Fvp reaction conditions showed that this process should
be performed at high temperatures and a mixture of products
was always observed. We decided to study the possibility
of lowering the energy of activation and increasing the
selectivity without changing the type of reaction, and
therefore we chose to use MCM-41 mesoporous molecular
sieves as solid catalysts. Heterogeneous fvp systems using
catalytic materials have potential applications than can
provide new general synthetic methodologies. In our group
we have already studied fvp reactions using zeolites and
hydrotalcites as catalytic materials in the pyrolysis of NH-
pyrazoles4 and azolyl-malonamates5 and this work is the first
report of MCM-catalyzed systems. In this case we compare
the fvp reactions of benzotriazole 1 in homogeneous system6
(without catalyst) and heterogeneous system using Al-MCM-
41 (Si/Al: 20) as the solid catalyst.7
Scheme 1. Mechanism Proposed for Fvp Reactions of
Benzotriazole 1
When homogeneous fvp experiments of 2-(1H-1,2,3-
benzotriazol-1-yl)phenylethanone8 (1) were performed be-
tween 400 and 500 °C, the products mainly identified in the
solid crude were 7H-dibenzo[b,d]azepin-7-one (4), phenan-
thridine (5), and 2-phenyl-4H-benzo[d]1,3oxazin-4-one (12)
(Table 1, Schemes 1 and 2).
cyclization followed by 1,5-hydrogen shift or from carbene
3 by C-H insertion followed by 1,3-H migration.
As in other reactions of alkylbenzotriazoles,9 the N-
phenylimine 7 was obtained by 1,4-hydrogen transfer in the
intermediate 2. However, in this case the imine did not
undergo electrocyclization reaction to give 2-phenyl-1,4-
benzoxazine as was observed in the reaction of allylbenzo-
triazoles.10
Table 1. Results of the Homogeneous Fvp Experiments
temp (°C)
% 1
% 4
% 5
% 12
% other
400
450
500
48
24
0
29
36
30
2
5
15
12
10
15
9a
25a
40b
2-Phenyl-4H-benzo[d][1,3]oxazin-4-one (12) was detected
at all temperatures, and an explanation for this observation
is given in Scheme 2. Benzotriazole 1 can undergo an acyl
migration to give the zwitterionic specie 8, which eliminates
diazomethane (detected in the volatile fractions by GC/MS)
to afford diradical 9. Rearrangement of this intermediate
could afford 10, a hybrid of oxazirene, acylnitrene, and nitrile
oxide.11 The coupling reaction of 10 with diradical 11 could
result in the formation of benzoxazinone 12. It is known that
oxazirenes collapse with many species to give more stable
structures.12 Compound 12 could also be formed by a [2+3]
cycloaddition of 1H-cyclopropabenzen-1-one 16 with the
nitrile oxide, as proposed in the formation of other benzox-
azinones.13 Benzocyclopropenone 16 was not detected in the
reaction crude; however, this key intermediate, in equilibrium
with diradical 11 under reaction conditions, can arise from
ring fragmentations in azepinones 4 and 6. In a control
experiment, the fvp reaction of the pure azepinone 4 at 450
°C gave small quantities of products such as benzonitrile
and biphenylene (14) supporting the presence of 11. Ben-
zonitrile could be formed by rearrangement of the isonitrile
fragment generated in the ring fragmentation, while biphen-
a Minor products: 1,2-dihydrodibenzo[b,d]azepin-7-one (6), 1-phenyl-
2-(phenylimino)ethanone (7), 2-phenylbenzoxazole (13), benzonitrile, and
biphenylene (14). b Minor products: 7, 13, benzonitrile, and biphenylene
(14).
The formation of the major product 4 is explained by
hydrogen loss from the intermediate 2 resulting from nitrogen
extrusion form 1 and followed by intramolecular cyclization
(Scheme 1). As the temperature was raised, the decarbony-
lation reaction of azepinone 4 was favored to afford the
aromatic ring phenanthridine (5). Between 400 and 450 °C
the azepinone 6 was detected by GC/MS as a minor product.
This compound can be formed from the diradical 2 by
(3) (a) Ishida, M.; Muramura, N.; Kato, S. Synthesis 1989, 562. (b)
Paterson, W.; Proctor, G. R. J. Chem. Soc. 1962, 3468.
(4) (a) Moyano, E. L.; Yranzo, G. I. J. Org. Chem. 2001, 66, 2943. (b)
Moyano, E. L.; del Arco, M.; Rives, V.; Yranzo, G. I. J. Org. Chem. 2002,
67, 8147.
(5) Pela´ez, W. J.; Gafarova, I. T.; Yranzo, G. I. ARKIVOC 2003, 10,
262.
(6) Reactions were carried out in vycor glass fvp equipment with use of
a GAYNOR PRDH temperature controller and a Thermolyne 21100 tube
furnace. Oxygen free dry nitrogen was used as the carrier gas. Samples to
be pyrolyzed were 30-50 mg. Contact times were around 10-2 s with
pressures of 0.2 to 0.1 Torr . Products were trapped at the liquid air
temperature, extracted with solvent, and submitted to different analyses or
separation techniques.
(7) In a typical run 0.50 g of fractured catalyst was placed along the
reactor (30 cm length, 1 cm diameter) with use of ceramic wool fiber as an
inert support. All catalysts were preactivated in air at 500 °C for 4 h before
each reaction.
(9) Prager, R. H.; Baradarani, M. M.; Khalafy, J. J. Heterocycl. Chem.
2000, 37, 631.
(10) Baker, S. J.; Jones, G. B.; Randles, K. R.; Storr, R. C. Tetrahedron
Lett. 1988, 29 (8), 953.
(11) (a) Wentrup, C.; Bornemann, H. Eur. J. Org. Chem. 2005, 4521.
(b) Poppinger, D.; Radom, L.; Pople, J. A. J. Am. Chem. Soc. 1977, 99,
7806.
(8) Prepared by a reported procedure, see: Katritzky, A. R.; Wu, J.
Synthesis 1994, 597.
(12) Poppinger, D.; Radom, L. J. Am. Chem. Soc. 1978, 100, 3674.
(13) Lown, J. W.; Matsumoto, K. Can. J. Chem. 1972, 50, 584.
2180
Org. Lett., Vol. 9, No. 11, 2007