Mode selectivity in the intramolecular cyclization of ketenimines bearing N-acylimino units: A computational and experimental study

Article abstract of DOI:10.1021/jo0482716

(Chemical Equation Presented) The mode selectivity in the intramolecular cyclization of a particular class of ketenimines bearing N-acylimino units has been studied by ab initio and DFT calculations. In the model compounds the carbonyl carbon atom and the keteniminic nitrogen atom are linked either by a vinylic or an o-phenylene tether. Two cyclization modes have been analyzed: the [2+2] cycloaddition furnishing compounds with an azeto[2,1-b]pyrimidinone moiety and a 6?r-electrocyclic ring closure leading to compounds enclosing a 1,3-oxazine ring. The [2+2] cycloaddition reaction takes place via a two-step process with formation of a zwitterionic intermediate, which has been characterized as a cross-conjugated mesomeric betaine. The 6π-electrocyclic ring closure occurs via a transition state whose pseudopericyclic character has been established on the basis of its magnetic properties, geometry, and NBO analysis. The 6π-electrocyclic ring closure is energetically favored over the [2+2] cycloaddition, although the [2+2] cycloadducts are the thermodynamically controlled products. A quantitative kinetic analysis predicts that 1,3-oxazines would be the kinetically controlled products, but they should transform rapidly and totally into the [2+2] cycloadducts at room temperature. In the experimental study, a number of N-acylimino-ketenimines, in which both reactive functions are supported on an o-phenylene scaffold, have been successfully synthesized in three steps starting from 2-azidobenzoyl chloride. These compounds rapidly convert into azeto[2,1-b]quinazolin-8-ones in moderate to good yields as a result of a formal [2+2] cycloaddition.

Full text of DOI:10.1021/jo0482716

Mode Selectivity in the Intramolecular Cyclization of Ketenimines
Bearing N-Acylimino Units: A Computational and Experimental
Study
Mateo Alajar´ın,* Pilar Sa´nchez-Andrada,* Angel Vidal, and Fulgencio Tovar
Departamento de Quı´mica Orga´nica, Facultad de Qu´ımica, Campus de Espinardo,
Universidad de Murcia, 30100 Murcia, Spain
alajarin@um.es
Received October 1, 2004
The mode selectivity in the intramolecular cyclization of a particular class of ketenimines bearing
N-acylimino units has been studied by ab initio and DFT calculations. In the model compounds
the carbonyl carbon atom and the keteniminic nitrogen atom are linked either by a vinylic or an
o-phenylene tether. Two cyclization modes have been analyzed: the [2+2] cycloaddition furnishing
compounds with an azeto[2,1-b]pyrimidinone moiety and a 6π-electrocyclic ring closure leading to
compounds enclosing a 1,3-oxazine ring. The [2+2] cycloaddition reaction takes place via a two-
step process with formation of a zwitterionic intermediate, which has been characterized as a cross-
conjugated mesomeric betaine. The 6π-electrocyclic ring closure occurs via a transition state whose
pseudopericyclic character has been established on the basis of its magnetic properties, geometry,
and NBO analysis. The 6π-electrocyclic ring closure is energetically favored over the [2+2]
cycloaddition, although the [2+2] cycloadducts are the thermodynamically controlled products. A
quantitative kinetic analysis predicts that 1,3-oxazines would be the kinetically controlled products,
but they should transform rapidly and totally into the [2+2] cycloadducts at room temperature. In
the experimental study, a number of N-acylimino-ketenimines, in which both reactive functions
are supported on an o-phenylene scaffold, have been successfully synthesized in three steps starting
from 2-azidobenzoyl chloride. These compounds rapidly convert into azeto[2,1-b]quinazolin-8-ones
in moderate to good yields as a result of a formal [2+2] cycloaddition.
Introduction
species as electron-deficient heterodienes (in neutral or,
more commonly, protonated form) with electron-rich
dienophiles.3 However, the participation of the CdN bond
of N-acylimines in [2+2] cycloaddition reactions is rare.
To our knowledge, the only reported reaction of this type
is the catalyzed [2+2] cycloaddition of N-acylimines with
ketenes yielding â-lactams, recently disclosed by Lectka
and co-workers.4
N-Acylimines and N-acyliminium ions1 behave as
valuable imino dienophiles when confronted with all-
carbon dienes in [4+2] cycloaddition reactions leading to
tetrahydropyridines.2 They may also act as the 4π
component in hetero-Diels-Alder reactions of inverse
electronic demand involving the combination of these
Our interest in the study of the intramolecular [2+2]
(1) For a recent review about the cyclizations of N-acyliminium ions
see: Maryanoff, B. E.; Zhang, H.-C.; Cohen, J. H.; Turchi, I. J.;
Maryanoff C. A. Chem. Rev. 2004, 104, 1431-1628.
cycloaddition reactions between imines and ketenimines,5
(2) (a) Boger, D. L.; Weinreb, S. M. Hetero Diels-Alder Methodology
in Organic Synthesis; Academic Press: San Diego, CA, 1987; Chapter
2. (b) Organic Synthesis Highlights II; Waldmann, H., Ed.; VCH
Verlagsgesellschaft: Weinheim, Germany, 1995; pp 37-47. (c) Ka-
metani, T.; Hibino, S. Adv. Heterocycl. Chem. 1987, 42, 245-333. (d)
Whiting, A.; Windsor, C. M. Tetrahedron 1998, 54, 6035-6050. (e)
Khatri, N. A.; Schmitthenner, H. F.; Shringarpure, J.; Weinreb, S. J.
Am. Chem. Soc. 1981, 103, 6387-6393.
(3) (a) Weinreb, S. M.; Scola, P. M. Chem. Rev. 1989, 89, 1525-
1534. (b) Gizecki, P.; Dhal, R.; Poulard, C.; Gosselin, P.; Dujardin, G.
J. Org. Chem. 2003, 68, 4338-4344. (c) Gong, Y.; Bausch, M. J.; Wang,
L. Tetrahedron Lett. 2001, 42, 1-4. (d) Michels, G.; Hermesdorf, M.;
Schneider, J.; Regitz, M. Chem. Ber. 1988, 121, 1775-1783.
(4) (a) Dudding, T.; Hafez, A. M.; Taggi, A. E.; Wagerle, T. R.; Lectka,
T. Org. Lett. 2002, 4, 387-390. (b) Hafez, A. M.; Taggi, A. E.; Dudding,
T.; Lectka, T. J. Am. Chem. Soc. 2001, 123, 10853-10859.
10.1021/jo0482716 CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/15/2005
1340
J. Org. Chem. 2005, 70, 1340-1349

Cyclization of Ketenimines
SCHEME 1. Planned Intramolecular [2+2]
Cycloaddition of N-Acylimino-ketenimines
SCHEME 2. [2+2] Cycloaddition of the
Imino-ketenimine 3 Leading to Bicycle 4
and the particular electronic characteristics of N-
acylimines (see below), led us to explore, theoretically and
experimentally, if the CdN bond of an N-acylimine would
participate in a [2+2] intramolecular cycloaddition with
a ketenimine function. To this end, we planned placing
both functionalities on a Z-1,2-vinylic or o-phenylene
scaffold as represented in structure 1 (Scheme 1), as our
previous work in the intramolecular cyclization of other
imino-ketenimines supported in similar frameworks oc-
curred successfully.5 If successful, these cycloadditions
should lead to adducts 2.
It has been demonstrated that the intermolecular [2+2]
cycloadditions of ketenimines with imines yielding aze-
tidin-2-imines only takes place if the ketenimines bear
electron-withdrawing substituents at the nitrogen atom,
which increase their electrophilic character.6 By contrast,
we demonstrated that the intramolecular [2+2] cycload-
dition of imino-ketenimines is considerably easier than
the intermolecular version if both functional groups are
incorporated into an appropriate scaffold, and does not
require the presence of electron-withdrawing groups on
the ketenimine nitrogen.5
close to 50°. By contrast, the s-trans conformer is the
global minimum energy conformation of 2-azabutadiene.
Wiberg8a stated that the delocalization index of the lone
pair at the nitrogen atom in methyleneformamide is
higher than that of the 2-azabutadiene. This fact was
explained by the greater electron-withdrawing character
of the CdO bond when compared with the CdC bond,
accounting for the calculated minimum energy conforma-
tion that allows the partial delocalization of the nitrogen
lone pair into the CdO bond as well as some conjugation
between the CdN and the CdO π-systems. Other reports
on the structure and conformation of more complex
N-acylimines support these hypotheses.9
Accordingly, the iminic nitrogen atom of N-acylimines
is probably less nucleophilic than that of simple imines
where it is linked to an sp3-hybridized carbon atom and,
hence, the [2+2] cycloaddition reactions of N-acylimino-
ketenimines type 1 seem a priori more difficult than those
of the previously essayed imino-ketenimines represented
by structure 3. Nevertheless, the tether linking the
N-acylimine and the ketenimine moieties in the imino-
ketenimines 1, an o-phenylene or vinylic fragment, may
increase the electrophilic character of the ketenimine
moiety by communicating across its π-system the CdO
function with the NdC π-system of the ketenimine
component. This feature might compensate the presum-
ably poor nucleophilic character of their iminic nitrogen
atom.
On the other hand, an alternative cyclization mode of
N-acylimino-ketenimines 1 that can be envisaged is the
6π-electrocyclic ring closure leading to the [1,3]-oxazine
5 (Scheme 3). Moreover, due to its likely pseudopericyclic
character,10 a low-energy barrier can be presumed for this
transformation.
A
common nonconcerted mechanism has been
established5c-e,7 for all the [2+2] cycloaddition reactions
of imino-ketenimines that have been approached by
computational methods. For example, the conversion of
N-(4-azapenta-1,4-dienyl)ketenimine (3) into its [2+2]
cycloadduct 4 takes place in two steps. The first one
involves the nucleophilic attack of the iminic nitrogen
lone pair onto the central carbon atom of the ketenimine
moiety, leading to a zwitterionic intermediate INT that,
in the second step, undergoes a 4π-conrotatory ring
closure (Scheme 2). The conjugated CdC bond linked to
the keteniminic nitrogen enhances the electrophilic
character of the ketenimine moiety favoring the cycliza-
tion process.5c
We have found in the literature several reports dealing
with the electronic description and the rotational profile
of the simplest N-acylimine, methyleneformamide.8 In-
terestingly, its computed global minimum energy con-
formation is s-cisoid, and it shows a torsional angle θCNCO
(5) (a) Alajar´ın, M.; Molina, P.; Vidal, A. Tetrahedron Lett. 1996,
37, 8945-8948. (b) Alajar´ın, M.; Molina, P.; Vidal, A.; Tovar, F.
Tetrahedron 1997, 53, 13449-13472. (c) Alajar´ın, M.; Vidal, A.; Tovar,
F.; Arrieta, A.; Lecea, B.; Coss´ıo, F. P. Chem. Eur. J. 1999, 5, 1106-
1117. (d) Coss´ıo, F. P.; Arrieta, A.; Lecea, B.; Alajar´ın, M.; Vidal, A.;
Tovar, F. J. Org. Chem. 2000, 65, 3633-3643. (e) Alajar´ın, M.; Vidal,
A.; Tovar, F.; Coss´ıo, F. P.; Arrieta, A.; Lecea, B. J. Org. Chem. 2000,
65, 7512-7515. (f) Alajar´ın, M.; Vidal, A.; Orenes, R.-A. Eur. J. Org.
Chem. 2002, 4222-4227.
(6) (a) Arnold, B.; Regitz, M. Angew. Chem., Int. Ed. Engl. 1979,
18, 320. (b) Van Camp, A.; Goossens, D.; Moya-Portugez, M.; March-
and-Brynaert, J.; Ghosez, L. Tetrahedron Lett. 1980, 21, 3081-3084.
(7) (a) Alajar´ın, M.; Sa´nchez-Andrada, P.; Vidal, A.; Tovar, F. Eur.
J. Org. Chem. 2004, 2636-2643. (b) Alajarin, M.; Vidal, A.; Tovar, F.;
Sa´nchez-Andrada, P. Tetrahedron Lett. 2002, 43, 6259-6261.
(8) (a) Wiberg, K. B.; Rablen, P. R.; Marquez, M. J. Am. Chem. Soc.
1992, 114, 8654-8668. (b) See also: Allmann, R.; Kupfer, R.; Nagel,
M.; Wu¨rthwein, E.-U. Chem. Ber. 1984, 117, 1597-1605. (c) Wu¨rth-
wein, E.-U.; Kupfer, R.; Kaliba, C. Angew. Chem., Int. Ed. Engl. 1983,
22, 252-253. (d) McAllister, M. A.; Tidwell, T. J. Chem. Soc., Perkin
Trans. 2 1994, 2239-2248.
(9) See for example: (a) Gross, L. A.; Baird, G. S.; Hoffman, R. C.;
Baldridge, K. K.; Tsien, R. Y. Proc. Natl. Acad. Sci. 2000, 97, 11990-
11995. (b) Yarbrough, D.; Wachter, R. M.; Kallio, K.; Matz, M. V.;
Remington, S. J. Proc. Natl. Acad. Sci. 2001, 98, 462-467.
(10) (a) Birney, D. M. J. Org. Chem. 1996, 61, 243-251. (b) Alajar´ın,
M.; Sa´nchez-Andrada, P.; Coss´ıo, F. P.; Arrieta, A.; Lecea, B. J. Org.
Chem. 2001, 66, 8470-8477.
J. Org. Chem, Vol. 70, No. 4, 2005 1341

Alajar´ın et al.
TABLE 1. Energy Barriers (∆E, kcal‚mol-1) Computed for the Reactions Included in Schemes 2, 4, and 5a
1a f 2a and 1a f 5a
∆E1a
∆EINTa
∆E2a
∆Erxn2a
∆E3a
∆Erxn5a
HF/6-31G* b
20.10
10.25
7.43
0.99
-10.40
-9.49
23.80
12.18
15.14
-45.38
-51.06
-43.82
15.89
8.33
4.88
-12.19
-14.25
-14.36
MP2/6-31G* c
B3LYP/6-31G* d
1b f 2b and 1b f 5b
∆E1b
∆EINTb
∆E2b
∆Erxn2b
∆E3b
∆Erxn5b
HF/6-31G* b
23.51
14.64
11.38
12.91
4.66
0.08
17.26
4.08
9.06
-46.41
-50.62
-44.78
21.71
15.61
9.94
5.70
7.11
-0.10
MP2/6-31G* b
B3LYP/6-31G* d
3 f 45c
∆E1
∆EINT
∆E2
∆Erxn4
HF/6-31G* b
32.18
18.25
19.73
24.49
12.83
14.04
20.00
7.44
11.06
-24.38
-30.34
-22.57
MP2/6-31G* b
B3LYP/6-31G* d
a See Figure 1 for the notation of the energy barriers. b Energies computed on the fully optimized HF/6-31G* geometries. The ZPVE
corrections, computed at the same level, have been included. The ZPEs were not scaled. c Energies computed on the fully optimized
MP2/6-31G* geometries. The ZPVE corrections, computed at the same level, have been included. The ZPEs were not scaled. d Energies
computed on the fully optimized B3LYP/6-31G* geometries. The ZPVE corrections, computed at the same level, have been included. The
ZPEs were not scaled.
SCHEME 3. The Two Cyclization Modes of
N-Acylimino-ketenimines 1
SCHEME 4. The Two-Step Pathway Found for the
[2+2] Cycloaddition of the
N-Acylimino-ketenimines 1a,b Leading to the
Cycloadducts 2a,b
Here, we report the computational and the experimen-
tal study undertaken with the aim of exploring the mode
selectivity ([2+2] cycloaddition vs 6π-electrocyclic ring
closure) in the intramolecular cyclization of N-acylimino-
ketenimines 1 and its mechanistic scheme.
ing the discussion we will comment on only the results
obtained at the B3LYP/6-31G* theoretical level, unless
otherwise stated. Houk et al. have shown that this
computational level is adequate for pericyclic reactions,
its quality being comparable or even superior to that of
comparatively more expensive methods.11
The Two-Step [2+2] Cycloaddition Pathway. For
the transformations of 1a,b into the [2+2] cycloadducts
2a,b this computational study provides reaction path-
ways similar to those previously described for the in-
tramolecular [2+2] cycloaddition of other imino-keten-
imines,5c involving the zwitterionic intermediates INTa,b
shown in Scheme 4 and the corresponding transition
structures TS1a,b and TS2a,b.
For the sake of comparison, in Table 1 we also gathered
the energy barriers computed previosly5c for the conver-
sion of N-(4-azapenta-1,4-dienyl)ketenimine (3) into its
[2+2] cycloadduct 4 (Scheme 2), where a methylene sp3-
carbon is linked to the iminic N atom instead of the
carbonyl sp2-carbon in 1a,b.
Computational Results
For the theoretical study we selected the most simple
systems that model the N-acylimino-ketenimines 1, those
where the iminic carbon atom and the terminal atom of
the ketenimine moiety are unsubstituted and the two
reactive functionalities are linked either by a vinylic (1a)
or an o-phenylene (1b) tether (Scheme 4). We carried out
an intensive search along the RHF/6-31G*, B3LYP/
6-31G*, and MP2/6-31G* potential energy surfaces for
the transformations of 1a into 2a and 5a, whereas for
the transformations of 1b into 2b and 5b the search was
made along the RHF/6-31G* and the B3LYP/6-31G*
potential energy surfaces, and the MP2/6-31G* calcula-
tions were computed on the fully optimized RHF/6-31G*
geometries. The energy barriers computed at the B3LYP
and MP2 theoretical levels were similar, in general
somewhat higher at the MP2 level, and are gathered in
Table 1. The qualitative reaction profiles are depicted in
Figure 1. In Figures 2 and 3 the chief geometrical
features of the corresponding stationary points optimized
at the B3LYP level are shown. With the aim of simplify-
The computed energy barriers associated to the first
step of the [2+2] cycloaddition of N-acylimino-keten-
(11) Guner, V.; Khuong, K. S.; Leach, A. G.; Lee, P. S.; Bartberger,
M. D.; Houk, K. N. J. Phys. Chem. A 2003, 107, 11445-11459.
1342 J. Org. Chem., Vol. 70, No. 4, 2005

Cyclization of Ketenimines
FIGURE 1. Qualitative reaction profiles at the B3LYP/6-31G* level of the intramolecular [2+2] cycloaddition and the
6π-electrocyclic ring closure of N-acylimino-ketenimines 1a,b leading to 2a,b and 5a,b, respectively.
FIGURE 2. Computer plot of the stationary points found in the transformation of N-acylimino-ketenimine 1a into 2a and 5a
optimized at the B3LYP/6-31G* level. Plain numbers correspond to the geometrical parameters, and the numbers between
parentheses are the relative energies. Bond lengths and angles are given in angstroms and degrees, respectively. In this and the
following figure that include ball-and-stick representations, atoms are represented by increasing depth of shading in the order H,
C, O, and N.
imines 1a,b through the transition structures TS1a,b
were 7.43 and 11.38 kcal‚mol-1 respectively, both being
lower than the barrier computed for the transformation
of imino-ketenimine 3 into the polar intermediate INT
(19.73 kcal‚mol-1). The following reasonings on electron-
ics grounds may be argued to account for the relative
weights of these energy barriers:
In N-acylimino-ketenimines 1a,b the conformation of
the N-acylimino units is s-cisoid, showing a torsional
angle θCNCO close to 40°, a similar value to that of
methyleneformamide (see Figures 2 and 3). The conjuga-
tion of the C1dN3 π-system occurs not only with the
adjacent C4dC5 π-system, but also with the CdO func-
tion, resulting in a notable increase of the electrophilic
character of the ketenimine moiety: the natural charge
on the central carbon of the ketenimine function is lower
in the imino-ketenimine 3 (0.40) than in 1a,b (0.45 in
both compounds). Interestingly, the nucleophilicity of the
iminic nitrogen atom seems to be higher in the N-
acylimino-ketenimines 1a,b than in 3: the natural
charge on the iminic atom is more negative in 1a,b (-0.49
and -0.50, respectively) than in the imino-ketenimine 3
(-0.42). In consequence a better nucleophile-electrophile
interaction between the two reacting fragments is ex-
pected for 1a,b when compared with 3. On the other
hand, the conjugation of the C1dN3 π-system with the
CdO group in 1b involves some loss of aromaticity of the
benzene ring, and consequently the conjugation between
these π-systems is probably lower than that in 1a. This
idea could account for the slightly lower electrophilicity
of the ketenimine moiety of 1b when compared with that
of 1a (see Figures 2 and 3 and Tables S2-S5 of the
Supporting Information).
As a consequence of the attack of the iminic nitrogen
on the ketenimine central carbon, partial or total positive
and negative charges are generated on the iminic and
the heterocumulenic parts, respectively, of the first
transition states and, obviously, of the polar intermedi-
ates. The CdO group present in TS1a,b and INTa,b
allows for a more efficient charge delocalization than
their CH2-analogous TS1 and INT, as represented by the
resonance hybrids in Chart 1. In turn, the delocalization
J. Org. Chem, Vol. 70, No. 4, 2005 1343

Alajar´ın et al.
FIGURE 3. Computer plot of the stationary points found in the transformation of N-acylimino-ketenimine 1b into 2b and 5b.
See the caption of Figure 2 for additional details.
CHART 1. Some Canonical Forms of the
Resonance Hybrids 1a,b and INTa,b
CHART 2. Charge Distribution According to the
VB Method of the Cross-Conjugated Mesomeric
Betaines INTa,b Showing the Anionic and Cationic
Segments and the Two Open-Valence Tautomers
1a,b and 6a,b
structural types of heterocyclic mesomeric betaines. A
detailed analysis of the polar intermediates INTa,b, and
their resonance forms, reveals that the opposite charges
are delocalized in separated parts of a common π-electron
system, the carbonyl carbon atom acting as an isolator
that prevents the charge neutralization. The anionic
segment is isoconjugated with an odd alternant hydro-
carbon anion (odd-AH, 9 atoms in INTa and 13 in INTb),
therefore these betaines belong to a subclass of cross-
conjugated mesomeric betaines (CCMB-odd-AH). In Chart
2 the starred positions indicate the possible sites for the
negative charge in the canonical formulas. The positive
segment is joined to the negative one by two union bonds
(C1-N7 and C6-N7) through two unstarred sites. There
are two open-chain valence tautomers of these intermedi-
is more effective in TS1a and INTa than in TS1b and
INTb (see Σq in Figures 2 and 3), probably as a result of
the slight loss of aromaticity in the benzene ring neces-
sary for allowing the delocalization of negative charge
on the carbonyl oxygen via o-quinoid canonical forms.
This fact may account for the higher energy barrier
computed for the conversion 1b f INTb with respect to
that of 1a f INTa.
Before describing the second step of the [2+2] cyclo-
addition of the N-acylimino-ketenimines 1a,b, we will
focus now into the special characteristics of the zwitte-
rionic intermediates INTa,b, which are real heterocyclic
mesomeric betaines. Depending on their electronic na-
ture, Ollis classified these compounds into four major
classes: conjugated, cross-conjugated, pseudo-cross-
conjugated, and heterocyclic N-ylides.12 The subdivision
on the basis of the isoconjugated equivalents (see below)
led to four additional subclasses for a total of 16 distinct
(12) (a) Ollis, W. D.; Stanforth, S. P.; Ramsden, C. A. Tetrahedron
1985, 41, 2239-2329. See also: (b) Schmidt, A.; Habeck, T.; Kinder-
man, M. K.; Nieger, M. J. Org. Chem. 2003, 68, 5977-5982. (c) Potts,
K. T.; Murphy, P. M.; Kuehnling, W. R. J. Org. Chem. 1988, 53, 2889-
2898.
1344 J. Org. Chem., Vol. 70, No. 4, 2005

Cyclization of Ketenimines
SCHEME 5. The 6π-Electrocyclic Ring Closure of
the N-Acylimino-ketenimines 1a,b Leading 5a,b
ates: the N-acylimino-ketenimines 1a,b and the N-(2-
azabutadien-3-yl)imino-ketenes 6a,b (Chart 2).
Somewhat unexpectedly, we noted a slight similarity
between the calculated structure of INTa and that
presumed for their open-chain valence tautomer 6a. This
finding is inferred from the following observations: (i) a
notable increase of the C6-N7 bond distance on going
from the imino-ketenimine 1a to INTa (from 1.436 Å to
1.572 Å); (ii) the computed bond order of this bond is low
in INTa (0.74); and (iii) the C5-C6-O bond angle of
INTa is wide (135.30°), while the N7-C6-O bond angle
is small (114.9°) (see also the bond lengths and the
computed bond orders of the bonds forming the six-
membered ring in Tables S4 and S5 of the Supporting
Information).13 In contrast, INTb is less similar to the
structure expected for its valence tautomer, the o-
quinomethane imine 6b, than in the preceding case,
probably as a result of the partial loss of aromaticity of
the benzene ring that such similarity would require.
In relation to the second step of the [2+2] cycloaddition,
the computed energy barrier associated to the conrotatory
ring closure of INTa leading to 2a via the transition
structure TS2a was 15.14 kcal‚mol-1, a value higher than
those computed for the conversion of the intermediates
INTb and INT into their respective [2+2] cycloadducts
2b (9.06 kcal‚mol-1) and 4 (11.06 kcal‚mol-1). The most
effective charge delocalization is found in intermediate
INTa (see above) and, consequently, their interacting
terminal carbon atoms C2 and C8 are less polar than
those of INTb and INT (see Σq in Figures 2 and 3), thus
making the 4π-electrocyclic ring closure of these later
intermediates easier than that of INTa. The partial
recovery of aromaticity on going from INTb to 2b can
also account for the lower energy barrier of this conver-
sion in comparison with that of INTa into 2a.
vibration of TS3a,b reveals that the nuclear motion has
no component associated with the rotation around the
CdO group (see the dihedral angles shown in Figures 2
and 3). Instead, the forming six-membered ring in the
transition state is nearly planar, and the electronic
motions correspond to the attack of a lone pair of the
oxygen atom on the LUMO plane of the ketenimine
component, both placed in the molecular plane, and to
the slight rotation of the cumulenic CdC bond. Second,
the NBO analyses of TS3a,b show bonding between an
oxygen lone pair and the π* C1-N3 natural localized
orbital (the second-order perturbation energy associated
with this donation being ∆E(2)LpOfπ*C1-N3 > 42 kcal
mol-1). Therefore, the new, partially formed σ-bond is due
to the interaction between an sp2-lone pair of the carbonyl
oxygen atom and a p-orbital of the CdN bond placed in
the molecular plane, instead of involving the two parallel
p-orbitals in both termini of the cyclic array of six atoms
(see Figure 4). Besides, the p-orbital at the nitrogen atom
is becoming the new lone pair in the final product while
the original lone pair will be the new p-orbital. Conse-
quently, nonbonded pairs are converted into bonded pairs
thus giving rise to “disconnections” in the cyclic array of
overlapping orbitals, because the atomic orbitals switch-
ing functions are mutually orthogonals. These charac-
teristics are consistent with a pseudopericyclic process.
The pseudopericyclic reactions represent a special kind
It is worth emphasizing that the first energy barrier
of the transformation 1a f 2a is lower than the second
one, similar to what has been calculated for the two-step
Staudinger reaction between ketenes and imines14 and
in sharp contrast with the rest of the [2+2] cycloadditions
between imines and ketenimines that we have previously
studied by computational methods. The particular ther-
modynamic stability of the cross-conjugated mesomeric
betaine INTa may well account for this fact.
Pseudopericyclic 6π-Electrocyclic Ring Closure.
The alternative reaction pathway that has been compu-
tationally analyzed for the cyclization of N-acylimino-
ketenimines 1a,b is the 6π-electrocyclic ring closure, via
the transition structures TS3a,b, leading to the 1,3-
oxazines 5a,b (Scheme 5).
We have established the pseudopericyclic orbital topol-
ogy of both transition states on the basis of their
geometries, the results of the NBO analyses, and their
magnetic behavior. First, the animation of the imaginary
(13) A related tilting of a CdO group in the direction of a ketene
valence isomer in mesoionic pyridopyrimidinylium olates and nonme-
soionic pyridopyrimidinones has been described in: Plu¨g, C.; Wallfisch,
B.; Andersen, H. G.; Bernharddt. P. V.; Baker, L.-J.; Clark, G. R.; Wong,
M. W.; Wentrup, C. J. Chem. Soc., Perkin Trans. 2 2000, 2096-2108.
(14) (a) Hegedus, L. S.; Montgomery, J.; Narukawa, Y.; Snustad,
D. C. J. Am. Chem. Soc. 1991, 113, 5784-5791. (b) Coss´ıo, F. P.;
Ugalde, J. M.; Lo´pez, X.; Lecea, B.; Palomo, C. J. Am. Chem. Soc. 1993,
115, 995-1004. (c) Lecea, B.; Arrastia, I.; Arrieta, A.; Roa, G.; Lo´pez,
X.; Arriortua, M. I.; Ugalde, J. M.; Coss´ıo, F. P. J. Org. Chem. 1996,
61, 3070-3079.
FIGURE 4. Basic atomic orbital interactions for the 6π-
electrocyclic ring closure of N-acylimino-ketenimines 1: (A)
pericyclic pathway; (B) pseudopericyclic pathway; and (C)
calculated transition state.
J. Org. Chem, Vol. 70, No. 4, 2005 1345

Alajar´ın et al.
of pericyclic reaction that has been placed on a solid
foundation by the works of Birney and others.10,15
The computed energy barrier for the transformation
of imino-ketenimine 1a into 5a was very low, 4.88
kcal‚mol-1, as expected for a pseudopericyclic reaction,
and the process is predicted to be exothermic by 14.36
kcal‚mol-1 (Table 1). Probably as a result of the loss of
the aromaticity in the benzene ring taking place in the
conversion 1b f 5b (note the o-quinoid type structure of
5b), the calculated energy barrier of this transformation
is somewhat higher than that of the preceding case (9.94
kcal‚mol-1). The DFT approach predicts that this process,
1b f 5b, is nearly isoenergetic (∆Erxn ) -0.1 kcal‚mol-1),
while at the MP2/6-31G*//HF/6-31G* level it is predicted
to be endoergonic (∆Erxn ) 7.11 kcal‚mol-1).
To confirm the pseudopericyclic nature of the 6π-
electrocyclic ring closures of 1a,b via TS3a,b affording
5a,b, as well as to prove the partial loss of aromaticity
of the benzene ring in the conversions 1b f INTb and
1b f 5b, we have computed the nucleus-independent
chemical shifts (NICS) at the B3LYP/6-31G* theoretical
level at different points of structures TS3a,b, 1b, INTb,
2b, and 5b. According to Schleyer,16 large negative NICS
values at the center of the ring under consideration are
associated with aromatic character, whereas positive
NICS values are a convenient indicator of antiaromatic-
ity. In addition, Coss´ıo et al.17 demonstrated that the
aromatic behavior of compounds and transition states can
be described by the variation of the NICS at different
points above and below the molecular plane, the intersec-
tion point being the (3,+1) ring critical point of the
electron density (RCP) defined by Bader.18 Two ring
currents circulating at a certain distance from the
molecular plane characterize π-aromaticity, benzene be-
ing the archetypal example.17
FIGURE 5. Plot of the calculated NICS versus z for (A) the
forming six-membered ring of the transition structures TS3a
and TS3b corresponding to the 6π-electrocyclic ring closure
of 1a,b and (B) the benzene ring of 1b, 2b, INTb, and 5b.
On the other hand, our results show that the benzene
ring of 1b exhibits negative NICS at all the points
calculated, the maximum (-11.9 ppm/mol) found at 0.73
Å above and below the ring critical point. This same
pattern is shown by the benzene ring of the [2+2]
cycloadduct 2b (NICSmax ) -12.0 ppm/mol at (0.75 Å),
and consequently, the benzene ring in both structures is
aromatic. By contrast, the NICSmax computed in the
benzene ring of benzoxazine 5b was 3.19 ppm/mol,
therefore proving to be nonaromatic. Interestingly, the
zwitterionic intermediate INTb showed low negative
NICS, the largest value being -5.25 ppm/mol, found at
1.01 Å above and below the ring critical point, indicating
that its benzene ring is only slightly aromatic (see Figure
5B).
Kinetic Analysis and Mode Selectivity in the
Cyclization of Imino-ketenimines 1a,b. After having
discussed the two modes of intramolecular cyclization of
imino-ketenimines 1a,b, [2+2] cycloaddition, and 6π-
electrocyclic ring closure, let us now analyze the heights
of the different energy barriers gathered in Table 1 to
predict which would be the preferred reaction path. From
the reaction profile in Figure 1, it seems that the [2+2]
cycloadducts 2a,b are predicted to be the products of
thermodynamic control, while the oxazines 5a,b should
be the kinetically controlled products. In the transforma-
tion 1b f 2b and 5b, the energy barriers ∆E1b, ∆E2b, and
∆E3b are quite similar, and the energy barrier for the
electrocyclic ring opening of the oxazine 5b affording the
imino-ketenimine 1b, ∆E-3b, is very low (10.04 kcal‚mol-1).
For these reasons, it seems reasonable to assert that even
if 5b is the product of the kinetic control, it would be
easily (and rapidly) transformed into the [2+2] cyclo-
adduct 2b. However, this assumption is not obvious in
the transformation of 1a into 2a and/or 5a, as the energy
barriers ∆E2a and ∆E3a are quite different, and because
∆E-3a is high (19.24 kcal‚mol-1). Therefore, we decided
to carry out a quantitative kinetic analysis to predict,
In relation to the magnetic properties of the transition
structures TS3a,b, the results support its pseudoperi-
cyclic nature, given that at the RCP of TS3a and TS3b
the computed NICS were 0.16 and 1.67 ppm‚mol-1
,
respectively, and both plots of the calculated NICS
against z show a similar shape (see Figure 5A) to those
described for the 6π-electrocyclic ring closure of 5-oxo-
2,4-pentadienal and N-formyliminovinyl ketene to pyran-
2-one and 1,3-oxazin-6-one, respectively, two processes
unequivocally pseudopericyclic.10a,b,19 By contrast, an
aromatic disrotatory ring closure shows one ring current
circulating on the side where the disrotatory movement
allows a close proximity between the terminal p atomic
orbitals, this kind of aromaticity being defined by Coss´ıo
as π1-aromaticity.17c
(15) (a) Birney, D. M.; Xu, X.; Ham, S. Angew. Chem., Int. Ed. 1999,
38, 189-193. (b) Birney, D. M.; Ham, S.; Unruh, G. R. J. Am. Chem.
Soc. 1997, 119, 4509-4517. (c) Fabian, W. M. F.; Kappe, C. O.; Bakulev,
V. A. J. Org. Chem. 2000, 65, 47-53. (d) Liu, R. C.-Y.; Lusztyk, J.;
McAllister, M. A.; Tidwell, T. T.; Wagner, B. D. J. Am. Chem. Soc.
1998, 120, 6247-3251. (e) Luo, L.; Bartberger, M. D.; Dolbier, W. R.
J. Am. Chem. Soc. 1997, 119, 12366-12367.
(16) Schleyer, P. v. R.; Maerker, C.; Dransfeld, A.; Jiao, H.; Hommes,
N. J. R. v. E. J. Am. Chem. Soc. 1996, 118, 6317-6318.
(17) (a) Morao, I.; Coss´ıo, F. P. J. Org. Chem. 1999, 64, 1868-1874.
(b) Coss´ıo, F. P.; Morao, I.; Jiao, H.; Scheleyer, P. v. R. J. Am. Chem.
Soc. 1999, 121, 6737-6746. (c) R. de Lera, A.; Alvarez, R.; Lecea, B.;
Torrado, A.; Coss´ıo, F. P. Angew. Chem., Int. Ed. 2001, 40, 557-561.
(18) Bader, R. F. W. Atoms in Molecules-A Quantum Theory;
Clarendon Press: Oxford, UK, 1990; pp 13-52.
(19) Rodr´ıguez-Otero, J.; Cabaleiro-Lago, E. M. Chem. Eur. J. 2003,
9, 1837-1843.
1346 J. Org. Chem., Vol. 70, No. 4, 2005

Cyclization of Ketenimines
TABLE 2. Variation of the Ratio of Products 2a,b/5a,b
more precisely, the ratio of final products, 2a,b versus
5a,b, considering the mechanistic scheme represented
below. It was necessary to solve the system of differential
equations outlined in eqs 1-4, with the environment
conditions represented in eqs 5 and 6. For simplicity, the
steady-state approximation was applied to [INT].20 The
solutions were eqs 7-9, where λ1 and λ2 are a combina-
tion of the rate constants k1, k2, k3, k-1, and k-3, which
were calculated by means of the eqs 10-12 (see the
Supporting Information).
with the Reaction Time (t)
t (s)
2a/5a
% 2a
t (s)
2b/5b
% 2b
1 × 10-4 1.271 × 10-2
1 × 10-2 1.272 × 10-2
1 × 100 1.334 × 10-2
1 × 102 7.688 × 10-2
1.27 1 × 10-20 4.877 × 10-2
1.27 1 × 10-10 8.648 × 10-2
1.32 1 × 10-7 8.767 × 10-2
4.65
7.96
8.06
7.14 1 × 10-5 2.932 × 10-1 22.67
3 × 102 2.178 × 10-1 17.88 3 × 10-5 8.832 × 10-1 46.90
1 × 103 8.745 × 10-1 46.65 7 × 10-5 2.610
72.30
82.14
95.31
2 × 103 2.477
71.24 1 × 10-4 4.599
3 × 103 5.457
84.51 2 × 10-4 2.032 × 101
1 × 104 4.947 × 102
99.80 1 × 10-3 4.492 × 105 100
1 × 105 8.752 × 1026 100
1 × 10-2 1.293 × 1054 100
1 × 106 2.576 × 10269 100
1 × 10-1
∞
100
results, at 298 K, after a reaction time of 10-4 s the ratio
2b/5b would be 4.6, i.e. 82.14% of 2b vs 17.86% of 5b; at
10-3 s 100% of 2b is predicted to be in the reaction
mixture. By contrast, after 3 × 103 s the ratio 2a/5a
would reach a value of 5.5, i.e. 84.5% of 2a vs 15.5% of
5a, and after 104 s the proportion of 2a would rise to
99.8% (see Table 2 and Figure 6).
d[1]
dt
) -(k1 + k3)[1] + k-1[INT] + k-3[5]
(1)
(2)
d[INT]
dt
) k1[1] - (k-1 + k2)[INT] ) 0;
[INT] )
k1[1]
(k-1 + k2)
d[2]
dt
) k2[INT]
(3)
(4)
d[5]
dt
) k3[1] - k-3[5]
[1]0 ) [1] + [INT] + [2] + [5]
[2]0 ) [INT]0 ) [5]0 ) 0
k3[1]0
(5)
(6)
[5] )
(eλ t - e λ t
)
(7)
(8)
1
2
(λ2 - λ1)
[1]0
FIGURE 6. Plot of percent [2a] and [2b] (the [2+2] cyclo-
adducts) in relation to [5a] and [5b], respectively (the products
of the 6π-electrocyclic ring closure) against log t (time in
seconds).
λ2t
1
[1] )
[2] )
[(λ2 + k-3)eλ t - (λ1 + k-3)e ]
(λ2 - λ1)
2
k1k2[1]0
(λ2 + k-3)(eλ t - 1)
-
[
λ2
(k2 + k-1)(λ2 - λ1)
Accordingly, this kinetic analysis shows that the ox-
azines 5 are the kinetically controlled products, but they
should transform rapidly and totally into the [2+2]
cycloadducts 2 at room temperature. The reaction time
required for the total transformation of oxazine 5a into
the [2+2] cycloadduct 2a is approximately 3 h. By
contrast, due to the different kinetic behavior of the
system 1b f 2b/5b, both the formation of the [2+2]
cycloadduct 2b and the transformation of the initially
formed oxazine 5b into 2b are predicted to be completed
after extremely short reaction times.
In summary, this computational study predicts that
the N-acylimino-ketenimines 1 should undergo easily an
intramolecular cyclization leading to the corresponding
[2+2] cycloadducts 2 via a two-step pathway. The pres-
ence of the CdO group attached to the iminic nitrogen
allows for the increase of the electrophilic character of
the ketenimine moiety, thus facilitating the first reaction
step in relation to other imino-ketenimines supporting
an sp3-carbon atom on the iminic nitrogen. The alterna-
tive cyclization of the N-acylimino-ketenimines 1 consist-
ing of a pseudopericyclic 6π-electrocyclic ring closure
leading to 1,3-oxazines 5 has been mechanistically char-
(λ1 + k-3)(eλ t - 1)
1
(9)
]
λ1
∆E-1 ) ∆E1 - ∆EINT
∆E-3 ) ∆E3 - ∆Erxn5
KBT
(10)
(11)
k )
[e-(∆E/RT)
]
(12)
h
In contrast with other quantitative kinetic analyses
that we have previously performed,5c,7a the present one
shows a strong dependence of the ratio 2a,b/5a,b with
the reaction time. This is a consequence of the revers-
ibility at room temperature of the 6π-electrocyclic ring
closure of 1a,b leading to 5a,b allowing the conversion
of 5a,b into 2a,b under the reaction conditions.
The calculated values for the ratio [2]/[5] at various
reaction times show remarkable differences between the
systems 1a f 2a/5a and 1b f 2b/5b. According to our
(20) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms;
McGraw-Hill: New York, 1981; pp 72-87.
J. Org. Chem, Vol. 70, No. 4, 2005 1347

Alajar´ın et al.
TABLE 3. 1,2-Dihydroazeto[2,1-b]quinazolin-8-ones 12
SCHEME 6. Synthesis of
Azeto[2,1-b]quinazolin-8-ones 12a
R1
R2
R3
yield (%)
12a
12b
12c
12d
12e
12f
C6H5
C6H5
C6H5
CH3
C6H5
CH3
C6H5
C6H5
65
44
67
44
58
36
C6H5
C6H5
4-CH3-C6H4
4-CH3-C6H4
C6H5
4-CH3-C6H4
4-CH3-C6H4
CH3
C6H5
OCH2CH3
with the ketene.5b Azeto[2,1-b]quinazolin-8-ones 12 were
isolated from the final reaction mixtures in moderate to
good yields after purification by column chromatography
(Table 3). No other reaction products could be separated
in appreciable yield in the chromatographic purification.
Compounds 12 were unequivocally characterized by their
analytical and spectral data (see the Experimental Sec-
tion).
Obviously the formation of azetoquinazolinones 12 is
rationalized as occurring by a formal intramolecular
[2+2] cycloaddition between the imine and the keten-
imine functions of the putative imino-ketenimines 11,
thus confirming the predictions of the computational and
kinetic analyses.
a Reagents and conditions: (i) Et3N, toluene, 0 °C f rt, 2 h; (ii)
PMe3, toluene, rt, 30 min; (iii) PhR3CdCdO, toluene, rt, 30 min.
acterized. The quantitative kinetic analysis shows that
despite the oxazines 5 being predicted to be the kineti-
cally controlled products, they will transform into the
[2+2] cycloadducts 2 at room temperature in short
reaction times.
Conclusions
Ab initio and DFT calculations predict that N-acylimino-
ketenimines 1, where the carbonyl carbon atom and the
keteniminic nitrogen atom are linked either by a vinylic
or an o-phenylene tether, should undergo easily an
intramolecular cyclization leading to the corresponding
[2+2] cycloadducts 2 via a two-step pathway, with
formation of cross-conjugated mesomeric betaines as
intermediates. The CdO group attached to the iminic
nitrogen increases the electrophilic character of the
ketenimine moiety, thus facilitating the first reaction
step.
The alternative cyclization of the N-acylimino-keten-
imines 1 via a 6π-electrocyclic ring closure leading to 1,3-
oxazines 5 is the kinetically favored process. The pseudo-
pericyclic character of the corresponding transition states
has been established on the basis of their geometric
parameters, the NBO analyses, and their nonaromatic
character, as determined by the NICS values.
The quantitative kinetic analysis shows a strong
dependence of the ratio [2]/[5] with the reaction time. In
spite of the fact that oxazines 5 are the kinetically
controlled products, they should transform into the [2+2]
cycloadducts 2 at room temperature in short reaction
times.
Experimental Study
We next explored the preparation of a series of N-
acylimino-ketenimines to check experimentally the re-
sults of the computational study.
It has been described that N-acylimines may be easily
prepared by reaction of acyl halides either with ketimines
nonsubstituted at the nitrogen atom8b,21 or with N-
trimethylsilylaldimines,22 and we usually prepare keten-
imines by sequential treatment of organic azides with
tertiary phosphines and ketenes.5,7 Thus, we combined
these synthetic methodologies for the preparation of the
requisite heterocumulenes 11 (Scheme 6). Whereas the
reaction of 2-azidobenzoyl chloride 7 with N-trimethyl-
silylaldimine only gave complex mixtures, it cleanly
reacted with C,C-diaryl ketimines, C-methyl-C-phenyl
ketimine, and ethyl benzimidate in toluene solution at 0
°C in the presence of triethylamine, yielding N-acylimines
9 (Scheme 6). Treatment of a toluene solution of com-
pounds 9 with 1 equiv of 1 M trimethylphosphane in
toluene was followed by evolution of dinitrogen, indicat-
ing the conversion of the azide function into the corre-
sponding λ5-phosphazene 10. The isolation and purifica-
tion of compounds 10 were prevented by the extreme
hydrolytic sensitivity of the phosphazene group. When
these compounds were treated in the same reaction flask,
at room temperature, with diphenyl ketene or methyl
phenyl ketene the pale yellow reaction mixture turned
orange, and immediately faded to a colorless solution. IR
spectra of the reaction mixtures did not show the strong
absorption around 2000 cm-1 expected for the CdCdN
grouping of ketenimines 11, which should reasonably
result from the aza-Wittig reaction of the phosphazene
N-Acylimino-ketenimines 11, with both reactive func-
tions placed onto an o-phenylene scaffold, have been
synthesized. They rapidly converted into azeto[2,1-b]-
quinazolin-8-ones 12 in moderate to good yields, by a
formal [2+2] cycloaddition between the imine and keten-
imine functions.
Experimental Section
Materials. 2-Azidobenzoyl cloride 7,23 diphenyl ketimine,24
bis(4-methylphenyl) ketimine,23 phenylmethyl ketimine,23 eth-
(21) Kupfer, R.; Nagel, M.; Wu¨rthwein, E.-U.; Allmann, R. Chem.
Ber. 1985, 118, 3089-3104.
(22) Kupfer, R.; Meier, S.; Wu¨rthwein, E.-U. Synthesis 1984, 688-
699.
(23) Porter, T. C.; Samalley, R. K.; Teguiche, M.; Purwono, B.
Synthesis 1997, 773-777.
(24) Pickard, P. L.; Tolbert, T. L. J. Org. Chem. 1961, 26, 4886-
4888.
1348 J. Org. Chem., Vol. 70, No. 4, 2005

Cyclization of Ketenimines
yl iminobenzoate hydrochloride,25 diphenylketene,26 and me-
thylphenylketene27 were prepared by published procedures.
Harmonic frequency calculations at each level of theory
verified the identity of each stationary point as a minimum
or a transition state, and were used to provide an estimation
of the zero-point vibrational energies (ZPVE), which were not
scaled. Bond orders32 and natural charges were calculated with
the NBO (natural bond orbital) method.33 NICS values were
obtained at the B3LYP/6-31G*// B3LYP/6-31G* level with the
GIAO (Gauge-Independent Atomic Orbital) method.34
Preparation of 1,2-Dihydroazeto[2,1-b]quinazolin-8-
ones 12 (General Procedure). The corresponding imine or
imidate (4 mmol) was dissolved in dry toluene (25 mL) and
cooled at 0 °C before the dropwise addition of a solution of
2-azidobenzoyl chloride 7 (0.73 g, 4 mmol) in the same solvent
(5 mL). The reaction mixture was stirred at 0 °C for 30 min.
At this point, triethylamine (0.6 g, 6 mmol) was added, and
the stirring was continued for 2 h at room temperature. The
precipitated triethylammonium chloride was separated by
filtration, and the solvent was removed from the filtrate under
reduced pressure. The resulting oil was purified by column
chromatography on silica gel, using hexanes/ethyl acetate
(7:3, v/v) as eluent, except for 9e (R1 ) C6H5; R2 ) CH3) where
the crude material was used us such in the following step.
Acknowledgment. This work has been supported
by the MCYT-FEDER (Project BQU2001-0010) and the
Fundacio´n Seneca-CARM (Proyecto PI-100749/FS/01).
P.S.-A. thanks the Universidad de Murcia for a stu-
dentship. The valuable comments of Prof. F. P. Coss´ıo
are also acknowledged.
To a solution of the corresponding N-acylimine 9 (3 mmol)
in dry toluene (15 mL) was added trimethylphosphane (1 M
toluene solution, 3 mL, 3 mmol), and the reaction mixture was
stirred at room temperature for 30 min. Then a solution of
the appropriate ketene (3 mmol) in dry toluene (2 mL) was
added, and the stirring was continued for 30 min. The solvent
was removed under reduced pressure and the crude material
was purified by column chromatography on silica gel, using
hexanes/ethyl acetate (4:1, v/v) as eluent.
Computational Methods. The calculations were per-
formed with the Gaussian98 suite of programs.28 Geometry
optimizations were carried out at the RHF, Becke3LYP,29 and
MP230 theoretical levels with the internal 6-31G* basis set.31
Supporting Information Available: Tables S1-S7 in-
cluding the chief geometric, electronic, and energetic features
for all stationary points discussed in the text; Cartesian
coordinates of local minima and transition structures discussed
in the text, selected orbital interactions of the NBO analysis,
mathematical treatment of the kinetic analysis and analytical
and spectroscopic data for compounds 12a-f. This material
is available free of charge via the Internet at http://pubs.acs.org.
JO0482716
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