Chemistry Reference
In-Depth Information
45a
- R = COCH
3
45b
- R = COCF
3
45c
- R = CO
2
CH
3
45d
- R = Ts
+
S
-
NR
Structure of the N-substituted dibenzothiophene sulfilimines.
26
SCHEME 12.19.
Shortly after
26
Toscano et al. synthesized a series of N-substituted dibenzothio-
phene sulfilimines (
, Scheme 12.19) and studied their photochemistry. Time-
resolved infrared (TRIR) spectroscopy, product studies, and computational methods
were applied to study the photoreactivity of sulfilimines derived from dibenzothio-
phene that were expected to release acetyl nitrene (
1
3d
45a-d
),
mesyl nitrene, and tosyl nitrene. Reactions of the mesyl- and tosyl nitrenes are
discussed in Section 12.3.
As aforementioned, the electronic structure and properties of acetyl nitrene
1
3d
were analyzed theoretically by Liu et al.
18
The free energy of activation for the
Curtius rearrangement of this nitrene was predicted to be sufficiently high (
), trifluoroacetyl nitrene (
46
G
#
D
21.7 kcal/mol at the CBS-QB3 level).
18
Thus, this reaction does not compete with
the trapping of
1
3d
ΒΌ
by solvent molecules. The PES for the reactions of
1
3d
with
propane, ethylene, and methanol were calculated by both the DFT and the CBS-QB3
methods, and the activation free energies of these reactions were controlled by
entropic factors. It
is noteworthy that
the singlet-triplet splitting for
3d
was
calculated at the B3LYP, CCSD(T), CBS-QB3,
18
and G2
101
levels. The B3LYP
and CCSD(T) methods predict that
will have triplet ground state, as opposed to
the G2 and CBS-QB3 theories, which indicate that
3d
3d
has a singlet ground state.
, the B3LYP method predicts
3
3d
3c
Similar to
to be too stable by approximately 8-
9 kcal/mol. The
4.0 kcal/mol at the CBS-QB3 level
(
5.7 kcal/mol at the G2 level), and nitrene
3d
is most likely a ground state singlet.
Based on the TRIR data and aforementioned theoretical results, Toscano and
coworkers proposed the following mechanism for the photoreactivity of sulfilimines
45a
D
E
ST
value was predicted to be
(Scheme 12.20). TRIR spectroscopy made it possible to detect an IR band at
1518 cm
1
that was assigned to triplet nitrene
3
3d
. Unfortunately, singlet nitrene
1
3d
was not observed in CH
2
Cl
2
due to overlapping of its band with an IR band at
1776 cm
1
, which grows in at the same rate as the triplet nitrene, and the rate depends
on the initial concentration of the precursor
45a
. This band was attributed to the
reaction between the singlet acetyl nitrene and
. A band
at 1680 cm
1
was also observed and assigned to the product of the singlet acetyl
nitrene reaction with CH
2
Cl
2
.InCD
3
CN, the adduct band at 1776 cm
1
was very
weak and a new, strong band at 1648 cm
1
was detected and attributed to the ylide
46a
44
or the starting material
45a
(Scheme 12.20).
26
The decrease of the 1776 cm
1
band was attributed to the
competition between ylide
formation and reaction of
1
3d
46a
with
45a
or
44
.
Product study
26
is consistent with a singlet ground state of
3d
, and the TRIR data
suggest that the triplet state of
3d
is populated slowly (
280 ns). The latter fact was
attributed to the thermal population of
3
3d
from
1
3d
. From the intensity and rate of
