Radicals from fragmentation of benzyloxymethoxycarbenes in solution

Article abstract of DOI:10.1139/v00-021

2-Benzyloxy-2-methoxy-5,5-dimethyl-Δ³-1,3,4-oxadiazolines, including the parent as well as p-substituted analogues, undergo thermolysis at 100°C in benzene to afford a mixture of products. Two primary fragmentations of the oxadiazolines were identified. The major pathway involves 1,3-dipolar cycloreversion to N₂ and the corresponding carbonyl ylides. The latter dissociate to acetone and the corresponding benzyloxy(methoxy)carbenes, which undergo fragmentation to ArCH₂ and MeOCO radical pairs that recombine to afford methyl arylacetates. Carbene dimers were not observed, showing that the fragmentation process is faster than carbene dimerization. A second fragmentation pathway observed for the oxadiazolines is an alternative cycloreversion to the corresponding benzyl methyl carbonate and 2- diazopropane. Products from diazopropane included acetone azine and, in some instances, traces of propene.

Full text of DOI:10.1139/v00-021

356
Radicals from fragmentation of
benzyloxymethoxycarbenes in solution
Nadine Merkley, Manal El-Saidi, and John Warkentin
Abstract: 2-Benzyloxy-2-methoxy-5,5-dimethyl-∆3-1,3,4-oxadiazolines, including the parent as well as p-substituted
analogues, undergo thermolysis at 100°C in benzene to afford a mixture of products. Two primary fragmentations of
the oxadiazolines were identified. The major pathway involves 1,3-dipolar cycloreversion to N2 and the corresponding
carbonyl ylides. The latter dissociate to acetone and the corresponding benzyloxy(methoxy)carbenes, which undergo
fragmentation to ArCH2 and MeOCO radical pairs that recombine to afford methyl arylacetates. Carbene dimers were
not observed, showing that the fragmentation process is faster than carbene dimerization. A second fragmentation
pathway observed for the oxadiazolines is an alternative cycloreversion to the corresponding benzyl methyl carbonate
and 2-diazopropane. Products from diazopropane included acetone azine and, in some instances, traces of propene.
Key words: benzyloxy(methoxy)carbene, carbene, radical pair, rearrangement.
Résumé : Les 2-benzyloxy-2-méthoxy-5,5-diméthyl-∆3-1,3,4-oxadiazolines, y compris le produit fondamental ainsi que
les analogues substitués en position para, donnent lieu à une thermolyse, à 100°C dans le benzène, qui conduit à un
mélange de produits. On a identifié les deux fragmentations primaires des oxadiazolines. La voie principale implique
une cycloréversion 1,3-dipolaire en N2 et en ylures de carbonyles correspondants. Ces derniers se dissocient en acétone
et en benzyloxy(méthoxy)carbènes correspondants qui se fragmentent en paires de radicaux ArCH2 et MeOCO qui se
recombinent pour donner des arylacétates de méthyle. On n’a pas observé la formation de dimères de carbènes ce qui
démontre que le processus de fragmentation est plus rapide que la dimérisation des carbènes. Une deuxième voie de
fragmentation observée pour les oxadiazolines est une cycloréversion alternative en 2-diazopropane et en carbonate de
benzyle et de méthyle correspondant. Les produits résultant du diazopropane comprennent l’azine de l’acétone et, dans
quelques cas, des traces de propène.
Mots clés : benzyloxy(méthoxy)carbène, carbène, paire de radicaux, réarrangement.
[Traduit par la Rédaction] Merkley et al. 361
Introduction
generate esters (25). Hoffmann et al. reported in 1972 that
allyloxymethoxycarbene dissociates to allyl and methoxy-
carbonyl radicals in the gas phase (26). Intramolecular reac-
tions of dialkoxycarbenes appeared to be slow in solution,
the major products formed in the absence of a trap being the
carbene dimers (4, 5, 27). However, a few cases of fragmen-
tation of allyloxymethoxycarbenes to radical pairs, in solu-
tion at 110°C, were recently reported (28). In one of those
cases, the carbene was intercepted with tert-butyl alcohol
and the radicals were trapped with TEMPO. Thus the se-
quence oxadiazoline → carbene → radicals was established
for the first time, ruling out a potential direct pathway from
precursor to radicals.
We now report that the carbenes with a benzyloxy or
p-substituted benzyloxy substituent (ArCH2O(MeO)C:) also
fragment to radicals in solution. Thermolysis of 2-benzyl-
oxy-2-methoxy-5,5-dimethyl-∆3-1,3,4-oxadiazolines at 100°C
in the presence of alcohols (ROH) affords orthoformates
(ArCH2O)(MeO)(RO)CH, formed by trapping (ArCH2O)(MeO)C:.
In the absence of a carbene trap however, the carbenes frag-
ment to benzylic radicals and methoxycarbonyl radicals,
both of which can be trapped with TEMPO. The radicals
couple to afford methyl arylacetates, both in the presence or
in the absence of TEMPO. Competing fragmentation of the
oxadiazolines to the corresponding benzyl methyl carbonate
and 2-diazopropane also occurs.
There has been a resurgence of interest in the chemistry of
dialkoxycarbenes (1) since the early work of Hoffmann (2),
and Lemal et al. (3) with dialkoxycarbenes derived from
norbornadien-7-one acetals. Much of the subsequent work
was done by R.A. Moss and his group, who first generated
dialkoxycarbenes from diazirines (4–10). Recently oxadia-
zolines (11) have also been used to generate dialkoxycar-
benes, which were shown to react with a variety of
unsaturated moieties including anhydrides (12), C60
(13–15), methylenecyclopropanes (16), alkynes (17, 18),
fluorenone (19), α , β-unsaturated isocyanates (20, 21), a
bisketene (22), and hexachlorocyclopentadiene (23), for ex-
ample. They also attack aromatics substituted with electron-
withdrawing groups to effect nucleophilic aromatic substitu-
tion (24).
Dialkoxycarbenes also undergo intramolecular rearrange-
ments, at least in the gas phase in a mass spectrometer, to
Received October 14, 1999.
N. Merkley, M. El-Saidi, and J. Warkentin.1 Department of
Chemistry, McMaster University, Hamilton, ON L8S 4L8,
Canada.
1Author to whom correspondence may be addressed.
Fax: (905) 525 2509. e-mail: warkent@mcmaster.ca
Can. J. Chem. 78: 356–361 (2000)
© 2000 NRC Canada

Merkley et al.
357
para substituent, and in the order p-NO2 > p-CF3 > p-Cl >
p-H > p-Me > p-OMe (2.83, 2.62, 2.55, 2.35, 2.11, and 2.09,
all × 10–5 s–1). The reverse order would be expected if the
thermolyses gave carbenes in one step, as proposed by
Smith (30). The step forming 3 was assigned by analogy to
instances in which a carbonyl ylide intermediate is manda-
tory, because a ketene acetal was among the products (27).
Ylide 3 fragments to carbene 4, possibly in competition with
electrocyclic ring closure to oxirane (9) (19), from which a
ketene acetal (10) might be generated by abstraction of the
ring oxygen by the carbene (31). In the present cases, ketene
acetals were not detectable in the crude reaction mixtures by
1H NMR spectroscopy nor were any isolated. Thus, they
were probably either formed in minor yields or they were
absent altogether.
Scheme 1.
Methods, results, and discussion
Oxadiazolines 2 were prepared from the methoxycarbonyl
hydrazone of acetone via 2-acetoxy-2-methoxy-5,5-
dimethyl-∆3-1,3,4-oxadiazoline (1) by the exchange method
(29), illustrated in Scheme 1. They were purified by chroma-
tography on silica with a Chromatotron apparatus, and they
were identified by means of 1H and 13C NMR spectroscopies
at 200 and 50 MHz, respectively. Diastereotopicity of the
CH2 protons was evident. Moreover, the 13C spectra are di-
agnostic for oxadiazolines of that type, with δ values near
137.5 for C-2 and 118.5 for C-5 (11). In the mass spectra, 2
afforded neither molecular ions nor M – N2 ions.
Thermolyses were carried out with degassed solutions in
benzene-d6 (initial oxadiazoline concentration 0.1 M) at 100
± 0.1°C. Reaction products were examined by 1H NMR
spectroscopy before the tube was opened (in cases of small
scale runs with NMR tubes) to facilitate recognition of ini-
tial products rather than those that might have arisen from
rearrangements or hydrolyses during separation procedures.
The reported yields (isol) are probably considerably lower
than the chemical yields because reactions were run on a
small scale (one mmol or less) and because of some difficul-
ties with the separation of recognized products from one an-
other and from minor co-products.
Intermediacy of carbenes 4 was conveniently demonstrated
in a few cases by running thermolyses of oxadiazolines from
which the alcohol (ArCH2OH) used in the exchange reaction
had not been removed. The excess alcohol trapped the car-
bene to afford the appropriate orthoformate (7, R = CH2Ar).
In other cases, tert-butyl alcohol (0.5 M) was used to inter-
cept carbenes by converting them into analogous orthofor-
mates 7 (R = t-Bu). Fragmentation of the carbenes to radicals
5 and 6 is apparently slower than trapping with t-BuOH
at [t-BuOH] = 0.5 M. To estimate the rate constant for frag-
mentation of 4, we modeled trapping of 4 with t-BuOH at
100°C with analogous trapping of (p-MeOC6H4)2C: at ambi-
ent temperature (k = 0.4 × 109 M–1 s–1) (32). Taking k100°C
=
109 M–1 s–1 and the efficiency of trapping to be at least 98%,
the estimated fragmentation rate constant for 4 is ≤107 s–1. A
fragmentation rate constant of 105 s–1 would be more than
sufficient to prevent carbene dimerization in the absence of a
carbene trap, because the carbene is generated very slowly
from the oxadiazoline and its concentration must be ex-
tremely low. Dialkoxycarbenes that do not fragment to radi-
cals generally do afford carbene dimers when generated in
unreactive solvents (4, 5, 27).
Scheme 2 outlines the overall thermolysis chemistry of
the benzyloxymethoxy oxadiazolines, including some reac-
tions that are included on the basis of analogy. The major
first step is 1,3-dipolar cycloreversion to N2 and carbonyl
ylide 3, supported by the finding that the rate constants for
oxadiazoline thermolysis are relatively insensitive to the
Radicals 5 and 6 must be born as a singlet pair (28), be-
cause the precursor carbene is a singlet lying below the trip-
let by more than 70 kcal mol–1 (4), and they couple to afford
methyl arylacetates (8), Scheme 2. Only those radicals that
have diffused out of the solvent cage in which they were
born are trapped with TEMPO; in-cage coupling converts
Scheme 2.
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358
Can. J. Chem. Vol. 78, 2000
Scheme 3.
Scheme 4.
pairs to methyl arylacetate. In accord with that mechanism,
small amounts (ca. 8% in the case of 1a) of 1,2-diaryl-
ethanes, from coupling of benzylic radicals (6) in the ab-
sence of TEMPO, were indicated by GCMS although none
could be isolated. Several other minor products, indicated in
GC traces, were not identified.
Scheme 5.
Most of the methyl arylacetates (8) are well known al-
though some had not been characterized by NMR spectros-
copy. The NMR data are therefore included here. Esters 8b
and 8d were also prepared independently by acid catalysed
esterification of appropriate arylacetic acids. Their spectra
agreed well with the spectra of corresponding 8 from
oxadiazoline thermolysis.
Thermolysis of 2 in the presence of 2,2,6,6-tetramethyl-1-
piperdinyloxy (TEMPO, 0.5 M)) afforded 11 and 12. Those
adducts, which arise from the coupling of methoxycarbonyl-
and benzylic radicals with TEMPO, Scheme 3, constitute
firm evidence for the intermediacy of radicals. The fact that
ester 8a and 11 were formed in approximately 1:1 ratio is
not surprising, because 0.5 M TEMPO is not expected to
compete with in-cage coupling. Formation of a fraction of
the 8a by a 1,2-rearrangement of carbene 4a cannot be ex-
cluded from the experimental evidence. However, a compu-
tational study of allyloxyhydroxycarbene clearly indicated
that homolysis of that carbene is strongly favoured over 1,2-
rearrangement2 and we assume that the same would be true
for carbenes 4.
to exclude low yields of the ketene acetals, which might not
totally survive the reaction conditions in any case (27).
Authentic methyl benzyl carbonates 14b and 14e, pre-
pared from the appropriate ArCH2OH and methyl chloro-
formate, gave NMR spectra in agreement with those of the
corresponding 14 from oxadiazoline thermolysis.
The alternative cycloreversion reaction of the oxadiazo-
lines to a carbonate ester and diazopropane is not well un-
derstood. It may be a general, although minor, competitor in
other oxadiazoline thermolyses, which commonly produce
traces of by-products that have not been identified.
A minor product obtained in some cases was acetone az-
ine, 13. It cannot be accommodated easily within the ylide
mechanism of Scheme 1. Although fragmentation of the ylide
intermediate to dimethylcarbene and carbonate, in competi-
tion with its fragmentation to dialkoxycarbene and acetone,
could be proposed, the dimethylcarbene would then have to
attack the oxadiazoline starting material to generate acetone
azine, as in Scheme 4. An analogous process was proposed
earlier for reactions with much higher initial oxadiazoline
concentrations (33). It is now known that dimethylcarbene
undergoes 1,2-H migration with k = 1.4 × 108 s–1 at room
temperature (34–36). That fast rearrangement, which is
probably still faster at 100°C, makes it unlikely that acetone
azine arises according to Scheme 4 and, for that reason, a
second 1,3-dipolar cycloreversion (Scheme 5) of some oxa-
diazolines (37) is proposed. The mechanism is not firmly es-
tablished, however. Although the appropriate carbonate 14
was obtained, its formation is not necessarily diagnostic for
the alternate cycloreversion because reaction of 9 with 4
would afford the same carbonate, together with 10. A com-
parison of the yields of azine and 10, which might have
Summary
2-Benzyloxy-2-methoxy-5,5-dimethyl-∆3-1,3,4-oxadiazolines
undergo thermolysis at 100°C in benzene, by competing
cycloreversions, affording either N2 and a carbonyl ylide or
2-diazopropane and a carbonate ester. The carbonyl ylide
fragments to acetone and a benzyloxy(methoxy)carbene, which
fragments further to the methoxycarbonyl radical and a
benzylic radical. Those radicals couple to afford methyl
arylacetates. Formation of 2-diazopropane was inferred from
the presence of acetone azine among the products.
Experimental
For product isolation, thermolyses were carried out with
degassed solutions of substrate (1 mmol in 30 mL of dry
benzene, 96 h) in a constant temperature oil bath at 100 ±
0.1°C. Products were isolated by means of centrifugal chro-
matography. For examination of a crude reaction mixture by
NMR spectroscopy, thermolyses were run with degassed so-
lutions in sealed NMR tubes (ca. 10 mg in 0.5 mL of C6D6).
Unless otherwise indicated, the reported NMR data refer to
1
helped, was not possible because the composite H NMR
spectra of the crude reaction mixture were too complicated
2 D.L. Reid and J. Warkentin. Manuscript in preparation.
© 2000 NRC Canada

Merkley et al.
359
CDCl3 solutions containing TMS. In the MS data, numbers
in brackets are relative intensities.
109 (23), 84 (12), 73 (94), 59 (29). MS (CI, NH3) m/z: 322
((M + NH4)+, 6), 193 (10), 176 (100), 131 (15), 60 (52).
1
2f: Oil, 64% yield. H NMR δ: 1.55 (3H, s), 1.58 (3H, s),
Synthesis of oxadiazolines
3
3.47 (3H, s), 4.94 (2H, m), 7.52 (2H, d, J = 8.7 Hz), 8.21
Oxadiazolines 2 were prepared from a mixture of 2-ace-
toxy-2-methoxy-5,5-dimethyl-∆3-1,3,4-oxadiazoline (1, ca.
75%) and its acyclic isomer, methyl-2,3-diaza-4-methyl-4-
acetoxypent-2-enoate (29) (ca. 25%) by treatment with the
appropriate benzylic alcohol in dichloromethane containing
acetic acid. For example, 3.6 mmol of 2 and 1.0 mmol of al-
cohol in CH2Cl2 (35 mL) containing acetic acid (1 mL) was
refluxed for 48 h. Most of the solvent was removed with a
rotary evaporator and the residue was stirred for one hour
with 5% aqueous KOH. Extraction with CH2Cl2, drying over
MgSO4, and evaporation of the volatile components left the
oxadiazolines in nearly pure form. Yields for the exchange
step ranged from 60 to 90%, based on the alcohol. In some
cases the analogous procedure, but with the alcohol in ex-
cess, gave an oxadiazoline containing about 20% of the alco-
hol. Thermolysis of those mixtures served as probes for the
intermediacy of a carbene with the alcohol acting as a trap
for it.
3
(2H, d, J = 8.7 Hz). 13C NMR δ: 23.9, 24.3, 52.1, 65.5,
119.9, 123.6, 127.8, 136.9, 144.4, 147.6. MS (EI) m/z: 211
(8), 152 (17), 136 (40), 120 (16), 106 (10), 84 (31), 73
(100). MS (CI, NH3) m/z: 299 ((M + NH4)+, 100), 282
((M + H)+, 8).
1
Orthoformate 7a: (R = t-Bu, Ar = benzyl). Yield 61%. H
NMR (C6D6) δ: 1.26 (9H, s), 3.21 (3H, s), 4.60 (2H, s), 5.36
(1H, s), 7.03–7.15 (5H, m). 13C NMR (C6D6) δ: 28.8, 49.8,
65.1, 73.9, 109.5, 127.5, 128.4, 128.6, 139.6. MS (EI) m/z:
193 (17), 181 (61), 151 (36), 91 (100).
1
7c: (R = Ar = 4-methoxybenzyl) H NMR δ: 3.36 (3H, s),
3
3.79 (6H, s), 4.56 (4H, s), 5.24 (1H, s), 6.86 (2H, d, J =
8.7 Hz), 7.26 (2H, d, 3J = 8.7 Hz). HRMS: calcd for
C18H22O5: 318.1467; found 318.1452.
1
7f: (R = Ar = 4-nitrobenzyl) H NMR δ: 3.44 (3H, s), 4.78
3
(4H, s), 5.39 (1H, s), 7.52 (4H, d, J = 8.7 Hz), 8.20 (4H, d,
3J = 8.7 Hz).
1
Oxadiazoline 2a: Oil, yield 89%. H NMR δ: 1.48 (3H, s),
2
1.51 (3H, s), 3.43 (3H, s), 4.68 (1H, d, J = –11.5 Hz), 4.78
1
Methyl phenylacetate 8a: (38) Oil, 20% yield. H NMR δ:
2
(1H, d, J = –11.5 Hz), 7.24–7.27 (5H, m). 13C NMR δ:
3.60 (2H, s), 3.66 (3H, s), 7.25–7.35 (5H, m).
23.9, 24.0, 51.9, 66.6, 119.3, 127.7, 127.8, 128.3, 136.6,
137.1. MS (CI, NH3) m/z: 207 (40), 184 (29), 179 (12), 108
(100), 73 (12).
1
8b: (39) H NMR δ: 2.30 (3H, s), 3.56 (2H, s), 3.64 (3H, s),
7.12 (5H, m). MS (EI) m/z: 164 (M+, 27), 105 (95). MS (CI,
NH3) m/z: 182 (M + NH4)+.
1
2b: Oil, 60% yield. H NMR δ: 1.54 (3H, s), 1.57 (3H, s),
2
8c: (40) Oil, 35% yield. 1H NMR δ: 3.55 (2H, s), 3.67
2.34 (3H, s), 3.48 (3H, s), 4.69 (1H, d, J = –11 Hz), 4.79
2
3
3
(1H, d, J = –11 Hz), 7.14 (2H, d, J = 7 Hz), 7.24 (2H, d,
3J = 7 Hz). 13C NMR δ: 21.1, 24.0, 52.0, 66.6, 119.4, 127.9,
129.0, 133.6, 137.6, 142.6. MS (EI) m/z: 105 (100), 73 (59),
59 (10). MS (CI, NH3) m/z: 198 (25), 147 (43), 122 (100),
73 (18), 58 (14).
(3H, s), 3.77 (3H, s), 6.84 (2H, d, J = 8.5 Hz), 7.18 (2H, d,
3J = 8.5 Hz). 13C NMR δ: 40.1, 51.8, 55.1, 113.9, 125.9,
130.2, 158.6, 172.2.
8d: (39) Oil, 21% yield. 1H NMR δ: 3.59 (2H, s),
3
3
3.69(3H, s), 7.20 (2H, d, J = 8.5 Hz), 7.29 (2H, d, J =
8.5 Hz). MS (EI) m/z: 186 ((M + 2)+, 11), 184 (M+, 22), 127
(40), 125 (80), 49 (100). MS (CI, NH3) m/z: 204 (15), 202
((M + NH4)+, 80), 125 (100).
1
2c: Oil, 77% yield. H NMR δ: 1.55 (3H, s), 1.57 (3H, s),
2
3.48 (3H, s), 3.80 (3H, s), 4.66 (1H, d, J = –13 Hz), 4.74
2
3
(1H, d, J = –13 Hz), 6.87 (2H, d, J = 9 Hz), 7.28 (2H, d,
3J = 9 Hz). 13C NMR δ: 23.9, 24.1, 51.9, 55.1, 66.4, 113.7,
119.2, 129.5, 130.2, 136.9, 159.3. MS (EI) m/z: 196 (10),
138 (32), 121 (100). MS (CI, NH3) m/z: 267 ((M + H)+, 4),
163 (100), 121 (78).
1
8e: (footnote 3)3Oil, 20% yield. H NMR δ: 3.68 (2H, s),
3
3
3.70 (3H, s), 7.40 (2H, d, J = 8.6 Hz), 7.58 (2H, d, J =
8.6 Hz). 19F NMR δ: (CDCl3, CFCl3): –62.5 (s). MS
(EI) m/z: 159 (100). MS (CI, NH3) m/z: 219 ((M + H)+, 38),
176 (100).
1
2d: Oil, 90% yield. H NMR δ: 1.53 (3H, s), 1.58 (3H, s),
2
2
3.47 (3H, s), 4.72 (1H, d, J = –11.8 Hz), 4.81 (1H, d, J =
3
3
1
–11.8 Hz), 7.30 (2H, d, J = 5 Hz), 7.33 (2H, d, J = 5 Hz).
13C NMR δ: 24.0, 24.1, 51.9, 65.9, 119.4, 128.5, 129.0,
133.7, 135.3, 137.6. MS (EI) m/z: 127 (30), 125 (100), 89
(16), 73 (77), 59 (10). MS (CI, NH3) m/z: 288 ((M + NH4)+, 8),
218 (11), 195 (8), 167 (5), 144 (33), 142 (100), 125 (25), 73
(26), 60 (26).
8f: (41) Yield 19%. H NMR δ: 3.72 (2H, s), 3.75 (3H, s),
7.46 (2H, d, 3J = 8 Hz), 8.17 (2H, d, 3J = 8 Hz). 13C NMR δ:
40.7, 52.3, 123.5, 130.2, 145.2, 147.9. MS (EI) m/z: 195
(M+, 12), 136 (79). MS (CI, NH3) m/z: 196 ((M + H)+, 40);
HRMS: calcd for C9H9NO4: 195.0532; found: 195.0531.
11: Thermolysis of 2a (0.1 M) was carried out in the pres-
ence of excess TEMPO (0.5 M) in benzene at 110°C. After
the solvent was evaporated some of the excess TEMPO was
removed at ambient temperature with a vacuum pump. The
TEMPO adducts of benzyl and of methoxycarbonyl were
isolated by chromatography on silica, with a Chromatotron
apparatus. It was difficult to separate 11 from excess
1
2e: Oil, 88% yield. H NMR δ: 1.54 (3H, s), 1.59 (3H, s),
2
2
3.48 (3H, s), 4.83 (1H, d, J = –8.2 Hz), 4.93 (1H, d, J =
–8.2 Hz), 7.46 (2H, d, 3J = 8.1 Hz), 7.60 (2H, d, 3J =
8.1 Hz). 19F NMR (CDCl3, CFCl3) δ: –62.5 (s). 13C NMR δ:
23.9, 24.2, 52.0, 65.9, 119.7, 125.3, 125.4, 127.6, 130.3,
136.9, 140.9. MS (EI) m/z: 159 (100), 140 (10), 127 (11),
3 P. Stahly. U.S. Patent 5,302,752 (1994).
© 2000 NRC Canada

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Can. J. Chem. Vol. 78, 2000
TEMPO but rechromatography of an initial enriched fraction
was successful. Yield 12%. H NMR (C6D6) δ: 1.17 (6H, s),
References
1
1.28 (6H, s), 1.28–1.49 (6H, m), 4.89 (2H, s), 7.08–7.35
(5H, m). 13C NMR (C6D6) δ: 17.4, 20.4, 33.3, 39.9, 79.3,
127.8, 128.5, 128.3, 138.7. GC/MS (EI) m/z: 247 (M+, <1),
156 (100), 91 (45). Authentic 11 was prepared by heating a
solution of di-tert-butylperoxide in toluene containing
TEMPO. The NMR spectra in CDCl3 matched those re-
ported (42).
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2. R.W. Hoffmann. Acc. Chem. Res. 18, 248 (1985).
3. D.M. Lemal, E.P. Gosselink, and S.D. McGregor. J. Am.
Chem. Soc. 88, 582 (1966).
4. R.A. Moss, M. Wlostowski, S. Shen, K. Krogh-Jespersen, and
A. Matro. J. Am. Chem. Soc. 110, 4443 (1988).
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7549 (1993).
6. R.A. Moss, M. Wlostowski, J. Terpinski, G. Kmiecik-Lawrynowicz,
and K. Krogh-Jespersen. J. Am. Chem. Soc. 109, 3811 (1987).
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950 (1976).
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1
12: H NMR (C6D6) δ: 1.15 (6H, s), 1.21 (6H, s), 1.24–1.77
(6H, m), 3.48 (3H, s). GC/MS (EI) m/z: 215 (M+, 3), 200
(100), 83 (76), 55 (84).
1
14a: (43) 10% yield. H NMR δ: 3.76 (3H, s), 5.14 (2H, s),
7.25–7.35 (5H, m). MS (EI) m/z: 155 (12), 107 (26), 91
(100). MS (CI, NH3) m/z: 184 ((M + NH4)+, 100).
9. R.A. Moss, S. Shen, and M. Wlostowski. Tetrahedron Lett. 29,
6417 (1988).
1
14b: 21% yield. H NMR δ: 2.34 (3H, s), 3.76 (3H, s), 5.11
10. X.-M. Du, H. Fan, J.L. Goodman, M.A. Kesselmayer, K.
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5871 (1994).
15. R. González, F. Wudl, D.L. Pole, P.K. Sharma, and J.
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(2H, s), 7.15 (2H, d, 3J = 8 Hz), 7.26 (2H, d, 3J = 8 Hz). MS
(EI) m/z: 180 (M+, 95), 105 (100), 104 (97). MS (CI,
NH3) m/z: 198 ((M + NH4)+, 34), 122 (100).
1
14c: 25% yield. H NMR δ: 3.76 (3H, s), 3.79 (3H, s), 5.08
3
3
(2H, s), 6.87 (2H, d, J = 8.6 Hz), 7.31 (2H, d, J = 8.6 Hz).
MS (EI) m/z: 196 (M+, 15), 180 (10), 122 (10), 121(100).
HRMS: calcd for C10H12O4: 196.0736; found 196.0744.
1
14e: 10% yield. H NMR δ: 3.81 (3H, s), 5.21 (2H, s), 7.49
3
3
(2H, d, J = 8 Hz), 7.63 (2H, d, J = 8 Hz). 13C NMR δ:
1
55.0, 68.5, 123.9 (q, JC-F = 271 Hz), 126.6, 128.1, 130.6 (q,
2JC-F = 32.4 Hz), 139.2, 155.6. 19F NMR (CF3Cl ref) δ:
–62.7. MS (EI) m/z: 235 ((M + 1)+, 21), 215 (18), 159 (100);
MS (CI, NH3) m/z: 252 (M + NH4)+.
17. K. Kassam, P. Venneri, and J. Warkentin. Can. J. Chem. 75,
1256 (1997).
18. K. Kassam and J. Warkentin. Can. J. Chem. 75, 120 (1997).
19. D.L. Pole and J. Warkentin. J. Org. Chem. 62, 4065 (1997).
20. J.H. Rigby, A. Cavezza, and G. Ahmed. J. Am. Chem. Soc.
118, 12848 (1997).
1
14f: 12% yield. H NMR δ: 3.83 (3H, s), 5.26 (2H, s), 7.55
(2H, d, J = 8.7 Hz), 8.23 (2H, d, J = 8.7 Hz). 13C NMR δ:
55.1, 67.8, 123.7, 128.3, 142.4, 155.3 (one signal not seen,
presumably superimposed). HRMS: calcd for C9H9NO5
211.0481; found 211.0482.
3
3
21. J.H. Rigby, A. Cavezza, and M.J. Heeg. J. Am. Chem. Soc.
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J. Org. Chem. 64, 4344 (1999).
24. J.P. Ross, P. Couture, and J. Warkentin. Can. J. Chem. 75,
1331 (1997).
25. D. Suh, D.L. Pole, J. Warkentin, and J.K. Terlouw. Can. J.
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26. R.W. Hoffmann, R. Hirsch, R. Fleming, and M.T. Reetz.
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326 (1997).
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1
Acetone azine. H NMR (C6D6) δ: 1.79 (3H, s), 1.81 (3H, s).
The same spectrum was obtained with an authentic sample.
Rate constants for thermolysis of 2a–f in C6D6 at 100°C
were determined by means of H NMR spectroscopy, with
1
toluene as internal standard. Toluene did not react apprecia-
bly with radicals produced during the thermolyses. A sealed
NMR tube containing the oxadiazoline (0.1 mmol) in C6D6
was withdrawn from the oil bath from time to time and
cooled rapidly to room temperature before integration of the
1H NMR spectrum. Typically, seven points were collected,
covering the range from 0 to 87% of completion. The time
scale was not corrected for the cooling and warm-up times.
First order plots, which were linear, gave the rate constants
reported in the text.
29. K. Kassam, D.L. Pole, M. El-Saidi, and J. Warkentin. J. Am.
Chem. Soc. 116, 1161 (1994).
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The authors gratefully acknowledge financial support in
the form of a grant-in-aid from the Natural Sciences and
Engineering Research Council of Canada. The assistance of
Joseph P. Ross, who did some exploratory work on this pro-
ject, is also greatly appreciated.
© 2000 NRC Canada

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© 2000 NRC Canada