Chemistry Reference
In-Depth Information
Δ
E
/kcal mol
-1
TS
70
R
1
R
2
N(1)
O
60
N(2)
N(3)
C(2)
C(6)
50
2
3
1
4
R
1
R
2
40
C(2)
N(1)
syn
-
ArC(O)N
3
R
1
30
N(1)
O
N(3)
C(6)
N(2)
20
+
R
2
N(2)
N(3)
O
10
ArNCO + N
2
FIGURE 12.4.
The stationary points on the PES for the rearrangement of
syn
conformers
of benzoyl azide (1) and its
o
-methyl (2),
o
-iso-propyl (3), and
o
,
o
-dimethyl (4)
substituted derivatives.
Source
: Reprinted with permission from Ref. 46. Copyright 2007 Springer
Science
þ
Business Media.
1
H NMR spectroscopy in toluene-d
8
. The yields of the isocyanates (
by
6a,b
)
95%).
56
Based on the results of previous studies,
18,45
Tarwade et al. calculated barriers for
the concerted rearrangement of
syn
conformers of azides
estimated by
1
H NMR spectra were found to be quantitative (
>
at the B3LYP/6-
311
þ
G(
d
,
p
) level.
56
The barriers in a gas phase (25.1 and 27.8 kcal/mol, respec-
tively) were found to be in a good agreement with experimental enthalpies of
activation in toluene (Scheme 12.3). In addition, they calculated the barrier
(26.3 kcal/mol) for the concerted rearrangement of the acetyl azide (
syn
-
5a,b
1d
)at
the CCSD(T)/6-311
þ
G(
d
,
p
) level, which is in good agreement with the free energy
O
O
O
H
#
= 23.9 kcal/mol
N
C
O
Δ
MeO
N
3
MeO
S
#
= 0.3 cal/mol K
Δ
5a
6a
O
O
O
N
C
O
H
#
= 25.5 kcal/mol
Δ
MeO
MeO
N
3
S
#
= 0.2 cal/mol K
Δ
6b
5b
SCHEME 12.3.
Curtius rearrangement of cyclopropenoyl (
5a
) and cyclopropanoyl (
5b
)
azides.
56
