Pd2dba3/Bippyphos: A robust catalyst system for the hydroxylation of aryl halides with broad substrate scope

Article abstract of DOI:10.1002/adsc.201300088

A mixture of tris(dibenzylideneacetone)dipalladium(0) (Pd ₂dba₃) and 5-(di-tert-butylphosphino)-1′,3′, 5′-triphenyl-1′H-[1,4′]bipyrazole (Bippyphos) is shown to be a robust and efficient catalyst system for the hydroxylation of structurally diverse (hetero)aryl halides under mild conditions and with broad substrate scope. Included in this reactivity survey is the successful synthesis of substituted benzofurans and related heteroatomic derivatives, which are formed via the hydroxylation of 2-haloalkynylarenes. Notably, a significant number of the reactions reported herein proceed at room temperature, and we have demonstrated that it is possible to conduct reactions on the benchtop under air using unpurified solvents with negligible loss in reactivity versus related transformations conducted under inert atmosphere conditions. We also report herein the first crystallographically characterized (Bippyphos)Pd(II) complex, which confirms the ability of this synthetically useful ligand to adopt a bidentate binding motif in a manner similar to Buchwald's biarylphosphine ligand class. Copyright

Full text of DOI:10.1002/adsc.201300088

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DOI: 10.1002/adsc.201300088
Pd2dba3/Bippyphos: A Robust Catalyst System for the
Hydroxylation of Aryl Halides with Broad Substrate Scope
Christopher B. Lavery,a Nicolas L. Rotta-Loria,a Robert McDonald,b
and Mark Stradiottoa,*
a
Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada, B3H 4J3
Fax : (+1)-902-494-1310; e-mail: mark.stradiotto@dal.ca
X-Ray Crystallography Laboratory, Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada,
b
T6G 2G2
Received: January 29, 2013; Published online: March 19, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300088.
Abstract: A mixture of tris(dibenzylideneacetone)di- reactions on the benchtop under air using unpurified
palladium(0) (Pd2dba3) and 5-(di-tert-butylphos- solvents with negligible loss in reactivity versus relat-
ACHTUNGTRENNUNGphino)-1’,3’,5’-triphenyl-1’H-[1,4’]bipyrazole (Bippy- ed transformations conducted under inert atmos-
phos) is shown to be a robust and efficient catalyst phere conditions. We also report herein the first crys-
system for the hydroxylation of structurally diverse tallographically characterized (Bippyphos)Pd(II)
(hetero)aryl halides under mild conditions and with complex, which confirms the ability of this syntheti-
broad substrate scope. Included in this reactivity cally useful ligand to adopt a bidentate binding motif
survey is the successful synthesis of substituted ben- in a manner similar to Buchwaldꢀs biarylphosphine
zofurans and related heteroatomic derivatives, which ligand class.
are formed via the hydroxylation of 2-haloalkynylar-
enes. Notably, a significant number of the reactions
reported herein proceed at room temperature, and Keywords: benzofurans; Bippyphos; hydroxylation;
we have demonstrated that it is possible to conduct ligands; palladium; phenols
Introduction
Phenols are among the most important synthons for
the construction of a variety of naturally occurring
and biologically active products.[1] Moreover, the
phenol moiety itself is present in a number of top sell-
ing pharmaceuticals; selected examples are depicted
in Figure 1.
Classical non-oxidative methods for preparing phe-
nols include nucleophilic aromatic substitution of acti-
vated aryl halides, copper-promoted transformation of
diazoarenes, and synthetic routes utilizing reactive
benzyne intermediates.[2] However, these protocols
are limited in scope and utility owing to the relatively
harsh reaction conditions that are required, as well as
the electronic demands and limited availability of the
requisite starting materials.[1,2] While the metal-cata-
lyzed cross-coupling of aryl halides and hydroxide
anions represents a conceputally attractive method
for the preparation of phenols,[1a,b,3,4] the catalyst em-
ployed must be judiciously chosen so as to overcome
potential challenges, including but not restricted to: that contain the phenol moiety.
Figure 1. Selected examples of top-selling pharmaceuticals
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catalyst inhibition by free hydroxide anions; difficult ents, which can be prone to hydrolysis at elevated
À
C O bond reductive elimination owing to the small temperatures under typical catalytic conditions, to be
size of the hydroxide group; and uncontrolled aryla- accomodated. More recently, Chern and co-workers
tion of the target phenol to afford the undesired reported on the use of a catalyst system comprised of
diaryl ether. While copper catalysts have been identi- Herrmannꢀs palladacycle and L1 under microwave ir-
fied for the hydroxylation of aryl halides,[5] the need radiation; however, elevated temperatures were re-
for high metal/ligand loadings and harsh reaction con- quired in order to achieve suitable reactivity (115–
ditions, as well as their typically poor performance 1508C).[7a]
with synthetically useful aryl chlorides,[6] represent im-
Notwithstanding these developments, the relative
portant practical drawbacks. In this context, the use scarcity of palladium-based catalysts that have proven
of palladium-based catalysts for such transformations effective in promoting the hydroxylation of (hetero)-
has been shown to offer significant reactivity advan- aryl halides provides motivation for further investiga-
tages, including increased scope, lower catalyst load- tion. From a practical perspective, the development of
ings, and milder reaction conditions.[7]
alternative catalysts based on a single commercially
The first report of the hydroxylation of aryl halides available palladium precursor/ligand pair, which
(bromides and chlorides) using a hydroxide salt was offers broad scope in the (hetero)aryl halide under
published by Buchwald and co-workers in 2006 utiliz- mild conditions and without the need for microwave
ing a palladium catalyst supported by either tBuX- irradiation is of particular interest. Moreover, the
Phos (L1) or Me4tBuXPhos (L2) at elevated tempera- identification of catalyst systems of this type that
tures (80–1008C), whereby the optimal ligand was prove capable of operating under air using bench-top
identified on a substrate-dependent basis (Figure 2).[7f] synthetic protocols would represent an important ad-
Following their initial report in 2009 regarding the vance in terms of enabling the broader uptake of such
successful application of L3 in analogous catalytic hy- hydroxylation protocols by synthetic chemists. Herein
droxylation chemistry (100–1208C),[7b] Beller and co- we report on such a catalyst system comprised of
workers subsequently disclosed that [PdACTHNUTRGNEUNG
(cod)- Pd2dba3 and Singerꢀs Bippyphos[8] ligand (L4,
ACHTUNGTRENNUNG(CH2SiMe3)2]/L3 mixtures catalyze the hydroxylation Figure 2), which is capable of furnishing a broad spec-
of aryl halides at room temperature.[7c] Such mild con- trum of phenols and phenol-derived benzofurans
ditions enabled nitrile and trifluoromethyl substitu- from (hetero)aryl bromides and chlorides in a manner
that is competitive with the best catalyst systems re-
ported to date (vide supra), while also demonstrating
for the first time the viability of employing bench-top
reaction protocols in such transformations. Also dis-
closed herein is the first example of a crystallographi-
cally characterized (Bippyphos)Pd(II) complex, which
establishes the ability of this useful ligand to bind pal-
ladium in a bidentate fashion.
Results and Discussion
In keeping with our interest in addressing current lim-
itations associated with the use of small and/or chal-
lenging substrate classes in palladium-catalyzed cross-
coupling chemistry,[9] we initially focused our atten-
tion on the development of a hydroxylation catalyst
system based on one of our DalPhos ligands.[9b,10] In
seeking a catalyst system that would have the ability
to operate at room temperature, and using conditions
inspired by the work of Buchwald[7f] and Beller,[7c] we
selected the hydroxylation of bromomesitylene as the
test reaction using commercially available Pd2dba3 as
the palladium source (4 mol% Pd; 8 mol% ligand;
CsOH·H2O, 3 equiv.; THF, 0.5M; 248C; 12 h). After
Figure 2. Previously reported ligands (L1–L3) used in the
screening a selection of our DalPhos ligands including
palladium-catalyzed hydroxylation of aryl halides, the Bip-
Mor-DalPhos,[9d] Pyr-DalPhos,[9a] and OTips-Dal-
pyphos ligand (L4) used in the present catalyst system, and
below other screened ligands that proved unsuccessful
under the test conditions employed.
Phos[9b], that had been shown to perform well in vari-
À
ous palladium-catalyzed C N cross-coupling applica-
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Pd2dba3/Bippyphos: A Robust Catalyst System for the Hydroxylation of Aryl Halides
Figure 3. Scope of Pd2dba3/L4-catalyzed cross-coupling of aryl halides with CsOH·H2O to form phenols. Reactions at 248C
unless stated; 20 h reaction time (unoptimized); yields are of isolated product unless stated.
tions, along with a selection of other commercially to exploring the scope of reactivity (Figure 3). Grati-
À
available ligands of note in palladium-catalyzed C O fyingly, a wide array of (hetero)aryl bromides and
cross-coupling chemistry [L1, IPr, JosiPhos-CyPF-tBu, chlorides were converted to the respective phenols in
Bippyphos (L4), Cy-Bippyphos, and Trippyphos; synthetically useful isolated yields (24–1008C; 63–
Figure 2], only the L4-based catalyst system proved 93%, 2–17). Bromobenzene and chlorobenzene, along
capable of achieving non-negligible conversions of the with their methylated derivatives, were each success-
starting bromomesitylene under the stringent test con- fully employed as substrates, affording the target phe-
ditions employed, affording the target phenol (2,4,6- nols in good to excellent isolated yields (2–5). Naph-
trimethylphenol, 1) in 89% isolated yield. Notably, thyl and biphenyl halides were also successfully ac-
less than 30% yield of the target phenol was obtained commodated as reaction partners (6–8), as were aryl
under similar conditions when using L4 in combina- halides featuring tert-butyl or methoxy groups (9 and
tion with either PdACHTNUTRGENNUG(OAc)2 or [PdACHUTGNTREN(NUNG cinnamyl)Cl]2. Fur- 10). While no appreciable difference in reactivity was
thermore, negligible conversion was achieved when observed between structurally analogous aryl bro-
using the Pd2dba3/L4 catalyst system under analogous mides and chlorides (4–6), the ability to selectively
conditions employing LiOH, NaOH, KOH or hydroxylate the bromide position in 1-bromo-4-
NMe4OH·
N
ACHTUNGTRENUNbNG enzene was demonstrated (11). Furthermore,
role of CsOH·H2O in this reaction system.
Having found the Pd2dba3/L4 catalyst system to be (12), trifluoromethyl (13), cyano (14), or keto (15)
effective for the hydroxylation of bromomesitylene functionality, which have proven to be particularly
under mild conditions, we then turned our attention challenging in such transformations (vide supra), were
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Figure 4. Scope of Pd2dba3/L4-catalyzed cross-coupling of 2-alkynylhaloarenes with CsOH·H2O to form benzofuran deriva-
tives; yields are of isolated product unless stated.
each accommodated under mild reaction conditions were also well-tolerated, affording 19 and 20. 2-Bro-
(24 or 658C). Nitrogen-containing heterocycles (8- moalkynylarene substrates featuring n-propyl or thio-
bromoquinoline and 2-chloropyridine) were also phen-3-yl substitution at the alkyne terminus also
transformed into the corresponding phenols (16 and proved to be suitable substrates in combination with
17) in high isolated yields. In keeping with the expect- the Pd2dba3/L4 catalyst system, producing the corre-
À
ed slow C O reductive elimination from intermedi- sponding substituted benzofurans (21 and 22) in high
ates of the type [(L4)Pd
N
ing methyl substituents in the ortho position reacted involving heteroaryl halides, we were pleased to ob-
most efficiently to furnish the corresponding phenols serve that the use of 2-bromo-3-(phenylethynyl)thio-
in excellent yield at room temperature, as did 1- phene enabled the formation of the thienofuran 23,
bromo- and 1-chloronaphthalene. While higher tem- while the use 2-bromo-3-(phenylethynyl)pyridine pro-
peratures were required in order to achieve efficient ceeded smoothly to give the furopyridine 24, each in
conversion for alternative substrates, such reactions high isolated yield.
proceeded smoothly affording the target phenol.
For convenience, the reactions presented herein
Phenols can serve as synthons in the formation of were typically set-up within a dinitrogen-filled glove-
sought-after oxygen-containing heterocycles such as box using purified solvents. However, control experi-
benzofurans.[1,11] In this context we became interested ments employing representative substrates confirmed
in investigating the utility of the Pd2dba3/L4 catalyst that hydroxylation reactions employing the Pd2dba3/
À
system in enabling tandem C O cross-coupling/hydro- L4 catalyst system can be conducted under signifi-
phenoxylation processes starting from 2-haloalkynyl- cantly less rigorous conditions, including under air
ACHTUNGTRENNUNGarenes (Figure 4). To the best of our knowledge, re- and using benchtop quality solvent that had not been
ports of such transformations are limited to a single pre-treated or purified in any way. To the best of our
publication from Buchwald and co-workers, wherein knowledge this represents the first report of palladi-
three examples in total (one using L1, two using L2) um-catalyzed aryl halide hydroxylation to be conduct-
are featured.[7f] Gratifyingly, the use of the Pd2dba3/L4 ed under such non-inert conditions. Our initial investi-
catalyst system allowed for the preparation of benzo- gations confirmed that the hydroxylation of bromo-
furans and related heteroaromatic derivatives featur- mesitylene to give 1 (248C, THF) proceeds quantita-
ing diverse substitution in high isolated yields (78– tively when the reaction is conducted under air, in-
93%, 18–24). Under the conditions outlined in cluding when using unpurified, benchtop-quality
Figure 4, 1-bromo- and 1-chloro-2-(phenylethynyl)- solvent. Having confirmed subsequently that reactions
benzene were each readily converted to the corre- conducted in air leading to 2, 4 and 6 were also suc-
sponding C-2-phenyl substituted benzofuran (18); cessful (Figure 3), we re-examined the synthesis of
methyl or fluoro substituents on the aryl bromide 21–24 under benchtop conditions. Gratifyingly, in all
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Pd2dba3/Bippyphos: A Robust Catalyst System for the Hydroxylation of Aryl Halides
cases the Pd2dba3/L4 catalyst system proved to be relative to what would be expected to be a compara-
2
À
robust, maintaining high activity and conversions to tively weak Pd C-5 (ipso) interaction. The k -P,C
the target furan, even while operating under air and/ bonding motif in C1 is reminiscent of that observed
or when using benchtop quality solvents (Figure 4). within analogous Pd(II) complexes of Buchwaldꢀs
These results further establish the practical utility of family of biarylphosphine ligands.[13] The observation
the Pd2dba3/L4 catalyst system for the hydroxylation that L4 is capable of ligating Pd(II) fragments in a bi-
of aryl halides under a range of experimental condi- dentate fashion is in keeping with Beller and co-work-
tions.
erꢀs assertion that a bidenate bonding motif engenders
Given the broadly useful nature of the Pd2dba3/L4 desirable catalytic performance in palladium-cata-
catalyst system for the hydroxylation of aryl halides, lyzed phenol synthesis under mild conditions.[7c]
we became interested in understanding how L4 binds
to palladium, as it has been proposed that ligand den-
ticity can dramatically influence catalyst performance Conclusions
in this class of transformations.[7c] Perhaps surprising-
ly, while L4 has proven useful in a number of palladi- In summary, we have identified a robust and efficient
um-catalyzed transformations,[8] the palladium coordi- catalyst system for the hydroxylation of aryl halides
nation chemistry of this ligand remains unexplored. comprised of Pd2dba3 and Bippyphos (L4). In utilizing
While our initial efforts to prepare an isolable coordi- Pd2dba3/L4, a diversity of (hetero)aryl halides was
nation complex derived from Pd2dba3 and L4 have transformed to the corresponding phenols, with a sig-
thus far proven unsuccessful, we have observed that nificant number of these transformations proceeding
treatment of L4 with [PdACTHNUTRGEN(UNG cod)Cl2] in CH2Cl2 at 248C at room temperature. We have also successfully ap-
over the course of 1 h affords the crystallographically plied this catalytic chemistry in the synthesis of struc-
characterized [(k2-P,C-Bippyphos)PdCl2] complex turally diverse benzofurans and related heteroatomic
(C1) as an orange solid in 84% isolated yield derivatives, which are formed via the hydroxylation of
(Figure 5).[12] Notably, L4 is coordinated in a k2-P,C- 2-haloalkynylarenes. Notably, the performance of
bidentate fashion to palladium in C1 via phosphorus Pd2dba3/L4 is competitive with the best catalyst sys-
and the ipso carbon of the lower pyrazole ring. The tems reported thus far in the literature for such trans-
À
observation that the Pd Cl-1 is longer than the corre- formations. Moreover, by using the Pd2dba3/L4 cata-
sponding Pd Cl-2 distance is in keeping with the an- lyst system we also achieved the first examples of pal-
À
ticipated stronger trans-directing ability of phosphorus ladium-catalyzed aryl halide hydroxylation to be con-
ducted under air and using unpurified, as-received re-
action solvents, which proceed with negligible loss in
reactivity relative to transformations conducted under
inert atmosphere conditions, thereby offering a useful
practical advantage. We also report herein the first
crystallographically characterized (Bippyphos)Pd(II)
complex, which confirms the ability of this syntheti-
cally useful ligand to adopt a bidentate binding motif
in a manner similar to Buchwaldꢀs biarylphosphine
ligand class. We envision that the desirable perfor-
mance of the commercially available Pd2dba3/L4 cata-
lyst system will facilitate the broader uptake of palla-
dium-catalyzed hydroxylation protocols by end-users
in both academic and industrial settings.
Experimental Section
General Information
Unless otherwise noted, all reactions were set up inside a di-
nitrogen-filled inert atmosphere glove-box and worked up in
air using benchtop procedures. Unless otherwise noted, 1,4-
dioxane and THF used in the catalytic transformations were
dried over Na/benzophenone followed by distillation under
an atmosphere of dinitrogen. Solvents used in the synthesis
of C1 were deoxygenated by sparging with dinitrogen fol-
lowed by passage through an mBraun double column sol-
Figure 5. ORTEP diagram of [(k2-P,C-Bippyphos)PdCl2]
(C1) shown with 50% displacement ellipsoids; all hydrogen
atoms have been omitted for clarity. Selected interatomic
À
À
À
distances (ꢂ): Pd Cl-1 2.3443(5); Pd Cl-2 2.3020(6); Pd P
À
2.2629(5); Pd C-5 2.220(2).
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vent purification system equipped either with one alumina-
packed column and one column packed with copper-Q5 re-
actant (pentane), or two alumina-packed columns (dichloro-
11 Hz), 29.5 (d, J=54 Hz), 29.4 (d, J=54 Hz); 31P{1H} NMR
(CDCl3): d=45.5.
methane and diethyl ether). [PdACHTUNRTGNEUNG(cinnamyl)Cl]2 used in the
Catalytic Protocol A (Phenol Synthesis)
palladium source evaluation experiments was prepared ac-
cording to a literature protocol,[14] and Pd2dba3 was pur-
chased from Strem Chemicals. Haloalkynylarene substrates
were prepared by using literature synthetic protocols involv-
ing Sonogashira reactions of aryl iodides[15] or bromides[16]
with appropriate terminal alkyne precursors. Bippyphos
(L4), Cy-Bippyphos and Trippyphos were provided as a gift
from Digital Specialty Chemicals. CDCl3 used for NMR
analysis of C1 was degassed by using at least three repeated
freeze-pump-thaw cycles and stored over 4 ꢂ molecular
sieves for 24 h prior to use. All other chemicals were ob-
tained from commercial sources in high purity and used as
received. Column chromatography was carried out using
Silicycle SiliaFlash 60 with particle size 40–63 mm (230–400
mesh). Gas chromatography (GC) data were obtained on
a Shimadzu GC-2014 equipped with a SGE BP-5 30 m,
0.25 mm I.D. column. In the case where yields are given on
the basis of gas chromatography experiments, the data were
corrected by calibration using dodecane as an internal stan-
dard and product identity was confirmed by comparison
with authentic samples. All 1H, 13C, and 31P NMR characteri-
zation data were collected at 300 K on a Bruker AV-500
spectrometer operating at 500.1, 125.8, and 202.5 MHz (re-
spectively) with chemical shifts reported in parts per million
To an oven-dried screw-capped vial was added a stir bar,
Pd2dba3 (2 mol%), L4 (8%), CsOH·H2O (3 mmol), aryl
halide (1 mmol) and 2 mL of THF (248C or 658C) or 1,4-di-
oxane (1108C). The vial was then sealed under dinitrogen
with a cap containing a PTFE septum, removed from the
glove-box and placed in a temperature-controlled aluminum
heating block set at the required temperature and stirred
vigorously. Reaction progress was monitored by use of TLC
or GC methods and after complete consumption of the aryl
halide had been observed (8–20 h), the reaction mixture was
allowed to cool, if necessary, and then acidified with 2N
aqueous HCl and diluted with water. The resulting solution
was extracted with either ethyl acetate or diethyl ether (3ꢃ
2 mL). The collected organic extractions were dried over
sodium sulfate, filtered and concentrated to afford crude
product which was further purified by use of column chro-
matography.
Catalytic Protocol B (Benzofuran Synthesis)
To an oven-dried screw-capped vial was added a stir bar,
Pd2dba3 (2 mol%), L4 (8%), CsOH·H2O (3 mmol), haloalk-
AHCTUNGTREGyNNUN nylarene (1 mmol) and 2 mL of 1,4-dioxane. The vial was
then sealed under dinitrogen with a cap containing a PTFE
septum, removed from the glove-box, placed in a tempera-
ture-controlled aluminum heating block set at 1108C and
stirred vigorously. Reaction progress was monitored by use
of TLC or GC methods and after complete consumption of
the haloalkynylarene had been observed (12 h), the reaction
mixture was cooled, diluted with ethyl acetate (50 mL) and
washed with water (50 mL). The layers were separated and
the organic layer was washed with water (3ꢃ50 mL). The or-
ganic layer was dried over sodium sulfate, filtered and con-
centrated to afford crude product which was further purified
by use of column chromatography.
1
downfield of SiMe4 (for H and 13C) or 85% H3PO4 in D2O
(for 31P). NMR data were acquired with the technical assis-
tance of Dr. Michael Lumsden (NMR-3, Dalhousie Univer-
sity), while mass spectrometric data were acquired by Mr.
Xiao Feng (Mass Spectrometry Laboratory, Dalhousie Uni-
versity). Elemental analyses were performed by Midwest
Microlab, LLC, Indianapolis, IN (USA). Characterization
1
data (HR-MS, H and 13C{1H} NMR; line data and spectra)
for the catalytic reaction products are provided in the Sup-
porting Information.
Synthesis and Characterization of Compound C1
Experiments Conducted Under Air
To a magnetically stirred solution of [PdACTHNUTRGENUG(N cod)Cl2] (0.10 g,
Catalytic protocols A and B were followed in the case of ex-
periments conducted under air with the exception that the
reactions were set up on the benchtop (using where noted
unpurified, as-received solvents). The yields reported for
these experiments are given on the basis of calibrated GC
data.
0.35 mmol) in CH2Cl2 (2 mL) was added a CH2Cl2 (2 mL)
solution of L4 (Bippyphos, 0.18 g, 0.35 mmol) resulting in
a transparent orange solution. After 1 h of magnetic stirring
at ambient temperature, 31P NMR analysis of the reaction
mixture confirmed the consumption of L4 and the quantita-
tive formation of a new phosphorus-containing product
(C1). The solvent and other volatiles were removed under
vacuum and the residual solid was washed with pentane (3ꢃ
2 mL) and then Et2O (3ꢃ2 mL). Subsequent removal of the Acknowledgements
solvents under vacuum afforded (C1) as an analytically pure
orange solid; yield: 0.20 g (0.29 mmol, 84%). Anal. calcd.
for C32H35Cl2N4PPd: C 56.19, H 5.19, N 8.19; found: C
55.87, H 4.82, N 8.21; 1H NMR (CDCl3): d=8.17 (d, J=
2.1 Hz, 1H), 7.76–7.74 (m, 2H), 7.41–7.49 (m, 2H), 7.45–
7.44 (m, 2H), 7.40–7.37 (m, 2H), 7.35–7.31 (m, 4H), 7.30–
7.27 (m, 2H), 6.57 (d, J=2.1 Hz, 1H), 1.10 (d, J=16.2 Hz,
9H), 0.99 (d, J=16.0 Hz, 9H); 13C{1H} NMR (CDCl3): d=
163.7, 159.9, 147.0 (d, J=13 Hz), 143.0 (d, J=39.9 Hz),
138.0, 131.3, 130.5, 130.1, 130.0, 129.9, 129.2, 129.0, 128.8,
128.7, 128.1, 127.0, 114.8, 95.9, 39.2 (d, J=9 Hz), 39.1 (d, J=
We are grateful to NSERC of Canada and Dalhousie Univer-
sity for their support of this work. Digital Specialty Chemi-
cals is thanked for the gifts of Bippyphos, Cy-Bippyphos, and
Trippyphos and Drs. Scott Laneman (DSC) and Sarah Craw-
ford (Dalhousie) are thanked for helpful discussions.
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