N-Mesityl-C-acylketenimines: 1,5-Sigmatropic Shifts and Electrocyclization to Quinolines

Article abstract of DOI:10.1021/jo9722562

Flash vacuum thermolysis (FVT) of triazoles 6a- c generates α-oxoketenimines 10, the ester 10a being isolable. FVT of pyrroledione 8 generates the isomeric imidoylketene 9a. Ketenes 9 and ketenimines 10 undergo thermal interconversion by 1,3-shifts of methoxy and dimethylamino groups under mild FVT conditions (ca. 350-400°C). Both 9 and 10 are directly observable by IR spectroscopy at either 77 K or on Ar matrix isolation at 12 K. On FVT at temperatures above ca. 400°C, the ketenimines 10 undergo a 1,5-H shift to o-quinoid imines 12/13, followed by electrocyclization to dihydroquinolines 14 (unobserved) and 15 (observed by NMR). The latter are easily oxidized to alkylquinoline-3-carboxylates or quinoline-3-carboxamides 16 by atmospheric oxygen. Ab initio calculations on model compounds 18-23 predict an energy barrier of ca. 38 kcal mol^-1 (161 kJ mol^-1) for the 1,5-H shift in N-(o-methylphenyl)ketenimines via the transition state TS19 followed by an electrocyclization barrier to dihydroquinoline 23a via TS22a of ca. 16 kcal mol^-1.

Full text of DOI:10.1021/jo9722562

J . Org. Chem. 1998, 63, 5779-5786
5779
N-Mesityl-C-a cylk eten im in es: 1,5-Sigm a tr op ic Sh ifts a n d
Electr ocycliza tion to Qu in olin es
V. V. Ramana Rao, Belinda E. Fulloon, Paul V. Bernhardt, Rainer Koch,1 and Curt Wentrup*
Department of Chemistry, The University of Queensland, Brisbane, Qld 4072, Australia
Received December 12, 1997
Flash vacuum thermolysis (FVT) of triazoles 6a -c generates R-oxoketenimines 10, the ester 10a
being isolable. FVT of pyrroledione 8 generates the isomeric imidoylketene 9a . Ketenes 9 and
ketenimines 10 undergo thermal interconversion by 1,3-shifts of methoxy and dimethylamino groups
under mild FVT conditions (ca. 350-400 °C). Both 9 and 10 are directly observable by IR
spectroscopy at either 77 K or on Ar matrix isolation at 12 K. On FVT at temperatures above ca.
400 °C, the ketenimines 10 undergo a 1,5-H shift to o-quinoid imines 12/13, followed by
electrocyclization to dihydroquinolines 14 (unobserved) and 15 (observed by NMR). The latter are
easily oxidized to alkylquinoline-3-carboxylates or quinoline-3-carboxamides 16 by atmospheric
oxygen. Ab initio calculations on model compounds 18-23 predict an energy barrier of ca. 38 kcal
mol-1 (161 kJ mol-1) for the 1,5-H shift in N-(o-methylphenyl)ketenimines via the transition state
TS19 followed by an electrocyclization barrier to dihydroquinoline 23a via TS22a of ca. 16 kcal
mol-1
.
In tr od u ction
It was demonstrated in previous work that imi-
doylketenes 1 and oxoketenimines 2 undergo an extraor-
dinarily facile 1,3-shift of the methoxy group at temper-
atures as low as 200 °C under flash vacuum thermolysis
(FVT) conditions. These temperatures do not reflect the
energy barriers for the interconversions 1 a 2, but are
the mildest conditions possible for the generation of 1
and 2 from their precursors (1,2,3-triazoles, pyrrole-2,3-
diones, and Meldrum’s acid derivatives).2 The calculated
activation barriers for 1,3-shifts of electron rich groups
possessing lone pairs (e.g., 1, X ) R′O, R′2N, and Cl) are
very low in the oxoketenes, oxoketenimines, and imi-
doylketenes.3 For example, the 1,3-Cl shift in chlorocar-
midoketenimines 10 carrying N-mesityl and N-(2,6-
dimethylphenyl) groups, generated by FVT of triazole and
pyrroledione precursors 6 and 8 (Scheme 1). The out-
come is another cyclization reaction, involving an aro-
matic o-methyl group and leading to quinolines 16 as the
major products.
bonylketenes (PhC(O)C(Cl)dCdO) takes place at -30 °C
in solution.3c The C-alkoxycarbonylketenimines 2 (X )
OR′) are in some cases isolable at room temperature,
being susceptible to nucleophilic attack by water or
methanol at the electrophilic central carbon atom of the
cumulene moiety. The imidoylketenes 1 are not isolable.
The N-arylimidoylketenes (4, X ) OMe or NMe2) undergo
a very facile cyclization to quinoline-4-ones (5).2
Resu lts
1. F VT Exp er im en ts. FVT of the 1,2,3-triazole 6a
was carried out with product isolation on a BaF2 disk at
77 K for observation by IR spectroscopy. At a FVT
temperature of 400 °C, weak signals ascribed to N-
mesityl-3-carbomethoxyketenimine 10a and imidoylketene
9a were observed (Figure 1a; Scheme 1). The identity
of the ketenimine 10a was established by isolation (see
Experimental Section). At 500 °C, a strong signal due
to the oxoketenimine 10a together with a weak signal
due to the ketene 9a was observed (Figure 1b). As the
temperature increased to 600 °C, both intermediates 9a
and 10a decreased in absolute intensity (Figure 1c), and
further decrease occurred at higher FVT temperatures.
Neither 9a nor 10a was observable after FVT at 800 °C.
Upon warming the deposition disk to room temperature,
the imidoylketene 9a disappeared at -10 °C, whereas
the oxoketenimine 10a remained observable and isolable
In an attempt to hinder the cyclization to a quinolin-
4-one, we have studied C-alkoxycarbonyl- and C-carboxa-
(1) Present address: Fachbereich Chemie, Universita¨t Oldenburg,
D-26111 Oldenburg, Germany.
(2) (a) Fulloon, B. E.; Wentrup, C. J . Org. Chem. 1996, 61, 1363. (b)
Fulloon, B. E.; El-Nabi, H. A. A.; Kollenz, G.; Wentrup, C. Tetrahedron
Lett. 1995, 36, 6547.(c) Clarke, D.; Mares, R. W.; McNab, H. J . Chem.
Soc., Chem. Commun. 1993, 1020. (d) Clarke, D.; Mares, R. W.; McNab,
H. J . Chem. Soc., Perkin Trans. 1 1997, 1799. (e) Synthesis of 3b and
5b: Rao, V. V. R.; Wentrup, C. J . Chem. Soc., Perkin Trans 1 1998, in
press.
(3) (a) Wong, M. W.; Wentrup, C. J . Org. Chem. 1994, 59, 5279-
5285. (b) Bibas, H.; Wong M. W.; Wentrup C. Chem. Eur. J . 1997, 3,
237-248. (c) Finnerty, J .; Andraos, J .; Wong, M. W.; Yamamoto, Y.;
Wentrup, C. J . Am. Chem. Soc. 1999, 120, 1701-1704.
S0022-3263(97)02256-1 CCC: $15.00 © 1998 American Chemical Society
Published on Web 08/05/1998

5780 J . Org. Chem., Vol. 63, No. 17, 1998
Ramana Rao et al.
F igu r e 1. Partial IR spectra (77 K; 1900-2200 cm-1 range; absorbance 0.04-0.33) of the products of FVT (a-c) of triazole 6a at
400-600 °C and (d-h) of pyrroledione 8 at 300-700 °C. K ) ketene 9a (2135 cm-1). I ) oxoketenimine 10a (2081, 2069 cm-1).
Sch em e 1

N-Mesityl-C-acylketenimines
J . Org. Chem., Vol. 63, No. 17, 1998 5781
Sch em e 2
at room temperature. FVT above 700 °C resulted in the
appearance of a new species absorbing at 1721 cm-1 and
assigned as quinoline 16a , which was subsequently
isolated. Quinoline 16a actually started appearing al-
ready at ca. 400 °C but condensed in the cooler parts of
the apparatus before reaching the BaF2 disk under these
conditions.
Preparative FVT of 6a at 450 °C afforded the keten-
imine 10a (isolated in 21% yield) and the quinoline 16a
(56%). Ketenimine 10a was stable at room temperature
for short periods of time and can be stored under N2 at
-5 °C for 1-2 weeks but reacts with atmospheric
moisture to give amide 11a . The 1H and 13C NMR
spectra of this compound indicate that it exists in the
amide rather than the tautomeric imide form.
As indicated in Scheme 1, the formation of quinoline
16 is explained in terms of a 1,5-H shift in ketenimine
10 to give the o-quinoid imine 12. Electrocyclization of
13 affords dihydroquinolines 14 (unobserved) and 15
(observed) prior to oxidation to 16.
imidoylketene 9 was converted to oxoketenimine 10 and
the latter was totally converted to the cyclized products
15a and 16a (combined yield 68%).
Evidence for the presence of the dihydroquinoline
1
intermediate 15a was obtained by GCMS and H NMR
Preparative FVT of the isolated and distilled keten-
imine 10a also resulted in the formation of the quinolines
15a and 16a . Direct evidence for the isomerization of
ketenimine 10a to ketene 9a was obtained by IR spec-
troscopy of the 77 K pyrolysate resulting from FVT of
7a at 400 °C: a small amount of the ketene was observed
in the presence of a large amount of unchanged keten-
imine. The quinoline mixture 15a /16a also started
forming at this mild temperature (400 °C). At 700 °C
and after complete oxidation of 15a , the quinoline 16a
was isolable in 84% yield.
spectroscopy of the product of the preparative FVT of 6a
at 450 °C; this gave a ratio of ca. 1:2.8 of 15a to 16a . The
amount of 15a rapidly decreased, and that of 16a
increased correspondingly due to oxidation by atmo-
spheric oxygen. After ca. 24 h only the quinoline 16a
was present, and it was isolated in 56% yield. The
quinoline mixture started forming at 350 °C (preparative
FVT), when much of the starting material was recovered
unchanged, giving a ratio of 6a :10a :(15a + 16a ) ) 24:
33:9.
On preparative FVT of 6a at 700 °C, a mixture of 15a
and 16a was again obtained. The quinoline 16a was
isolated in 65% yield after full oxidation of 15a . The H
Similar FVT of the analogous 1-mesityltriazole-4-
carboxamide 6b above 500 °C gave rise to an absorption
at 2058 cm-1 (weak at 500 °C; strong at 600 °C) ascribed
to the corresponding ketenimine 10b, which persisted on
warm up to 0 °C. The ketene 9b was not observable at
77 K, but on Ar matrix isolation at 12 K it appeared at
2133/2138 cm-1 (weak at 540 °C; strong at 700 °C FVT
temperature). The ketenimine 10b absorbed strongly at
1
NMR spectrum of 15a (in admixture with 16a ) features
a singlet at 3.71 ppm, ascribed to the CH2 group, a
doublet at 7.34 ppm (H-C(2)), and a broad singlet at 5.85
ppm (NH). Exchange with D2O caused the latter to
disappear and the doublet at 7.34 ppm to collapse to a
singlet. The IR spectrum of the mixture of 15a and 16a
showed a strong, broad signal at 3337 cm-1, assigned to
the N-H stretch in the dihydroquinoline 15a .
The structure of the quinoline 16a was ascertained by
an X-ray crystal structure determination (Figure S1).
Crystal data and bond lengths and angles are reported
in Tables S1 and S2 (Supporting Information).
2061 cm-1
.
Preparative FVT of triazole 6b at 600 °C resulted in
quinoline 16b (56%) together with the unexpected 6,8-
dimethylquinoline 17 (29%). The ketenimine 10b was
not isolable at room temparature. GCMS of the crude
product gave the ratio of 16b and 17 as 2.8:1.
FVT of the N-(2,6-dimethylphenyl)triazole 6c under the
same conditions as used for 6b at 600 °C gave rise to a
strong absorption at 2059 cm-1 in the IR spectrum (77
K), assigned to ketenimine 10c. Preparative FVT at
600-700 °C afforded only quinoline 16c (67% isolated
yield) and no methylquinoline corresponding to 17. FVT
of quinolinecarboxamide 16b at temperatures up to 800
°C did not produce the quinoline 17; the source of this
compound in the thermolysis of 6b remains unknown.
2. Th eor y. To examine the energy requirement for
the rearrangement of oxoketenimines to the quinolines,
ab initio molecular orbital calculations4 were carried out
on the 1,5-sigmatropic shift from the ketenimine 18 to
the o-quinoid imine 20 via the transition state TS19
(Scheme 2) using the GAUSSIAN 94 system of pro-
grams.5 Structures and energies were examined at the
G2(MP2,SVP) level of theory6 for 18a , TS19a , 20a , 22a ,
FVT of the N-mesityl-5-methoxypyrroledione 8 (Scheme
1) produced results similar to those described above
(Figures 1d-h). At 300 °C a weak band due to the
imidoylketene 9a appeared in the IR spectrum. When
the temperature was increased to 350 °C, the oxoketen-
imine 10a appeared, and the imidoylketene 9a increased
in intensity. At 400 °C both oxoketenimine 10a and
imidoylketene 9a increased in intensity. At 500 °C, there
was a strong signal due to oxoketenimine 10a , together
with a weak one due to ketene 9a . The IR spectrum
(Figure 1g) was now similar to the one shown in Figure
1b. The only observable species at 800 °C was the
quinoline 16a (1723 cm-1).
Preparative FVT of the pyrroldione 8 at 450 °C again
yielded the oxoketenimine 10 (23%) and a mixture of the
dihydroquinoline 15a and quinoline 16a . Over a short
time period in the presence of air, 15a was totally
converted to 16a (53%). FVT of 8 at 600 °C gave 15a
and 16a as the only species, thus indicating that the
(4) Hehre, W. J .; Radom, L.; Schleyer, P. v. R.; Pople, J . A. Ab Initio
Molecular Orbital Theory; Wiley: New York, 1986.

5782 J . Org. Chem., Vol. 63, No. 17, 1998
Ramana Rao et al.
Ta ble 1. Ca lcu la ted Rela tive En er gies (k ca l m ol-1) of
Keten im in es, Tr a n sition Str u ctu r es, Vin ylim in es, a n d
Electr ocycliza tion P r od u cts (Sch em es 2 a n d 3)
computational level
MP2/6-31G*//
MP2/6-31G*+ZPVE
G2(MP2,SVP)+ZPVE
18a
TS19a
20a
18b
TS19b
20b
0
0
40.8
20.4
0
42.0
26.8
0
39.7
26.9
38.6a
20.4a
b
21a
TS22a
23a
24
36.5
-23.5
0.0
TS25
26
27
TS28
29
30.2
2.03
0.23
22.6
-22.8
a
b
Estimated value; see text. Not located as a minimum com-
putationally.
and TS23a (R ) H). This corresponds effectively to
QCISD(T)/6-311+G(3df,2p)//MP2/6-31G* energies to-
gether with zero-point vibrational and isogyric correc-
tions. In the G2(MP2,SVP) theory, the basis-set exten-
sion energy contribution is calculated at the MP2 level,
and the QCISD(T) energy is evaluated using the 6-31G*
basis set. It has been shown that the accuracy of the
G2(MP2,SVP) method is comparable with that of the G2-
(MP2) theory7 but computationally more efficient.6 The
frozen core approximation was employed for all correlated
calculations. Zero-point vibrational energy calculations
were performed at the HF/6-31G* level.
The influence of a CHO group in 18b-20b was
calculated only at the MP2/6-31G*//MP2/6-31G* level of
theory. However, estimated G2(MP2,SVP) values for the
relative energies of these structures can be obtained by
applying the corresponding changes in relative energies
for 18a -20a when going from MP2/6-341G* to G2(MP2,-
SVP) theory. For this purpose, the difference between
the G2(MP2,SVP) and the MP2/6-31G*//MP2/6-31G*
energies for 18a -20a without ZPVE corrections is used,
since the latter are different in the two cases. The
resulting energy increments are then added to the MP2/
6-341G*//MP2/6-31G*+ZPVE values for 18b-20b. The
calculated relative energies are summarized in Table 1,
and selected geometries are given in Figure 2. The MP2/
6-31G*//MP2/6-31G* value for the activation barrier (18b
(TS19b) is 39.7 kcal mol-1 (166 kJ mol-1), and the
estimated G2(MP2,SVP) value is 39 kcal mol-1 (161 kJ
mol-1).
F igu r e 2. Computed structures of 18a and 20a (upper),
TS19a (middle; planar projection and perspective views; the
H atom is migrating ca. 0.5 Å above the molecular plane), and
TS22a (bottom; planar and perspective views) (MP2/6-31G*).
Bond lengths in Å and angles in degrees.
conjugation in the forming CdNCdC moiety in 20
(Figure 2). As the hydrogen atom is being transferred
toward the in-plane allenic p orbital, one could expect a
pseudopericyclic nature of the reaction.8 However, TS19
is not appreciably different from those of ordinary peri-
cyclic reactions such as the 1,5-H shift in 1,3-pentadiene,9
which we have recalculated at the same level of theory.
On the other hand, a comparison of the reaction temper-
atures required for a 1,5-H shift to an allene (32) (170
°C) and to a vinylic carbon in the analogous 1-mesityl-
propene (36) (> 300 °C) (Scheme 3) reveals a very
significant acceleration in the cumulenic systems. The
exact nature of this “allene effect”19 (see Discussion)
requires further theoretical investigation. Likewise, the
1,5-H shift in allene 38 has a barrier some 10 kcal mol-1
below that in 1,3-pentadiene 40, as discussed below.
The second step in the formation of dihydroquinolines
23 is formulated as a [2π + 2π + 2π] electrocyclization of
the Z-vinylimine 21 (Scheme 2). Such reactions are
expected to have activation barriers of the order of 26-
29 kcal mol-1 in simple, i.e., not o-quinoid, systems.9,10
In the transition structures TS19, the migrating
hydrogen atom is ca. 0.5 Å above the plane of the
molecule. The terminal CH2 group has twisted to provide
(5) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
J ohnson, B. G.; Robb, M. A.; Cheeseman, J . R.; Keith, T. A.; Peterson,
G. A.; Montgomery, J . A.; Raghavachari, K.; Al-Laham, M. A.;
Zakrzewski, V. G.; Ortiz, J . V.; Foresman, J . B.; Cioslowski, J .;
Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala,
P. Y.; Chen, W.; Wong, M. W.; Andres, J . L.; Replogle, E. S.; Gomperts,
R.; Martin, R. L.; Fox, D. J .; Binkley, J . S.; DeFrees, D. J .; Baker, J .;
Stewart, J . J . P.; Head-Gordon, M.; Gonzalez, C.; Pople, J . A. Gaussian
94, Gaussian Inc., Pittsburgh, PA, 1995.
(6) (a) Smith, B. J .; Radom, L. J . Phys. Chem. 1995, 99, 6468. (b)
Curtiss, L. A.; Redfern, P. C.; Smith, B. J .; Radom, L. J . Chem. Phys.
1996, 104, 5148.
(7) Curtiss, L. A.; Raghavachary, K.; Pople, J . A. J . Chem. Phys.
1993, 98, 1293.
(8) (a) Birney, D. M. J . Org. Chem. 1996, 61, 243-251. (b) Koch,
R.; Wong, M. W.; Wentrup, C. J . Org. Chem. 1996, 61, 6809-6813. (c)
Bibas, H.; Koch, R.; Wentrup, C. J . Org. Chem. 1998, 63, 2619-2626..
(9) Houk, K. N.; Li, Y.; Evanseck, J . D. Angew. Chem., Int. Ed. Engl.
1992, 31, 682-708.

N-Mesityl-C-acylketenimines
Sch em e 3
J . Org. Chem., Vol. 63, No. 17, 1998 5783
similar to or just a little above those resulting in 1,3-
MeO shifts. (These temperatures are upper limits; the
1,3-MeO shifts in ketenes take place as soon as the
ketenes can be observed.) The calculated activation
barrier of ca. 38 kcal mol-1 (ca. 161 kJ mol-1) for the 1,5-H
shift leading to quinolone formation is thus in qualitative
agreement with the mechanistic interpretation given in
Scheme 1.
The ratios of intensities of oxoketenimine (10) and
imidoylketene (9) peaks in the IR spectra (Figure 1)
cannot be taken as a true indication of equilibrium ratios
(even disregarding the unknown differences in extinction
coefficients), because the ketenimines are being removed
from the equilibria by cyclization to the quinolones.
However, it should be noted that the observed peak ratios
are very similar to those found in the analogous N-phenyl
derivatives, where the ketenes (and not the ketenimines)
are being removed from the equilibria by cyclization to
quinoline-4-ones.2a-c It appears, therefore, that the
strong ketenimine peaks in the IR spectra in fact do
reflect a preponderance of the ketenimines in equilibrium
with the ketenes. This, too, is in agreement with ab initio
calculations, which predict the oxoketenimines to be
lower in energy than the imidoylketenes by ca. 2 kcal
As mentioned below, only a minimum for E-20a , not for
Z-21a , was actually located.
To evaluate the consequences of the loss of the benzene
resonance energy in TS19, 20, and 21, we have calculated
the activation barriers for the 1,5-H shift in the nonaro-
matic model N-vinylketenimine 24 and the electro-
cyclization of Z-3-azahexa-1,3,5-triene 27 via transition
structures TS25 and TS28 at the G2(MP2,SVP) level
(Scheme 3 and Table 1). The 1,5-H shift barrier of 30
kcal mol-1 is ca. 10 kcal mol-1 below the aromatic case
(18-20) in Scheme 2.
For the electrocyclization 27 f 29, the computed
activation energy is ca. 23 kcal mol-1, and the reaction
is exothermic by ca. 23 kcal mol-1. For the benzo system
20a , the transition state TS22a for electrocyclization lies
mol-1 11
.
The 1,5-H shifts/electrocyclizations of ketenimines 10
to quinolines 15-16 can be compared with analogous
reactions in the literature. Motoyoshiya et al. reported
rearrangement of C-sulfenyl-C-ethoxycarbonyl-N-phen-
ylketenimines to quinoline-4-ones on reflux in 1,2-dichlo-
robenzene (180 °C) and postulated imidoylketene inter-
mediates formed by 1,3-shifts of the ethoxy groups.12 The
ketene intermediates could not, however, be observed
under such reaction conditions. It was also found that
the 2,6-disubstituted ketenimine 30 disappeared on
heating to 250 °C, but the product of this reaction could
not be identified.13 The results reported in the present
paper suggest that cyclization to the dihydroquinoline 31
took place.
only ca. 16 kcal mol-1 above the E-imine 20a .
A
minimum for the Z-imine 21a was not located. This
cyclization to isoquinoline 23a is exothermic by ca. 43
kcal mol-1 due to the recovery of aromaticity (Scheme 2
and Figure 2). The very low barrier suggests a very early
transition state TS22a in terms of the Hammond pos-
tulate. This is borne out in the calculated structure
(Figure 2), which features a distance of 2.53 Å between
the two carbon atoms where the new single bond will
form. This is only a little shorter than the van der Waals
radii (2.9 Å). In the nonbenzo system 27, the cyclization
is much less exothermic (22 kJ mol-1; Table 1; Scheme
3) and the TS28 is more advanced (the corresponding
C-C distance is 2.25 Å). When going from imine 20a to
23a , the bonds involved change by an average of 12 pm.
In TS22a the changes are only ca. 3 pm. In TS28 they
are 5-6 pm (all geometry data are given in Tables S3
and S5 in the Supporting Information).
Heimgartner et al. reported thermal cyclization of
mesitylallene (32a ) to dihydronaphthalene (34) (Scheme
4), and isomerization of allene 32b to 35, taking place at
170 °C in decane with ∆Hq ) 28.8 (28.7) kcal mol-1 and
(∆Sq ) 14.0 (-12.8) cal K-1 mol-1, corresponding to
∆Gq(298) ) 33.0 (32.5) kcal mol-1 or 138 (136) kJ mol-1
for 32a and 32b, respectively.14 In contrast, the mesit-
ylpropene 36 required a temperature > 300 °C for the
analogous 1,5-H shift equilibrating it with 37.14 The
open-chain system 38 undergoes a 1,5-H shift to 39 at
100 °C in the gas phase, with Ea ) 24.6 kcal mol-1 [from
which ∆Hq ) 23.9 kcal mol-1, and ∆Gq(298) (estimated)
∼28 kcal mol-1 (117 kJ mol-1)].15 This is a very low
barrier for a 1,5-H shift; the normal value in open-chain
Discu ssion
The isomerization of ketenimines 10 to dihydroquino-
lines 14 (and then 15 and 16) takes place under very mild
FVT conditions. The 1,3-shift of the methoxy group,
converting the ketenimine 10a to ketenes 9a , is detect-
able at FVT temperatures > 400 °C. In related earlier
work we established such shifts taking place at temper-
atures > 380 °C from the ketenimine side and > 200 °C
from the ketene side.2a-b The calculated activation
barrier for the 1,3-MeO shift in methoxycarbonylketen-
imine is ca. 35 kcal mol-1 (ca. 32 kcal/mol from the
methoxyimidoylketene side; at the G2(MP2,SVP) level of
theory).11 The preparative and analytical experiments
with 6 and 10a reported here reveal that quinolone
formation commences at ca. 400 °C (analytical conditions)
or 350 °C (preparative conditions), i.e., at temperatures
1,3-pentadienes (40) is on the order of 36-38 kcal mol-1 9
.
The low barrier for 38 has been corroborated by deter-
mination of similar activation parameters for cycloalk-
enylallenes in solution.16
(12) Motoyoshiya, J .; Yamamoto, I.; Gotoh, H. J . Chem. Soc., Perkin
Trans. 1 1981, 2727-2730.
(13) Motoyoshiya, J .; Takagi, A.; Hirakawa, K.; Kakurai, T. J .
Heterocycl. Chem. 1986, 23, 597-599.
(14) Heimgartner, H.; Asindely, J .; Hansen, H.-J .; Schmid, H. Helv.
Chim. Acta 1970, 53, 1212-1215.
(10) Marvell, E. N. Thermal Electrocyclic Reactions; Academic: New
York, 1980.
(11) Wong, M. W.; Wentrup, C. Unpublished results.
(15) Skattebøl, L. Tetrahedron 1969, 25, 4933-4938.
(16) Shen, G.-Y.; Tapia, R.; Okamura, W. H. J . Org. Chem. 1987,
52, 7499-7506.

5784 J . Org. Chem., Vol. 63, No. 17, 1998
Ramana Rao et al.
Sch em e 4
gous 1,5-H shift in 1-mesitylpropene (36) (reaction > 300
°C in solution),14 might be ascribed to a pseudopericyclic
nature of the former reaction (32 f 33). The difference
is also seen clearly in the open-chain systems, 38 f 39
versus 40 f 40. An explanation of the “allene effect” has
been given by J ensen,19 who demonstrated computation-
ally that this effect is specific for 1,3- and 1,5-H shifts
but negligible for 1,7- and 1,9-H shifts. J ensen concluded
that the allene effect is due to extra conjugation in a
biradicaloid transition state.
The activation barriers for allenes 32 and ketenes 41
are similar (32.5-37 kcal mol-1). Our calculated activa-
tion barrier for the 1,5-H shift in ketenimine 18, 38 kcal
mol-1 (161 kJ mol-1) fits nicely at the upper end of this
range. The electrocyclic reactions 13 f 14 (and the
model reactions 20 f 23 and 27 f 29) are normal
pericyclic processes of the 3-azahexatriene type with
computed activation energies of the order of 16-22 kcal
mol-1
.
Con clu sion
FVT of triazoles 6a -c generates ketenimines 10
(Scheme 1). FVT of pyrroledione 8 generates the isomeric
imidoylketene 9a . Ketenes 9 and ketenimines 10 un-
dergo a facile thermal interconvertion by 1,3-shifts of
methoxy and dimethylamino groups. Both 9 and 10 are
directly observable by low-temperature IR spectroscopy.
When using FVT at temperatures above ca. 400 °C
(analytical conditions) or 350 °C (preparative conditions),
the ketenimines 10 undergo a 1,5-H shift to o-quinone
imine methides 12, followed by electrocyclization to
dihydroquinolines 14 and then 15. Compounds 15 are
detectable by NMR spectroscopy but are easily oxidized
to quinolines 16 by atmospheric oxygen. Ab initio
calculations on model compounds 18-20 (Scheme 2)
predict an energy barrier of ca. 38 kcal mol-1 (161 kJ
mol-1) for the 1,5-H shift via the transition state TS19
and a very early transition state for the cyclization to 23
(Table 1; Figure 2).
The oxoketenimine 10a is isolable at room tempera-
ture. It was fully characterized spectroscopically and
chemically via its hydrolysis to amide 11. FVT of this
ketenimine regenerates a small amount of the isomeric
imidoylketene 9a . Both experiments and theory indicate
that the oxoketenimines 10 dominate in the equilibria
with imidoylketenes, 10 a 9.
An analogous cyclization of ketene 41a to isochromene
43a was reported as taking place at 250 °C without any
further details.17 Recently, Rappoport et al.18 described
the cyclization of 41b to 43b at 170 °C (decane solution,
< 1 h). From the reported first-order rate constant, k ∼
8 × 10-4 s-1, the free energy of activation can be derived
with the aid of transition state theory (k ) kBT/h
exp(-∆Gq/RT)), yielding ∆Gq ∼ 32.6 kcal mol-1 (136 kJ
mol-1). Compound 41c was reported to react ca. 200
times slower than 41b, from which a rough ∆Gq ∼ 37.2
kcal mol-1 (155 kJ mol-1) may be derived.
The above data indicate that the penalty for sacrificing
the aromaticity in the transition state leading to o-
quinodimethide 33 is only ca. 5 kcal mol-1 (in comparison
with 38 f 39). The calculated difference between the
aromatic and open-chain ketenimines (Schemes 2 and 3)
was ca. 10 kcal mol-1 (section 2). The significantly lower
activation energy for 32 f 33 compared with the analo-
Exp er im en ta l Section
The pyrolysis apparatus was as previously reported for Ar
matrix (12 K),20 neat film (77 K) deposition,21 and preparative
scale work (77 K isolation).22 BaF2 disks were used for
depositions. Due to different designs, temperatures required
for a reaction to occur are often ca. 100 °C lower in the
preparative than in the analytical (spectroscopic) apparatus.
Details of the X-ray crystal structure analysis of 16a are given
in the Supporting Information. Reactions with azides (de-
scribed below) are potentially hazardous, and due precautions
against explosions should be taken. No adverse observations
were made.
(19) J ensen, F. J . Am. Chem. Soc. 1995, 117, 7487-7492.
(20) Kappe, C. O.; Wong, M. W.; Wentrup, C. J . Org. Chem. 1995,
60, 1686.
(21) Briehl, H.; Lukosch, A.; Wentrup, C. J . Org. Chem. 1984, 49,
2772.
(17) Hug, R.; Hansen, H.-J .; Schmid, H. Helv. Chim. Acta 1972, 55,
10-15. The work is quoted as Gilgen, P. Diplomarbeit, Universita¨t
Zurich, 1971.
(18) Sigalov, M.; Rappoport, Z. J . Chem. Soc., Perkin Trans. 2 1997,
1911-1912.
(22) Wentrup, C.; Blanch, R.; Briehl, H.; Gross, G. J . Am. Chem.
Soc. 1988, 110, 1874.

N-Mesityl-C-acylketenimines
J . Org. Chem., Vol. 63, No. 17, 1998 5785
Meth yl 1-(2,4,6-Tr im eth ylp h en yl)-1H-1,2,3-tr ia zole-4-
ca r boxyla te (6a ). This compound was prepared23 from 2,3,4-
trimethylphenyl azide (3.5 g; 21.7 mmol) and methyl propiolate
(1.7 g; 20.2 mmol), yielding 4.05 g (75%), mp 158-160 °C: IR
(KBr) ν 3137, 2953, 1740, 1545, 1442, 1374, 1340, 1261, 1248,
F VT of Tr ia zole 6a . (a) The compound (100 mg) was
subjected to preparative FVT at 450 °C, being sublimed into
the apparatus at 50-60 °C in the course of 2 h. The products
were collected in a U-tube cooled in liquid N2. Upon comple-
tion of the pyrolysis, the system pressure was equalized with
N2, and the U-tube warmed to room temperature. The oily
residue in the U-tube was dissolved in CCl4 and immediately
subjected to vacuum distillation using a Kugelrohr apparatus.
Distillation at 50 °C (4.5 × 10-5 mbar) afforded 16 mg (21%)
of the oxoketenimine 10a as a clear oil: IR (CCl4) ν 2951, 2077,
1
1229, 1195, 1143, 1048 cm-1; H NMR (CDCl3) δ 1.93 (s, 6H)
2.34 (s, 3H), 3.98 (s, 3H), 6.98 (s, 2H), 8.12 (s, 1H); 13C NMR
δ 17.3, 21.2, 52.4, 129.3, 129.6, 133.0, 134.9, 140.1, 140.7, 161.3;
MS m/z 245 (M+), 214, 185, 158, 143, 115, 91, 77, 55; HRMS
calcd for C13H15N3O2 m/z 245.1164; found 245.1161. Anal.
Calcd for C13H15N3O2: C, 63.67; H, 5.17; N, 17.14. Found: C,
63.79; H, 5.14; N, 17.34.
1
2069, 1710, 1445, 1238, 1145 cm-1; H NMR (CDCl3) δ 2.27
(s, 3H), 2.34 (s, 6H), 3.69 (s, 3H), 4.24 (s, 1H), 6.88 (s, 2H); 13
C
N,N-Dim eth yl-1-(2,4,6-tr im eth ylp h en yl)-1H-1,2,3-tr ia -
zole-4-ca r boxa m id e (6b). A mixture of compound 6a (1.2
g, 4.63 mmol) and 20% aqueous KOH (20 mL) was stirred
overnight at room temperature. The reaction mixture was
diluted to 50 mL and acidified with dilute HCl, and the solid
which separated was filtered, washed with water, and dried
to yield a white solid (1.0 g, 94%). GCMS analysis of this
product indicated that the corresponding carboxylic acid had
been formed. Without further purification, a mixture of this
acid (0.9 g; ca. 3.9 mmol) and SOCl2 (10 mL) was refluxed for
1 h. The excess thionyl chloride was removed on a rotary
evaporator, and the residue was dissolved in CH2Cl2 (20 mL)
and cooled to 10 °C in an ice-water bath. A solution of
dimethylamine (2 equiv) in CH2Cl2 (15 mL) was added drop-
wise with stirring. After addition, stirring was continued for
30 min at 20 °C. The reaction mixture was washed with water
(50 mL). The organic phase was dried over anhydrous MgSO4,
and the solvent was removed under reduced pressure at 40
°C. The residue was stirred with hexane (10 mL), and the
solid was filtered, washed with hexane, and dried to afford a
white solid (0.9 g, 90%), mp 116-118 °C: IR (KBr) ν 1624,
NMR (CDCl3) δ 18.8, 21.0, 45.4, 51.3, 129.2, 129.7, 134.4, 138.3,
165.7, 170.1 (see also Supporting Information); HRMS calcd
for C13H15NO2 m/z 217.1103, found 217.1103. Anal. Calcd for
C
13H15NO2: C, 71.85; H, 6.96; N, 6.45. Found: C, 71.46; H,
6.91; N, 6.51.
A pink solid which had deposited at the entrance of the
U-tube was found to be a mixture of two compounds having
Rf values of 0.49 and 0.29 (TLC, SiO2, hexane/CHCl3 3:2).
Column chromatography (hexane/CHCl3 3:2) yielded only a
yellow solid, 42 mg (56%), mp 98-100 °C which was identified
as methyl 6,8-dimethylquinoline-3-carboxylate (16a): IR (CDCl3)
ν 2955, 1723, 1605, 1440, 1275, 1237, 1115, 1007 cm-1 1H
;
NMR (CDCl3) δ 2.49 (s, 3H), 2.77 (s, 3H), 3.99 (s, 3H), 7.50 (s,
2H), 8.69 (d, 1H), 9.36 (d, 1H); 13C NMR (CDCl3) δ 18.0, 21.5,
52.4, 122.6, 125.8, 127.0, 134.4, 136.8, 137.1, 138.3, 147.5,
148.0, 166.2; HRMS calcd for C13H13NO2 m/z 215.0946, found
215.0943.
(b) To determine the composition of the above pink solid,
the products of a similar pyrolysis of 6a at 450 °C were
examined by GCMS. This allowed the identification of quino-
line 16a (tR 10.04 min, m/z 215) and the dihydroquinoline 15a
(tR 11.70 min, m/z 217). The ratio of intensities of the peaks
at m/z 215 and 217 was ca. 2.8:1, variable because 15 is rapidly
oxidized to 16a . Spectral data for 15a in admixture with
16a : 1H NMR (CDCl3) δ 2.11 (s, 3H), 2.19 (s, 3H), 3.70 (s, 2H),
3.71 (s, 3H), 5.85 (bs, 1H), 6.70 (s, 2H), 7.34 (d, 1H); IR (KBr)
1537, 1391, 1209, 1109, 1039, 858, 763, 685 cm-1 1H NMR
;
(CDCl3) δ 1.97 (s, 6H), 2.36 (s, 3H), 3.17 (s, 3H), 3.67 (s, 3H),
7.00 (s, 2H), 8.15 (s, 1H); 13C NMR (CDCl3) 17.2, 21.0, 36.4,
38.7, 129.1, 130.1, 132.8, 134.8, 140.2, 144.5, 161.1; HRMS
calcd for C14H18N4O m/z 258.1481, found 258.1480. Anal.
Calcd for C14H18N4O: C, 65.08; H, 7.03; N, 21.70. Found: C,
64.99; H, 7.04; N, 21.66.
ν 3337, 1719, 1663, 1640 cm-1 13C NMR (CDCl3) δ 16.6, 20.6,
;
25.9, 51.0, 97.9, 120.7, 121.8, 128.1, 129.1, 132.1, 136.9, 150.0,
168.8; HRMS calcd for C13H15NO2 m/z 217.1103, found 217.1095.
The 1H NMR spectra of 15a and 16a are reproduced in the
Supporting Information.
N,N-Dim eth yl-1-(2,6-dim eth ylph en yl)-1H-1,2,3-tr iazole-
4-ca r boxa m id e (6c). The procedure was similar to that used
above except for the purification of the final product. The
crude product was stirred with hexanes-ether (4:1) to yield a
white solid (1.14 g, 85%), mp 89-91 °C: IR (KBr) ν 1615, 1538,
1390, 1204, 1120, 1039, 772, 686 cm-1; 1H NMR (CDCl3) δ 2.01
(s, 6H), 3.17 (s, 3H), 3.67 (s, 3H), 7.17-7.34 (m, 3H), 8.18 (s,
1H); 13C NMR (CDCl3) δ 17.3, 36.4, 38.7, 128.4, 130.0, 130.2,
135.1, 135.3, 144.6, 161.0; HRMS calcd for C13H16N4O m/z
244.1324, found 244.11324. Anal. Calcd for C13H16N4O: C,
63.92; H, 6.60; N, 22.93. Found: C, 63.96; H, 6.57; N, 22.81.
5-Me t h oxy-1-(2,4,6-t r im e t h ylp h e n yl)p yr r ole -2,3-d i-
on e (8). The procedure was analogous to that used in ref 24.
To a stirred and cooled (0 °C) mixture of N-2,4,6-trimethyl-
phenylacetimidic ester25 (1.4 g, 7.3 mmol) and dry triethyl-
amine (1.5 g, 14.6 mmol) in dry ether (50 mL) was added a
solution of oxalyl chloride in dry ether (10 mL) over 30 min.
After the solution was stirred for 3.5 h at 0 °C, hexane (50
mL) was added and stirring continued for 30 min at room
temperature. The separated solid was filtered, washed quickly
with ice-cold water, and crystallized from benzene to give
yellow crystals, yield 70%: mp 146-148 °C; IR (KBr) ν 2956,
2925, 1772, 1714, 1578, 1573, 1488, 1449, 1395, 1320, 1245,
942 cm-1; 1H NMR (CDCl3) δ 2.09 (s, 6H), 2.29 (s, 3H), 4.03 (s,
3H), 4.95 (S, 1H), 6.94(s, 2H); 13C NMR δ 17.9, 21.1, 59.0, 96.5,
126.4, 129.2, 136.9, 139.7, 159.3, 179.4, 179.8; HRMS calcd
for C14H15NO3 m/z 245.1052, found 245.1052. Anal. Calcd for
(c) Similar FVT at 350 °C afforded a mixture of 6a , 10a ,
and (15a + 16a ) in the ratio 24:33:9 as determined by 1H NMR
spectroscopy.
(d ) Compound 6a (80 mg) was pyrolyzed at 700 °C as above
to yield 40 mg of 16a (65%) after complete oxidation of 15a .
Rea ction s of Keten im in e 10a . (a ) The freshly distilled
ketenimine 10a (60 mg) was subjected to FVT at 400 °C. The
products were collected on the BaF2 disk at 77 K. The IR
spectrum revealed the presence of a large amount of keten-
imine 10a as well as a small amount of ketene 9a (if the
extinction coefficients of 10a and 9a were identical, less than
10% of 9a was formed). The quinoline 16a (<5% yield) was
identified in the yellow material deposited between the py-
1
rolysis tube and the 77 K BaF2 disk by H NMR spectroscopy
and comparison with the previously described sample.
(b) Preparative FVT of 10a (18 mg) at 700 °C gave an 84%
yield of 16a after complete oxidation of 15a (initial mixture of
1
15a and 16a ) ca. 1:3 by H NMR).
(c) Following a similar pyrolysis of 6a (100 mg) at 450 °C,
the pure, distilled oxoketenimine 10a (15 mg) was left exposed
to the atmosphere for 18 h at room temperature, whereby it
reacted with H2O to give N-mesitylmalonamide monomethyl
ester 11a as yellow crystals, 17.5 mg (95%), mp 132-134 °C:
IR (CCl4) ν 3351, 3275, 2956, 1752, 1647, 1535, 1324, 1232,
1
1157, 1057, 1025 cm-1; H NMR (CDCl3) δ 2.17 (s, 6H), 2.27
C
14H15NO3: C, 68.54; H, 6.17; N, 5.71. Found: C, 68.64; H,
6.31; N, 5.58.
(s, 3H), 3.51 (s, 2H), 3.80 (s, 3H), 6.88 (s, 2H), 8.35 (bs, 1H);
13C NMR (CDCl3) δ 18.3, 20.9, 40.9, 52.6, 128.9, 130.8, 134.9,
137.1, 163.3, 170.4; HRMS calcd for C13H17NO3 m/z 235.1208,
found 235.1211. Anal. Calcd for C13H17NO3: C, 66.35; H, 7.29;
N, 5.96. Found: C, 66.29; H, 7.27; N, 6.11.
(23) Abu-Orabi, S. T.; Atfah, I.; Mari´ıi, F. M.; Ali, A. A. J . Heterocyl.
Chem. 1989, 26, 1461. Huisgen, E.; Knorr, R.; Mo¨bius, L.; Szeimies,
G. Chem. Ber. 1965, 98, 4014.
(24) Kappe, C. O.; Kollenz, G.; Leung-Toung, R.; Wentrup, C. J .
Chem. Soc., Chem. Commun. 1992, 487.
F VT of P yr r oled ion e 8. The compound (75 mg) was
pyrolyzed at 450 °C, being sublimed into the apparatus at 75
(25) DeWolfe, R. H. J . Org. Chem. 1962, 27, 490.

5786 J . Org. Chem., Vol. 63, No. 17, 1998
Ramana Rao et al.
°C in the course of 3 h. Similar workup as with 6a yielded
the oxoketenimine 10a (15 mg, 23%) along with the quinoline
16a (35 mg, 53%) after complete oxidation of 15a (initial
mixture of 15a and 16a ) ca. 1:3 by 1H NMR). At 600 °C,
16a was obtained analogously in 68% yield. The spectral data
for 10a , 15a , and 16a were as reported above.
N,N-Dim et h yl-6,8-d im et h ylq u in olin e-3-ca r b oxa m id e
(16b) a n d 6,8-Dim eth ylqu in olin e (17). (a ) F VT of 6b. This
compound (200 mg) was pyrolyzed as described for 6a . At 600
°C an oil was obtained on the coldfinger. GCMS indicated the
presence of two compounds, 16b (m/z 228) and 17 (m/z 157).
Column chromatography of the oil (SiO2, hexane/benzene 1:1)
gave 17 as an oil (36 mg, 29%), followed by 16b as an oil (100
mg, 56%, eluting with CHCl3). Spectral data for 16b: IR
residue was dissolved in CH2Cl2 and cooled to 10 °C, and 2
equiv of dimethylamine in CH2Cl2 was added slowly. The
resulting mixture was stirred at 20 °C for 30 min. Water was
added (20 mL) and the organic phase was separated and
washed successively with water and brine, dried over MgSO4,
concentrated in vacuo, and chromatographed on SiO2. The
desired amide 16b (15 mg) was eluted with CHCl3-MeOH
(1:1). Its NMR spectra were identical with the ones reported
in (a) above.
F VT of Tr ia zole 6c. This compound (100 mg) was simi-
larly pyrolyzed at 600 °C to give compound 16c (60 mg, 67%):
IR (CCl4) ν 2928, 1645, 1548, 1492, 1392, 1261, 1095, 1008
cm-1 1H NMR (CDCl3) δ 2.75 (s, 3H), 3.01 (s, 3H), 3.12 (s,
;
3H), 7.34-7.67 (m, 3H), 8.17 (d, 1H), 8.92 (d, 1H); 13C NMR
(CDCl3) δ 18.0, 35.6, 39.7, 126.3, 127.0, 127.1, 127.6, 128.8,
130.8, 135.4, 137.2, 147.3, 169.3; HRMS calcd for C13H14N2O
m/z 214.1106, found 214.1103.
(CCl4) ν 2926, 1645, 1605, 1490, 1392, 1266, 1099, 1001 cm-1
;
1H NMR (CDCl3) δ 2.49 (s, 3H), 2.78 (s, 3H), 3.07 (s, 3H), 3.18
(s, 3H), 7.46 (s, 2H), 8.14 (d, 1H), 8.91 (d, 1H); 13C NMR (CDCl3)
δ 17.9, 21.5, 35.5, 39.6, 125.0, 127.1, 128.7, 133.2, 134.7, 136.6,
140.0, 145.8, 146.3, 169.4; HRMS calcd for C14H16N2O m/z
228.1262, found 228.1261.
Ack n ow led gm en t. This research was supported by
the Australian Research Council. We thank the Deut-
sche Forschungsgemeinschaft for a Fellowship for R.K.
and Dr. J ens Morawietz for asistance with the matrix
isolation experiments.
17 is a known compound:26 spectral data: IR (CCl4) ν 2926,
1590, 1495, 1373, 1125, 858 cm-1; 1H NMR (CDCl3) δ 2.47 (s,
3H), 2.77 (s, 3H), 7.31-7.39 (s, 2H), 7.98-8.02 (dd, 2H), 8.85-
8.87 (m, 1H); 13C NMR (CDCl3) δ 18.0, 21.4, 120.8, 124.6, 128.3,
131.9, 135.6, 135.9, 136.5, 145.9, 148.3; HRMS calcd for
C
11H11N m/z 157.0891, found 157.0879.
(b) Con ver sion of 16a to 16b. For proof of the structure
Su p p or tin g In for m a tion Ava ila ble: X-ray crystal struc-
ture and packing diagram, crystal data, and bond lengths and
angles for 16a (Figure S1, Tables S1 and S2). Calculated
geometries of 18a , TS19a , and 20a (MP2/6-31G*), absolute
energies and zero-point vibrational energies for 18-20a ,b and
22-29 (Tables S3-S6). 1H and 13C NMR spectra of 10a , and
1H NMR spectra of 15a and 16a (12 pages). This material is
contained in libraries on microfiche, immediately follows this
article in the microfilm version of the journal, and can be
ordered from the ACS; see any current masthead page for
ordering information.
of 16b, this compound was also synthesized from the fully
characterized ester 16a (30 mg) which was hydrolyzed with
10% aqueous KOH at 80 °C for 30 min. After cooling to room
temperature, the mixture was acidified with 6 M HCl, the solid
was filtered and washed with water to give 20 mg of the
carboxylic acid, this was taken up in 2 mL of freshly distilled
SOCl2, and the mixture was refluxed for 30 min under N2.
After removal of excess SOCl2 on the rotatory evaporator, the
(26) Claret, P. A.; Osborne, A. G. Org. Magn. Reson. 1976, 8, 147.
Draper, P. M.; Maclean, D. B. Can. J . Chem. 1968, 46, 1487. Schaefer,
T. P. Can. J . Chem. 1961, 39, 1864.
J O9722562