supra).
2.3. Reduction of N-benzylideneaniline with zinc
As the reaction appeared to follow a different path in the room
temperature reaction of PhSiH3 with titanium isopropoxide, we
wished to examine the protocol developed by Periasamy and
Vairaprakash.8 The products, coupled diamine 3 and reduced imine
4, formed in the reduction of the imine using a combination of
Ti(OiPr)4/TiCl4/Zn at room temperature was critically dependent on
the solvent. For example, the reaction carried out in THF at 0 ꢁC
followed by hydrolytic work up resulted in exclusive formation of
the coupled product 3 (entry 1, Table 1).
However a significant amount of the reduced product 4 was
observed when either dichloromethane or chloroform was used as
the solvent instead of THF (entries 2 and 3, Table 1). This led to the
suspicion that hydrogen from the solvent was involved in the re-
duction. However, when CDCl3 was used as solvent, no deuterium
incorporation was observed in the reduced product 4 (entry 4,
Table 1). Hence it is clear that the solvent does not act as a proton
source in the formation of reduced products. Use of D2O instead of
water to quench the reaction resulted in deuterium incorporation
Scheme 3. Plausible mechanism for the formation of reduced imine 4 in the titanium
isopropoxide mediated reaction of imine 1 with PhSiD3.
The reaction of deuterium labeled phenylsilane (PhSiD3) with
imine 1 was carried out in the presence of 1 equiv of titanium
isopropoxide. Analysis of the product after aqueous work up
showed the formation of coupled product 3 and reduced product 4

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A. Kumar et al. / Tetrahedron 70 (2014) 3185e3190
Fig. 2. Comparison of the 13C{1H} NMR spectrum (benzylic carbon of 4) in the crude product obtained from (a) Reaction of 1 with Ti(OiPr)4 and PhSiH3 on deuterolysis (b) Reaction
of 1 with Ti(OiPr)4 and PhSiD3 on hydrolysis.
Table 1
sodium/benzophenone prior to use. Grignard grade Mg turnings,
phenylsilane, trichlorophenylsilane, deuterium oxide, LiAlD4, and
titanium(IV) isopropoxide were obtained from SigmaeAldrich
U.S.A. Ethyl bromide was purchased from Spectrochem India.
Titanium(IV) tetrachloride and zinc were obtained from Merck
Germany. N-Benzylideneaniline was prepared by a standard liter-
ature procedure.9 The 1H, 2H, and 13C{1H} NMR spectra were
recorded on BRUKER AMX 400 operating at 400 MHz for 1H,
61.3 MHz for 2H, and 100 MHz for 13C, with tetramethylsilane or
CDCl3 as internal reference.
Ratio of products formed in the reduction of 1 by Zn mediated by a mixture of
Ti(OiPr)4/TiCl4 and quenching with X2O (X¼H/D)
Entry
Solvent
X
Conversion (%)
Ratio of productsa (%)
3
4 (% D)b
1
2
3
4
5
6
THF
H
H
H
H
D
D
95
98
83
74
92
92
95
58
51
68
57
80
d (d)
42 (d)
49 (d)
32 (d)
43 (>99)
20 (>90)
CH2Cl2
CHCl3
CDCl3
CH2Cl2
CHCl3
a
Ratio of products is determined by 1H NMR and is average of two runs.
% of deuterium incorporation as determined by 13C{1H} NMR.
b
4.2. Synthesis of deuterated phenylsilane (PhSiD3)10
in the reduced product 4, which is suggestive of the involvement of
a titanacycle intermediate 11 (entries 5 and 6, Table 1). A compar-
ison of the peaks corresponding to reduced product 4 in 13C{1H}
NMR for CH2Cl2/CDCl3 on hydrolysis and CHCl3/CH2Cl2 on deuter-
olysis is given in Fig. 3.
LiAlD4 (1.2 g, 31.22 mmol) was mixed with dry Et2O (100 ml) and
cooled to 0 ꢁC in an ice bath. Trichlorophenylsilane (2.5 ml,
15.61 mmol) was added slowly to the above suspension in a drop-
wise manner. After the addition the reaction mixture was brought
to room temperature and refluxed for 24 h. The solvent was evac-
uated and PhSiD3 was distilled into a cold trap. Yield¼1.47 g (87%).
1H NMR (400 MHz, CDCl3)
2H NMR (61.3 MHz, CDCl3)
128.6, 128.7, 130.3, 136.4.
d
7.59e7.61 (m, 2H), 7.34e7.42 (m, 3H).
3. Conclusion
d
4.24 (s, 3D). 13C NMR (100 MHz, CDCl3):
d
In summary, we have carried out mechanistic studies on the
titanium mediated reduction of N-benzylideneaniline using
Grignards, phenylsilane, and zinc. In the reduction of N-benzyli-
deneaniline employing Ti(OiPr)4/EtMgBr and Ti(OiPr)4/TiCl4/Zn, the
use of deuterium oxide confirmed presence of LVT intermediates
leading to the formation of a reduced imine through the traditional
titanacycle pathway. Interestingly, the reduction with zinc to give
the amine is possible only when chloroform or dichloromethane is
used. In the case of phenylsilane mediated reduction, a direct
transfer of the silane hydrogen to the imine was observed. This was
confirmed by using deuterated phenylsilane. Thus a third pathway
exists in which imines are reduced to the amine by a direct transfer
of the hydrogen in the presence of titanium reagents.
4.3. Reduction of N-benzylideneaniline with Grignard
reagents
Ethyl bromide (0.4 ml, 5.35 mmol) was dissolved in 5 ml dry THF
and a small portion was added to magnesium turnings (0.65 g,
26.8 mmol) in 10 ml dry THF. The reaction was initiated with gentle
warming. The remaining amount of ethyl bromide/THF solution
was added in a dropwise manner with constant stirring over a pe-
riod of half an hour while cooling the reaction mixture to maintain
the temperature around 50 ꢁC. After the addition of ethyl bromide,
the reaction mixture was refluxed for 1 h to ensure complete for-
mation of Grignard reagent. The Grignard was added dropwise to
a solution of imine 1 (0.49 g, 2.67 mmol) and Ti(OiPr)4 (0.76 g,
2.67 mmol) in 10 ml of dry THF maintained at ꢀ78 ꢁC using an
acetone slush bath. After addition, the reaction mixture was
allowed to attain room temperature and stirred for 12 h. It was
quenched by addition of D2O and filtered through a Celite pad. The
filtrate was extracted with diethyl ether. The organic layer was
separated and dried over anhydrous Na2SO4. Solvent was then
4. Experimental section
4.1. Materials and methods
All reactions were carried out under an atmosphere of dry ni-
trogen using standard Schlenk techniques and oven-dried glass-
ware. Tetrahydrofuran was freshly distilled from LiAlH4 followed by

A. Kumar et al. / Tetrahedron 70 (2014) 3185e3190
3189
Fig. 3. Comparison of the 13C{1H} NMR spectrum (benzylic carbon of 4) in the crude product obtained from (a) Reaction of 1 with Ti(OiPr)4 and Zn in CDCl3 on hydrolysis, (b)
Reaction of 1 with Ti(OiPr)4 and Zn in CHCl3 on deuterolysis, (c) Reaction of 1 with Ti(OiPr)4 and Zn in CH2Cl2 on hydrolysis, and (d) Reaction of 1 with Ti(OiPr)4 and Zn in CH2Cl2 on
deuterolysis.
removed under reduced pressure to yield yellow oil that contained
compounds 2, 3, and 4 in the ratio 34:31:35.
NH). 13C NMR (100 MHz, CDCl3):
128.9, 129.6, 140.4, 147.5.
d 64.4, 114.6, 118.6, 127.8, 128.0,
Benzenemethanamine, N-phenyl-4: 1H NMR (400 MHz, CDCl3):
7.33 (m, 4H), 7.2 (t, 2H, J¼8.0 Hz), 6.75 (t, 2H, J¼7.6 Hz), 6.62 (d, 2H,
d
4.4. Reduction of N-benzylideneaniline with phenylsilane
J¼8.4 Hz), 4.32 (s, 2H). 13C NMR (100 MHz, CDCl3):
d 48.3, 113.4,
118.1, 127.5, 128.1, 129.1, 129.3, 139.8, 148.7.
To a solution of N-benzylideneaniline 1 (0.142 g, 0.79 mmol) in
5 ml of dry THF was added 1 equiv of titanium(IV) isopropoxide
(0.24 ml, 0.79 mmol). The reaction mixture was cooled to 0 ꢁC in an
ice bath and allowed to equilibrate for half an hour. To the above
cooled solution 1 equiv of phenylsilane (0.1 ml, 0.79 mmol) was
added in a dropwise manner. The resulting reaction mixture was
allowed to warm up to room temperature. While stirring at room
temperature it attained a brownish black color. Stirring was con-
tinued for 24 h. The reaction mixture was then quenched by the
addition of either H2O or D2O. The solution was then filtered
through a Celite pad and the filtrate was extracted with diethyl
ether (10 ml). The organic portion was separated and dried over
anhydrous sodium sulfate. Solvent was then removed from the
organic fraction to give product mixture that contained compounds
3 and 4 in the ratio 70:30.
(b) PhSiD3 followed by hydrolysis
d,l 1,2-Ethanediamine, N,N0,1,2-tetraphenyl-3: 1H NMR
(400 MHz, CDCl3):
7.24e7.21 (m, 10H), 7.06 (t, 4H, J¼8.0 Hz), 6.6
d
(t, 2H, J¼8.0 Hz), 6.53 (d, 4H, J¼8.0 Hz), 4.58 (br s, 4H, d,l NCH, d,l
NH). 13C NMR (100 MHz, CDCl3):
128.9, 129.6, 140.4, 147.5.
d 64.4, 114.6, 118.6, 127.8, 128.0,
a
-Deuterio-benzenemethanamine, N-phenyl-4: 1H NMR
(400 MHz, CDCl3):
d
7.38 (m, 4H), 7.18 (t, 2H, J¼8.0 Hz), 6.70 (t, 2H,
J¼7.4 Hz), 6.60 (d, 2H, J¼8.4 Hz), 4.30 (s, 1H). 2H NMR (61.3 MHz,
CDCl3):
d d 48.0
4.4 (s, 1D). 13C NMR (100 MHz, CDCl3):
(t, JCeD¼21 Hz), 113.3, 118.0, 127.6, 128.1, 129.1, 129.7, 139.9, 148.6.
Spectral data of the crude reaction mixture obtained in the
above reaction using
4.5. Reduction of N-benzylideneaniline with zinc
TiCl4 (0.11 ml, 1.0 mmol) was added to a solution of Ti(OiPr)4
(0.3 ml, 1.0 mmol) in dichloromethane or chloroform (4 ml) and the
resulting yellow colored solution was stirred for 10 min. Activated
zinc powder (0.325 g, 5 mmol) was added in three portions and
stirred for another 10 min. The resulting reaction mixture was then
cooled to 0 ꢁC and N-benzylideneaniline 1 (0.09 g, 0.5 mmol) in
(a) PhSiH3 followed by deuterolysis
d,l 1,2-Ethanediamine, N,N0,1,2-tetraphenyl-3: 1H NMR
(400 MHz, CDCl3):
(t, 2H, J¼8.0 Hz), 6.53 (d, 4H, J¼8.0 Hz), 4.58 (br s, 4H, d,l NCH, d,l
d
7.24e7.21 (m, 10H), 7.06 (t, 4H, J¼8.0 Hz), 6.6

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Acknowledgements
A.G.S. thanks DST, New Delhi (SR/S5/MBD-02/2007) for the
award of a research grant, A.K. gratefully acknowledges a PDF from
IISc and P.A.K acknowledges a senior research fellowship from
Council of Scientific and Industrial Research (_501100001412). We
thank the DST, New Delhi, for providing us funds through its FIST
program for purchase of a 400 MHz NMR spectrometer.
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Supplementary data
Supplementary data containing the 1H, 2H, 13C{H} spectra of the
products obtained in the reactions discussed in this article.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2014.03.035.
References and notes
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