Mechanism of ipso aromatic substitution by reaction of aryloxy(methoxy)carbenes and diaryloxycarbenes with DMAD

Article abstract of DOI:10.1139/v01-029

Some aryloxy(methoxy)carbenes and diaryloxycarbenes attack dimethyl acetylenedicarboxylate (DMAD) with aryl group transfer to an alkyne carbon of DMAD. In this study diaryloxycarbenes with different aryl groups that could be transferred competitively, were generated in the presence of DMAD to probe for the mechanism of that ipso aromatic substitution. It was found that a para electron-withdrawing substituent, relative to an electron-donating substituent, facilitated migration of an aryl group. Mechanisms in accord with these findings involve initial nucleophilic attack by the carbene at an alkyne carbon of DMAD. That step is followed by either nucleophilic, ipso attack on the aromatic ring or by electron transfer, from the side chain of the aromatic ring into the ring itself.

Full text of DOI:10.1139/v01-029

364
Mechanism of ipso aromatic substitution by
reaction of aryloxy(methoxy)carbenes and
diaryloxycarbenes with DMAD
Xiaosong Lu and John Warkentin
Abstract: Some aryloxy(methoxy)carbenes and diaryloxycarbenes attack dimethyl acetylenedicarboxylate (DMAD) with
aryl group transfer to an alkyne carbon of DMAD. In this study diaryloxycarbenes with different aryl groups that could
be transferred competitively, were generated in the presence of DMAD to probe for the mechanism of that ipso aro-
matic substitution. It was found that a para electron-withdrawing substituent, relative to an electron-donating
substituent, facilitated migration of an aryl group. Mechanisms in accord with these findings involve initial nucleophilic
attack by the carbene at an alkyne carbon of DMAD. That step is followed by either nucleophilic, ipso attack on the
aromatic ring or by electron transfer, from the side chain of the aromatic ring into the ring itself.
Key words: aromatic substitution, diaryloxycarbene, DMAD, ipso, nucleophilic.
Résumé : Quelques aryloxy(méthoxy)carbènes et diaryloxycarbènes attaquent l’acétylènedicarboxylate de diméthyle
(ACDM) avec un transfert de groupe aryle vers un atome de carbone alcyne du ACDM. Dans le présent travail, dans le
but d’étudier le mécanisme de cette réaction de substitution aromatique ipso, on a généré en présence du ACDM des
diaryloxycarbènes portant divers groupes aryles qui peuvent être en compétition pour un transfert. On a trouvé que, par
opposition à un substituant électrodonneur, un substituant électroaffinitaire facilite la migration d’un groupe aryle. Les
mécanismes qui correspondent à ces faits impliquent une attaque nucléophile initiale par le carbène au niveau du car-
bone de l’alcyne du ACDM. Cette étape est suivie par une attaque nucléophile ipso du noyau aromatique ou par un
transfert d’électron à partir de la chaîne latérale du noyau aromatique vers le cycle lui-même.
Mots clés : substitution aromatique, diaryloxycarbène, ACDM, ipso, nucléophile.
[Traduit par la Rédaction] Lu and Warkentin 369
Introduction
the more electron-deficient aryl group from oxygen to an
alkyne carbon atom of DMAD.
Recently, we (1) reported the results of thermolysis of bis-
oxadiazoline 1 in benzene containing dimethyl acetylene-
dicarboxylate (Scheme 1). The multifunctional product (2)
must have arisen from a cascade of reactions, including
transfer of the aryl ring from oxygen to carbon. Analogous
substitution reactions were also observed with a simpler
oxadiazoline (3a) (1), and with 5,5-dimethyl-2,2-diphenoxy-
∆3–1,3,4-oxadiazoline (3b) (2, 3) (Scheme 2). The only re-
lated reaction that we are aware of is that between an amino-
thiocarbene and diethyl acetylenedicarboxylate, reported by
Scherowsky et al. (4) in 1977 (Scheme 3). Although a vari-
ety of ipso substitutions on benzene derivatives are known
(5), oxy-substituents are not generally among the leaving
groups unless there are activating nitro groups in the ring
also. It was therefore of interest to try to establish the mech-
anism of the substitution reactions of Schemes 1 and 2. We
now report that competition reactions, with diaryloxycarbenes
((ArO)(Ar′O)C:), gave evidence for preferential transfer of
Methods, results, and discussion
The strategy for determining the effect of aryl substituents
involved generation of unsymmetric diarylcarbenes (5) so as
to have internal competition, between the aryl groups in a
dipolar (or other) intermediate (6a, 7a), in the step that gen-
erates isomeric esters (Scheme 4). In all but one case the
substituents in the aryl groups were in the p-position, and it
was assumed that a conformational preference (6a, 7a)
would be small at most. It is probably irrelevant because the
Curtin–Hammett principle is expected to apply (6). Al-
though intermediates 6a and 7a presumably epimerize to 6b
and 7b, through a small barrier, that too should be irrelevant
because intramolecular substitution cannot occur with the
ester groups trans, as in 6b and 7b.
5,5-Dimethyl-2,2-diphenoxy-∆3-1,3,4-oxadiazoline
(11,
Ar = Ar′ = Ph), the first 2,2-diaryloxy oxadiazoline (2), was
prepared by hydrazinolysis of diphenyl carbonate, subse-
quent reaction of the hydrazide (8) with acetone to afford 9,
oxidative cyclization of 9 to 10 with lead tetraacetate (LTA)
in CH2Cl2, and acid-catalyzed reaction of 10 with phenol
(Scheme 5) (3, 7). Unsymmetric analogues (11, one or both
aryl groups bearing a substituent) were prepared similarly
(3).
Received November 16, 2000. Published on the NRC
Research Press Web site on April 27, 2001.
X. Lu and J. Warkentin.1 Department of Chemistry,
McMaster University, Hamilton, ON L8S 4M1, Canada.
1Corresponding author (telephone: (905) 525-9140; fax: (905)
522–2509. e-mail: warkent@mcmaster.ca).
Can. J. Chem. 79: 364–369 (2001)
© 2001 NRC Canada
DOI: 10.1139/cjc-79-4-364

Lu and Warkentin
365
Scheme 1.
Scheme 3.
Scheme 2.
esters from 11e (Scheme 6). The geometry (ArOCO and
CO2Me groups trans) was established for 12a by single
crystal X-ray diffraction and that geometry was assumed for
the other triesters 12, and for 13e, on the basis of analogy.
The effects of substituents on the ease of ipso substitution,
given by the yield ratios (13:12) rather than by the absolute
yields, are in the order p-CN >> o-MeO > H > Me > p-MeO
(Table 1).
It is clear from Table 1 that an electron-withdrawing
group in the p-position increases the migratory aptitude of
an aryl group whereas an electron-donating group decreases
it. Unfortunately, most substrates gave only one of the two
possible esters and it was not possible to construct a
Hammett plot and to estimate the value of ρ. However, the
sense of the substituent effect is unmistakable, and the effect
cannot be small. We suggest that a likely mechanism to ac-
count for the migratory aptitudes is that illustrated with
Scheme 7, for the case X=H, Y=CN. The steps to the
carbene are well-precedented (8), as are the attack of nucleo-
philic carbenes on DMAD and other alkynes (9–12) and the
reversible ring-opening and ring-closing of cyclopropenes
and (or) vinylcarbenes (13–16). Moreover, Boger et al. (17)
and Boger and Brotherton-Pleiss (18) have reported reac-
tions of an intermediate (a π-delocalized vinylcarbene) anal-
ogous to 15 in Scheme 7. We assume that 15 is equilibrated
with cyclopropene 16, in view of the fact that diphenoxy
OAr
E
RO
RO2C
heat
N
O
DMAD
N
Ar
E
3
4
(a) R= Me, Ar= o-MeOC6H4; (b) R = Ar = Ph
(DMAD = dimethyl acetylenedicarboxylate;
E = CO2Me)
Thermolysis of oxadiazolines 11 as benzene solutions
containing DMAD, at 110°C in sealed tubes, gave one or
both of the isomeric esters (12, 13) in yields (or composite
yields) between 28 and 37% (of isolated material) and a
diaryl carbonate (14) in 25–35% yield (Scheme 6). The es-
ters were separated from other products by means of radial
chromatography, which also separated the isomers in the one
case that gave both. Assignment of structure followed from
the NMR spectra of the products and from the GC–MS pat-
terns. In their EI mass spectra, aryl esters afford base peaks
from loss of the fragment ArO in the case of
ArOCOC(E)=C(Ar′)E, and from loss of Ar′O in the case of
Ar′OCOC(E)=C(Ar)E. It was therefore easy to assign the
gross structures of the esters from 11a–d and of the isomeric
Scheme 4.
© 2001 NRC Canada

366
Can. J. Chem. Vol. 79, 2001
Table 1. Yields (%) of esters (12, 13) and carbonate (14) from
oxadiazolines (11).
Scheme 5.
11
12
13
13:12
14
a
b
c
d
e
37
28
37
38
11
<2a
<2a
<2a
<2a
25
<0.05
<0.07
<0.05
<0.05
2.3
29
26
30
32
35
aNot detected. We did not experience any difficulties with detection of
1
the esters by H NMR spectroscopy, and we estimate that the yield of the
detected isomer was at least 15-fold greater than that of the isomer that
was not detected.
Scheme 6.
Scheme 7.
Scheme 8.
OC6H4CN
PhO OC6H4CN
PhO
PhO
OC6H4CN
DMAD
_
+
..
O
N
O
N
E
PhO
E
OC6H4CN PhO
OC6H4CN
+
OPh
result (Scheme 9) cannot be ruled out with confidence.
Some intermolecular chemistry of intermediates similar to
15 is thought to involve electron transfer from a π-
delocalized vinylcarbene intermediate, and subsequent cou-
pling of a radical cation and radical anion pair (Scheme 10)
(20).
A special interaction must be invoked for the o-methoxy
substituent, which promoted migration whereas the p-
methoxy group was not effective. Given that a likely mecha-
nism (Scheme 7) involves a dipolar intermediate, it is logical
to propose that the o-methoxy group exerts its effect through
space, rather than through bonds (Scheme 11). A through-
space interaction of the ortho methoxy group with the posi-
tive center of a dipolar intermediate would help to orient the
aryl group for attack, as shown in Scheme 11. Such an in-
volvement would differentiate strongly between the o- and
p-methoxy groups, which presumably have similar
conjugative effects.
_
:
E
OC6H4CN
16
E
E
15
E
PhO
PhO
E
O
CN
O
_
+
CN
E
E
E
17
18
oxadiazoline 3b, when thermolyzed in the presence of
DMAD, affords not only 19 but also 20 and 21 (2)
(Scheme 8).
Intermediate 15 also affords 18, presumably via 17, which
is analogous to the intermediate usually invoked in describ-
ing nucleophilic aromatic substitutions (5). Dipolar interme-
diates in ipso nucleophilic aromatic substitution have been
demonstrated many times (19).
Summary
A concerted reaction, leading directly from the carbene
and DMAD to the product, is unlikely given that aromatic
substitutions generally occur through intermediates. How-
ever, a more complex stepwise process, involving electron
transfer (e.t.) before ring closure but leading to the observed
Diaryloxycarbenes attack DMAD to afford unsaturated
triesters, by attack at the sp-carbon and migration of an aryl
group from oxygen to the other sp-carbon of DMAD. If
the aryloxy groups are para-substituted, the aryl group
that is transferred preferentially is the one that is more
© 2001 NRC Canada

Lu and Warkentin
367
Scheme 9.
Scheme 10.
E
PhO
E
OC6H4CN
+
PhO
E
OC6H4CN
_
E
E
E
E
E
OPh
O
O
O
O
e.t.
_
+
+
.
+
:
OC6H4CN
+
MeO
MeO
_
.
15
e.t.
E
E
E
PhO
E
O
.
_
.
(E = CO2R)
PhO
E
O
CN
+
O
.
CN
CN
MeO
.
E
O
E
E
Scheme 11.
PhO
E
O
E
electron-deficient. The ortho- but not the para-methoxy
group also promotes migration, presumably by stabilizing an
electrophilic site by donation of electron density from the
oxygen atom through space. Aryloxymethoxycarbenes
(1) probably react with DMAD by an analogous mechanism.
1
1038s, 1010s. H NMR (200 MHz, CDCl3) δ: 3.76 (s, 3H,
OMe), 3.87 (s, 3H, OMe), 3.92 (s, 3H, OMe), 6.67–6.85 (m,
4H, ArH), 7.58–7.74 (m, 4H, ArH). 13C NMR (75 MHz,
CDCl3) δ: 53.3 (2 × OMe), 55.5, 113.9, 114.5, 117.8, 121.4,
128.5, 128.9, 132.4, 137.2, 143.1, 145.2, 157.7, 162.3,
162.4, 165.8.
Experimental section
General
NMR spectra were recorded with a Bruker AC-200 or
AC-300 NMR spectrometer and IR spectra were obtained
with a Biorad FTS-40 spectrophotometer, with the sample in
CCl4. The IR bands are labeled qualitatively with the sym-
bols br, s, and m for broad, strong, and medium intensity
bands. Weak bands are not listed. The eluting solvent for ra-
dial chromatography was a gradient of ethyl acetate (2.5–10
vol. %) in hexanes. Melting points were obtained with a
Thomas Hoover capillary melting point apparatus, and are
uncorrected.
The E-configuration of 12a (ArOCO and CO2Me groups
trans) was confirmed by means of single crystal X-ray dif-
fraction.
p-Cyanophenyl p-methoxyphenyl carbonate (14a): white solid,
mp: 103–105°C. EI-MS m/z (%): 269 (M, 34), 254 (31), 225
(9), 210 (23), 154 (6), 123 (100), 107 (84), 77 (77), 65
(32). CI-MS (NH3) m/z (%): 287 (M + 18, 48), 269 (M, 89),
123 (100). IR (CCl4) cm–1: 3004m, 2932m, 2838m, 2232m,
1781s, 1717m, 1605s, 1505s, 1465m, 1296m, 1225s, 1183s,
1
1041m. H NMR (200 MHz, CDCl3) δ: 3.79 (s, 3H, OMe),
Synthesis of diaryl carbonates
Diphenyl carbonate is commercially available. The other
diaryl carbonates were prepared as described earlier (3).
6.88–7.72 (m, 8H, ArH). 13C NMR (75 MHz, CDCl3) δ:
55.5, 110.1, 114.5, 117.9, 121.5, 121.9, 133.7, 144.2, 151.4,
153.8, 157.7.
Thermolysis of 11a with DMAD
The procedure described below for thermolysis of 11a
with DMAD is typical.
2-(p-Cyanophenoxy)-2-(p-methoxyphenoxy)-5,5-dimethyl-
∆3-1,3,4-oxadiazoline (1.03 g, 3.04 mmol) was heated in the
presence of DMAD (0.52 g, 3.66 mmol) in 25 mL of ben-
zene (sealed tube) at 110°C for 24 h. Radial chromatography
following evaporation of the solvent gave 12a (37%) and
14a (29%).
Thermolysis of oxadiazoline (11b) with DMAD
Thermolysis of 2(p-cyanophenoxy)-5,5-dimethyl-2(p-
methylphenoxy)-∆3-1,3,4-oxadiazoline in the presence of
DMAD gave 12b (26%) and 14b (28%).
Triester 12b: pale yellow oil. EI-MS m/z (%): 348 (M –
Me), 272 (M – OC6H4Me, 100), 244 (23), 200 (23), 176
(14), 154 (46), 107(8), 77 (23), 59 (32), 51 (9). CI-MS
(NH3) m/z (%): 397 (M + 18, 33), 380 (M + 1), 272 (M –
OC6H4Me, 100). HRMS m/z calcd. for C21H17NO6 : 379.1056;
found: 379.1073. IR (CCl4) cm–1: 2956m, 2234m, 1746s,
1639m, 1610m, 1507s, 1437s, 1269s, 1229s, 1189s, 1075s,
Triester 12a: white crystals, mp: 121 to 122°C. EI-MS m/z
(%): 395 (M, 7), 364 (M – 31, 6), 272 (100), 244 (40), 200
(42), 154 (96), 123 (71), 95 (40), 59 (80). CI-MS (NH3) m/z
(%): 413 (M + 18, 19), 396 (M + 1, 17), 272 (100). HRMS
m/z (%): calcd. for C21H17NO7: 395.1005; found: 395.1026.
IR (CCl4) cm–1: 2955m, 2839m, 2234m, 1746br, 1637m,
1608m, 1505s, 1437m, 1270br, 1232br, 1188br, 1073s,
1
1010s. H NMR (200 MHz, CDCl3) δ: 2.30 (s, 3H, Me),
3.88 (s, 3H, OMe), 3.92 (s, 3H, OMe), 6.66–7.13 (m, ArH
of C6H4Me), 7.58–7.73 (m, ArH of C6H4CN). 13C NMR
(75 MHz, CDCl3) δ: 20.7, 53.3 (2 × OMe), 99.8, 113.9,
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Can. J. Chem. Vol. 79, 2001
117.8, 120.3, 120.6, 128.6, 130.0, 132.4, 136.3, 137.2,
145.2, 147.5, 162.1, 162.4, 165.8.
(30), 41 (24). CI-MS (NH3) m/z (%): 262 (M + 18, 100),
244 (M, 100). IR (CCl4) cm–1: 3006m, 2956m, 2838m,
1780s, 1596m, 1507s, 1465m, 1442m, 1298m, 1229s, 1182s,
1103m, 1071m, 1041m, 1010m. 1H NMR (200 MHz,
CDCl3) δ: 3.80 (s, 3H, OMe), 6.88–7.44 (m, 9H, ArH). 13C
NMR (75 MHz, CDCl3) δ: 55.6, 114.5, 120.9, 121.8, 126.2,
129.5, 144.6, 151.1, 152.4, 157.6.
p-Cyanophenyl p-tolyl carbonate 14b: white solid, mp: 91
to 92°C. EI-MS m/z (%): 65 (30), 77 (33), 91 (100), 102
(19), 209 (13), 253 (M+, 23). CI-MS (NH3) m/z (%): 253
(M + 1, 37), 271 (M + 18, 18). HRMS m/z calcd. for
C15H11NO3: 253.0739; found: 253.0745. IR (CCl4) cm–1:
2234m, 1783s, 1604m, 1507s, 1287m, 1229s, 1189s, 1167s,
1018m, 1006m. 1H NMR (CDCl3) δ: 2.36 (s, 3H, Me), 7.12–
7.25 (m, 4H, C6H4Me), 7.40–7.73 (m, 4H, C6H4CN). 13C
NMR (50 MHz) δ: 20.8, 110.3, 117.9, 120.4, 122.0, 130.2,
133.8, 136.4, 148.6, 151.3, 154.0.
Thermolysis of oxadiazoline 11e with DMAD
Thermolysis of 2(o-methoxyphenoxy)-5,5-dimethyl-2-
phenoxy-∆3–1,3,4-oxadiazoline in the presence of DMAD
gave 12e (11%) and 13e (25%) as well as 14e (35%).
Triester 12e: pale yellow oil. EI-MS m/z (%): 371 (M + 1,
4), 339 (10), 277 (M – 93, 100), 249 (100), 219 (10), 205
(28), 181 (18), 159 (20), 131 (45), 103 (12), 65 (40). CI-MS
(NH3) m/z (%): 388 (M + 18, 34), 371 (M + 1, 40), 277 (M
– 93, 100). HRMS m/z calcd. for C20H19O7 (M + H):
371.1131; found: 371.1108. IR (CCl4) cm–1: 3005m, 2954s,
2841m, 1745br, 1592s, 1550br, 1493s, 1461m, 1436m,
1226br, 1162m, 1117m. 1H NMR (200 MHz, CDCl3) δ: 3.80
(s, 3H, OMe), 3.84 (s, 3H, OMe), 3.90 (s, 3H, OMe), 6.77–
7.46 (m, 9H, ArH). 13C NMR (75 MHz, CDCl3) δ: 52.9,
53.0, 55.8, 111.3, 120.8, 121.0, 123.0, 126.1, 129.3, 130.0,
130.1, 131.8, 144.2, 150.1, 156.9, 162.7, 163.9, 166.9.
Thermolysis of oxadiazoline (11c) with DMAD
Thermolysis of 2(p-cyanophenoxy)-5,5-dimethyl-2-
phenoxy-∆3–1,3,4-oxadiazoline in the presence of DMAD
gave 12c (37%) and 14c (30%).
Triester 12c: pale yellow oil. EI-MS m/z (%): 365 (M, 2),
334 (M – OMe, 7), 272 (M – OPh, 100), 244 (17), 200
(13), 176 (8), 154 (14), 77 (9), 59 (37). CI-MS (NH3) m/z
(%): 383 (M + 18, 84), 272 (M – OPh, 100). IR (CCl4) cm–1:
2955m, 2233m, 1742b, 1638m, 1593m, 1550m, 1492s,
1436s, 1307m, 1267b, 1233b, 1189s, 1160m, 1074s, 1010s.
1H NMR (200 MHz, CDCl3) δ: 3.89 (s, 3H, OMe), 3.93 (s,
3H, OMe), 6.77–7.76 (m, 9H, ArH). 13C NMR (75 MHz,
CDCl3) δ: 53.4 (2 × OMe), 114.0, 117.8, 120.7, 126.6,
128.6, 128.7, 129.6, 132.5, 137.1, 145.4, 149.6, 162.0,
165.9.
Triester (13e): pale yellow oil. EI-MS m/z (%): 371 (M + 1,
5), 247 (M – 123, 100), 219 (53), 175 (19), 151 (16), 129
(31), 95 (22), 77 (28), 59 (33). CI-MS (NH3) m/z (%): 388
(M + 18, 38), 371 (M + 1, 12), 247 (M – 123, 100). HRMS
m/z calcd. for C20H19O7 (M + H): 371.1131; found:
371.1108. IR (CCl4) cm–1: 2954m, 2841m, 1760m, 1742br,
1633m, 1609m, 1501s, 1436m, 1308m, 1263br, 1232br,
p-Cyanophenyl phenyl carbonate (14c): white solid, mp: 79
to 80°C. EI-MS m/z (%): 239 (M, 33), 195 (22), 167 (28),
140 (5), 121 (13), 102 (21), 77 (100), 65 (35), 49 (30), 43
(76). CI-MS (NH3) m/z (%): 257 (M + 18, 21), 239 (M, 89).
IR (CCl4) cm–1: 2235m, 1782s, 1603m, 1550m, 1507m,
1
1198br, 1174s, 1110s, 1069m, 1010m. H NMR (200 MHz,
CDCl3) δ: 3.74 (s, 3H, OMe), 3.90 (s, 3H, OMe), 3.92 (s,
3H, OMe), 6.63–7.60 (m, 9H, ArH). 13C NMR (75 MHz,
CDCl3) δ: 53.0 (2 × OMe), 55.8, 112.6, 120.7, 122.3, 126.1,
127.3, 128.1, 128.7, 130.3, 132.6, 138.9, 148.2, 151.2,
162.5, 163.0, 167.2.
1
1495m, 1221br, 1186s, 1163s, 1008m. H NMR (200 MHz,
CDCl3) δ: 7.26–7.74 (m). 13C NMR (75 MHz, CDCl3) δ:
110.3, 118.0, 120.7, 122.0, 126.7, 130.0, 133.9, 150.7,
151.1, 153.9.
o-Methoxyphenyl phenyl carbonate (14e): white solid, mp:
58 to 59°C. EI-MS m/z (%): 244 (M, 100), 200 (28), 185
(10), 151 (20), 124 (20), 95 (33), 77 (84), 65 (37). CI-MS
(NH3) m/z (%): 262 (M + 18, 100), 244 (M, 100). IR
(CCl4) cm–1: 2962m, 2841m, 1784s, 1550br, 1503br, 1465m,
Thermolysis of p-methoxyphenoxy phenoxy oxadiazoline
(11d) with DMAD
Thermolysis of 2-(p-methoxyphenoxy)-2-phenoxy-5,5-
dimethyl-∆3–1,3,4-oxadiazoline in the presence of DMAD
gave 12d (38%) and 14d (32%).
1
1310m, 1233br, 1191s, 1173s, 1113m. H NMR (200 MHz,
CDCl3) δ: 3.89 (s, 3H, OMe), 6.92–7.44 (m, 9H, ArH). 13C
NMR (75 MHz, CDCl3) δ: 56.0, 112.6, 120.7, 120.9, 122.2,
126.1, 127.3, 129.5, 140.0, 151.0, 151.2, 151.6.
Triester 12d: light yellow oil. EI-MS m/z (%): 370 (M, 2),
339 (M – OMe, 5), 311 (7), 277 (6), 247 (100), 219 (41),
151 (16), 129 (22), 95 (10), 59 (27). CI-MS (NH3) m/z
(%): 388 (M + 18, 8), 371 (M + 1, 15), 247 (100). IR
(CCl4) cm–1: 3004m, 2954m, 2839m, 1743br, 1633m, 1505s,
Acknowledgements
1
1437m, 1269s, 1232s, 1188s, 1070m, 1039m, 1012m. H
The authors gratefully acknowledge the financial support
of the Natural Sciences and Engineering Research Council
of Canada (NSERC) and the assistance of Mr. Brian Sayer
and Dr. Kirk Green with NMR spectroscopy and mass spec-
trometry.
NMR (200 MHz, CDCl3) δ: 3.75 (s, 3H, OMe), 3.90 (s, 6H,
2 × OMe), 6.65–7.49 (m, 9H, ArH). 13C NMR (75 MHz,
CDCl3) δ: 53.0, 53.1, 55.5, 114.4, 121.7, 126.3, 127.7,
128.9, 130.4, 132.8, 143.4, 148.0, 157.6, 162.7, 163.4,
167.0.
p-Methoxyphenyl phenyl carbonate (14d): light yellow oil.
EI-MS m/z (%): 245 (M + 1, 21), 244 (M, 100), 200 (15),
185 (32), 157 (11), 124 (60), 123 (46), 95 (25), 77 (95), 65
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© 2001 NRC Canada

Lu and Warkentin
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