Aminoxyl radical addition to arylketenes

Article abstract of DOI:10.1002/poc.1036

Kinetic studies of the addition of the aminoxyl radical TEMPO (2,2,6,6-tetramethylpiperidinylaminoxyl, TO) to the 4-substituted phenylketenes 8 (4-RC₆H₄CH=C=O, R=NO₂, CN, Cl, H, CH ₃, CH₃O) and to 3-p

Full text of DOI:10.1002/poc.1036

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2006; 19: 841–846
Published online 3 November 2006 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.1036
Aminoxyl radical addition to arylketenesy
*
Austin Acton, Annette D. Allen, Andrei V. Fedorov, Huda Henry-Riyad and Thomas T. Tidwell
Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada
Received 8 August 2005; revised 26 January 2006; accepted 28 January 2006
ABSTRACT: Kinetic studies of the addition of the aminoxyl radical TEMPO (2,2,6,6-tetramethylpiperidinylami-
— —
noxyl, TOꢀ) to the 4-substituted phenylketenes 8 (4-RC H CH
C
O, R¼NO , CN, Cl, H, CH , CH O) and to 3-
—
—
6
4
2
3
3
pyridylketene (15) reveal a correlation of logk2(TEMPO) with the Hammett sigma parameters of the substituents
(including the aza substituent of 15), with rho ¼ 1.58 (r ¼ 0.94). A better correlation is obtained with the rate constants
for hydration of the same substrates: logk2(TEMPO) ¼ 1.64logk(H2O)ꢁ5.76 (r ¼ 0.98). These results fit within a
previously established correlation of ketene hydration reactivity with aminoxyl radical reactivity, and confirm that the
reactivity of TEMPO in additions to ketenes is dominated by the nucleophilic character of the aminoxyl oxygen, with a
strong trend of increasing reactivity for more electrophilic ketenes. Copyright # 2006 John Wiley & Sons, Ltd.
KEYWORDS: ketenes; TEMPO; free radical addition; substituent effects
INTRODUCTION
reactivity with conjugating ketene substituents,1 con-
jugate additions to substrates with unsaturated substi-
tuents Eqn (2)],1c,d cyclopropylcarbinyl radical ring
opening in substrates with cyclopropyl substituents,1f
trapping by oxygen of stabilized enolic radicals from
diarylketenes,1b and a correlation of ketene reactivity
with aminoxyl reactivity compared to hydration reactions
involving data for16 ketenes Eqn (3)].1c,d,h
Previous studies of the reactivity of ketenes with the
aminoxyl radical 2,2,6,6-tetramethylpiperidinylaminoxyl
(TEMPO, TOꢀ) have provided convincing evidence that
this reaction proceeds by initial oxygen attack by TEMPO
in the ketene plane on the carbonyl carbon to give enolic
radical intermediates which are ordinarily captured by
addition of a second TEMPO radical at the enolic carbon
[Eqn (1)].1 For ketenes with unsaturated substituents such
as vinyl or alkynyl groups conjugate addition may occur
[Eqn (2)].1c–e
logkðTEMPOÞ
¼ 1:18 logkðH2OÞ ꢁ 4:45ðr ¼ 0:94Þ
(3)
(1)
n
n-Bu
O
n-Bu
-Bu
O
TO•
TO•
C
O
•
TO
OT
OT
1
3
2
+•
N
N
_
O•
TO•
O
4
Ph
Ph
Ph
O
O
TO•
TO•
(2)
C
O
TO
•
OT
OT
5
7
6
Evidence for this reaction pathway includes compu-
tational studies,1b,g the large kinetic accelerations of the
This correlation, however, may seem surprising, in
that radicals are electron deficient species, and might
be expected to be electrophilic in character, and not
nucleophilic. However, nitroxyl radicals are often
named as ‘nitroxides,’ emphasizing the high-electron
density on oxygen, as shown in the resonance structure 4.
Indeed detailed study of the factors controlling
*Correspondence to: T. T. Tidwell, Department of Chemistry, Univer-
sity of Toronto, Toronto, Ont. M5S 3H6, Canada.
E-mail: ttidwell@chem.utoronto.ca
yDedicated to Professor Norma Nudelman. This article is published as
part of the special issue Festschrift for Norma Nudelman.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 841–846

842
A. ACTON ET AL.
reactivity in additions of carbon centered radicals to
alkenes emphasizes that the variable polar character of
different types of radicals plays a major role in these
processes.2
substituted benzoyl chlorides with diazomethane in ether.
Photolysis of the diazo ketones in hexane with 254 or 300
nm light gave the corresponding ketenes 8, identified using
conventional FTIR by their distinctive absorption at 2116,
2116, 2119, 2123, and 2123 cmꢁ1, respectively [Eqn (5)].
Qualitatively it was apparent that the ketenes were
progressively shorter lived as the substituents became
increasingly more electron withdrawing. Thus, for the
4-MeO and 4-Me substituents the IR absorptions were
strong and relatively long-lived, while those for the 4-NO2
and 4-CN substituents were weaker, and transient. The UV
spectra of the ketenes 8 in isooctane were also measured,
and minimum values of the extinction coefficients were
estimated based on the assumption that the diazo ketones 11
were completely converted to ketenes 8 (Table 1).
One of the most useful criteria for the interpretation of
addition reactions to alkenes has been the effect of aryl
substituents on the reactivity of styrenes, and this criterion
has been especially applied to electrophilic additions,
including styrene protonation.3 Arylketenes may be
considered as a special class of styrenes, and indeed
the reported correlation of the hydration reactivity of
substituted phenylketenes 8 using Hammett sigma
parameters with a slope rho ¼ 1.2 has been taken as
strong evidence that these reactions are polar additions
with nucleophilic attack on carbonyl carbon with
R
R
R
O
N2
O
O
TO•
TO•
hν
(5)
C O
•
OT
TO
OT
R
12
8
13
11
zwitterionic transition states 9 leading to acid enol
intermediates 10, which lead to the carboxylic acids
[Eqn (4)].4 This result argues against an earlier proposal
that these were concerted reactions with rate limiting
proton transfer to C(2) through cyclic transition states,
forming the acids directly.4c–d The present study reports
the examination of the influence of different substituted
aryl groups on the addition of TEMPO to arylketenes to
further elucidate the nature of the transition state of this
reaction.
Product studies of the reactions of the ketenes 8 with
TEMPO in hexane were carried out by adding TEMPO to
solutions of the ketenes generated by photolysis of the
diazo ketones in hexane for the 4-MeO, 4-Me, and 4-Cl
substituted derivatives, while for the 4-CN and 4-NO2
substituted compounds photolysis was carried out in the
presence of TEMPO for in situ trapping. The 1,2-
diaddition products 13 were isolated in 25–49% yields
[Eqn (5)]. 3-Pyridylketene (15) was also generated by
photochemical Wolff rearrangement of the diazo ketone
R
R
R
R
H O
2
δ−
O
OH
OH
O
(4)
C O
δ+
OH
OH
2
9
8
10
14, as we have done previously for examination of its
reactivity in amination5 and hydration,5b and reacted with
TEMPO to give the 1,2-diaddition product 16 [Eqn (6)].
This substrate may be included in a Hammett type
correlation by using a sigma constant for the 3-aza group
of 0.55.5c,d
RESULTS
4-Substituted diazoacetylbenzenes 11 with MeO, Me, Cl,
CN, and NO2 substituents were prepared by reaction of the
N
N
N
O
N2
O
O
TO•
TO•
N
hν
C O
•
(6)
OT
TO
OT
15
16
14
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 841–846

AMINOXYL RADICAL ADDITION TO ARYLKETENES
Table 1. Ketenyl IR absorption and UV spectra of arylke-
843
tenes 8 and 15
UV (lmax nm),
" (isooctane)b
R
IR (cmꢁ1 a
)
MeO (8a)
Me (8b)
H (8c)
Cl (8d)
CN (8e)
2116
2116
2118
2119
2123
251 (6.3 ꢂ 103), 390 (25)
250 (1.8 ꢂ 104), 380 (120)
248 (1.2 ꢂ 104), 378 (47)
255 (1.4 ꢂ 104), 370 (72)
218 (1 ꢂ 104), 224 (1 ꢂ 104),
273 (1.6 ꢂ 104), 370 (93)
NO2 (8f)
2123 225 (5.4 ꢂ 103), 261 (4 ꢂ 103),
317 (7.4 ꢂ 103)
Ar ¼ 3-Pyridyl (15) 2123
a Hexane.
b Minimum values of " assuming complete conversion of the diazo ketones
to the ketenes.
250 (4.3 ꢂ 103), 290 (7 ꢂ 102)
Rate constants were measured for the reaction of the
4-substituted phenylketenes 8 and for 3-pyridylketene (15)
with different concentrations of excess TEMPO under
pseudo first order conditions by measuring the decrease in
the UVabsorption of the ketenes as carried out previously,1c–h
and plots of the measured rate constants versus the TEMPO
concentration gave second order rate constants, as reported
in Table 2, together with previous results1c,d for phenylke-
tene (8c, R¼ H). Also included in Table 2 are the reported
rate constants for hydration of these 4-substituted arylke-
tenes, as measured by the change in conductivity of the
solutions.6a When the hydration of phenylketene (8c) is
observed by UV spectroscopy, two processes are observed,
conversion of the ketene to the acid enol, and conversion of
the acid enol to the carboxylic acid.5e The observed rate
constants for generation of the carboxylic acids measured by
conductivity should correspond closely to the rate constants
for nucleophilic attack of water on the ketene.6a
Figure 1. Plot of logk2(TEMPO) versus sigma (para) con-
stants for arylketenes 8 and 15
vary by a factor of 52 for the different substituents,
and give a correlation with the Hammett sigma constants
(Fig. 1) with a slope of 1.6 and a correlation coefficient
r ¼ 0.94 [Eqn (7)]. This compares with the rho value of
1.2 found for the rate constants for hydration of the
phenylketenes 8 with the sigma constants.6 The
correlation coefficient of 0.94 is only fair, but the trend
in reactivity with higher reactivity for the ketenes with the
more strongly electron withdrawing substituents is
unmistakable, and agrees with the previously found
parallel between ketene reactivity with nitroxyl radicals
and with water, showing the nucleophilic character of the
reaction.1c–h
log k2ðTEMPOÞ ¼ 1:56s þ 0:42 ðr ¼ 0:94Þ (7)
Correlation of logk2 for reaction of ketenes with
TEMPO with logk for reaction with water (Fig. 2) gives a
better correlation, with a correlation coefficient of 0.98
[Eqn (8)]. This result confirms the utility of the study of
arylketenes for the examination of the parallel between
hydration reactions and TEMPO addition, and shows
the strong nucleophilic character of the aminoxyl radical
in this reaction. The formation of radical intermediates in
these reactions of TEMPO with ketenes is confirmed
by the addition of two TEMPO molecules, the ring
opening of cyclopropylcarbinyl intermediates, and the
reaction of the intermediates with molecular oxygen.1b
DISCUSSION
The rate constants for reaction of the 4-substituted
phenylketenes 8 and 3-pyridylketene 15 with TEMPO
Table 2. Rate constants for TEMPO addition and for
hydration of arylketenes 8 (4-RC H CH C O) and 3-
pyridylketene (15)
–
–
–
–
6
4
R
Sigmaa k2(TEMPO) 258C, sꢁ1 Mꢁ1 k(H2O) sꢁ1b
CH3O (8a) ꢁ0.27
CH3 (8b) ꢁ0.17
2.26
1.23
1.26
5.55
43.4
63.6
11.0
4.5 ꢂ 103
3.8 ꢂ 103
4.9 ꢂ 103
9.6 ꢂ 103
2.56 ꢂ 104
4.95 ꢂ 104
1.6 ꢂ 104e
H (8c)c
Cl (8d)
CN (8e)
NO2 (8f)
3-Py (15)
0.0
0.23
0.66
0.78
0.55d
log k2ðTEMPOÞ
¼ 1:64 log kðH2OÞ ꢁ 5:76 ðr ¼ 0:98Þ
(8)
a Ref. unless noted.
3c
A new correlation of rate constants for ketene reactions
with TEMPO and with water including the previous
results and the results for the reaction of the arylketenes
measured is given in Fig. 3 [Eqn (9)]. This includes 22
b Ref. 6a, unless noted.
1c,d
c Refs.
d Ref.
e Ref.
.
5c,d
.
5b
.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 841–846

844
A. ACTON ET AL.
In summary, there is a correlation of the rate constants
for addition of the aminoxyl radical TEMPO to arylketenes
with the rate constants for hydration (Fig. 2), confirming
the nucleophilic character of the aminoxyl radical. The
results also fit with the general correlation of TEMPO
reactivity with ketenes compared to hydration reactivity.
EXPERIMENTAL
The diazo ketones 11 are all known compounds,6a and
were prepared by reaction of the acyl chlorides with
diazomethane.
Reaction of 4-Methoxyphenylketene
(8a) with TEMPO
Figure 2. Plot of logk2(TEMPO) versus logk(H2O) for arylk-
etenes 8 and 15
4-Methoxydiazoacetophenone (11a, 0.10 g, 0.57mmol)
and TEMPO (0.18g, 1.2mmol, 2.1 equiv) in hexane
(75 mL) were photolyzed 1 h with 300 nm light and left
stirring for 1 h at room temperature. The solution was
concentrated and chromatographed using CH2Cl2 to give
the bis(TEMPO) adduct 13a (0.090 g, 0.20 mmol, 34%).
1H NMR (400mHz, CDCl3) d 0.47 (s, 3), 0.65 (s, 3), 0.84
(s, 3), 1.03 (s, 3), 1.12 (s, 3), 1.17 (s, 3), 1.23 (s, 3), 1.26 (s,
3), 1.36–1.62 (m, 12), 3.80 (s, 3), 5.28 (s, 1), 6.83 (d, 2,
J ¼ 8.6 Hz), 7.38 (d, 2, J ¼ 8.6 Hz). 13C NMR (100 mHz,
CDCl3) 17.1, 17.3, 20.2, 20.8, 22.9, 31.2, 32.2, 33.7, 34.5,
39.4, 39. 6, 40.5, 55.4, 60.1, 60.6, 87.3, 113.7, 128.5, 129.0,
129.7, 132.0. IR (CDCl3) 1786 cmꢁ1. EIMS m/z 304
(Mþ-OT), 243, 165, 126, 83, 69, 56. HREIMS m/z calcd for
C18H26NO3 (Mþ-TO) 304.1912, found 304.1913. Pho-
tolysis of 11a in hexane in the absence of TEMPO gave a
ketenes, and covers a range of 107 in hydration reactivity,
and 109 in reactivity with TEMPO.
log k2ðTEMPOÞ
¼ 1:22 log kðH2OÞ ꢁ 4:93 ðr ¼ 0:94Þ
(9)
A possible reason for the even greater variation in
reactivity as a function of structure for the reaction with
TEMPO than for the reaction with water is that in
hydration reactions more than one water molecule
participates, leading to an enol intermediate 10 by addition
to the ketene carbonyl [Eqn (4)].4a,5e In the addition of
TEMPO an enolic radical intermediate is formed Eqn (6)],
and so there is more conjugation possible with the aryl
substituent compared to the hydration reaction.
solution displaying the ketenyl IR at 2116 cmꢁ1
.
4
2
Reaction of 4-Methylphenylketene
(8b) with TEMPO
4-Methyldiazoacetophenone (11b, 0.060 g, 0.38 mmol) in
hexane (25 mL) was photolyzed 25 min with 254 nm light
to generate the ketene 8b (IR 2116 cmꢁ1). TEMPO
(0.12 g, 0.79 mmol, 2.1 equiv) was added, and the solution
was stirred for 2 h at room temperature. The solution was
concentrated and chromatographed using CH2Cl2 to give
the bis(TEMPO) adduct 13b (0.080 g, 0.18 mmol, 47%).
1H NMR (400 mHz, CDCl3) d 0.48 (s, 3), 0.69 (s, 3), 0.87
(s, 3), 1.06 (s, 3), 1.09 (s, 3), 1.16 (s, 3), 1.27 (s, 3), 1.30 (s,
3), 1.35–1.63 (m, 12), 2.36 (s, 3), 5.23 (s, 1), 7.15 (d, 2,
J ¼ 8.1 Hz), 7.36 (d, 2, J ¼ 7.9 Hz). 13C NMR (100 mHz,
CDCl3) 17.0, 17.2, 20.2, 20.6, 21.3, 30.9, 32.0, 33.5, 34.2,
39.3, 39.8, 40.9, 59.2, 60.4, 87.5, 127.5, 128.8, 136.4,
137.5, 171.1. IR (CDCl3) 1773 cmꢁ1. HREIMS m/z calcd
for C27H45N2O3 (Mþ) 445.3428, found 435.3424.
0
-2
-4
-6
-8
-2
-1
0
1
2
3
4
5
6
7
log kH O
2
Figure 3. Plot of logk2(TEMPO) versus logk(H2O) for
ketenes (filled circle is for 3-pyridylketene, 15)
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 841–846

AMINOXYL RADICAL ADDITION TO ARYLKETENES
845
Reaction of 4-Chlorophenylketene
(8d) with TEMPO
(100 mHz, CDCl3) 16.9, 17.0, 20.1, 20.3, 20.6, 31.0, 31.9,
33.3, 34.2, 39.2, 39.4, 40.3, 60.1, 60.6, 87.2, 111.9, 118.7,
128.3, 132.2, 144.6. IR (CDCl3) 1776 cmꢁ1. HREIMS m/
z calcd for C27H42N3O3 (Mþ) 456.3220, found 456.3222.
4-Chlorodiazoacetophenone (11d, 0.080 g, 0.44 mmol) in
hexane (75 mL) was photolyzed 20 min with 254nm light
to generate the ketene 8d (IR 2119 cmꢁ1). TEMPO (0.14 g,
0.92 mmol, 2.1 equiv) was added, and the solution was
stirred for 2 h at room temperature. The solution was
concentrated and chromatographed using 1:9 EtOAc/
hexane to give the bis(TEMPO) adduct 13d (0.10 g,
Reaction of 4-Nitrophenylketene
(8f) with TEMPO
4-Nitrodiazoacetophenone (11f, 0.050 g, 0.26 mmol) and
TEMPO (0.090 g, 0.55 mmol, 2.1 equiv) in hexane
(100 mL) were photolyzed 35 min with 300 nm light
and left stirring for 1 h. The solution was concentrated and
chromatographed using CH2Cl2 to give the bis(TEMPO)
adduct 13f (0.030 g, 0.063 mmol, 25%). 1H NMR
(400 mHz, CDCl3) d 0.46 (s, 3), 0.61 (s, 3), 0.86 (s, 3),
1.06 (s, 3), 1.10 (s, 3), 1.16 (s, 3), 1.28 (s, 3), 1.31 (s, 3),
1.36–1.65 (m, 12), 5.40 (s, 1), 7.69 (d, 2, J ¼ 8.6 Hz), 8.27
(d, 2, J ¼ 8.6 Hz). 13C NMR (100 mHz, CDCl3) 16.8,
17.0, 20.2, 20.3, 20.6, 31.1, 32.0, 33.3, 34.2, 39.2, 39.4,
40.3, 60.2, 60.7, 87.0, 123.7, 128.4, 130.7, 146.6, 169.7.
IR (CDCl3) 1789 cmꢁ1. EIMS m/z 476 (Mꢁ), 319 (Mþ-
TO) 291, 270, 194, 164, 142, 89, 69, 56. HREIMS m/z
calcd for C26H42N3O5 (Mþ) 476.3124, found 476.3124.
Photolysis of 11a in hexane in the absence of TEMPO
1
0.21 mmol, 49%). H NMR (400 mHz, CDCl3) d 0.50 (s,
3), 0.67 (s, 3), 0.88 (s, 3), 1.06 (s, 3), 1.09 (s, 3), 1.14 (s, 3),
1.25 (s, 3), 1.29 (s, 3), 1.30–1.58 (m, 12), 5.28 (s, 1), 7.36
(d, 2, J ¼ 8.5Hz), 7.47 (d, 2, J ¼ 8.3Hz). 13C NMR
(100 mHz, CDCl3) 17.0, 17.2, 20.3, 20.7, 21.0, 31.1, 32.1,
33.5, 34.4, 39.2, 39.4, 40.4, 60.1, 60.6, 87.0, 128.9, 130.9,
138.0, 141.8, 175.2. IR (CDCl3) 1777 cmꢁ1. HREIMS m/z
calcd for C26H42ClN2O3 (Mþ) 465.2878, found 465.2862.
Reaction of 4-Cyanophenylketene
(8e) with TEMPO
4-Cyanodiazoacetophenone (11e, 0.050 g, 0.29 mmol)
and TEMPO (0.090 g, 0.61 mmol, 2.1 equiv) in hexane
(100 mL) were photolyzed 25 min with 254 nm light and
left stirring for 1 h at room temperature. The solution was
concentrated and chromatographed using 3:7 EtOAc/
hexane to give the bis(TEMPO) adduct 13e (0.030 g,
0.066 mmol, 23%). 1H NMR (400 mHz, CDCl3) d 0.44 (s,
3), 0.60 (s, 3), 0.84 (s, 3), 1.04 (s, 3), 1.08 (s, 3), 1.14 (s, 3),
1.26 (s, 3), 1.29 (s, 3), 1.37–1.65 (m, 12), 5.33 (s, 1), 7.63
(d, 2, J ¼ 8.2 Hz), 7.67 (d, 2, J ¼ 8.3 Hz). 13C NMR
gave a solution displaying the keteneyl IR at 2123 cmꢁ1
.
Reaction of 3-Pyridylketene (3–15)
with TEMPO
3-Diazoacetylpyridine (3–14, 14 mg, 0.095 mmol)5a
and TEMPO (31 mg, 0.200 mmol, 2.1 equiv) in hexane
Table 3. Kinetics of ketene reactions with TEMPO (isooctane, 258C)
Ketene
Ketene [TEMPO] M ꢂ 103 kobs (sꢁ1) ꢂ 102a
Ketene
[TEMPO] M ꢂ 103 kobs (sꢁ1) ꢂ 102
8a (R ¼ MeO)b
4.11
3.29
2.46
1.644
0.822
4.11
3.29
2.46
0.934
0.766
0.567
0.373
0.202
0.562
0.485
0.389
0.274
0.164
2.72
8ee (R ¼ CN)c
1.644
0.822
0.358
0.286
0.215
0.143
0.358
0.286
0.215
0.143
0.0715
1.90
6.88
4.05
1.38
1.14
0.881
0.661
2.89
2.49
2.12
1.82
1.010
1.83
2.97
4.40
5.31
7.91
8b (R ¼ Me)c
8d (R ¼ Cl)d
8ff (R ¼ NO2)f
1.644
0.822
4.11
g
–
–
–
–
3.29
2.46
2.32
1.62
3–15 (3-PyCH C O)
3.81
4.76
5.71
7.62
1.644
0.821
1.20
0.994
a Average of duplicate runs at each concentration.
b kobs ¼ (2.26 ꢃ 0.04)sꢁ1 Mꢁ1[TOꢀ]þ(1.20 ꢃ 1.01) ꢂ 10ꢁ4 sꢁ1
.
c kobs ¼ (1.23 ꢃ 0.04)sꢁ1 Mꢁ1[TOꢀ]þ(7.2 ꢃ 1.0) ꢂ 10ꢁ4 sꢁ1
.
d kobs ¼ (5.50 ꢃ 0.34)sꢁ1 Mꢁ1[TOꢀ]þ(4.0 ꢃ 1.0) ꢂ 10ꢁ3 sꢁ1
.
ekobs ¼ (43.4 ꢃ 1.7)sꢁ1 Mꢁ1[TOꢀ]þ(1.25 ꢃ 12.3.) ꢂ 10ꢁ4 sꢁ1
.
f kobs ¼ (63.6 ꢃ 4.6)sꢁ1 Mꢁ1[TOꢀ]þ(6.9 ꢃ 1.0) ꢂ 10ꢁ3 sꢁ1
.
gkobs ¼ (11.0 ꢃ 1.13)sꢁ1 Mꢁ1[TOꢀ]ꢁ(7.33 ꢃ 6.12) ꢂ 10ꢁ4 sꢁ1
.
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 841–846

846
A. ACTON ET AL.
(30 mL) was degassed with argon over powdered
molecular sieves and photolyzed 8 min with 300 and
350 nm light. The solution was filtered, concentrated, and
chromatographed with 2:8 EtOAc/hexane containing 1%
Et3N to give the bis(TEMPO) adduct 3-15 (3 mg,
TEMPO by UV.1b–h Second order rate constants for
reaction were obtained from plots of kobs versus
[TEMPO] as reported in Table 3.
1
0.007 mmol, 7%). H NMR (400 mHz, CDCl3) d 0.45
REFERENCES
(s, 3), 0.62 (s, 3), 0.75 (s, 3), 0.85 (s, 3), 1.03 (s, 3), 1.05 (s,
3), 1.06 (s, 3), 1.14 (s, 3), 1.2–1.7 (m, 12), 5.32 (s, 1), 7.30
(m, 1), 7.84 (d, 1), 8.56 (d, 1), 8.70 (2, 1). 13C NMR
(100 mHz, CDCl3) 16.9, 17.0, 20.16, 20.23, 20.6, 20.8,
29.7, 31.1, 32.0, 33.3, 34.4, 39.5, 40.3, 60.5, 85.4, 123.5,
127.3, 129.7, 134.8, 149.5. IR (CDCl3) 1780 cmꢁ1. EIMS
m/z 276, 247, 182, 156, 140, 119, 91, 83, 69, 55.
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Reaction of 4-Pyridylketene (4–15)
with TEMPO
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4-Diazoacetylpyridine (21.3 mg, 0.145 mmol)5a and
TEMPO (46 mg, 0.30 mmol, 2.1 equiv) in hexane
(14 mL) were purged 30 min with argon, and photolyzed
15 min with 300 and 350 nm light. The solution was stirred
for 2 h at room temperature. The solution was filtered,
concentrated, and chromatographed with EtOAc/hexane,
1:9–3:7 to give the bis(TEMPO) adduct 4–15 (1.9 mg,
1
0.018 mmol, 3%). H NMR (400 mHz, CDCl3) d 0.4–1.6
(m, 36), 5.26 (s, 1), 7.41 (d, 2, J¼ 6.0 Hz), 8.60 (d, 2,
J¼ 6.0 Hz). 13C NMR (100 mHz, CDCl3) 17.1, 17.2, 20.3,
20.4, 20.8, 31.2, 32.1, 33.5, 34.4, 39.4, 39.6, 40.5, 59.9, 60.3,
60.8, 60.9, 86.9, 122.5, 148.0, 150.0, 169.8. IR (CDCl3)
1776 cmꢁ1. EIMS m/z 432, 156, 140, 69. HREIMS m/z
calcd for C25H42N3O3 (MHþ) 432.3203, found 432.3226.
Kinetic measurements
The kinetics of the reactions of ketenes were measured as
reported previously by monitoring the disappearance of
ketene in the presence of various concentrations of excess
Copyright # 2006 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2006; 19: 841–846