Cyanoketene and Iminopropadienones

Article abstract of DOI:10.1021/jo9701288

Cyanoketene (8) is generated in high yields on flash vacuum thermolysis (FVT) of suitably substituted Meldrum's acid derivatives (5-[(alkylamino)(methylthio or alkylamino)methylene]-2,2-dimethyl-1,3-dioxane-4,6-diones) (3e-j), and also on FVT of cyanoacetic acid derivatives 9e,f,g,j,k,m. The major reaction pathway from 3 proceeds via ketenimines 6 and (alkylimino)propadienones 7, the latter undergoing a retro-ene reaction to 8. A minor pathway is via imidoylketenes 4e,h and oxoketenimines 5e,h, which undergo retro-ene reactions to 9. All intermediates were characterized by Ar matrix FTIR and tandem mass spectrometry (collisional activation MS). Trapping of 4, 5, and 8 with nucleophiles is also reported. The preference of 1,3-X shifts over 1,5-H shifts in imidoylketenes 12 (X = SMe or NMe₂) is corroborated by the calculated activation barriers. Neat cyanoketene is highly reactive, reacting at or below 80 K, and this is attributed to the availability of a low-lying ketene LUMO. The IR spectrum of cyanoketene (Ar, 14 K) is dominated by two absorptions at 2163 (s; C=C=O) and 2239 (w; CN) cm^-1 in excellent agreement with density functional (B3-LYP/6-31G*) and ab initio (QCISD/6-31G*) calculations.

Full text of DOI:10.1021/jo9701288

4240
J . Org. Chem. 1997, 62, 4240-4247
Cya n ok eten e a n d Im in op r op a d ien on es
Daniel W. J . Moloney,1a Ming Wah Wong,1a Robert Flammang,1b and Curt Wentrup*,1a
Department of Chemistry, The University of Queensland, Brisbane, Queensland 4072, Australia, and
Organic Chemistry Laboratory, University of Mons-Hainaut, 19 Avenue Maistriau, B-7000 Mons, Belgium
Received J anuary 24, 1997X
Cyanoketene (8) is generated in high yields on flash vacuum thermolysis (FVT) of suitably
substituted Meldrum’s acid derivatives (5-[(alkylamino)(methylthio or alkylamino)methylene]-2,2-
dimethyl-1,3-dioxane-4,6-diones) (3e-j), and also on FVT of cyanoacetic acid derivatives 9e,f,g,j,k ,m .
The major reaction pathway from 3 proceeds via ketenimines 6 and (alkylimino)propadienones 7,
the latter undergoing a retro-ene reaction to 8. A minor pathway is via imidoylketenes 4e,h and
oxoketenimines 5e,h , which undergo retro-ene reactions to 9. All intermediates were characterized
by Ar matrix FTIR and tandem mass spectrometry (collisional activation MS). Trapping of 4, 5,
and 8 with nucleophiles is also reported. The preference of 1,3-X shifts over 1,5-H shifts in
imidoylketenes 12 (X ) SMe or NMe2) is corroborated by the calculated activation barriers. Neat
cyanoketene is highly reactive, reacting at or below 80 K, and this is attributed to the availability
of a low-lying ketene LUMO. The IR spectrum of cyanoketene (Ar, 14 K) is dominated by two
absorptions at 2163 (s; CdCdO) and 2239 (w; CN) cm-1 in excellent agreement with density
functional (B3-LYP/6-31G*) and ab initio (QCISD/6-31G*) calculations.
In tr od u ction
efficient methods of generating cyanoketene by flash
vacuum thermolysis (FVT). These reactions are detailed
in the present paper.
Cyanoketene (NCsCHdCdO) has been the subject of
several studies,2 but due to its presumed high reactivity,
there is little direct spectroscopic information available.
Its photoelectron spectrum was obtained by using ther-
molysis of cyanoacetyl chloride at 920 K.3 The mass and
collisional activation mass spectra, the ion molecule
reactions of cyanoketene, and its relationship with the
unsubstituted iminopropadienone (HCdCdCdCdO) have
been investigated.4 Recently, flow pyrolysis of cyano-
acetyl chloride has also been employed to obtain the
microwave spectrum of cyanoketene and several of its
isotopomers.5 However, an infrared spectrum of cyano-
ketene has never been reported.
The reported2b isolation of cyanoketene as a not highly
reactive liquid with a boiling point of -34 °C, and the
attribution of 1H and 13C NMR spectra to this compound,
obtained by treatment of ethyl cyanoacetate with P2O5,
are in disagreeement with the high reactivity of cy-
anoketene reported below; for example, neat cyanoketene
disappears at temperatures below 80 K. Dicyanoketene
is even more reactive.2c
Work in collaboration with J . Chuche on the synthesis
of R-cyano carbonyl compounds such as cyanoacetic acid
derivatives 9 led to the realization that facile, thermal
1,3-shifts of electron rich migrating groups X (SMe or
RR′NH, and later also OMe and Cl) was taking place in
the postulated imidoylketene intermediates 4e-j, inter-
converting them with 5e-j.8a This was subsequently
proven by direct IR spectroscopic monitoring of the
intermediates in FVT/matrix isolation experiments,
whereby these intermediates were generated from both
Meldrum’s acid derivatives (2,2-dimethyl-1,3-dioxane-4,6-
diones (3)) and pyrrole-2,3-dione precursors.8b The high
migratory aptitude of electron rich groups in the inter-
conversion 1 f 2 has been corroborated by ab initio
calculations9 and can be understood in terms of a favor-
able interaction between a high-lying lone pair orbital
During our investigations of (alkylimino)propadi-
enones6 (RNdCdCdCdO) and the oxoketene-oxoketene7
(1a f 2a ) and imidoylketene-oxoketenimine8 (1b f 2b)
rearrangements, we have discovered several facile and
X Abstract published in Advance ACS Abstracts, J une 1, 1997.
(1) (a) University of Queensland. (b) University of Mons.
(2) (a) For a summary, see Tidwell, T. T. Ketenes; Wiley: New York,
1995, p 196 and references therein. (b) Zavlin, P. M.; Efremov, D. A.
Zh. Obshch. Khim. 1988, 58, 2403. J . Gen. Chem. USSR 1988, 58, 2139.
Zavlin, P. M.; Efremov, D. A.; Essentseva, N. S. Zh. Obshch. Khim.
1991, 61, 1269. J . Gen. Chem. USSR 1991, 61, 1153. Efremov, D. A.;
Zavlin, P. M.; Essentseva, N. S.; Tebby, J . C. J . Chem. Soc., Perkin
Trans 1 1994, 3163-3168. (c) For dicyanoketene, see Gano, J . E.;
J acobson, R. H.; Wettach, R. H. Angew. Chem., Int. Ed. Engl. 1983,
22, 165. Angew. Chem. Suppl. 1983, 193-197.
(6) (a) Mosandl, T.; Kappe, C. O.; Flammang, R.; Wentrup, C. J .
Chem. Soc., Chem. Commun. 1992, 1571-1573. (b) Flammang, R.;
Laurent, S.; Flammang-Barbieux, M.; Wentrup, C. Rapid Commun.
Mass Spectrom. 1992, 6, 667-670. (c) Mosandl, T.; Stadtmu¨ller, S.;
Wong, M. W.; Wentrup, C. J . Phys. Chem. 1994, 98, 1080-1086.
(7) Wentrup, C.; Netsch, K.-P. Angew. Chem., Int. Ed. Engl. 1984,
23, 802.
(8) (a) Ben Cheikh, A.; Chuche, J .; Manisse, N.; Pommelet, J . C.;
Netsch, K.-P.; Lorencak, P.; Wentrup, C. J . Org. Chem. 1991, 56, 970-
975. (b) Kappe, C. O.; Kollenz, G.; Leung-Toung, R.; Wentrup, C. J .
Chem. Soc., Chem. Commun. 1992, 487-488. (c) Kappe, C. O.; Kollenz,
G.; Netsch, K.-P.; Leung-Toung, R.; Wentrup, C. J . Chem. Soc., Chem.
Comun. 1992, 488-490. (d) Fulloon, B.; El-Nabi, H. A. A.; Kollenz,
G.; Wentrup, C. Tetrahedron Lett. 1995, 36, 6547-6550. (e) Fulloon,
B. E.; Wentrup, C. J . Org. Chem. 1996, 61, 1363-1368.
(3) Bock, H.; Hirabayashi, T.; Mohmand, S. Chem. Br. 1981, 114,
2595-2608.
(4) (a) Flammang, R.; Haverbeke, Y. Van; Wong, M. W.; Ru¨hmann,
A.; Wentrup, C. J . Phys. Chem. 1994, 98, 4814-4820. (b) Holmes, J .
L.; Mayer, P. M.; Vasseur, M.; Burgers, P. C. J . Phys. Chem. 1993, 97,
4865-4870.
(5) Hahn, M.; Bodenseh, H. Presented at the Tenth Colloquium on
High Resolution Molecular Spectroscopy, Dijon, France, Sept. 1987,
paper M16. Bodenseh, H. Private communication, 1996.
(9) Wong, M. W.; Wentrup, C. J . Org. Chem. 1994, 59, 5279-5285.
Bibas, H.; Wong, M. W.; Wentrup, C. J . Am. Chem. Soc. 1995, 117,
9582. Koch, R.; Wong, M. W.; Wentrup, C. J . Org. Chem. 1996, 61,
6809-6813. Bibas, H.; Wong, M. W.; Wentrup, C. Chem. Eur. J . 1997,
3, 151-162.
S0022-3263(97)00128-X CCC: $14.00 © 1997 American Chemical Society

Cyanoketene and Iminopropadienones
J . Org. Chem., Vol. 62, No. 13, 1997 4241
of the migrating group X and the low-lying ketene
LUMO; the latter lies in the same plane as X and has a
high coefficient at the electrophilic central carbon atom
of the ketene function. Thermodynamics will usually
displace the equilibrium between 4 and 5 toward the
latter, which can be isolated in several cases, particularly
when X ) OMe.8d,e However, when R ) Ph, imi-
doylketenes 4 efficiently cyclize to quinolones, and the
equilibrium is therefore displaced toward 4 at high
temperatures, when quinolones can be isolated in almost
quantitative yields.8b,d,e
Furthermore, we have demonstrated that iminopro-
padienones 7a and 7b can be isolated at temperatures
below -50 and -70 °C, respectively, following FVT of
Meldrum’s acid precursors of type 3a ,b or isoxazolo-
pyrimidinones.6,7e Some (N-arylimino)propadienones are
even isolable at room temperature.10 The fragmentation
of Meldrum’s acid derivatives 3 to ketenes 4 dominates
with X ) SMe or OMe, which are good migrators (giving
5) but relatively poor leaving groups (giving 6 and 7).
The competing formation of 7 is observed in both of these
cases at high temperatures, but it becomes the near-
exclusive reaction when X is an amine function.6a These
derivatives, therefore, are excellent precursors of imino-
propadienones 7a ,b.
F igu r e 1. Partial FTIR spectra (14 K; 2000-2320 cm-1 range)
of the products of FVT of 3e as a function of temperature. I )
oxoketenimine 5e (2054 cm-1); P ) (isopropylimino)propadi-
enone 7e (2221, 2234 cm-1); C ) cyanoketene 8 (2163 (s), 2239
(w) cm-1; K ) imidoylketene 4e (2112 cm-1).
Sch em e 1
Consequently, there are two ways that the final
products, cyanoacetic acid derivatives 9e-j, may be
formed in FVT reactions of N-alkyl-substituted Mel-
drum’s acid derivatives 3e-j: (i) the ketene-ketenimine
route via 4 and 5 followed by retro-ene type alkene
elimination to give 9; or (ii) initial formation of imino-
propadienone 7 followed by alkene elimination to give
cyanoketene (8). As will be shown below, the latter is
reactive enough to react with the nucleophile (RSH or
RR′NH) in the cold trap to regenerate 9. The present
study seeks to unravel these mechanistic alternatives.
Resu lts a n d Discu ssion
Im id oylk et en es, Ket en im in es, Im in op r op a d ie-
n on es, Cya n ok eten e, a n d Retr o-En e Rea ction s. The
Meldrum’s acid derivatives 3e-j all gave rise to a strong
absorption at 2163 cm-1 together with a very weak one
at 2239 cm-1 upon FVT with Ar matrix isolation of the
products at 14 K for IR spectroscopy. As detailed below,
these absorptions are due to cyanoketene (8) (Scheme 1).
In the cases 3e and 3h , cyanoketene became the domi-
nant species above ca. 600 °C (cf. Figure 1). In the cases
3f,g,i,j, cyanoketene was always the major species, even
under the mildest FVT conditions, starting at 300-400
°C. An example is shown in Figure 2b.Detailed IR
monitoring of the products of FVT of 3e as a function of
temperature revealed several intermediates. Analogous
monitoring of the reaction by mass spectrometry permit-
ted a direct correlation between IR, mass, and collisional
activation mass spectra (CAMS) (Figures 1-5). As seen
in Figure 1 and Table 1, the first-formed species is a
ketenimine (5e; 2054 cm-1; m/z 157), which starts ap-
pearing already at 200 °C, at which temperature there
is very little reaction taking place. As the temperature
increases, a signal ascribed to the isomeric ketene 4e
(2112 cm-1) becomes visible, while that due to 5e remains
stronger. This is the normal intensity ratio for intercon-
verting imidoylketenes and oxoketenimines.8c,d The sec-
ond intermediate detected by MS has a mass of 211 and
corresponds to the transient ketenimine 6e, formed by
thermal elimination of MeSH (m/z 48; also detected). 6e
rapidly loses CO2 and acetone to furnish the third
intermediate, (isopropylimino)propadienone (7e; 2234,
2221 cm-1; m/z 109). The cumulenic stretching vibrations
of iminopropadienones in the 2200 cm-1 region have
extremely high extinction coefficients and often appear
as multiple bands due to matrix site splitting.6a,c We
have previously calculated the structures and IR spectra
of iminopropadienones 7 (R ) H, Me, and Ph) at the HF-
and MP2/6-31G* (MP2/3-21G for Ph) levels.6e New B3-
LYP/6-31G* calculations confirm very strong antisym-
(10) Moloney, D. J . W.; Wentrup, C. To be published. Moloney, D.
J . W. Ph.D. Thesis, The University of Queensland, Brisbane, 1997.

4242 J . Org. Chem., Vol. 62, No. 13, 1997
Moloney et al.
F igu r e 2. FTIR spectra (14 K; abscissa in wavenumbers) of
cyanoketene 8 produced by (a) FVT of 9m at 800 °C; (b) FVT
of 3j at 700 °C. A ) acetone; B ) tert-butylamine; C )
cyanoketene 8 (2163, 2239 cm-1); D ) CO2; E ) CO, F ) 3,5-
dimethylpyrazole; G ) 2-methylpropene; W ) water.
Ta ble 1. Rela tive Abu n d a n cies of Som e Ion s of In ter est
in th e Ma ss Sp ectr a of 3e d u r in g F VT
temp,
°C
m/z 259
(M+, 3e)
211
(6e)
157
(4/5)
115
(9e)
109
(7)
67
(8)
200
300
400
600
100
36
3
6
54
13
0
65
100
76
10
5
60
3
4
61
100
17
8
33
73
0
3
100
metric absorptions of 7 (R ) H, Me, iPr, t-Bu, Ph) in the
2200 cm-1 range (ca. 4000 km mol-1) together with very
weak symmetric ones in the 2100 cm-1 range (ca. 60 km
mol-1). Consequently, the signals seen in Figure 1 do
not correspond to large amounts of 7e. These signals
disappear above 700 °C, with concomitant increased
signals for propene and cyanoketene in both the IR and
mass spectra. However, cyanoketene (8; 2163s, 2239w
cm-1; m/z 67) and propene start appearing below 500 °C,
and cyanoketene is the only species remaining at 800 °C
(together with CO2, acetone, methanethiol, and propene).
The formation of methyl cyanothiolacetate (9e; m/z 115)
is observed by MS at 350-400 °C. In the low pressure
gas phase of the mass spectrometer, this must be due to
thermal elimination of propene from 5e via a six-centered
six-electron cyclic transition state of the retro-ene type8a
(eq 1), giving 9e directly; however, the signal (m/z 115)
due to 9e never becomes strong. As shown below,
compounds 9 themselves undergo thermal elimination of
HX above 400-500 °C to give cyanoketene (8), but this
process is inefficient below ca. 600 °C. Thus, cyanoketene
always becomes the near-exclusive product when using
FVT temperatures of 700-800 °C.
Chemical evidence for the assignment of ketene 4e and
ketenimine 5e was obtained in a preparative trapping
experiment with MeOH at 77 K, whereby esters 10 and
11 were isolated in 19 and 58% yields, respectively,
reflecting the predominance of 5e in the thermal equi-
librium.
F igu r e 3. FVT/mass spectra of 3e at (a) 100 °C (no thermal
decomposition); (b) 250 °C (ketenimine 6e, m/z 211 appears);
(c) 350 °C (iminopropadienone 7e, m/z 109 at optimal intensity;
m/z 115 (9) and 67 (8) present); (d) 400 °C (m/z 109, 115, and
157 disappearing; m/z 67 (cyanoketene 8) dominant; at 600
°C this becomes the only significant species in the spectrum).
already at 200 °C. Nevertheless, all the intermediates,
5h (2054 cm-1; m/z 171), 7h (2214, 2239 cm-1; m/z 123),
6h (m/z 225), and 9e (m/z 115), as well as isobutene,
methanethiol, CO2, and acetone, were detectable by Ar
matrix IR and/or mass spectrometry. The most impor-
The tert-butyl analogue 3h is more sensitive than 3e.
Decomposition with formation of cyanoketene (8) starts

Cyanoketene and Iminopropadienones
J . Org. Chem., Vol. 62, No. 13, 1997 4243
ture of 400 °C (cf. 700 °C for 3e). From 450 °C onward,
essentially only cyanoketene was formed.
The amine derivatives 3f,g,i,j gave cyanoketene im-
mediately on FVT over the whole temperature range
(300-800 °C), with little evidence for the other interme-
diates being present according to Ar matrix IR spectros-
copy. The appropriate alkene (propene or isobutene) was
also immediately detected (cf. Figure 2b). This reaction,
too, is formulated as a six-centered six-electron retro-ene
type reaction (eq 2): Both of these retro-ene reactions
(eqs 1 and 2) and other, related pericyclic reactions in
cumulenes are under theoretical investigation in our
group. the orbital topology of the cumulene systems
makes it possible to transfer the hydrogen atom and
create the third bond of the CN triple bond in the plane
of the molecules.
1,3-X ver su s 1,5-H Sh ifts in Im id oylk eten es 4. It
was shown previously8a,11 that (N-alkylimidoyl)ketenes
12 possessing at least one R-hydrogen atom undergo
1,5-H shifts to produce enaminoacroleins 14 as final
products in good yields (Scheme 2). However, no trace
of such 1,5-H shift products was obtained from 4b or 4e,
which instead underwent the 1,3-shifts of the methylthio
group, giving 5 (Scheme 1). Thus, one should expect the
activation barriers for the 1,3-shifts (12 f 13) of the most
efficient migrators9 (X ) NMe2 or SMe) to be lower than
that of the 1,5-H shift (12 f 14). Conversely, with poor
1,3-migrators (e.g. X ) Ph, H, or Me8a), the 1,5-H shift
will dominate. These expectations are nicely supported
by our theoretical calculations of ground state and
transition structure energies at the MP2/6-311+G(2d,p)/
/MP2/6-31G* level (Table 2). The calculated 1,5-H shift
barriers are similar for the three model systems consid-
ered, of the order of 100 kJ mol-1. However, the barrier
for the 1,3-X shift falls dramatically from the high values
for H and CH3, which are known to be poor migrators
(the methyl group not migrating at all),9 to ca. 90 kJ
mol-1 for X ) NH2. Thus, already the 1,3 shift of the
amino group will compete effectively with the 1,5-H shift.
We have shown in other studies9 that the 1,3-shift
barriers of SMe and NMe2 are lower than that of NH2
by 20-40 kJ mol-1. Therefore, only the 1,3-X shift will
be observed in compounds 12 (X ) SMe or NMe2). The
F igu r e 4. Collisional activation mass spectra [CA(O2)] of
isopropyl-substituted intermediates; (a) m/z 157 (4e and/or 5e);
(b) m/z 211 (6e); (c) m/z 109 (7e); (d) m/z 67 (8), generated by
FVT/MS of 3e (cf. Figure 3).
tant CA mass spectra are illustrated in Figure 5. The
CAMS of 8 and 9 were identical with those obtained from
3e (vide supra). In the IR spectra, cyanoketene (8) was
the major product from 3h already at an FVT tempera-
(11) Briehl., H.; Lukosch, A.; Wentrup, C. J . Org. Chem. 1984, 49,
2772. Gordon, H. J .; Martin, J . C.; McNab, H. J . Chem. Soc., Perkin
Trans. 1 1984, 2129. Maujean, A.; Marcy, G.; Chuche, J . Tetrahedron
Lett. 1980, 21, 519.

4244 J . Org. Chem., Vol. 62, No. 13, 1997
Moloney et al.
F igu r e 5. CA(O2) mass spectra of tert-butyl-substituted intermediates; (a) m/z 225 (6h ); (b) m/z 171 (4h /5h ); (c) m/z 123 (7h ),
generated by FVT/MS of 3h . The spectrum of m/z 67 (8) was identical with the one shown in Figure 4.
Sch em e 2
supported by theoretical calculations, at the B3-LYP and
QCISD levels in particular. HF/ and MP2/6-31G* cal-
culations were also carried out (Table 3). Note that all
three methods predict only the two observed bands with
a reasonable intensity in the range 700-3000 cm-1, in
agreement with observations (apart from the two absorp-
tions at 2163 and 2239 cm-1 described above, a weak
band at 3076 cm-1 is always observed in the matrix IR
spectra of 8, but it is more hazardous to assign this since
alkenes and other intermediates also give rise to absorp-
tions in this region). The MP2 calculated intensities of
the CCO and CN vibrations are drastically different and
in disagreement with experiment. This is due to the fact
that these vibrations are strongly coupled at both the HF
and MP2/6-31G* levels of theory. The MP2 CCO and CN
frequencies are also in poor agreement with experiment
and with the other theoretical results. We have shown
elsewhere that the standard scaling factor13 of 0.9427 is
not applicable to cyano compounds.14
Ta ble 2. Ca lcu la ted Rela tive En er gies (k J m ol-1) for 1,3
a n d 1,5 Sh ifts in Im id oylk eten esa
speciesb
X ) H X ) CH3 X ) NH2
The CdCdO stretching frequency of cyanoketene (ν3;
exptl in Ar: 2163; B3LYP/6-31G*: 2165 cm-1) is higher
than that of the parent ketene (Ar: 2142; gas: 2151;
B3LYP/6-31G*: 2153 cm-1), roughly midway between
this and dicyanoketene2b (Ar: 2175 cm-1). This is
consistent with the expectation that resonance structure
15 of ketene is favored by the electron-withdrawing cyano
group, thus leading to a higher frequency.15
s-trans CH3NdCXsCHdCdO
s-cis CH3NdCXsCHdCdO
1,5-H shift transition structure
1,3-X shift transition structure
s-trans OdCXCHdCdNCH3
s-cis OdCXCHdCdNCH3
E CH2dNsCXdCHCHO
0.0
0.5
0.0
4.0
97.3
233.2
6.5
7.9
24.2
17.7
0.0
-0.2
96.7
112.4
179.8
22.3
25.3
20.1
23.4
90.2
-19.2
-19.8
29.5
Z CH2dNCXdCHCHO
27.0
a
Cf. Scheme 2. MP2/6-311+G(2d,p)//MP2/6-31G* values are
b
reported. Imidoylketenes and ketenimines are defined as s-trans
or s-cis based on the π systems, regardless of the nature of X.
same can be expected for SH and Cl groups. The low
barriers for the 1,3-shifts of these electron rich groups
was rationalized in terms of a favorable interaction
between a lone pair on the migrating group and the
vacant central carbon p orbital of the ketene LUMO.9
Cya n ok eten e fr om Cya n oa cetic Acid Der iva tives
9. Further proof for the identity of cyanoketene was
adduced by FVT of the reaction products 9e,f (X ) SMe
or NMe2) as well as methyl cyanoacetate (9k ) and
1-(cyanoacetyl)-3,5-dimethylpyrazole (9m ) (Scheme 1),4a,12
all of which produced cyanoketene (9e starting at 500
°C, but reaction still incomplete at 800 °C; 9f starting at
400 °C and reaction complete at 700 °C; 9k starting at
550 °C; 9m starting at 450 °C and reaction complete at
800 °C). The Ar matrix IR spectrum obtained from 9m
is shown in Figure 2a.
Our best calculated structure of cyanoketene is shown
in Figure 6. The very short CdO bond (and slightly
elongated CdC bond) are again in agreement with an
enhancement of resonance structure 15 as in a highly
electronegatively substituted ketene.16 For comparison,
the calculated structural parameters for ketene at the
(13) Scaling factors: (a) for HF (0.9229) and MP2 (0.9427): Pople,
J . A.; Scott, A. P.; Wong, M. W.; Radom, L. Isr. J . Chem. 1993, 33,
345. (b) For B3-LYP (0.9613): Wong, M. W. Chem. Phys. Lett. 1996,
256, 391. (c) For QCISD (0.9538): Scott, A. P.; Radom, L. J . Phys.
Chem. 1996, 100, 16502-16513.
(14) Kappe, C. O.; Wong, M. W.; Wentrup, C. Tetrahedron Lett. 1993,
34, 6623.
(15) Gano, J . E.; J acob, E. J . Spectrochim. Acta 1987, 43A, 1023-
1025. Cf. also McAllister, M. A.; Tidwell, T. T. Can. J . Chem. 1994,
72, 882.
(16) For the X-ray structure of a pivaloylketene with an extremely
short CdO bond, see: Kappe, C. O.; Evans, R. A.; Kennard, C. H. L.;
Wentrup, C. J . Am. Chem. Soc. 1991, 113, 4234-4237. For ketene,
see: Runge, W. In The Chemistry of Ketenes, Allenes, and Related
Compounds; Patai, S., Ed.; Wiley: Chichester, 1980; Chapter 2, pp
45-98.
In fr ar ed Spectr u m an d Str u ctu r e of Cyan oketen e.
The assignment of the IR spectrum of cyanoketene (8) is
(12) For the use of pyrazolides to generate ketenes, see Besida, J .;
Brown, R. F. C.; Colmanet, S.; Leach, D. N. Aust. J . Chem. 1982, 35,
1373.

Cyanoketene and Iminopropadienones
J . Org. Chem., Vol. 62, No. 13, 1997 4245
Ta ble 3. Ca lcu la ted Vibr a tion a l F r equ en ciesa (cm -1) a n d IR In ten sitiesb (k m m ol-1) of Cya n ok eten e
mode
HF/6-31G*
MP2/6-31G*
B3-LYP/6-31G*
QCISD/6-31G*
assignment
CH stretch
CN stretch
CCO stretch
in phase CH wag
out of phase CH wag
CC stretch
sym CCO + CCN bend
asym CCO + CCN bend
CCC wag
CH oop bend
CCO oop bend
CCN oop bend
A′ ν1
ν2
3048 (26)
2322 (40)
2145 (1253)
1364 (13)
1109 (0)
945 (1)
644 (8)
420 (4)
147 (11)
612 (113)
528 (1)
3088 (33)
2128 (487)
2099 (49)
1336 (6)
1094 (1)
937 (1)
618 (2)
390 (1)
136 (7)
528 (73)
489 (0)
3084 (24)
2258 (21)
2165 (730)
1358 (5)
1106 (2)
949 (1)
631 (3)
401 (1)
139 (7)
577 (67)
494 (2)
3110 (24)
2231 (20)
2126 (699)
1361 (5)
1113 (2)
947 (1)
625 (3)
389 (2)
141 (8)
547 (77)
491 (2)
ν3
ν4
ν5
ν6
ν7
ν8
ν9
A′ ν10
ν11
ν12
400 (5)
363 (7)
391 (5)
370 (5)
a
b
Scaled values.13 IR intensities are given in parentheses.
Ta ble 4. Ca lcu la ted HOMO a n d LUMO En er gies (eV) of
Som e Keten es (HF /6-311+G**)
species
HOMO
LUMO
ketene
-10.07
-9.50
-10.72
-10.67
-11.24
1.80
1.66
1.44
1.20
0.71
methylketene
formylketene
cyanoketene 8
dicyanoketene
F igu r e 6. Optimized structure of cyanoketene (QCISD/6-
311+G**). Bond lengths in angstroms and angles in degrees.
The calculated rotational constants for this geometry, A, B,
and C are 28.968, 2.7858, and 2.5414 GHz, respectively.
energy and lowers the LUMO by a much smaller amount
than do the formyl or cyano groups. The in-plane LUMO
plays an important role in governing the reactivity of
ketenes, in particular cycloaddition reactions19 and, as
shown above, 1,3-X shifts.9 Hence, the high reactivity
of cyanoketenes2c may be attributed to the availability
of a low-lying ketene LUMO.
same level (QCISD/6-311+G**) are: r(CdO) ) 1.165;
r(CdC) ) 1.321; r(CsH) ) 1.080 Å, HCH ) 120.6°.
Reactivity of Cyan oketen e. Cyanoketene is a highly
reactive molecule. After preparation by FVT of e.g. 3j
at 600 °C, with isolation as a neat film at 14 K,
subsequent warmup in 10 K intervals demonstrated that
8 vanishes at 80 K. In other words, it is practically
impossible to isolate cyanoketene at liquid nitrogen
temperature (77 K). Therefore, although all the FVT
reactions manifestly produce cyanoketene, preparative
thermolysis experiments with product isolation at 77 K
invariable gives acetic acid derivatives 9.8a Thus, even
when the products of FVT (600 °C) of 3i were isolated
on a 77 K cold finger previously coated with methanol,
subsequent warmup to rt gave only N,N-dimethyl-
cyanoacetamide (9f; 69%); 9k was not detected. A likely
explanation is that cyanoketene reacts at 77 K with the
dimethylamine with which it is cocondensed before it has
time to diffuse through this medium to the methanol
surface. We have demonstrated elsewhere that other
ketenes react with amines (pyridine) at temperatures
between 15 and 40 K via zwitterionic (“ketene-pyridine
ylide”) intermediates.17
Tidwell et al.18 have shown that the stabilization
energy of cyanoketene is similar to that of alkylketenes
(e.g. methylketene), but the cyano group is less stabilizing
as a π acceptor than is formyl (by about 4 kcal mol-1).
Our calculations (Table 4) demonstrate that, as expected,
the cyano group lowers both the HOMO and the LUMO
energies of ketene, in particularly the latter, making the
ketene more electrophilic. The cyano group is more
effective than the formyl group in this respect, and the
two cyano groups in dicyanoketene have a very pro-
nounced effect. A methyl group increases the HOMO
Con clu sion
Cyanoketene (8) is formed efficiently by flash vacuum
thermolysis of Meldrum’s acid derivatives 3. The amine
derivatives 3f,g,i,j give cyanoketene particularly cleanly,
without any other intermediates being detectable in
significant quantites. The methylthio derivatives 3e,h
fragment by two routes: (i) elimination of acetone and
CO2 starting at ca. 300 °C leads to imidoylketenes 4
which undergo a facile 1,3-shift of the methylthio group
to give oxoketenimines 5, detected by both matrix IR and
mass spectrometry. At higher temperatures, retro-ene
type alkene elimination from 5 gives cyanoacetic acid
derivatives 9, which eliminate methanethiol to produce
cyanoketene (8). (ii) The major route is HX elimination
from the starting material 3, which creates a transient
Meldrum’s acid-ketenimine 6, detectable by mass spec-
trometry. Facile elimination of CO2 and acetone from 6
furnishes the iminopropadienones 7, detectable by MS
and IR (giving strong absorptions in the 2200 cm-1
region). However, above 300 °C, the iminopropadienones
undergo a facile retro-ene type alkene elimination to give
cyanoketene (8). Thus, in all cases, cyanoketene becomes
the exclusive ultimate product.
The 1,3-shifts of methylthio (and amino) groups con-
verting imidoylketenes 4(12) to oxoketenimines 5(13)
compete efficiently with 1,5-H shifts to give enaminoac-
roleins 14 (Scheme 2).
(19) Inter alia the following and references cited therein (a) Cossio,
F. P.; Ugalde, J . M.; Lopez, X.; Lecea, B.; Palomo, C. J . Am. Chem.
Soc. 1993, 115, 995. (b) Wang, X.; Houk, K. N. J . Am. Chem. Soc. 1990,
112, 1754-1756. (c) Valenti, E.; Pericas, M. A.; Moyano, A. J . Org.
Chem. 1990, 55, 3582. (d) Sonveaux, E.; Andre´, J . M.; Delhallo, J .;
Fripiat, J . G. Bull. Soc. Chim. Belg. 1985, 94, 831-849. (e) Houk, K.;
Strozier, R. W.; Hall, J . A. Tetrahedron Lett. 1974, 897. (f) Woodward,
R. B.; Hoffmann, R. The Conservation of Orbital Symmetry; Verlag
Chemie: Academic Press: New York, 1970; p 163.
(17) Qiao, G. G.; Andraos, J .; Wentrup, C. J . Am. Chem. Soc. 1996,
118, 5634-5638. Visser, P.; Zuhse, R.; Wong, M. W.; Wentrup, C. J .
Am. Chem. Soc. 1996, 118, 12598-12602.
(18) Gong, L.; McAllister, M. A.; Tidwell, T. T. J . Am. Chem. Soc.
1991, 113, 6021.

4246 J . Org. Chem., Vol. 62, No. 13, 1997
Moloney et al.
Ap p a r a tu s a n d P r oced u r e for F VT/Ma tr ix Isola tion
a n d Ma ss Sp ectr om ety. The apparatus for FVT/matrix
isolation26 was as previously reported. Samples were deposited
with Ar on BaF2 disks at 14 K for FTIR spectroscopy. The
preparative FVT apparatus26 employed a 25 × 1.8 (i.d.) cm
quartz tube in an electrically heated oven. The system was
continuously pumped at better than 10-4 mbar. The six-sector
mass spectrometer (EBEEBE configuration) and the FVT/MS
unit have been described elsewhere.28 Collisional activation
(CA) mass spectra were obtained using oxygen (80% transmit-
tance). The resulting spectra are referred to as CA(O2) spectra.
Matrix IR and mass spectra of known compounds were
recorded for comparison with FVT products. Acetone29 was
monitored by Ar matrix IR spectroscopy at 3018, 1721, 1361,
The cyanoacetic acid derivatives 9 (X ) OMe, SMe,
NMe2, and 3,5-dimethylpyrazol-1-yl) can also be used as
cyanoketene precursors, cleanly on FVT at 600-800 °C.
Since the Meldrum’s acid derivatives 3 generally produce
cyanoketene at lower temperatures than do the acetic
acid derivatives 9, the route via (alkylimino)propadi-
enones 7 is the major one in all cases. This is in
agreement with the fact that mehyl- (7b), phenyl- (7a ),
and other (arylimino)propadienones (7)10 can be isolated
from FVT of the corresponding Meldrum’s acid deriva-
tives 3.
The IR spectrum of cyanoketene is well reproduced by
DFT and ab initio calculations, and the high reactivity
of cyanoketene is attributed to the availability of a very
low energy LUMO.
1217, 1092 cm-1; CO2 at 2344, 2340 cm-1; CO at 2141 cm-1
MeSH30 at 2941, 2547, 1446, 1436, 1327, 1072, 962 cm-1
;
;
propene31 at 3091, 2983, 1453, 998 cm-1; 2-methylpropene31
at 2984, 2942, 1463, 1444 cm-1; MeOH29 at 3666, 3005, 2848,
1474, 1466, 1333, 1076, 1034 cm-1; dimethylamine at 3193,
2973, 2832, 1482, 1479, 1159, 1148, 1025, 861 cm-1; isopro-
pylamine at 1383, 1346, and 794 cm-1; tert-butylamine at 1388,
1249, 1219, and 889 cm-1; 3,5-dimethylpyrazole at 3500, 2997,
2926, 1812, 1582, 1419, 1354, 1273, 996, 780 cm-1; and N,N-
dimethylcyanoacetamide at 3023, 2943, 2261, 1697, 1672,
Exp er im en ta l Section
Com p u ta tion a l Meth od s. Ab initio20 and density func-
tional21 calculations were carried out using the GAUSSIAN
92/DFT series of programs.22 The equilibrium structure,
charge distribution, dipole moment, ionization potential and
thermochemistry (not reported here), and the vibrational
spectrum of cyanoketene were examined with the Hartree-
Fock (HF), MP2,20 QCISD,23 and B3-LYP24 methods using the
6-31G* and 6-311+G** basis sets.20
1506, 1458, 1401, 1260, 1139, 1065, 980, 936 cm-1
.
Cya n ok eten e (8). (a ) F r om Meld r u m ’s Acid P r ecu r -
sor s 3e-j. A sample of the precursor (ca. 10 mg) was
sublimed at 80 °C in the FVT apparatus using the desired oven
temperature. The FVT product was matrix isolated with Ar
at 10-4 mbar over a 20 min period and examined by FTIR
spectroscopy. Cyanoketene was observed at 2239 (w), 2163
(s) cm-1, as discussed in the text and shown in Figures 1 and
2b.
(b) F r om N,N-Dim eth ylcya n oa ceta m id e (9f). A 10 mg
sample was sublimed at 40 °C and FVT/matrix isolation was
carried out as above. Cyanoketene was observed at 2239 (w),
2163 (s) cm-1, commencing at an FVT temperature of 400 °C;
the reaction was complete at 700 °C.
(c) F r om Met h yl Cya n oa cet a t e (9k ). Methyl cyano-
acetate (20 mg) was cooled in an ice/salt bath, evacuated, and
subjected to FVT by allowing the sample to warm to room
temperature over a 20 min period. Ar matrix isolated cy-
anoketene was observed at 2239 (w), 2163 (s) cm-1, commenc-
ing at 550 °C. The conversion was complete at 850 °C.
(d ) F r om 1-(Cya n oa cetyl)-3,5-d im eth ylp yr a zole (9m ).
The precursor (10 mg) was sublimed at ca 50 °C and FVT/
matrix isolation was performed as above. Cyanoketene was
observed together with dimethylpyrazole starting at ca. 450
°C. The conversion was complete at 800 °C (Figure 2a).
P r ep a r a tive F VT of 5-[(Isop r op yla m in o)(m eth ylth io)-
Ma ter ia ls. 3e-h and 3j were prepared as previously
reported.8a
5-[(t er t -Bu t yla m in o)(d im e t h yla m in o)m e t h yle n e ]-2-
d im eth yl-1,3-d ioxa n e-4,6-d ion e (3i). To 5-[bis(methylthio)-
methylene]-2,2-dimethyl-1,3-dioxane-4,6-dione8a,25 (2.48 g; 10
mmol) in 30 mL of THF was added tert-butylamine (0.73 g;
10 mmol), and the mixture was allowed to stir overnight. To
the stirring solution was added dimethylamine solution (4 g;
10 mmol, as a 10% w/v solution in ethanol), mercuric oxide
(2.16 g; 10 mmol), and mercuric chloride (2.71 g; 10 mmol),
the mixture was stirred for a further 2 days and filtered, and
the filtrate was evaporated. The resulting solid was recrystal-
lized from THF to yield colorless crystals (0.41 g, 15%), mp
174-176 °C; 1H NMR (CDCl3) δ 1.43 (s, 9 H), 1.72 (s, 6 H),
3.16 (s, 6 H), 6.36 (s 1 H); 13C NMR (CDCl3) δ 26.9, 30.2, 40.5,
56.7, 71.8, 163.5, 165.0; IR (CHCl3) δ 1653, 1635 cm-1; MS
m/z 270.1580 (M+, 18%), 213 (11), 212 (11), 167 (60), 153 (11),
112 (100). Anal. Calcd for C13H21N2O4: C, 57.75; H, 8.21; N,
10.37. Found: C, 57.97; H, 8.41; N, 10.38.
N-(Cyan oacetyl)-3,5-dim eth ylpyr azole (9m ). To a stirred
suspension of 3,5-dimethylpyrazole (1.94 g, 20 mmol) and
dicyclohexylcarbodiimide (4.17 g, 20 mmol) was added cy-
anoacetic acid (1.38 g, 20 mmol) at 0 °C. The mixture was
then stirred at room temperature overnight. The resulting
suspension was filtered and the solid washed several times
with dry acetone. Evaporation of the filtrate gave an orange
gum, which was distilled (40 °C, 3.5 × 10-4 mbar) to give an
orange oil, 34 mg (69%); 1H NMR (CDCl3) δ 2.25 (s, 3 H), 2.60
(s, 3 H), 4.91 (s, 2 H), 6.05 (s, 1 H); IR (CHCl3) ν 1735, 1654,
1602 cm-1. Anal. Calcd for C8H9N3O: C, 59.25; H, 4.97; N,
25.91. Found: C, 59.24; H, 5.05; N, 25.82.
m eth ylen e]-2,2-d im eth yl-1,3-d ioxa n e-4,6-d ion e (3e).
A
sample (440 mg, 1.9 mmol) was gently sublimed at ca. 80 °C
and subjected to FVT at 600 °C/1.5 × 10-3 mbar in the course
of 3 h, using a dry ice/acetone (-78 °C) cooled U-tube as a
cold trap. Upon completion of the thermolysis, methanol (2
mL) was injected onto the pyrolysate. The apparatus was
flushed with nitrogen and the U-tube allowed to warm to room
temperature. The methanol solution was collected and the
solvent removed by rotary evaporation. The resulting yellow
oil was filtered through a plug of silica gel (ca. 1 cm3) and
purified by column chromatography (silica gel; ether/hexane).
The following products were isolated:
(20) Hehre, W. J .; Radom, L.; Schleyer, P. v. R.; Pople, J . A. Ab Initio
Molecular Orbital Theory; Wiley: New York, 1986.
(21) (a) Parr, R. G.; Yang, W. Density Functional Theory of Atoms
and Molecules; Oxford University Press: New York, 1989. (b) Density
Functional Methods; Labanowski, J .; Andzelm, J ., Eds.; Springer:
Berlin, 1991.
(22) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
J ohnson, 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.
(26) Kappe, C. O.; Wong, M. W.; Wentrup, C. J . Org. Chem. 1995,
60, 1686-1695.
(27) Cf. Wentrup, C.; Blanch, R.; Briehl, H.; Gross, G. J . Am. Chem.
Soc. 1988, 110, 1874-1880.
(28) (a) Wong, M. W.; Wentrup, C.; Flammang, R. J . Phys. Chem.
1995, 99, 16849-16856. (b) Brown, J .; Flammang, R.; Govaert, Y.;
Plisnier, M.; Wentrup, C.; Haverbeke, Y. Van. Rapid Commun. Mass
Spectrom. 1992, 6, 249-253. (c) Bateman, R. H.; Brown, J .; Lefevere,
M.; Flammang, R.; Haverbeke, Y. Van. Int. J . Mass Spectrom. Ion
Processes 1992, 115, 205.
(23) Pople, J . A.; Head-Gordon, M.; Raghavachari, K. J . Chem. Phys.
1987, 87, 5968.
(24) (a) Becke, A. D. J . Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(29) Han, S. W.; Kim, K. J . Phys. Chem. 1996, 100, 17124-17132.
(30) Barnes, A. J .; Hallam, H. E.; Howells, J . D. R. J . Chem. Soc.,
Faraday Trans 2 1972, 68, 737.
(31) Barnes, A. J .; Howells, J . D. R. J . Chem. Soc., Faraday Trans
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(25) Huang, X.; Chen, B.-C. Synthesis 1986, 967. 1987, 481.

Cyanoketene and Iminopropadienones
Met h yl 3-Met h oxy-3-(isop r op yla m in o)p r op -2-en e-
J . Org. Chem., Vol. 62, No. 13, 1997 4247
the FVT tube at 600 °C/1.5 × 10-3 mbar in the course of 2 h.
The products were collected in a liquid nitrogen (-196 °C)
cooled U-tube which had been previously coated with methanol
(2 mL). Upon completion of the pyrolysis, additional methanol
(2 mL) was injected onto the pyrolysate. The apparatus was
flushed with nitrogen and the U-tube allowed to warm to room
temperature. The methanol solution was collected and the
solvent removed in vacuo. The resulting yellow oil was filtered
through a plug of silica gel (ca. 1 cm3), purified by column
chromatography (silica gel; ethyl acetate/hexane), and shown
to be identical with the previously described N,N-dimethyl-
cyanoacetamide,8a 34 mg (69%).
1
th iola te (11). Rf ) 0.80, 186 mg (58%), yellow oil; H NMR
(CDCl3) δ 1.73 (d, 6 H), 2.26 (s, 3 H), 3.57 (s, 3 H), 3.76 (sept,
1 H), 4.33 (s, 1 H), 8.76 (br s, 1 H); 13C NMR (CDCl3) δ 14.2;
23.4; 45.5; 50.0; 76.1; 165.2; 169.6; IR (CHCl3) ν 2957, 1693,
1439, 1399, 1273, 1012 cm-1; MS m/z 189 (M+, 67%), 174 (54),
158 (36), 142 (44), 100 (100), 84 (54), 68 (35); HRMS: calcd
for C8H15NO2S m/z 189.0822; found 189.0822. Anal. Calcd
for C8H15NO2S; C, 50.77; H, 7.99; N, 7.40. Found: C, 50.62;
H, 8.06; N, 7.57.
Meth yl 3-(Isopr opylam in o)-3-(m eth ylth io)pr op-2-en oate
1
(10). Rf ) 0.60, 61 mg (19%), yellow oil; H NMR (CDCl3) δ
1.39 (d, 6 H), 2.23 (s, 3 H), 3.39 (s, 3 H), 3.65 (s, 1 H), 8.75 (br
s, 1 H); IR (CHCl3) ν 2960 (CH), 1694 (CO), 1444, 1340, 1001
cm-1; MS m/z 189 (M+, 32%), 174 (16), 158 (100), 142 (65),
124 (64), 82 (51), 68 (91); HRMS: calcd for C8H15NO2S m/z
189.0822; found 189.0822. Anal. Calcd for C8H15NO2S: C,
50.77; H, 7.99; N, 7.40. Found: C, 50.71; H, 7.86; N, 7.36.
P r ep a r a tive F VT of 5-[(ter t-Bu tyla m in o)(m eth ylth io)-
Ack n ow led gm en t. The Brisbane laboratory thanks
the Australian Research Council for financial support
and for a Fellowship for M.W.W. The Mons laboratory
thanks the Fonds National de la Recherche Scientifique
for its contribution toward the acquisition of the VG
(Micromass) Autospec 6F tandem mass spectrometer.
m eth ylen e]-2,2-d im eth yl-1,3-d ioxa n e-4,6-d ion e (3h ).
A
sample (120 mg, 0.48 mmol) was sublimed at ca. 80 °C through
J O9701288