New C2-symmetrical 1,2-diphosphanes for the efficient rhodium-catalyzed asymmetric hydroboration of styrene derivatives

Article abstract of DOI:10.1002/1521-3773(20010401)40:7<1235::AID-ANIE1235>3.0.CO;2-Y

A double [2,3] sigmatropic rearrangement enables the fast synthesis of novel C₂-symmetrical 1,2-diphosphanes in good yields. These phosphanes (for example, 1; c-Hex = cyclohexyl) are highly efficient ligands for the rhodium-catalyzed asymmetric hydroboration of a wide variety of styrenes (see scheme). cod = 1,5-cyclooctadiene; DME = 1,2-dimethoxyethane.

Full text of DOI:10.1002/1521-3773(20010401)40:7<1235::AID-ANIE1235>3.0.CO;2-Y

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[14] Crystal structure analyses: 1: monoclinic, space group C2/c, a ˆ
2511.8(5), b ˆ 1309.2(3), c ˆ 4291.8(9) pm, b ˆ 101.37(3)8, Vˆ
13.84(1) nm3, Z ˆ 4; 1calcd ˆ 1.473 Mgm 3, m ˆ 7.109 mm 1, F(000) ˆ
6040. Data collection: 2q ˆ 3.86 ± 48.148, 18 ꢀ h ꢀ 28, 13 ꢀ k ꢀ 13,
49 ꢀ l ꢀ 49, 24666 reflections, of which 9010 were independent
(Rint ˆ 0.0781) and 611 refined. All non-hydrogen atoms were refined
anisotropically and the H atoms were included in calculated positions,
R1 ˆ 0.0663, wR2 ˆ 0.1794 (F > 4s(F)), GOF(F 2) ˆ 1.083; max. resid-
rearrangement of diphenylphosphinites that are readily
available from the corresponding unsaturated 1,2-diols
(Scheme 1). Herein we report the use of this rearrangement
for a practical preparation of new chiral 1,2-diphosphanes and
their application in highly efficient rhodium-catalyzed asym-
metric hydroborations.
3
Å
ual electron density 2.556 eŠ . 2: triclinic, space group P1, a ˆ
1548.9(3), b ˆ 1738.7(4), c ˆ 2097.4(4) pm, a ˆ 78.37(3), b ˆ 68.79(3),
OPPh2
3
g ˆ 77.12(3)8, Vˆ 5.0877(18) nm3, Z ˆ 2; 1calcd ˆ 1.473 Mgm
,
m ˆ
1
OPPh2
OPPh2
OH
OH
9.621 mm
,
F(000) ˆ 2396. Data collection: 2q ˆ 4.22 ± 51.848,
a)
b)
18 ꢀ h ꢀ 18, 21 ꢀ k ꢀ 21, 25 ꢀ l ꢀ 25, 55309 reflections, of which
18432 were independent (Rint ˆ 0.0939) and 829 refined. All non-
hydrogen atoms were refined anisotropically and the H atoms were
included in calculated positions, R1 ˆ 0.1313, wR2 ˆ 0.4136 (F >
4s(F)), GOF(F 2) ˆ 1.851; max. residual electron density
17.506 eŠ 3. The crystals of 2 are extremely thin, very small platelets,
and therefore the data set was strongly influenced by absorption
effects. An absorption correction could not be performed. Several
maxima of similar size occur which are all located at the periphery of
the molecule (60 ± 150 pm from the H atoms). If only data up to 2q ˆ
448 are considered for the refinement of the structure, the structural
parameters are altered only slightly: R1 ˆ 0.128; max. residual electron
density 13.40 Š 3. The intensities were measured with a STOE-IPDS
PPh2
O
5a
4
P(O)Ph2
P(O)Ph2
6a: 85 % from 4
Scheme 1. Tandem [2,3] sigmatropic rearrangement of diphenylphosphin-
ite. a) ClPPh2, 4-DMAP, Et2O, RT; b) toluene, reflux, 42 h.
diffractometer with
a
CCD area detector (MoKa radiation, l ˆ
0.71073 Š). The crystals were mounted in perfluoropolyether oil;
Tˆ 183(2) and 193(2) K, respectively. The structures were solved by
using direct methods and refined against F 2 for all observed
reflections. (Structure solution with SHELXS 94, refinement using
SHELXL 93). Crystallographic data (excluding structure factors) for
the structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publica-
tion nos. CCDC-150538 (1) and CCDC-150539 (2). Copies of the data
can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK (fax: (‡44)1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk). .
The racemic mixture of the diacetate 1 is prepared in three
steps in about 40% overall yield starting from 1,3-cyclo-
hexadiene.[3] This diacetate is easily resolved using Pseudo-
monas fluorescens lipase[4] to afford the chiral diacetate (S,S)-
2 (43% yield, 99.5% ee) and a mixture of monoacetates
(R,R)-3 (45% yield, 94% ee; Scheme 2). The two enantio-
OAc
OAc
(S,S)-2: 43 %, 99.5 % ee
OAc
d)
a)–c)
OH
OAc
OAc
+
rac-1: 40 %
OAc
OH
New C2-Symmetrical 1,2-Diphosphanes for the
Efficient Rhodium-Catalyzed Asymmetric
Hydroboration of Styrene Derivatives**
(R,R)-3a
(R,R)-3b
45 %, 94 % ee
Â
Stephane Demay, Florence Volant, and Paul Knochel*
OH
OAc
(R,R)-3a
OAc
OH
OH
OH
e)
Chiral 1,2-diphosphanes are important ligands for asym-
metric metal catalysis.[1] Their synthesis is challenging espe-
cially if the stereoselective formation of secondary carbon ±
phosphorus bonds is desired. Recently, we have developed a
method[2] that provides a stereoselective synthesis of cyclic
1,2-diphosphane oxides using a tandem [2,3] sigmatropic
+
(R,R)-3b
(R,R)-4: 29 % from 1, 99.5 % ee
Scheme 2. Preparation of the optically pure diol (R,R)-4. a) Br2 (1 equiv),
CHCl3, 58C; b) 1m KOH, H2O, RT, 96 h; c) Ac2O (2 equiv), pyridine, RT,
12 h; d) Pseudomonas fluorescens lipase, pH 7, buffer, 388C; e) NaOMe
(1 equiv), MeOH, RT, 1 h; f) recrystallization from AcOEt.
[*] Prof. Dr. P. Knochel, Dipl.-Ing. S. Demay, Dipl.-Chem. F. Volant
Department Chemie, Ludwig-Maximilians-Universität
Butenandstrasse 5 ± 13, Haus F, 81377 Munich (Germany)
Fax : (‡49)89-2180-7680
meric diols (R,R)-4 and (S,S)-4 are obtained in optically pure
forms after saponification and recrystallization from AcOEt
in yields of 29 and 31%, respectively (from 1). These
compounds have been prepared on a multigram scale and
are a convenient starting material for a range of new chiral
phosphanes of interest for metal catalysis.
The reaction of (R,R)-4 with several diarylchlorophos-
phanes[5] provides the corresponding diphosphinites 5a ± c
which undergo a smooth [2,3] sigmatropic rearrangement in
E-mail: paul.knochel@cup.uni-muenchen.de
[**] We thank the Deutsche Forschungsgemeinschaft (Leibniz program),
The Institut de Recherches Servier (Suresnes, France), and PPG-
SIPSY for financial support. We thank also BASF AG (Ludwigsha-
fen), Chemetall GmbH (Frankfurt), and Degussa ± Hüls AG (Hanau)
for the generous gift of chemicals.
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
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toluene (for 5a and 5b; reflux, 42 h) or in mesitylene (for 5c;
1708C, 17 h) to afford the corresponding phosphane oxides
6a ± c in 63 ± 85% yield (Scheme 3). These phosphane oxides
OH
a), b)
Scheme 4. Rhodium-catalyzed asymmetric hydroboration of styrene with
ligands 7a ± f. a) [Rh(cod)2]BF4 (1.0 mol%), 7a ± f (1.2 mol%), catechol-
borane (1.2 equiv), solvent; b) 3m KOH, H2O2.
PAr2
PAr2
OH
OH
P(O)Ar2
P(O)Ar2
c), d)
a), b)
(R,R)-4
7a: Ar = Ph: 75 %
7b: Ar = 3,5-xylyl: 80 %
7c: Ar = 2-furyl: 60 %
6a: Ar = Ph: 85 %
6b: Ar = 3,5-xylyl: 63 %
6c: Ar = 2-furyl: 81 %
Table 1. Rhodium-catalyzed enantioselective hydroboration of styrene
using ligands 7a ± f.
Entry Ligand
Solvent
T [8C] ee [%] (config.)
PPh2
1
2
Et2O:CH2Cl2 (4:1)
60
60
8 (S)
PPh2
PPh2
P(c-hex)2
P(O)Ph2
7a
d)
e), d)
P(3,5-xylyl)2
PPh2
P(c-hex)2
P(O)Ph2
Et2O:CH2Cl2 (4:1)
Et2O:CH2Cl2 (4:1)
65 (R)
P(3,5-xylyl)2
(R,R)-7e: 70 %
(R,R)-7d: 72 %
(R,R)-6a
7b
P(2-furyl)2
f)–h)
3
P(2-furyl)2
7c
60
65 (S)
PPh2
P(c-hex)2
PPh2
O
4
5
DME
DME
35
40
92 (S)
8 (R)
P(c-hex)2
O
7d
7f: 50 %
PPh2
Scheme 3. Preparation of the diphosphanes 7a ± f. a) ClPAr2 (2.05 equiv),
4-DMAP (2.1 equiv), Et2O, RT, 30 min; b) toluene or mesitylene, reflux;
c) Pd/C, H2, AcOH, 608C, 18 h; d) HSiCl3 (20 equiv), toluene, autoclave,
1108C, 14 h; e) Raney Ni in excess, 40 bar H2, EtOH, 908C, 12 h; f) OsO4
(0.1 equiv), NMO (3 equiv), pyridine (3 equiv), tBuOH, H2O, reflux, 6 h;
g) HSiCl3 (30 equiv), toluene, autoclave, 1308C, 18 h; h) Me2C(OMe)2
(excess), PTSA (cat.), CH2Cl2, THF, RT, 12 h.
PPh2
7e
PPh2
PPh2
O
6
DME
40
15 (R)
O
7f
are readily converted into the corresponding saturated
phosphanes (7a ± c) in a two-step sequence (1. H2, Pd/C,
AcOH, 658C, 18 h; 2. HSiCl3 (20 equiv), toluene, 1108C,
autoclave, 14 h) in satisfactory overall yield (60 ± 80%).
Alternatively, the phenyl rings can also be readily reduced
with a Raney nickel catalyst[6] in 90% yield to give the (R,R)-
1,2-bis(dicyclohexylphosphanyl)cyclohexane (7d) in 80%
yield after subsequent reduction with HSiCl3.[7] The direct
reduction of 6a with HSiCl3 provides the unsaturated
diphosphane 7e. The treatment of 6a with a catalytic amount
of OsO4 (10 mol%), N-methylmorpholine N-oxide (NMO;
3 equiv), and pyridine (3 equiv) in tBuOH/H2O (reflux, 6 h)
affords stereoselectively[8] a cis 1,2-diol, which leads to the
diphosphane 7 f in 50% yield after reduction of the phos-
phane oxide (HSiCl3, 30 equiv, toluene, 1308C, 18 h) and ketal
formation (Me2C(OMe)2, p-toluenesulfonic acid (PTSA;
cat.), CH2Cl2, THF, RT, 12 h).
The catalytic activity of rhodium complexes of these new
phosphanes in enantioselective hydroboration reactions was
investigated.[9] The hydroboration of styrene with catechol-
borane in the presence of [Rh(cod)2]BF4 (1 mol%; cod ˆ
1,5-cyclooctadiene) and the chiral diphosphanes 7a ± f
(1.2 mol%) furnishes, after oxidative workup (KOH, H2O2),
1-phenylethanol with high regioselectivity (>99:1) and vari-
able enantioselectivity (Scheme 4 and Table 1). The diphos-
phanes 7a, 7e, and 7 f (entries 1, 5, and 6 of Table 1) produce
low enantioselectivities which are independent of the reaction
conditions. However, the electron-rich diphosphane 7b pro-
vides (R)-1-phenylethanol with 65% ee using a mixture of
diethyl ether and CH2Cl2 (4:1) as the solvent for the reaction.
This mixture was found to be superior to THF, dimethoxy-
ethane (DME), tert-butyl methyl ether, toluene, and mixtures
of these solvents. Interestingly the electron-poor diphosphane
7c (entry 3, Table 1) provides the (S)-alcohol; this result
shows the importance of the electron density of the phospho-
rus center on the enantioselectivity.[10] Finally, the best results
are obtained with the very electron-rich diphosphane 7d. In
this case, (S)-1-phenylethanol is obtained in 92% ee (DME,
358C, 3 h) with a regioselectivity greater than 99:1 (Table 1,
entry 4). A lower reaction temperature results in no reaction,
while a higher temperature results in lower enantioselectivity
and regioselectivity.
The diphosphane 7d has been applied to the hydroboration
of other styrene derivatives to evaluate the scope of this new
ligand under standard conditions (1 mol% of catalyst in DME
at
358C; Table 2). In all cases the regioselectivity is
excellent. Irrespective of the electronic nature of the sub-
stituents, their position and size have a profound effect on the
enantioselectivity. Indeed, all the styrene derivatives with a
1236
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Table 2. Rhodium-catalyzed enantioselective hydroboration of styrene
derivatives using ligand 7d.
Table 3. Rhodium-catalyzed enantioselective hydroboration of 2-substi-
tuted styrene derivatives using ligand 7c.[a]
Entry Substrate
Conversion Regio-
ee [%]
Yield[c]
Entry
Substrate
Regio-
selectivity
ee [%]
(config.)
Yield[b]
[%]
[%] (t [h])
selectivity (config.) [%]
1
100 (3)
> 99:1
> 99:1
92 (S)
93 (S)
85
81
1
Me
86:14
97:3
97:3
94:6
77 (S)
82 (S)
81 (S)
82 (S)
75
84
86
81
8a
8k
2
100 (3)
86 (15)
F
2
3
4
8b
F
8l
3
> 99:1
76 (S)
62
F
Cl
8c
8m
4
5
100 (2)
98 (4)
> 99:1
85 (S)
58 (S)
84
80
Cl
CF3
8d
8e
8f
8n
[a] Reaction is performed with 2 mmol of styrene derivatives, 1 mol% of
[Rh(cod)2]BF4, 1.05 mol% of ligand 7c, and 1.2 equivalents of catechol-
borane in a mixture of DME and toluene (3:2) at 758C; the conversion
was 100% after 1 h. [b] Yield of analytically pure product.
98:2
F3C
6
7
84 (17)
98 (4)
99:1
98:2
93 (S)
92 (S)
69
82
MeO
Me
In summary, the [2,3] sigmatropic rearrangement of chiral
unsaturated 1,2-diphosphinites allows an effective and orig-
inal preparation of chiral diphosphanes. We have also shown
the high efficiency of these diphosphanes in the asymmetric
hydroboration of styrene derivatives. The modular approach
for the preparation of these ligands is a great advantage since
various substituents can be attached to the phosphorus center.
8g
8
9
100 (5)
> 99:1
97:3
91 (S)
74 (S)
84 (S)
83
64
50
Me
8h
Experimental Section
74 (17)[a]
62 (17)[b]
Typical procedure for the double [2,3] sigmatropic rearrangement: Prep-
aration of (1R,2R)-1,2-bis(diphenylphosphanyl)-3-cyclohexene (6a):
(1R,2R)-1,2-dihydroxy-3-cyclohexene (4, 10 mmol, 1.14 g), 4-dimethylami-
nopyridine (DMAP; 21 mmol, 2.1 equiv, 2.57 g), and diethyl ether
(120 mL) were added under argon to a 250-mL flask equipped with a
magnetic stirrer. Pure chlorodiphenylphosphane (20.5 mmol, 2.05 equiv,
4.60 g) was added dropwise to the homogeneous solution. The resulting
suspension was stirred for 0.5 h and then filtered under argon over a short
pad of dry silica gel. The precipitate was washed twice with diethyl ether
(20 mL). The ether solution was evaporated under reduced pressure and
the resulting crude diphosphinite was diluted with toluene (80 mL). This
solution was heated under argon to reflux for 42 h. The solvent was
evaporated after cooling the reaction mixture to RT. Crude 6a was
recrystallized from AcOEt to yield a colorless crystalline solid (4.09 g,
m.p. 201 ± 2028C, 85% yield from 4).
8i
10
97:3
8j
[a] Reaction performed with 2 mol% of catalyst at 288C. [b] Reaction
performed with 2 mol% of catalyst at 358C. [c] Yield of the analytically
pure product.
substituent in the para position give enantioselectivities
between 84 and 93% ee (Table 2, entries 2, 4, 6, 7, and 10),
except for the para-trifluoromethylstyrene (58% ee; Table 2,
entry 5), which is the only styrene bearing a pure electron-
withdrawing group (only I, no ‡M effect). Enantioselectiv-
ities lay between 74 and 91% ee when the substituents are in
the meta position (Table 2, entries 3, 8, and 9). ortho-
Fluorostyrene is hydroborated under these conditions with a
regioselectivity greater than 99:1, but with only 56% ee.
Remarkably, the use of ligand 7c solves the problem of the
hydroboration of 2-substituted styrenes (Table 3). Reactions
were performed with 1 mol% of catalyst in a mixture of DME
and toluene (3:2) at 758C. The high rate of these hydro-
borations is surprising, with most reactions being completed
within 1 h at this low temperature. Enantioselectivities lay
between 77 and 82% and regioselectivities are also good.
Typical procedure for the preparation of (1R,2R)-1,2-bis(dicyclohexyl-
phosphanyl)cyclohexane (7d): Unsaturated phosphane oxide 6a (2.41 g,
5 mmol), Raney nickel (5 g, rinsed with ethanol prior to use), and ethanol
(20 mL) were placed in an autoclave under argon. The autoclave was
heated at 908C for 20 h with vigorous magnetic stirring under hydrogen
(40 bar). After cooling the autoclave and purging with argon, the Raney
nickel was filtered off and rinsed with AcOEt. The solvents were
evaporated under reduced pressure. The reaction was complete and gave
only one product as judged by 31P NMR spectroscopic analysis of the crude
reaction mixture. The white and waxy residue was recrystallized from n-
heptane/CH2Cl2 to give 6 h as colorless needles (2.29 g, 90% yield). The
phosphane oxide 6 h (2 mmol) was placed under argon in an autoclave with
toluene (30 mL) and trichlorosilane (4 mL, 40 mmol, 20 equiv) and heated
for 14 h at 1108C. After cooling the reaction mixture to RT, it was
transferred in a 100-mL flask filled with argon. The excess toluene and
trichlorosilane were evaporated under a high vacuum (10 ± 2 Torr). The
residue was dissolved in toluene (25 mL) and carefully hydrolyzed at 08C
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with degassed 3m KOH (10 mL). The mixture was stirred at RT for 30 min.
The two layers were then separated and the organic phase dried (MgSO4)
under argon. The resulting clear yellow organic phase was filtered and
transferred by cannulation into a second flask flushed with argon. The
toluene was evaporated and the crude diphosphane 7d washed with dried
degassed methanol (2 Â 5 mL). After filtration, traces of the solvent were
removed under high vacuum (2 h) to yield 7d as a white microcrystalline
solid (m.p. 126 ± 1288C), which was stored under argon (0.76 g, 80% yield).
The First Crystallographic Evidence for the
Structures of ortho-Lithiated Aromatic Tertiary
Amides**
Jonathan Clayden,* Robert P. Davies, Mark A. Hendy,
Ronald Snaith², and Andrew E. H. Wheatley*
Typical procedure for the hydroboration with 7d:
A
mixture of
Directed metalation is arguably the most selective way of
making regiospecifically substituted aromatic rings. The use of
directed ortho-metalation involving amide-type functional
groups (secondary and tertiary amides, carbamates, and
oxazolines) has revolutionized the synthesis of complex
benzenoid aromatic compounds over the last 15 years:[1]
several recent total syntheses have involved important
ortho-lithiation steps.[2] The four classes of substituents
mentioned are not only the best directors of lithiationÐwith
their electron-rich oxygen centers which promote the ªcom-
plex-induced proximity effectº[3] and withdraw electron
density from the ringÐbut also the most versatile.[2c]
It is, therefore, surprising how little is known of the
products of ortho-lithiation reactions. Kinetic-isotope-effect
evidence[4] suggests the reaction proceeds by a rate-determin-
ing deprotonation of an initial substrate ± organolithium
complex. It is assumed that O Li coordination is maintained
from reactive complex through to products, though for
tertiary amides (the best directing group of all[5]) such
coordination poses severe geometric difficulties. Even in the
simple benzamide 1, the tertiary amide group lies twisted out
of the aromatic ring plane for steric reasons,[6, 7] inhibiting
direct O-coordination to a 2-lithio group. The angle of twist
affects the rate of lithiation,[6] but even amides which have
little flexibility to rotate far from perpendicular, for example,
2 and 3, still undergo efficient ortho-lithiation.[7, 8]
[Rh(cod)2]BF4 (8.1 mg, 0.020 mmol) and diphosphane 7d (11.5 mg,
0.024 mmol) in dry DME (5 mL) was stirred for 10 min at RT in a 10-mL
Schlenk tube under argon. Styrene or a derivative (2 mmol) was added to
the resulting orange solution. The homogeneous mixture was cooled to
358C and stirred at this temperature for 15 min before adding freshly
distilled catecholborane dropwise (2.4 mmol, 0.26 mL). The catecholbor-
ane dissolved in the DME and some gas evolved from the reaction mixture.
The reaction was monitored by sampling. Aliquots were taken, treated with
KOH (3 m) and 30% H2O2, and extracted with diethyl ether or dichloro-
methane. The samples were then analyzed by chiral GC (Chiralsil DEX-CB
column) or chiral HPLC (OD or OJ columns) to determine the conversion
(using n-decane as an internal reference) and enantiomeric excess. The
1
regioselectivity was determined by H NMR spectroscopic analysis of the
final crude reaction mixture after oxidative work-up. The products were
purified by chromatography on silica (pentane/diethyl ether) to afford the
corresponding alcohols.
Received: November 8, 2000 [Z16060]
[1] R. Noyori, Asymmetric Catalysis in Organic Synthesis, Wiley-Inter-
science, New York, 1994.
[2] S. Demay, K. Harms, P. Knochel, Tetrahedron Lett. 1999, 4981, and
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[3] T. Posternak, H. Friedli, Helv. Chem. Acta 1953, 36, 251.
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K. Sakai, Tetrahedron: Asymmetry 1991, 2, 389.
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[7] For the preparation of the five membered ring homologue, see a) C.
Eckert, L. Dahlenburg, A. Wolski, Z. Naturforsch B 1995, 50, 1004;
b) A. Saare, L. Dahlenburg, Z. Naturforsch. B 1995, 50, 1009; c) L.
Dahlenburg, A. Kaunert, Eur. J. Inorg. Chem. 1998, 885; d) L.
Dahlenburg, V. Kurth, J. Organomet. Chem. 1999, 585, 315.
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Int. Ed. Engl. 1985, 24, 878; b) K. Burgess, M. J. Ohlmeyer, J. Org.
Chem. 1988, 53, 5178; c) T. Hayashi, Y. Matsumoto, Y. Ito, J. Am.
Chem. Soc. 1989, 111, 3426; d) M. Sato, N. Miyaura, A. Suzuki,
Tetrahedron Lett. 1990, 31, 231; e) T. Hayashi, Y. Matsumoto, Y. Ito,
Tetrahedron: Asymmetry 1991, 2, 601; f) K. Burgess, W. A. Van der
Donk, M. J. Ohlmeyer, Tetrahedron: Asymmetry 1991, 2, 613; g) T.
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h) J. M. Brown, D. I. Hulmer, T. P. Langzell, J. Chem. Soc. Chem.
Commun. 1993, 1673; i) J. Sakaki, W. B. Schweizer, D. Seebach, Helv.
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Spindler, H. Landert, A. Tijani, J. Am. Chem. Soc. 1994, 116, 4062;
k) A. Schnyder, L. Hintermann, A. Togni, Angew. Chem. 1995, 107,
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N
N
N
O
O
MeO O
1
2
3
Herein we report the first crystal structures of the products
of tertiary-amide-directed ortho-metalation reactions. These
go some way towards clarifying the nature of the O Li
coordination in ortho-lithiated amides, and also towards
[*] Dr. A. E. H. Wheatley, Dr. R. P. Davies, Dr. M. A. Hendy,
Dr. R. Snaith²
Department of Chemistry
University of Cambridge
Lensfield Road, Cambridge, CB2 1EW (UK)
Fax : (‡44)1223-336362
E-mail: aehw2@cam.ac.uk
Dr. J. Clayden
Department of Chemistry
University of Manchester
Oxford Road, Manchester, M13 9PL (UK)
Fax : (‡44)161-2754939
E-mail: j.p.clayden@man.ac.uk
[**] This work was supported by the UK EPSRC (M.A.H.), and St.
Catharineꢁs (R.P.D.) and Gonville & Caius (A.E.H.W.) Colleges,
Cambridge.
[10] For a discussion, see reference [9l].
1238
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