Facile 1,3-shift of chlorine in a chlorocarbonylketene

Article abstract of DOI:10.1021/ja973242g

Chlorocarbonyl(phenyl)ketene (2) undergoes a degenerate 1,3-shift of chlorine, as determined by ¹³C NMR spectroscopy. The two carbonyl signals (183 and 157 ppm) coalesce at -30°C, and from this as well as line-shape analysis, an activation barr

Full text of DOI:10.1021/ja973242g

J. Am. Chem. Soc. 1998, 120, 1701-1704
1701
Facile 1,3-Shift of Chlorine in a Chlorocarbonylketene
Justin Finnerty,1a John Andraos,1a Yohsuke Yamamoto,1b Ming Wah Wong, and
1c
Curt Wentrup*
Contribution from the Department of Chemistry, The UniVersity of Queensland,
Brisbane, Qld 4072, Australia
ReceiVed September 15, 1997
Abstract: Chlorocarbonyl(phenyl)ketene (2) undergoes a degenerate 1,3-shift of chlorine, as determined by
1
3
C NMR spectroscopy. The two carbonyl signals (183 and 157 ppm) coalesce at -30 °C, and from this as
‡
well as line-shape analysis, an activation barrier for the 1,3-Cl shift interconverting 2a and 2a′ of ∆G ) 41.8
-
1
-1
(
4 kJ mol (10.0 ( 1 kcal mol ) is calculated. These data are in excellent agreement with calculated
G2(MP2,SVP) and B3-LYP/6311+G(3df,2p)//6-31G* + ZPVE) 1,3-Cl shift and rotational barriers. Analogous
,3-halogen shifts in acyl isocyanates are predicted.
(
1
Introduction
barrier for the 1,3-shift, but rather by the barriers required to
generate the ketenes or ketenimines from the requisite precur-
sors. Taking advantage of the fact that chlorocarbonyl(phenyl)-
In previous work from this laboratory it was demonstrated
that R-oxo ketenes 1a can undergo degenerate 1,3-shifts of the
substituents R under the conditions of flash vacuum thermolysis
7
ketene (2) is an isolable compound, we have carried out a
1
3
variable-temperature C NMR investigation of this compound
and report direct experimental proof of the rapid 1,3-chlorine
(
FVT).2
-
1
shift, taking place at -30 °C with a barrier of ∼42 kJ mol .
Results and Discussion
The 13C NMR spectrum of 2 features a single, slightly
broadened signal in the carbonyl region at 171 ppm at room
temperature (Figure 1a). At higher temperatures, this signal
sharpens, but otherwise the spectrum remains unchanged up to
∼
150 °C, when the compound starts decomposing (in nitroben-
Experiments on the related imidoylketene-R-oxo ketenimine
3
4
zene-d5 solution). In contrast, cooling the solution (in CD2-
Cl2) causes the carbonyl signal to all but vanish near -30 °C,
with formation of a very broad, flat-topped coalescence peak
spanning a range of some 40 ppm (Figure 1b). At -40 °C
there is already decoalescence, and at -60 °C two well-defined
carbonyl signals appear at 183 and 157 ppm (Figure 1c). From
the coalescence temperature (Tc) of -30 °C, a free energy of
(
1b/1′b) and vinylketene-acylallene (1c/1′c) rearrangements
5
as well as ab initio calculations on the R-oxo ketenes revealed
an accelerating effect of electron-rich substituents, e.g., MeO,
MeS, and Me2N, which was ascribed to a favorable interaction
between a lone pair located on the substituent and the low-
lying ketene LUMO, which has a large coefficient at the central
ketene carbon atom. The same effect was also predicted for
‡
8
-
1 6
activation ∆G ) 41.6 kJ/mol is calculated. Line-shape
chlorine, with a calculated 1,3-shift barrier of 53 kJ mol . In
other words, we should expect such 1,3-shifts of chlorine (and
other lone-pair donors such as NMe2) to take place at temper-
atures well below ambient. However, all our previous experi-
ments have been performed in the gas phase under FVT
conditions, and the lowest temperature where such observations
8
a,b
‡
analysis of these signals from +35 to -60 °C afforded ∆G
41.8 kJ/mol and Tc ) -29 °C.
)
3
,4
were practicable were ∼200 °C (for MeO and MeS groups).
This temperature was not necessarily limited by the activation
(
1) (a) University of Queensland Postdoctoral Fellow. (b) Department
of Chemistry, Faculty of Science, Hiroshima University, Higashi-Hiroshima
39, Japan. (c) ARC Australian Research Fellow. Present address: Depart-
ment of Chemistry, National University of Singapore, Kent Ridge, Singapore
The aromatic carbon signals of 2 appeared at 124.5 (ipso-
C), 128.8, 128.7, and 129.7 ppm (0 °C). This region of the
7
1
3
C NMR spectrum remained unchanged at -60 °C. The
1
3
1
8
19260.
terminal ketene C signal remained constant and sharp at 63.5
(2) Wentrup, C.; Netsch, K.-P. Angew, Chem., Int. Ed. Engl. 1984, 23,
00-802.
(
(7) (a) Nakanishi, S.; Butler, K. Org. Prep. Proced. Int. 1975, 7, 155-
158. (b) Butler, K. US Pat. No. 3,557,090, Pfizer Inc., 1971. (c) Butler, K.
US Pat. No. 3,697,506, Pfizer Inc., 1972. (d) Butler, K. US Pat. No. 3,-
790,620, Pfizer & Co., Inc., 1974.
3) (a) Kappe, C. O.; Kollenz, G.; Leung-Toung, R.; Wentrup, C. J.
Chem. Soc., Chem. Commun. 1992, 487. (b) Fulloon, B. E.; Wentrup, C. J.
Org. Chem. 1996, 61, 1363-1368.
(
48.
4) Bibas, H.; Wong, M. W.; Wentrup, C. Chem. Eur. J. 1997, 3, 237-
(8) Line-shape analysis and kinetic treatment: (a) Bovey, F. A. Nuclear
Magnetic Resonance Spectroscopy, 2nd Ed.; Academic: New York, 1988,
Chapter 5; in particular p 297-302. (b). G u¨ nther, H. NMR Spectroscopy;
Wiley: New York, 1995; Chapter 9, pp 336-389 f. (c) The estimated error
2
(
5) Wong, M. W.; Wentrup, C. J. Org. Chem. 1994, 59, 5279-5285.
(6) Koch, R.; Wong, M. W.; Wentrup, C. J. Org. Chem. 1996, 61, 6809-
-
1
6
813.
in our NMR-derived energy (enthalpy) values is (4 kJ mol
.
S0002-7863(97)03242-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/14/1998

1702 J. Am. Chem. Soc., Vol. 120, No. 8, 1998
Andraos et al.
Table 1. Calculated Relative Energies (kJ mol-1) of R-Oxo
Ketenes (RC(dO)CHdCdO)a
R
s-trans
s-cis
1,3-shift barrier
CH
H
SiH
OH
OCH
F
NH
3
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.3
1.8
206.0
142.7
119.7
114.5
97.9
88.8
69.6
58.9
52.7
51.5
47.2
38.9
33.9
3
0.0
1.8
3
1.8
2.2
2
-5.2
-1.3
4.2
PH
Cl
2
SH
-2.7
-3.3
4.2
SCH
Br
N(CH
3
3
)
2
-7.2
Figure 1. 13C NMR spectra of the carbonyl group region of 2 at various
temperatures: (a) 25 °C, (b) -30 °C; the divisions are 5 ppm; and (c)
a
G2(MP2,SVP) values.
-
60 °C.
expected from the relative nucleophilicities. In addition, the
four-membered ring transition structures containing Cl or Br
may be significantly less strained as judged from the larger
calculated CCC bond angles (105, 111, and 114° for R ) F,
Cl, and Br, respectively).
Since it is not feasible to perform G2(MP2,SVP) calculations
for chlorocarbonyl(phenyl)ketenes (2a and 2b), we have ex-
amined the energies of this system, using a density functional
method, at the B3-LYP/6-311+G(3df,2p)//B3-LYP/6-31G* +
ZPVE level. At this level of theory, the calculated 1,3-H shift
barrier of the parent formylketene (1a, R ) H) is 139 kJ mol ,
in good agreement with the G2(MP2,SVP) value (143 kJ mol ).
The rotational barriers are virtually identical at the B3-LYP and
G2(MP2) levels. This lends confidence to the B3-LYP barriers
ppm throughout the temperature range. The downfield shift
observed for all 13C NMR signals at lower temperatures is a
common occurrence.
‡
Values for ∆G (cis-trans) will be the subject of future studies
-
1
of other ketenes. However, a rotational barrier of ∼48 kJ mol
(
see below) is perfectly reasonable and comparable with those
in diazocarbonyl compounds (38-75 kJ mol ). The fact that
-1 9
1
3
14
the C NMR peaks for the aryl carbons and for the terminal
ketene carbon atom remained unsplit means either that the
chemical shift differences between these signals in 2a and 2b
are too small to cause resolution at the temperatures reached
here or that only 2a is present, in agreement with the calculations
below.
-
1
-1
-
1
for 2. The calculated 1,3-Cl shift barrier in 2 is 36 kJ mol ,
and for chloroformylketene (1a, R ) Cl) it is 53 kJ mol . The
lowering by 17 kJ mol may be understood in terms of extra
-
1
Theory
-1
stabilization of the planar transition structure (Figure 2) due to
conjugation with the phenyl ring. Both the s-trans (2a) and
the s-cis conformation (2b) are significantly distorted from
planarity. The dihedral angles between the phenyl group and
the ketene moiety are 37° and 67° in 2a and 2b, respectively
We have previously reported the effect of substituents on the
1
(
,3-migration in R-oxo ketenes at the QCISD(T)/6-311+G-
5
2d,p)//MP2/6-31G* + ZPVE level. Here, we have calculated
the 1,3-shift barriers at a higher level of theory, G2(MP2,-
1
0,11
SVP),
and extended the list of substituents (Table 1). It is
(
Figure 2). The s-cis rotamer (2b) is computed to be 10 kJ
seen that substituents with unshared pairs of electrons are good
migrators, which is understood in terms of the favorable
interaction between the lone pair of the migrating atom and the
-
1
mol less stable than the s-trans form. The calculated cis-
trans isomerization barrier is 48 kJ mol , and the barrier for
-
1
4
,5
rotation of the phenyl group in the s-trans form (2a), via a planar
vacant central carbon p orbital of the ketene LUMO. Thus,
the dimethylamino group is predicted to be the best migrator
-
1
transition structure, is ∼3 kJ mol . The reason for this
exceptionally low barrier may be found in the stabilization of
the planar transition structure due to conjugation.
-
1
with a 1,3-shift barrier of only 34 kJ mol . Indirect evidence
for rapid 1,3-shifts of the dimethylamino group in dimethyla-
In previous computational work we found that different
substituents may stabilize either the s-cis or the s-trans form of
acylketenes, acylketenimines, imidoylketenes, and vinylketenes
midoketenimines (1′b, R ) NMe2) below room temperature will
be published elsewhere.12 1,3-Migrations involving PH2, SH,
SCH3, Cl, and Br can also be expected to be very facile
processes, of which only the SMe shift has been observed so
-
1
4-6
within a ∼10 kJ mol range.
3
The calculated ∆E, ∆H, ∆S, and ∆G values for chlorocar-
bonyl(phenyl)ketene are summarized in Table 2. The entropy
values and temperature corrections (HT - H0) were derived from
B3LYP/6-31G* frequency calculations. The enthalpies of
reaction (∆H) were obtained by adding the thermal correction
to ∆E, and the final ∆G values were computed from the equation
far, and then only in the gas phase above 200 °C. The
calculated barriers for the 1,3-halogen shifts follow the order
(9) Kaplan, F., Meloy, G. K. J. Am. Chem. Soc. 1966, 88, 950. Kessler,
H. Angew. Chem., Int. Ed. Engl. 1970, 9, 219-235. Regitz, M.; Maas, G.
Diazo Compounds; Academic Press: New York, 1986. Lauer, W.; Krause,
V.; Wengenroth, H.; Meier, H. Chem. Ber. 1988, 121, 465-469. Nikolaev,
V. A.; Popik, V. V.; Korobitsyna, I. K. Russ. J. Org. Chem. 1990, 27, 437-
∆
G ) ∆H - T∆S. Since the B3-LYP barrier is slightly
4
50; Zh. Org. Khim. 1990, 27, 505-521.
10) Curtiss, L. A.; Redfern, P. C.; Smith, B. J.; Radom, L. J. Chem.
Phys. 1996, 104, 5148.
11) Caculations were performed using the GAUSSIAN G92/DFT
-
1
underestimated by ∼4 kJ mol (vide supra), our best estimate
(
‡
-1
of ∆G for the 1,3-Cl shift is 39 kJ mol , in excellent agreement
-
1
(
with the experimental value (42 kJ mol ).
programs: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Wong, M. W.; Foresman, J. B.; Robb, M. A.; Head-Gordon,
M.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley,
J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; DeFrees, D. J.; Baker, J.;
Stewart, J. J. P.; Pople, J. A. GAUSSIAN 92/DFT; Gaussian Inc.: Pittsburgh,
PA, 1992.
(13) For the interconversion of (alkylthio)vinylketenes and -acylallenes,
see: Himbert G.; Fink, D. Z. Naturforsch. 1994, 49b, 542-550. Himbert,
G.; Fink, D. J. Prakt. Chem. 1994, 336, 654-657.
(14) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (c) The B3-LYP/6-31G* zero-
point vibrational energies were scaled by 0.9804: Wong, M. W. Chem.
Phys. Lett. 1996, 256, 391.
(12) Moloney, D. J. W.; Wentrup, C. To be published. Moloney, D. J.
W. Ph.D. Thesis, The University of Queensland, 1997.

Shift of Cl in a Chlorocarbonylketene
J. Am. Chem. Soc., Vol. 120, No. 8, 1998 1703
1
3
C NMR spectroscopy, even though the rearrangement is in
fact expected to take place at room temperature. Our 13C NMR
measurements for this compound at temperatures up to the
boiling point showed two carbonyl signals (δ 141.0 and 129.8
in CDCl3) and no sign of line broadening. For the analogous
fluoro compound 3a, one calculates in the same manner Tc
)
344 °C at 100.4 MHz; the observation of this 1,3-shift by
C NMR spectroscopy is out of the question, although the
1
3
reaction should take place not far above room temperature.
16
It remains uncertain whether the reported broadness of the
1
9
F NMR signal for this compound is due to such fluorine
migration. However, the bromide 3c is a candidate for
experimental observation of a 1,3-Br shift at a calculated Tc of
∼
150 °C.
4
As in the case of the imidoylketenes and vinylketenes, the
higher barriers in the acyl isocyanates relative to R-oxo ketenes
can be explained by the energy of the acceptor orbital (the
LUMO of the cumulene). For instance, the LUMO energies
(HF/6-31G*) calculated for formylketene and formyl isocyanate
are 2.54 and 2.77 eV, respectively.
Conclusion
We have experimentally verified a remarkable theoretical
prediction, viz. the degenerate 1,3-Cl shift in chlorocarbon-
-
1
ylketene 2, with an activation barrier of ∼42 kJ mol (10 kcal
-
1
mol ). Other facile 1,3-shifts of this type, e.g., of Br, SMe,
NMe2, PR2, F, and OR groups are to be expected, and such
reactions are the subject of ongoing research in our laboratories.
The question of stabilizing four-membered ring zwitterionic
transition states or intermediates of the type 4 in such reactions
is under continuing investigation.
*
Figure 2. B3-LYP/6-31G calculated structures of s-cis (2b) and s-trans
(
1
2a) chlorocarbonyl(phenyl)ketene, and the transition structures for the
,3-chlorine shift and for cis-trans isomerization in 2. Bond lengths
are in angstroms, and angles, in degrees.
Table 2. Calculated Energies of Chlorocarbonyl(phenyl)Ketene 2a
∆
H
∆G
∆
E
b
species
(0 K) -30 °C 25 °C
∆S
-30 °C 25 °C
s-trans (2a)
s-cis (2b)
0.0
10.1
35.9
0.0
10.2
34.8
46.3
0.7
0.0
10.3
34.3 -2.60
45.5 -3.31
1.5 -6.42
0.00
1.23
0.0
10.0
35.5
47.1
2.3
0.0
10.0
35.1
46.5
3.4
1
,3-Cl shift TS
cis-trans rot. TS 47.7
phenyl rot. TS 2.8
Experimental Section
(in s-trans 2a)
Chlorocarbonyl(phenyl)ketene (2). The literature procedure7a gave
various mixtures of the desired ketene 2 and phenylmalonyl dichloride.
a
In kJ mol-1, except for ∆S in J mol-1 K-1. b B3-LYP/6-
3
11+G(3df,2p)//B3-LYP/6-31G* + ZPVE.
7b-d
Procedures using PCl
5
were found to be undesirable as significant
decomposition occurred. The following procedure gave reproducible
results. A mixture of 50 g (0.28 mol) of phenylmalonic acid and 250
mL of freshly distilled thionyl chloride in a 500-mL three-necked round-
Acyl Isocyanates
The degenerate 1,3-shifts observed in R-oxo ketenes may also
be expected in acyl isocyanates 3, but the calculated activation
barriers are higher: H (207), F (116), Cl (84.5), and Br (72 kJ
2
bottomed flask equipped with a condenser and a CaCl drying tube
was refluxed at 85-95 °C for 48 h. A second batch starting from 25
g (0.14 mol) of phenylmalonic acid was prepared analogously. After
the two batches were combined and excess thionyl chloride distilled,
-
1
13
mol ) at the G2(MP2,SVP) level. From the measured
C
1
13
the resultant oil (60 g) was found by H and C NMR (vide infra)
to be a mixture of ketene 2 (50-70%) and phenylmalonyl dichloride
(
50-30%). This crude mixture was refluxed without solvent for 5 h
2
at 2 mbar using a KOH trap and a liquid N trap in series to collect
the evolved HCl. The resulting oil was purified by repeated distilla-
tion (first at 80-94 °C/2 mbar, then at 82-86 °C/2 mbar) using a 25
cm Vigreux column to give 56 g (74%) of ketene 2 as an orange oil.
NMR analysis of different batches demonstrated that the ketene
was of 95-98% purity, the remaining 5-2% being phenylmalonyl
dichloride.
1
5
‡
-1
NMR spectrum of 3b and the calculated ∆G ) 84.5 kJ mol ,
8a
one estimates a coalescence temperature Tc of 188 °C at 100.4
MHz. The high Tc is due to the large ∆ν for the two carbonyl
signals for this compound (1131 Hz in a field of 100.4 MHz).
The low boiling point of this compound (63 °C) unfortunately
puts this experiment outside the range accessible by liquid-phase
A sample of phenylmalonyl dichloride was prepared for comparison
using the method of Stensrud et al.1 This procedure yields a mixture
7a
(
16) Glemser, O.; Biermann, U.; Fild, M. Chem. Ber. 1967, 100, 1082-
086.
17) (a) Stensrud, T.; Bernatek, E.; Johnsgaard, M. Acta Chem. Scand.
1
(
(
16.
15) Klapstein, D.; Nau, W. M. Spectrochim. Acta 1994, 50A, 307-
1971, 25, 523-528. (b) Sorm, F.; Beranek, J.; Smrt, J.; Sicher, J. Collect.
Czech. Chem. Commun. 1955, 20, 593-597.
3

1704 J. Am. Chem. Soc., Vol. 120, No. 8, 1998
Andraos et al.
consisting of ∼82% of the dichloride and 18% of ketene 2. Thealter-
Acknowledgment. This work was supported by the Aus-
tralian Research Council. We are indebted to Professor Kin-ya
Akiba, Hiroshima University, for the use of his NMR spec-
trometer and variable-temperature probe, and to Dr. I. Brereton
and Ms. L. Lambert for recording the spectra at Brisbane.
1
7b
5
native procedure of Sorm et al. using PCl is undesirable because of
low yields and much decomposition.
1
H NMR (CDCl , 200 MHz): (phenylmalonyl dichloride) δ 5.40
3
13
(
(
1
s, 1H), 7.35-7.48 (m, 5H); (ketene 2): δ 7.25-7.46 (m). C NMR
CDCl , 50 Hz) (phenylmalonyl dichloride): δ 76.8, 128.3, 129.5,
29.6, 130.2, 166.4; (ketene 2 (0 °C)): 63.5 (CdCdO), 124.5 (ipso-
C), 128.7, 128.8, 129.7 (arom.), 170.3 (CdCdO). IR (film): ν 2128
3
Supporting Information Available: Calculated E0 energies
(G2(MP2,SVP)) of R-oxo ketenes and acyl isocyanates (in
hartrees); calculated total energies, ZPVEs, temperature cor-
rections, and entropies of 2a and 2b; optimized B3-LYP/6-31G*
equilibrium and transition structures of 2 in Cartesian coordi-
nates, and energy profile for rotation of the phenyl ring in the
s-trans 2a (5 pages). See any current masthead page for ordering
information and Web access instructions.
-
1
cm
.
Variable-temperature NMR spectroscopy was performed on a
JEOL EX-400 spectrometer (100.5 MHz for carbon). The temperatures
1
were calibrated using the H NMR chemical shift difference of the
signal of neat MeOH (low-temperature region) or 1,3-propanediol (high-
temperature region). CD Cl and nitrobenzene-d were used as solvents
2 2 5
for low and high temperatures, respectively. Solutions of 2 (200-250
mg) in 0.5-0.6 mL of the solvent were sealed in NMR tubes under
N
2
. Spectra were recorded between -60 and +180 °C.
JA973242G