Observable azacyclobutenone ylides with antiaromatic character from 2-diazoacetyl-azaaromatics

Article abstract of DOI:10.1021/ja0686920

Azacyclobutenone ylides 2 and 11 were generated in solution by laser flash photolysis of 2-diazoacetylpyridine (1) and 3-diazoacetylpyridazine (10), respectively, together with the corresponding ketenes. The ylides were identified by their characteristic IR and UV spectra: 2, ν (CH₃CN) 1725 cm^-1, λ_max 360 and 550 (br) nm; 11, ν (CH₃CN) 1776 cm^-1, λ_max 370 and 550 (br) nm. 2-Triisopropylsilyldiazoacetylpyridine 20 upon photolysis at 5°C in CH₃CN forms the ylide 21 as a rather persistent (T_1/2 2 h at 25°C) purple solution, ν (CH₃CN) 1718 cm^-1z, λ_max 245, 378 and 546 (br) nm, but no ketene is observed. Quinolyl ylide 14 and pyridyl ylides 17 and 19 with Me and 2-pyridyl substituents, respectively, with characteristic IR and UV spectra were also generated. The ¹H NMR spectrum of the pyridyl ring of 21 shows substantial upfield shifts relative to those of 20. Calculated nucleus-independent chemical shifts (NICS) for 2, 11, and 21 are comparable to those for benzocyclobutadiene (22) and benzocyclobutenone enolate (23), with substantial positive values for the 4-membered rings, while the NICS values for the 6-membered rings are significantly more positive than for benzene or pyridine. Significant bond alternation is also found in the calculated ylide structures, and these results suggest strong antiaromatic character for the 4-membered rings of 2, 11, 14, 17, 19, and 21, and greatly reduced aromatic character for the 6-membered rings.

Full text of DOI:10.1021/ja0686920

Published on Web 04/24/2007
Observable Azacyclobutenone Ylides with Antiaromatic
Character from 2-Diazoacetyl-azaaromatics
Nanyan Fu,† Annette D. Allen,† Shinjiro Kobayashi,† Pearl A. Sequeira,†
Muhong Shang,† Thomas T. Tidwell,*,† and Masaaki Mishima‡
Contribution from the Department of Chemistry, UniVersity of Toronto, Toronto,
Ontario M5S 3H6, Canada, and Institute for Materials Chemistry and Engineering,
Kyushu UniVersity, 6-10-1 Hakozaki, Fukuoka 812-8581, Japan
Received December 4, 2006; E-mail: ttidwell@chem.utoronto.ca
Abstract: Azacyclobutenone ylides 2 and 11 were generated in solution by laser flash photolysis of
2-diazoacetylpyridine (1) and 3-diazoacetylpyridazine (10), respectively, together with the corresponding
ketenes. The ylides were identified by their characteristic IR and UV spectra: 2, ν (CH3CN) 1725 cm-1
,
λmax 360 and 550 (br) nm; 11, ν (CH3CN) 1776 cm-1, λmax 370 and 550 (br) nm. 2-Triisopropylsilyldiaz-
oacetylpyridine 20 upon photolysis at 5 °C in CH3CN forms the ylide 21 as a rather persistent (T1/2 2 h at
25 °C) purple solution, ν (CH3CN) 1718 cm-1, λmax 245, 378 and 546 (br) nm, but no ketene is observed.
Quinolyl ylide 14 and pyridyl ylides 17 and 19 with Me and 2-pyridyl substituents, respectively, with
characteristic IR and UV spectra were also generated. The 1H NMR spectrum of the pyridyl ring of 21
shows substantial upfield shifts relative to those of 20. Calculated nucleus-independent chemical shifts
(NICS) for 2, 11, and 21 are comparable to those for benzocyclobutadiene (22) and benzocyclobutenone
enolate (23), with substantial positive values for the 4-membered rings, while the NICS values for the
6-membered rings are significantly more positive than for benzene or pyridine. Significant bond alternation
is also found in the calculated ylide structures, and these results suggest strong antiaromatic character for
the 4-membered rings of 2, 11, 14, 17, 19, and 21, and greatly reduced aromatic character for the
6-membered rings.
Photolysis of 2-diazoacetylpyridine 1 in an argon matrix was
reported to give the ylide 2, as identified by the IR and UV
spectra, and upon further photolysis in the matrix this rearranged
to ketene 3 with IR bands at 2123 and 2132 cm-1 for the E and
Z conformations, respectively (eq 1).1a Upon warming, 3 formed
by X-ray.2a Open-chain zwitterions 7 and 8 were observed from
intermolecular addition of pyridines to ketenes.2b,c Computations
showed that the 4-membered ring ketene-ylide form of 3 is not
an energy minimum.1a
a [4 + 2]-cycloaddition dimer.1 Flash vacuum thermolysis (FVT)
of triazoles 4 (R ) Ph, Me, MeO) at 680-750 °C with collection
of the pyrolysates at 7 K was suggested to form diazo ketones
which then formed the corresponding ketenes 5, as identified
by their ketenyl IR absorptions (eq 2).1c,d Heating of 4 (R )
2-pyridyl) in benzylamine gave N-benzyl bis(2-pyridyl)aceta-
mide, possibly via ketene 5 (R ) 2-pyridyl).1e,f However in these
Recent investigations in our laboratory have examined the
generation and direct observation of 2-pyridylketene (3)
generated in solution by laser flash photolysis (LFP) of 1,3a-c
with confirmation of the structure by the observation of the
characteristic ketenyl absorption in solution at 2127 cm-1 1a
. The
studies further examples of ylides analogous to 2 were not
observed.1c,d
The ylide 2 may be compared to a ketene-derived
5-membered ring zwitterions 6, with the structures established
† University of Toronto.
(2) (a) Chelain, E.; Goumont, R.; Hamon, L.; Parlier, A.; Rudler, M.; Rudler,
H.; Daran, J.; Daran, J. C.; Vaissermann, J. J. Am. Chem. Soc. 1992, 114,
8088-8998. (b) Qiao, G. G.; Andraos, J.; Wentrup, C. J. Am. Chem. Soc.
1996, 118, 5634-5638. (c) Kollenz, G.; Holzer, S.; Kappe, C. O.; Dalvi,
T. S.; Fabian, W. M.; Sterk, H.; Wong, M. W.; Wentrup, C. Eur. J. Org.
Chem. 2001, 1315-1322.
‡ Institute for Materials Chemistry and Engineering.
(1) (a) Kuhn, A.; Plu¨g, C.; Wentrup, C. J. Am. Chem. Soc. 2000, 122, 1945-
1948. (b) Plu¨g, C.; Kuhn, A.; Wentrup, C. J. Chem. Soc., Perkin Trans. 1
2002, 1366-1368. (c) Anderson, H. G.; Bednarek, P.; Wentrup, C. J. Phys.
Org. Chem. 2003, 519-524.
9
6210
J. AM. CHEM. SOC. 2007, 129, 6210-6215
10.1021/ja0686920 CCC: $37.00 © 2007 American Chemical Society

Azacyclobutenone Ylides with Antiaromatic Character
A R T I C L E S
Figure 2. UV absorption of ylide 11 from laser flash photolysis of
3-diazoacetylpyridazine (10) in CH3CN (lower trace) and isooctane (upper
trace).
in the UV spectrum and to lesser wave number in the IR are
also observed in some other ylides (vide infra), as tabulated in
Table S-5, Supporting Information.
In H2O LFP of 1 with 308 nm excitation also gives rise to
transient absorption at 520 nm assigned to 2, and this decays
with first-order kinetics, with a rate constant of (3.22 ( 0.13)
× 105 s-1 at room temperature. Rate constants in H2O/CH3CN
mixtures reported in Table S-1 (Supporting Information) give
an empirical correlation of (log k) with the molar [H2O], as has
been previously demonstrated for ketenes.3c
Figure 1. UV spectrum and IR band of ylide 2 in solution.
reactions of 3 with n-butylamine and with water were found
to proceed with formation of intramolecularly H-bonded
dihydropyridine transients
9 (eq 3), which in slower
processes were converted to the observable amide and acid,
respectively.3a,b
Observation of ylide 2 may seem improbable, as this was
calculated1a to be less stable than the ketene 3 by 43.4 kcal/
mol. However, as shown in Figure S-2 (Supporting Information)
there is a direct route for forming 2 upon photolysis of 1, and
the conversion of 2 to 3 through a carbene-like transition state
is calculated to be 47.6 kcal/mol above 2 in energy. Experi-
mentally the formation of 2 and the photochemical conversion
to 3 was observed to occur upon photolysis in a matrix.1a The
half-life of 2 formed by photolysis of 1 in CH3CN was between
300-500 × 10-6 s at 22 °C when followed by IR and UV,
respectively. In H2O the rate of disappearance of 2 (Table S-1)
was 140-230 times greater than in CH3CN, but as no products
from these reactions could be identified after removal of the
solvent, the nature of the decay of 2 is not known.
During these investigations it was observed that in addition
to UV absorptions attributed to 2-pyridylketene (3) and the
intermediates 9, another absorption at longer wavelength was
also detected in solution. Further studies of the photolysis of 1
and other 2-diazoacetyl azaaromatics in solution have now been
carried out. Ylides such as 2 have not previously been detected
in solution, and we now report experimental observations of
this and related ylides in solution at room temperature, and find
that these are surprisingly long-lived despite their thermody-
namic instability.
3-Diazoacetyl pyridazine (10) was prepared from the known
acid,4a and LFP in CH3CN using 266 nm light resulted in
immediate disappearance of the infrared absorption of the diazo
ketone at 2112 cm-1 and formation of a new band at 1776 cm-1
,
Results and Discussion
(3) (a) Acton, A. W.; Allen, A. D.; Antunes, L. M.; Fedorov, A. V.; Najafian,
K.; Tidwell, T. T.; Wagner, B. D. J. Am. Chem. Soc. 2002, 124, 13790-
13794. (b) Allen, A. D.; Fedorov, A. V.; Tidwell, T. T.; Vukovic, S. J.
Am. Chem. Soc. 2004, 126, 15777-15783. (c) Allen, A. D.; Tidwell, T. T.
J. Am. Chem. Soc. 1987, 109, 2774-2780.
(4) (a) Karapanayiotis, T.; Dimopoulos-Italiano, G.; Bowen, R. D.; Terlouw,
J. K. Int. J. Mass. Spectrom. 2004, 236, 1-9. (b) Murti, A.; Bhandari, K.;
Ram, S.; Prabhakar, Y. S.; Saxena, A. K.; Jain, P. C.; Gulati, A. K.; Srimal,
R. C.; Dhawan, B. N.; Nityanand, S.; Anand, N. Indian J. Chem. 1989,
28B, 934-942. (c) Eistert, B.; Schade, W. Chem. Ber. 1958, 91, 1411-
1415. (d) Boyer, J. H.; Goebel, N. J. Org. Chem. 1960, 25, 304-305. (e)
Maas, G.; Bru¨ckmann, R. J. Org. Chem. 1985, 50, 2801-2802. (f)
Bru¨ckmann, R.; Schneider, K.; Maas, G. Tetrahedron 1989, 45, 5517-
5530.
Upon LFP with 308 nm excitation of 1 in isooctane at 22 °C
a transient absorption is observed with maxima at 380 and 570
nm (Figure 1). A similar absorption is observed by LFP in CH3-
CN. These closely resemble that reported by Wentrup et al.,1a
during matrix photolysis of 1, and are also assigned as due to
2. With LFP in CH3CN at 25 °C, IR detection revealed a band
at 1725 cm-1 (Figure 1), similar to the keto absorption for 2
noted by Wentrup, et al.1a of 1759 cm-1 in an Ar matrix at 10
K. The shifts to shorter wavelength in the more polar CH3CN
9
J. AM. CHEM. SOC. VOL. 129, NO. 19, 2007 6211

A R T I C L E S
Fu et al.
Figure 4. 1H NMR of pyridyl H of diazo ketone 20 (left) and ylide 21
(right). The signal at δ 7.3 ppm is due to CHCl3.
In CH3CN the ketene 18 was also observed and identified by
the IR absorption at 2112 cm-1. Photolysis of the triazole 4 (R
) 2-pyridyl)4c,d in CH3CN led to the ylide 19 (eq 7), identified
by the characteristic UV absorption with λmax 385 and 470 (br)
nm, and IR absorption at 1730 cm-1 (Figure S-13). However,
the corresponding ketene was not observed.
Figure 3. UV spectrum of quinolyl substituted ylide 14 in CH3CN (lower)
and in isooctane (upper).
attributed to the azacyclobutenone ylide 11, and at 2132 cm-1
attributed to the ketene 12 (eq 4). The ylide absorption decayed
,
Reaction of diazo ketone 11,3c,d with triisopropylsilyl triflate
following a reported general procedure4e,f gave the diazo ketone
20 (eq 8). Continuous photolysis of 20 with 300 nm light in
CH3CN or pentane resulted in formation of ylide 21 identified
by the carbonyl absorption at 1718 cm-1 or 1746 cm-1
,
respectively (eq 8) (Figures S-8 and S-9, respectively). No
ketene IR absorption was detected.
with a half-life of approximately 5 × 10-4 s. The UV absorption
of ylide 11 was observed in both CH3CN and isooctane (Figure
2). Details of the lifetime measurements for 2, 11, and other
ylides (vide infra) in CH3CN are given in Table S-4, Supporting
Information.
Similarly LFP of 2-diazoacetylquinoline (13)4b in CH3CN
with 266 nm light gave immediate disappearance of the diazo
ketone infrared absorption at 2107 cm-1 and formation of a
new absorption at 1725 cm-1, attributed to the azacyclobutenone
ylide 14, and at 2124 cm-1, due to the ketene 15 (eq 5). The
ylide absorption at 1725 cm-1 decayed with a half-life of
approximately 5 × 10-6 s (IR) or 3 × 10-5 s (UV). The UV
absorption of ylide 14 was also observed in both CH3CN and
in isooctane (Figure 3).
The ylide was further characterized by the UV spectrum, with
a strong absorption at 378 nm and broad absorption at 546 nm
in CH3CN (Figure S-10). In pentane UV absorption was
observed at 397 nm and at 582 (br) nm with fine structure
(Figure S-11).
Ylide 21 is remarkably long lived in solution and decayed
with a half-life of 2 h at 25 °C after generation in CH3CN as
monitored by UV spectroscopy. The decay gave approximately
first-order kinetics, but upon removal of the solvent at room
temperature no recognizable products could be identified.
Photolysis of 20 in pentane and cooling to -78 °C led to
precipitation of a solid, and removal of the solvent at -78 °C
gave 21 as a dark-purple solid, but efforts to obtain an X-ray
structure were unsuccessful. Analysis by UV and IR of solutions
obtained by photolysis indicated approximately 85% conversion
1
of 20 to 21, while dissolution of the solid in CDCl3 and H
NMR analysis at -30 °C showed 70% conversion.
Upon LFP 2-pyridyl 1-diazoethyl ketone (16) gave the ylide
17 (eq 6), identified by the characteristic UV spectrum in CH3-
CN and in isooctane (Figure S-16, Supporting Information) and
The 1H NMR spectrum of 21 (Figure 4 and Supporting
Information) has the signals for the pyridyl ring at δ (CDCl3)
6.18, 6.56, 6.73, and 6.87, which show substantial upfield shifts
compared to those of the precursor diazo ketone 20, δ (CDCl3)
7.42, 7.83, 7.89, and 8.59, for an average upfield shift for 21
of 1.35 ppm (17%).
the IR absorption at 1722 cm-1
.
The 13C NMR shifts for the pyridyl ring for 21 are at 109.7,
125.6, 125.7, 131.8, and 163.1, while those of the diazo ketone
20 are 122.2, 126.4, 137.2, 147.8, and 155.3 (Table 1). Thus,
the ring carbons in 21 are upfield in four cases and downfield
9
6212 J. AM. CHEM. SOC. VOL. 129, NO. 19, 2007

Azacyclobutenone Ylides with Antiaromatic Character
A R T I C L E S
Table 1. NMR Chemical Shift Assignments for 20 and 21
Table 2. GIAO-B3LYP/6-31G(d)//B3LYP/6-31G(d) Calculated
NICS Values and ∆R Values (Å)
20
δ
1Hst
21
δ
1H
δ
20
−
21a
20
δ
13C
21
δ
13C
δ 20 −
21a
NICS(0)
6-ring
NICS(0)
4-ring
NICS(1)
6-ring
NICS(1)
4-ring
∆
∆
R
R
H(21)
H(22)
H(23)
H(24)
8.59
7.42
7.83
7.89
6.87
6.56
6.73
6.18
1.72
0.86
1.10
1.71
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
147.8
126.4
137.2
122.2
155.3
189.8
52.3
125.6
125.7
131.8
109.7
163.1
173.8
127.9
22.2
0.7
5.4
12.5
-7.8
16.0
-75.6
6 ring
4 ring
benzene
pyridine
benzocyclo-
butadiene (22)
benzocyclo-
butenone
-9.64
-7.97
-1.81
-11.21
-10.99
-3.52
0.000
0.057
0.076
23.54
14.71
13.92
8.62
0.172
0.155
-0.05
-1.87
0.082
enolate (23)
2
11
a Difference in chemical shifts of 20 and 21.
1.44
4.53
1.07
14.80
15.85
16.80
13.58
-0.78
0.48
-1.21
10.27
10.08
10.07
8.59
0.109
0.141
0.104
0.205
0.214
0.196
21
in one by an average of 6.6 ppm (4.8%), in contrast to the
cyclobutenone
enolate (26)
cyclobutadienea
cyclooctatetraeneb
1
average 17% for the H shifts. The greater upfield effects on
the 1H shifts are consistent with these being due to aromaticity/
antiaromaticity effects, which are not expected to have a
21.5
13.3
5.74
2.30
0.12811
significant effect on the 13C shifts. The H and 13C chemical
1
a Reference 6b. b 8-membered ring, nonplanar.
shifts (Table 1) were assigned using gCOSY45, ROESY, HSQC,
and HMBC techniques. The carbonyl carbon (C7) showed an
upfield shift of 16.0 ppm, while the ylide carbon (C8) was
downfield by 75.6 ppm, consistent with ylide character for this
carbon. Calculated GIAO chemical shifts (Supporting Informa-
tion, Table S3) were consistently further upfield from the
8.62 for the 6- and 4-membered rings, respectively, is indicated
to be a little more aromatic (or less antiaromatic) than the ylides
2, 11, and 21.
1
experimental values by δ 0.76 ( 0.04 ppm for H and 8.1 (
1.1 ppm for 13C and showed a similar pattern to the measured
shifts.
Benzocyclobutenone enolate (23), which shares structural and
electronic properties with the ylides, is derived from the ketone
24 by proton abstraction (eq 9) and has previously been subject
to examination as a possibly antiaromatic species.7a,b On the
basis of experimental studies of 23 generated in the gas phase
combined with ab initio calculations it was concluded there was
significant double bond character in the C1-C2 bond with
electron delocalization to oxygen and into the 6-membered ring,
which tended to alleviate antiaromaticity in the 4-membered
ring.7a Kinetic studies of the enolization of 24 show this is less
reactive than 2-indanone (25) by a factor of 3.1 × 106,
corresponding to an activation energy difference of 8-9 kcal/
mol.7b Other model ketones were more reactive by factors of
1.6 × 103 to 5.3 × 105.7b The large energy difference implied
considerable destabilization of 23, for which ring strain and
antiaromaticity were proposed as the cause. Since cyclobutanone
has a pKa similar to that of acetone, it was concluded that
antiaromaticity must play a large role in the destabilization of
23.7b
1
The remarkable upfield H NMR shifts in 21 suggest a loss
of aromaticity and to elucidate this phenomenon nucleus-
independent chemical shifts (NICS)5,6 were calculated for ylides
2, 11, and 21 and for the model compounds benzene, pyridine,
non-planar cyclooctatetraene, benzocyclobutadiene (22) and for
benzocyclobutenone enolate 23. The results are given in Table
2, where NICS(0) is the calculated value at the ring center, and
NICS(1) is the calculated value 1 Å above the ring center.
Aromatic rings are characterized by negative NICS values,
whereas positive values are characteristic of antiaromatic rings.5
The NICS(1) values avoid effects due to σ electrons and are
preferred for the analysis of aromaticity, although for the
substrates considered here the trends are similar. The ylides 2,
11, and 21 have NICS(1) values of -1.21 to 0.48 for their
6-membered rings, indicating much weaker aromatic character
than that found for pyridine. The NICS(1) values of 10.07 to
10.27 for their 4-membered rings are indicative of substantial
antiaromatic character in these rings. By comparison benzo-
cyclobutadiene (22), with NICS(1) values of -3.52 for the
6-membered ring and 13.92 for the 4-membered ring, is
indicated to have modestly more aromatic character for the
6-membered ring and less for the 4-membered ring. Benzo-
cyclobutenone enolate, 23, with NICS(1) values of -1.87 to
Previously the role of aromaticity/antiaromaticity in 23 had
not been analyzed using NICS calculations, and these results
found for 23 support the presence of significant antiaromatic
effects in this enolate, especially in the 4-membered ring, with
weak aromaticity in the 6-membered ring. Similarly, cy-
(5) (a) Allen, A. D.; Tidwell, T. T. Chem. ReV. 2001, 101, 1333-1348. (b)
Wiberg, K. B. Chem. ReV. 2001, 101, 1317-1331.
(6) (a) Schleyer, P. v. R.; Maerker, C.; Dransfield, A.; Jiao, H.; Hommes, N.
J. R. v. E. J. Am. Chem. Soc. 1996, 118, 6317-6318. (b) Chen, Z.; Wannere,
C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P. v. R. Chem. ReV. 2005,
3842-3882. (c) Oddershede, J.; Larsen, S. J. Phys. Chem. A 2004, 108,
1057-1063.
(7) (a) Broadus, K. M.; Kass, S. R. J. Chem. Soc., Perkin Trans. 2 1999, 2389-
2396. (b) Nagorski, R. W.; DeAtley, A. D.; Broadus, K. M. J. Am. Chem.
Soc. 2001, 123, 8428-8429.
9
J. AM. CHEM. SOC. VOL. 129, NO. 19, 2007 6213

A R T I C L E S
Fu et al.
It may be noted that the calculated geometries of the planar
ylides resemble those of polyalkenes. Cyclooctatetraene, which
1
is nonplanar, has δ 13C of 131.5 and δ H of 5.6910 and C-C
bond lengths of 1.334 and 1.462 Å, giving ∆R of 0.128.11 We
have obtained NICS(0) and NICS(1) values for the nonplanar
structure as 5.74 and 2.30, respectively, where NICS(0) is taken
as the center of the molecule and NICS (1) is 1 Å above this
(Table 2). This calculation indicates that nonplanar cyclooc-
tatetraene is nonaromatic, as expected. However, the ylides
considered here have the geometric properties of planar non-
aromatic nonconjugated polyenes, and this may reasonably be
attributed to a significant contribution from antiaromaticity.
In summary, observable azacyclobutenone ylides 2, 11, 14,
17, 19, and 21 have been prepared in solution at ambient
temperatures and characterized by their distinctive IR and UV
spectra. The NMR spectra of the longer-lived triisopropylsilyl-
substituted species 21 and calculated NICS values and bond
alternations for 2, 11, and 21 indicate substantial antiaromatic
character for these species. Benzocyclobutenone enolate (24)
and cyclobutenone enolate (26) share some of these properties
with the ylides and with benzocyclobutadiene (23).
Figure 5. MP2/6-31G(d) calculated7a bond distances (Å) of cyclobutenone
enolate (26).
clobutenone enolate (26) has been found to be strongly
destabilized (as indicated by the energy change in eq 10
calculated at the MP2/6-31G(d) level) and a calculated geometry
(Figure 5) described as having allylic anion character with the
C2 and C4 hydrogens bent out of plane by 43°.7a We calculate
essentially the same geometry for 26 using DFT methods, and
calculate a NICS(1) value of 8.59 (Table 2) indicative of
significant antiaromatic character for 26. This may be compared
to the value of 13.3 for cyclobutadiene (Table 2).
Another criterion of antiaromaticity is bond alternation, and
the B3LYP calculated geometries of ylides 2, 11, and 21 and
model compounds are shown in Table S-3 (Supporting Informa-
tion). Values of the difference ∆R between the longest and
shortest bonds in each ring provide a measure of bond
alternation, and as shown in Table 2, these are larger in 2, 11,
and 21 than in benzocyclobutadiene (23)8 and are also larger
than in naphthalene (0.047 Å at 205 K).6c Heteroatoms in 2,
11, and 21 affect these distances, but the differences are
Experimental Section
Diazo ketones 11 and 13,4b and triazole 4 (R ) 2-pyridyl),4c,d are
known compounds prepared by the reported procedures. UV-visible
LFP experiments used a Xe-light source (150 W), spectrograph,
photomultiplier, and ICCD camera, with 248 nm KrF and 308 nm XeCl
excimer lasers, and a 266 and 355 nm Nd:YAG laser. Time-resolved
IR experiments were conducted in Japan with a dispersive-type IR
spectrometer utilizing 266 nm pulses of a Nd:YAG laser (15 Hz
repetition rate, 2 mJ per pulse) as the excitation source. A reservoir of
sample solution was continuously circulated between two calcium
fluoride plates with 2 mm path length. Computations were carried out
using Gaussian 98.12
3-Diazoacetylpyridazine (10). To 3-pyridazinylcarboxylic acid4a
(765 mg, 6.2 mmol) stirred in THF (15 mL) and cooled in an ice bath
under argon was added Et3N (0.945 mL, 6.7 mmol) and isobutyl
chloroformate (0.805 mL, 6.2 mmol). The mixture was stirred 1 h with
cooling, and then added to a cold ether solution of excess diazomethane,
left cold for 1 h, and allowed to warm to room temperature overnight.
The ether solution was washed with saturated NaCl solution and dried
over anhydrous Na2SO4; after evaporation of the solvent, the resulting
solid was chromatographed (alumina, CH2Cl2/hexanes, 1:1 v/v) to give
1
consistent with the other evidence from H NMR and NICS
calculations. The ∆R values for enolate 23 are a little greater
than for 21 for the 6-ring, and a little less for the 4-ring, but the
6-ring values are significantly larger than that for pyridine. Thus
the ∆R values indicate a significant lack of aromaticity in the
4-membered rings of both the ylides 2, 11, and 21, as well as
in benzocyclobutadiene 22 and the enolate 23, and more
aromatic character in the 6-membered rings than in the
4-membered, but less than in benzene or pyridine.
The question of aromatic and antiaromatic character in the
azacyclobutenone ylides 2, 11, 14, 17, 19, and 21 is closely
related to similar considerations regarding cyclobutadiene, and
the continuing great interest in the latter species has recently
been considered in a thoughtful essay.9 There it is noted
“Various criteria have been proposed over time to judge whether
a molecule is aromatic or not... and it was generally assumed
that several of these criteria have to be met for a molecule to
qualify for this distinction.” The magnetic criteria, includ-
ing NICS calculations, indicate antiaromaticity for cyclo-
butadiene, but energetic considerations are more problematic.9
For the ylides considered here the criteria used are the
1
10 (435 mg, 48%) as opaque crystals, mp 127-131 °C dec. H NMR
(400 MHz, CDCl3) δ 9.32 (dd, J ) 1.6, 4.9 Hz, 1H), 8.22 (dd, J )
1.6, 8.4 Hz, 1H), 7.67 (dd, J ) 4.9, 8.4 Hz, 1H), 6.96 (s, 1H). 13C
NMR (100 MHz, CDCl3) δ 184.1, 155.2, 153.7, 127.6, 124.3, 55.0. IR
(CHCl3) 2112, 1634 cm-1. UV-vis (CH3CN) λmax (ꢀ) 217 nm (9.8 ×
103), 307 nm (8.1 × 103). EI-MS m/z 148 (11, M+), 120 (60, [M -
N2]+), 92 (100), 69 (19), 65 (49). HR-EI-MS m/z calcd for C6H4N4O
148.0385, found 148.0389.
2-Pyridyl 1-Diazoethyl Ketone (16). 2-Pyridylcarboxylic acid (0.2
g, 1.62 mmol) in THF (10 mL) was cooled in an ice bath, Et3N (0.25
mL, 1.78 mmol) and isobutyl chloroformate (0.21 mL, 1.62 mmol)
were added, and the mixture stirred for 1 h. A solution of diazoethane
prepared from N-nitroso-N-ethylurea (1.7 g, 14.5 mmol) was added to
the cold solution, which was left standing for 1 h, and then allowed to
warm to room temperature overnight. Saturated NaCl solution was
added and extracted three times with ether. The combined ether layers
1
measured H chemical shifts, and the calculated bond alterna-
tions and NICS parameters, and all of these are consistent
with significant antiaromatic character. While computations
indicate that ylide 2 is strongly destabilized, any relevance of
this result to the antiaromatic character of 2 and its derivatives
is difficult to quantify and beyond the scope of this investigation.
(10) van Eikema Hommes, N. J. R.; Clark, T. J. Mol. Model. 2005, 11, 175-
(8) The calculated geometry of 22 at the ab initio MP2 level (ref 6a) gives ∆R
values of 0.146 for the 4-membered ring and 0.065 for the 6-membered
ring.
(9) Bally, T. Angew. Chem., Int. Ed. 2006, 45, 6616-6619.
185.
(11) Schro¨der, G. Cyclooctatetraene; Verlag Chemie: Weinheim, 1965.
(12) Frisch, M. J.; et al. Gaussian 98, Revision A.9; Gaussian, Inc.: Pittsburgh,
PA, 1998.
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6214 J. AM. CHEM. SOC. VOL. 129, NO. 19, 2007

Azacyclobutenone Ylides with Antiaromatic Character
A R T I C L E S
were dried over anhydrous CaSO4 and evaporated. The crude product
Ylide 21. Diazo ketone 20 (23 mg, 0.075 mmol) was dissolved in
pentane (50 mL, dried over molecular sieves 4 A), and degassed by
argon for 20 min at 5 °C. The solution was irradiated using 300 nm
light at 5 °C for 10 min, and the color of the solution changed to deep
purple. The solution was cooled with dry ice, and a dark-purple solid
precipitated. The pentane solvent was withdrawn carefully with a
syringe, and residual pentane was evaporated with a stream of argon,
giving the ylide 21 as a dark-purple solid, which was kept with dry ice
under argon. 1H NMR (500 MHz, CDCl3, -30 °C) δ 6.87 (d, J ) 4.8
Hz, 1H), 6.75-6.72 (m, 1H), 6.57-6.54 (m, 1H), 6.18 (d, J ) 6.8 Hz,
1H), 1.20-1.16 (m, 3H), 1.11 (d, J ) 7.0 Hz, 18H). 13C NMR (125
MHz, CDCl3, -30 °C) δ 173.8, 163.1, 131.8, 127.9, 125.7, 125.6, 109.7,
18.9, 11.5. IR (CDCl3) 1713 cm-1; IR (CH3CN) 1718 cm-1; IR
(pentane) 1746 cm-1. UV-vis (pentane) λmax (ꢀ) 249 nm (8.6 × 103),
397 nm (4.9 × 103), 421 nm (3.6 × 103), 582 nm (9.8 × 102); UV-
vis (CH3CN) λmax (ꢀ) 245 nm (7.9 × 103), 378 nm (3.6 × 103), 546
nm (8.2 × 102).
(0.286 g) was recrystallized twice from cold pentane to give 16 as
1
yellow needles (28 mg, 11%), mp 37-38 °C. H NMR (400 MHz,
CDCl3) δ 8.54-8.53 (m, 1H), 7.96-7.94 (m, 1H), 7.86-7.82 (m, 1H),
7.43-7.40 (m, 1H), 2.18 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 185.3,
154.3, 147.8, 137.3, 126.5, 122.4, 62.8, 11.0. IR (CH3CN) 2089, 1630
(sh), 1606 cm-1. EI-MS m/z 161 (5, M+), 133 (70, [M - N2]+), 105
(100, [M - N2 - CO]+), 79 (62). HR-EI-MS m/z calcd for C8H7N3O
161.0589, found 161.0593.
2-(Triisopropylsilyldiazoacetyl)pyridine (20). To a solution of
2-diazoacetylpyridine (1, 294 mg, 2 mmol) and i-Pr2NEt (348 µL, 2
mmol) in ether (30 mL) was added dropwise at 0 °C under N2 a solution
of triisopropylsilyl triflate (538 µL, 2 mmol) in ether (10 mL). A white
solid precipitated, leaving a light-yellow solution. The mixture was
allowed to warm to room temperature and was stirred overnight. After
filtration the solvent was evaporated to afford a yellow oil, which was
chromatographed (silica gel, CH2Cl2/hexanes, 2:1 v/v with 1% Et3N)
1
to give 20 as a pale-yellow oil (312 mg, 51%). H NMR (400 MHz,
CDCl3) δ 8.59-8.58 (m, 1H), 7.91-7.88 (m, 1H), 7.85-7.81 (m, 1H),
7.43-7.40 (m, 1H), 1.50-1.42 (m, 3H), 1.16 (d, J ) 7.5 Hz, 18H).
13C NMR (100 MHz, CDCl3) δ 189.8, 155.3, 147.8, 137.2, 126.4, 122.2,
52.3, 18.7, 11.8. IR (CDCl3) 2084, 1602 cm-1. UV-vis (pentane) λmax
(ꢀ) 225 nm (9.9 × 103), 261 nm (5.1 × 103), 313 nm (4.6 × 103);
UV-vis (CH3CN) λmax (ꢀ) 226 nm (8.9 × 103), 262 nm (5.2 × 103),
310 nm (4.7 × 103). EI-MS m/z 303 (0.7, M+), 287 (2, [M - CH3]+),
275 (69, [M - N2]+), 260 (50, [M - N2 - CH3]+), 232 (100, [M - N2
- CH(CH3)2]+), 190 (32), 146 (22), 131 (74), 115 (23), 103 (64), 87
(22), 75 (78), 61 (45). HR-EI-MS m/z calcd for C16H25N3OSi 303.1767,
found 303.1763.
Acknowledgment. Financial support by the Natural Sciences
and Engineering Research Council of Canada and the Petroleum
Research Fund administered by the American Chemical Society
is gratefully acknowledged.
Supporting Information Available: Experimental and com-
putational details, with NMR, UV, and IR spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
JA0686920
9
J. AM. CHEM. SOC. VOL. 129, NO. 19, 2007 6215