Chemistry Reference
In-Depth Information
TABLE 12.1. The Relative Electronic Energies (in Kilocalories per Mole) of the
Stationary Points on the PES for the Curtius Rearrangement of Carbonyl Azides
(R-C(O)N
3
) Calculated at the PBE/TZ2P Level
Stationary Points
1c [R
¼
H]
1d [R
¼
CH
3
]
1a [R
¼
Ph]
syn
-
1
57.1
37.9
40.4
TS1
79.7
70.8
74.9
anti
-
1
52.6
38.9
47.1
TS2
87.2
71.8
79.4
1
3 þ
N
2
86.1
66.6
72.6
TS3
þ
N
2
107.0
85.5
86.2
2þ
N
2
0.0
0.0
0.0
The RNCO
þ
N
2
state was chosen as a reference.
45
electronic energies of different species and transitions states are presented (Fig. 12.3,
Table 12.1).
45
According to previous results,
18
Figure 12.3 demonstrates that
syn
-
carbonyl azides undergo concerted rearrangement, whereas the
anti
conformers
undergo stepwise decomposition through singlet nitrenes. The relative electronic
energies of the stationary points for the rearrangement of azides
1a,c,d
are given in
Table 12.1.
Analysis of the results shown in Figure 12.3, and Table 12.1 demonstrates that for
azides
calculations predict that the concerted Curtius rearrangement is a
preferable mechanism of decomposition. Moreover, the calculations for
1a
,
c
1a
agree
fairly well with experiment. Indeed,
the calculated barrier for the concerted
rearrangement of
in a gas phase (34.5 kcal/mol) is close to the activation energy
of its Curtius rearrangement in hexane (31.6 kcal/mol).
31,44
In the case of acetyl azide
1d
1a
, preference of concerted versus stepwise rearrangement is negligible (Table 12.1),
which contradicts with the results of high-level calculations.
18
This is mainly due to
the pronounced difference in the enthalpy of
anti-syn
isomerization as calculated by
the two methods.
In a subsequent paper,
46
Zabalov and Tiger analyzed the influence of the
ortho
-
alkyl substituents on the thermal decomposition of benzoyl azide (
). Using the
PBE/TZ2P level of theory, they localized a number of
syn
and
anti
conformers for
o
-
methyl,
o
-isopropyl,
o
-
tert
-butyl, and
o
,
o
-dimethyl derivatives of
1a
. The
syn
conformers were found to be more stable than
anti
conformers, the electronic
energy difference between the most stable
syn
and
anti
conformers being in the range
3-7 kcal/mol.
Similar to
1a
, the
syn
conformers of
ortho
-substituted benzoyl azides were found
to undergo concerted rearrangement. The PES displayed in Figure 12.4 demonstrates
that
ortho-alkyl
substituents destabilize the structure of benzoyl azide and hence
reduce the barrier of the Curtius rearrangement. This agrees well with experimental
finding.
31,44
Recently,
56
Tarwade et al. performed a combined experimental and theoretical
study of the Curtius rearrangement of cyclopropenoyl (
1a
5a
) and cyclopropanoyl (
5b
)
azides (Scheme 12.3). The kinetics of the Curtius rearrangement of
5a,b
were studied
