Reversible double insertion of aryl isocyanates into the Ti-O bond of titanium(IV) isopropoxide

Article abstract of DOI:10.1016/j.jorganchem.2004.11.038

The insertion of phenyl isocyanate into titanium isopropoxide leads to the formation of a dimeric complex [Ti(OⁱPr)₂(μ-O ⁱPr){C₆H₅N(OⁱPr)CO}]₂ (1) which has been structurally characterized. Reaction of titanium isopropoxide with two and more than 2 equiv. of phenyl isocyanate is complicated by competitive, reversible insertion between the titanium carbamate and titanium isopropoxide. The ligand formed by insertion of phenyl isocyanate into the titanium carbamate has been structurally characterized in its protonated form C₆H₅N{C(OⁱPr)O}C(O)N(H)C₆H ₅ (3aH). Insertion into the carbamate is kinetically favored whereas insertion into isopropoxide gives the thermodynamically favored product.

Full text of DOI:10.1016/j.jorganchem.2004.11.038

Journal of Organometallic Chemistry 690 (2005) 1282–1293
www.elsevier.com/locate/jorganchem
Reversible double insertion of aryl isocyanates into the Ti–O bond
of titanium(IV) isopropoxide
*
Rajshekhar Ghosh, Munirathinam Nethaji, Ashoka G. Samuelson
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Malleswaram, Bangalore 560 012, India
Received 6 October 2004; accepted 22 November 2004
Abstract
The insertion of phenyl isocyanate into titanium isopropoxide leads to the formation of a dimeric complex [Ti(OiPr)2(l-
OiPr){C6H5N(OiPr)CO}]2 (1) which has been structurally characterized. Reaction of titanium isopropoxide with two and more than
2 equiv. of phenyl isocyanate is complicated by competitive, reversible insertion between the titanium carbamate and titanium iso-
propoxide. The ligand formed by insertion of phenyl isocyanate into the titanium carbamate has been structurally characterized in
its protonated form C6H5N{C(OiPr)O}C(O)N(H)C6H5 (3aH). Insertion into the carbamate is kinetically favored whereas insertion
into isopropoxide gives the thermodynamically favored product.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Aryl isocyanates; Multiple insertion; Titanium isopropoxide; Reversible insertion
1. Introduction
larly, incorporation of neutral polar substrates like
CO2, CS2, isocyanates, and isothiocyanates, into poly-
mers could be of great value in synthesizing functional-
ized polymers [5]. While insertions of polar
heterocumulenes into M–N [6], M–C [7] and M–O [8]
have been studied with a variety of metals, especially
with titanium [9,10], and zirconium [11], multiple inser-
tions of heterocumulenes have not been studied exten-
sively [12,13]. Well characterized reactions involving
multiple insertions are mainly those of alkynes
[12a,14], nitriles [15], isocyanides [16], and carbon mon-
oxide [17]. Apart from these instances, only a few exam-
ples of alkenes that stop at double insertions [18] are
known. Notably, structurally characterized products
from titanium alkoxide insertion into heterocumulenes
are very few [19].
Homogeneous catalytic processes often involve inser-
tion of neutral substrates into transition metal-X bonds
as one of the key steps, where X is carbon, oxygen, or
nitrogen [1]. In the metal promoted polymerization of
olefins for example, multiple insertion of the olefin into
the metal alkyl group is the key step in the propagation
of the polymer chain [2]. Consequently, mechanistic
studies involving the insertion of a M–X group into a
neutral substrate are of great importance in unraveling
factors that promote or deter multiple insertion [3]. Re-
cently the elegant work of Jordan and coworkers [4]
involving the multiple insertion of chloro alkenes has
brought to light complications involved in polymeriza-
tion. A good understanding of this key reaction would
permit one to control this reaction and enable design
of catalysts for polymerization of new substrates. Simi-
In this study we show how insertion reactions of aryl
isocyanates follow complex pathways due to opposing
electronic and steric requirements. In the presence of
isopropoxide and carbamate, insertion of aryl isocya-
nate into carbamate is kinetically favored over direct
insertion into isopropoxide. This results in a product
*
Corresponding author. Tel.: +09180 2293 2663; fax: +09180 2360
1552.
E-mail address: ashoka@ipc.iisc.ernet.in (A.G. Samuelson).
0022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.11.038

R. Ghosh et al. / Journal of Organometallic Chemistry 690 (2005) 1282–1293
1283
that has undergone double insertion of aryl isocyanate
and is thermodynamically uphill. The product of double
insertion slowly reverts to give the carbamate (the mono
insertion product).
2. Results and discussion
2.1. Insertion of phenyl isocyanate
Mehrotra and coworkers [9b] had shown by ebulio-
scopic measurements that treatment of titanium iso-
propoxide with 1 equiv. of an isocyanate leads to the
formation of a dimeric complex. However, Meth-Cohn
Fig. 1. Structure of compound 1. Methyl groups on the isopropyl
moiety have been omitted for clarity.
1
et al. [10]. had recorded the H NMR of the reaction
of titanium isopropoxide with 1 equiv. of phenyl isocy-
anate (Eq. (1)) in carbon tetrachloride. They could dis-
tinguish only two methine protons contrary to what is
expected for 1.
˚
with the Ti–N distance being 2.11 A and Ti–O distance
˚
2.24 A. The N–C–O angle was found to be 115.6ꢁ and
i Pr
Ph
i Pr
i Pr
i Pr
i Pr
i Pr
O
O
the C–Ti–N angle was found to be 60.2ꢁ. The C–N
and the C–O distance of the NCO unit was intermediate
N
O
O
O
O
O
THF
O
Ti
Ph
N O
C
+
O
Ti
Ti
˚
between a single and a double bond at 1.33 and 1.25 A,
O
Room Temp
i Pr
O
iPr
i Pr
N
O
O
respectively, due to delocalization of the double bond.
The plane of the phenyl ring was found to be 36ꢁ with
respect to the plane containing the Ti–N–C–O. In an
analogous reaction of carbon dioxide, it was recently
found that the carbonate prefers a bridging mode [20].
Carboxylate groups also prefer to have a bridging mode
O
i Pr
i Pr
i Pr
Ph
1
ð1Þ
We recorded 1H NMR of this reaction mixture in
CDCl3 and found three septets at 4.49, 4.76 and 5.10
ppm. We assign these to the methine proton of isoprop-
oxide which is either terminal, bridged, or on the carba-
mate (obtained by the insertion of phenyl isocyanate
into the titanium isopropoxide), respectively. The dis-
tinctive resonances in CDCl3 are consistent with the di-
meric structure and permit us to follow the reaction
much better. The assignments of the resonances are
based on the preferential hydrolysis of the carbamate
in complex 1.
1
[21]. The higher symmetry in solution required by H
NMR spectrum is presumably due to fluxional behavior.
Addition of a second equivalent of phenyl isocyanate
led to the formation of a complex (Eq. (2)) which had
four sets of methine resonances: one at 5.10 ppm, two
sets between 4.88 and 4.70 ppm and one at 4.49 ppm
in the ratio 3.6:3:3. The peaks at 5.10 and 4.49 ppm
are assigned to the isopropoxide on the carbamate in a
chelated fashion and the terminal isopropoxide on the
titanium in accordance with what had been observed
for complex 1. In the presence of trace quantities of
moisture, 1H NMR of the reaction mixture also showed
a peak at 5.01 ppm due to partial hydrolysis. The com-
When a solution of complex 1 in petroleum ether was
cooled to ꢀ20 ꢁC, single crystals suitable for X-ray anal-
ysis were obtained (Fig. 1). A crystallographic investiga-
tion was taken up to confirm the nuclearity and bonding
modes proposed for the complex. The molecular struc-
ture of compound 1 is centro-symmetric. (Crystallo-
graphic parameters are given in Table 1.) The most
interesting feature of this molecule is the chelating nat-
ure of the carbamate. (Selected bond lengths and bond
angles are given in Table 2.) The asymmetric unit con-
tains only one half of the complete structure, containing
two terminal isopropoxides, one bridging isopropoxide
and one carbamate. Both titanium atoms are six coordi-
nate. The terminal isopropoxides are at a distance of
1
plex H NMR pattern observed is not consistent with a
simple structure proposed for 2. As this complex could
not be crystallized by cooling or precipitation for carry-
ing out further characterization, it was decomposed by
1
addition of water. On deliberate hydrolysis, H NMR
of the solution showed two sets of methine resonances
at 5.01 and at 4.00 ppm assigned to the isopropyl group
of the carbamic acid ester and isopropanol in the ratio
1:1. This shows that the reaction mixture indeed has
two isocyanates inserted into two of the isopropoxides
on the titanium.
1
˚
1.77 A from the titanium. The bridging isopropoxides
are not equidistant from both titanium atoms, the dis-
On the basis of the new peaks in H NMR in the
range 4.85–4.70 ppm, it is unlikely that the proposed
complex 2 has a static structure as shown in Eq. (2).
˚
tances are 1.93 and 2.11 A. The carbamate is chelated

1284
R. Ghosh et al. / Journal of Organometallic Chemistry 690 (2005) 1282–1293
Table 1
Crystallographic data for compound 1 and 3aH
Compound
1
3aH
Empirical formula
Formula weight
Temperature (K)
C19H33NO5Ti
403.36
293(2)
C17H18N2O3
298.33
293(2)
˚
Wavelength (A)
0.71073
Orthorhombic
Pccn
0.71073
Orthorhombic
Pbca
Crystal system
Space group
Unit cell dimensions
˚
a (A)
18.521(4)
23.791(7)
8.894(4)
16.300(7)
˚
b (A)
˚
c (A)
10.340(5)
4556(3)
22.681(10)
3288(3)
3
˚
Volume (A )
Z
8
8
Color and shape
Colorless, hexagonal
0.401
1728
Colorless, needle
0.084
1264
Absorption coefficient (mmꢀ1
F(000)
Diffractometer
)
Enraf Nonius CAD4
0.40 · 0.39 · 0.38
1.39–24.98
SMART CCD Area Detector
0.44 · 0.18 · 0.06
1.80–26.06
Crystal size (mm3)
Theta range for data collection (ꢁ)
Reflections collected
4003
24,130
Independent reflections (Rint
Completeness to theta
Absorption correction
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
)
4003 (0.0000)
24.98ꢁ = 100.0%
Empirical (DIFABS)
0.923 and 0.4824
4003/0/235
3231 (0.1060)
26.06ꢁ = 99.7%
Empirical
0.995 and 0.961
3231/0/271
1.012
1.050
Final R indices [I > 2r(I)]
R indices (all data)
Largest diff. peak and hole (e/A )
R1 = 0.0794, wR2 = 0.2078
R1 = 0.1882, wR2 = 0.3109
0.456 and ꢀ0.480
R1 = 0.0599, wR2 = 0.1317
R1 = 0.1142, wR2 = 0.1554
0.130 and ꢀ0.119
3
˚
Table 2
Bond lengths and bond angles for compound 1
out and also the relative intensities of these peaks de-
crease. The bridged isopropoxide appears at 4.76 ppm,
as in complex 1. Ratios of the peaks in the three regions
keep changing and at ꢀ40 ꢁC it was found to be 2:1:2.
Concomitant gain in the peaks corresponding to the
chelated species is observed. This process was found to
be reversible and on returning to room temperature,
the ratio of the peaks in the three regions was found
to be 3.6:3:3 again.
Ti(1)–O(1)
1.934(4)
2.112(4)
2.116(6)
2.236(5)
3.233(2)
1.258(8)
1.303(8)
1.328(8)
O(1)–Ti(1)–O(1)#1
O(1)–Ti(1)–N(1)#1
O(1)#1–Ti(1)–N(1)#1
O(1)–Ti(1)–O(4)#1
O(1)#1–Ti(1)–O(4)#1
N(1)#1–Ti(1)–O(4)#1
Ti(1)–O(1)–Ti(1)#1
C(10)–N(1)–Ti(1)#1
74.0(2)
145.6(2)
87.2(2)
Ti(1)–O(1)#1
Ti(1)–N(1)#1
Ti(1)–O(4)#1
Ti(1)–Ti(1)#1
O(4)–C(10)
O(5)–C(10)
N(1)–C(10)
89.00(19)
84.33(18)
60.1(2)
106.0(2)
94.1(4)
The proposed complex 2 may be in dynamic equilib-
rium with other complexes as shown in Scheme 1.
The new peaks present in the proposed complex 2,
which were not present in complex 1, were reduced in
intensity on lowering the temperature.
2.2. Multiple insertions
i Pr
Ph
Titanium has two isopropoxides on the proposed
complex 2 that are still capable of undergoing further
insertion reactions. The reaction mixture obtained on
treatment of titanium isopropoxide with 3 equiv. of phe-
O
i Pr
i Pr
N
C
THF
O
1
Ph
N
C
O
O
O
+
Ti
Room Temp
O
1
nyl isocyanate was analyzed by H NMR. It showed
N
C
O
three sets of methine resonances at 5.10, 5.01 and 4.76
ppm in the ratio of 3:6:4 whereas the reaction mixture
from the reaction of 4 equiv. of phenyl isocyanate had
the same sets in the ratio 2:7:2. The peak at 5.10 ppm
is for the chelated carbamate and the peak at 4.76
ppm is a mixture of the bridged isopropoxide and the
monodentate carbamate based on assignments made
for the proposed complex 2. The peak at 5.01 ppm is
iPr
Ph
2
ð2Þ
To aid structural assignment, variable temperature
NMR of 2 was attempted. At temperatures below ꢀ20
ꢁC, the two sets of peaks at 4.85–4.70 ppm separate

R. Ghosh et al. / Journal of Organometallic Chemistry 690 (2005) 1282–1293
1285
i Pr
i Pr
Pr
Ph
N
Ph
N
i
i Pr
O
O
Pr
O
iPr
i Pr
O
Ph
N
iPr
O
Ph
O
Ph
N
O
O
O
O
O
i Pr
O
O
O
Ti
N
2
O
Ti
O
Ti
O
i
O
O
O
N
O
Pr
Ti
Ti
N
O
O
N
Ph
O
i Pr
N
O
O
i Pr
i Pr
O
Ph
O
O
N
i Pr
O
i
i Pr
i Pr
Ph
i Pr
iPr
Pr
Ph
iPr
2b
2a
Ph
2c
Scheme 1. Hypothetical solution-structures of dinuclear alkoxide-bridged species in equilibrium with the proposed compound 2.
Ph
i Pr
O
N
Ph
N
i Pr
i Pr
N
i Pr
O
O
Ph
i
i
C
Pr
C
C
O
O
Pr
Pr
O
O
O
N
X
O
Ti
N C O
n
O
+
O
O
i
O
O
Pr
Pr
O
+
Ti
Ti
i Pr
C
N
O
O
N
C
O
i
i
Pr
Ph
Ph
i
i Pr
Ph
n = 3 or 4
H2O
X
O
X
O
C
N
+
X
TiO2
N
C
C
+
N
O
O
H
H
O
3aH X = H
3bH X = 4-Me
3cH X = 4-OMe
3dH X = 4-F
4aH X = H
4bH X = 4-Me
4cH X = 4-OMe
4dH X = 4-F
4eH X = 2-Me
3eH X = 2-Me
Scheme 2. Double insertion of aryl isocyanate and the organic products isolated.
obtained by the superimposition of septets resulting
from the protonated carbamate (4aH) and the com-
pound obtained upon insertion of the phenyl isocyanate
into the titanium carbamate resulting in the formation
of the ligand {C6H5NꢀC(O)N(C6H5)C(O)OiPr} bound
to the titanium through the nitrogen. These assignments
were supported by the hydrolysis studies on these reac-
tion mixtures. The complexes were hydrolyzed (Scheme
intramolecularly hydrogen bonded to O2 in compound
3aH and resonates at 10.9 ppm (selected bond lengths
and bond angles are given in Table 3).
The double insertion of aryl isocyanate into isoprop-
oxide leading to the formation of coordinated 3a
1
2) and H NMR of the organic products showed peaks
at 10.9 ppm, in addition to those expected for the car-
bamic acid and isopropanol.
Fortuitously, the organic product having the unusu-
ally low field peak at 10.9 ppm, crystallized out. X-ray
analysis showed it to be the product 3aH (Fig. 2),
formed by the insertion of a second phenyl isocyanate
into the carbamate. Multiple insertion of alkyl isocya-
nate into isopropoxide had been suggested by Meth-
Cohn et al. [10] as a product formed in the presence of
a large excess of the isocyanate. Crystallographic char-
acterization also revealed the cause of the low field res-
onance, the hydrogen on the nitrogen (H2) is
Fig. 2. Molecular structure of compound 3aH. Hydrogen atoms on
carbon have been omitted for clarity.

1286
R. Ghosh et al. / Journal of Organometallic Chemistry 690 (2005) 1282–1293
Table 3
Bond lengths and bond angles for compound 3aH
O(1)–C(1)
O(2)–C(2)
O(3)–C(2)
N(1)–C(2)
N(1)–C(1)
N(2)–C(1)
N(2)–H(2)
H(2)ꢁ ꢁ ꢁO(2)
1.210(3)
1.203(2)
1.329(3)
1.385(3)
1.421(3)
1.342(3)
0.88(3)
C(2)–N(1)–C(1)
O(1)–C(1)–N(2)
O(1)–C(1)–N(1)
N(2)–C(1)–N(1)
O(2)–C(2)–O(3)
O(2)–C(2)–N(1)
O(3)–C(2)–N(1)
N(2)–H(2)ꢁ ꢁ ꢁO(2)
125.79(19)
125.0(2)
118.2(2)
116.8(2)
124.8(2)
125.0(2)
110.2(2)
133.9(19)
1.98(2)
Fig. 3. Concentration of 3aH as a function of time on addition of
PhNCO to (a) 2 ! 5 and (b) 5 ! 6.
suggests a competitive insertion of phenyl isocyanate
between the titanium-isopropoxide and titanium-
carbamate (4a) (Scheme 2).
These concentration profiles show that the multiple
insertion is a reversible process and not an irreversible
competitive reaction to give 5 and 50. In fact formation
of 5, having coordinated 4a, probably follows two differ-
ent pathways (Scheme 3). One of them involves a revers-
ible insertion of the incoming isocyanate into the Ti–N
bond in preference to the Ti–O bond (process A) to give
50. This complex then gets converted into a complex
having two coordinated 4a ligands (proposed compound
5) by an intramolecular rearrangement (process B). The
alternative competing pathway (process C), is a revers-
ible double insertion followed by a direct insertion to
give a complex containing three coordinated 4a ligands
(proposed compound 5).
We then checked to see if the concentration ratios of
coordinated 3a and 4a are invariant as a function of
time. The reaction of compound 2 with 1 equiv. of phe-
nyl isocyanate (Eq. (3)) led to the formation of a mixture
of products (5 and 50). Concentration of coordinated 3a
and 4a was followed by hydrolysis with stoichiometric
1
quantities of water and H NMR analysis. The concen-
tration of 3aH reached a maximum and then showed a
steady decay (graph a, Fig. 3), whereas the concentra-
tion of 4aH increased with time. Ultimately the complex
had only 3 equiv. of 4a and 1 equiv. of isopropoxide
coordinated to titanium after a period of about 48 h.
This suggests that coordinated 3a has transferred one
phenyl isocyanate molecule into isopropoxide to form
a molecule with three coordinated 4a ligands.
The concentration profiles of 4aH showed a jump
upon addition of 1 equiv. of the phenyl isocyanate to
Ph
i Pr
O
N
Ph
N
O
Ph
i
i
N
Pr
THF
Pr
Pr
O
N
O
+
Ph
N
C
O
48 hours
2
O
O
O
i
O
O
Pr
Pr
O
5
+
Ti
Room Temp
ð3Þ
Ti
N
O
O
N
O
i
i
Pr
Ph
Ph
i
O
i Pr
5'
Ph
5
A similar formation and decay of 3aH (graph b, Fig. 3)
was observed when the proposed complex 5 containing
three coordinated 4a ligands and one isopropoxide was
treated with 1 equiv. of the isocyanate (Eq. (4)).
either 2 or 5. This jump corresponded to 57% of the
added isocyanate in the case of 2 and 38% in the case
of 5. Pathway A is competing with C favorably although
it leads to a less stable compound 50 (proposed). Subse-
iPr
Ph
iPr
i Pr
O
Ph
Ph
N
i Pr
O
O
N
Ph
O
O
N
O
O
O
N
N
Ph
THF
80 hours
Ti
O
O
Ph
N C O
5
+
6
O
Ti
+
O
O
i
ð4Þ
Room Temp
30 mins.
O
O
N
Pr
O
N
N
i Pr
Ph
i
Ph
Ph
Pr
i
Pr
6
6'

R. Ghosh et al. / Journal of Organometallic Chemistry 690 (2005) 1282–1293
1287
iPr
Ar
O
i
O
Pr
Ar
N
N
O
O
i Pr
i Pr
N
Ti
O
Ar
A
O
+
Ar
N
C
O
O
O
O
Ti
N
O
i
O
O
Ar
Pr
i Pr
N
Ar
2
iPr
O
5'
i Pr
C
Intra molecular
transfer of
isopropoxide
B
i Pr
Ar
O
O
i Pr
N
O
N
O
O
O
Ti
O
N
Pr
Ar
iPr
Ar
5
Scheme 3. Proposed intramolecular and reversible reinsertion pathway for the conversion of the double insertion product to the mono insertion
product.
quently the conversion of coordinated 3a into coordi-
nated 4a can take place either through pathways C or
B. The conversion of 2:5 and subsequently to 6 were also
studied with the 4-fluorophenyl isocyanate and similar
concentration changes were observed.
The double insertion is favored by kinetic factors
arising from the comparable nucleophilicity of the coor-
dinated nitrogen in comparison with the coordinated
oxygen. This proposal concurs with a few reports in
the literature where the preference for insertion of isocy-
anate into M–N bonds over insertions into M–O bonds
[22,23] has been reported. These studies had been con-
ducted with coordinated OR and NR2 (where R is an al-
kyl group). It is interesting that in the present system,
even an NAr (Ar = aryl) group attached to a COOR is
nucleophilic enough to compete with an OR (R = alkyl)
group. Subsequently, thermodynamic factors convert
the intermediate to form the more stable 5. Thus coordi-
nated 4a ligand functions as a molecular ferry, to trans-
fer the phenyl isocyanate to an isopropoxide.
Fig. 4. Steric requirements of two mono inserted ligands versus a
double inserted ligand and isopropoxide.
experiment was conducted (Scheme 4). The reaction of
titanium isopropoxide was carried out with 4 equiv. of
4-methylphenyl isocyanate. Conversion of 3b:4b was
1
1
confirmed by H NMR and took 108 h. A H NMR
spectrum of the organic fraction after deliberate hydro-
lysis of an aliquot of the reaction mixture showed the ra-
tio of 4bH to free isopropanol to be 7.4:1 and the ratio
of 3bH:4bH was 1:52. Concentration of 3bH was ob-
tained from NMR analysis of the peak at 10.9 ppm.
At this stage 2 equiv. of 4-fluorophenyl isocyanate was
added to the reaction mixture. The reaction was moni-
tored with time and showed the formation of the mixed
double insertion product 3f. As time progressed, forma-
tion of 3d and even 3g coordinated to titanium, was in-
ferred from analysis of the hydrolysis products.
Compound 3gH can only be formed if a second inser-
tion of 4-methylphenyl isocyanate was carried out on
4d. This reaction was monitored for 60 h. After which
the ratios of 4bH, 4dH and 3(b,d,f,g)H formed on
The conversion of 50 to 5, where the coordinated 3a
and isopropoxide is transformed to two 4a ligands, is
probably driven by the steric differences between the
two ligands. This steric demand of 3a can be visualized
by modeling studies based on the structure of complex 1.
The structure of a simulated complex is shown in Fig. 4
ꢂ
using the coordinates of 3aH in the program WEBLAB
VIEWER [24]. Coordinated 4a has a cone angle of 104ꢁ,
whereas coordinated 3a is estimated to have a cone an-
gle of 171ꢁ based on the crystal structure of 3aH. A cone
angle of 64ꢁ for the isopropoxide ligand is estimated in
the structure of complex 1. The steric congestion is re-
lieved by conversion of coordinated 3a with an isoprop-
oxide into two 4a ligands (Fig. 4).
1
hydrolysis with water were estimated by H NMR and
gas chromatography and are discussed below.
It was found that the reaction mixture contained 3
and 4 in the ratio 1:18. The products from the double
insertion of the aryl isocyanate (3dH, 3fH, 3bH and
To verify the reversibility of mono and double inser-
tion an experiment similar to the classical crossover

1288
R. Ghosh et al. / Journal of Organometallic Chemistry 690 (2005) 1282–1293
X'
O
X
i Pr
i Pr
i Pr
N
O
(i)
O
O
N
O
O
O
+
TiO2
N
H
Ti
+
Me
4
N O
C
O
+
H
O
(ii)
i Pr
X
3bH X = X' = Me
3dH X = X' = F
3fH X = F, X' = Me
4bH X = Me
4dH X = F
3gH X = Me, X' = F
Scheme 4. A pseudo crossover experiment to find out the reversibility of mono and double insertion. (i) After 108 h, 2 eq. 4-fluorophenyl isocyanate
was added and stirred for 60 h. (ii) Organic fraction extracted after hydrolysis with water.
3gH) were present in the ratio 1:0.9:1.6:1.2. The ratio of
4bH and 4dH was estimated by gas chromatography
and was found to be in the ratio 1:0.61. This shows that
the insertion of aryl isocyanate both into Ti–OiPr bond
and into the coordinated carbamate is completely
reversible. Electron withdrawing substituents in the para
position of the isocyanate appear to have favored the
insertion leading to a bias in the ratio of 4bH and
4dH. Further exploration of the electronic requirements
in the reaction follow.
4b (as simulated by WebLab Viewer shown in the sup-
porting information). This has interesting consequences
on the ratio of 3e:4e as a function of time. When tita-
nium isopropoxide was treated with 4 equiv. of 2-meth-
ylphenyl isocyanate, the ratio of 3eH:4eH in the organic
fraction reached 1:1.5 against 1:3.4 in the reaction with
4-methylphenyl isocyanate. Over time, the ratio of
3eH:4eH only fell to about 1:3.7 in 60 h. Whereas in
the reaction of 4-methylphenyl isocyanate, the ratio
3bH:4bH became 1:9.5 in the same time period. There
was no further drop in the concentration of 3eH,
whereas the 3bH:4bH ratio fell to 1:47 in 108 h. This
experiment shows how the conversion from 3 to 4 is
an equilibrium process controlled by steric require-
ments. Conversion from coordinated 3 to coordinated
4 does not provide as much steric relief in the case of
2-methylphenyl isocyanate as in the case of 4-methyl-
phenyl isocyanate.
2.3. Electronic effects on the formation of 3 and 4
A competition experiment was carried out with a
mixture of 4-methoxyphenyl isocyanate, 4-fluorophenyl
isocyanate and titanium isopropoxide in a 1:1:1 ratio
at 193 K. It was found that after 10 min., the ratio of
4dH:4cH was 72:28 which became 64:36 after 30 min
and 1:1 after 1 h. The insertion reaction is clearly faster
with 4-fluorophenyl isocyanate than with 4-methoxy-
phenyl isocyanate. A similar competition experiment be-
tween 4-methoxyphenyl isocyanate and 4-methylphenyl
isocyanate and titanium isopropoxide was carried out
by mixing them in the ratio 1:1:1 at 193 K. After 10
min the ratio of 4cH:4bH was found to be 64:36, which
slowly equilibrates to 1:1 ratio in 1 h showing that the
insertion is faster with 4-methoxyphenyl isocyanate
compared to 4-methylphenyl isocyanate. This implies
that only the inductive effect of the 4-methoxy group is
operational. If a resonance effect is in operation, then
it would have behaved as an electron-donating group
and its rate would have been slower. This contrasts with
the reactions between aryl isocyanates and 2-ethylhexa-
nol where a resonance effect is operative [25]. The pres-
ent result implies that the aryl isocyanate binds to
titanium with the lone pair prior to insertion, so that
the resonance effect is not operative.
3. Conclusion
We have shown that the insertion of a second mole-
cule of heterocumulene into a Ti–OiPr bond, takes place
by two different paths. The coordinated carbamate picks
up the free isocyanate and transfers it to a coordinated
isopropoxide. The comparable nucleophilicity of the
coordinated nitrogen makes insertion of aryl isocyanate
into the carbamate facile. The ligand formed from dou-
ble insertion of aryl isocyanate readily converts to two
coordinated carbamates if an isopropoxide is present.
The insertion reactions are favored by the polariza-
tion of the heterocumulene to form a positive charge
on the carbon. Thus the reaction is favored by electron
withdrawing groups on the para position of the isocya-
nates. The electron withdrawing effect is through
induction.
2.4. Steric effects on the formation of 3 and 4
The presence of an ortho-substituted arene prevents
the complete conversion of the coordinated double in-
serted product to the coordinated carbamate. Significant
amounts of the double inserted product remain at the
end of the reaction. Thus the second step is primarily
driven by steric requirements.
Surprisingly the steric demands of 3 and 4 do not
vary with substituents on the aromatic group in a simple
fashion. Steric requirements of 3e are about the same as
3b whereas 4e requires significantly greater space than

R. Ghosh et al. / Journal of Organometallic Chemistry 690 (2005) 1282–1293
1289
4. Experimental
an Enraf-Nonius CAD4 single crystal diffractometer
equipped with graphite-monochromatized Mo Ka radi-
ation, whereas data for compound 3aH was collected on
a Bruker AXS single crystal diffractometer equipped
with SMART APEX CCD detector and a sealed Mo
Ka source working at 1.75 kW. Intensity data were col-
lected at 293(2) K. For complex 1, accurate unit cell
parameters and orientation matrices were determined
by least-squares refinement of 25 well-centered reflec-
tions in the range 9ꢁ 6 h 6 13ꢁ. Three periodically mea-
sured reference reflections showed no significant decay
during the time of data collection. For compound 3aH
the SMART software was used for data acquisition
and the SAINT software for data extraction. The data
were corrected for Lorentz and polarization effects.
Absorption correction for complex 1 was done with DI-
FABS [26] and for compound 3aH SADABS [27] was
used for the absorption correction. All computations
were performed using the WINGX package [28]. The
4.1. General
All manipulations were carried out under an inert
atmosphere of dry nitrogen using a standard double
manifold. Tetrahydrofuran and petroleum ether were
freshly distilled from sodium/benzophenone prior to
use. Titanium tetraisopropoxide, phenyl isocyanate, 4-
methoxyphenyl isocyanate, 4-fluorophenylisocyanate,
2-methylphenyl isocyanate were obtained from Aldrich
and 4-methylphenyl isocyanate was obtained from Lan-
caster and used as supplied.
4.2. Physical measurements
1H NMR and 13C{1H} spectra were recorded
either on
a
Bruker ACF 200 MHz, Bruker
AMX400 or on a Bruker Avance 400 operating at
200 MHz for 1H NMR and 50 MHz for 13C
NMR (Bruker ACF 200 MHz) and at 400 and
100 MHz, respectively, for the 400 MHz. spectrom-
eters, with tetramethylsilane (TMS) as the internal
reference. All spectra were recorded in CDCl3 as
the solvent. For compounds 3aH, 4aH and 4bH,
representative coupled 13C NMR were recorded as
additional evidence for identifying the compounds.
NMR spectra could only be obtained for a mixture
of 5 and 6 as no separation could be accomplished.
HRESMS was recorded on a Micromass Q-Tof mi-
cro instrument (Electrospray ionization, ESI). Ele-
mental analyses were carried out on a Carlo Erba
elemental analyzer model 1106. GC measurements
were made using a Chemito GC 7610 instrument
with a flame ionization detector and a packed col-
umn (0.32 mm i.d., 2.44 m length) having 5% SE-
30 as the liquid phase and Ch-W(HP) as the solid
phase. Nitrogen was used as a carrier gas at a flow
rate of 30 cm3/min. A temperature program (1 min
at 100 ꢁC ramp to 120 ꢁC at 10 ꢁC/min then ramp
to 250 ꢁC at 25 ꢁC/min and hold at 250 ꢁC for 2
min) run was employed with the injector being
maintained at 200 ꢁC and the detector at 260 ꢁC,
to analyze the organic products in the crossover
experiments.
positions of heavy atoms were determined by SHELXS
-
86 [29]. The remaining atoms were located from the dif-
ference Fourier map using SHELXL-97 [30]. For complex
1 the H-atoms were geometrically fixed with the respec-
tive non H-atoms and allowed to ride.
4.4. Synthesis of compound 1
Titanium tetraisopropoxide (1 ml, 3.38 mmol) was
dissolved in 20 ml of tetrahydrofuran and into this solu-
tion, phenyl isocyanate (0.37 ml, 3.38 mmol) was added.
The mixture was stirred for 8 h and the solvent evapo-
rated to dryness under vacuum. The residue was washed
with 2–3 ml of cold petroleum ether. The remaining
white solid was redissolved in 25 ml of warm petroleum
ether which on slow cooling resulted in the formation of
colorless crystals (950 mg, 70%). These crystals were
highly sensitive to moisture and decomposed immedi-
1
ately on exposure to air. H NMR (200 MHz) d 7.28–
7.22 (m, 8H), 7.04–7.00 (m, 2H), 5.10 (sept, 2H, J =
6.2 Hz), 4.76 (sept, 2H, J = 6.2 Hz), 4.49 (sept, 4H,
J = 6.2 Hz), 1.31 (d, 12H, J = 6.2 Hz), 1.25 (d, 24H,
J = 6.2 Hz), 1.12 (d, 12H, J = 6.2 Hz). 13C NMR (100
MHz) 154.1 {NC@O(OiPr)}, 139.4 (Ph), 129.1 (Ph),
123.1 (Ph), 119.1 (Ph), 68.5 (terminal OiPrCH), 68.1
(bridged OiPrCH), 64.3 (inserted OiPrCH), 25.9 (termi-
nal OiPrCH3), 25.5 (bridged OiPrCH3), 22.3 (inserted
OiPrCH3).
4.3. X-ray crystallography
The single crystal of complex 1 were highly sensitive
to air and decomposed even in the mother liquor, hence
the crystal were taken out in paraffin oil for mounting. A
good quality crystal of complex 1 was chosen and
mounted in a Lindemann capillary with paraffin oil
and a crystal of compound 3aH was glued to the tip
of a glass fiber along the largest dimension for indexing
and data collection. Data for complex 1 was collected on
4.5. Reaction of titanium isopropoxide with 2 equiv. of
phenyl isocyanate
Titanium tetraisopropoxide (1 ml, 3.38 mmol) was
dissolved in 20 ml of tetrahydrofuran. Into this solution,
phenyl isocyanate (0.74 ml, 6.76 mmol) was added. The

1290
R. Ghosh et al. / Journal of Organometallic Chemistry 690 (2005) 1282–1293
mixture was stirred for 9 h and the solvent evaporated to
dryness under vacuum to give a dark orange paste. This
residue was extracted with minimum quantity of petro-
leum ether and dried under vacuum to give a red orange
paste. This gave a mixture of complexes 2 (800 mg,
45%). 1H NMR (200 MHz) d 7.41–7.27 (m, 32H),
7.12–6.97 (m, 16H), 5.11 (OCH sept, 3.6H, J = 6.2
Hz), 4.76 (OCH sept, 3H, J = 6.2 Hz), 4.49 (OCH sept,
3H, J = 6.2 Hz), 1.31 (CHCH3 d, 22H, J = 6.2 Hz), 1.26
(CHCH3 d, 18H, J = 6.2 Hz), 1.17 (CHCH3 d, 18H,
J = 6.2 Hz). On attempting to crystallize out the pure
compound, hexagonal crystal of complex 1 were formed.
Anal. Calc. for C10H13NO2: C, 67.02; H, 7.31; N, 7.81.
Found: C, 66.92; H, 7.33; N, 7.55%.
4.8. Compound 4bH
Carbamic acid, (4-methylphenyl)-, 1-methylethyl es-
1
ter: H NMR (400 MHz) d 7.28 (d, 2H, J = 8.0 Hz),
7.08 (d, 2H, J = 8.0 Hz), 6.84 (s, NH), 5.02 (OCH sept,
1H, J = 6.2 Hz), 2.29 (PhCH3 s, 3H) 1.28 (CHCH3 d,
6H, J = 6.2 Hz) 13C NMR (100 MHz) d 153.6
{PhNC@O(OiPr)}, 135.7 (ipso), 132.7 (ipso-CCH3 m,
1
2JCH = 5.9 Hz), 129.5 (Ph ddm, JCH = 156.0 Hz,
2JCH = 5.1 Hz 3JCH = 6.8 Hz), 119.0 (Ph d,
1
4.6. Compound 3aH
1JCH = 160.2 Hz), 68.5 (OCH d, JCH = 150.0 Hz),
1
22.1 (CHCH3 qd, JCH = 126.3 Hz, JCH = 5.1 Hz),
2
1
2
Carbamic acid, phenyl[(phenylamino) carbonyl]-, 1-
methylethyl ester: titanium tetraisopropoxide (1 ml,
3.38 mmol) was dissolved in 20 ml of tetrahydrofuran.
After addition of phenyl isocyanate (1.48 ml, 13.52
mmol), the mixture was stirred for 3 h, and the solvent
evaporated to dryness under vacuum to give an orange
red paste. This mixture was redissolved in methylene
chloride and taken in a separating funnel washed with
distilled water and the organic part extracted. This or-
ganic mixture contains both 4aH and 3aH in the ratio
7:3. They were separated by running a preparative
TLC with 1% ethyl acetate in petroleum ether (total
20.7 (PhCH3qt, JCH = 126.3 Hz, JCH=4.2 Hz). Anal.
Calc. for C11H15NO2 C, 68.37; H, 7.82; N, 7.24. Found:
C, 68.64; H, 7.91; N, 6.86%.
4.9. Compound 4cH
Carbamic acid, (4-methoxyphenyl)-, 1-methylethyl
1
ester: H NMR (400 MHz) d 7.27 (d, 2H, J = 8.8 Hz),
6.84 (d, 2H, J = 8.8 Hz), 6.41 (s, NH), 4.99 (OCH sept,
1H, J = 6.4 Hz), 3.77 (OCH3 s, 3H), 1.28 (CHCH3 d,
6H, J = 6.4 Hz) 13C NMR (100 MHz) d 155.6 (ipso-
COCH3), 153.7 {PhNC@O(OiPr)}, 131.2 (ipso), 120.5
(Ph), 114.0 (Ph), 68.3 (OCH), 55.3 (OCH3), 22.0
(CHCH3). Anal. Calc. for C11H15NO3: C, 63.14; H,
7.22; N, 6.69. Found: C, 63.47; H, 7.20; N, 6.13%.
1
yield 80%). H NMR (400 MHz) d 10.88 (s, 1H, NH),
7.47 (d, 2H, J = 7.6 Hz), 7.31 (m, 3H), 7.22 (t, 2H,
J = 7.6 Hz), 7.11 (d, 2H, J = 7.6 Hz), 6.99 (t, 1H,
J = 7.6 Hz), 4.91 (OCH sept, 1H, J = 6.2 Hz), 1.07
(CHCH3 d, 6H, J = 6.2 Hz). 13C NMR (100 MHz) d
155.6 {PhNC@O(OiPr)}, 151.6 {PhNC@O(NPh)},
4.10. Compound 4dH
2
2
137.8 (ipso t, JCH = 9.5 Hz), 137.2 (ipso t, JCH = 9.5
Carbamic acid, (4-fluorophenyl)-, 1-methylethyl es-
1
1
Hz), 128.9 (Ph dd, JCH = 156.9 Hz, JCH = 3.3 Hz),
2
ter: H NMR (400 MHz) d 7.33 (dd, 2H, J = 9.4 Hz,
1
2
128.8 (Ph dd, JCH = 159.9 Hz, JCH = 8.0 Hz), 128.7
3
4JHF = 4.8 Hz), 6.99 (dd, 2H, J = 9.4 Hz, JHF = 8.4
1
(Ph dd, JCH = 159.2 Hz, JCH = 7.7 Hz), 128.0 (Ph dt,
2
Hz), 6.50 (s, NH), 5.01 (OCH sept, 1H, J = 6.6 Hz),
1.28 (CHCH3 d, 6H, J = 6.6 Hz) 13C NMR (100
1JCH = 161.0 Hz, JCH = 7.3 Hz), 123.8 (Ph dt,
2
1JCH = 157.7 Hz, JCH = 8.8 Hz), 119.8 (Ph d,
MHz) d 158.8 (ipso-CF d, JCF = 241.0 Hz), 153.4
2
1
1JCH = 162.5 Hz), 71.7 (OCH d sept, JCH = 149.7 Hz,
{PhNC@O(OiPr)}, 134.1 (Ph d, JCF = 2.0 Hz), 120.34
(Ph br), 115.5 (Ph d, JCF = 22.7 Hz), 68.8 (OCH),
1
4
2JCH = 4.0 Hz), 21.5 (CHCH3 dd, JCH = 126.9 Hz,
1
2
2JCH = 4.3 Hz). Anal. Calc. for C17H18N2O3C, 68.43;
H, 6.08; N, 9.39. Found: C, 68.09; H, 6.23; N, 9.17%.
22.0 (CHCH3). HRESMS (M + Na)+ obsd. m/z
220.0751. Anal. Calc. for C10H12FNO2Na: 220.0749.
4.7. Compound 4aH
4.11. Compound 4eH
Carbamic acid, phenyl-, 1-methylethyl ester: 1H
NMR (400 MHz) d 7.42 (d, 2H, J = 7.6 Hz), 7.29 (t,
2H, J = 7.6 Hz), 7.05 (t, 1H, J = 7.6 Hz), 6.92 (s, NH),
5.05 (OCH sept, 1H, J = 6.2 Hz), 1.30 (CHCH3 d, 6H,
Carbamic acid, (2-methylphenyl)-, 1-methylethyl es-
1
ter: H NMR (400 MHz) d 7.83 (d, 1H, br), 7.21 (t,
1H, J = 7.8 Hz), 7.16 (d, 1H, J = 7.8 Hz), 7.01 (t, 1H,
J = 7.8 Hz), 6.34 (NH), 5.03 (OCH sept, 1H, J = 6.4
Hz), 2.26 (PhCH3 s, 3H), 1.31 (CHCH3 d, 6H, J = 6.4
Hz). 13C NMR (100 MHz) d 153.3 {PhNC@O(OiPr)},
135.7 (ipso), 129.9 (ipso-CCH3), 127.7 (Ph), 126.2 (Ph),
123.6 (Ph), 121.1 (Ph), 68.1 (OCH), 21.6 (CHCH3),
17.2 (PhCH3). HRESMS (M + Na)+ obsd. m/z
216.0999, calc. for C11H15NO2Na: 216.1000.
J = 6.2 Hz) 13C NMR (100 MHz)
d
153.7
{PhNC@O(OiPr)}, 138.4 (ipso), 129.0 (Ph dd,
2
1JCH = 161.0 Hz, JCH = 9.2 Hz), 123.2 (Ph dt,
2
1JCH = 159.4 Hz, JCH = 8.4 Hz), 119.0 (Ph d,
1
1JCH = 166.3 Hz), 68.7 (OCH d, JCH = 150.3 Hz),
1
2
22.1 (CHCH3 qd, JCH = 126.6 Hz, JCH = 4.6 Hz).

R. Ghosh et al. / Journal of Organometallic Chemistry 690 (2005) 1282–1293
1291
4.12. Compound 3bH
1H, J = 7.8 Hz), 7.21 (m, 4H), 7.16 (d, 1H, J = 7.8
Hz), 7.03 (t, 1H, J = 7.8 Hz), 5.04 (OCH sept, 1H,
J = 6.4 Hz), 2.40 (PhCH3 s, 3H), 2.21 (PhCH3 s, 3H),
1.18 (CHCH3 d, 3H, J = 6.4 Hz), 1.16 (CHCH3 d, 3H,
Carbamic acid, 4-methylphenyl[(4-methylphenyl-
amino)carbonyl]-, 1-methylethyl ester: 1H NMR (400
MHz) d 10.86 (s, 1H, NH), 7.42 (d, 2H, J = 8.0 Hz),
7.19 (d, 2H, J = 8.0 Hz), 7.09 (d, 2H, J = 8.0 Hz), 7.05
(d, 2H, J = 8.0 Hz), 5.01 (OCH sept, 1H, J = 6.2 Hz),
2.37 (PhCH3 s, 3H), 2.29 (PhCH3 s, 3H), 1.16 (CHCH3
d, 6H, J = 6.2 Hz). 13C NMR (100 MHz) d 155.6
{PhNC@O(OiPr)}, 151.6 {PhNC@O(NPh)}, 137.6
(ipso), 135.2 (ipso), 134.4 (ipso-CCH3), 133.1 (ipso-
CCH3), 129.2 (Ph), 129.2 (Ph), 128.2 (Ph), 119.7 (Ph),
71.4 (OCH), 21.3 (CHCH3), 20.9 (PhCH3), 20.6
(PhCH3). HRESMS (M + H)+ obsd. m/z 327.1734, calc.
for C19H23N2O3: 327.1709.
J = 6.4 Hz). 13C NMR (100 MHz)
d
155.3
{PhNC@O(OiPr)}, 150.9 {PhNC@O(NPh)}, 136.2
(ipso), 136.1 (ipso), 135.6 (ipso-CCH3), 130.2 (ipso-
CCH3), 129.9 (Ph), 128.4 (Ph), 128.1 (Ph), 127.3 (Ph),
126.3 (Ph), 123.7 (Ph), 120.7 (Ph), 120.6 (Ph), 71.1
(OCH), 21.2 (CHCH3), 17.9 (PhCH3), 17.0 (PhCH3).
HRESMS (M + Na)+ obsd. m/z 349.1534, calc. for
C19H22N2O3Na: 349.1528.
4.16. Reaction of titanium isopropoxide with 3 equiv. of
phenyl isocyanate
4.13. Compound 3cH
Titanium tetraisopropoxide (1 ml, 3.38 mmol) was
dissolved in 20 ml of tetrahydrofuran. Into this solution,
phenyl isocyanate (1.1 ml, 10.1 mmol) was added. The
mixture was stirred for 54 h and the solvent evaporated
to dryness under vacuum resulting in an orange–red
paste (1.42 g, 68%). This reaction mixture was found
to be mostly the proposed compound 5 with trace quan-
tities of the complex with coordinated 3a. 1H NMR (400
MHz) d 7.55–6.86 (m, 65H), 5.10 (OCH sept, 3H,
J = 6.2 Hz), 5.01 (OCH sept, 6H, J = 6.2 Hz), 4.80
(OCH sept, 4H, J = 6.2 Hz), 1.31 (CHCH3 d, 54H,
J = 6.2 Hz), 1.19 (CHCH3 d, 24H, J = 6.2 Hz).
Carbamic acid, 4-methoxyphenyl[(4-methoxyphe-
nylamino)carbonyl]-, 1-methylethyl ester: 1H NMR
(400 MHz) d 10.78 (s, 1H, NH), 7.45 (d, 2H, J = 8.8
Hz), 7.09 (d, 2H, J = 8.8 Hz), 6.91 (d, 2H, J = 8.8 Hz),
6.83 (d, 2H, J = 8.8 Hz), 4.98 (OCH sept, 1H, J = 6.4
Hz), 3.83 (OCH3 s, 3H), 3.77 (OCH3 s, 3H), 1.16
(CHCH3 d, 6H, J = 6.4 Hz). 13C NMR (100 MHz) d
158.9(ipsoCOCH3),
156.1
(ipso-COCH3),
155.9
{PhNC@O(OiPr)}, 152.0 {PhNC@O(NPh)}, 133.5
(ipso), 131.1 (ipso), 130.0 (Ph), 129.6 (Ph), 121.5 (Ph),
114.1 (Ph), 71.5 (OCH), 55.5 (OCH3), 55.4 (OCH3),
21.6 (CHCH3). HRESMS (M + Na)+ obsd. m/z
381.1434, calc. for C19H22N2O5Na: 381.1426.
4.17. Reaction of titanium isopropoxide with 4 equiv. of
phenyl isocyanate
4.14. Compound 3dH
Titanium tetraisopropoxide (1 ml, 3.38 mmol) was
dissolved in 20 ml of tetrahydrofuran. Into this solution,
phenyl isocyanate (1.48 ml, 13.52 mmol) was added. The
mixture was stirred for 80 h and the solvent evaporated
to dryness under vacuum resulting in an orange–red
paste (1.72 g, 65%). This reaction mixture was found
to be the proposed compound 6 with a trace quantity
of the complex containing coordinated 3a. 1H NMR
(400 MHz) d 7.55–6.86 (m, 55H), 5.1 (OCH sept, 2H,
J = 6.2 Hz), 5.01 (OCH sept, 7H, J = 6.2 Hz), 4.80
(OCH sept, 2H, J = 6.2 Hz), 1.31 (CHCH3 d, 54H,
J = 6.2 Hz), 1.19 (CHCH3 d, 12H, J = 6.2 Hz).
Carbamic acid, 4-fluorophenyl[(4-fluorophenyla-
mino)carbonyl]-, 1-methylethyl ester: 1H NMR (400
MHz) d 10.92 (s, 1H, NH), 7.48 (dd, 2H, J = 9.4 Hz,
4JHF = 4.7 Hz), 7.16 (dd, 2H, J = 9.4 Hz, JHF = 4.7
4
3
Hz), 7.07 (dd, 2H, J = 9.4 Hz, JHF = 8.2 Hz), 6.94
3
(dd, 2H, J = 9.4 Hz, JHF = 8.2 Hz), 5.00 (OCH sept,
1H, J = 6.2 Hz), 1.16 (CHCH3 d, 6H, J = 6.2 Hz). 13C
1
NMR (100 MHz) d 161.9 (ipso-CF d, JCF = 244.0
Hz), 159.1 (ipso-CF d, JCF = 244.0 Hz), 155.4
1
{PhNC@O(OiPr)}, 151.8 {PhNC@O(NPh)}, 133.6
(ipso, d, JCF = 3.0 Hz), 132.9 (ipso d, JCF = 3.0 Hz),
4
4
3
3
130.3 (Ph d, JCF = 8.0 Hz), 121.6 (Ph d, JCF = 8.0
4.18. General procedure followed to carry out the kinetic
measurements
2
Hz), 115.4 (Ph d, JCF = 23.0 Hz), 115.3 (Ph d,
2JCF = 23.0 Hz), 72.0 (OCH), 21.3 (CHCH3). HRESMS
(M + Na)+ obsd. m/z 357.1038, calc. for C17H16F2N2O3-
Na: 357.1027.
Titanium tetraisopropoxide (0.25 ml, 0.845 mmol.)
was dissolved in 30 ml of tetrahydrofuran. Into this
solution aryl isocyanate (3.38 mmol) was added. The
mixture was stirred at room temperature and the reac-
tion was monitored at regular intervals by withdrawing
aliquots of 0.2 ml. The complex was decomposed by
4.15. Compound 3eH
Carbamic acid, 2-methylphenyl[(2-methylphenyla-
mino)carbonyl]-, 1-methylethyl ester: 1H NMR (400
MHz) d 10.98 (NH), 8.08 (d, 1H, J = 7.8 Hz), 7.27 (d,
1
addition of water and H NMR of the organic fraction
was recorded. Concentrations of the organic ligands 4H

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R. Ghosh et al. / Journal of Organometallic Chemistry 690 (2005) 1282–1293
(m) J. Zhang, R. Cai, L. Weng, X. Zhou, Organometallics 22
(2003) 5385;
and 3H were estimated from the integrals of the corre-
sponding methine and NH protons.
(n) X.G. Zhou, L.X. Zhang, C.M. Zhang, J. Zhang, M. Zhu,
R.F. Cai, Z.E. Huang, Z.X. Huang, Q.J. Wu, J. Organomet.
Chem. 655 (2002) 120.
Acknowledgment
[7] (a) E.B. Brouwer, P. Legzdins, S.J. Rettig, K.J. Ross, Organo-
metallics 12 (1993) 4234;
(b) C.C. Chang, J.H. Chen, B. Srinivas, M.Y. Chiang, G.H. Lee,
Organometallics 16 (1997) 4980;
We gratefully acknowledge generous financial sup-
port from the Department of Science and Technology,
New Delhi.
(c) D.L. Clark, S.K. Grumbine, B.L. Scott, J.G. Watkin,
Organometallics 15 (1996) 949;
(d) D.A. Gately, J.R. Norton, P.A. Goodson, J. Am. Chem. Soc.
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Appendix A. Supplementary data
Cone angles of ortho-substituted ligands are marked
on a simulated molecular structure diagram. Crystallo-
graphic data for the structural analysis has been depos-
ited with the Cambridge Crystallographic Data Centre,
CCDC No. 251819 for compound 1 and CCDC No.
251820 for compound 3aH. Supplementary data associ-
ated with this article can be found, in the online version,
at doi:10.1016/j.jorganchem.2004.11.038.
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