The synthesis of P-stereogenic MOP analogues and their use in rhodium catalysed asymmetric addition

Article abstract of DOI:10.1016/j.jorganchem.2011.08.010

The synthesis is reported of novel P-stereogenic binaphthyl substituted monophosphines via a short five-step synthesis using a nickel coupling reaction with separation of the borane-protected diastereomeric products. Extensive coordination studies of these ligands with a number of well-known metal precursors were performed to more effectively understand their behaviour during catalysis. These ligands and some previously reported P-stereogenic ligands were tested in the rhodium catalysed asymmetric addition of phenyl boronic acid to napthaldehyde. These studies in asymmetric catalysis were used to compare the chiral induction of ligands that combine both axial and central chirality with ligands lacking P-stereogenicity.

Full text of DOI:10.1016/j.jorganchem.2011.08.010

Journal of Organometallic Chemistry 696 (2011) 3608e3615
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
The synthesis of P-stereogenic MOP analogues and their use in rhodium catalysed
asymmetric addition
,
Eoin F. Clarkea, Eoin Raftera, Helge Müller-Bunza, Lee J. Highamb, Declan G. Gilheanya
*
a School of Chemistry and Chemical Biology, Centre for Synthesis and Chemical Biology, Conway Institute for Biomolecular and Biomedical Research, University College Dublin,
Belfield, Dublin 4, Ireland
b School of Natural Sciences-Chemistry, Bedson Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis is reported of novel P-stereogenic binaphthyl substituted monophosphines via a short five-
step synthesis using a nickel coupling reaction with separation of the borane-protected diastereomeric
products. Extensive coordination studies of these ligands with a number of well-known metal precursors
were performed to more effectively understand their behaviour during catalysis.
These ligands and some previously reported P-stereogenic ligands were tested in the rhodium cata-
lysed asymmetric addition of phenyl boronic acid to napthaldehyde. These studies in asymmetric
catalysis were used to compare the chiral induction of ligands that combine both axial and central
chirality with ligands lacking P-stereogenicity.
Received 9 September 2010
Received in revised form
11 August 2011
Accepted 14 August 2011
Keywords:
P-Stereogenic
P-Chiral
P-Chirogenic
MOP analogues
Asymmetric catalysis
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
Previously our group have reported the novel synthesis of
P-stereogenic MOP analogues (Fig. 1) [8,9]. These ligands contain
In metal catalysed asymmetric synthesis, two of the best-known
sources of chirality are ligands with axially chiral backbones or with
P-stereogenic (also called P-chiral or P-chirogenic) centres. The
huge success of these ligand types is apparent from the awarding of
the 2001 Nobel prize in chemistry to Noyori and Knowles for their
development of the axially chiral BINAP ligand and the P-stereo-
genic DiPAMP ligands respectively [1,2].
Despite the clear success of such ligands, the reported cases of
ligands combining both these elements are relatively few. This is
due to the difficulty that synthesising such ligands often presents
[3]. The few cases where such hybrid ligands were studied have
proved inconclusive. Thus, for example, in the case of asymmetric
hydroformylation, Pizzano and co-workers found that their PeOP
ligands which combined both axial and P-stereogenicity were less
selective than their ligands without P-stereogenicity, suggesting
that the chiralities were not co-operative [4,5].Conversely, Buch-
wald and co-workers have reported significantly improved ees
when their P-stereogenic binaphthyl substituted monophosphines
were tested in palladium catalysed asymmetric vinylation and
arylation reactions [6,7].
anisyl and phenyl substituents on the stereogenic phosphorus,
creating a hybrid of the P-chiral DiPAMP and Hayashi’s axially chiral
MOP [10,11] ligand. Herein we report the synthesis of a number of
other P-stereogenic MOP analogues with different P-substituents and
in-depth analysis of their coordination behaviour. These ligands,
along with those previously reported by our group, were then tested
in asymmetric catalysis to probe the effect of changes to the chiral
environment around the phosphorus donor relative to their axially
chiral backbones. These analyses have given us an insight into both
the coordination behaviour of the new ligands and, more impor-
tantly, what is the influencing factor in the chiral induction of these
ligands and the MOP ligand.
2. Results and discussion
2.1. Synthesis of ligands
The ligands were synthesised using a variation of the method-
ology previously developed by our group (Scheme 1) [8]. This
method proceeds via the mono-triflation of optically pure (R)-BINOL
using N-phenyl-bis(trifluoromethanesulfonamide). The singly tri-
flated compound [2] is then methylated using methyl iodide to give
the methoxy triflate [3]. This compound can then be directly coupled
with a secondary phosphine using nickel catalysis to give the target
* Corresponding author. Fax: þ353 1 716 2127.
E-mail address: declan.gilheany@ucd.ie (D.G. Gilheany).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.08.010

E.F. Clarke et al. / Journal of Organometallic Chemistry 696 (2011) 3608e3615
3609
Table 1
MeO
Ph
MeO
Yields from nickel coupling reactions of secondary phosphines to
Ph
methoxy triflate [3] according to Scheme 1.a
P
P
OMe
OMe
R
Yield from coupling reactionb
cyclohexyl
o-tolyl
o-anisyl
methyl
63%
30%
46%
93%c
Fig. 1. Our previously reported P-stereogenic MOP analogues.
a
NiCl2(dppe) 0.217 mmol, phosphine 2.6 mmol, BINOL triflate [2]
2.17 mmol, BH3.THF 1 M solution 12.0 mmol, DMF 11 mL, 16 h [12].
monophosphines [4aed], which were separated as the protected
phosphine boranes [5aed] by flash chromatography. The borane
protecting groups could then be removed byaddition of a base to give
the desired diastereomericaly pure phosphine products.
b
based on phosphine borane (4).
c
product diastereomers inseparable.
This route offers several advantages over previous methods: it
requires no repetitive crystallisations and it is convergent: in the
sense that the two stereogenic entities (substituted binaphthyl and
P-stereogenic secondary phosphine) are constructed separately.
Also deboronation is much more convenient and reliable for
retention of configuration than phosphine oxide reduction, which
was required when we previously separated similar compounds as
the oxides [8]. The route also offered the advantage that a number
of phosphines could be tested in the nickel coupling reaction to give
a series of chiral ligands (Table 1).
Despite the advantages of this route there were still limitations
present as to which phosphines could be successfully used. We
found that certain secondary phosphines, notably those most steri-
cally hindered, were not successful in the coupling to the methoxy
triflate [3] (the phosphines tert-butyl-phenylphosphine and
m-xylylphenylphosphine could not be coupled using these condi-
tions). In addition, although separation of the diastereomers as
boranes was much more successful than as their oxides, the meth-
ylphenyl derivatives [5d] proved to be completely inseparable on
silica and their very low solubility also precluded their production by
preparative HPLC using cellulose based columns. Ironically the
analogous oxides had been separated easily by us previously,
however after testing these oxides with series of reduction methods
we were unable to selectively reduce both diastereomers [8]. This
was therefore an unviable route for this study as both diastereomeric
products were required for a study of potential cooperative effects.
Thus the potentially most interesting cases (5d), which would
provide a small/medium/large group set at phosphorus, are currently
unavailable by any method.
2.2. Assignment of absolute configuration
Previously the stereochemistry of the phosphine-boranes [5a]
had to be assigned by deprotection of the phosphine and oxidation
to the phosphine oxide. This was then crystallised to find the
chirality of the P-chiral centre. For the cyclohexyl phenyl phosphine
borane [5b] it was possible to directly crystallise the phosphine
borane to assign the chirality (Fig. 2). Unfortunately, we were
unable to crystallise the diastereomers of [5c] either as the boranes
or as the phosphine oxides. Metal complexation and subsequent
crystallisation of these compounds also proved unsuccessful,
therefore after exhaustive attempts the phosphorus chirality of
these compounds could not be assigned.
2.3. Coordination chemistry of P-chiral MOP ligands
In order to probe more effectively the factors determining the
asymmetric outcome of the reactions tested, a coordination study
was carried out employing some of the common metal precursors
used in catalysis. It is well known that the nature of the metal
precursor is very important in catalysis [14]. Therefore we employed
several such precurosrs: Rh(acac)(ethylene)2, [Rh2Cl2(CO)4], Rh
(COD)2BF4 and[Pd(acac)(CH3CN)2]BF4 and studied theircoordination
complexes with MOP and our P-stereogenic derivatives, in particular
the ligands whose chirality was assigned (6a/b, 7a/b) as these were
the most important in studying the method of chiral induction. We
tested both diastereomers of these ligands (see Fig. 3) and compared
them with the complexation of the (R)-MOP ligand.
Tf2NPh, DMAP
Collidine
MeI
OH
OH
OTf
OH
OTf
O
K2CO3
[3]
[2]
[1]
(R)
(R)
(R)
1) [NiCl2(dppe)]
DABCO
DMF
100 oC
PhRPH
2) BH3.THF
BH3
Ph
BH3
BH3
Ph
5a: R=An
5b: R=Cy
Ph
R
Separation
P
R
P
O
P
R
5c: R=o-tolyl
O
O
5d R=Me
[5]
[4]
(R)
(R)
(R)
Scheme 1. Synthesis of diastereomericaly pure phosphine boranes.

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E.F. Clarke et al. / Journal of Organometallic Chemistry 696 (2011) 3608e3615
(8.8)
(8.8)
H
H
(145)
(147)
H
H
(161)
(166)
(88)
O
P
(87)
O
Pd(acac)
An
Pd(acac)
Ph
P
Ph
Ph
9a
MOP-Pd
Fig. 4. Analysis of our AnPhMOP with Pregosin’s data for MOP (on right).
Fig. 2. X-ray crystal structure of (R,S)-CyPhMOPBH.3, hydrogens omitted for clarity
[13].
observed in the 1H NMR spectrum (cf.
AnPhMOP), along with a low shift of
showed coupling to aryl protons in the COSY spectrum (at
and 7.20 respectively). The CeH coupling (HSQC) spectrum showed
d
8.8 ppm for MOP and
5.82 ppm, both of which
7.58 ppm
d
2.4. Palladium complexes
d
d
Previous studies by Kocovsky et al. and Pregosin et. al. reported
the unusual bonding modes in Pd complexes of their MAP and
CN-MOP ligands respectively [15,16]. Pregosin et. al. found that the
most abundant species in solution exhibited coordination of the
coupling of the proton at
Fig. 5).
d 8.9 ppm with the carbon at d 147 ppm (see
The above observations strongly suggest that our P-chiral MOP
ligands form the [Pd(acac)(Ligand)] complexes with the ligand
ligand through phosphorus and a s-bond to carbon on the methoxy-
bonding to the palladium through the
s-bond to carbon on the
naphthyl ring [17]. These results were used in our study to probe and
compare the coordination modes of our P-chiral MOP ligands.
Our (R,R)-AnPhMOP (6a, An ¼ ortho-anisyl) ligand was com-
plexed to [Pd(acac)(CH3CN)2] by adding an acetone solution of the
ligand to acetone/DCM solution of the palladium precursor stirring
for 15 min. The solvents were removed under reduced pressure and
the NMR analyses of the resulting red solid (9a) were performed in
CD2Cl2.
methoxy-naphthyl ring as well as the phosphine, in a manner
similar to that detailed by Pregosin and co-workers. Efforts to obtain
crystals for X-ray analysis however have so far proven unsuccessful.
2.5. Rhodium complexes
To date, there have been relatively few examples in the litera-
ture of rhodium MOP complexes. Gladiali and co-workers have
reported the rhodium complexes of phosphorus-sulfur-MOP type
ligands [18]. The presence of sulfur in place of oxygen in MOP lead
to a greater propensity for the metal to bond to the heteroatom,
given the softness of the sulphur relative to the oxygen. As a result,
bidentate coordination occured via the phosphorus and sulfur
atoms with no unusual coordination modes observed.
However, another example of a MOP derivative bonded to
rhodium was reported by Chauvin and co-workers [19] who reacted
a phosphine-phosphonium moiety (formed by the reaction of BINAP
with MeI) with [Rh(COD)2]BF4. The resulting complex formed by
The 31P NMR showed a single peak at 38.8 ppm and the 1H and
d
13C shifts indicated that our complex had a very similar coordination
behaviour to the corresponding MOP complex (see Fig. 4) [17]. The
low shift for the ipso-bonded carbons (
sp3 conformations whilst the high shift (
145.147 ppm) of the carbons alpha and gamma to the ipso-carbon is
d
87.88) reflects their pseudo
d
161.166 ppm and
d
reflective of delocalisation of positive charge from the metal into the
ring system. This is also reflected in the high shift of the proton on
the carbon at 145 ppm in our system. The CeH coupling (HSQC)
spectrum (see Fig. 5) showed coupling of this proton at
d 8.8 ppm
with the carbon at d 145 ppm. The methoxy signal was unchanged in
the 13C spectrum, suggesting no coordination through the methoxy
group.
reductive PeC bond cleavage gave
h5-(seP,peArene) chelating
rhodium ligand complex. Therefore it was of great interest for us to
investigate the rhodium complexes of our ligands to clarify whether
we were forming chelating ligands and if so whether they bonded
through the oxygen or through an aryl carbon.
Mass spectrometry was also consistent with coordination of one
phosphine to palladium (m/z 707.1). This confirmed the formation
of the [Pd(acac)(6a)] complex.
The corresponding (R,S)-AnPhMOP 6b palladium complex also
showed 1 main peak in 31P NMR, at
d 31.4 ppm.
For both isomers of our CyPhMOP (7a/b), on coordination to
palladium, the 31P showed predominantly one peak - ((R,S) at
d
54.0 ppm and (R,R) at 48.6 ppm). However, minor amounts of other
species were also observed in solution. The (R,S) isomer 7b complex
was subjected to high-resolution mass spectrometry and the ion
found (679.614) agreed with theory (679.1593) for coordination of 1
ligand to palladium. Again, a characteristic high shift (d 8.9) was
P
O
O
P
O
Ph
Ph
6a/b
7a/b
Fig. 5. HSQC CeH correlation spectrum for AnPhMOP(6a)ePd. Key CeH correlation
Fig. 3. P-chiral ligands used in coordination studies.
between proton at
d 8.8 and carbon at d 145.

E.F. Clarke et al. / Journal of Organometallic Chemistry 696 (2011) 3608e3615
3611
2.5.1. Rh(acac)(C2H4)2 in CDCl3
cases where positive charge is delocalised into the aryl ring via PeC
-bonding. The 13C NMR shows multiple split peaks, which are
Studies were commenced with this rhodium complex as it was
the precursor to our rhodium catalysed boronic acid experiments,
which are discussed later. Complexation of the MOP ligand and our
(R,R)-AnPhMOP were tested (carried out by dissolving both
species in CDCl3 in an NMR tube). Both showed the formation of
two doublets in 31P NMR initially, however in both case the
complexes proved unstable in solution forming their respective
oxides over time.
s
consistent with rhodium bonding to several carbons in possibly an
h2 or h4 manner.
The mass spectrum shows 2:1 coordination of phosphine to
rhodium (Found m/z 1039 corresponding to the [Rh(MOP)2]þ ion)
suggesting two ligands with different bonding modes.
(R,R)-AnPhMOP 6a yielded two doublets, at 37.3 ppm (236.6 Hz)
and 26.7 ppm (234. Hz).
(R,R)-CyPhMOP 7a showed a more complex pattern with
a doublet of doublets at 50.8 ppm (32.4 Hz, 209.1 Hz) and multiple
peaks at 41.0 ppm.
2.5.2. Rh(acac)(C2H4)2 in C6D6
Our study again started with MOP which showed (as before)
two doublets in the 31P spectrum at 61.3 ppm (J ¼ 189 Hz) and
51.3 ppm (J ¼ 185 Hz) in a 1:1 ratio which would likely correspond
to the Rh(acac)(MOP)2 and Rh(acac)(C2H4)(MOP) complexes. When
(R,R)-AnPhMOP was complexed, similar species to MOP was
observed with doublets at 55.2 ppm (J ¼ 199 Hz) and 48.3 Hz
(J ¼ 193 Hz), in a 1:2 ratio. No tendency to form oxides was
observed here, but again multiple species are present in solution.
Given the tendency of this catalyst precursor to form oxides over
time in CDCl3, and the presence of multiple species in both CDCl3
and C6D6, it was decided to examine other precursors also.
(R,S)-CyPhMOP 7 b showed a similar characteristic splitting
pattern to MOP with a doublet of doublets observed at 50.0 ppm
(30.7 Hz, 204.4 Hz) and 40.7 ppm (31.1 Hz, 191.1 Hz).
The similar pattern observed in the 31P NMR spectra of these
ligands with [Rh(COD)2]BF4 (especially for MOP and 7a/b)
suggest that in each case two phosphines are on each rhodium
atom, with each coordinating in a different manner, as gauged
by the two different phosphorus shifts. The complexation of
AnPhMOP seems to behave differently. Repeated attempts to
form crystals of these complexes have met with limited
success.
2.5.3. [Rh2Cl2(CO)4] in CDCl3
Complexation of MOP with [Rh2Cl2(CO)4] resulted in multiple
species again being observed in the 31P NMR. A broad peak at
47e48 ppm was accompanied by doublets at 40.75 ppm
(J ¼ 181.9 Hz), 33.4 ppm (J ¼ 128.3 Hz) and 21.1 ppm (J ¼ 126.9 Hz).
When (R,R)-AnPhMOP (6a) was coordinated, a single doublet
was observed at 47.1 ppm (J ¼ 159.3 Hz). This shift and coupling
correspond to previously reported [RhCl(CO)(Ligand)]2 dimers
[20e22]. This was further confirmed by mass analysis showing
a 1:1 ratio of of ligand to metal (m/z 630 [MHþ e Cl]; HRMS calc. for
[MHþ e Cl] (C35H28O3PRh) 630.0831, found 630.0800). (It is note-
worthy that on a separate run of this complex, the broad peak at
47e48 ppm was observed).
2.6. Rhodium catalysed asymmetric addition to aldehyde
In order to test these ligands in asymmetric catalysis, we sought
a reaction where the ligand MOP had previously been employed. It
was also desirable to choose a system where there was still good
scope for improvement. With these considerations in mind we
chose rhodium catalysed asymmetric addition of phenyl boronic
acid to napthaldehyde as our initial test reaction. Miyaura and co-
workers had described such addition of organoboronic acids to
aldehydes and showed that Hayashi’s MOP ligand was capable of
inducing an ee of 41% with a yield of 78% for this particular
example [26].
(R,S)-AnPhMOP (6b) initially showed a doublet at 13.3 ppm
(J ¼ 127 Hz), and over time (7 days) showed also a doublet at
51.6 ppm (J ¼ 156.2 Hz). The mass spectrum for this complex was
also consistent with formation of a [RhCl(CO)(6b)]2 complex (m/z
629.2) and loss of carbonyl (m/z 601.2).
We were able to reproduce Miyaura and co-workers findings,
obtaining an ee of 42% and a yield of 78% and we then tested our
P-stereogenic derivatives in the reaction with the results shown
(Table 2). Unfortunately, the results failed to show significant
enhancement of the selectivity of the reaction, with only one case
giving higher ee. This was achieved using the (R,S)-AnPhMOP 6b
ligand. It is important to consider the sense of enantioselectivity
obtained in this series of tests. Hayashi’s (R)-MOP ligand
produces (R)-naphthylphenylcarbinol. In both the anisylphenyl-
and cyclohexylphenyl- derived MOPs, the two diastereomers
produce the opposite hands of product, suggesting that the new
chiral environment around the phosphorus atom is influencing
the stereoselectivity.
(R,R)-CyPhMOP (7a) shows a predominant peak at 38.2 ppm
(J ¼ 125 Hz) with a minor doublet at 58.8 ppm (J ¼ 182 Hz) which
remains unchanged over 7 days.
(R,S)-CyPhMOP (7b) afforded a single complex by 31P NMR with
a doublet at 33.2 ppm (J ¼ 125.5 Hz). These shifts and coupling
would normally correspond to the bisphosphine complex [23e25],
however as mass analysis shows the formation of the mono ligand
complex (m/z 605.2) we speculate that this complex is possibly has
both phosphorus coordination and coordination through the
s
-
The ortho-tolyl derivatives 8a/b were also tested in this reaction
despite their unknown P-configurations. The presence or absence
of a cooperative effect however would still be apparent when both
diastereomers were tested. However these o-tolyl ligands showed
quite different behaviour to the other ligands tested with both
diastereomers giving the same product configuration with only 16%
ee in difference. This was surprising considering that the anisyl
derivative which is structurally similar gave a significant difference
in products for the two diastereomers.
It is important to note also the ligands which were most selec-
tive in forming a single ligand rhodium complex presented the
selectivities comparable with the MOP ligand whilst those which
formed a mixture of complexes give lower enantioselectivities and
lower yields. This leads use to conclude that the best complex for
this catalytic reaction is the singly bound phosphine to rhodium
complex.
bond to carbon on the methoxy-naphthyl ring. Analysis of the
methoxy carbon in the 13C NMR suggested that this is not the point
of coordination to the metal since it appears as a singlet.
2.5.4. [Rh(COD)2]BF4 in CDCl3
The common hydrogenation precursor [Rh(COD)2]BF4 was also
examined. With this precursor, displacement of two cyclooctadiene
groups by two phosphines would be expected giving the [Rh(Li-
gand)2]BF4 complex. However, complexation of the MOP ligand
initially gave a broad doublet at 43.5 ppm (J ¼ 143 Hz), which is
replaced over two days by two doublets of doublets at 49.5 ppm
(J ¼ 32 Hz, 218 Hz) and 36.6 ppm (J ¼ 32 Hz, 194 Hz) in a 1:1 ratio.
The 1H NMR spectrum is noteworthy for the observance of a shift at
8.55 ppm (which couples to a proton at 6.7 ppm in COSY) and is
analogous to the characteristic shifts observed in the palladium

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E.F. Clarke et al. / Journal of Organometallic Chemistry 696 (2011) 3608e3615
Table 2
Results for rhodium catalysed asymmetric addition of phenyl boronic acid to napthaldehyde.a
R1
R2
O
O
H
PhB(OH)2 (2eqv.)
HO
Rh(acac)(C2H4)2 (3 mol%)
P
*
L* =
L* (6 mol%)
DME/H2O (1:1), 60 oC, 36 h
Phosphine L*
R1/R2 (config)
eeb (config)
Yield %
MOP
6a
6b
7a
7b
Ph, Ph (R)
42 (R)
9 (S)
46 (R)
22 (R)
4 (S)
78
42
20
47
21
17
7
An, Ph (R,R)
An, Ph (R,S)
Cy, Ph (R,R)
Cy, Ph (R,S)
o-Tol, Ph (nd)c
o-Tol, Ph (nd)c
8a
8b
36 (R)
20 (R)
a
Reactions were carried out according to the procedure of Miyaura and co-workers [26].
ees were determined by chiral HPLC (see experimental section for details).
(nd) ¼ not determined.
b
c
Comparative studies of this with the palladium complexes
X-ray diffraction experiments on were carried out at 100 K on
a Bruker SMART diffractometer using Mo Ka X-radiation,
a ¼ 0.71073 Å.
suggest that this may be due to the formation of a rigid ligand
rhodium complex bound both at the phosphine and the -bond to
s
carbon on the methoxy-naphthyl ring. This behaviour is in line with
the results of Pregosin and Kocovsky and the complexes which they
observed.
4.1. Synthesis of (R)e2e((trifluoromethanesulfonyl)oxy)e20ehyd-
roxye1,10ebinaphthyl [2]
Bolm and Rudolph have since published a report with selectiv-
ities of up to 95% ee [27] for this type of transformation using chiral
ferrocenyl based ligands.
The procedure of Katsuki and co-workers was employed with
some modifications [30].
A solution of (R)-BINOL (342 mg,
1.19 mmol), 2,4,6-collidine (145 mg, 1.19 mmol), DMAP (17.5 mg,
0.143 mmol) and N-phenyl-bis(trifluoromethanesulfonamide)
(427 mg, 1.19 mmol) was refluxed in DCM (5 mL) overnight. After
cooling the solvent was removed in vacuo and the resulting thick
orange oil was purified by column chromatography on silica gel
using toluene as eluant to yield the mono-triflate material as a thick
3. Conclusion
We have presented the expansion of our previously developed
synthesis of P-stereogenic MOP analogues, which shows the
versatility of the method. We have also tested the effects of P-
stereogenicity on an axially chiral ligand. These studies have shown
that the chirality of the phosphorus donor is of very great signifi-
cance for the selectivity of the asymmetric catalytic process,
wherein reversed or increased ees were observed when opposite
diastereomers were used in catalysis.
yellow oil (352 mg, 70%). 1H (300 MHz, CDCl3)
ArH), 7.62e6.99 (m, 8H, ArH), 5.19 (br s, 1H, OH).
d 8.10e7.80 (m, 4H,
4.2. Synthesis of (R)e2e((trifluoromethanesulfonyl)oxy)e20eme-
thoxye1,10ebinaphthyl [3]
Unfortunately we have not as yet been able to improve on the
previous results of the MOP ligand. In cases where the chirality of
the P-stereogenic centre appears to be the deciding factor for the
chirality of the product the selectivities of the reaction do not
significantly surpass that of the original MOP ligands.
From analysis of the coordination behaviour of the P-stereogenic
derivatives of MOP, it can be concluded that investigation of
compounds of this type is worthwhile since the derivatives syn-
thesised differ in behaviour not only from MOP itself, but also
significant differences can be seen between the diastereomers of the
same compound also, which is as a direct result of making the envi-
ronment around phosphorus chiral. Such differences in coordination
behaviour can be expected to propagate very different outcomes in
catalysis reactions, which is the ultimate goal of this research.
To a solution of (R)-2-((trifluoromethanesulfonyl)oxy)-20-hydroxy-
1,10-binaphthyl (4.64 g, 11.1 mmol) in acetone (40 mL) was added
potassium carbonate (2.30 g, 16.6 mmol) and methyl iodide (1.04 mL,
16.7 mmol). The mixture was stirred for 24 h by which time TLC
analysis (silica, dichloromethane/pentane, 1:3) showed the starting
material to be completely consumed. After removal of solvent,10% HCl
was added until the pH of the solution was approximately 3. The
product was extracted with ethyl acetate (2 ꢀ 75 mL), washed with
water and brine, and dried over magnesium sulphate. Removal of
solvent yielded a brown oil (4.85 g) which was recrystallised from
ethanol/water to yield white needles (3.06 g, 64%). A further crop of
crystals was recovered from the mother liquor after standing for a few
days.
M.P. 99e101 ꢁC; 1H (300 MHz, CDCl3)
d 8.05 (m, 1H, ArH), 8.02
(m, 1H, ArH), 7.97 (m, 1H, ArH), 7.88 (m, 1H, ArH), 7.57e7.22 (m, 7H,
ArH), 7.00 (m, 1H, ArH), 3.82 (s, 3H, OCH3) (same as lit [31].).
4. Experimental section
All reactions were carried out under a N2 atmosphere in dry
glassware using Schlenk-line techniques. Air and moisture sensitive
liquids and solutions were transferred via syringe. All solvents were
dried and distilled by standard procedures [28].
The metal complexes for catalysis di-m-chlorotetrakis(ethylene)
dirhodium(I) and 2,4-pentanedionatobis(ethylene)rhodium(I) were
prepared according to literature procedures [29].
4.3. Synthesis of cyclohexylphenylphosphine
To a solution of dichlorophenylphosphine (5.31 g, 29.7 mmol) in
dry Et2O (150 mL) at ꢂ78 ꢁC was added cyclohexylmagnesium
bromide (2 M soln. in Et2O, 15 mL, 30.0 mmol) over 3 h via syringe
pump. After complete addition the solution was warmed to room

E.F. Clarke et al. / Journal of Organometallic Chemistry 696 (2011) 3608e3615
3613
temperature and 31P NMR at this point showed the presence of one
major peak at 100.1 ppm with a small impurity (double addition) at
5 ppm. The solution was cooled to 0 ꢁC and LiAlH4 (1 M in Et2O,
48 mL, 48 mmol) was added over 30 min. The 31P NMR now showed
the desired peak for secondary phosphine at ꢂ30.0 ppm. The
solution was quenched by the slow addition of degassed H2O
(10 mL), degassed 10% HCl (10 mL) and degassed H2O (20 mL). The
ether layer was separated and the aqueous layer extracted with
Et2O (20 mL). The combined ether extracts were dried over MgSO4
and the solvent removed in vacuo. The resulting yellow oil was
distilled under high vacuum to yield the secondary phosphine as
a colourless oil (2.25 g, 39%).
washed with degassed methanol (2 mL ꢀ 2) and degassed pentane
(2 mL). The white solid (20 mg, 70%) was dried under high vacuum.
(R,S)-CyPhMOP; M.p. 158e160 ꢁC; [
a
]
D
¼ 160.0ꢁ (c 0.48, CHCl3);
1H (300 MHz, CDCl3)
d 7.93e7.76 (m, 5H, ArH), 7.39e6.89 (m, 12H,
ArH), 3.15 (s, 3H, OCH3) 2.15 (m, 1H, PCH) 1.61e0.79 (m, 10H, CH2);
13C (126 MHz, CDCl3)
153.9e111.4 (multiple P-coupled signals,
ArC), 54.1 (OCH3), 28.7e25.3 (multiple P-coupled signals, CH2); 31
d
P
(121 MHz, CDCl3)
d
ꢂ15.46; MS (Electrospray) m/z (% intensity):
477.2 (6), 476.2 (20), 475.2 (100, M D HD); HRMS calc. for [MH]þ
(C33H32OP) 475.2191, found 475.2205.
4.3.3. (R,R)-CyPhMOP.BH3 was treated in a similar manner to above
to yield (R,R)-CyPhMOP
B.p.102 ꢁC @ 0.2 mmHg; 1H (300 MHz, CDCl3)
d 7.55e7.50 (m, 2H,
ArH), 7.36e7.34 (m, 2H, ArH), 1.92e1.16 (m, 11H, CH2); 31P (121 MHz,
(R,S)-CyPhMOP; M.p. 148e150 ꢁC; [
a
]
¼ ꢂ110.0ꢁ (c 0.2, CHCl3);
D
CDCl3)
d
ꢂ30.01 ppm (lit [32].
d
- 29.0 ppm) (1JPeH ¼ 192 Hz).
1H (300 MHz, CDCl3)
d
7.95 (d, J ¼ 9.0 Hz, 1H, ArH), 7.85e7.73 (m,
4H, ArH), 7.40e7.32 (m, 2H, ArH), 7.18e6.96 (m, 8H, ArH), 6.80 (t,
J ¼ 7.6 Hz, 1H, ArH), 6.46 (d, J ¼ 8.5 Hz, 1H, ArH), 3.73 (s, 3H, OCH3),
2.28 (m, 1H, PCH) 1.72e0.79 (m, 10H, CH2); 13C (126 MHz, CDCl3)
4.3.1. Synthesis of (R,R) and (R,S)e2e(cyclohexylphenylphosphino-
borane)e20emethoxye1,10ebinaphthyl [4b]
The procedure of Cai and co-workers was employed with some
modifications [12]. To a red solution of [NiCl2(dppe)] (0.114 g,
0.217 mmol) in DMF (4 mL) under N2 was added cyclo-
hexylphenylphosphine (0.250 g, 1.30 mmol) via a syringe, resulting
in a slight colour change to dark red. The solution was stirred at
room temperature for 1 h and then heated to 100 ꢁC for 30 min. At
this point a solution of (R)-2-((trifluoromethanesulfonyl)oxy)-20-
methoxy-1,10-binaphthyl (0.937 g, 2.17 mmol) and powdered
DABCO (0.972 g, 8.67 mmol) in DMF (7 mL) was added, causing an
immediate colour change from dark red to dark green. After 1 h the
remainder of the secondary phosphine (0.250 g, 1.30 mmol) was
added. After stirring overnight at 100 ꢁC, 31P NMR showed the
disappearance of all secondary phosphine starting material and the
d
153.2e111.9 (multiple P-coupled signals, ArC), 55.0 (OCH3),
28.5e25.5 (multiple P-coupled signals, CH2); 31P (121 MHz, CDCl3)
d
ꢂ17.28; MS (Electrospray) m/z (% intensity): 477.2 (4), 476.2 (25),
475.2 (100, M D HD); HRMS calc. for [MH]þ (C33H32OP) 475.2191,
found 475.2199.
4.4. Synthesis of phenyl-o-tolylphosphine
To a solution of dichlorophenylphosphine (4.0 g, 22.3 mmol) in
dry THF (20 mL) at ꢂ78 ꢁC was added o-tolylmagnesium bromide
(1 M soln. in THF, 22.3 mL, 22.3 mmol) over 2 h via syringe pump.
After complete addition the solution was warmed to room
temperature and 31P NMR at this point showed the presence of one
major peak at 77.8 ppm with minor impurities at ꢂ22 ppm (double
addition) and 160.7 ppm (starting material). The solution was
cooled to 0 ꢁC and LiAlH4 (1 M in Et2O, 36 mL, 36 mmol) was added
over 30 min. The 31P NMR now showed mainly the desired peak for
secondary phosphine at ꢂ49.10 ppm. The solvent was removed
under reduced pressure and the mixture was quenched by the slow
addition of degassed H2O (8 mL), degassed 15% NaOH (8 mL) and
degassed H2O (24 mL). The product was extracted into Et2O
(2 ꢀ 50 mL), the combined ether extracts were dried over MgSO4
and the solvent removed in vacuo. The resulting yellow oil was
distilled under high vacuum to yield the secondary phosphine as
a colourless oil (1.73 g, 39%).
emergence of 2 peaks at
sponding to the 2 desired diastereomers. The volatiles were
d
ꢂ16.8 ppm and
d
ꢂ17.7 ppm, corre-
removed under high vacuum and
1 M BH3.THF (12.0 mL,
12.0 mmol) was added. 31P showed the disappearance of the 2
diastereomer peaks and the emergence of a broad peak at
d
24.0 ppm. Si gel (5.5 g) was added (some bubbling observed on
addition) and the solvent was removed to yield a yellow powder
which was chromatographed (Si, DCM) to yield a mixture of
boranes (0.940 g, 89%). The mixture was subjected to further
chromatography on Si gel (cyclohexane/ethyl acetate; 19:1) which
resulted in successful separation of the two diastereomers (R,S)-
CyPhMOP.BH3 (413 mg, 39%) and (R,R)-CyPhMOP.BH3 (252 mg, 24%)
with the remainder eluting as a mixture of fractions. A small sample
of the second borane off the column was crystallised from ethyl
acetate and analysed by X-ray crystallography.
B.p. 104e106 ꢁC @ 0.2 mmHg; 1H (300 MHz, CDCl3)
d 7.45e7.08
(m, 9H, ArH), 5.17 (d, 1H, 1JPeH ¼ 221 Hz, PH), 2.35 (s, 3H, CH3); 31
P
(121 MHz, CDCl3)
d
ꢂ48.60 ppm (1JPeH ¼ 218 Hz); 13C (126 MHz,
(R,S)-CyPhMOP.BH3; 1H (300 MHz, CDCl3)
d
8.42e8.35 (m, 1H,
CDCl3) d 141.6e125.6 (multiple P-coupled signals, ArC), 21.6 (d,
ArH), 8.02 (d, 1H, J ¼ 8.69 Hz, ArH), 7.92e7.85 (m, 2H, ArH), 7.55 (d,
1H, J ¼ 8.15 Hz, ArH), 7.46 (t, 1H, J ¼ 6.99 Hz, ArH), 7.36 (d, 1H,
J ¼ 9.07 Hz, ArH), 7.16e7.04 (m, 2H, ArH), 6.96e6.60 (m, 7H, ArH),
6.32 (d, 1H, J ¼ 8.48 Hz, ArH), 3.71 (s, 3H, OCH3), 2.55 (m, 1H, PCH)
JPeC ¼ 14.3 Hz, CH3).
4.4.1. Synthesis of (R,R) and (R,S)e2e(phenyl-o-
tolylphosphinoborane)e20emethoxye1,10ebinaphthyl [4c]
1.80e1.14 (m, 10H, CH2); 31P (121 MHz, CDCl3)
(R,R)-CyPhMOP.BH3; 1H (300 MHz, CDCl3)
d
23.2 (br s).
The procedure of Cai and co-workers was employed with some
modifications [12]. To a red solution of [NiCl2(dppe)] (0.171 g,
0.324 mmol) in DMF (6 mL) under N2 was added phenyl-o-tolyl-
phosphine (0.389 g, 1.95 mmol) via a syringe, resulting in a slight
colour change to dark red. The solution was stirred at room
temperature for 2 h and then heated to 100 ꢁC for 30min. At thispoint
a solution of (R)-2-((trifluoromethanesulfonyl)oxy)-20-methoxy-1,10-
binaphthyl (1.40 g, 3.24 mmol) and powdered DABCO (1.45 g,
13.0 mmol) in DMF (10.5 mL) was added, causing an immediate
colour change from dark red to green. After 30 min the remainder of
the secondary phosphine (0.389 g, 1.95 mmol) was added. After
stirring overnight at 100 ꢁC, 31P NMR showed the disappearance of all
secondary phosphine starting material and the emergence of a single
d
8.40e8.33 (m, 1H,
ArH), 8.01 (d, 1H, J ¼ 8.62 Hz, ArH), 7.87 (m, 3H, J ¼ 8.77 Hz, ArH),
7.42 (t, 1H, J ¼ 7.90 Hz, ArH), 7.47e6.73 (m, 11H, ArH), 3.20 (s, 3H,
OCH3), 1.90 (m, 1H, PCH) 1.70e0.79 (m, 10H, CH2); 31P (121 MHz,
CDCl3)
d 23.1 (br s).
4.3.2. Deprotection of borane adducts to give (R,R) and (R,S)e
2e(cyclohexylphenylphosphino)e20emethoxye1,10ebinaphthyl [5b]
To (R,S)-CyPhMOP.BH3 (30 mg, 0.06 mmol) in a Schlenk vessel
under N2 was added anhydrous Et2NH (2 mL). The resulting solu-
tion was stirred overnight at 50 ꢁC. After cooling, 31P NMR showed
the presence of a single peak at
removed under high vacuum and the resulting white solid was
d
ꢂ15.6 ppm. The amine was
peak at
d
ꢂ22.3 ppm, corresponding to a mixture of the 2 desired

3614
E.F. Clarke et al. / Journal of Organometallic Chemistry 696 (2011) 3608e3615
diastereomers. The volatiles were removed under high vacuum and
1 M BH3.THF (28 mL, 28 mmol) was added. 31P showed the appear-
stirred for 1 h and the remainder of the secondary phosphine
(0.35 g, 1.04 mmol) was added. The temperature was maintained at
100 ꢁC for 2 days, after which a colour change to light brown was
apparent. 31P NMR of the solution at this point revealed that the
major components (>99% by 31P integration) were the desired
ance of the 2 diastereomeric borane peaks at
d
21.1 and
d 20.1 ppm
and also some residual phosphines at
d
ꢂ21.7 and
d
ꢂ22.1 ppm. Si gel
(7.0 g) was added (some bubbling observed on addition) and the
solvent was removed to yield a brown powder which was chroma-
tographed (Si, DCM) to yield a mixture of both protected and
unprotected diastereomers (0.940 g, 89%). The mixture was subjected
to further chromatography on Si gel (cyclohexane/diethyl ether;
19:1) which resulted in separation of the phosphine mixture (0.380 g,
30%) from the borane mixture. The borane mixture was then chro-
matographed on Si gel (pet. spirits/diethyl ether; 50:1) which resul-
ted in successful separation (0.08 g combined, 5%) of the desired
protected diastereomers.
diastereomers at
mate bias of 60:40 in favour of the peak at
d
ꢂ33.6 ppm and
d
ꢂ34.1 ppm with an approxi-
d
ꢂ33.6. After removal of
the volatiles under high vacuum the resulting black residue was
dissolved in dry THF (20 mL) and 1 M BH3.THF (25 mL, 25 mmol)
was added. 31P NMR at this stage showed no trace of the phosphine
products. After addition of Si gel (7.0 g) the solvent was removed
under high vacuum and the resulting brown powder was chro-
matographed on silica gel (ethyl acetate/cyclohexane, 1:10) to
separate the mixture of desired diastereomers (1.79 g, 92%) from
a small amount of triflate starting material. Despite exhaustive
attempts, the mixture of diastereomers could not be separated
under any standard chromatographic conditions.
1st off column: PhTolMOPBH3.D(A); 1H (300 MHz, CDCl3)
d
7.90e7.82 (m, 3H, ArH), 7.65 (d, 1H, J ¼ 8.16 Hz, ArH), 7.55e6.93
(m, 14H, ArH), 6.75e6.69 (m, 1H, ArH), 6.41 (t, 1H, J ¼ 8.44 Hz, ArH),
6.07 (d, 1H, J ¼ 8.47 Hz, ArH), 3.72 (s, 3H, CH3), 2.27 (s, 3H, CH3); 31
P
(121 MHz, CDCl3)
d 20.2 (br s).
2nd off column: PhTolMOPBH3.D(B); 1H (300 MHz, CDCl3)
4.5. Coordination chemistry studies
d
7.95e7.86 (m, 3H, ArH), 7.79 (d, 1H, J ¼ 8.05 Hz, ArH), 7.66e6.85
(m, 17H, ArH), 3.04 (s, 3H, CH3), 2.24 (s, 3H, CH3); 31P (121 MHz,
Representative procedures of the metal complexes studied.
CDCl3) d 21.2 (br s); MS (Electrospray) m/z (% intensity): 499.0 (100,
M þ Hþ-BH3þO), 483.0 (90, M þ HþeBH3).
4.5.1. [Pd((R,R)-AnPhMOP)(acac)]
To a stirred solution of [Pd(acac)(CH3CN)2]BF4 (20 mg, 0.053 mmol)
in acetone (1.5 mL) and CH2Cl2 (0.5 mL) was added slowly (R,R)-
AnPhMOP (26.5 mg, 0.053 mmol) in acetone (1.5 mL) at room
temperature. Stirring was continued for 15 min, followed by removal
of the solvents under reduced pressure. The resulting red solid was
dissolved in anhydrous degassed CD2Cl2 and the following analyses
performed.
4.4.2. Deprotection of borane adducts to give (R,R) and (R,S)e2e(o-
tolylphenylphosphino)e20emethoxye1,10ebinaphthyl [5c]
To PhTolMOPBH3.D(A) (50 mg, 0.101 mmol) in a Schlenk vessel
under N2 was added anhydrous Et2NH (2 mL). The resulting
mixture was stirred overnight at room temperature. 31P NMR
showed the presence of a single peak at
d
ꢂ20.9 ppm. The amine
was removed under high vacuum and the resulting white solid was
washed with degassed methanol (2 mL ꢀ 2). The white solid
(21 mg, 43%) was dried under high vacuum. Since the reaction is
essentially quantitative (since no other product is visible by 31P
NMR), the loss of yield can be attributed to the difficulty in
manipulating such a small quantity of material.
MS (Electrospray) m/z (% intensity): 708.1 (15), 707.1 (35), 705.1
(70), 703.1 (100, Mþ), 702.1 (60), 701.1 (25); 1H (500 MHz, CD2Cl2)
d
8.75 (d, J ¼ 9.5 Hz, 1H), 8.10e6.91 (multiple signals, 19H, ArH),
5.95 (d, J ¼ 8.5 Hz, 1H), 5.03 (s, 1H, acac-H), 3.65 (s, 3H, -OMe), 3.34
(s, 3H, eOMe), 1.71 (s, 3H, acac-Me), 1.61 (s, 3H, acac-Me); 13C
(126 MHz, CD2Cl2)
d 188.8 (acac-C), 187.4 (acac-C), 185.5 (acac-C),
PhTolMOP.D(A); [
a
]
¼ 46.0ꢁ (c 0.5, CHCl3); 1H (300 MHz, CDCl3)
162.0, 161.5, 155.6, 146.0e111.3 (multiple P-coupled signals), 101.45,
D
d
7.99e7.80 (m, 4H, ArH), 7.49e6.93 (m, 16H, ArH), 6.74 (d,
99.4 (acac-C), 88.6, 68.1, 57.1 (OeCH3), 56.9 (OeCH3); 31P (121 MHz,
J ¼ 8.5 Hz, 1H, ArH) 3.44 (s, 3H, OCH3) 2.17 (s, 3H, CH3); 13C
CDCl3) d 38.8 ppm.
(126 MHz, CDCl3)
d 153.9e111.4 (multiple P-coupled signals, ArC),
54.4 (OCH3), 20.1 (d, JPeC ¼ 20.7 Hz, CH3); 31P (121 MHz, CDCl3)
4.5.2. [Pd((R,R)-CyPhMOP)(acac)]
The procedure above was also applied to (R,S)-CyPhMOP and the
following data obtained.
2
d
ꢂ20.87; MS (Electrospray) m/z (% intensity): 484.0 (50), 483.0
(100, M D HD); HRMS calc. for [MH]þ (C34H28OP) 483.1878, found
483.1895.
MS (Electrospray) m/z (% intensity): 484.0 (50), 483.0 (100,
PhTolMOPBH3.D(B) was treated in a similar manner to above to
yield PhTolMOP.D(B).
M þ Hþ); HRMS calc. for [MH]þ (C38H38O3PPd) 679.614, found
679.1593. 1H (500 MHz, CD2Cl2)
d
8.93 (d, J ¼ 9.6 Hz, 1H), 8.16e7.14
PhTolMOP.D(B); [
CDCl3)
a
]
¼ 36.7ꢁ (c 0.45, CHCl3); 1H (300 MHz,
(multiple signals, 15H, ArH), 5.82 (d, J ¼ 8.5 Hz, 1H), 5.42 (s, 1H,
D
d
7.98e7.84 (m, 4H, ArH), 7.49e6.72 (m, 17H, ArH), 3.10 (s,
acac-H), 3.60 (s, 3H, -OMe), 2.07 (s, 3H, acac-Me), 1.80 (s, 3H, acac-
3H, OCH3) 2.00 (s, 3H, CH3); 13C (126 MHz, CDCl3)
d
155.1e112.5
Me); 13C (126 MHz, CD2Cl2)
d 189.2 (acac-C), 187.2 (acac-C), 183.7
(multiple P-coupled signals, ArC), 54.9 (OCH3), 21.2 (d,
(acac-C), 165.2, 161.0, 152.8, 147.1e124.0 (multiple P-coupled
signals), 117.8, 114.0, 110.9, 101.6, 99.7, 85.1, 65.8, 57.8 (OeCH3),
36.4e25.4 (multiple P-coupled signals); 31P (121 MHz, CDCl3)
2JPeC ¼ 21.2 Hz, CH3); 31P (121 MHz, CDCl3)
d
ꢂ21.65; MS (Elec-
trospray) m/z (% intensity): 484.2 (30), 483.2 (100, M D HD); HRMS
calc. for [MH]þ (C34H28OP) 483.1878, found 483.1888.
d 54.0 ppm.
4.4.3. Synthesis of (R,R) and (R,S)e2e(methylphenylphosphinobor-
ane)e20emethoxye1,10ebinaphthyl [4d]
4.5.3. [Rh(MOP)2]BF4
MOP (20 mgs, 43 mmol) and [Rh(COD)2]BF4 (8.7 mgs, 22 mmol)
were dissolved in CDCl3 (1 mL) and the following analyses were
taken.
To a red solution of [NiCl2(dppe)] (0.244 g, 0.463 mmol) in DMF
(8.7 mL) under N2 was added methylphenylphosphine (0.35 g,
1.04 mmol) via a syringe. The solution immediately underwent
a colour change from red to dark green. After stirring at room
temperature overnight the solution was heated to 100 ꢁC for 0.5 h,
and a solution of (R)-2-((trifluoromethanesulfonyl)oxy)-20-metho-
xy-1,10-binaphthyl (2.00 g, 4.63 mmol) and DABCO (2.08 g,
18.5 mmol) in DMF (15 mL) was added. The green solution was
MS(Electrospray)m/z(%intensity):1041.0(30),1040.0(80),1039.0
(100, Mþ); 1H (300 MHz, CDCl3)
d
8.78 (d, J ¼ 7.2 Hz, 1H), 7.20e5.95
(multiple signals, ArH), 3.64 (s, 3H, -OMe), 3.49 (s, 3H, eOMe); 13C
(126 MHz, CD2Cl2) 155.3, 154.5, 143.6e118.8 (multiple P-coupled
signals), 112.9, 112.6, 90.0, 57.5, 55.9, 55.7; 31P (121 MHz, CDCl3)
49.5 ppm (dd, 32 Hz, 218 Hz), 36.6 ppm (dd, 32 Hz, 194 Hz).
d
d

E.F. Clarke et al. / Journal of Organometallic Chemistry 696 (2011) 3608e3615
3615
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(1 eqv) were dissolved in degassed DME (3 mL) and degassed water
(3 mL) under nitrogen (the reaction was generally carried out on
such a scale as to use 15e20 mg of phosphine). The mixture was
stirred at 60 ꢁC for 36 h. The reaction was worked up by extracting
into toluene (2 ꢀ 10 mL) and dried over MgSO4. The product was
isolated by silica gel chromatography (cyclohexane/ethyl acetate,
10:1), the product being the spot closest to the baseline. The yield
was determined by mass analysis of the isolated product.
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[13] For Details of Crystalographic Data see Supporting Information.
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with the 1st isomer eluting at 7.7 min and 2nd at 13.8 min.
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The authors thank Science Foundation Ireland for support under
grant 06/RFP/CHO013, Enterprise Ireland for a Scholarship (E F C),
the European Commission for a Marie Curie Development Host
Fellowship HPMD-CT-2000-00053 (L.J.H.) and Johnson Matthey for
the loan of metal salts.
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