Chemistry Reference
In-Depth Information
O
O
>310 nm
-N
2
, Ar, 12 K
h
v
R
′
CH
2
O
+ O
C
N
R
′
CHO + HN
C
O
H
2
CO
3
N
3
56a-c
R
′
R
′
H
2
CO
N
3
58a-c
57
53a-c
SCHEME 12.27.
Photolysis of azide formates
53a-c
with a built-in intramolecular triplet
sensitizers in cryogenic matrices.
129
Photolysis of azidoformate
53d
in the argon matrix yields isocyanate
59d
,
presumably via secondary photolysis of triplet nitrene
3
56d
yielding again alkoxy
and cyano radicals.
129
These radicals recombine, since the aryloxy radical does not
have any abstractable H-atom (Scheme 12.28). Note that the formation of the triplet
(4-acetylphenoxy)carbonyl nitrene
3
56d
upon irradiation of
53d
was detected
previously at 8 K by EPR spectroscopy.
51,52
To understand the difference in the properties of carbonyl nitrenes (R
CO
N)
and nitrenoformates (RO
CO
N),
the structures and singlet-triplet splitting
D
E
ST
) for simplest nitrenes (R
¼
(
H and Me) were analyzed at the CCSD(T) and
CBS-QB3 levels of theory.
17,18
According to the calculations, both nitrenoformates
(CH
3
O
N) have triplet ground state, whereas carbonyl
nitrenes (CH
3
CO
N and H
CO
N) are singlet ground state species.
The calculated structures in Figure 12.18 demonstrate that singlet nitrenes
1
3d
CO
N and HO
CO
and
1
have very similar geometries of the CON-fragment, and the ON bond is only
slightly longer and the NCO bond angle slightly larger in the latter case. Geometries
of the corresponding triplet nitrenes are also very similar, although the NCO bond
angle and NO distance are also noticeably larger in the latter nitrene. Nevertheless,
carbonyl nitrenes (R-CON) and nitreneformates (RO
49b
CON) have ground states of
different multiplicity. The nitreneformates have triplet ground state, although their
singlet excited states have the same structure as the singlet ground states of carbonyl
nitrenes, namely, a structure of a cyclic oxazirene.
17-19
The difference in the
E
ST
between carbonyl nitrenes and nitreneformates was attributed to the smaller bonding
interaction between the nitrogen and the oxygen atoms in the latter case.
19
However,
Liu and coworkers
18
demonstrated using isodesmic reaction calculations that
methoxycarbonyl nitrene has a triplet ground state because the oxygen atom
stabilizes the triplet state of the carbonyl nitrene more than the corresponding
singlet state.
Note that the simple B3LYP calculations reproduce very well the geometry
of singlet acyl nitrenes.
17-19
However,
D
the B3LYP calculations overestimates
O
O
>310 nm
-N
2
, Ar, 12 K
h
v
Ar-O
+ O
C
N
ArO
N
C
O
3
N
3
56d
ArO
ArO
N
3
59d
53d
SCHEME 12.28.
Photolysis of
(4-acetylphenoxy)carbonyl azide
53d
in cryogenic
matrices.
129
