6-oxocyclohexa-2,4-dienylideneketene: A highly reactive α-oxoketene

Article abstract of DOI:10.1021/ja980060t

The title ketene 2 has been generated by laser flash photolysis and observed in solution for the first time, using time-resolved infrared spectroscopy. The kinetics of the reactivity of 2 with H₂O, MeOH, and Et₂NH with relative reactivities of 1.0, 2.0, and 73 have been measured. These are the first direct measurements of the reactivities of a ketene with these different nucleophiles under the same conditions. Ab initio molecular orbital calculations indicate the hydration occurs by in-plane attack of the H₂O molecule on the ketenyl carbonyl through a pseudopericyclic transition state with assistance by coordination to the keto-carbonyl.

Full text of DOI:10.1021/ja980060t

J. Am. Chem. Soc. 1998, 120, 6247-6251
6247
6
-Oxocyclohexa-2,4-dienylideneketene: A Highly Reactive
R-Oxoketene
†
,‡
§
,†
Regina C.-Y. Liu, Janusz Lusztyk,* Michael A. McAllister, Thomas T. Tidwell,* and
Brian D. Wagner‡
,⊥
Contribution from the Department of Chemistry, UniVersity of Toronto, Toronto,
Ontario M5S 3H6, Canada, Department of Chemistry, UniVersity of North Texas, Denton, Texas 76203,
Steacie Institute for Molecular Sciences, National Research Council, Ottawa, Ontario K1A OR6, Canada,
and Department of Chemistry, UniVersity of Prince Edward Island, Charlottetown C1A 4P3, Canada
ReceiVed January 5, 1998
Abstract: The title ketene 2 has been generated by laser flash photolysis and observed in solution for the first
time, using time-resolved infrared spectroscopy. The kinetics of the reactivity of 2 with H2O, MeOH, and
Et2NH with relative reactivities of 1.0, 2.0, and 73 have been measured. These are the first direct measurements
of the reactivities of a ketene with these different nucleophiles under the same conditions. Ab initio molecular
orbital calculations indicate the hydration occurs by in-plane attack of the H2O molecule on the ketenyl carbonyl
through a pseudopericyclic transition state with assistance by coordination to the keto-carbonyl.
R-Oxoketenes have been the subject of continuing chemical
study because of their interesting structural features, high
reactivity, and synthetic utility.1 Photolysis of 2-phenyl-3,1-
O
O
C
O
O
CO Me
2
O
hν, 77 K
PhCH
MeOH
(1)
–
O
-
1
benzoxan-4-one (1) at 77 K gave an IR band at 2118 cm
O
Ph
O
OH
attributed to the title ketene 2 (Chemical Abstracts name 2,4-
cyclohexadien-1-one, 6-carbonyl [21083-33-0]), which was
trapped by MeOH to give methyl salicylate (eq 1).2 Photolysis
of 3 was also reported to give 2, as evidenced by capture with
water, phenol, or benzoic acid,2b and matrix photolysis of the
peroxide 4 was suggested to give 2, as identified by comparison
1
2
1
O
a
2
5
O
1
7
O
3
4
O
O
O
6
2
O
2
c
3
4
5
with the UV spectrum observed from photolysis of 1 or 3.
This latter transformation was suggested to involve the inter-
mediacy of lactone 5.2c Photolysis of 4 at 8 K gave rise to IR
The 1,4-isomer 6 of 2 and its 2,6-dimethyl analogue have
been suggested to be intermediates in the base induced hy-
-
1
2d
bands at 2139 and 1650 cm assigned to 2, and an IR
3
a,b
absorption at 1904 cm was assigned to 5.2d The photoelectron
-1
drolysis of aryl 4-hydroxybenzoates 7 (eq 2).
, identified by its IR bands at 2110 and 1635 cm at 10 K in
an Ar matrix, was also reported from CO addition to the
Formation of
-
1
6
spectrum of 2 generated by pyrolysis of salicylic acid has also
been reported.2e The formation of some polycyclic analogues
3
c
3a
2f
corresponding carbene. It was also proposed by analogy to
of 2 from diazoketones has also been suggested.
3
d
calculations on the exomethylene analogue of 6 that nucleo-
philic attack on such ketenes would be favored to occur from
a perpendicular direction at the ketenyl carbonyl carbon. This
is contrary to the accepted mechanism of nucleophilic addition
†
University of Toronto.
Steacie Institute for Molecular Sciences.
University of North Texas.
University of Prince Edward Island.
‡
§
⊥
1
a
to other ketenes.
(1) (a) Tidwell, T. T. Ketenes; Wiley: New York, 1995. (b) Wentrup,
C.; Heilmayer, W.; Kollenz, G. Synthesis 1994, 1219-1248. (c) Chiang,
Y.; Guo, H.-X.; Kresge, A. J.; Tee, O. S. J. Am. Chem. Soc. 1996, 118,
OH–
–ArO–
3
6
386-3391. (d) Koch, R.; Wong, M. W.; Wentrup, C. J. Org. Chem. 1996,
1, 6809-6813. (e) Visser, P.; Zuhse, R.; Wong, M. W.; Wentrup, C. J.
–O
HO
CO2Ar
CO Ar
2
Am. Chem. Soc. 1996, 118, 12598-12602. (f) Birney, D. M.; Wagenseller,
P. E. J. Am. Chem. Soc. 1994, 116, 6262-6270. (g) Eisenberg, S. W. E.;
Kurth, M. J.; Fink, W. H. J. Org. Chem. 1995, 60, 3736-3742. (h) Nakatani,
K.; Shirai, J.; Sando, S.; Saito, I. J. Am. Chem. Soc. 1997, 119, 7626-
7
O
C
O
(2)
7635. (i) Wentrup, C.; Fulloon, B. E.; Moloney, D. W. J.; Bibas, H.; Wong,
6
M. W. Pure Appl. Chem. 1996, 68, 891-894. (j) Cartwright, G. A.; Gould,
R. O.; McNab, H. Chem. Commun. 1997, 1293-1294. (k) Birney, D. M.;
Xu, X.; Ham, S.; Huang, X. J. Org. Chem. 1997, 62, 7114-7120.
1
R-Oxoketenes are characterized by their high reactivity,
including unusual rearrangements, concerted additions involv-
1i,j
(
2) (a) Chapman, O. L.; McIntosh, C. L. J. Am. Chem. Soc. 1970, 92,
7
1
3
001-7002. (b) Horspool, W. M.; Khandelwal, G. D. Chem. Commun.
970, 257-258. (c) Dvorak, V.; Kolc, J.; Michl, J. Tetrahedron Lett. 1972,
443-3446. (d) Chapman, O. L.; McIntosh, C. L.; Pacansky, J.; Calder,
G. V.; Orr, G. J. Am. Chem. Soc. 1973, 95, 4061-4062. (e) Schulz, R.;
Schweig, A. Tetrahedron Lett. 1979, 59-62. (f) McMahon, R. J.; Chapman,
O. L.; Hayes, R. A.; Hess, T. C.; Krimmer, H. P. J. Am. Chem. Soc. 1985,
(3) (a) Thea, S.; Cevasco, G.; Guanti, G.; Kashefi-Naini, N.; Williams,
A. J. Org. Chem. 1985, 50, 1867-1872. (b) Isaacs, N. S.; Najem, T. S. J.
Chem. Soc., Perkin Trans. 2 1988, 557-562. (c) Sander, W.; Bucher, G.;
Reichel, F.; Cremer, D. J. Am. Chem. Soc. 1991, 113, 5311-5322. (d)
Kuzuya, M.; Miyake, F.; Kamiya, K.; Okuda, T. Tetrahedron Lett. 1982,
2593-2596.
1
07, 7597-7606.
S0002-7863(98)00060-2 CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/11/1998

6248 J. Am. Chem. Soc., Vol. 120, No. 25, 1998
Liu et al.
3
Figure 2. Kinetic traces obtained upon 308 nm LFP of 1 in CH CN.
-
1
(
a) Irreversible disappearance of absorption of 1 at 1751 cm . (b)
-
1
Exponential decay of absorption of 2 at 2135 cm . (c) Generation of
the benzaldehyde absorption at 1703 cm . (d) Exponential growth of
-
1
absorption of salicylic acid (10) formed from 2 and H
2
O, monitored at
-
1
1
693 cm .
Figure 1. IR spectra observed upon 308 nm LFP of 1 in CH
3
CN.
Top: Ketenyl absorption of 2 at 2135 cm . Bottom: Spectra observed
µs (filled circles) and 25 µs (open circles) after the laser pulse
indicating instantaneous bleaching of the lactone band of 1 at 1751
-1
1
-
1
cm , instantaneous formation of benzaldehyde absorption at 1703
-
1
-1
cm , and growth of absorptions in the 1680-1700 cm region
corresponding to salicylic acid.
Figure 3. Quenching plot of 2 in CH
3
CN by Et
2
NH.
1f,k
ing the R-oxo and ketenyl groups, and their ability to promote
at 2135 cm- disappeared with first-order kinetics (Figure 2),
even in CH3CN freshly distilled from CaH2, and this reaction
was shown to involve hydration by adventitious H2O by the
observation of the concurrent growth of the IR absorption of
1
1h
DNA cleavage. The latter reaction has been shown to involve
amide formation by nucleophilic attack of the NH2 group of
1h
guanine residues with a highly electrophilic R-oxoketene. The
ketene 2 also possesses a restricted geometry and upon conver-
sion to salicylic acid derivatives (eq 1) gains in aromatic
stabilization, and might be expected to have enhanced reactivity.
There have, however, been no quantitative studies on the
structure and reactivity of this interesting intermediate, and no
studies of the relative stabilities of 2 and the ring-closed form
-
1
salicylic acid at 1693 cm , and by the observation of a linear
dependence on [H2O] of the rate of reaction of 2, which
indicated a [H2O] of 0.0026 M in the distilled CH3CN. No IR
band near 1900 cm attributable to the lactone 5 was observed.
The observed rate constants for first-order disappearance of 2
in CH3CN gave linear plots as a function of the concentration
of H2O, MeOH, and Et2NH (Figure 3), and these gave rate
-1
5
2
. Reported herein are the first generation and observation of
in solution at room temperature, quantitative reactivity studies
7
7
constants at 22 °C, k2 ) (1.5 ( 0.3) × 10 , (3.0 ( 0.6) × 10 ,
of this species with nucleophiles, and theoretical studies of its
structure and reaction mechanism with H2O.
9
-1 -1
and (1.1 ( 0.2) × 10 M s , respectively.
The reaction of photogenerated 2 with MeOH has been shown
2a
Results
to give methyl salicyclate (eq 1) and the formation of salicylic
acid upon hydration has also been reported previously.2b The
assignment of the salicylic acid IR absorption in CH3CN was
confirmed by measurement of an absorption of an authentic
sample at 1684 cm . The reaction of R-oxoketenes with
amines to form amides is also well-known.
products from 2 have been found by us or others, and the
observed first-order kinetics for the disappearance of 2 in the
presence of nucleophiles show that dimerization is not significant
under these conditions.
4
Laser flash photolysis of 1 in CH3CN with 308 nm light
5
and time-resolved infrared detection (TRIR) resulted in the
-
1
irreversible disappearance of the IR band of 1 at 1751 cm
and the instantaneous formation of new IR bands at 2135 and
-
1
1
b,k
-
1
No dimeric
1
703 cm , identified as due to the ketene absorption of 2, and
2
the benzaldehyde carbonyl, respectively (Figure 1). The band
(
4) (a) Mowry, D. T. J. Am. Chem. Soc. 1947, 69, 2362-2363. (b)
Freeman, F.; Karchefski, F. M. J. Chem. Eng. Data 1977, 22, 355-357.
c) Perlmutter, P.; Buniani, E. Tetrahedron Lett. 1996, 37, 3755-3756.
5) (a) Allen, A. D.; Colomvakos, J. D.; Diederich, F.; Egle, I.; Hao, X.;
(
(
Ab initio molecular orbital calculations of the structure and
energy of 2 and 5, an initial complex 8 of 2 with H2O, a
transition state 9 for hydration of 2, and two conformations 10a
and 10b of salicylic acid (the reaction product of 2) were carried
Liu, R.; Lusztyk, J.; McAllister, M. A.; Rubin, Y.; Sung, K.; Wagner, B.
D. J. Am. Chem. Soc. 1997, 119, 12125-12130. (b) Wagner, B. D.; Arnold,
B. R.; Brown, G. S.; Lusztyk, J. J. Am. Chem. Soc. 1998, 120, 1827-
1
834.

6-Oxocyclohexa-2,4-dienylideneketene
J. Am. Chem. Soc., Vol. 120, No. 25, 1998 6249
Table 1. Calculated Energies and ZPVE (hartrees) and Relative Energies (kcal/mol) of Ketene 2 and Related Structures
levela
2
5
8
9
10a
10b
12
HF/6-31G*
-417.0923
0.0882
0.0
-417.0864
0.0895
-493.1112
0.1108
-493.1004
0.1127
-493.1835
0.1169
-493.1892
0.1172
-493.1520
b
ZPVE (×0.9)
0.1150
c
d
-3.9e
4.0e
-45.5e
-48.9e
e
E
rel
4.5 (5.8)
-26.9
MP2//HF
-418.3060
-418.3098
-494.5121
-494.5191
-494.5872
-494.5924
-494.5581
c
d
-5.2e
-8.4e
-48.5e
-51.5e
e
E
rel
0.0
-1.6 (-0.3)
-418.3213
-31.4
MP3//HF
-418.3184
0.0
c
d
E
rel
-1.0 (0.3)
MP4 (SDQ)//HF
-418.3402
-418.3384
c
d
E
rel
0.0
1.9 (3.2)
QCISD//HF
-418.3414
0.0
-418.3408
c
d
E
rel
1.2 (2.5)
MP2//MP2
-418.3137
-418.3153
-494.5213
-6.8e
-494.5931
-46.5e
-494.5983
-49.8e
c
d
E
rel
0.0
-0.2 (1.1)
MP3//MP2
-418.3202
0.0
-418.3236
c
d
E
rel
-1.3 (0.0)
MP4(SDQ)//MP2
-418.3453
-418.3427
c
d
E
rel
0.0
2.4 (3.7)
QCISD//MP2
-418.3463
0.0
-418.3452
c
d
E
rel
1.5 (2.8)
a
HF/6-31G*//HF/6-31G*; MP2/6-31G*//HF/6-31G*; MP3/6-31G*//HF/6-31G*; MP4(SDQ)/6-31G*//HF/6-31G*; QCISD/6-31G*//HF/6-31G*;
b
MP2/6-31G*//MP2/6-31G*; MP3/6-31G*//MP2/6-31G*; MP4(SDQ)/6-31G*//MP2/6-31G*; QCISD/6-31G*//MP2/6-31G*. Zero-point vibrational
energy for HF/6-31G* geometry. Including 0.9 ZPVE (HF/6-31G*). d Including the T∆S term at 300 K. e Energy relative to 2 + H
c
2
2
O. H O
energy (ZPVE): HF//HF -76.0107 (ZPVE 0.0207); MP2//HF -76.1960; MP2//MP2 -76.1968.
6
stretch), 1813.6 (198.1, CdC stretch), and 2163.4 cm-1 (733.3,
out with Gaussian 92 and 94, and the resulting energies and
geometries obtained are given in Tables 1 and 2 respectively,
in the Supporting Information. The geometries for 2, 5, 8, and
CdO stretch). Application of the scaling factors of 0.895 and
0.85 used above to the calculated CdO band leads to values of
-
1
2d
1
0 were optimized at both the HF/6-31G* and MP2/6-31G*
1936 and 1839 cm , on either side of the assigned value of
-
1
levels, but the transition structure 9 could only be located at
HF/6-31G*, and at the MP2 level no barrier for conversion of
the complex 8 to form 10a could be located. The structure and
energy of the enol 12 from H2O addition to the ketenyl group
was also calculated at the HF/6-31G*//HF/6-31G* and MP2/
1904 cm . No bands attributable to 5 were detected in our
study.
Discussion
The formation of 2 and its hydration to salicylic acid (10)
6
(
2
-31G*//HF/6-31G* levels. The relative energies reported
Table 1) include scaled HF/6-31G* ZPVE corrections, and for
and 5, energies including T∆S terms at 300 K were also
obtained.
The HF/6-31G* calculated frequency of the ketene band in
are clearly demonstrated by the instantaneous appearance of the
-
1
ketene band of 2 at 2135 cm and of benzaldehyde at 1703
-
1
cm upon photolysis, and the subsequent first-order disap-
pearance of 2 as a function of H2O concentration, with formation
-
1
of the IR band of 10 at 1693 cm .
-
1
2
is 2383.4 cm , which when scaled by the factor of 0.895
The kinetics of the hydration of 2 are first order in [H2O],
and infrared studies of H2O in CH3CN indicate there is extensive
hydrogen bonding between the two, and that at mole fractions
6c
used in a previous study of ketene IR gives a predicted value
of 2133.1 cm , in good agreement with our observed value of
2
for benzaldehyde requires a scaling factor of 0.85 to reproduce
the experimental value of 1703 cm . For the ketone band of
2
-
1
-
1
-1
7a
135 cm in CH3CN. Our calculated value of 2003.4 cm
below 0.04 no water dimers are present. Calculations at the
7
b
HF/6-31G**//HF/6-31G** level indicate that the favored
geometry of the CH3CN- - -H2O complex involves a linear
arrangement of the CtN- - -H-O bonds, with an interaction
energy of 4.3 kcal/mol, so the oxygen is free for nucleophilic
attack.
-
1
-1
we calculate a value of 1919.6 cm , with an intensity relative
to the ketenyl band of 0.25, and scaling of the frequency by
-1
0.85 leads to a predicted value of 1631.7 cm . In our measured
spectrum (Figure 1) there is no observable band below 1650
The calculations favor a reaction pathway for hydration of 2
analogous to that proposed for formylketene in which there is
-
1
-1
1f
cm . However, 1630 cm is the spectral limit of the diode
used in the experiment and the region below this frequency has
initial formation of a hydrogen bonded complex 8. At the HF/
6-31G*//HF/6-31G* level there is a barrier of 7.9 kcal/mol for
conversion of 8 to a cyclic transition structure 9, which then
forms salicylic acid (10) (Scheme 1). The initial hydrogen
bonded complex 8 is calculated to be 6.8 kcal/mol below the
reactants at the MP2//MP2 level. However, at the MP2/6-31G*/
-
1
not been explored. Absorption at 1650 cm in Ar at 8 K was
2
d
-1
attributed to the ketone band of 2, while a band at 1635 cm
was observed for the isomer 6.3c For the lactone 5 the
significant HF/6-31G* calculated infrared frequencies with
intensities and assignments (parentheses) are 1620.8 (70.8, C-H
/
8
HF/6-31G* level the energy of 9 is less than that of the complex
by 3.2 kcal/mol and there is no barrier to the formation of
(6) (a) Gaussian 92; Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.;
Gill, P. M. W.; Wong, M. W.; Foresman, E. S.; Gomperts, G.; Andres, J.
L.; Raghavachari, K.; Binkley, J. S.; Gonzales, C.; Martin, R. L.; Fox, D.
J.; Defrees, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A.; Gaussian, Inc.:
Pittsburgh, PA, 1992. (b) Gaussian 94; ReVision B3. Frisch, M. J.; Trucks,
G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.;
Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.;
Raghavachari, K.; Al-Laham, M. A.; Zukrzewski, V. G.; Ortiz, J. V.;
Foresman, J. B.; 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.; Gonzales,
C.; Pople, J. A.; Gaussian Inc.: Pittsburgh, PA, 1995. (c) McAllister, M.
A.; Tidwell, T. T. Can. J. Chem. 1994, 72, 882-887.
10a. At the MP2/6-31G*//MP2/6-31G* level no transition
structure corresponding to 9 could be located, and there is no
barrier for conversion of the complex 8 to 10a, which is more
stable than 8 by 39.7 kcal/mol. Studies of other pseudoperi-
cyclic reactions such as the hydration of formylketene1 show
similar results.
f
(7) (a) Jamroz, D.; Stangret, J.; Lindgren, J. J. Am. Chem. Soc. 1993,
115, 6165-6168. (b) Damewood, J. R.; Kumpf, R. A. J. Phys. Chem. 1987,
91, 3449-3452.

6250 J. Am. Chem. Soc., Vol. 120, No. 25, 1998
Liu et al.
Scheme 1
The rate constant for the hydration of acetylketene (18) in
1
c
6
-1
H2O has been determined as 1.5 × 10 s , and the
extrapolated rate constant for 2 in 55.5 M H2O based on the
7
-1 -1
rate constant determined here of 1.5 × 10 M
s
is 8.3 ×
8
-1
1
0 s , or 550 times greater than that for 18. Normally the
hydration reactivity of ketenes in H2O/CH3CN is strongly
accelerated as the fraction of H2O in the medium increases, and
this has been attributed to the increase in solvent polarity with
8
a-c
increasing water content
in addition to the mass law effect.
Thus the reactivity of 2 in pure H2O may be even closer to the
diffusion controlled limit. However, for the ketene t-BuC(CO2-
Et)dCdO the increase in rate with increasing [H2O] in H2O/
CH3CN mixtures was strongly attenuated, and this may arise
from a less polar transition state when addition to an acylketene
8
a
by a cyclic transition state is involved. However, 2 is still
indicated to be much more reactive than 18, as expected for
the gain in aromatic stabilization.
O
C
O
1
8
The atoms C7, C1, C6, O2, H2, and O3 involved in the six-
membered ring for H2O transfer in 9 at the HF/6-31G* level
are in an almost coplanar arrangement, with dihedral angles
Solvent isotope effects have been taken to indicate that
hydration of many ketenes occurs with the participation of a
second molecule of H2O acting as a general base.
study of the hydrolysis of bis(2,4-dinitrophenyl) carbonate with
a wide range of H2O/CH3CN mixtures has been interpreted
(Table 2, Supporting Information) H2O3C7C1 of 10.8°, O3C7C1C6
8a-c
A recent
of -9.3°, C7C1C6O2 of 1.5°, O3H2O2C6 of -0.4°, and C7O3H2O2
of -7.9°. This type of process involving in plane attack
orthogonal to the π-system has been described by Birney and
8d
similarly, and in this latter study a close parallel between the
1
f
Wagenseller as a pseudopericyclic reaction, and does not
contain a cyclic array of overlapping orbitals.
effect of the mole fraction of H2O in CH3CN on the reaction
rate and upon the solvent polarity parameter ET was noted, and
interpreted as showing similar solute-solvent interaction mech-
anisms in the two systems, namely H-bonding and dipolar
interactions.
An alternative pathway for hydration of 2 is by attack of H2O
on the ketene from the side opposite the keto group, leading to
a zwitterionic transition structure 11 and then to salicylic acid
enol 12 (Scheme 1). An analogus pathway has been proposed
for the hydration of 6-methylenecyclohexa-2,4-dienylideneketene
13), and the in-plane attack opposite the methylene group was
confirmed by the rate depressing effect of substituents ortho to
the ketenyl group. However, the calculations of the interaction
of 2 with H2O give 8 as the only detectable complex, and as
discussed above this leads to 10a with no barrier at the MP2/
There is no enthalpic barrier to the addition of H2O to 2, and
the transition state 9 is 8.4 kcal/mol below the reactants (2 +
H2O) at the MP2/6-31G*//HF/6-31G* level. However, experi-
mentally the unfavorable entropy of attaining this rather ordered
transition state evidently gives rise to the experimentally
observed free energy barrier. These results are rather similar
to those1 found at the MP2/6-31G*+ZPE//MP2/6-31G* level
for the addition of H2O to formylketene (14), in which there is
initial formation of a hydrogen bonded complex 15 5.9 kcal/
mol below the reactants, and this complex has a barrier of 3.1
kcal/mol for forming the transition state 16, which is 2.8 kcal/
mol below the reactants, and then forms the product 17, which
is 26.0 kcal/mol below the reactants (eq 3). At the highest level
of theory utilized (MP4(SDQ)/6-31G*+ZPE//MP2/6-31G*) the
complex 15 was 5.4 kcal/mol below the reactants, and this
complex had a barrier of 6.4 kcal/mol to form 16, and 17 is
f
(
9
6
-31G*//MP2-6-31G* level. An optimized geometry for 12
was found at the HF/6-31G* level, but the HF/6-31G* and MP2/
-31G* energies for this structure are 18.6 and 17.1 kcal/mol,
6
2
6.3 kcal/mol below the reactants.
respectively, above those for 10a. No transition structure 11
was located for H2O attack on the ketene opposite the keto
carbonyl, and this pathway suffers not only from the lack of
aromaticity in 12, but also from oxygen-oxygen repulsion and
a lack of H-bonding in 11.
O
O
O
O
H
C
O
C
H2O
OH
H
OH
H
OH
(3)
O
O
O
3
a
It has also been proposed that nucleophilic addition to the
14
15
16
17
ketene 6 would occur perpendicular to the ketene plane, largely
3
d
There is no barrier for H2O addition to 2 at the MP2/6-31G*/
MP2/6-31G* level and salicylic acid 10a is 46.5 kcal/mol more
on the basis of MO calculations that the LUMO lies in this
perpendicular plane. However, these calculations used the low-
level CNDO/2 method for the analogous structure 13, and do
not appear to be reliable for analyzing the hydration mechanisms
of either 2 or 6.
/
stable than the reactants, as compared to 2.8 and 26.3 kcal/mol
greater stabilities for the structures 16 and 17 relative to 14 +
H2O. The greater propensity for hydration of 2 compared to
14, by 20.2 kcal/mol relative to the products, is expected because
(8) (a) Allen, A. D.; McAllister, M. A.; Tidwell, T. T. Tetrahedron Lett.
of the gain in aromatic stabilization. The hydration of 2 to the
enediol 12 is calculated to be exothermic by 31.4 kcal/mol at
the MP2/6-31G*//HF/6-31G* level, but this is 17.1 kcal/mol
less than for direct formation of 10a, and this difference again
reflects the aromatic stabilization of 10a.
1
993, 34, 1095-1098. (b) Allen, A. D.; Tidwell, T. T. J. Am. Chem. Soc.
1987, 109, 2774-2780. (c) Tidwell, T. T. Acc. Chem. Res. 1990, 23, 273-
2
1
79. (d) El Seoud, O. A.; El Seoud, M. I.; Farah, J. P. S. J. Org. Chem.
997, 62, 5928-5933.
(9) Schiess, P.; Eberle, M.; Huys-Francotte, M.; Wirz, J. Tetrahedron
Lett. 1984, 25, 2201-2204.

6-Oxocyclohexa-2,4-dienylideneketene
J. Am. Chem. Soc., Vol. 120, No. 25, 1998 6251
The rate constant for addition of MeOH exceeds that for
competition experiment the relative reactivity of acetylketene
(18) with n-PrNH2 and n-BuOH was found to be 2.3/1.1k Thus
the amine was more reactive, as predicted by the calculation,
but a possible cause of the rather low selectivity was proposed
to be hydrogen bonding of the amine which diminished its
hydration by a factor of 2. There is also infrared data supporting
1
0a
the formation of H bonds between MeOH and CH3CN, and
the calculated structure of this complex1 resembles that for
H2O and CH3CN. Thus the MeOH pathway for reaction with
0b
1
k
‡
2
evidently is similar to that for H2O.
reactivity. For the addition of NH3 to 14 a ∆G value of 7.3
Studies of amines in CH3CN by H NMR and IR11b indicate
1
11a
kcal/mol may be derived, indicating that the unfavorable
1k
that hydrogen bonding interactions of the type R2NH- - -NCCH3
are present. In this complex the amine lone electron pair is
available for nucleophilic attack, and by analogy to the H2O
reaction (Scheme 1) a transition state 19 for addition of Et2NH
to 2 leading directly to the salicylamide appears reasonable. This
resembles the calculated transition state 20 recently reported
by Birney et al.1k for the addition of NH3 to formylketene (14).
entropy leads to a net free energy barrier for the reaction.
Calculations at different levels of theory (Table 1) consistently
predict that the acylketene 2 and the ring-closed benzodioxetane
5 are close in energy, but particularly when the free energy is
considered 2 is more stable by 2.8 kcal/mol at the highest level
considered. This difference is much less than the 17.5 kcal/
mol higher energy calculated for the ring-closed structure 21
1
4
from formylketene, and the greater relative stability of 5 may
O
be attributed to aromatic stabilization, partially offset by ring
strain. Previously the photochemical interconversion of 2 and
O
CO2NEt2
NEt2
H
NH2
H
2d
5
in an argon matrix at 8 K was reported, with identification
-1
of 5 by its IR band at 1904 cm . At low temperatures the
entropy factor disfavoring 5 is reduced, and this species may
also be stabilized by the matrix. Thus the calculations offer
confirmatory evidence for the possible observation of 5.
O
OH
O
20
19
The rate constant for addition of Et2NH to 2 exceeds those
for H2O and MeOH by factors of 73 and 37, respectively, and
O
12a
for comparison competition studies of the reactivity of a
dienylketene in CH3CN lead to a rate ratio k(n-BuNH2)/k(n-
BuOH) of 50, while measured rates1 for Ph2CdCdO in H2O
O
2b
2
1
4
give the rate ratio k(n-BuNH2)/k(H2O) ) 7.1 × 10 . For
In summary the reactive acylketene 2 has been observed in
solution for the first time, and calculations and kinetic studies
favor a pseudopericyclic process for the hydration of this species.
By analogy the reactions with CH3OH and Et2NH follow similar
pathways. The calculated energy of the lactone 5 is close to
that of 2, and provides supporting evidence for the reported2d
matrix observation of 5.
reactions of both H2O and n-BuNH2 in H2O the solvent is
expected to be strongly hydrogen bonded to the nucleophile,
and it has been suggested that a desolvation step may affect
1
3
9
-1
the observed reactivities. The rate constant of 1.1 × 10 M
-1
s
for the reaction of Et2NH with 2 is near the diffusion
controlled limit, and the rather small difference between Et2-
NH, MeOH, and H2O rate constants for reaction with 2 may be
attributed to low selectivity because of the high reactivity of
this substrate.
Experimental Section
The precursor 1 was prepared as described.4 Laser flash photolyses
of 1 with time-resolved IR detection were carried out with a 308 nm
Lumonics Excimer-500 laser and a Mutek MPS-1000 diode laser as a
monitoring source. Details of this system and the methods for kinetic
There is calculated to be no enthalpic barrier for addition of
1k
NH3 to formylketene (14) to give 20, whereas there is a 6.3
kcal/mol calculated enthalpic barrier for conversion to the H2O/
4 complex 15 to 17 (eq 3).1f This calculated greater reactivity
5b
1
measurements are provided elsewhere.
of 14 with NH3 relative to H2O is consistent with the observed
greater reactivity of 2 with Et2NH compared to H2O. In a
Acknowledgment. Financial support by the Natural Sciences
and Engineering Research Council of Canada is gratefully
acknowledged. Issued as NRC Publication No. 40872.
(10) (a) Farwaneh, S. S.; Yarwood, J.; Cabaco, I.; Besnard, M. J. Mol.
Liq. 1993, 56, 317-332. (b) Sathyan, N.; Santhanam, V.; Sobhanadri, J. J.
Mol. Struct. 1995, 333, 179-189.
Supporting Information Available: Quenching data and
rate versus concentration plots for 2 and calculated energy and
geometry for 2 and related structures (7 pages, print/PDF). See
any current masthead page for ordering information and Web
access instructions.
(11) (a) Peshekhodov, P. B.; Petrov, A. N.; Al’per, G. A. Russ. J. Gen.
Chem. 1993, 63, 854-856. (b) Sinsheimer, J. E.; Keuhnelian, A. M. Anal.
Chem. 1974, 46, 89-93. (c) Oh, H. K.; Woo, S. Y.; Shin, C. H.; Park, Y.
S.; Lee, I. J. Org. Chem. 1997, 62, 5780-5784.
(
12) (a) Barton, D. H. R.; Chung, S. K.; Kwon, T. W. Tetrahedron Lett.
1
1
3
996, 37, 3631-3634. (b) Andraos, J.; Kresge, A. J. J. Am. Chem. Soc.
992, 114, 5643-5646. (c) Quinkert, G. Pure Appl. Chem. 1973, 33, 285-
JA980060T
16.
(
13) McClelland, R. A.; Kanagasabapathy, V. M.; Banait, N. S.; Steenken,
(14) Nguyen, M. T.; Ha, T.; More O’Ferrall, R. A. J. Org. Chem. 1990,
55, 3251-3256.
S. J. Am. Chem. Soc. 1992, 114, 1816-1823.