R. Devendra et al. / Journal of Molecular Catalysis A: Chemical 395 (2014) 72–86
75
3.2. Energy diagrams for the two pathways
Fig. 4 shows the energy diagram for the two types of interactions
that were discussed previously. Pathway 1 lead to the formation of
N-substituted phenyl carbamate (methyl phenyl(trimethylstannyl)
carbamate) and pathway 2 leads to the formation of O-substituted
phenyl carbamate (methyl trimethylstannyl phenylcarbonimi-
carbamate (methyl phenyl(trimethylstannyl)carbamate). The acti-
vation energy for the first transition state for pathway 1 is
23.9 kJ mol−1 less than for pathway 2.
From Fig.
transition states and for pathway
can be found. For pathway
TS11 (95.5 kJ mol−1
4
it can be seen that pathway
three transition states
in the
1 has two
2
1
)
interaction between trimethyl tin methoxide and phenyl iso-
cyanate form the trimethyl tin N-phenyl carbamate (methyl
phenyl(trimethylstannyl)carbamate), where the oxygen of the
methoxy group is coordinated with tin atom. TS12 (79.8 kJ mol−1
)
represents the rotation of the isocyanate carbonyl to form
the trimethyl tin N-phenyl carbamate. In pathway
2 TS21
(132.9 kJ mol−1) is to form trimethyl tin O-phenyl carbamate
(methyl trimethylstannyl phenyl carbonimidate). For pathway 2,
TS22 (90.1 kJ mol−1) corresponds to the rearrangement of the
carbamate to give N-coordinated species by rotating around
Fig. 3. Potential energy surface for the rotation of the phenyl isocyanate molecule
the Sn
O C
N dihedral angle, whereas TS23 (42.68 kJ mol−1) is
the transition state for the change from O-substituted (methyl
trimethylstannyl phenylcarbonimidate) to N-substituted (methyl
phenyl(trimethylstannyl)carbamate) carbamate. This leads to
the formation of the N-substituted phenyl carbamate (methyl
phenyl(trimethylstannyl)carbamate) as given in literature [1]. The
value in the bracket is the free energy difference from the starting
material at 298 K in gas phase.
Structure 1 (Table 1) as the initial orientation of the two
molecules has a distance of 5.12 A between the alkoxy oxygen
˚
and isocyanate carbon. In structure 2 the distance between the
˚
same atoms is reduced to 3.52 A. Fig. S2a shows the electro-
static isopotential surface for structure 2. It can be seen that
the electrostatic fields of the two oxygen atoms are interact-
ing. At this distance the isocyanate molecule tends to rotate to
adopt the new orientation of structure 3 (O-coordination). The
reason may be due to the interaction between the electrostatic
fields as explained in the previously. Fig. S2b shows the electro-
static isopotential surface for structure 3. The interaction between
electrostatic fields around individual atoms is now less than in
structure 2. This can be further explained as the Lewis action
between the organotin center and the isocyanate oxygen. Finally,
the insertion reaction takes place with the movement of the
alkoxy group of the organotin and latching on to the positive
charged isocyanate carbon. Structure 4 represents the transi-
tion state for the insertion reaction. The distance between the
The free energy is an indication of the conversion of reactants
to products. It is interesting to see that the free energy difference
between the starting materials trimethyltin methoxide and phenyl
isocyanate and the final product trimethyltin N-phenyl carbamate
is positive, which indicate that the reactants are more stable
than the product. In our calculation we used DFT (B3LYP and
PBE) and ab initio MP2 with various basis sets such as 6-31G*,
6-31+G** and DGDZVP for light elements. DFT-B3LYP and MP2
using 6-31G* basis set resulted in positive Free energies 12.51 and
6.79 kJ mol−1, respectively; the DFT-BPE functional gave values
0.69 kJ mol−1 using 6-31G* basis set which was close to zero. How-
ever, with DFT/B3LYP using DGDZVP basis sets a negative value
of −2.33 kJ mol−1was obtained. Steps were also taken to calculate
the free energy difference between triethyltin methoxide and
phenyl isocyanate and the reaction product triethyltin-N-phenyl
carbamate, which gave a positive value of 18.39 kJ mol−1 as well
using DFT B3LYP level of theory with 6-31G* basis set. However,
the product formation from reacting TBTM and phenyl isocyanate
seems to be more stable than the information from thermochemi-
cal data from model compounds. 13C NMR and FTIR measurements
indicate that the reaction between TBTM and phenyl isocyanate
(1:1 mole) leads to the formation of tributyltin phenyl carbamate
without any residual phenyl isocyanate. This may be due to the sta-
bilization of the product due to hydrogen bonding from urethane
formed because of methanol being present as an impurity. If phenyl
isocyanate is replaced with methyl isocyanate the reaction product
with TMTM gives a negative free energy difference −8.78 kJ mol−1
using DFT B3LYP with 6-31G* basis set. The free energy difference
for the reaction was also calculated using the optimized geometries
obtained from DFT B3LYP LANL2DZ/6-31G* level of theory with
DFT B3LYP and PBE with LANL2DZ/6-311+G(2df,2p) level of theory
using single point gave 12.5 and −9.1 kJ mol−1. The calculations do
not result in a uniform picture as the free energy has sometimes
a positive and sometimes a negative value. Experimentally, the
˚
isocyanate carbon and alkoxy oxygen is 1.71 A. With the inser-
utylstannyl phenylcarbonimidate). From this point further steps
will take place as explained in the subsequent sections. We
have also simulated the rotational barrier between the two reac-
tion pathways. Fig. 3 shows the potential energy surface for the
rotation of the phenyl isocyanate molecule from O-coordination
to N-coordinate orientation. The barrier was calculated for the
molecular assembly, where the distance between the C O of
the O-coordinated interaction was set to a pre-selected value to
change to the N-coordinated orientation. The pre-selected dis-
˚
tance of 1.97 A was taken as the distance between the C O for
transition state of N-coordinated interaction, which is approxi-
mately the distance between isocyanate carbon and the alkoxide
oxygen at N-coordinated transition state. At this distance rota-
tional barrier is 32.1 kJ mol−1 for the O-coordinated interaction
to rearrange to N-coordinated interaction. This is also about
16.3 kJ mol−1 higher than the O-coordinated transition state. This
data suggest that the O-coordination has a rotational energy bar-
rier, which prevents it from entering to the N-coordinated reaction
pathway.