Chemistry Reference
In-Depth Information
Calculations at the B3LYP/6-31G
level indicate that the phenyl substitution
stabilizes ylide
172,
hence it is only about 14 kcal/mol higher in energy than the
cyclic carbodiimide
, and the barrier between them is only about 18 kcal/mol
from the carbodiimide side. The s-
E
ylide
170
172
can cyclize via a very low barrier
175
(about 8 kcal/mol) to the 1-cyanobenzimidazole
.
Conclusive evidence for the ylide as well as the seven-membered cyclic carbo-
diimide and their interconversion was obtained by adding a nitro group to the
benzene ring of quinazolyl nitrene (Scheme 8.28). The extended conjugation
resulted in a spreading of the UV-Vis spectrum, which facilitated selective photo-
chemistry. Thus, 7-nitro-2-phenyl-4-quinazolinyl nitrene
168-NO
2
was generated in
the same manner as
.
Photolysis of this nitrene in its visible absorption band at
168
l
>
695 nm converted it
n
max
2013 cm
1
). Photolysis
of the latter at 308 nm caused reversible ring opening to the nitrile ylide
into the cyclic carbodiimide
170-NO
2
(
l
max
340 nm;
172-NO
2
(
l
max
479-540 nm). The two
E
and
Z
isomers of this ylide were observed in the IR
and UV-Vis spectra, and it was possible to bleach them selectively on irradiation at
l
>
550 and
l
>
495 nm, respectively, when they both reverted to the cyclic
carbodiimide
170-NO
2
. It was possible to cycle several times between the ylides
and the seven-membered ring, but eventually a small amount of 1-cyanobenzimidazole
175-NO
2
was also formed. This compound is formed in quantitative yield on FVT at
400-500
C.
45
The calculated energy difference between the
Z
-nitrile ylide
172-NO
2
and the
cyclic carbodiimide
170-NO
2
has decreased further to 11 kcal/mol in this system.
The activation barrier between the two is 14 kcal/mol from the carbodiimide side and
only 5 kcal/mol from the ylide side, while the barriers to cyclization of the ylide to
the 1-cyanobenzimidazole has increased to 12 kcal/mol. Thus, the nitrile ylides are
protected in a deeper potential energy well.
N
CN
N
N
Ph
N
NO
2
NO
2
N
Ph
175-NO
2
168-NO
2
CN
N
N
N
CN
N
NO
2
N
NO
2
N
NO
2
N
C
Ph
C
Ph
Ph
170-NO
2
Z
-
172-NO
2
E
-
172-NO
2
SCHEME 8.28.
