Room temperature metathesis of aryl isocyanates and aromatic aldehydes catalyzed by group(IV) metal alkoxides: An experimental and computational study

Article abstract of DOI:10.1016/j.jorganchem.2009.10.044

Aromatic aldehydes and aryl isocyanates do not react at room temperature. However, we have shown for the first time that in the presence of catalytic amounts of group(IV) n-butoxide, they undergo metathesis at room temperature to produce imines with the extrusion of carbon dioxide. The mechanism of action has been investigated by a study of stoichiometric reactions. The insertion of aryl isocyanates into the metal n-butoxide occurs very rapidly. Reaction of the insertion product with the aldehyde is responsible for the metathesis. Among the n-butoxides of group(IV) metals, Ti(OⁿBu)₄ (8aTi) was found to be more efficient than Zr(OⁿBu)₄ (8aZr) and Hf(OⁿBu)₄ (8aHf) in carrying out metathesis. The surprisingly large difference in the metathetic activity of these alkoxides has been probed computationally using model complexes Ti(OMe)₄ (8bTi), Zr(OMe)₄ (8bZr) and Hf(OMe)₄ (8bHf) at the B3LYP/LANL2DZ level of theory. These studies indicate that the insertion product formed by Zr and Hf are extremely stable compared to that formed by Ti. This makes subsequent reaction of Zr and Hf complexes unfavorable.

Full text of DOI:10.1016/j.jorganchem.2009.10.044

Journal of Organometallic Chemistry 695 (2010) 338–345
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Room temperature metathesis of aryl isocyanates and aromatic aldehydes catalyzed
by group(IV) metal alkoxides: An experimental and computational study
*
Akshai Kumar, Ashoka G. Samuelson
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Aromatic aldehydes and aryl isocyanates do not react at room temperature. However, we have shown for
the first time that in the presence of catalytic amounts of group(IV) n-butoxide, they undergo metathesis
at room temperature to produce imines with the extrusion of carbon dioxide. The mechanism of action
has been investigated by a study of stoichiometric reactions. The insertion of aryl isocyanates into the
metal n-butoxide occurs very rapidly. Reaction of the insertion product with the aldehyde is responsible
for the metathesis. Among the n-butoxides of group(IV) metals, Ti(OnBu)4 (8aTi) was found to be more
efficient than Zr(OnBu)4 (8aZr) and Hf(OnBu)4 (8aHf) in carrying out metathesis. The surprisingly large
difference in the metathetic activity of these alkoxides has been probed computationally using model
complexes Ti(OMe)4 (8bTi), Zr(OMe)4 (8bZr) and Hf(OMe)4 (8bHf) at the B3LYP/LANL2DZ level of theory.
These studies indicate that the insertion product formed by Zr and Hf are extremely stable compared to
that formed by Ti. This makes subsequent reaction of Zr and Hf complexes unfavorable.
Received 1 August 2009
Received in revised form 29 September
2009
Accepted 28 October 2009
Available online 14 November 2009
Keywords:
Aryl isocyanates
Aromatic aldehydes
Titanium n-butoxide
Titanium isopropoxide
Insertion
Ó 2009 Elsevier B.V. All rights reserved.
Metathesis
1. Introduction
with carbon dioxide to give isocyanates and uranium oxo com-
plexes [9b].
Formation of new carbon–carbon bonds by a metal carbene cat-
alyzed olefin metathesis has found wide utility in synthetic chem-
istry [1]. Stoichiometric and catalytic exchange between multiply
bonded substrates has been extended to metathesis of carbon–het-
eroatom double bonds recently [2,3]. Carbodiimide metathesis is
catalyzed by a number of complexes leading to formation of
unsymmetrical carbodiimides [4]. Group 14 complexes catalyze
metathesis of phenyl isocyanates to give N,N0 diphenyl carbodii-
mides and carbon dioxide [5]. The reverse reaction, metathesis of
carbon dioxide with carbodiimide leading to formation of isocya-
nates has also been demonstrated [6].
While the metathesis coupling of heterocumulenes are well ex-
plored, there are few reports that describe the metathesis of isocy-
anates and carbonyl compounds leading to imines [7]. These
reactions proceed under drastic conditions with poor yields in
the absence of catalysts. The yields can be improved by the use
of metal carbonyls as catalysts, but they still require high temper-
atures [8]. Guiducci et al. have reported the use of cyclopentadi-
enyl–amidinate titanium imido compounds for the conversion of
carbonyl compounds to imines at elevated temperatures [9a]. Very
recently Meyer et al. have shown that mid and high valent uranium
amido and imido derivatives promote multiple bond metathesis
Contrary to the high temperature metathesis reported till date,
we report here a room temperature metathesis of aromatic alde-
hydes and aryl isocyanates to give imines and carbon dioxide cat-
alyzed by simple group(IV) metal alkoxides. Theoretical
calculations are performed on the model complexes Ti(OMe)4,
Zr(OMe)4 and Hf(OMe)4 at B3LYP/LANL2DZ level of theory to get
an insight into these reactions.
2. Results and discussion
2.1. Reaction of phenyl isocyanate with benzaldehyde in the presence
of Ti(OnBu)4
The reaction of equivalent amounts of phenyl isocyanate
(PhNCO) and benzaldehyde (PhCHO) was initially carried out in
the presence of 10 mol% of titanium(IV) n-butoxide {Ti(OnBu)4} at
room temperature (Entry 1, Table 1). The reaction was monitored
by 1H NMR and was found to be complete in 60 h. The products
were analyzed and found to contain significant amounts of the
metathesis product N-benzylidene(aniline) 3a. However the major
product, butyl N-phenylcarbamate 4, and trace amounts of butyl
2,4-diphenylallophanate 5, arose from the insertion of the butoxy
group into PhNCO [10]. The amount of imine 3a formed was found
to increase when Ti(OnBu)4 was reduced to a smaller amount (En-
try 2, Table 1). Subsequent reactions showed that titanium n-
* Corresponding author. Tel.: +91 80 2293 2663; fax: +91 80 2360 1552.
E-mail address: ashoka@ipc.iisc.ernet.in (A.G. Samuelson).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.10.044

A. Kumar, A.G. Samuelson / Journal of Organometallic Chemistry 695 (2010) 338–345
339
Table 1
Hydrolysis of complex 9 gives rise to product 4. The reaction of
complex 9 with benzaldehyde can proceed via two paths, either
by insertion of aldehyde into the C–N bond (path X) or by insertion
of aldehyde into the C–O bond (bond between the carbonyl carbon
and alkoxy oxygen in the carbamate group) (path Y). If initial inser-
tion is followed by insertion of aldehyde into the C–N bond of 9
then it results in complex 10 (path X). Complex 10 may then ex-
trude CO2 to give 11 which in turn loses the coordinated imine
to generate the metal alkoxide 8. Alternatively, if there is aldehyde
insertion into the C–O bond of 9, then it leads to complex 100 (path
Y). This is similar to the second insertion observed with isocya-
nates at elevated temperatures in a head to head fashion permit-
ting CO2 extrusion (Scheme 2) [6e,10].
Reaction of phenyl isocyanate with benzaldehyde mediated by Ti(OnBu)4.
2.3. Reaction of aryl isocyanates with aromatic aldehydes with varying
electronic demands
Entry
Ti(OnBu)4 (mol%)
Ratio of productsa (%)
Yieldb 3a (%)
TON
The metathesis reaction was carried out with aryl isocyanates
and aryl aldehydes having different electronic demands (Table 2).
The metathesis reaction was found to proceed most efficiently
for a combination of an aryl isocyanate with an electron donating
group and an aryl aldehyde with an electron withdrawing group.
Based on these results, one might conclude that the insertion of
alkoxide into isocyanate is not a rate limiting step. If it was, then
electron withdrawing groups on isocyanate would have acceler-
ated the reaction [6e,10]. Insertion of the aldehyde into the C–O/
C–N bond of 9 must have a higher energy of activation. This ex-
plains why electron deficient aldehydes react faster with the elec-
tron rich isocyanates which have undergone insertion.
In the reaction of salicylaldehyde with phenyl isocyanate, one
would have expected the formation of 6 and 7 (Scheme 3). Product
6 is formed by the nucleophilic addition of hydroxyl group of 3j to
the C@O of isocyanate. Similarly 7 will be the product arising from
the nucleophilic addition of hydroxyl group of salicylaldehyde to
the C@O of isocyanate. However in the above reaction, both 6
and 7 were absent and 3j was formed as the only product. This
indicates that coupling is faster than nucleophilic addition in this
case. Hence the reaction is tolerant to aldehydes with nucleophilic
functional groups such as a hydroxyl group.
3a
4
5
1
2
3
4
5
10.0
1.0
0.1
100.0
25.0
41
73
95
17
28
58
27
5
83
72
1
–
–
–
–
44
72
76
18
24
4.4
72.0
760.0
0.2
1.0
Ratio of products were determined by 1H NMR.
All yields are wt% with respect to aldehyde after 60 h of reaction.
a
b
butoxide can be utilized in even smaller quantities to convert iso-
cyanates to imines under ambient conditions (Entry 3, Table 1).
To understand these reactions, we examined the stoichiometric
reactions of metal alkoxides with the reactants (Entry 4, Table 1).
Metal alkoxides do not react with benzaldehyde. However they un-
dergo very rapid insertion reaction with isocyanates to produce a
carbamate ligand coordinated to titanium which has been charac-
terized [10]. A control experiment was carried out to determine
whether the carbamate generated from Ti(OnBu)4 could carry out
the metathesis reaction. Ti(OnBu)4 was treated with four equiva-
lents of PhNCO and stirred at room temperature. The reaction
was followed by 1H NMR and when all the PhNCO had undergone
insertion, four equivalents of benzaldehyde were added. Analysis
of the reaction mixture after work up indicated the formation of
metathesis product 3a in 24% yield (Entry 5, Table 1).
2.4. Metathesis reaction catalyzed by group(IV) metal alkoxides
2.2. Plausible mechanism
The reaction of p-methoxyphenyl isocyanate with p-cyanobenz-
aldehyde was then attempted with various group(IV) metal alkox-
ides as catalysts (Scheme 4). All these reactions resulted in the
formation of imine 3e and trace amounts (<1%) of carbamate 40.
Insertion of phenyl isocyanate into the M–OR bond of metal alk-
oxide 8 would generate a metal carbamate complex 9 (Scheme 1).
Scheme 1. Proposed mechanism for the metathesis coupling of isocyanate and aldehyde.

340
A. Kumar, A.G. Samuelson / Journal of Organometallic Chemistry 695 (2010) 338–345
Scheme 2. Insertion of two molecules of phenyl isocyanate in head to head fashion and elimination of carbon dioxide.
Table 2
NMR spectra of the crude products obtained in the reaction of p-
methoxyphenyl isocyanate with p-cyanobenzaldehyde in the pres-
ence of titanium(IV) n-butoxide, zirconium(IV) n-butoxide and haf-
nium(IV) n-butoxide is given in Fig. 1. The peak at 10.09 ppm
corresponds to the C–H of the aldehyde and that at 8.52 ppm is
for the C–H of the imine. Comparison of the relative intensities of
these two peaks in the products obtained with Ti(OnBu)4, Zr(OnBu)4
and Hf(OnBu)4 clearly indicates that the metathesis reaction is
more favored with Ti than with Zr and Hf. In addition all the spec-
tra show formation of trace amounts of 40. A distinct triplet is ob-
served at 0.95 ppm which corresponds to the methyl proton of the
butoxy group in the carbamate 40. The carbamate 40 results from
hydrolysis of insertion intermediate such as 9. It is likely that in
the case of Zr and Hf, the reaction of 9 with aldehyde to give 10/
11 is less favored and hence after the reaction one observes unre-
acted aldehyde and 40 which is the hydrolysis product of 9. Hence
one may assume that the formation of 9 quenches the catalyst in
case of Zr and Hf. However in the case of Ti, it happens after many
cycles.
Further, Zirconium(IV) n-butoxide was found to be less efficient
than its heavier congener hafnium(IV) n-butoxide. The observed
reactivity pattern appears to follow the trend in the ionic sizes of
these elements. The ionic sizes for the four coordinate Ti, Zr and
Hf in their +4 oxidation state are 56, 73 and 72 pm, respectively
[11]. Ti being the smallest among the group(IV) elements is found
to be the most efficient metathesis catalyst. On the other hand Hf
which is slightly smaller than Zr is found to be a better metathesis
catalyst than the latter. So it is likely that the five coordinate inter-
mediates 9, 10 and 11 involved in the metathesis reaction are sta-
bilized to a greater extent for Zr and Hf compared to that of Ti.
Between Zr and Hf, these intermediates are more stabilized for Zr
compared to Hf owing to the smaller size of Hf.
Metathesis reaction with substrates of varying electronic demands.
Entry
R
R0
Product
Yielda (%)
TON
1
2
3
4
5
6
7
8
9
H
H
H
F
OMe
OMe
OMe
F
H
3a
3b
3c
3d
3e
3f
3g
3h
3i
70
70
78
70
83
68
70
61
68
70
70
78
70
83
68
70
61
68
OMe
CN
CN
CN
H
OMe
OMe
H
F
a
All yields are wt% with respect to aldehyde after 54 h of reaction.
The yield of 3e with Ti as catalyst was found to be better than Zr
and Hf (Table 3). In the case of Zr and Hf significant amounts of
unreacted aldehyde was observed in the crude product. The 1H
Scheme 3. Reaction of phenyl isocyanate with salicylaldehyde in the presence of 0.1 mol% Ti(OnBu)4.
Scheme 4. Metathesis reaction of p-methoxyphenyl isocyanate with p-cyanobenzaldehyde mediated by group(IV) metal alkoxides.

A. Kumar, A.G. Samuelson / Journal of Organometallic Chemistry 695 (2010) 338–345
341
Table 3
coordination of the other [6e]. It is also known that the insertion
reaction of carbon dioxide with titanium(IV) isopropoxide takes
place only in the presence of one fourth equivalents of water
[12]. In this study it has been found that the imine never undergoes
insertion into the Ti–O bond. Hence both the substrates aldimine
3a and carbon dioxide are capable of only coordinating to the Ti
centre. This results in low yields of the metathesis products. These
observations clearly indicate that insertion is the key step to
metathesis reaction.
Metathesis reaction of p-methoxyphenyl isocyanate with p-cyanobenzaldehyde
mediated by group(IV) metal alkoxides.
Entry
M(OR)4
% Conversiona
Yieldb 3e (%)
TON
1
2
3
4
Ti(OnBu)4
Zr(OnBu)4
Hf(OnBu)4
Ti(OiPr)4
88
9
20
78
86
8
16
77
860
80
160
770
a
% Conversion is the % of aldehyde converted to imine and is obtained from 1H
NMR of the crude product.
b
All yields are wt% with respect to aldehyde after 96 h of reaction.
2.6. Reaction of phenyl isocyanate with acetophenone in the presence
of Ti(OnBu)4
2.5. Reaction of aldimine with carbon dioxide in the presence of
Ti(OnBu)4
The metathesis reaction could not be extended to ketones
(Scheme 6). Phenyl isocyanate did not react with acetophenone
at room temperature. However at 110 °C, the expected metathesis
reaction occurred with the formation of the carbodiimide. The
reaction was followed by IR and after 54 h of reflux, the band at
2250 cmꢀ1 corresponding to –N@C@O stretch of isocyanate had
completely disappeared and a prominent band at 2137 cmꢀ1 was
observed. This corresponds to the –N@C@N– stretch of the carbo-
diimide. NMR analysis indicated that the product contained 69% of
carbodiimide and 31% of ketimine. The carbodiimide was isolated
as diphenyl urea in 60% yield after work up. At this temperature,
the carbodiimide was formed as a major product and arises due
to metathesis of isocyanates [6e].
A reverse reaction with carbon dioxide (Scheme 5) was at-
tempted. Aldimine 3a was treated with one equivalent of Ti(OnBu)4
in the presence of 6 equivalents of carbon dioxide. The reaction
was carried out in a sealed vial using toluene as solvent at 180 °C
for 72 h. However the reaction did not proceed to completion
and only 12% conversion to the metathesis product benzaldehyde
was observed. The other product phenyl isocyanate was only iso-
lated as the insertion product 4 in an equivalent amount with re-
spect to benzaldehyde.
Earlier it has been shown that if the metathesis of heterocumu-
lenes with carbon dioxide has to take place then it requires inser-
tion of at least one of the substrate into the Ti–O bond and
Fig. 1. Comparison of 1H NMR spectra of the crude products formed in the reaction of p-methoxyphenyl isocyanate with p-cyanobenzaldehyde in the presence of 0.1 mol% of
(i) titanium(IV) n-butoxide, (ii) zirconium(IV) n-butoxide and (iii) hafnium(IV) n-butoxide.

342
A. Kumar, A.G. Samuelson / Journal of Organometallic Chemistry 695 (2010) 338–345
Scheme 5. Metathesis reaction of aldimine 3a with carbon dioxide mediated by Ti(OnBu)4.
Scheme 6. Metathesis reaction of phenyl isocyanate with acetophenone mediated by Ti(OnBu)4.
of 10b is much higher in the case of zirconium and hafnium in
3. Computational studies
comparison with Ti. On the other hand, if the reaction proceeds
through path Y, the energy required for formation of 10b and
11b is higher for Ti. Irrespective of the path followed, the formation
of aldimine 3a with the regeneration of 8bTi/8bZr/8bHf by the
reaction of trigonal bipyramidal 9bTi/9bZr/9bHf with PhCHO is
an energetically favored process for Ti. In the case of Zr and Hf,
the intermediate 9b is more stable than the products according
to the calculation. This makes the metathesis reaction with tita-
nium alkoxide catalytically feasible but not with Zr or Hf alkoxide.
The C–O (bond between the carbonyl carbon and alkoxy oxygen
in the carbamate group) and C–N bond distances of the optimized
structure 9 for isocyanates with varying electronic demands were
compared (Table 4). In all cases, the C–O bond (bond between
the carbonyl carbon and alkoxy oxygen in the carbamate group)
was found to be weaker than the normal C–O bond. The C–N bond
had partial double bond character. This indicates that the reaction
is likely to proceed through path Y that involves insertion of alde-
hyde into the C–O bond to give 10’. The electronic effects of groups
present on aryl isocyanates on the rate of the reaction can also be
explained if we consider the reaction to occur via path Y. For isocy-
anates with electron donating group, the C–O bond was much
weaker compared to those with an electron withdrawing group.
This is consistent with the greater reactivity observed for isocya-
nates with electron donating groups.
3.1. Comparison of energies of intermediates for the two possible paths
Following the recent successful prediction of reactivity patterns
in group(IV) metal alkoxides using computational methods
[10b,13], we sought reasons for the excellent activity of Ti vs. Zr
and Hf. The structures numbered 8–11 in Scheme 1 were investi-
gated. For this purpose Ti(OnBu)4 (8aTi), Zr(OnBu)4 (8aZr) and
Hf(OnBu)4 (8aHf) were modeled by Ti(OMe)4 (8bTi), Zr(OMe)4
(8bZr) and Hf(OMe)4 (8bHf) where bulky nBu groups are replaced
by Me groups to reduce computational load. The letters a and b are
used to indicate metal complexes with OnBu ligand and OMe li-
gand, respectively. Ti, Zr and Hf are added to indicate the metal
present in the complex. For example, 10aTi implies structure 10
with M = Ti and OnBu as the alkoxy group.
There are two pathways for which various intermediates are gi-
ven in Fig. 2 (path X and path Y) along with the computed energy
levels. The reaction of PhNCO with 8bTi leads to the formation of a
stable intermediate 9bTi. The formation of 9bZr/9bHf is energeti-
cally more favored by 6.0 kcal/mol than the formation of 9bTi. Sub-
sequent insertion of aldehyde can take place either through path X
requiring the cleavage of C2–N1 bond (see complex 9 in Table 4) or
through path Y requiring the cleavage of C2–O3 bond. If the reac-
tion proceeds through path X, the energy required for formation
Fig. 2. Computed energy profile for the reaction of phenyl isocyanate with benzaldehyde in the presence of M(OMe)4 (8b, M = Ti, Zr, Hf) through path X and path Y done at the
B3LYP/LANL2DZ level of theory.

A. Kumar, A.G. Samuelson / Journal of Organometallic Chemistry 695 (2010) 338–345
343
Table 4
5.3. Metathesis reaction of phenyl isocyanate and benzaldehyde using
stoichiometric amounts of titanium(IV) n-butoxide (100 mol%)
Comparison of the C–N and C–O bond distances in intermediate 9 for isocyanates
with varying electronic demands computed at the B3LYP/LANL2DZ level of theory.
Phenyl isocyanate (0.035 g, 0.3 mmol) and benzaldehyde
(0.03 g, 0.3 mmol) were mixed in dry THF (10 ml) and to this solu-
tion was added Ti(OnBu)4 (100
l
l, 0.3 mmol). The reaction mixture
was stirred at room temperature for 60 h. It was quenched by a
stoichiometric amount of water (10 l, 0.6 mmol) and stirred for
l
1 h. The solvent was then removed under vacuum to give a solid
residue. This residue contains 3a and 4. Their ratio in the crude
product was determined by 1H NMR. These compounds were sep-
arated by preparative TLC using 1% ethyl acetate in petroleum
ether as eluent. Yield of imine 3a = 0.010 g (18%).
0
0
Entry
1
Complex
Ar
d(C2–O3) (AÅ)
d(C2–N1) (AÅ)
9bTiO
1.49
1.35
2
3
9bTi
1.45
1.36
9bTiF
1.44
1.36
5.4. Metathesis reaction of phenyl isocyanate and benzaldehyde using
substoichiometric amounts of titanium(IV) n-butoxide (25 mol%)
4. Conclusions
Phenyl isocyanate (0.035 g, 0.3 mmol) and benzaldehyde
(0.03 g, 0.3 mmol) were mixed in dry THF (10 ml) and to this solu-
A catalytic room temperature metathesis reaction of aromatic
aldehydes and aryl isocyanates has been discovered. The metathe-
sis reaction proceeds via insertion of an alkoxide into a heterocu-
mulene. Contrary to conventional synthesis of imines from
aldehydes and amines where water is the by-product, this method
involves only carbon dioxide elimination. This makes water sensi-
tive and heat sensitive aldimines accessible through this route.
Titanium alkoxides were found to promote catalytic metathesis
(isocyanates and aldehydes) more efficiently than zirconium alk-
oxides and hafnium alkoxides. Computational studies on model
metal alkoxides done at the B3LYP/LANL2DZ level of theory are
helpful in understanding this reactivity pattern and the reaction
path. There are two ways by which metathesis could be formally
achieved. The reactivity pattern for substituted isocyanates is con-
sistent with an insertion into the C–O bond (bond between the car-
bonyl carbon and alkoxy oxygen in the carbamate group). Based on
computational results, it is likely that the C–O bond breaks and
inserts an aldehyde. The insertion product formed by Zr and Hf
complex is extremely stable compared to that formed by Ti. Subse-
quent insertion of aldehyde to give imine is computed to be ener-
getically unfavorable for Zr and Hf but not for Ti. This explains why
titanium alkoxides are better metathesis catalysts in comparison
with their heavier congeners.
tion was added Ti(OnBu)4 (25
l
l, 0.075 mmol). The reaction mix-
ture was stirred at room temperature for 60 h. It was quenched
by a stoichiometric amount of water (2.5 l, 0.15 mmol) and stir-
l
red for 1 h. The solvent was then removed under vacuum to give
a solid residue. This residue contains 3a and 4. Their ratio in the
crude product was determined by 1H NMR. These compounds were
separated by preparative TLC using 1% ethyl acetate in petroleum
ether as eluent. Yield of the imine 3a = 0.013 g (24%).
5.5. Metathesis reaction of phenyl isocyanate and benzaldehyde using
substoichiometric amounts of titanium(IV) n-butoxide (10 mol%)
Phenyl isocyanate (0.035 g, 0.3 mmol) and benzaldehyde
(0.03 g, 0.3 mmol) were mixed in dry THF (10 ml) and to this
solution was added Ti(OnBu)4 (10
l
l, 0.03 mmol). The reaction
mixture was stirred at room temperature for 60 h. It was quenched
by stoichiometric amount of water (1 l, 0.06 mmol) and
a
l
stirred for 1 h. The solvent was then removed under vacuum to
give a solid residue. This residue contains 3a, 4 and 5. Their ratio
in the crude product was determined by 1H NMR. These
compounds were separated by preparative TLC using 1% ethyl ace-
tate in petroleum ether as eluent. Yield of the imine 3a = 0.023 g
(44%).
5. Experimental
5.1. General
5.6. Metathesis reaction of phenyl isocyanate and benzaldehyde using
catalytic amounts of titanium(IV) n-butoxide (1 mol%)
All manipulations were carried out under an inert atmosphere
of dry nitrogen using a standard double manifold. Tetrahydrofuran
was freshly distilled from sodium/benzophenone prior to use.
Zirconium(IV) n-butoxide (84 wt% in 1-butanol), titanium(IV)
n-butoxide, titanium(IV) isopropoxide, hafnium(IV) n-butoxide,
phenyl isocyanate, benzaldehyde, p-anisaldehyde, p-cyanobenzal-
dehyde, salicylaldehyde, p-methoxyphenyl isocyanate, p-flurophe-
nyl isocyanate and acetophenone were obtained from Aldrich USA.
The butanol solution of zirconium(IV) n-butoxide was evaporated
to dryness under vacuum at 110 °C prior to use.
Phenyl isocyanate (0.35 g, 2.9 mmol) and benzaldehyde (0.31 g,
2.9 mmol) were mixed in dry THF (10 ml) and to this solution was
added Ti(OnBu)4 (10
l
l, 0.03 mmol). The reaction mixture was stir-
red at room temperature for 60 h. It was quenched by a stoichiom-
etric amount of water (1 l, 0.06 mmol) and stirred for 1 h. The
l
solvent was then removed under vacuum to give a solid residue.
This residue contains 3a and trace amount of 4. Their ratio in the
crude product was determined by 1H NMR. The solid was dissolved
in petroleum ether containing 1% ethyl acetate and passed through
a silica plug. The solvent was then removed by distillation under
reduced pressure to give the imine 3a. Yield of the imine
3a = 0.383 g (72%).
Similar procedure was used for the metathesis of other imines
and aldehydes with varying electronic demands to obtain imines
3b–j and the corresponding yields are given in Table 2.
5.2. Physical measurements
1H NMR and 13C NMR spectra were recorded on Bruker AMX
400 operating at 400 MHz for 1H NMR and 100 MHz for 13C NMR,
with tetramethylsilane as internal reference. All spectra were re-
corded in CDCl3.

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A. Kumar, A.G. Samuelson / Journal of Organometallic Chemistry 695 (2010) 338–345
5.7. Metathesis reaction of phenyl isocyanate and benzaldehyde using
catalytic amounts of titanium(IV) n-butoxide (0.1 mol%)
5.10.2. Compound 3b: N-(4-methoxybenzylidene)aniline
1H NMR (400 MHz) d 8.36 (s, 1H), d 7.83 (d, 2H, J = 8.0 Hz), d
7.35 (m, 2H), d 7.18 (m, 3H), d 6.95 (d, 2H, J = 8.0 Hz), d 3.84 (s,
Phenyl isocyanate (0.35 g, 2.9 mmol) and benzaldehyde (0.31 g,
2.9 mmol) were mixed in dry THF (10 ml) and to this solution was
3H). 13C NMR (100 MHz)
d 162.20, 159.71, 152.24, 130.50,
129.21, 129.15, 125.56, 120.80, 114.13, 55.47.
added Ti(OnBu)4 (1
l
l, 0.003 mmol). The reaction mixture was stir-
red at room temperature for 60 h. It was quenched by a stoichiom-
etric amount of water (0.1 l, 0.006 mmol) and stirred for 1 h. The
5.10.3. Compound 3c: N-(4-cyanobenzylidene)aniline
l
1H NMR (400 MHz) d 8.49 (s, 1H), d 8.00 (d, 2H, J = 6.4 Hz), d
solvent was then removed under vacuum to give a solid residue.
This residue contains 3a and trace amount of 4. Their ratio in the
crude product was determined by 1H NMR. The solid was dissolved
in petroleum ether containing 1% ethyl acetate and passed through
a silica plug. The solvent was then removed by distillation under
reduced pressure to give the imine 3a. Yield of the imine
3a = 0.404 g (76%).
7.76 (d, 2H, J = 6.4 Hz), d 7.41 (m, 2H), 7.23 (m, 3H). 13C NMR
(100 MHz)
d 158.38, 151.48, 140.42, 133.03, 129.82, 129.60,
127.43, 121.44, 118.98, 114.85.
5.10.4. Compound 3d: N-(4-cyanobenzylidene)4-fluoroaniline
1H NMR (400 MHz) d 8.49 (s, 1H), d 7.99 (d, 2H, J = 8.0 Hz), d
7.75 (d, 2H, J = 12.0 Hz), d 7.24 (m, 2H), d 7.09 (m, 2H). 13C NMR
(100 MHz) d 163.07, 160.62, 157.48, 146.98, 139.86, 132.58,
129.08, 122.60, 118.41, 116.24, 116.01, 114.52.
5.8. Metathesis reaction of p-methoxyphenyl isocyanate and p-
cyanobenzaldehyde using catalytic amounts of titanium(IV) n-
butoxide (0.1 mol%)
5.10.5. Compound 3e: N-(4-cyanobenzylidene)4-methoxyaniline
1H NMR (400 MHz) d 8.52 (s, 1H), d 7.98 (m, 2H), d 7.73 (m, 2H),
d 7.29 (m, 2H), d 6.94 (m, 2H), d 3.84 (s, 3H). 13C NMR (100 MHz) d
159.24, 155.51, 143.84, 140.43, 132.62, 128.96, 124.53, 122.64,
118.67, 114.72, 114.06, 55.65.
p-Methoxyphenyl isocyanate (0.43 g, 2.9 mmol) and p-cyano-
benzaldehyde (0.38 g, 2.9 mmol) were mixed in dry THF (10 ml)
and to this solution was added Ti(OnBu)4 (1
l
l, 0.003 mmol). The
reaction mixture was stirred at room temperature for 60 h. It
was quenched by a stoichiometric amount of water (0.1 l,
l
5.10.6. Compound 3f: N-benzylidene-4-methoxyaniline
1H NMR (400 MHz) d 8.47 (s, 1H), d 7.90 (m, 2H), d 7.45 (m, 3H),
d 7.23 (m, 2H), d 6.92 (m, 2H), d 3.81 (s, 3H). 13C NMR (100 MHz) d
158.30, 144.91, 136.47, 131.04, 128.75, 128.52, 122.21, 114.30,
55.47.
0.006 mmol) and stirred for 1 h. The solvent was then removed un-
der vacuum to give a solid residue. The solid was dissolved in
petroleum ether containing 1% ethyl acetate and passed through
a silica plug. The solvent was then removed by distillation under
reduced pressure to give the imine 3e. Yield of the imine
3e = 0.581 g (86%).
When the same reaction when repeated with 0.1 mol% of tita-
nium(IV) isopropoxide, zirconium(IV) n-butoxide and hafnium(IV)
n-butoxide the yields of imine 3e were 77%, 8% and 16%, respec-
tively. Comparison of the 1H NMR spectra of the crude product
for these reactions that indicate the % conversion of aldehyde 2e
to the imine to 3e are given in Fig. 1 and Table 3.
5.10.7. Compound 3g: N-(4-methoxybenzylidene)4-methoxyaniline
1H NMR (400 MHz) d 8.39 (s, 1H), d 7.82 (d, 2H, J = 6.8 Hz), d
7.21 (d, 2H, J = 8.8 Hz), d 6.96 (d, 2H, J = 7.2 Hz), d 6.92 (d, 2H,
J = 6.8 Hz), d 3.85 (s, 3H), d 3.81 (s, 3H). 13C NMR (100 MHz) d
162.48, 158.39, 145.75, 130.73, 129.97, 122.56, 114.83, 114.63,
55.97, 55.89.
5.10.8. Compound 3h: N-(4-methoxybenzylidene)4-fluoroaniline
1H NMR (400 MHz) d 8.34 (s, 1H), d 7.83 (d, 2H, J = 6.8 Hz), d
7.16 (m, 2H), d 7.07 (m, 2H), d 6.98 (d, 2H, J = 7.2 Hz), d 3.86 (s,
5.9. Reaction of N-(benzylidene)aniline with carbon dioxide in the
presence of titanium(IV) n-butoxide
3H). 13C NMR (100 MHz)
d 162.78, 160.27, 148.85, 130.97,
Into 8 ml THF in a sealable vial flushed with nitrogen, carbon
dioxide (117.0 ml, 4.8 mmol) from a gas burette was condensed.
A mixture of titanium(IV) n-butoxide (0.3 ml, 0.8 mmol) and N-
(benzylidene)aniline (0.16 g, 0.8 mmol) in 2 ml THF was added
while maintaining the vial under liquid nitrogen. The vial was sub-
sequently evacuated and sealed. The sealed vial was heated in an
oil bath maintained at 180 °C for 72 h. The vial was broken open
and the reaction mixture evacuated to dryness under vacuum.
The mixture was then worked up by addition of stoichiometric
amount of water (to precipitate out TiO2), followed by extraction
of organic fraction with dichloromethane. Solvent was then evap-
orated to dryness from organic fraction. 1H NMR analysis of the
crude product indicated that the reaction did not proceed to com-
pletion and only 12% conversion to the metathesis product benzal-
dehyde 2a was observed. The other product phenyl isocyanate 1a
was only isolated as the insertion product 4 in an equivalent
amount.
129.59, 122.75, 122.67, 116.38, 116.16, 114.70, 55.91.
5.10.9. Compound 3i: N-benzylidene-4-fluoroaniline
1H NMR (400 MHz) d 8.46 (s, 1H), d 7.89 (m, 2H), d 7.44 (m, 3H),
d 7.19 (m, 2H), d 7.07 (m, 2H). 13C NMR (100 MHz) d 162.94,
160.69, 148.53, 136.54, 131.96, 129.29, 122.84, 122.77, 116.47,
116.25.
5.10.10. Compound 3j: N-(2-hydroxybenzylidene)aniline
1H NMR (400 MHz) d 13.29 (s, 1H), d 8.63 (s,1H), d 7.42 (m, 4H),
d 7.30 (m, 3H), d 7.02 (d, 1H, J = 8.0 Hz), d 6.93 (t, 1H, J = 7.6 Hz). 13C
NMR (100 MHz) d 162.73, 161.15, 148.53, 133.19, 132.32, 129.45,
126.94, 121.21, 119.23, 119.11, 117.29.
5.10.11. Compound 4: butyl N-phenylcarbamate [10]
1H NMR (400 MHz) d 7.37 (d, 2H, J = 8 Hz), d 7.30 (t, 2H,
J = 8 Hz), d 7.05 (t, 1H, J = 7.3 Hz), d 6.63 (s, 1H, NH), d 4.17 (t, 2H,
J = 6.7 Hz),
d 1.65 (m, 2H), d 1.42 (m, 2H), d 0.95 (t, 3H,
5.10. Spectral data
J = 7.3 Hz). 13C NMR (100 MHz) d 153.78, 138.02, 129.10, 123.37,
118.63, 65.18, 31.02, 19.14, 13.30.
5.10.1. Compound 3a: N-(benzylidene)aniline
1H NMR (400 MHz) d 8.50 (s, 1H), d 7.95 (d, 2H, J = 4.0 Hz), d
7.48 (d, 3H, J = 4.0 Hz), d 7.45 (t, 2H, J = 8.0 Hz), d 7.28 (m, 3H).
13C NMR (100 MHz) d 160.50, 152.11, 136.23, 131.44, 129.33,
129.20, 125.99, 120.91.
5.10.12. Compound 5: butyl 2,4-diphenylallophanate [10]
1H NMR (400 MHz) d 10.91 (s, 1H, NH), d 7.53 (d, 2H, J = 7.6 Hz),
d 7.41 (m, 3H), d 7.31 (t, 2H, J = 7.6 Hz), d 7.22 (d, 2H, J = 7.6 Hz), d
7.09 (t, 1H, J = 7.6 Hz), d 4.15 (t, 2H, J = 6.8 Hz), d 1.48 (m, 2H), d

A. Kumar, A.G. Samuelson / Journal of Organometallic Chemistry 695 (2010) 338–345
345
1.18 (m, 2H), d 0.82 (t, 3H, J = 7.6 Hz). 13C NMR (100 MHz) d 156.08,
151.58, 137.77, 137.10, 128.96, 128.70, 128.26, 123.96, 119.92,
67.22, 30.31, 18.78, 13.47.
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Organometallics 18 (1999) 5729;
5.11. Computational details
For theoretical studies, Ti(OMe)4, Zr(OMe)4 and Hf(OMe)4 were
taken as model complexes in place of Ti(OnBu)4, Zr(OnBu)4 and
Hf(OnBu)4 where bulky nBu groups are replaced by Me groups for
computational efficiency. All structures were optimized using the
DFT method (B3LYP/LANL2DZ), based on Becke’s three-parameter
functional [14]. The LANL2DZ basis set uses the effective core
potentials (ECP) of Hay and Wadt [15]. The nature of the stationary
points was characterized by vibrational frequency calculations. The
GAUSSIAN 03 program package was used for all calculations [16].
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(d) R. Xi, L.R. Sita, Inorg. Chim. Acta 270 (1998) 118;
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Acknowledgements
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53;
A.G.S. thanks DST for the award of a research grant and A.K.
gratefully acknowledges a senior research fellowship from CSIR.
We also thank Prof. E.D. Jemmis, Susmita De and Shrabani Dinda
for their valuable suggestions with Gaussian calculations. We
thank the DST, New Delhi, for providing us funds through its FIST
program for purchase of a 400 MHz NMR spectrometer.
(b) H. Heinz, S. Klaus, Z. Naturforsch., Teil B: Anorg. Chem., Org. Chem. 39B
(1984) 1032;
(c) B. Pierre, D. Nobel, Chem. Rev. 89 (1989) 1927.
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(b) S.C. Bart, C. Anthon, F.W. Heinemann, E. Bill, N.M. Edelstein, K. Meyer, J.
Am. Chem. Soc. 130 (2008) 12536.
[10] (a) R. Ghosh, M. Nethaji, A.G. Samuelson, J. Organomet. Chem. 690 (2005)
1282;
(b) A. Kumar, S. De, A.G. Samuelson, E.D. Jemmis, Organometallics 27 (2008)
955.
Appendix A. Supplementary data
[11] Values obtained from <http://www.webelements.com>.
[12] R. Ghosh, M. Nethaji, A.G. Samuelson, Chem. Commun. (2003) 2556.
[13] (a) P.N.V. Pavan Kumar, E.D. Jemmis, J. Am. Chem. Soc. 110 (1988) 126;
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[14] (a) A.D. Becke, J. Chem. Phys. 98 (1993) 5648;
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2009.10.044.
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