Journal of the American Chemical Society
ARTICLE
Scheme 5. Calculated Energies of Ground and Transition
States (in kcal/mol) at the B3LYP/6-31G** þ ZPVE Level
Figure 5. Final photolysis product: acyclic carbodiimide 22. (a) Spec-
trum after 878 min of broadband photolysis of Ar matrix isolated
tetrazolo[1,5-a]quinoxaline 11 at 15 K (selected wavenumbers: 3423,
2157, 2131, 1505, 851, 788, 757, 640, 517, 475, 411 cmꢀ1). (b)
Calculated spectrum of acyclic carbodiimide s-Z-22 (3446, 2150,
2112, 1482, 833, 744, 470, 408 cmꢀ1). (c) Calculated spectrum of
acyclic s-E-22 (3438, 2151, 2119, 1485, 824, 743, 467, 395 cmꢀ1 (all
calculations at B3LYP/6-31þG** level, wavenumbers scaled by 0.9613).
Ordinate in arbitrary absorbance units. The same compound (22) is
obtained by photolysis of the cyclic carbodiimide 15 originating from
tetrazoloquinazoline 13 (see Figure S9).
We propose that essentially the same mechanism is followed in
the FVT reactions (Schemes 3 and 5). Here, there will be plenty
of energy available to form N-cyanobenzimidazole 17 by the
reaction 21 f 17, and o-cyanamidobenzonitrile 16 by the
reactions 21E f 23 f 26 and 21Z f 22 f 23, whereby the
last step will be a solid- or liquid-phase tautomerization rather
than a ‘forbidden’ 1,3-H shift with a high calculated barrier
(82 kcal/mol) (Scheme 5).
Substituted Quinoxalinyl- and Quinazolinylnitrenes. The
methoxy- and chlorotetrazoloquinoxalines 24T underwent simi-
lar matrix photolysis via the azides and nitrenes to afford the
cyclic carbodiimides 26 (26a: 2000 cmꢀ1; 26b: 2005 cmꢀ1
)
give either the diazirine 14Az (Scheme 5) or the transition state
leading to 15. The two processes have similar activation barriers
on the singlet energy surface, but the formation of 15 dominates
the subsequent chemistry because 15 is a relatively stable
molecule. The ring-opening of 15 to the nitrile ylide 21 has a
calculated barrier of ca. 24 kcal/mol (Scheme 5), and the
reversion of the ylide 21 to the seven-membered ring 15 is
almost barrier-free. The two geometric isomers of 21 can
interconvert via a low 14 kcal/mol barrier, and the E-isomer
21E cyclizes to 1-cyanobenzimidazole in an almost barrier-free
process (Scheme 5). The Z-isomer 21Z can form the same
product via a 7.5 kcal/mol barrier. The 1,7-H shift1,3,4 converting
21Z to the final ring-opened carbodiimide 22 also has a very low
barrier (2.5 kcal/mol; Scheme 5). To summarize, the reactions
12 and 14 f 15 f [21] f 22 are calculated to be very facile
processes, and they correspond to the experimentally observed
processes, whereby ylide 21 is still unobserved, and the very low
activation barrier for its reversion to 15 will make it extremely
difficult to detect. Nevertheless, direct evidence for substituted
derivatives of 21 will be given below.
(Scheme 6). Thus, matrix deposition of the 5-methoxytetrazo-
loquinoxaline 24Ta through an FVT oven at ca. 250 °C affords a
mixture of 24Ta and the azide 24Aa. Photolysis of the azide at
308 nm generated the nitrene 25a, characterized by its IR
spectrum (Figure 6), and the UVꢀvis spectrum which features
a red shift of 32 nm compared to 12 (449 nm band f481 nm;
Figure S13). Photolysis in the visible absorption band of 25a
caused disappearance of the nitrene and formation of the seven-
membered ring carbodimide 26a. The experimental spectra of
both the nitrene and the carbodiimide show excellent agreement
with the calculated spectra (Figure 6).
Mild FVT of the chloro derivative 24Tb at 240 °C afforded the
corresponding azide 24Ab (2141, 2137, 1333, and 1330 cmꢀ1).
Ar matrix photolysis of the azide at 308 nm led rapidly and cleanly to
the nitrene, absorbing up to 650 nm in the UVꢀvisible (Figure 7).
The nitrene was observed simultaneously in the IR (Figure 8 and
Figure S14). Further photolysis at λ > 610 nm or at 395ꢀ460 nm
caused disappearance of the nitrene and clean formation of the cyclic
carbodiimide 26b, whose IR spectrum is again in excellent agree-
ment with calculations (Figure 8; Scheme 6).
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dx.doi.org/10.1021/ja111155r |J. Am. Chem. Soc. 2011, 133, 5413–5424