Chemistry Reference
In-Depth Information
Δ
E
/kcal mol
-1
10
TSIc
R
R
C
O
TSId
O
C
N(1)
N(2)
5
N(1)
N(2)
TSIa
N(3)
TS
N(3)
1a, 1c,
1d
1c, 1d
R
0
anti
-RC(O)N
3
C
N(1)
syn
-RC(O)N
3
O
N(3)
N(2)
-5
1a
FIGURE 12.2.
Stationary point on the PES for
syn
-
anti
transformation of formyl (
1c
), acetyl
(
1d
), and benzoyl (
1a
) azides calculated at the PBE/TZ2P level of theory.
45
Source
: Reprinted
with permission from Ref. 45. Copyright 2005 Springer Science
þ
Business Media.
singlet acetyl nitrene (
1
3d
G
#
) for this process was
found to be 32.2 kcal/mol at the CBS-QB3 level (Fig. 12.1). The lower energy
conformer of
). The free energy of activation (
D
1d
has a
syn
relationship between the carbonyl and the azide groups
(
syn
-
1d
). Calculations predict that
syn
-
1d
will decompose by concerted migration of
methyl group along with nitrogen extrusion. The free energy of activation for this
concerted process is only 27.1 kcal/mol (at CBS-QB3 level). Hence,
syn
-
is the
predominating conformer, and it undergoes transformation much faster than
anti
-
1d
.
Thus, calculations predict that Curtius rearrangement of acetyl azide is a concerted
process.
Independently,
45
Zabalov and Tiger investigated structures of formyl, acetyl, and
benzoyl azides (
1d
, respectively) and explored stationary points on the
PES for the Curtius rearrangement of these azides using density functional theory at
the PBE/TZ2P level. As previously described,
18
they localized for these azides two
stable conformations,
syn
and
anti
, with the
syn
conformations being more stable
than the
anti
(Fig. 12.2). For
1c
,
1d
, and
1a
1c
and
1d
, the enthalpy of
anti-syn
isomerization was
predicted to be low (about
0.9 kcal/mol), whereas for benzoyl azide
1a
it is
6.7 kcal/mol. The barriers for the
syn
-
anti
isomerization are predicted to be 9.2,
9.4, and 7.0 kcal/mol for
1a
,
1c
, and
1d
, respectively. Note, that values calculated for
1d
at the PBE/TZ2P level are noticeably lower than those predicted at high level of
theory (CBS-QB3).
18
Zabalov and Tiger
45
also localized stationary points on the PES for the thermal
decomposition of carbonyl azides
. There are, however, no information
on the free energies of activation for the processes under study; only relative
1a
,
1c
, and
1d
