2-Pyrazinylnitrene and 4-Pyrimidylnitrene
P h otolysis of Tetr a zolo[1,5-a ]p yr a zin e 9T/2-Azid op y-
r a zin e 9A in Ar Ma tr ix a t 7 K. Tetrazole 9T was sublimed
at 70 °C in a stream of Ar and deposited on a CsI window at
25 K in the course of 20 min. The matrix was cooled to 7 K
and the IR spectrum of 9T/9A recorded: IR (Ar, 7K) 3091 w,
3018 w, 2311 w, 2209 w, 2165 m, 2152 m, 2129 s, 2119 w,
1852 w, 1583 m, 1579 m, 1523 m, 1518 m, 1477 s, 1474 s, 1467
m, 1462 m, 1446 w, 1436 w, 1403 s, 1369 w, 1356 m, 1345 s,
1307 w, 1301 w, 1291 s, 1232 w, 1205 w, 1187 w, 1174 w, 1168
w, 1157 w, 1151 w, 1140 w, 1128 w, 1086 w, 1080 m, 1030 w,
1025 w, 1020 m, 1018 w, 1016 w, 1011 s, 993 w, 913 w, 908 w,
842 w, 822 w, 798 m, 796 m, 794 s, 718 w, 646 w, 644 w, 597
m, 544 w, 430 w, 426 w, 416 w cm-1. UV irradiation (λ > 260
nm; 45 min) using the unfiltered light from a 1000-W high-
pressure Hg/Xe lamp resulted in complete conversion to
1-cyanoimidazole 11 and carbodiimide 22: IR (Ar, 7K) 3430
m, 3420 m, 2282 w, 2268 m, 2204 w, 2135 s, 2118 m, 2084 w,
2044 w, 1642 w, 1625 w, 1532 w, 1485 m, 1428 w, 1378 w,
1281 m, 1252 w, 1240 w, 1234 m, 1226 w, 1105 w, 1103 w,
1070 w, 1067 w, 1065 w, 1015 w, 1011 w, 1008 w, 1001 m, 890
m, 851 w, 826 w, 819 w, 810 m, 794 w, 768 m, 761 w, 737 w,
733 m, 725 w, 645 w, 642 m, 608 w, 597 w, 593 w, 587 w, 544
than the corresponding singlet nitrenes, and they are
significantly more stable than 21 (Table 2). Ring expan-
sion of phenylnitrene to didehydroazepine has been
shown to be a facile process.24 We believe that similar
ring expansion reactions of nitrenes 10 and 25 to 21 are
also relatively straightforward processes.
We have cursorily considered the alternate seven-
membered-ring cumulenes 31 and 32 that might have
been formed by ring expansion of nitrenes 10 and 25. At
the B3LYP/6-31+G* level the bond lengths of the former
indicate that it exists as an ylide (31′) with its major IR
absorption at 1761 cm-1; however, the MP2/6-31G*
geometry is that of structure 31 with a long N-N single
bond (1.49 Å). This observation has prompted us to
undertake more extensive calculations on seven-mem-
bered-ring cumulenes in order to determine the reality
of ylides of the type 31′. The results will be published
elsewhere. Compound 32 exists as a ketenimine with a
major IR absorption at 1868 cm-1 as expected at the
B3LYP/6-31G* level. No experimental observations make
it necessary to consider compounds 31-32 further in the
present chemical context.26
w, 538 w, 507 w, 478 w, 418 w cm-1
.
Wa r m u p of Ma tr ix-Isola ted 11 a n d 22: Con ver sion of
22 to Cya n a m id e 23. The Ar matrix containing 11 and 22
was warmed to 36 K, at which temperature Ar started to
evaporate as indicated by a slight pressure increase. The
temperature was raised slowly by 0.5 K every 10-15 min in
the course of several hours, until all Ar had evaporated. The
temperature at this stage was 41-42 K. Broadened IR bands
resulted for the neat 11 and 23: IR (Ar, 41-42 K) 3128 m,
2268 s, 2238 s, 2126 m, 2030 w, 1534 w, 1480 m, 1401 w, 1374
w, 1328 w, 1282 m, 1261 m, 1240 m, 1185 m, 1102 m, 1068 m,
1026 w, 1001 m, 930 w, 907 m, 892 w, 832 m, 733 m, 642 m,
The relative energies of the cyanoimidazoles at the
B3LYP/6-31+G* level are the following: 1-CN, 15.4 kcal/
mol; 2-CN, 2.3 kcal/mol; 4-CN, 0.0 kcal/mol; 5-CN, 0.2
kcal/mol. High-temperature FVT causes sigmatropic 1,5-
shifts of cyano groups in cyanocyclopentadienes, pyrroles,
imidazoles, and their benzo analogues.25 Such processes
have not been observed under the conditions of the
present paper but takes place very easily in 2-chloro-1-
cyanoimidazole.27
In summary, selective ring opening of the seven-
membered-ring cyclic carbodiimide 21 to the nitrile ylide
zwitterion 28 provides a mechanistic rationale for the
formation of the final products, 11 and 22. Further work
in our laboratory indicates that ring opening of het-
eroarylnitrenes may be a rather common reaction under
the conditions of matrix-isolation photolysis, and in
several cases also on gas-phase thermolysis. Some het-
eroarylcarbenes undergo analogous ring opening reac-
tions. The ring openings may take place in either the
nitrene (carbene) itself or in the seven-membered-ring
cumulene, depending on the nature of the system.
592 m, 553 w, 505 w, 484 w cm-1
.
Dep osition of Au th en tic 1-Cya n oim id a zole 11 a t Cr yo-
gen ic Tem p er a tu r es. (a ) With Ar . A stoppered Pyrex vessel
containing 11 was immersed in an ice-salt bath (-15 °C), the
stopper was opened, and 11 was volatilized and co-deposited
with a stream of Ar onto a CsI window at 25 K in the course
of 16 min. The coolant was replaced with ice (-2 °C), and
deposition continued for another 15 min, after which the ice
bath was removed and the vessel allowed to warm to room
temperature (5 min). The sample was stoppered and the
matrix cooled to 7 K. Some absorptions of 11 were split into
doublets: IR (Ar, 7 K) 3135 w, 2282 w, 2278 w, 2269 s, 2266
s, 1617 w, 1597 w, 1593 w, 1531 w, 1526 w, 1483 s, 1480 s,
1378 w, 1375 w, 1289 w, 1281 m, 1278 m, 1252 w, 1234 m,
1233 m, 1226 w, 1106 w, 1101 m, 1067 w, 1064 m, 1001 m,
894 w, 889 w, 888 w, 811 m, 727 w, 725 m, 723 m, 642 m, 595
w, 587 w, 586 w, 498 w, 478 w cm-1
.
(b) With ou t Ar . The stoppered vessel containing 11 was
cooled with an ice-salt bath (as above), the stopper was
opened, and 11 was sublimed onto the deposition window at 7
K for 20 min. The vessel was closed, and the IR spectrum
recorded. All bands were broadened: IR (neat, 7 K) cm-1 3126
m, 2266 s, 1530 w, 1483 m, 1375 w, 1281 m, 1239 m, 1192 w,
1101 w, 1064 m, 999 m, 979 w, 894 w, 831 w, 752 w, 642 m,
589 w, 485 w. Warming of 11 to 55 K caused no changes in
the IR spectrum.
Exp er im en ta l Section
Compounds 9T,28 11,28 and 24T29 were prepared according
to literature procedures.
P h otolysis of Tetr a zolo[1,5-c]p yr im id in e 24T/4-Azi-
d op yr im id in e 24A in Ar Ma tr ix a t 7 K. Tetrazole 24T was
sublimed at 50-52 °C in a stream of Ar and co-deposited with
Ar on a CsI window at 25 K in the course of 20 min. The matrix
was cooled to 7 K, and the IR spectrum of 24T/24A was
recorded: IR (Ar, 7K) 3065 w, 3045 w, 2449 w, 2436 w, 2295
w, 2289 w, 2253 w, 2203 w, 2177 w, 2171 w, 2166 m, 2155 w,
2133 s, 2009 w, 1621 m, 1611 w, 1578 w, 1590 m, 1576 s, 1566
w, 1561 w, 1549 w, 1530 w, 1504 w, 1495 w, 1463 m, 1458 m,
1402 w, 1396 w, 1387 m, 1376 w, 1371 w, 1354 w, 1349 w,
1339 w, 1321 w, 1313 w, 1309 w, 1303 s, 1293 m, 1244 w, 1231
w, 1224 w, 1200 w, 1164 w, 1148 m, 1140 w, 1114 w, 1099 w,
(24) J ohn, W. T. G.; Sullivan, M. B.; Cramer, C. J . Int. J . Quantum
Chem. 2001, 85, 492.
(25) Wentrup, C.; Crow, W. D. Tetrahedron 1970, 26, 3965. Wentrup,
C.; Crow, W. D. Tetrahedron 1970, 26, 4915.
(26) For further calculations on zwitterionic and neutral heterocu-
mulenes see: Kuhn, A.; Vosswinkel, M.; Wentrup, C. J . Org. Chem.
2002, 67, in press.
(27) Addicott, C.; Wentrup, C. Unpublished results. Addicott, C.
Ph.D. Thesis, The University of Queensland, Brisbane, Australia, 2002.
(28) Wentrup, C.; Crow, W. D. Tetrahedron 1970, 26, 4969.
(29) Temple, C.; McKee, R. L.; Montgomery, J . A. J . Org. Chem.
1965, 30, 829.
J . Org. Chem, Vol. 67, No. 24, 2002 8545