Differing reactivities of zirconium and titanium alkoxides with phenyl isocyanate: An experimental and computational study

Article abstract of DOI:10.1021/om701004f

The insertion reaction of 1 and 2 equiv of PhNCO (2) into Ti(O ⁿBu)₄ (1aTi) and Zr(OⁿBu)₄ (1aZr) results in the formation of a carbamate ligand (PhNCOOⁿBu) referred to as a monoinsertion ligand (MIL). Reaction of metal n-butoxide (1a) with more than 2 equiv of PhNCO leads to the competitive insertion of PhNCO into the M-N bond of metal carbamate or into the M-O bond of the alkoxide. The ligand formed by the insertion of PhNCO into a metal carbamate is an allophanate ligand (PhNCONPhCOOⁿBu), referred to as a double insertion ligand (DIL). The complexes containing these ligands were hydrolyzed, and the organic products 5 (from MIL) and 6 (from DIL) were spectroscopically characterized. The reactivity of Ti(OⁿBu)₄ is very similar to the reactivity of Ti(OⁱPr)₄ reported earlier by us. The formation of DIL over MIL is clearly favored in the case of Ti(OⁿBu)₄, whereas the formation of DIL is less favored with Zr(OⁿBu) ₄. At -80°C, Zr(OⁿBu)₄ gives only MIL, and so only 5 is isolated on hydrolysis. In contrast, Ti(OⁿBu) ₄ initially produces DIL and then it slowly decays to MIL, and both 5 and 6 are isolated on hydrolysis. The unusual preference for monoinsertion or double insertion has been probed computationally using model complexes Ti(OMe)₄ (1bTi) and Zr(OMe)₄ (1bZr) at the B3LYP/LANL2DZ level of theory. The relative thermodynamic stabilities of the intermediate model metal complexes (10b, 11b, 12b, and 13b) are indicative of the diverse behavior shown by Ti and Zr. The barrier for the formation of 11bZr is assigned to a high-energy intermediate 10bZr, which results in the reduced preference for DIL formation. The smaller size of Ti precludes higher coordination numbers; thus the heptacoordinated intermediate 12bTi is higher in energy compared to 12bZr. This facilitates the formation of MIL in the case of Zr(O ⁿBu)₄.

Full text of DOI:10.1021/om701004f

Organometallics 2008, 27, 955–960
955
Differing Reactivities of Zirconium and Titanium Alkoxides with
Phenyl Isocyanate: An Experimental and Computational Study
Akshai Kumar, Susmita De, Ashoka G. Samuelson,* and Eluvathingal D. Jemmis*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, 560 012, India
ReceiVed October 8, 2007
The insertion reaction of 1 and 2 equiv of PhNCO (2) into Ti(OnBu)4 (1aTi) and Zr(OnBu)4 (1aZr)
results in the formation of a carbamate ligand (PhNCOOnBu) referred to as a monoinsertion ligand (MIL).
Reaction of metal n-butoxide (1a) with more than 2 equiv of PhNCO leads to the competitive insertion
of PhNCO into the M-N bond of metal carbamate or into the M-O bond of the alkoxide. The ligand
formed by the insertion of PhNCO into a metal carbamate is an allophanate ligand (PhNCONPhCOOnBu),
referred to as a double insertion ligand (DIL). The complexes containing these ligands were hydrolyzed,
and the organic products 5 (from MIL) and 6 (from DIL) were spectroscopically characterized. The
reactivity of Ti(OnBu)4 is very similar to the reactivity of Ti(OiPr)4 reported earlier by us. The formation
of DIL over MIL is clearly favored in the case of Ti(OnBu)4, whereas the formation of DIL is less
favored with Zr(OnBu)4. At -80 °C, Zr(OnBu)4 gives only MIL, and so only 5 is isolated on hydrolysis.
In contrast, Ti(OnBu)4 initially produces DIL and then it slowly decays to MIL, and both 5 and 6 are
isolated on hydrolysis. The unusual preference for monoinsertion or double insertion has been probed
computationally using model complexes Ti(OMe)4 (1bTi) and Zr(OMe)4 (1bZr) at the B3LYP/LANL2DZ
level of theory. The relative thermodynamic stabilities of the intermediate model metal complexes (10b,
11b, 12b, and 13b) are indicative of the diverse behavior shown by Ti and Zr. The barrier for the formation
of 11bZr is assigned to a high-energy intermediate 10bZr, which results in the reduced preference for
DIL formation. The smaller size of Ti precludes higher coordination numbers; thus the heptacoordinated
intermediate 12bTi is higher in energy compared to 12bZr. This facilitates the formation of MIL in the
case of Zr(OnBu)4.
been extensively explored in the case of heterocumulenes.11
Multiple insertions in this context are of three types as illustrated
(Scheme 1) using R′NCO insertions into M-OR: (1) reaction
where insertion of more than one substrate takes place into more
than one group in a metal complex (eq a); (2) reaction where
Introduction
Homogeneous catalytic processes often involve insertion of
neutral substrates into transition metal-X bonds as one of the key
steps, where X is carbon, oxygen, or nitrogen.1,2 A good under-
standing of this key step would help to control this reaction and
enable the design of catalysts for polymerization of new substrates.
Similarly, incorporation of neutral polar substrates like CO2, CS2,
isocyanates, and isothiocyanates into polymers could be of great
value in synthesizing functionalized polymers.3
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Insertions of polar heterocumulenes into M-N,4 M-C,5 and
M-O6 have been studied with a variety of metals, especially
with titanium7,8 and zirconium.9 The initial product of insertion
could react further with another molecule of the heterocumulene,
leading to multiple insertions. While there are many instances
where multiple insertions occur with alkynes,10 they have not
* Corresponding authors. (A.G.S.) Ph: 91-80-2293-2663(off). Fax: 91-
80-2360-1552. E-mail: ashoka@ipc.iisc.ernet.in. (E.D.J.) Ph: 91-80-2293-
3347(off). Fax: 91-80-2360-1552. E-mail: jemmis@ipc.iisc.ernet.in.
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Brombacher, H.; Vahrenkamp, H. Eur. J. Inorg. Chem. 2002, 1, 153. (c)
Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis: The Applications and
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10.1021/om701004f CCC: $40.75
2008 American Chemical Society
Publication on Web 02/05/2008

956 Organometallics, Vol. 27, No. 5, 2008
Kumar et al.
Scheme 1. Types of Reactions through Which Multiple Inser-
tions Occur
Scheme 2. Formation of Dimeric Complex 3 by the Insertion
of PhNCO into Ti(OnBu)4
structurally characterized.20 Head-to-tail insertion (eq c) occurs
with heterocumulenes21a,16b–d and alkynes,12 and head-to-head
insertion (eq d) occurs with heterocumulenes21b and isocy-
anides.16a
This study reports the experimental observation of the
differences in the reaction of Ti(OnBu)4 and Zr(OnBu)4 with
PhNCO. Theoretical calculations were performed on the model
complexes Ti(OMe)4 and Zr(OMe)4 at the B3LYP/LANL2DZ
level of theory to get an insight into these reactions. The
structures are numbered from 1 to 13. The letters a and b are
used to indicate metal complexes with a OnBu ligand and a
OMe ligand, respectively. Ti and Zr 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.
insertion of one substrate takes place into more than one group
in a metal complex (eq b); (3) reaction where insertion of more
than one substrate takes place into one group in a metal complex
(eqs c and d); multiple insertions according to the third pathway
can further take place in either head-to-tail (eq c) or head-to-
head fashion (eq d).
Well-characterized multiple insertions are mainly those of
alkynes,10a,12,13 which undergo insertion by eq a, whereas carbon
disulfide,14 nitriles,15 isocyanides,16a,17 and carbon monoxide18
undergo multiple insertion by eq b. Apart from these unusual
double insertions, a few examples of alkenes that show only
double insertions and not polymerization are also known.19
Surprisingly, very few complexes obtained by insertion of
isocyanates and isothiocyanates into metal alkoxides have been
Results and Discussion
The insertion reaction of one molecule of PhNCO (2) is
discussed first. This is followed by the discussion of double
insertion. The concentration profiles of DIL and MIL are
discussed next, showing the difference in the reactivities of
Ti(OnBu)4 (1aTi) and Zr(OnBu)4 (1aZr). Results of the com-
putational studies with model substrates are given at the end.
Monoinsertion of Phenyl Isocyanate. The reaction of
PhNCO with titanium isopropoxide has been reported to give a
dimeric complex due to a facile insertion reaction.21a It has been
characterized by 1H NMR spectroscopy and single-crystal X-ray
diffraction studies. The reaction of Ti(OnBu)4 with 1 equiv of
PhNCO was carried out in tetrahydrofuran in a similar fashion
and is expected to give the analogous product (3) where one of
the butoxide groups at each metal center has undergone insertion
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1
(Scheme 2). H NMR of this reaction mixture in CDCl3 was
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Differing ReactiVities of Zr and Ti Alkoxides
Organometallics, Vol. 27, No. 5, 2008 957
Scheme 3. Insertion of 2 equiv of PhNCO into Zr(OnBu)4 Leading to 5 after Hydrolysis
Scheme 4. Insertion of more than 2 equiv of PhNCO into Zr(OnBu)4 leading to 5 and 6 after Hydrolysis
Table 1. Ratio of Hydrolyzed Products in the Reaction of Zr(OnBu)4
(Scheme 5). The concentrations of these compounds were
with Varying Amounts of PhNCO in Toluene, as Shown in
estimated by hydrolysis with stoichiometric quantities of
Scheme 4
1
water and H NMR analysis. While 11aTi having MIL and
ratio of products (%)
DIL would give 5 and 6 in the ratio 1:1, hydrolysis of 13aTi
entry
equiv of 2
time (h)
4
5
6
would give 3 equiv of 5 (Scheme 6). The ratio of 5 to 6 is
a measure of the relative concentration of 10aTi (or 11aTi)
and 12aTi (or 13aTi). The reaction was followed initially at
intervals of 1 h and later at intervals of 3 h. The concentration
of 6 reached a maximum and then showed a steady decay,
whereas concentration of 5 increased with time. Ultimately
the reaction mixture had only 3 equiv of MIL and one equiv
of butoxide coordinated to titanium after 54 h. This suggests
that 11aTi with coordinated DIL has transferred one of the
phenyl isocyanates into metal butoxide to form the complex
13aTi, with three coordinated MIL. Such a decay can be
explained by the reversibility of the step leading to 11aTi
so that the thermodynamically more stable 13aTi is formed
(Scheme 5). A similar concentration profile in the formation
and decay of DIL was observed when Ti(OnBu)4 was treated
with 4 equiv of PhNCO at 35 °C (Figure 1).
1
2
3
4
2
3
4
22
27
30
48
50
22
18
50
71
64
39
7
18
61
10
n-butoxide groups, i.e., the carbamate (obtained by insertion of
PhNCO into Ti(OnBu)4), bridging and terminal, respectively.
The distinctive resonances in CDCl3 are consistent with the
dimeric structure predicted for this complex (3).
The reaction of Zr(OnBu)4 with 1 equiv of PhNCO resulted
in a similar insertion reaction, but distinctive methylene
resonances were not discernible in the NMR spectrum. Reaction
of Zr(OnBu)4 with 2 equiv of PhNCO leads to the formation of
a product mixture with an even more complex 1H NMR pattern.
As products in this reaction could not be separated and
characterized, the ratio of various insertion products was
In the reaction of Zr(OnBu)4 with 4 equiv of PhNCO at 35
°C, the concentration of 5 and 6 obtained after hydrolysis
showed a different trend from what was seen with Ti(OnBu)4.
The concentration of 6 increased initially (Figure 1) and
remained constant after 30 h; the concentration of 5 also showed
an initial rise before remaining constant after 30 h. This implies
that both 11aZr (or 10aZr) and 13aZr (or 12aZr) are formed
through independent pathways and 11aZr does not decay to
13aZr.
Since the reactions of Ti and Zr are clearly different, there
is a possibility that the kinetically favored product obtained
with Ti is not favored with Zr. In order to confirm this,
reactions were carried out at different temperatures. At 45
°C the reaction of Zr(OnBu)4 with 4 equiv of PhNCO
produced a concentration profile similar to what was observed
at 35 °C (Figure 2). However, the ratios of 13aZr to 11aZr
were different, leading to the observed concentration profiles
of 5 and 6 shown in Figure 2. In the reaction of Zr(OnBu)4
with 4 equiv of PhNCO at 0 °C, significantly less double
insertion was observed (Figure 3).
1
obtained by hydrolyzing with water. On hydrolysis, H NMR
of the solution showed only two sets of methene resonances at
3.64 and 4.17 ppm assigned to the butanol (4) and butyl group
of butyl N-phenyl carbamate (5) in the ratio of 1:1 (Scheme 3).
This shows that the reaction mixture indeed has two isocyanate
groups inserted into two of the butoxides on zirconium.
Double Insertion of Phenyl Isocyanate. The reaction of
Zr(OnBu)4 with 3 equiv of PhNCO was carried out at 35 °C,
and the reaction mixture was analyzed by 1H NMR after
hydrolysis. 1H NMR of the products showed peaks for the butyl
N-phenyl carbamate (5) and butyl 2,4-diphenylallophanate (6)
(Scheme 4). The latter arises from the protonation of DIL, which
is formed by the addition of a second molecule of PhNCO to
the coordinated carbamate. The ratio of these products for
different amounts of PhNCO are given in Table 1. With the
increasing amounts of PhNCO, the product 6 arising from
double insertion of PhNCO increased. A mechanistic scheme
depicting the formation of various products, including possible
intermediates, is given in Scheme 5.
Not surprisingly, further lowering the temperature to -80
°C resulted in the formation of only 5 after hydrolysis of the
reaction mixture. The concentration of 5 increased initially and
remained constant after 48 h. This clearly indicates that MIL is
kinetically favored in comparison to the formation of DIL. This
explains the changes in the concentration of DIL and MIL as a
function of temperature.
The insertion of a third molecule of PhNCO to the compound
9a is possible either into the carbamate or into the alkoxide
ligand (Scheme 5). The preference of Zr and Ti for the two
insertions is different. Furthermore, the conversion of DIL to
MIL as a function of time, which is observed with titanium
Kinetic Studies. Previous results with insertion reactions of
titanium(IV) isopropoxide showed that the concentration of MIL
and DIL varied as a function of time.21a The concentration of
the product arising from the double insertion of aryl isocyanates
on Ti(OiPr)4 decreases with time. A kinetic preference for the
formation of the double-insertion product and thermodynamic
preference for the monoinsertion product were suggested as
possible reasons for the observed concentration profiles.21a
similar result is expected with Ti(OnBu)4 also.
A
The reaction of Ti(OnBu)4 with 3 equiv of PhNCO at 35
°C leads to the formation of a mixture of products containing
10aTi (or its isomer 11aTi) and 12aTi (or its isomer 13aTi)

958 Organometallics, Vol. 27, No. 5, 2008
Kumar et al.
Scheme 5. Schematic Representation of the Stepwise Insertion of PhNCO (2) into M(OR)4 Showing the Intermediates Involved
Scheme 6. Hydrolysis Products of the Intermediates Involved in the Reaction of Ti(OnBu)4 with 3 equiv of PhNCO
alkoxides, is not observed in the reaction of zirconium alkoxide.
To explain these differences, a theoretical study was undertaken.
The difference in reactivity of Ti and Zr had been observed in
several other reactions, and computational studies have helped
in understanding the differences.22
characterized.21a It is worth noting that the energy difference
between the two species 7b and 8b is quite small (1.1 kcal/
mol).
The reaction of octahedral metal complex 9b with PhNCO
leads to the square-pyramidal intermediate 10b (insertion into
carbamate) or heptacoordinated 12b (insertion into alkoxide by
[2+2] cycloaddition), which subsequently gives octahedral 11b
or 13b, respectively. Since 11b gives DIL and MIL in the ratio
1:1 whereas 13b gives 3 equiv of MIL, the ratio of 11b/13b
Computational Studies
Support for the mechanistic scheme and the product ratios
was sought from computational studies. For this purpose,
Ti(OnBu)4 (1aTi) and Zr(OnBu)4 (1aZr) were modeled by
Ti(OMe)4 (1bTi) and Zr(OMe)4 (1bZr), where bulky nBu
groups are replaced by Me groups for easy computation (Scheme
5). Experimentally, insertion of 2 equiv of PhNCO into metal
n-butoxide (1a) gives only MIL. This implies that the first two
steps (1 f 7 f 9) involve insertion of PhNCO into the metal
alkoxide bonds. The insertion of one molecule of PhNCO into
the M-O bond of 1b leads to a carbamate ligand coordinated
through N and the oxygen of the alkoxide. This gives structure
7b, which presumably rearranges to 8b. In the case of titanium
isopropoxide, insertion of PhNCO leads to the analogue of 8b,
which was isolated as a dimer and crystallographically
(21) (a) Ghosh, R.; Nethaji, M.; Samuelson, A. G. J. Organomet. Chem.
2005, 690, 1282. (b) Ghosh, R.; Samuelson, A. G. Chem. Commun. 2005,
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(22) (a) Pavan Kumar, P. N. V.; Jemmis, E. D. J. Am. Chem. Soc. 1988,
110, 126. (b) Jemmis, E. D.; Giju, K. T. Angew. Chem., Int. Ed. Engl.
1997, 36, 606. (c) Jemmis, E. D.; Giju, K. T. J. Am. Chem. Soc. 1998, 120,
6952. (d) Jemmis, E. D.; Phukan, A. K.; Giju, K. T. Organometallics 2002,
21, 2254.
Figure 1. Formation of insertion products 5 and 6 in the reaction
of Ti(OnBu)4 and Zr(OnBu)4 with 4 equiv of PhNCO in THF at
35 °C.

Differing ReactiVities of Zr and Ti Alkoxides
Organometallics, Vol. 27, No. 5, 2008 959
Figure 4. Reaction energy profile for the insertion of 3 equiv of
PhNCO (2) into M(OMe)4 (1b, M ) Ti, Zr), indicating the relative
energies of the intermediates.
Figure 2. Formation of insertion products 5 and 6 in the reaction
of Zr(OnBu)4 with 4 equiv of PhNCO in THF at 45 °C.
preference of Zr for a larger number of ligands as in 12b is a
well-precedented observation documented in other systems.22
Thermodynamic stabilities computed for the model substrates
are extremely valuable in understanding the differences in
product profiles. Although intermediate 13bTi is thermodynami-
cally more stable than 11bTi, the energy difference between
the two intermediates is only 1.7 kcal/mol. The reversibility of
these reactions is indicated by the formation of both 5 and 6
(hydrolysis products of 13Ti and 11Ti, respectively) in the
beginning followed by slow decrease in the concentration of 6
with concomitant increase in the concentration of 5. In contrast,
the formation of 11Zr involves a thermodynamically higher
energy intermediate 10Zr. This prevents the formation of 11Zr
and consequently isolation of 6 on hydrolysis of the reaction
mixture at low temperatures. Since the reverse reaction would
follow the same pathway, the decay of 11Zr to 12Zr is
unfavorable. The intermediate 12Zr, however, is very stable,
making it the more favorable product at low temperatures.
Hydrolysis of 12Zr leads to isolation of 5.
Figure 3. Formation of insertion products 5 and 6 in the reaction
of Zr(OnBu)4 with 4 equiv of PhNCO in THF at 0 °C.
determines the ratio of DIL/MIL. So the key questions that we
need to answer are the following. Are there differences in the
thermodynamic stabilities of 11b and 13b for Ti and Zr? If so,
are these reflected in the ratios of products? In the reaction of
9b with PhNCO, does the metal prefer formation of 10b or 12b
(precursors to 11b and 13b, respectively), and if so, why?
The energetics for the formation of various intermediates is
given in Figure 6 for Ti and Zr. In the reaction of Ti(OMe)4,
formation of square-pyramidal 10bTi appears to be an uphill
process. Rearrangement to octahedral 11bTi leads to a more
stable product. Similarly, formation of 11bZr is preceded by
the high-energy intermediate 10bZr. However, in comparison
to the Ti intermediate, the Zr intermediate is higher in energy.
This makes it understandable why the reaction of Ti leading to
11bTi is reversible but the corresponding reaction of Zr is not
reversible.
The energetics of the reaction leading to the formation of
12b and 13b give further insight into the product profiles
observed in the reactions of Ti and Zr. In the case of Zr,
formation of 12bZr is a downhill process (Figure 4). The
corresponding reaction of Ti leading to the stable 13bTi is
preceded by an intermediate 12bTi, which is heptacoordinated
and higher in energy. On the other hand, the heptacoordinated
structure 12bZr is more stable compared to 13bZr/10bZr.
Therefore, the 12bZr will be formed more readily in the reaction
compared to 10bZr/11bZr or 13bZr. But the smaller Ti does
not show this preference for the heptacoordinated 12bTi. The
Conclusions
Recent studies with insertion reactions of Ti(OiPr)4 and
heterocumulenes have shown that heterocumulenes CO2, PhNCO,
and PhNCS behave in a very similar fashion. However, there
are significant differences in the reactivity of Ti and Zr toward
phenyl isocyanate. Computational studies on model metal
alkoxides are helpful in understanding this reactivity pattern.
The difference in reactivity is primarily attributed to the
difference in size of the two metals; Zr prefers higher coordina-
tion numbers. Reactions leading to seven coordinated complexes
are stabilized more in the case of Zr than in the case of Ti.
This explains why only monoinsertion takes place with Zr at
low temperatures. The double insertion of 2 results in an
intermediate that involves formation of five-coordinated species.
This process is computed to be energetically unfavorable for
Zr but not as much for Ti, a fact that is borne out by the
experimental observation regarding reversibility of double
insertion in the case of titanium alkoxides.
Experimental Section
General Procedures. All manipulations were carried out under
an inert atmosphere of dry argon using a standard double manifold.
Toluene and tetrahydrofuran were freshly distilled from sodium/
benzophenone prior to use. Zirconium(IV) n-butoxide (84 wt % in
1-butanol) and titanium(IV) n-butoxide were obtained from Aldrich

960 Organometallics, Vol. 27, No. 5, 2008
Kumar et al.
USA. Butanol was removed from zirconium n-butoxide solution
under vacuum at 110 °C. Phenyl isocyanate was obtained from
Merck and used as supplied.
Computational Details. For theoretical studies, Ti(OMe)4 and
Zr(OMe)4 were taken as model complexes in place of Ti(OnBu)4
and Zr(OnBu)4, where bulky nBu groups are replaced by Me groups
for computational efficiency. All structures were optimized using
the hybrid HF-DFT method, B3LYP/LANL2DZ, based on Becke’s
three-parameter functional24a including Hartree–Fock exchange
contribution with a nonlocal correction for the exchange potential
proposed by Becke24b together with the nonlocal correction for the
correlation energy suggested by Lee24c et al. The LANL2DZ basis
set uses the effective core potentials (ECP) of Hay and Wadt.25
The nature of the stationary points was characterized by vibrational
frequency calculations. The Gaussian 03 program package was used
for all calculations.26
Physical Measurements. 1H NMR and 13C{H} NMR were
recorded on a Bruker AMX 400 operating at 400 MHz for 1H NMR
and 100 MHz for 13C NMR, with tetramethylsilane as internal
reference. All spectra were recorded in CDCl3. HRESMS was
recorded on a Micromass ESI-TOF MS instrument.
Synthesis of 3. Titanium(IV) n-butoxide (0.1 mL, 0.29 mmol)
was dissolved in 6 mL of tetrahydrofuran, and to this solution
phenyl isocyanate (0.32 mL, 0.29 mmol) was added. The mixture
was stirred at room temperature for 6 h. The solvent was evaporated
to dryness under vacuum to obtain an orange solid (0.10 g, 79%).
1H NMR (400 MHz): δ 7.36 (m, 8H), 7.15 (m, 2H), 4.15 (t, 4H,
J ) 7.6 Hz), 4.24 (t, 4H, J ) 6.8 Hz), 4.45 (t, 8H, J ) 6.8 Hz),
1.5–1.8 (m, 32 H), 0.9 (m, 24H). 13C NMR (100 MHz): δ 153.74,
148.76, 139.77, 129.45, 128.54, 123.52, 118.55, 67.77, 67.61, 65.00,
31.02, 30.84, 30.72, 19.19, 19.10, 18.96, 13.86, 13.56, 13.48.
Kinetic Measurements. Reaction of 1aZr with 4 equiv of 2
at -80 °C. Zirconium(IV) n-butoxide (0.14 g, 0.37 mmol) was
dissolved in 10 mL of tetrahydrofuran. This solution was kept in
an acetone/dry ice bath maintained at -80 °C. After 45 min of
equilibration, phenyl isocyanate (0.16 mL, 1.48 mmol) was added.
The mixture was stirred at -80 °C for 72 h. The reaction was
monitored at regular intervals by transferring aliquots of 0.1 mL
into an NMR tube, decomposing by addition of water, removal of
solvent under vacuum, and product dissolution in CDCl3 before
1H NMR analysis. To this sample was then added known amounts
p-bromoacetophenone as an internal standard. Concentrations of
organic products 5 and 6 were estimated from integrals of methene
protons of ligand with respect to methyl protons of the internal
standard. The procedures for other kinetic experiments are similar
and are given in the Supporting Information.
Acknowledgment. We thank the Centre for Modelling,
Simulation and Design (CMSD) and the High Performance
Computing Facility (HPCF) of the University of Hyderabad,
Maui High Performance Computing Center (MHPCC) at
Hawaii, and the Supercomputer Education and Research
Centre (SERC) of Indian Institute of Science for computa-
tional facilities. We also thank the DST, New Delhi, for
providing us funds for purchasing a 400 MHz NMR
spectrometer. A.K. and S.D. gratefully acknowledge senior
research fellowships from CSIR. A.G.S. and E.D.J. thank
the DST for the award of a research grant.
Supporting Information Available: Total energy, optimized
Cartesian coordinates, geometries, and detailed procedures for
kinetic measurements. This material is available free of charge via
the Internet at http://pubs.acs.org.
Compound 5, Butyl N-phenyl carbamate23a. Zirconium(IV)
n-butoxide (0.14 g, 0.37 mmol) was dissolved in 10 mL of
tetrahydrofuran. To this solution was added phenyl isocyanate (0.16
mL, 1.48 mmol) and stirred for 30 h. The solvent was evaporated
to dryness under vacuum to yield a yellow paste. This mixture was
redissolved in methylene chloride, taken in a separating funnel, and
washed with distilled water, and the organic layer was extracted
by evaporation. This organic layer contains 5 and 6 in the ratio
12:1. They were separated by preparative TLC using 1% ethyl
acetate in petroleum ether as eluent. Compound 5: mp ) 62-63.5
°C. 1H NMR (400 MHz): δ 7.37 (d, 2H, J ) 8 Hz), 7.30 (t, 2H, J
) 8 Hz), 7.05 (t, 1H, J ) 7.3 Hz), 6.63 (s, 1H, NH), 4.17 (t, 2H,
J ) 6.7 Hz), 1.65 (m, 2H), 1.42 (m, 2H), 0.95 (t, 3H, J ) 7.3 Hz).
13C NMR (100 MHz): δ 153.78, 138.02, 129.10, 123.37, 118.63,
65.18, 31.02, 19.14, 13.30.
OM701004F
(23) (a) McGhee, W.; Riley, D.; Christ, K.; Pan, Y.; Parnas, B. J. Org.
Chem. 1995, 60, 2820. (b) Wong, S.-W.; Frisch, K. C. J. Polym. Sci. Part
A: Polym. Chem. 1985, 24, 2867.
(24) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Becke, A. D.
Phys. ReV. A 1988, 38, 3098. (c) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV.
B 1988, 37, 785.
(25) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt,
W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt, W. R.
J. Chem. Phys. 1985, 82, 299.
(26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery,, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A. Gaussian 03,
ReVision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
Compound 6, Butyl 2,4-diphenylallophanate23b. Mp ) 64.5
-65.5 °C. 1H NMR (400 MHz): δ 10.91 (s, 1H, NH), 7.53 (d, 2H,
J ) 7.6 Hz), 7.41 (m, 3H), 7.31 (t, 2H, J ) 7.6 Hz), 7.22 (d, 2H,
J ) 7.6 Hz), 7.09 (t, 1H, J ) 7.6 Hz), 4.15 (t, 2H, J ) 6.8 Hz),
1.48 (m, 2H), 1.18 (m, 2H), 0.82 (t, 3H, J ) 7.6 Hz). 13C NMR
(100 MHz): δ 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. HRESMS:
found m/z ) 335.1356 (M + Na)+, calcd mass for C18H20N2O3Na
is 335.1372.