Bis(ketenyl)benzenes: Preparation, observation, and reaction with tetramethylpiperidin-1-yloxyl

Article abstract of DOI:10.1023/A:1015053418740

1,2- and 1,3-Bis(ketenyl)benzenes formed by double dehydrochlorination and by double Wolff rearrangement, respectively, gave ketenyl IR absorption at 2115, and 2122, and 2116 cm^-1, respectively. Reaction of these bisketenes with the aminoxyl ra

Full text of DOI:10.1023/A:1015053418740

2130
Russian Chemical Bulletin, International Edition, Vol. 50, No. 11, pp. 2130—2133, November, 2001
Bis(ketenyl)benzenes: preparation, observation,
and reaction with tetramethylpiperidin-1-yloxyl
A. D. Allen,a H. Rangwala,a K. Saidi,b Th. T. Tidwell,a« and J. Wanga
aDepartment of Chemistry, University of Toronto,
Toronto, M5S 3H6 Ontario, Canada.
Fax: +1 (416) 287 7279. E-mail: ttidwell@chem.utoronto.ca
bDepartment of Chemistry, Shahid Bahonor University of Kerman,
76169 Kerman, Iran
1,2- and 1,3-Bis(ketenyl)benzenes formed by double dehydrochlorination and by double
Wolff rearrangement, respectively, gave ketenyl IR absorption at 2115, and 2122, and
2116 cm–1, respectively. Reaction of these bisketenes with the aminoxyl radical
tetramethylpiperidin-1-yloxyl (TEMPO) gave the corresponding tetraadducts as mixtures of
meso- and d,l-isomers. The kinetics of the reaction of 1,3-bis(ketenyl)benzene with TEMPO
gave a rate constant comparable to that of the monoketene PhCH=C=O. The reactions
proceed by the initial attack of TEMPO on the carbonyl carbon of one ketenyl group
followed by fast capture of the intermediate radical by a second TEMPO, and then reaction
of the remaining ketene.
Key words: ketenes, diazoketones, Wolff rearrangement, nitroxyl radical TEMPO.
Diphenyketene (Ph2C=C=O) prepared1 from the re-
action of Ph2CClCOCl with zinc (Scheme 1) was the
first ketene to be isolated and characterized. This was
followed by the preparation of the isolable fluorenyl-
ideneketene by the same route.2 However, when
PhCH=C=O (1) was prepared by this method3 the
ketene could not be isolated, although there was evi-
dence for its presence in solution from trapping reac-
tions. The difficulty in isolation of ketene 1 is due to its
tendency for dimerization and its reactivity toward water
and other nucleophiles.
Scheme 2
Ph
O
O
H
H
H2O
PhCH=C=O
O
H
PhCH2CO2H
1
H
Scheme 3
∆H1
∆H2
•
+
CH2=C=O
H2NO
Scheme 1
•
O
H2NOCH2C=O
•
CH2C
Zn
Ph2CClCOCl
Ph2C=C=O
ONH2
Later4—8 the technique of flash photolysis was ap-
plied to the preparation of ketene 1, allowing direct
observation by fast UV spectroscopy5,6 and time-re-
solved IR spectroscopy.7 Kinetic studies of the reactivity
of compound 1 with water and with amines showed this
ketene was highly reactive, with a rate constant for
reaction with H2O greater than that for CH2=C=O by a
factor of 100.5,6 This rate effect is attributed to delocal-
ization of charge by the phenyl group in the transition
state for hydration (Scheme 2).8
–1
∆H1 = 7.5 kcal mol
–1
∆H2 = –18.7 kcal mol
This prediction was confirmed experimentally by the
discovery that ketenes such as 1 undergo facile reaction
with the aminoxyl radical tetramethyl-
piperidin-1-yloxyl (TEMPO, TO )
forming 1,2-bis(addition) products
(Scheme 4), or in some cases prod-
ucts resulting from allylic rearrange-
ments, cyclizations, or free radical
rearrangements.10—14 These reactions
•
N
•
O
Recently9—14 we have undertaken the study of radi-
cal reactions of ketenes, and predicted on the basis of
B3LYP/6-31G* calculations that the reaction of
CH2=C=O at the α-carbon would be exothermic
•
TEMPO (TO )
were all interpreted as involving rate
limiting attack of TEMPO on the carbonyl carbon as
shown in Scheme 4, and in the case of ketene 1 this was
–1
(∆H2 = –18.7 kcal mol ) (Scheme 3).10
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2034—2037, November, 2001.
1066-5285/01/5011-2130 $25.00 © 2001 Plenum Publishing Corporation

Bis(ketenyl)benzenes: properties and reactions
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 11, November, 2001 2131
followed by fast capture of the resulting radical by a
second TEMPO.11,12
for bisketene 2, and compound 3 has been prepared by
the classical Wolff rearrangement, as we have recently
demonstrated for monoketenes.11—13
Reaction of the bis(acyl chloride) 5 22 with amine 4
and Et3N in toluene at –78 °C gave immediate forma-
tion of an IR band at 2115 cm–1 indicative of formation
of bisketene 2. The successive addition of TEMPO gave
the tetraadduct 6 (23%), as a 2 : 1 mixture of meso- and
d,l-isomers that were not differentiated (Scheme 5). The
rapid formation of the ketene IR band, and the capture
of the intermediate as 6, are strong evidence for the
intermediacy of bisketene 2.
Scheme 4
•
•
TO
•
TO
PhCH=C=O
PhCHCO2T
PhCH(OT)CO2T
1
Our study of ketene 1 was greatly facilitated by the
discovery that this and other reactive ketenes could be
prepared by Wolff rearrangements in dilute solutions in
hydrocarbon solvents as relatively long-lived species at
room temperature, and their presence could be estab-
lished using conventional IR spectroscopy. Thus ketene
Scheme 5
1 was identified11,12 by its IR band at 2117 cm , in
–1
Me2N
NMe2
good agreement with the value found by TRIR studies.7
The preparation of bisketenes15 has been a major
goal of chemical research since first reports16 on system-
atic efforts to prepare α,ω-bisketenes of the type
O=C=CH(CH2)nCH=C=O beginning with the first term
of the series (CH=C=O)2 for n = 0, which may be
termed a 1,2-bisketene. These species have been of great
interest because of their potential as precursors for poly-
ester and polyamide preparation, but although carbon
suboxide (O=C=C=C=O) was reported previously,17,18
efforts to prepare α,ω-bisketenes of the type indicated
above as observable intermediates have been unsuccess-
ful.8,15 Bisketenes of the aromatic series have been
examined,19 including that of the type A,20 which can be
isolated and is the most highly studied aryl bisketene.
CH2COCl
CH2COCl
C
C
O
O
4
•
TO
5
2
CH(OT)CO2T
CH(OT)CO2T
6
Photolysis with 350-nm light of a 7•10–5 M solution
in isooctane of the bis(diazoketone) 7 23,24 derived from
isophthalic acid resulted in the disappearance of the IR
–1
absorption of diazoketone 7 at 2110 cm
and the
–1
appearance of a pair of bands at 2122 and 2116 cm
(rel. intensity 43 : 57) ascribed to bisketene 3. Addition
of BunNH2 to the preformed 3 gave bisamide 8 de-
scribed previously.25,26 Addition of TEMPO to a solu-
tion of 3 gave after chromatography adduct 9 (68%), as
a 55 : 45 mixture of meso- and d,l-isomers that were not
differentiated (Scheme 6). The complete conversion of
O
C
C
O
A
Our successful preparation of the highly reactive
aldoketene 1 as an observable intermediate in solution
encouraged us to believe that the replacement of more
than one ketenyl group on an aromatic scaffold might be
possible, and this goal has now been reached. We report
the successful preparation, observation, and study of
1,2- and 1,3-bis(ketenyl)benzenes (2, 3). The keys to
this success are the use of dilute solutions and moderate
temperatures to prevent dimerization reactions, and dry
hydrocarbon solvents to prevent reaction with water.
Scheme 6
O
O
•
TO
C
C
hν
O
O
N2
N2
7
3
TO
OT
Results and Discussion
TO2C
CO2T
Dehydrochlorination by a known procedure21 with
9
1,8-bis(dimethylamino)naphthalene (4) has been used

2132
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 11, November, 2001
Allen et al.
hexane (380 mL) was irradiated for 5 min with λ = 300 and
350 nm light. Then BunNH2 (5.1 mmol) was added, and the
solvent was evaporated. The known25,26 N,N´-dibutylbenzene-
1,3-bisacetamide (8) (m.p. 144—145 °C) was isolated
in 26% yield upon chromatography eluting first with
ether—CHCl3, 1 : 1 and then with MeOH—CHCl3, 1 : 9.
1H NMR (CDCl3), δ: 0.88 (t, 6 H); 1.22—1.32 (m, 4 H);
1.37—1.47 (m, 4 H); 3.17—3.25 (m, 4 H); 3.54 (s, 4 H); 5.40
(br.s, 2 H); 7.2—7.4 (m, 4 H). 13C NMR (CDCl3), δ: 13.7,
20.0, 31.6, 39.5, 43.7, 128.3, 129.5, 130.5, 135.8, 170.5.
IR (CDCl3), ν/cm–1: 1662. MS (EI), m/z (Irel (%)): 304 [M]+
(12), 232 (12), 205 (100). HRMS (EI), found: m/z 304.2153.
C18H28N2O2. Calculated: M = 304.2151. Similar irradiation of
a 0.07 mM solution of 7 in isooctane for 8 min resulted in the
the IR absorption of the bands of diazoketone 7 to
ketenyl bands, and the capture of the intermediate with
both BunNH2 and with TEMPO are convincing evi-
dence for the formation of 3.
The kinetics of this reaction (see Scheme 6) were
monitored as previously,11—13 and gave single exponen-
tial decay and a derived value of the rate constant (k2) of
–1 –1
2.44 L mol
s , which is comparable to a value for
PhCH=C=O (1.26 L mol s ).10,11 This result indi-
cates that bisketene 3 reacts with TEMPO by initial
reaction on one ketenyl moiety followed by reaction of
the second ketene with a rate constant either similar to
or faster than the first.
–1 –1
–1
disappearance of the IR absorption of 7 at 2110 cm and the
appearance of bands at 2122 and 2116 cm–1, which disap-
peared upon the addition of BunNH2.
In summary the highly reactive bis(ketenyl)benzenes
2 and 3 have been prepared as observable and relatively
long lived intermediates for the first time, and utilized in
synthetic and mechanistic investigations. The way is
now open for the preparation of further examples,
and for the exploitation of their multifunctional reac-
tivity.
A solution of diazoketone 7 (6 mg, 0.026 mmol) in hexane
(370 mL) was irradiated for 5 min with λ = 300 and 350 nm
light, and TEMPO (50 mg, 0.32 mmol) in hexane (0.05 mL)
was added. After 12 h the solvent was evaporated and the
residue chromatographed successively with CH2Cl2, ether, and
MeOH. In the MeOH fraction was found adduct 9 (14 mg,
68%) as a clear liquid which on addition of MeOH crystal-
lized, m.p. 183—186 °C (meso- and d,l-isomers in a ratio of
59 : 41, not separated). 1H NMR (CDCl3), δ: 0.4—1.7 (m,
72 H); 5.25 (s, CHOT, major isomer); 5.27 (s, CHOT, minor
isomer); 7.3—7.7 (m, 4 H). 13C NMR (CDCl3), δ: 16.6, 16.8,
19.6, 20.6, 31.3, 32.0, 33.3, 34.6, 39.1, 40.1, 59.2, 60.4, 87.3,
87.7, 126.0, 127.1, 127.2, 127.5, 127.7, 138.9, 139.1, 170.2,
170.25. IR (CDCl3), ν/cm–1: 1774, 1751 (sh). MS (EI),
m/z (Irel (%)): 626 [M – TO]+ (0.5), 156 [TO]+ (65), 69 (100).
HRMS, found: m/z 626.4550. C37H60N3O5. Calculated:
M = 626.4533. MS (electrospray), m/z: 783.5 [MH]+.
Experimental
Reactions were carried out under an atmosphere of argon
or nitrogen. 1H NMR spectra were obtained at 200 MHz
(Varian Gemini), 300 MHz (Varian Mercury), and 400 MHz
(Varian Unity). 13C NMR spectra were obtained on a Varian
Unity instrument (100 MHz). IR spectra were obtained on a
Perkin—Elmer FT-IR Spectrum 1000 spectrum. Bis(acyl chlo-
ride) 5 was synthesized22 from 1,2-benzenediacetic acid
(Aldrich) by reaction with SOCl2 and purified by sublimation.
Bis(diazo ketone) 7 (1H NMR (CDCl3), δ: 5.98 (s, 2 H);
7.50—7.56 (m, 1 H); 7.92—7.96 (m, 2 H); 8.13—8.16 (m, 1 H))
was obtained23,24 from bis(acyl chloride) of isophthalic acid
(Aldrich). Commercial 1,8-bis(dimethylamino)naphthalene
(Aldrich) was used. Photolyses were conducted using a Rayonet
photoreactor. Chromatography was carried out on SiO2
(Aldrich).
Kinetics of the reaction of bisketene 3 with TEMPO.
–6
Ketene solutions (2•10 M) in isooctane from irradiation of
–4
diazoketene 7 were reacted with (1.6—8.0)•10
M TEMPO
solutions and the decrease in the concentration of 3 at
–1
λ = 244 nm was monitored. The k2 rate constant (L mol–1 s
was derived from a plot of kobs (s–1) vs. [TEMPO] (mol L
described by the equation
)
)
–1
1,2-Bis(ketenyl)benzene (2) and its reaction with TEMPO.
To chloride 5 (100 mg, 0.4 mmol) in toluene (3 mL) at –78 °C
was added amine 4 (195 mg, 0.90 mmol) in toluene (2 mL),
and then Et3N (10 µL) was added. There was immediate
–4
kobs = (2.44±0.22)•[TEMPO] + (7.59±1.21)•10
(dimensionality of the last term of the equation is s–1).
formation of a yellow color and precipitation of the hydrochlo-
This work was financially supported by the Natu-
ral Sciences and Engineering Research Council of
Canada.
–1
ride salt of 4, and generation of a ketene IR band at 2115 cm
.
TEMPO (277 mg, 177 mmol) in toluene (2 mL) was added,
and the solution was stirred for ∼15 h at ∼20 °C, filtered, and
evaporated. Aqueous 1 N HCl was added to the residue, and
the mixture was extracted with ether. The organic layer
was dried, concentrated, and chromatographed (eluent
EtOAc—hexane, 1 : 3) to give tetraadduct 6 (78 mg, 23%) as a
2 : 1 mixture of the meso- and d,l-isomers (not differentiated).
1H NMR (CDCl3), δ: 0.6—1.6 (m, 72 H); 5.88 (s, CHOT,
minor isomer); 6.00 (s, CHOT, major isomer); 7.3—8.0 (m,
4 H, Ar). 13C NMR (CDCl3), δ: 16.8, 16.9, 17.0, 20.1, 20.4,
20.5, 20.6, 21.2, 31.5, 31.8, 33.2, 34.0, 39.2, 39.3, 39.5, 40.1,
40.2, 40.8, 60.0, 60.3, 60.5, 76.9, 77.2, 77.5, 82.2, 83.8, 127.4,
128.1, 128.9, 129.7, 135.9, 136.3, 169.0, 169.7. IR (CDCl3),
ν/cm–1: 1771, 1757. MS (EI), m/z (Irel (%)): 642 [Ì – Ò]+
(0.6), 156 (80), 140 (100). HRMS (EI), found: m/z 642.4492.
C37H60N3O6. Calculated: M = 642.4482.
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Received May 3, 2001