Asymmetric oxidation of 2-(arylsulfenyl)pyrroles

Article abstract of DOI:10.1139/V08-054

The asymmetric oxidation of prochiral 2-(arylsulfenyl)pyrroles has been investigated. A marked electronic effect within the substrate significantly influenced the degree of enantioselectivitv obtained, with very high enantio-selectivity being obtained for

Full text of DOI:10.1139/V08-054

676
Asymmetric oxidation of 2-(arylsulfenyl)pyrroles
Alison Thompson, Jose R. Garabatos-Perera, and H. Martin Gillis
Abstract: The asymmetric oxidation of prochiral 2-(arylsulfenyl)pyrroles has been investigated. A marked electronic
effect within the substrate significantly influenced the degree of enantioselectivity obtained, with very high enantio-
selectivity being obtained for 5-(nitrobenzensulfenyl)pyrrole-2-carboxaldehydes using Ti(i-PrO)4/(+)-(R,R)-
DET/H2O/CHP. This result bodes well for optimizing the asymmetric oxidation of other diaryl sulfides, substrates that
have previously given only low enantiomeric excesses.
Key words: asymmetric oxidation, sulfide, sulfoxide, pyrrole.
Résumé : On a étudié l’oxydation asymétrique de 2-(arylsulfényl)pyrroles prochiraux. On a noté qu’un effet électro-
nique marqué dans le substrat influence d’une façon significative le degré d’énantiosélectivité obtenu, la plus grande
énantiosélectivité étant obtenue avec les 5-(nitrobenzènesulfényl)pyrrole-2-carboxaldéhydes en utilisant le Ti(i-PrO)4/(+)-
(R,R)-DET/H2O/CHP. Les résultats obtenus sont de bon augure pour l’optimisation de l’oxydation asymétrique d’autres
sulfures de diaryles, des substrats qui n’ont donné antérieurement que de très faibles excès énantiomériques.
Mots-clés : oxydation asymétrique, sulfure, sulfoxyde, pyrrole.
[Traduit par la Rédaction] Thompson et al. 681
Introduction
functionalized pyrroles, 2-(arylsulfinyl)pyrroles (11) become
of interest.
The use of chiral sulfoxides as auxiliaries for the asym-
metric synthesis of biologically important compounds has
found many applications (1, 2). For example, the efficiency
of sulfoxides has been demonstrated for controlling the
stereoselectivities of alkylation reactions, Michael addition
reactions, aldol reactions, cycloaddition reactions, and
Pummerer rearrangements (3). Chiral sulfoxides are gener-
ally prepared using either organometallic reagents and re-
solved diastereomeric sulfinates (i.e., chiral sulfinyl transfer
reagents) (2, 4–6) or via the asymmetric oxidation of
prochiral sulfides (2, 7).
Synthetic strategies involving pyrroles often require the
introduction of a deactivating electron-withdrawing group
that reduces the electron-rich character of the pyrrole unit.
As such, the nucleophilicity of the pyrrolic core may be con-
trolled and undesirable side-reactions minimized (8). Whilst
this control has commonly been achieved by the use of
carboxylates at the 2-position, the use of a sulfenyl group to
mask the 2-position, rather than protect through electron-
withdrawal, has recently been demonstrated (9, 10). The
utility of 2-(sulfenyl)pyrroles in acylation reactions, nitration
reactions, and condensation reactions with various alde-
hydes, as well as the removal of the sulfenyl group using
Raney nickel, highlights the sulfenyl group as a valuable
masking/protecting group for pyrrolic compounds. With 2-
(arylsulfenyl)pyrroles showing promise for the synthesis of
Chiral 2-(arylsulfinyl)pyrroles of high enantiomeric purity
prepared by sulfinylation using diastereomerically pure
menthyl arylsulfinates have attracted attention because of
their use as removable chiral auxiliaries and as building blocks
for macromolecular chemistry (4). While this method has
been successfully applied for several 2-(arylsulfinyl)pyrroles
(4, 5), it lacks generality because of the difficulty in obtain-
ing a diverse set of diastereomerically pure menthyl aryl- or
alkylsulfinates (2). Indeed, the preparation of diastereo-
merically pure menthyl arylsulfinates requires multiple
recrystallizations, and, furthermore, menthyl alkylsulfinates
(e.g., menthyl methanesulfinate) are typically oils, which
thus prohibits recrystallization as a strategy by which to ob-
tain diastereomerically pure material (12, 13). In this article,
we report investigations towards an alternative approach to
chiral 2-(arylsulfinyl)pyrroles involving the asymmetric oxi-
dation of prochiral 2-(arylsulfenyl)pyrroles.
Although numerous methodologies have been reported for
the asymmetric oxidation of sulfides to sulfoxides (2, 6, 14–
16), so far, they have been largely limited to sulfides with
two stereochemically different substitutents (i.e., aryl and
alkyl). Substrates with similar groups such as dialkyl or
diaryl generally lead to little or no asymmetric induction.
For example, the asymmetric oxidation of methyl benzyl -
sulfide using a chiral titanium complex (17, 18), chiral
oxaziridine (19), chiral metallo(salen) complex (20), or en-
zymatic methods (21, 22) gives the desired sulfoxide in
modest–excellent yields (50%–97%), but with poor
enantioselectivities (14%–58%). Even a recent example for
the asymmetric oxidation of sulfides using hydrogen perox-
ide in water and a homochiral iron(III) catalyst focuses on
the oxidation of methyl phenyl sulfide, with a somewhat re-
duced enantioselectivity being obtained for even ethyl
Received 3 December 2007. Accepted 19 February 2008.
Published on the NRC Research Press Web site at
canjchem.nrc.ca on 13 May 2008.
A. Thompson,1 J.R. Garabatos-Perera, and H.M. Gillis.
Department of Chemistry, Dalhousie University, Halifax, NS
B3H 4J3, Canada.
1Corresponding author (e-mail: Alison.Thompson@dal.ca).
phenyl sulfide (23). Similarly,
a
very recent novel
Can. J. Chem. 86: 676–681 (2008)
doi:10.1139/V08-054
© 2008 NRC Canada

Thompson et al.
677
Scheme 1. Synthesis of 2-(arylsulfenyl)pyrroles.
oxaziridinium salt, derived from cholesterol, gives good–ex-
cellent enantioselectivity for a variety of aryl alkyl sulfides,
but no reports for diaryl sulfides are included (24).
38) and Modena (18). In our preliminary efforts, the asym-
metric oxidation of 2-(phenylsulfenyl)pyrrole 2a using tert-
butyl hydroperoxide (TBHP) according to both the Kagan
and Modena protocols resulted in essentially racemic
sulfoxide 6a (Table 1, entry 1), albeit in reasonable isolated
yield. Under similar conditions, pyrroles 2e and 2f, bearing
an electron-withdrawing group within the sulfenyl moiety,
were oxidized using cumyl hydroperoxide (CHP) at –20 °C
and 25 °C with a dramatic improvement of enantio-
selectivity (Table 1, entries 2 and 3 vs. entry 1). Interest-
ingly, 2e, bearing one nitro group, gave an enantioselectivity
of 29% under these conditions, whereas 2e, with two nitro
groups, gave an enantioselectivity of 78%, and so decreasing
the electron-density about the sulfide appeared to result in
an increased enantioselectivity. However, the use of 3a with
an electron-withdrawing tosyl group on the pyrrolic nitrogen
atom did not result in improved enantioselectivity, despite
attempts under various conditions (Table 1, entry 4). Main-
taining N-tosyl substitution, the introduction of the more
sterically demanding naphthyl group within 3b, rather than a
phenyl or tolyl group within 3a and 3c, respectively, on the
sulfenyl moiety did not significantly affect the oxidation re-
action (Table 1, entries 5 and 6A). Furthermore, and signifi-
cantly, other well-known methods for asymmetric sulfide
oxidation resulted in poor yields and enantiopurities with 2-
(p-tolylsulfenyl)pyrrole-N-tosylpyrrole 3c (Table 1, entry 6;
methods C–E). The poor enantioselectivities for 3c serve to
further emphasize that the preparation of enantiopure sul-
foxides by the oxidation of diaryl sulfides is far from trivial.
Results and discussion
To probe their asymmetric oxidation, we prepared a series
of 2-(arylsulfenyl)pyrroles with various substituents at the 2-
and 5-positions. Following an established procedure for the
preparation of 2-(phenylsulfenyl)pyrrole 2a (10, 25) from 2-
(thiocyanato)pyrroles (26–28) and phenylmagnesium bro-
mide, a series of 2-(arylsulfenyl)pyrroles 2a–2d were ob-
tained in good yields by simply varying the Grignard reagent
used (Scheme 1). We previously reported the preparation of
4e and 4f using Vilsmeier–Hack formylation conditions (9),
and extension of this work for the 2-(arylsulfenyl)pyrroles
2a–2d provided 5-(arylsulfenyl)pyrrole-2-carboxylaldehydes
4a–4d in moderate to good yields. The excellent directing
ability of the arylsulfenyl group for formylation at the vacant
α-position emphasizes a general feature of sulfenyl masking
groups. In our hands, 2-(arylsulfenyl)pyrroles 2a–2d pre-
pared via this route were found to be unstable to air (9) and
were immediately protected as their corresponding N-tosyl
derivatives 3a–3d for ease of manipulation and storage (8).
In all cases, the introduction of an electron-withdrawing
group (tosyl or formyl) resulted in enhanced product stability.
With several prochiral 2-(arylsulfenyl)pyrroles available,
we investigated their asymmetric oxidation (29–32) using
chiral titanium complexes prepared from Ti(i-PrO)4 and (+)-
(R,R)-diethyl tartrate (DET), as described by Kagan (17, 33–
© 2008 NRC Canada

678
Can. J. Chem. Vol. 86, 2008
Table 1. Asymmetric and racemic oxidation of 2-(arylsulfenyl)pyrroles.
Time
(h)
Temperature
(°C)
Isolated
yield (%)
eeb
(%)
Entry
1
Substrate
Product
R1
Ph
R2
H
R3
H
Methoda
Oxidant
0
9
29
2a
6a
A
B
A
TBHP
TBHP
CHP
20
20
48
–20
–20
–20
72
98
80
2
3
4
2e
2f
6e
6f
4-NO2Ph
2,4-NO2Ph
Ph
H
H
H
H
H
A
CHP
20
25
87
78
3a
5a
Ts
A
A
B
F
A
F
A
C
D
CHP
CHP
20
20
20
15 min
20
2
20
48
48
–20
25
25
0
25
25
–20
25
25
0
77
78
77
91
72
83
11
6
—
8c
7
—
12c
—
16c
31
0
TBHP
m-CPBA
CHP
m-CPBA
CHP
5
6
3b
3c
5b
5c
1-Naph
Tol
Ts
Ts
H
H
oxaziridine
H2O2
E
H2O2
48
25
0
—
F
m-CPBA
CHP
CHP
TBHP
CHP
TBHP
CHP
CHPd
TBHP
2
20
48
20
20
20
20
20
20
25
–20
25
–20
–20
–20
–20
–20
–20
45
0
—
—
27
69
54
53
53
17
52
7
8
3d
4a
5d
7a
Mes
Ph
Ts
H
H
A
A
A
A
B
B
B
A
37
62
79
95
72
70e
87
CHO
9
4b
4c
7b
7c
1-Naph
Tol
H
H
CHO
CHO
A
A
CHP
TBHP
20
20
–20
–20
87
78
53
65
10
A
A
A
CHP
CHP
CHP
20
48
48
–20
–20
–20
78
93
45
54
48
96
11
12
4d
4e
7d
7e
Mes
4-NO2Ph
H
H
CHO
CHO
F
A
m-CPBA
CHP
2
20
25
–20
45
0
—
—
13
4f
7f
2,4-NO2Ph
H
CHO
A
CHP
20
25
85
98
F
m-CPBA
2
25
76
—
aMethod A: Ti(i-PrO)4/(+)-(R,R)-DET/H2O/TBHP or CHP (1:2:1:2) in CH2Cl2. Method B: Ti(i-PrO)4/(+)-(R,R)-DET/TBHP or CHP (1:4:2) in
ClCH2CH2Cl. Method C: (+)-(1S)-(10-camphorsulfonyl)oxaziridine (1.05 equiv.) in CH2Cl2 (43). Method D: (–)-(S)-2-(N-3,5-diiodosalicyliden)amino-3,3-
dimethyl-1-butanol/[VO(acac)2]/H2O2 (1.5 equiv.) in CH2Cl2 (20, 44). Method E: chloroperoxidase/H2O2 (1.5 equiv.) in 20% acetonitrile and sodium citrate
buffer (0.05 mol/L, pH 5) (45). Method F: m-CPBA (1.0 equiv.) in CH2Cl2.
bDetermined by chiral HPLC analysis.
cDetermined by chiral HPLC analysis of the corresponding tosyl deprotected 6c.
dKinetic resolution of rac-7a using 0.5 equiv. of CHP.
eRecovered 7a.
Continuing the notion that the introduction of electron-
withdrawing groups enhances the stereoselectivity of oxida-
tion (Table 1, entries 2 and 3 cf. 1) the effect of the α-formyl
substituent was investigated. Thus, 5-(arylsulfenyl)pyrrole-2-
carboxylaldehydes 4a–4f were oxidized in moderate–good
yields using the Kagan and Modena protocols, and, in all
cases, the enantioselectivities were enhanced (Table 1, en-
tries 8–13) over attempts using analogous pyrroles lacking
the formyl group. In particular, the asymmetric oxidation of
4e and 4f provided chiral sulfoxides 7e and 7f, respectively,
in 45% yield (96% ee) and 85% yield (98% ee), respectively
(Table 1, entries 12 and 13).
Our results regarding the asymmetric oxidation of 2-
(arylsulfenyl)pyrroles possessing substitutents with varying
electronic effects allow us to rationalize the effectiveness of
the Kagan and Modena protocols for this transformation. Al-
though the Kagan and Modena procedures have been widely
used for the oxidation of many prochiral sulfides, the struc-
© 2008 NRC Canada

Thompson et al.
679
ture of the reagent responsible for the stereochemical course
of the reaction is still unknown (39, 40). In some cases, ad-
ditives and the preparation of the reagent are crucial in ob-
taining specific chiral sulfoxides with high enantiopurities
and (or) yields (37, 41, 42). More seriously, prochiral sul-
fides with two sterically different substitutents are generally
good substrates for asymmetric oxidation, whereas sub-
strates with similar groups often give products with low
enantiopurities. For example, Kagan and co-workers re-
ported the asymmetric oxidation of aryl methyl sulfides us-
ing a chiral titanium complex prepared from Ti(i-PrO)4/(+)-
(R,R)-DET/H2O/TBHP (1:2:1:1) to give chiral sulfoxides
with good–excellent enantiopurities (i.e., phenyl, 89% ee; p-
tolyl, 91% ee; p-methoxyphenyl, 86% ee; p-nitrophenyl,
77% ee); however, the asymmetric oxidation of alkyl methyl
sulfides under the same conditions resulted in chiral
sulfoxides with reduced enantiopurities (i.e., n-octyl, 71%
ee, t-butyl, 53% ee; cyclohexyl, 54% ee; benzyl, 58% ee)
(17). In the face of the poor enantioselectivities that have
traditionally been obtained for the asymmetric oxidation of
diaryl sulfides, the results from the present study suggest
that there is an important electronic effect that can influence
this transformation. Although the origin of enantioselectivity
is speculative at this point, one factor that could temper the
influence of an electronic effect is the substrate–reagent
binding affinity. If the mechanism involves a reversible as-
sociative step and electron-rich substrates have high associa-
tion constants, as would be expected, reactions involving
electron-rich substrates would give lower enantio-
selectivities, since these substrates bind more strongly (i.e.,
high reactivity and less selective). For electron-poor sub-
strates, low association constants would allow for the sub-
strate to dissociate and reassociate before reaction occurs
(i.e., low reactivity and more selective). The traditional
model used to predict the stereochemical outcome of sulfide
oxidation involves the differentiation of sterically different
substituents, and electronic effects are not typically consid-
ered (17, 39, 40). On the contrary, the results of the study
presented herein strongly suggest that electronic effects play
an important role in the asymmetric oxidation of diaryl sub-
strates.
Experimental section
Representative procedures
2-(p-Tolylsulfenyl)pyrrole (2c)
To a solution of p-tolylmagnesium bromide [prepared
from p-bromotoluene (12.0 mL, 16.8 g, 100 mmol) and
magnesium turnings (2.4 g, 100 mmol) in anhydrous THF
(150 mL) at 0 °C under N2], was added dropwise 2-
(thiocyanato)pyrrole (4.9 g, 39 mmol) in anhydrous THF
(150 mL). After stirring at 0 °C for 30 min, ice-cold water
(~100 mL) was added. The mixture was diluted with EtOAc
and washed with satd. aqueous NH4Cl and brine, before be-
ing dried over MgSO4 and concentrated to give crude 2-(p-
tolylsulfenyl)pyrrole 2c, which was purified using flash col-
umn chromatography (CH2Cl2 as eluent) to give the title
compound 2c (6.7 g, 38 mmol, 97%).
A solution of 2-(p-tolylsulfenyl)pyrrole-N-tosylpyrrole 3c
(0.52 g, 1.5 mmol) in 15% v/v 2 mol/L NaOH (5.5 mL,
11 mmol) in MeOH (35 mL) was heated to reflux for 2 h
until no starting material remained (TLC analysis). The mix-
ture was cooled to room temperature and diluted with
EtOAc, washed with aqueous 0.1 mol/L NaOH (3×) and
brine (3×) before being dried over MgSO4 and then concen-
trated to give crude 2-(p-tolylsulfenyl)pyrrole 2c (0.25 g,
1.4 mmol, 93%), which was used in the next reaction with-
1
out further purification. H NMR (500 MHz, CDCl3) δ: 8.17
(1H, br s), 7.00–6.97 (2H, m), 6.93–6.91 (2H, m), 6.84–6.82
(1H, m), 6.53–6.52 (1H, m), 6.27–6.26 (1H, m), 2.24
(3H, s). 13C NMR (125 MHz, CDCl3) δ: 135.5 (s), 135.5 (s),
129.9 (d × 2), 126.5 (d × 2), 121.8 (d), 118.4 (d), 116.4 (s),
110.4 (d), 21.0 (q). HRMS m/z calcd. for C11H11NS:
189.0612 (188.0534 for M – H); found: 188.0539 (ESI–).
2-(p-Tolylsulfenyl)pyrrole-N-tosylpyrrole (3c)
TsCl (8.1 g, 43 mmol) was added to a suspension of 2-(p-
tolylsulfenyl)pyrrole 2c (6.7 g, 35 mmol) and NaOH (5.6 g,
141 mmol) in 1,2-dichloroethane (130 mL) at 0 °C. The re-
sulting mixture was warmed to room temperature and stirred
for 12 h. Water was added, and the mixture was extracted
with CH2Cl2 (3×). The combined organic layers were
washed with brine, dried over MgSO4, and then concentrated
to give crude 2-(p-tolylsulfenyl)pyrrole-N-tosylpyrrole 3c,
which was purified using flash column chromatography
(50% CH2Cl2 in hexanes as eluent) to give the title com-
1
pound 3c (10.0 g, 29 mmol, 83%). H NMR (500 MHz,
CDCl3) δ: 7.73 (2H, d, J = 8.5 Hz), 7.61–7.59 (1H, m), 7.10
(2H, d, J = 8 Hz), 6.90 (2H, d, J = 8 Hz), 6.76 (2H, d, J =
8 Hz), 6.53–6.52 (1H, m), 6.33–6.31 (1H, m), 2.31 (3H, s),
2.24 (3H, s). 13C NMR (125 MHz, CDCl3) δ: 145.1 (s),
135.7 (s), 135.5 (s), 134.1 (s), 129.68 (d × 2), 129.64 (d ×
2), 128.2 (d × 2), 127.0 (d × 2), 126.6 (d), 125.8 (d), 121.0
(s), 111.8 (d), 21.7 (q), 21.1 (q). LRMS (EI): 343 ([M]+,
100), 188 (73). HRMS m/z calcd. for C18H17NO2S2:
343.0701; found: 342.9792 (EI).
Conclusions
We have demonstrated the asymmetric oxidation of 2-
(arylsulfenyl)pyrroles using a chiral titanium complex. Our
work suggests a strong electronic effect where the incorpora-
tion of electron-withdrawing groups to differentiate the two
sulfenyl substituents results in an enhancement of enantio-
selectivity. This method is particularly well-suited for
prochiral sulfides bearing two aryl groups and expands the
scope of substrates suitable for the Kagan and Modena pro-
tocols to include prochiral 2-(arylsulfenyl)pyrroles. Our cur-
rent research is directed towards the preparation of a range of
enantiopure 2-(arylsulfinyl)pyrroles using alternative strategies.
5-(p-Tolylsulfenyl)pyrrole-2-carboxylaldehyde (4c)
POCl3 (0.28 mL, 0.47 g, 3.1 mmol) was added dropwise
to DMF (0.24 mL, 0.23 g, 3.1 mmol) at 0 °C under N2. The
resulting mixture was heated to 100 °C and stirred for
15 min. After cooling to 0 °C, a solution of 2-(p-
tolylsulfenyl)pyrrole 2c (0.25 g, 1.4 mmol; 0.05 mol/L in
© 2008 NRC Canada

680
Can. J. Chem. Vol. 86, 2008
CH2Cl2) was slowly added and then heated at reflux for 1 h.
The resulting mixture was cooled to 0 °C, and aqueous
1 mol/L AcONa (50 mL) was added. The resulting suspen-
sion was heated to reflux and stirred for 1 h. After cooling to
room temperature, the mixture was extracted with CH2Cl2
(3×). The combined organic layers were washed with satd.
aqueous Na2CO3 (3×) and brine, before being dried over
MgSO4 and then concentrated to give crude 5-(p-
tolylsulfenyl)pyrrole-2-carboxylaldehydes 4c that was puri-
fied by flash column chromatography (CH2Cl2) to give the
title compound 4c (0.23 g, 1.1 mmol, 79%). 1H NMR
(500 MHz, CDCl3) δ: 9.62 (1H, br s), 9.38 (1H, s), 7.20 (2H,
d, J = 8 Hz), 7.09 (2H, d, J = 8 Hz), 6.92–6.91 (1H, m),
6.40–6.30 (1H, m), 2.31 (3H, s). 13C NMR (125 MHz,
CDCl3) δ: 178.5 (d), 137.1 (s), 135.1 (s), 131.5 (s), 130.6 (d
× 2), 130.4 (d × 3), 121.1 (s), 116.6 (d), 21.2 (q). HRMS m/z
calcd. for C12H11NOS: 217.0561 (216.0483 for M – H);
found: 216.0485 (ESI–).
calcd. for C18H17NO3S2: 359.0650 (382.0548 for M + Na);
found: 382.0542 (ESI+).
Acknowledgments
This work was supported by the Natural Sciences and En-
gineering Research Council of Canada (NSERC) and
AstraZeneca.
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50% wt% m-CPBA (0.22 g, 0.60 mmol) was added to a
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Asymmetric
Ti(i-PrO)4 (0.14 mL, 0.14 g, 0.50 mmol) was added
dropwise to a solution of (+)-(R,R)-DET (0.17 mL, 0.20 g,
1.00 mmol) and 2-(p-tolylsulfenyl)pyrrole-N-tosylpyrrole 3c
(0.17 g, 0.50 mmol) in CH2Cl2 (2 mL) under argon at room
temperature. After 5 min, water (9 µL, 0.50 mmol) was
added, and stirring was continued at room temperature for
45 min. 80% wt% Cumylhydroperoxide (0.19 mL, 0.15 g,
1.00 mmol) was then added under argon at –20 °C, and stir-
ring was continued for 20 h. Water was added, and the mix-
ture was allowed to stir for 30 min at room temperature. The
resulting mixture was diluted with CH2Cl2 and filtered
through a pad of silica and eluted with ethyl acetate to give
the crude product, which was purified using flash column
chromatography (10% ethyl acetate in CH2Cl2 as eluent) to
give the title 2-(p-tolylsulfinyl)pyrrole-N-tosylpyrrole 5c
(0.15 g, 40 mmol, 83%, 16% ee; determined by chiral HPLC
analysis of the corresponding tosyl deprotected 6c; see Sup-
plementary Data).2 1H NMR (500 MHz, CDCl3) δ: 7.81 (2H,
d, J = 8.5 Hz), 7.61 (2H, d, J = 8 Hz), 7.33 (1H, m), 7.29–
7.26 (4H, m), 6.62 (1H, dd, J = 2, 4 Hz), 6.31 (1H, dd, J =
3.5, 3.5 Hz), 2.40 (3H, s), 2.39 (3H, s). 13C NMR (125 MHz,
CDCl3) δ: 146.0 (s), 142.1 (s), 141.3 (s), 137.0 (s), 135.3 (s),
130.3 (d × 2), 129.9 (d × 2), 127.7 (d × 2), 126.4 (d), 126.0
(d × 2), 118.4 (d), 113.2 (d), 21.8 (q), 21.6 (q). HRMS m/z
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2 Supplementary data for this article are available on the journal Web site (canjchem.nrc.ca) or may be purchased from the Depository of Un-
published Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD 3754. For more in-
formation on obtaining material, refer to cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml.
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