Asymmetric rhodium carbene insertion into the Si-H bond: Identification of new dirhodium(II) carboxylate catalysts using parallel synthesis techniques

Article abstract of DOI:10.1016/S0957-4166(03)00035-1

Decomposition of methyl 2-diazophenylacetate in the presence of silanes and a chiral dirhodium(II) catalyst results in Si-H insertion of the intermediate carbenoid with varying degrees of enantioselectivity. New chiral dirhodium(II) carboxylate catalysts were identified using solution phase parallel synthesis techniques.

Full text of DOI:10.1016/S0957-4166(03)00035-1

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 14 (2003) 791–816
Asymmetric rhodium carbene insertion into the SiꢀH bond:
identification of new dirhodium(II) carboxylate catalysts using
parallel synthesis techniques
Richard T. Buck,a,b Diane M. Coe,c Martin J. Drysdale,c Leigh Ferris,a David Haigh,c
Christopher J. Moody,b,* Neil D. Pearsond and J. Bobby Sangheraa
aDepartment of Chemistry, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK
bSchool of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, UK
cGlaxoSmithKline, Medicines Research Centre, Gunnels Wood Road, Stevenage, Hertfordshire SG1 2NY, UK
dGlaxoSmithKline, New Frontiers Science Park, Coldharbour Road, Harlow, Essex CM19 5AD, UK
Received 6 November 2002; accepted 6 December 2002
Abstract—Decomposition of methyl 2-diazophenylacetate in the presence of silanes and a chiral dirhodium(II) catalyst results in
SiꢀH insertion of the intermediate carbenoid with varying degrees of enantioselectivity. New chiral dirhodium(II) carboxylate
catalysts were identified using solution phase parallel synthesis techniques. © 2003 Elsevier Science Ltd. All rights reserved.
tivity was possible using chiral diazoesters,8 all attempts
to effect enantioselective OꢀH insertions with chiral
dirhodium(II) carboxylates failed.9 Although, recently,
Doyle et al. have shown that chiral rhodium catalysts
can enhance the diastereocontrol in the OꢀH insertion
reactions of chiral diazoesters,10 the general lack of
enantioselectivity observed in insertions into polar XꢀH
bonds is thought to be associated with a stepwise
mechanism involving initial attack of the heteroatom X
on the electrophilic metal carbene to form an ylide
followed by hydrogen transfer.3 In contrast, carbene
insertions into less polar XꢀH bonds, such as SiꢀH, are
thought to be concerted, and therefore much more
likely to be influenced by chiral catalysts. We now
report the full details of a study on asymmetric inser-
tions into the SiꢀH bond.11,12
1. Introduction
The development of chiral catalysts for asymmetric
reactions of metal carbenoids has been widely studied
since the first report by Nozaki et al. that decomposi-
tion of ethyl diazoacetate in the presence of a cop-
per(II) complex with a chiral Schiff base ligand resulted
in enantioselective cyclopropanation of styrene.1
Although copper based catalysts are extremely effective
in cyclopropanation reactions, dirhodium(II) com-
pounds, first introduced by Teyssie´ et al.,2 are generally
superior catalysts for diazo compound decomposition
since they mediate a wider range of carbenoid processes
such as insertion.3 Although enantioselective CꢀH
insertion reactions using chiral dirhodium(II) carboxyl-
ates and carboxamides as catalysts are now fairly com-
monplace,4 attempts to effect enantioselective carbenoid
XꢀH (X=heteroatom) have been less successful. Thus
Brunner et al. observed up to 12% ee in the SꢀH
insertion reaction of 3-diazo-2-butanone with thiophe-
nol,5 and McKervey et al. have described an
intramolecular NꢀH insertion reaction which proceeds
in 45% ee using dirhodium(II) mandelate as catalyst.6
Our own work was initially directed at OꢀH insertion
reactions,7 and although we showed that diastereoselec-
2. Results and discussion
The rhodium(II) catalysed reaction of diazoesters with
silanes resulting in the formation of a-silylcarbonyl
compounds by SiꢀH insertion, was first reported by
Doyle et al. in 1988,13 although examples of copper
catalysed processes had been described earlier.14 The
reaction has recently been the subject of renewed inter-
est,15,16 and diastereoselective SiꢀH insertion was
achieved by using diazoesters of chiral alcohols.15b
Since the publication of our preliminary results,11 fur-
* Corresponding author.
0957-4166/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(03)00035-1

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R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
ther papers have appeared. Thus, asymmetric SiꢀH
insertions have been carried out using N-(arenesul-
fonyl)prolinates17 and N-phthaloyl-amino acids18 as
chiral ligands for rhodium; copper(I) catalysts bearing
chiral Schiff base ligands have also been used.19,20 The
mechanism of the rhodium carbene insertion into SiꢀH
bonds has also been discussed.21–23
confines of the pharmaceutical industry and the search
for new therapeutic agents. The benefits of parallel
techniques in the identification of new catalysts are
clear, since this methodology improves efficiency with-
out introducing complex analysis and a deconvolution
process to identify the catalyst of interest. A number of
groups have now published studies involving the prepa-
ration of ligands on a solid-support or in solution, and
evaluation of their potential as ligands for asymmetric
synthesis.30
Our initial work focused on the asymmetric synthesis of
methyl 2-(silyl)phenylacetates 2 from methyl diazo-
phenylacetate 1, a reaction that works well for a range
of silanes using dirhodium(II) acetate 3 as catalyst
(Table 1, entry 1). Thus a dichloromethane solution of
methyl diazophenylacetate 1 and the silane was treated
with the catalyst to give the SiꢀH insertion products 2
in good yield and in varying enantioselectivity (ee)
(Table 1). The ee was determined by HPLC analysis on
a chiral stationary phase by comparison with the
racemic methyl 2-(silyl)phenylacetates obtained from
the dirhodium(II) acetate catalysed reaction. A range of
known catalysts was used in the first instance: our
recently described catalysts9 based on phthalate half
esters 4 and 1-substituted pyrroles 5 gave relatively
poor results with ee’s in the range 8–23% (Table 1).
However, the known N-(benzenesulfonyl)proline
derived catalyst 624 gave significantly better results,
particularly with the bulkier silanes. Dirhodium(II)
mandelate 7,25 on the other hand, proved a poor cata-
lyst, although the O-alkyl derivatives 8–11 were some-
what better. Also significantly better was the catalyst 12
derived from 2-methoxy-2-(trifluoromethyl)phenylacetic
acid.26 Dirhodium(II) camphanate 1327 also gave >30%
ee with the bulky silane, whereas the related camphen-
ate ligand 14 was a poor catalyst. Finally, in this initial
screen, carboxamidate ligands were investigated:
Doyle’s catalyst, Rh2(MEPY)4 1528 consistently gave
40–50% ee, but our recently prepared difluoro-version
16,29 whilst exhibiting significantly greater catalytic
activity in terms of reduced reaction times, gave poorer
enantioselectivity (Table 1).
A set of 80 carboxylic acids ligands, L1–L80, was
selected for study; acetic acid was deliberately included
as L80 to act as control. The other ligands were chosen
by a cluster analysis of available acids. Thus the com-
mercially available carboxylic acids, ca. 30,000 com-
pounds (of which ca. 10,000 possess a ‘chiral flag’),
contained within the Available Chemical Directory
were identified and filtered to identify the mono-car-
boxylic acids. Compounds containing additional func-
tional groups which we reasoned might be incompatible
because of their ability to compete for the dirhodium
centre were then excluded; examples of such groups
were primary amines, amidines, phosphonic acids, and
nitriles. The resulting set, ca. 2000 compounds, was
then clustered and the centroid of each of 80 clusters
selected as a representative acid. The singletons pro-
duced during the process were not evaluated further
because of the self-evident limitation on second genera-
tion arrays.
The representative chiral carboxylic acid ligand from
one of each of the 79 clusters then underwent ligand
exchange with a previously prepared dirhodium(II) car-
bonate species.31 Typically, 20 chiral acids were sub-
jected to the reaction conditions in parallel. Subsequent
parallel vacuum filtration followed by drying allowed
the isolation of the desired chiral dirhodium(II) car-
boxylate catalysts. Although no further purification was
carried out, the structures were confirmed by auto-
1
mated H NMR and mass spectrometry analysis. From
The achievement of catalytic asymmetric SiꢀH insertion
using these diverse sets of chiral dirhodium(II) catalysts
suggested that SiꢀH insertion occurs in the same con-
certed fashion as has been demonstrated for CꢀH inser-
tion reactions. Evidence for the concerted nature of
these insertions has been provided subsequently.21 The
higher reactivity of the SiꢀH bond towards insertion
makes possible a range of intermolecular insertion that
are uncharacteristic of CꢀH insertion using the same
catalysts, but this higher reactivity dictates an earlier
transition state that limits catalyst control of enantiose-
lectivity. That enantiomeric excesses approaching 50%
were achieved encouraged us to continue these studies
as described below.
the original 80 acids used, 70 chiral dirhodium(II)
carboxylate catalysts were isolated in this manner, the
remaining 10 chiral carboxylic acid ligands were recov-
ered as unreacted starting material; these 10 ligands are
shown in Fig. 1. It is not clear why these ligands did
not form dirhodium(II) carboxylate species; although
some contain sites (e.g. sulfides, basic nitrogens) which
might also bind to the metal centre, this does not
necessarily prevent complex formation (cf. the ligands
in Fig. 2 and Table 2).
Subsequently a dichloromethane solution of methyl
2-diazophenylacetate 1 (10 mg) and one of 3 silanes,
with a varying degree of steric bulk, was then treated
with one of each of the 70 previously prepared chiral
dirhodium(II) carboxylate catalysts to effect the car-
benoid SiꢀH insertion reaction. Of the 70 chiral
dirhodium(II) carboxylate catalysts used, 48 catalysed
the desired SiꢀH insertion and gave the expected
product, the remaining 22 rhodium species showing no
catalytic activity (Fig. 2). The fact that these 22
rhodium species did not cause decomposition of the
In order to enhance further the enantioselectivity of the
rhodium carbene silane insertion reactions, a systematic
search for alternative chiral carboxylic acid ligands was
required, and therefore we decided to adopt parallel
array technology to evaluate a wide variety of acids.
The principles of combinatorial and parallel synthetic
techniques have rapidly found application beyond the

R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
793
Table 1. Dirhodium(II) catalysed asymmetric SiꢀH insertion reactions of methyl diazophenylacetate: initial screening of
catalysts

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R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
Table 1. (Continued)
Figure 1. Ten chiral carboxylic acids L which did not form isolable dirhodium(II) carboxylates.
2, along with the spectroscopic characterisation data
for the catalysts.
diazo ester 1 casts doubt on their structure, and sug-
gests that the desired dirhodium(II) carboxylate was
either not formed in these cases, or was isolated in an
impure state from the parallel synthesis reactions.
General relationships in this first set of parallel reac-
tions can be systematically deduced with regards to
chiral carboxylic acid ligand. For the majority of the
chiral dirhodium(II) carboxylate catalysts applied to
the enantioselective SiꢀH insertion reaction it was
apparent that in most cases there was a significant
increase in enantioselectivity on increasing the steric
bulk of the silane, i.e. the use of tri-isopropylsilane
generally gave the greatest enantioselectivities. Some
other significant relationships were the presence of
an N-(1-dimethylamino-5-naphthalenesulfonyl) group
With 48 catalytically active dirhodium(II) carboxylates,
one of which was the dirhodium(II) acetate control,
and using three silanes, 144 samples for analysis were
generated. Parallel purification of these silane insertion
products 2 through a small silica column (BondElut®)
allowed the isolation of sufficient material for the deter-
mination of the enantioselectivities of the SiꢀH inser-
tion products by automated HPLC on a chiral
stationary phase. The results are summarised in Table

R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
795
Figure 2. 22 Chiral carboxylic acids L which did not give dirhodium(II) species that were catalytically active in silane insertion
reactions with diazoester 1.
which generally gave good enantioselectivity, for exam-
ple the chiral carboxylic acid ligand N-(1-dimethyl-
amino-5-naphthalenesulfonyl)phenylalanine L4 in an
enantioselectivity of 48% and also N-(1-dimethylamino-
5-naphthalenesulfonyl)proline L6 in 47% ee. Chiral car-
boxylic acid ligands containing the N-(9-fluorenyl-
methoxycarbonyl) (Fmoc) protecting group also showed
good enantioselectivities, for example N-(9-fluorenyl-
position of diazoester 1 in the presence of dimethyl-
phenylsilane resulted in the formation of 2a in 51% ee,
increased to 85% at −78°C.17 N-Phthaloyl containing
chiral carboxylic acid ligands are also well documented.
In the enantioselective SiꢀH insertion reactions they also
induced reasonable enantioselectivities, for example with
Ikegami’s N-phthaloyl-(S)-phenylalanine L28 chiral car-
boxylic acid ligand,34 with an enantioselectivity of 33%
and N-phthaloyl-(S)-leucine L24 in 41% ee. Recent work
by the Japanese group has showed that the enantioselec-
tivity exhibited by ligand L28 increases to 74% if the
reaction is conducted at −90°C.18
methoxycarbonyl)-(4R)-tert-butoxy-(S)-proline
with an enantioselectivity of 46% and also N-(9-fluorenyl-
L32
methoxycarbonyl)-N-methylphenylalanine L35 in 44%
ee.
Chiral carboxylic acid ligands based on proline are well
documented in inducing enantioselectivity in a range of
asymmetric rhodium carbene transformations. This is
shown here for example with Davies’ N-(4-tert-
butylphenylsulfonyl)proline L61 chiral carboxylic acid
ligand,32,33 which achieved an enantioselectivity of 41%
and the valine-prolinate ligand L72 in 47% ee. Davies
subsequently reported that use of dirhodium(II) N-[(4-
dodecylphenyl)sulfonyl]prolinate in the catalysed decom-
There were also the more unusual chiral carboxylic acid
ligands that parallel synthesis methods allow to be
screened because of the diversity of the technique,
for example N-a-(benzyloxycarbonyl)-N%-o-(4-toluene-
sulfonyl)lysine L46 with an enantioselectivity of 41%.
More interestingly though, this first set of parallel
reactions also identified two chiral dirhodium(II)
carboxylate catalysts of significant interest (shaded in
Table 2). Firstly, the chiral carboxylic acid ligand based

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R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
Table 2. Parallel screening of chiral dirhodium(II) carboxylate catalysts in SiꢀH insertion reactions of methyl diazophenyl-
acetate

R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
797
Table 2. (Continued)

798
R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
Table 2. (Continued)

R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
799
Table 2. (Continued)

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R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
Table 2. (Continued)

R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
801
Table 2. (Continued)

802
R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
Table 2. (Continued)
on an N-arenesulfonyl a-amino acid substructure, N-
(1-dimethylamino-5-naphthalenesulfonyl)phenylalanine
L4, gave enantioselectivities up to 48% using the most
sterically hindered silane, tri-isopropylsilane. The chiral
carboxylic acid ligand based on an a-hydroxy acid
substructure, (S)-a-hydroxy-isocaproic acid L12, gave
enantioselectivities up to 53%, also with tri-isopropylsi-
lane. Already these results are an improvement on those
initially reported in Table 1, and therefore formed the
basis of the second generation parallel screen.
containing the N-arenesulfonyl a-amino acid and a-
hydroxy acid substructures. The results of these second
sets of parallel reactions are shown in Table 3 for the
N-arenesulfonyl a-amino acid ligands L4/1–L4/5 and
Table 4 for the a-hydroxy acid ligands L12/1–L12/8. The
dirhodium(II) carboxylates derived from ligands L4/2,
L4/3 and L4/5 were also prepared conventionally, as were
three additional catalysts from ligands L4/6–L4/8.35
These second sets of parallel reactions identified further
chiral dirhodium(II) carboxylate catalysts capable of
effecting enantioselective SiꢀH insertion reactions, and
although the a-hydroxy acid ligands gave generally poor
results in silane insertion reactions (Table 4), the N-are-
nesulfonyl a-amino acid ligands generally showed greater
promise (Table 3). In an attempt to improve enantiose-
lectivity, the SiꢀH insertion reactions were then repeated
at −78°C in dichloromethane, with the most hindered
Having now identified these two chiral carboxylic acid
ligands, L4 and L12, a second generation array was then
constructed to improve further the enantioselectivity of
the SiꢀH insertion reaction by utilising a substructure
search. This involved looking back over the array at the
specific sub-groups from which these two chiral car-
boxylic acid ligands were identified from and then to
synthesise further catalysts from these chiral ligands

R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
803
Table 3. Second generation parallel screening of chiral dirhodium(II) N-arenesulfonyl a-aminocarboxylates in SiꢀH insertion
reactions of methyl diazophenylacetate

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R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
Table 3. (Continued)
gave consistently the greatest enantioselectivities for the
SiꢀH insertion product, 2d in 50% ee, 2e in 55% ee and
2f in 44% ee respectively (Scheme 2).
silane, tri-isopropylsilane. As can be seen from Tables 3
and 4, in some, but not all cases, carrying out the reactions
at −78°C resulted in further improvement of the enan-
tioselectivity. The N-arenesulfonyl a-amino acids all
showed improvements in enantioselectivity under these
conditions, although no further improvement was
observed upon changing the solvent to pentane (Table 3).
The chiral carboxylic acid ligand N-(4-toluenesulfonyl)-
(S)-leucine L4/3 proved the best, improving from 49 to
76% ee at low temperature; this compares very favourably
with enantioselectivities observed in other studies,17,18
although we have not fully optimised the reaction condi-
tions (temperature, solvent). The resulting silylester 2c
(76% ee) was reduced with DIBAL to give the alcohol
17; acylation with 4-bromobenzoyl chloride gave the ester
18, obtained in 94% ee after a single recrystallisation
(Scheme 1)
Finally, the diazolactone, dihydro-3-diazo-4,4-dimethyl-
2-furanone 19, was also screened in the enantioselective
SiꢀH insertion reaction with the previous silanes.
Although the racemic silane insertion products 20a and
20b could be obtained in reasonable yield using dirhodi-
um(II) acetate as catalyst (Scheme 3), the system was not
suitable for parallel screening due to the non-trivial
purification of the SiꢀH insertion products.
2.1. Determination of the absolute stereochemistry of
the SiꢀH insertion product 2a
In order to determine the stereochemical outcome of the
silane insertion reactions, i.e. does an (S) chiral dirhodi-
um(II) carboxylate catalyst induce (S) or (R) enantiose-
lectivity in the SiꢀH insertion product, the conversion of
the methyl 2-(silyl)phenylacetates 2 into compounds of
known absolute configuration was investigated. This was
most conveniently done on the phenyldimethylsilyl
derivative2aandinvolvedreductionoftheestertoalcohol
21 followed by Tamao–Fleming oxidation of the CꢀSi
bond using the procedure published by Landais and
Planchenault36 to give phenyl-1,2-ethanediol (Scheme 4).
Other silanes were also screened in the enantioselective
SiꢀH
insertion
reaction,
tri-isobutylsilane,
tris(trimethylsilyl)silane and 1,1,1,3,5,5,5-heptamethyl-
siloxane, in the presence of the diazoester 1, and one of
each of the chiral dirhodium(II) carboxylate catalysts
from the first and second sets of parallel reactions, to yield
the expected SiꢀH insertion products 2d–f in each case.
The catalyst derived from the chiral carboxylic acid
ligand, N-(4-toluenesulfonyl)-(S)-phenylalanine L4/5,

R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
805
Table 4. Second generation parallel screening of chiral dirhodium(II) a-hydroxycarboxylates in SiꢀH insertion reactions of
methyl diazophenylacetate

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R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
Table 4. (Continued)
Scheme 1.
Hence the absolute configuration of the major enantiomer
of was readily determined by comparison of known
specific rotations of the product 1,2-diol. In common with
other studies,17,18 we find that there is no general pattern;
(S)-configured catalysts often give (S)-configured
silylester 2a but not always.
tion of improved catalysts for enantioselective SiꢀH
insertion reaction of diazoesters with silanes.
3. Conclusion
Thisparallel synthesis technique hasenabled ustoidentify
several chiral dirhodium(II) carboxylates capabIe of
effecting enantioselective SiꢀH insertion reactions using
methyl diazophenylacetate 1. It is interesting that cata-
lysts based on a-hydroxyacids and N-arenesulfonyl a-
amino acids emerged as the most generally useful families;
such catalysts were among the earliest chiral dirhodium-
(II) carboxylate catalysts investigated, although the first
studied ligands, mandelic acid and N-benzenesulfonyl-
prolinate, give relatively poor results in our silane inser-
tion reactions (8 and 12% ee, respectively, for Me2PhSiH).
Although the reactions have not been fully optimised in
terms of temperature and solvent, the use of parallel
synthesis techniques has resulted in the rapid identifica-
Scheme 2.
Scheme 3.

R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
807
Scheme 4.
4. Experimental
High-resolution mass spectra were recorded on a
Kratos MS80 instrument by electron impact (EI),
unless otherwise. Liquid chromatography–mass spectra
(LC–MS) were recorded on an automated Micromass
Platform LC instrument.
4.1. General information
Commercially available solvents and reagents were
utilised throughout with no further purification, other
than of those detailed below. ‘Light petroleum’ refers to
the fraction boiling between 40 and 60°C and was
distilled from calcium chloride through a 36 cm Vigreux
column before use. Diethyl ether and ethyl acetate were
purified in the same manner prior to use.
Dichloromethane was purified and dried by distillation
from phosphorus pentoxide. Toluene was distilled over
calcium hydride before being stored under nitrogen and
4.1.1. Methyl 2-diazophenylacetate 1. Prepared using the
Bamford–Stevens reactions.37
4.1.2. Dihydro-3-diazo-4,4-dimethyl-2-furanone 19. To a
stirred solution of dihydro-4,4-dimethyl-2,3-furandione
(2.50 g, 19.51 mmol) in dry toluene (100 mL) was
added 4-toluenesulfonylhydrazide (3.63 g, 19.51 mmol).
After heating under Dean and Stark conditions
overnight the reaction mixture was allowed to cool and
triethylamine (19.74 g, 195.11 mmol) was added drop-
wise. After stirring overnight the solvent was removed
under reduced pressure to yield a golden yellow oil.
This crude product was then subjected to flash silica gel
chromatography using light petroleum and ethyl ace-
tate (4:1) as eluant to yield the title compound as a
bright yellow oil (0.63 g, 23%); (Found: M+, 140.0585.
C6H8N2O2 requires 140.0586); wmax (film)/cm−1 2968,
2093, 1734, 1464, 1145; lH (400 MHz; CDCl3) 4.05
(2H, s, CH2), 1.42 (6H, s, Me); lC (100.6 MHz; CDCl3)
169.1 (CꢁO), 78.5 (CH2), 59.0 (CꢁN2), 38.9 (C), 25.9
(Me); m/z 140 (M+, 77%), 125 (76), 82 (20), 71 (11), 67
(100), 55 (52), 53 (62).
,
over activated 4 A molecular sieves. Tetrahydrofuran
was distilled from sodium benzophenone ketyl whilst
under nitrogen prior to use.
Analytical thin-layer chromatography (TLC) was car-
ried out using aluminium backed plates coated with
Merck Kieselgel 60 GF254. All developed plates were
visualised under UV light at 254 nm and/or by staining
with ammonium molybdate and permanganate dips.
Flash silica gel chromatography was carried out using
Merck Kieselgel 60 H silica and pressure was applied at
the column head using hand bellows. Samples were
applied to the separation column either pre-adsorbed
onto silica or as a saturated solution in an appropriate
solvent. Chiral high performance liquid chromatogra-
phy (HPLC) was performed using either a Thermo
Separation Products instrument or a Gilson automated
system at 265 nm. The Thermo Separation Products
chromatographic system used a Daicel Chemical Indus-
tries, Ltd. Chiralpak AD or a Chiralcel OD HPLC
column (diameter 0.46 cm, length 25 cm). The Gilson
automated chromatographic system used an (S,S)-
WHELK-01 HPLC column (diameter 0.46 cm, length
25 cm).
4.2. Catalysts: dirhodium(II) tetrakis(carbonate)
Prepared by the literature procedure.31 A stirred sus-
pension of dirhodium(II) acetate (0.50 g, 1.13 mmol) in
2 M aqueous sodium carbonate solution (13.33 mL)
was heated at 85°C for 30 min during which time the
green colour changed to a blue purple. Cooling and
filtration gave a purple solid which was subsequently
washed with water (25 mL), methanol (25 mL) and
ether (25 mL) and then dried to yield the title compound
as a purple powdery solid (0.63 g, 1.02 mmol, 90%).
This was then used in subsequent reactions without
further purification.
IR spectra were recorded in the range 4000–600 cm−1
using a Nicolet FT-205 spectrometer with internal cali-
bration. Spectra were recorded using potassium bro-
mide disks or as either solutions in chloroform or thin
1
films using sodium chloride plates. H NMR spectra
4.2.1. Dirhodum(II) tetrakis((1R,2S,5R)-(−)-menthyl (2-
carboxy)benzoate) 4. (a) To a stirred solution of (−)-
menthol (0.62 g, 3.97 mmol), phthalic anhydride (1.00
g, 6.75 mmol) and 4-dimethylaminopyridine (0.10 g,
0.81 mmol) in dioxane (5 mL) under a nitrogen atmo-
sphere was added triethylamine (0.60 g, 5.95 mmol)
dropwise. After stirring overnight ether (50 mL) was
added and the reaction mixture washed with aqueous
hydrochloric acid (2 M) (50 mL). The aqueous phase
was then extracted with ether (2×50 mL). The organic
extracts were combined, dried (MgSO4) and the solvent
was then removed under reduced pressure to yield a
were recorded using Bruker AC-250 (250 MHz), Bruker
AC-300 (300 MHz) and Bruker DPX-400 (400 MHz)
instruments. 13C NMR spectra were recorded on
Bruker AC-250 (62.9 MHz) and Bruker DPX-400
1
(100.6 MHz) instruments. H NMR spectra were refer-
enced against tetramethylsilane at 0.00 ppm and resid-
ual chloroform at 7.27 ppm. Chemical shift values lH
and lC are accurate to 0.01 ppm and 0.10 ppm
respectively. Signals are described as being broad (br),
singlets (s), doublets (d), triplets (t), quartets (q), multi-
plets (m), double doublets (dd), double triplets (dt), etc.

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R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
EtOH)); (Found: M+, 171.1046. C12H13N requires
171.1048); wmax (film)/cm−1 3029, 2980, 1491, 1450, 725;
lH (250 MHz; CDCl3) 7.27 (3H, m, ArH), 7.08 (2H, m,
ArH), 6.75 (2H, m, 2-H and 5-H), 6.18 (2H, m, 3-H
and 4-H), 5.25 (1H, q, J 7.0, CH), 1.82 (3H, d, J 7.0,
Me); lC (62.9 MHz; CDCl3) 143.5 (C), 128.6 (ArC),
127.4 (ArC), 125.8 (ArC), 119.4 (2-CH and 5-CH),
108.0 (3-CH and 4-CH), 58.1 (CH), 22.1 (Me); m/z 171
(M+, 70%), 117 (22), 105 (100), 77 (32), 67 (75).
colourless oily residue. Carbon tetrachloride (20 mL)
was added and the solution filtered. Removal of the
solvent under reduced pressure yielded a colourless
solid. Recrystallisation from chloroform and light
petroleum yielded (1R,2S,5R)-(−)-menthyl (2-car-
boxy)benzoate as a colourless solid (0.58 g, 48%), mp
108–109°C (lit.,38 mp 111°C); [h]2D0=−90.0 (c 1.0,
CHCl3) (lit.,38 [h]D20=−89.2 (c 5.0, CHCl3)); (Found: C,
70.7; H, 8.1. C18H24O4 requires C, 71.0; H, 8.0%);
(Found: M+, 304.1668. C18H24O4 requires 304.1674);
wmax (CHCl3)/cm−1 3500–2500, 3023, 2957, 1703, 1600,
1453, 1291, 739; lH (250 MHz; CDCl3) 11.25–10.75
(1H, bs, OH), 7.91 (2H, m, ArH), 7.59 (2H, m, ArH),
4.98 (1H, m, 1-Hax), 2.23 (1H, m, 6-Heq), 1.98 (1H, m,
6-Hax), 1.71 (2H, m, 3-Heq and 4-Heq), 1.50 (2H, m,
3-Hax and 4-Hax), 1.48 (2H, m, 2-Hax and 5-Hax), 1.10
(b) To a stirred solution of (S)-1-(1-phenylethyl)pyrrole
(5.00 g, 29.20 mmol) in dry tetrahydrofuran (100 mL)
under a nitrogen atmosphere at 0°C was added trifl-
uoroacetic anhydride (14.72 g, 70.08 mmol) dropwise.
After stirring for 2 h the solvent was removed under
reduced pressure to yield a yellow oil. This crude
product was then subjected to flash silica gel chro-
matography using light petroleum and ethyl acetate
(19:1) as eluant to yield (S)-1-(1-phenylethyl)-2-trifl-
uoroacetylpyrrole as a pale yellow oil (5.84 g, 75%),
[h]2D0=−96.4 (c 1.0, CHCl3); (Found: M+, 267.0872.
C14H12F3NO requires 267.0871); wmax (film)/cm−1 3032,
2985, 1668, 1495, 1450, 736; lH (250 MHz; CDCl3) 7.29
(5H, m, Ph), 7.16 (2H, m, 3-H and 5-H), 6.53 (1H, q, J
7.0, CH), 6.31 (1H, m, 4-H), 1.80 (3H, d, J 7.1, Me); lC
(62.9 MHz; CDCl3) 169.9 (CꢁO), 141.4 (C), 130.9 (3-
CH), 128.7 (ArC), 127.7 (ArC), 126.3 (ArC), 124.9
(5-CH), 124.8 (C), 110.5 (4-CH), 56.7 (CH), 22.0 (Me),
CF3 not observed; m/z 267 (M+, 23%), 198 (8), 105
(100), 77 (12).
(1H, m, CH6 Me2), 0.94 (3H, d, J 6.5, 5-Me), 0.90 (3H, d,
J 7.0, CHMe2), 0.84 (3H, d, J 7.0, CHMe2); lC (62.9
MHz; CDCl3) 172.4 (CꢁO), 167.6 (CꢁO), 134.1 (C),
134.0 (C), 132.1 (ArC), 130.5 (ArC), 129.7 (ArC), 128.5
(ArC), 75.9 (1-CH), 47.0 (2-CH), 40.2 (6-CH2), 34.1
(4-CH2), 31.4 (C6 HMe2), 26.1 (5-CH), 23.3 (3-CH2), 21.9
(5-Me), 20.7 (CHMe2), 16.1 (CHMe2); m/z 305 (M+,
24%), 167 (98), 149 (100), 138 (67), 123 (36), 95 (74), 81
(57), 55 (30).
(b) Dirhodium(II) acetate (100 mg, 0.23 mmol) and
(1R,2S,5R)-(−)-menthyl (2-carboxy)benzoate (825 mg,
2.71 mmol) were fused together at 160°C for a period
of 48 h in the absence of solvent. This crude product
was then subjected to flash silica gel chromatography
using light petroleum and ethyl acetate (4:1) as eluant
to yield the title compound as a green powdery solid
(195 mg, 61%), mp 165–166°C; [h]2D2=−90.0 (c 0.1,
CHCl3); (Found: C, 60.7; H, 6.3. C72H92O16Rh2
requires C, 60.9; H, 6.5%); (Found: M+, 1418.4544.
C72H92O16Rh2 requires 1418.4539); wmax (CHCl3)/cm−1
3020, 2957, 1724, 1598, 1453, 1216, 739; lH (250 MHz;
CDCl3) 7.90–7.33 (16H, m, ArH), 4.55 (4H, m, 1-Hax),
2.05 (4H, m, 6-Heq), 1.89 (4H, m, 6-Hax), 1.59 (8H, m,
3-Heq and 4-Heq), 1.33–0.93 (20H, m, 2-Hax, 3-Hax,
(c) To a stirred solution of (S)-1-(1-phenylethyl)-2-trifl-
uoroacetylpyrrole (1.00 g, 3.74 mmol) in N,N-dimethyl-
formamide (50 mL) was added lithium hydroxide (0.90
g, 37.42 mmol). After stirring at 70°C overnight the
reaction mixture was partitioned between water (100
mL) and ether (100 mL). The organic phase was
removed and the aqueous phase was acidified to pH 1
with aqueous hydrochloric acid (2 M) and then
extracted with ethyl acetate (3×100 mL). The organic
extracts were combined, washed with saturated brine
(100 mL), dried (MgSO4) and the solvent was removed
under reduced pressure to yield a pale brown oil. Ether
(50 mL) was added and the reaction mixture was
subsequently washed with water (3×50 mL), dried
(MgSO4) and the solvent was then removed under
reduced pressure to yield a pale yellow solid. Recrys-
tallisation from ethyl acetate and light petoleum yielded
(S)-1-(1-phenylethyl)pyrrole-2-carboxylate as colourless
crystals (0.55 g, 68%), mp 85–86°C; [h]2D0=−195.2 (c
1.0, CHCl3); (Found: M+, 215.0944. C13H13NO2
requires 215.0946); (Found: C, 72.5; H, 6.1; N, 6.4.
C13H13NO2 requires C, 72.5; H, 6.1; N, 6.5%) wmax
(CHCl3)/cm−1 3420, 3054, 2985, 1668, 1494, 1430, 739;
lH (250 MHz; CDCl3) 7.30 (2H, m, ArH), 7.25 (1H, m,
5-H), 7.14 (3H, m, ArH), 7.03 (1H, m, 3-H), 6.56 (1H,
q, J 7.1, CH), 6.21 (1H, m, 4-H), 1.80 (3H, d, J 7.0,
Me); lC (62.9 MHz; CDCl3) 165.9 (CꢁO), 142.6 (C),
128.5 (5-CH), 127.3 (ArC), 126.7 (ArC), 126.2 (ArC),
121.3 (C), 120.5 (3-CH), 108.8 (4-CH), 55.4 (CH), 21.9
(Me); m/z 215 (M+, 14%), 155 (2), 111 (13), 105 (100),
104 (38), 93 (3), 77 (19), 51 (7).
4-Hax, 5-Hax and CH6 Me2), 0.85 (12H, d, J 7.2, 5-Me),
0.82 (12H, d, J 6.9, CHMe2), 0.75 (12H, d, J 6.9,
CHMe2); lC (62.9 MHz; CDCl3) 187.3 (CꢁO), 166.3
(CꢁO), 135.4 (C), 131.3 (ArC), 130.3 (ArC), 129.4
(ArC), 129.1 (ArC), 127.7 (C), 75.2 (1-CH), 47.1 (2-
CH), 40.3 (6-CH2), 34.3 (4-CH2), 31.4 (C6 HMe2), 26.3
(5-CH), 23.4 (3-CH2), 22.0 (5-Me), 20.9 (CHMe2), 16.3
(CHMe2); m/z (FAB) 1418 (M+, 9%), 849 (50), 701 (42),
481 (99), 459 (99), 414 (100), 345 (84).
4.2.2.
Dirhodium(II)
tetrakis((S)-1-(1-phenylethyl)-
pyrrole-2-carboxylate) 5. (a) To a stirred solution of
(S)-(−)-1-phenylethylamine (5.00 g, 41.26 mmol) in tol-
uene (70 mL) was added acetic acid (70 mL) followed
by 2,5-dimethoxy tetrahydrofuran (6.54 g, 49.51 mmol).
After heating under reflux for 24 h the solvent was
removed under reduced pressure to yield a black oil.
This crude product was then subjected to flash silica gel
chromatography using light petroleum and ethyl ace-
tate
(97:3)
as
eluant
to
yield
(S)-1-(1-
phenylethyl)pyrrole as a colourless oil (6.62 g, 94%),
[h]D20=+40.0 (c 1.0, EtOH) (lit.,39 [h]2D5=+46.0 (c 0.9,

R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
809
(d) To a stirred solution of (S)-1-(1-phenylethyl)-
pyrrole-2-carboxylate (584 mg, 2.71 mmol) in chloro-
benzene (60 mL) was added rhodium(II) acetate (100
mg, 0.23 mmol). After heating under reflux for 24 h the
solvent was then removed under reduced pressure to
yield a green oil. This crude product was then subjected
to flash silica gel chromatography using light petroleum
and ethyl acetate (1:1) as eluant to yield the title
compound as a green powdery solid (176 mg, 72%), mp
130–131°C (dec.); [h]D21=−70.0 (c 0.1, CHCl3); (Found:
C, 58.7; H, 4.4; N, 5.3. C52H48N4O8Rh2 requires C,
58.8; H, 4.6; N, 5.3%); (Found: M+, 1062.1593.
C52H48N4O8Rh2 requires 1062.1582); wmax (CHCl3)/cm−1
3054, 2987, 1731, 1493, 1430, 737; lH (250 MHz;
CDCl3), 6.92 (20H, m, ArH), 6.70 (4H, m, 5-H), 6.64
(4H, m, 3-H), 6.23 (4H, q, J 7.5, CH), 5.92 (4H, m,
4-H), 1.58 (12H, d, J 7.5, Me); lC (62.9 MHz; CDCl3)
179.1 (CꢁO), 142.7 (C), 128.5 (5-CH), 128.5 (ArC),
128.2 (ArC), 123.2 (ArC), 118.8 (C), 117.7 (3-CH),
108.9 (4-CH), 55.1 (CH), 21.3 (Me); m/z (FAB) 1062
(M+, 26%), 546 (48), 476 (5), 441 (12), 412 (4), 374 (17),
345 (4), 308 (21), 271 (7), 218 (14), 198 (10), 156 (11),
121 (5).
wmax (film)/cm−1 3065, 2980, 1735, 1495, 1455, 1178,
753; lH (400 MHz; CDCl3) 7.46 (2H, m, ArH), 7.35
(3H, m, ArH), 4.87 (1H, s, CH), 4.21 (1H, dq, J 10.8,
7.1, CO2CH
CO2CHHMe), 3.61 (1H, dq, J 9.1, 7.0, OCH
3.52 (1H, dq, J 9.1, 7.0, OCHHMe), 1.28 (3H, t, J 7.0,
6
HMe), 4.15 (1H, dq,
J
10.8, 7.1,
6
6
HMe),
6
OCH2Me), 1.22 (3H, t, J 7.1, CO2CH2Me); lC (100.6
MHz; CDCl3) 171.0 (CꢁO), 136.8 (C), 128.5 (ArC),
128.5 (ArC), 127.1 (ArC), 81.0 (CH), 65.3 (OC6 H2Me),
61.1 (CO2C6 H2Me), 15.1 (OCH2Me), 14.1 (CO2CH2Me);
m/z 208 (M+, 1%), 163 (10), 135 (100), 107 (90), 105
(41), 79 (90), 77 (61), 51 (30).
(b) To a stirred solution of (S)-(+)-ethyl 2-ethoxy-2-
phenylacetate (0.50 g, 2.40 mmol) in methanol/water
(5:1) (36 mL) was added lithium hydroxide monohy-
drate (0.50 g, 12.00 mmol). After stirring overnight the
reaction mixture was then partitioned between water
(100 mL) and ether (100 mL). The organic phase was
removed and the aqueous phase was then acidified to
pH 1 with aqueous hydrochloric acid (2 M) and
extracted with ethyl acetate (3×100 mL). The organic
extracts were combined, washed with saturated brine
(100 mL), dried (MgSO4) and the solvent was then
removed under reduced pressure to yield (S)-(+)-h-
ethoxyphenylacetic acid as a pale yellow oil (0.42 g,
97%), [h]2D1=+102.0 (c 1.0, acetone) (lit.,40 [h]1D5=+88.5
(c 3.0, acetone)); (Found: M+, 180.0781. C10H12O3
requires 180.0786); wmax (film)/cm−1 3500–2500, 3065,
2980, 1724, 1495, 1455, 1175, 761; lH (400 MHz;
CDCl3) 8.41 (1H, bs, OH), 7.45 (2H, m, ArH), 7.36
(3H, m, ArH), 4.89 (1H, s, CH), 3.61 (1H, dq, J 9.2,
4.2.3. Dirhodium(II) tetrakis(N-phenylsulfonyl-(S)-proli-
nate) 6. Prepared by the literature procedure.24
4.2.4. Dirhodium(II) tetrakis((S)-(+)-mandelate) 7. Pre-
pared by the literature procedure.25
4.2.5. Dirhodium(II) tetrakis((S)-(+)-a-methoxyphenyl-
acetate) 8. A stirred suspension of dirhodium(II) tetra-
kis(carbonate) (50 mg, 0.08 mmol) and (S)-(+)-a-
methoxyphenylacetic acid (108 mg, 0.64 mmol) in water
(5 mL) was heated at 85°C for 1 h. During this time the
initial deep blue colour faded and the green product
precipitated out. Cooling and filtration gave a green
solid which was subsequently washed with water (10
mL) and dried under vacuum to yield the title com-
pound as a green powdery solid (54 mg, 77%), mp
250°C (dec.); [h]2D3=+60.0 (c 0.1, CHCl3); (Found: C,
49.8; H, 4.1. C36H36O12Rh2 requires C, 49.9; H, 4.2%);
(Found: M+, 866.0303. C36H36O12Rh2 requires
866.0317); wmax (CHCl3)/cm−1 3055, 2989, 1606, 1455,
1095, 740; lH (400 MHz; CD3OD) 7.17 (20H, m, ArH),
4.25 (4H, s, CH), 2.84 (12H, s, OMe); lC (100.6 MHz;
CD3OD) 190.0 (CꢁO), 137.1 (C), 127.8 (ArC), 127.7
(ArC), 126.5 (ArC), 82.8 (CH), 56.1 (Me); m/z (FAB)
866 (M+, 6%), 388 (11), 211 (15), 152 (29), 121 (100),
107 (69).
7.0, OCH6 HMe), 3.55 (1H, dq, J 9.2, 7.0, OCHH6 Me),
1.28 (3H, t, J 9.0, Me); lC (100.6 MHz; CDCl3) 174.6
(CꢁO), 135.8 (C), 128.9 (ArC), 128.7 (ArC), 127.2
(ArC), 80.4 (CH), 65.4 (CH2), 15.0 (Me); m/z 180 (M+,
1%), 135 (100), 107 (81), 79 (51), 77 (25).
(c)
A stirred suspension of dirhodium(II) tetra-
kis(carbonate) (50 mg, 0.08 mmol) and (S)-(+)-a-
ethoxyphenylacetic acid (117 mg, 0.64 mmol) in water
(5 mL) was heated at 85°C for 1 h. During this time the
initial deep blue colour faded and the green product
precipitated out. Cooling and filtration gave a green
solid which was subsequently washed with water (10
mL) and dried under vacuum to yield the title com-
pound as a green crystalline solid (29 mg, 39%), mp
250°C (dec.); [h]2D4=+40.0 (c 0.1, CHCl3); (Found: C,
52.0; H, 4.6. C40H44O12Rh2 requires C, 52.1; H, 4.8%);
(Found: M+, 922.0919. C40H44O12Rh2 requires
922.0933); wmax (CHCl3)/cm−1 3056, 2986, 1610, 1587,
1455, 1108, 746; lH (400 MHz; CDCl3) 7.19 (20H, m,
4.2.6. Dirhodium(II) tetrakis((S)-(+)-a-ethoxyphenylac-
etate) 9. (a) To a stirred solution of (S)-(+)-mandelic
acid (1.00 g, 6.57 mmol) in iodoethane (103 g, 657
mmol) was added silver(I) oxide (3.05 g, 13.14 mmol).
After stirring overnight the reaction mixture was
filtered and the solvent was removed under reduced
pressure to yield a colourless oil. This crude product
was then subjected to flash silica gel chromatography
using light petroleum and ethyl acetate (9:1) as eluant
to yield (S)-(+)-ethyl 2-ethoxy-2-phenylacetate as a
colourless oil (0.75 g, 55%), [h]2D3=+77.0 (c 1.0, CHCl3);
(Found: M+, 208.1101. C12H16O3 requires 208.1099);
ArH), 4.43 (4H, s, CH), 3.07 (4H, m, OCH
6 HMe), 2.93
(4H, m, OCHHMe), 1.05 (12H, t, J 9.3, Me); lC (100.6
6
MHz; CDCl3) 191.1 (CꢁO), 137.3 (C), 128.2 (ArC),
127.9 (ArC), 126.6 (ArC), 81.4 (CH), 65.1 (CH2), 15.0
(Me); m/z (FAB) 922 (M+, 6%), 376 (10), 295 (8), 255
(15), 181 (8), 152 (6), 135 (100), 133 (8), 107 (100).
4.2.7. Dirhodium(II) tetrakis((S)-(+)-a-npropoxyphenyl-
acetate) 10. (a) To a stirred solution of (S)-(+)-mandelic
acid (1.00 g, 6.57 mmol) in 1-iodopropane (111 g, 657
mmol) was added silver(I) oxide (3.05 g, 12.14 mmol).

810
R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
After stirring overnight the reaction mixture was
filtered and the solvent was removed under reduced
pressure to yield a colourless oil. This crude product
was then subjected to flash silica gel chromatography
using light petroleum and ethyl acetate (9:1) as eluant
to yield (S)-(+)-npropyl 2-npropoxy-2-phenylacetate as a
colourless oil (0.82 g, 53%), [h]2D3=+65.0 (c 1.0, CHCl3);
(Found: M+, 236.1410. C14H20O3 requires 236.1412);
wmax (film)/cm−1 3033, 2967, 1732, 1496, 1455, 1177,
754; lH (400 MHz; CDCl3) 7.47 (2H, m, ArH), 7.33
(3H, m, ArH), 4.86 (1H, s, CH), 4.08 (2H, t, J 6.7,
ArH), 4.41 (4H, s, CH), 3.03 (4H, m, OCH
2.87 (4H, m, OCHHCH2Me), 1.48 (8H, sextet, J 9.1,
OCH2CH2Me), 0.84 (12H, t, J 9.8, Me); lC (100.6
MHz; CDCl3) 191.1 (CꢁO), 137.4 (C), 128.2 (ArC),
127.9 (ArC), 126.6 (ArC), 81.5 (CH), 71.3 (OCH2), 22.8
(CH2), 10.6 (Me); m/z (FAB) 978 (M+, 1%), 323 (6),
295 (7), 239 (13), 218 (6), 181 (7), 149 (55), 133 (17), 107
(100).
6 HCH2Me),
6
6
4.2.8. Dirhodium(II) tetrakis((S)-(+)-a-isopropoxyphenyl-
acetate) 11. (a) To a stirred solution of (S)-(+)-mandelic
acid (0.50 g, 3.29 mmol) in 2-iodopropane (55 g, 329
mmol) was added silver(I) oxide (1.52 g, 6.57 mmol).
After stirring overnight the reaction mixture was
filtered and the solvent was removed under reduced
pressure to yield a colourless oil. This crude product
was then subjected to flash silica gel chromatography
using light petroleum and ethyl acetate (9:1) as eluant
to yield (S)-(+)-isopropyl 2-isopropoxy-2-phenylacetate
as a colourless oil (228 mg, 29%), [h]2D0=+68.0 (c 1.0,
CHCl3); (Found: M+, 236.1415. C14H20O3 requires
236.1412); wmax (film)/cm−1 3032, 2978, 1727, 1581,
1455, 1176, 724; lH (400 MHz; CDCl3) 7.46 (2H, m,
ArH), 7.32 (3H, m, ArH), 5.03 (1H, h, J 6.2,
CO2CH2
Me), 3.41 (1H, dt, J 9.0, 6.7, OCHH
sextet, J 6.9, OCH2CH2Me), 1.61 (2H, sextet, J 6.8,
CO2CH2CH2Me), 0.94 (3H, t, J 7.4, OCH2CH2Me),
6
CH2Me), 3.52 (1H, dt, J 9.0, 6.7, OCH
6 HCH2-
6
CH2Me), 1.68 (2H,
6
6
0.84 (3H, t, J 7.4, CO2CH2CH2Me); lC (100.6 MHz;
CDCl3) 171.2 (CꢁO), 137.0 (C), 128.5 (ArC), 128.4
(ArC), 127.1 (ArC), 81.1 (CH), 71.6 (OC
6
H2CH2Me),
66.6 (CO2C6 H2CH2Me), 22.8 (OCH2C6 H2Me), 21.9
(CO2CH2C
6
H2Me),
10.5
(OCH2CH2Me),
10.2
(CO2CH2CH2Me); m/z 236 (M+, 1%), 177 (2), 149 (62),
107 (100), 91 (9), 79 (27), 77 (17), 51 (5).
(b) To a stirred solution of (S)-(+)-npropyl 2-npropoxy-
2-phenylacetate (0.50 g, 2.12 mmol) in methanol/water
(5:1) (32 mL) was added lithium hydroxide monohy-
drate (0.44 g, 10.58 mmol). After stirring overnight the
reaction mixture was then partitioned between water
(100 mL) and ether (100 mL). The organic phase was
removed and the aqueous phase was then acidified to
pH 1 with aqueous hydrochloric acid (2 M) and
extracted with ethyl acetate (3×100 mL). The organic
extracts were combined, washed with saturated brine
(100 mL), dried (MgSO4) and the solvent was then
removed under reduced pressure to yield (S)-(+)-h-
npropoxyphenylacetic acid as a pale yellow oil (0.41 g,
99%), [h]2D0=+96.0 (c 1.0, CHCl3); (Found: M+,
194.0946. C11H14O3 requires 194.0943); wmax (film)/cm−1
3500–2500, 3033, 2965, 1724, 1496, 1455, 1186, 723; lH
(400 MHz; CDCl3) 7.44 (2H, m, ArH), 7.37 (3H, m,
ArH), 4.88 (1H, s, CH), 3.51 (1H, dt, J 9.0, 6.6,
CO2CH
6 Me2), 4.93 (1H, s, CH), 3.69 (1H, h, J 6.1,
OCHMe2), 1.26 (3H, d, J 6.2, OCHMe2), 1.21 (3H, d, J
6
6.2, OCHMe2), 1.19 (3H, d, J 6.2, CO2CHMe2), 1.13
(3H, d, J 6.2, CO2CHMe2); lC (100.6 MHz; CDCl3)
171.1 (CꢁO), 137.4 (C), 128.4 (ArC), 128.2 (ArC), 127.0
(ArC), 78.7 (CH), 71.0 (OC6 HMe2), 68.5 (CO2C6 HMe2),
22.2 (OCHMe2), 22.0 (CO2CHMe2), 21.7 (OCHMe2),
21.5 (CO2CHMe2); m/z 236 (M+, 1%), 177 (9), 149 (68),
107 (100), 104 (15), 79 (24), 77 (12), 51 (2).
(b) To a stirred solution of (S)-(+)-isopropyl 2-isopro-
poxy-2-phenylacetate (178 mg, 0.75 mmol) in methanol/
water (5:1) (12 mL) was added lithium hydroxide
monohydrate (158 mg, 3.77 mmol). After stirring
overnight the reaction mixture was then partitioned
between water (50 mL) and ether (50 mL). The organic
phase was removed and the aqueous phase was then
acidified to pH 1 with aqueous hydrochloric acid (2 M)
and extracted with ethyl acetate (3×50 mL). The
organic extracts were combined, washed with saturated
brine (50 mL), dried (MgSO4) and the solvent was then
removed under reduced pressure to yield a colourless
solid. Recrystallisation from ethyl acetate and light
petroleum yielded (S)-(+)-h-isopropoxyphenylacetic acid
as a colourless solid (140 mg, 96%), mp 53–54°C (lit.,40
mp 53–57°C); [h]2D0=+98.0 (c 1.0, CHCl3); (Found: C,
67.8; H, 7.3. C11H14O3 requires C, 68.0; H, 7.3%);
(Found: M+, 194.0946. C11H14O3 requires 194.0943);
wmax (CHCl3)/cm−1 3500–2500, 3065, 2975, 1724, 1582,
1456, 1177, 723; lH (400 MHz; CDCl3) 7.45 (2H, m,
ArH), 7.36 (3H, m, ArH), 4.99 (1H, s, CH), 3.74 (1H,
h, J 6.1, CH), 1.26 (3H, d, J 6.1, Me), 1.20 (3H, d, J
6.1, Me), OH not observed; lC (100.6 MHz; CDCl3)
174.1 (CꢁO), 136.3 (C), 128.8 (ArC), 128.7 (ArC), 127.1
OCH
OCHH
6
HCH2Me), 3.45 (1H, dt,
J
9.0, 6.8,
Me),
6
CH2Me), 1.68 (2H, sextet, J 6.8, OCH2CH2
6
0.94 (3H, t, J 7.4, Me), OH not observed; lC (100.6
MHz; CDCl3) 174.2 (CꢁO), 135.8 (C), 128.9 (ArC),
128.7 (ArC), 127.1 (ArC), 80.5 (CH), 71.7 (OCH2), 22.7
(CH2), 10.4 (Me); m/z 194 (M+, 1%), 149 (100), 135 (5),
122 (6), 107 (76), 91 (13), 79 (81), 63 (7), 51 (14).
(c)
A stirred suspension of dirhodium(II) tetra-
kis(carbonate) (50 mg, 0.08 mmol) and (S)-(+)-a-
npropoxyphenylacetic acid (126 mg, 0.64 mmol) in
water (5 mL) was heated at 85°C for 1 h. During this
time the initial deep blue colour faded and the green
product precipitated out. Cooling and filtration gave a
green solid which was subsequently washed with water
(10 mL) and dried under vacuum to yield the title
compound as a green crystalline solid (40 mg, 51%), mp
250°C (dec.); [h]2D3=+50.0 (c 0.1, CHCl3); (Found: C,
54.0; H, 5.6. C44H52O12Rh2 requires C, 54.0; H, 5.4%);
(Found: M+, 978.1522. C44H52O12Rh2 requires
978.1539); wmax (CHCl3)/cm−1 3058, 2966, 1682, 1599,
1454, 1194, 738; lH (400 MHz; CDCl3) 7.17 (20H, m,
(ArC), 77.9 (CH), 71.4 (C6 HMe2), 22.4 (Me), 21.6 (Me);
m/z 194 (M+, 1%), 149 (39), 135 (10), 107 (100), 79 (39),
77 (22).

R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
811
(c)
A
stirred suspension of dirhodium(II) tetra-
(30.9 mg, 0.05 mmol) and a chiral carboxylic acid ligand
(0.40 mmol) in distilled water (3 mL) was heated at 85°C
for 1 h. Cooling and filtration gave a solid which was
subsequently washed with water (10 mL) and dried under
vacuum to yield the desired rhodium species.
kis(carbonate) (50 mg, 0.08 mmol) and (S)-(+)-a-isopro-
poxyphenylacetic acid (126 mg, 0.64 mmol) in water (5
mL) was heated at 85°C for 1 h. During this time the initial
deep blue colour faded and the green product precipitated
out. Cooling and filtration gave a green solid which was
subsequently washed with water (10 mL) and dried under
vacuum to yield the title compound as a green powdery
solid (31 mg, 39%), mp 250°C (dec.); [h]2D4=+60.0 (c 0.1,
CHCl3); (Found: C, 53.9; H, 5.6. C44H52O12Rh2 requires
C, 54.0; H, 5.4%); (Found: M+, 978.1566. C44H52O12Rh2
requires 978.1569); wmax (CHCl3)/cm−1 3055, 2984, 1607,
1418, 1089, 738; lH (400 MHz; CDCl3) 7.21 (20H, m,
ArH), 4.52 (4H, s, CH) 3.17 (4H, h, J 6.0, CH), 1.03 (12H,
d, J 6.1, Me), 0.95 (12H, d, J 6.1, Me); lC (100.6 MHz;
CDCl3) 191.5 (CꢁO), 137.8 (C), 128.2 (ArC), 127.8 (ArC),
The structures of the formed chiral dirhodium(II) car-
boxylatecatalystswereconfirmedbyautomated 1HNMR
and mass spectrometry analysis where possible (Tables
2–4).
4.3.1. Dirhodium(II) tetrakis(N-(4-nitrophenylsulfonyl)-
(S)-phenylalaninate) L4/2. A stirred suspension of
dirhodium(II) tetrakis(carbonate) (50 mg, 0.08 mmol)
and N-(4-nitrophenylsulfonyl)-(S)-phenylalanine (227
mg, 0.64 mmol) in water (5 mL) was heated at 85°C for
1 h. During this time the initial deep blue colour faded
and the green product precipitated out. Cooling and
filtration gave a green solid which was subsequently
washed with water (10 mL) and dried under vacuum to
yield the title compound as a green powdery solid (123 mg,
95%), mp 250°C (dec.); [h]2D4=−110.0 (c 0.1, MeOH);
(Found: C, 44.8; H, 3.4; N, 6.5. C60H52N8O24Rh2S4
requires C, 44.9; H, 3.3; N, 6.7%); (Found: MH+,
1603.0165. C60H53N8O24Rh2S4 requires 1603.0166); wmax
(KBr)/cm−1 3281, 3055, 2987, 1730, 1603, 1533, 1454,
1351, 1310, 1163, 1093, 739; lH (400 MHz; CD3OD) 8.18
(8H, d, J 7.0, ArH), 7.80 (8H, d, J 7.0, ArH), 7.12 (20H,
m, ArH), 4.12 (4H, dd, J 9.5, 4.9, CH), 3.10 (4H, dd, J
126.8 (ArC), 79.0 (CH), 70.9 (C6 HMe2), 22.1 (Me), 21.9
(Me); m/z (FAB) 978 (M+, 3%), 922 (4), 785 (6), 400 (9),
376 (31), 323 (15), 295 (21), 239 (23), 208 (11), 181 (19),
149 (66), 108 (31), 107 (100).
4.2.9. Dirhodium(II) tetrakis((S)-(−)-a-methoxy-a-trifl-
uoromethylphenylacetate) 12. Prepared by the literature
procedure.26
4.2.10. Dirhodium tetrakis((1R)-(+)-camphanate) 13. Pre-
pared by the literature procedure.27
4.2.11. Dirhodium tetrakis((R)-(+)-camphene-1-carboxyl-
ate) 14. A solution of dirhodium(II) acetate (100 mg, 0.23
mmol) and (+)-camphene-1-carboxylic acid (652 mg, 3.62
mmol) in chlorobenzene (50 ml) was heated under reflux
for a period of 96 h. The chlorobenzene and residual acetic
acid were removed under high vacuum and the reaction
mixture was taken up in dichloromethane (100 ml). This
was washed with saturated sodium hydrogen carbonate
solution (100 ml) followed by water (2×100 ml). After
drying over sodium sulfate the mixture was concentrated
and purified by flash chromatography (light petroleum/
diethyl ether 5.7:1) to give the title compound as a dark
green solid (201 mg, 96%), mp >280°C; [h]2D4=+107.6 (c
13.8, 4.9, CH6 H), 2.81 (4H, dd, J 13.8, 9.5, CHH6 ), NH
not observed; lC (100.6 MHz; CD3OD) 172.7 (CꢁO),
149.7 (C), 146.6 (C), 136.5 (C), 129.0 (ArC), 128.0 (ArC),
127.7 (ArC), 126.4 (ArC), 123.6 (ArC), 57.7 (CH), 38.3
(CH2); m/z (FAB) 1603 (MH+, 6%), 664 (61), 483 (27),
413 (89), 391 (100), 373 (64), 232 (65).
4.3.2. Dirhodium(II) tetrakis(N-(4-toluenesulfonyl)-(S)-
leucinate) L4/3. A stirred suspension of dirhodium(II)
tetrakis(carbonate) (50 mg, 0.08 mmol) and N-(4-tolue-
nesulfonyl)-(S)-leucine (185 mg, 0.64 mmol) in water (5
mL) was heated at 85°C for 1 h. During this time the initial
deep blue colour faded and the green product precipitated
out. Cooling and filtration gave a green solid which was
subsequently washed with water (10 mL) and dried under
vacuum to yield the title compound as a green powdery
solid (88 mg, 81%), mp 250°C (dec.); [h]2D4=−90.0 (c 0.1,
CHCl3); (Found: C, 46.7; H, 5.3; N, 4.1.
C52H72N4O16Rh2S4 requires C, 46.5; H, 5.4; N, 3.9%);
(Found: MH+, 1343.2013. C52H73N4O16Rh2S4 requires
1343.2015); wmax (CHCl3)/cm−1 3287, 3055, 2961, 1725,
1599, 1455, 1324, 1160, 1094, 740; lH (400 MHz; CDCl3)
7.73 (8H, d, J 8.6, ArH), 7.27 (8H, d, J 8.5, ArH), 5.33
(4H, s, NH), 3.90 (4H, m, CH), 2.41 (12H, s, Me), 1.74
0.20,
CHCl3);
(Found:
C,
57.3;
H,
6.6.
C44H60O8Rh2:requires C, 57.3; H, 6.5%); wmax (CHCl3)/
cm−1 1605 (CꢁO), 1216 (CO); lH (250 MHz; CDCl3) 4.51
and 4.47 (each 1 H, s, camphene-2ꢁCH2), 1.99–1.37 (7
H, m, camphene-4, camphene-5,-6,-7), 1.02 and 1.00 (each
3 H, s, camphene-3-Me2); lC (62.9 MHz; CDCl3) 163.1
(CꢁO), 143.0 (camphene-2), 100.3 (camphene-2ꢁCH2),
61.7 (camphene-1), 46.8 (camphene-4), 42.4 (camphene-
3), 40.7 (camphene-7), 31.8 (camphene-6), 29.5 and 25.9
(camphene-3-Me2), 24.4 (camphene-5).
4.2.12. Dirhodium tetrakis[methyl 2-oxopyrrolidine-(5S)-
carboxylate] 15. Prepared by the literature procedure.28
4.2.13. Dirhodium tetrakis[methyl 3,3-difluoro-2-oxopy-
rrolidine-(5S)-carboxylate] 16. Prepared by the literature
procedure.29
(4H, m, CH6 Me2), 1.50 (8H, m, CH2), 0.88 (12H, d, J 6.6,
CHMe2), 0.80 (12H, d, J 6.5, CHMe2); lC (100.6 MHz;
CDCl3) 177.4 (CꢁO), 143.9 (C), 136.6 (C), 129.7 (ArC),
127.3 (ArC), 54.0 (CH), 42.0 (CH2), 24.3 (C6 HMe2), 22.7
4.3. General procedure for the synthesis of chiral
dirhodium(II) carboxylate catalysts by parallel synthesis
(CHMe2), 21.6 (Me), 21.2 (CHMe2); m/z (FAB) 1343
(MH+, 4%), 1102 (14), 1058 (20), 729 (39), 309 (60), 240
(100).
A stirred suspension of dirhodium(II) tetrakis(carbonate)

812
R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
4.4. Dirhodium(II) tetrakis(N-(4-toluenesulfonyl)-(S)-
phenylalaninate) L4/5
4.5.2. Methyl 2-(tert-butyldimethylsilyl)phenylacetate 2b.
To a stirred solution of methyl 2-diazophenylacetate
(100 mg, 0.57 mmol) in dry dichloromethane (2 mL)
A
stirred suspension of dirhodium(II) tetra-
under
a
nitrogen atmosphere was added tert-
kis(carbonate) (50 mg, 0.08 mmol) and N-(4-toluenesul-
fonyl)-(S)-phenylalanine (207 mg, 0.64 mmol) in water
(5 mL) was heated at 85°C for 1 h. During this time the
initial deep blue colour faded and the green product
precipitated out. Cooling and filtration gave a green
solid which was subsequently washed with water (10
mL) and dried under vacuum to yield the title com-
pound as a green powdery solid (113 mg, 94%), mp
250°C (dec.); [h]2D4=−80.0 (c 0.1, MeOH); (Found: C,
51.7; H, 4.5; N, 3.8. C64H64N4O16Rh2S4 requires C,
51.9; H, 4.4; N, 3.8%); (Found: MH+, 1479.1388.
C64H65N4O16S4Rh2 requires 1479.1389); wmax (KBr)/
cm−1 3283, 3054, 2987, 1730, 1598, 1455, 1325, 1157,
1092, 739; lH (400 MHz; CD3OD) 7.54 (8H, d, J 8.3,
ArH), 7.22 (8H, d, J 8.6, ArH), 7.15 (12H, m, ArH),
7.11 (8H, m, ArH), 4.01 (4H, dd, J 6.1, 4.2, CH), 3.03
butyldimethylsilane (73 mg, 0.63 mmol) followed by a
chiral dirhodium(II) catalyst (2 mol%). After stirring
for 30 min the solvent was removed under reduced
pressure to yield a green oil. This crude product was
then subjected to flash silica gel chromatography using
light petroleum and ethyl acetate (19:1) as eluant to
yield the title compound as a colourless oil; (Found: M+,
264.1546. C15H24O2Si requires 264.1546); wmax (film)/
cm−1 3026, 2953, 1723, 1600, 1453, 1148, 756; lH (250
MHz; CDCl3) 7.37 (2H, m, ArH), 7.25 (2H, m, ArH),
7.17 (1H, m, ArH), 3.67 (3H, s, OMe), 3.56 (1H, s,
CH), 0.88 (9H, s, CMe3), 0.12 (3H, s, Me), −0.11 (3H,
s, Me); lC (62.9 MHz; CDCl3) 173.7 (CꢁO), 137.0 (C),
128.7 (ArC), 128.0 (ArC), 125.6 (ArC), 51.3 (OMe),
42.9 (CH), 26.5 (CMe3), 17.7 (C6 Me3), −6.6 (Me), −7.0
(Me). m/z 264 (M+, 18%), 207 (50), 118 (100), 89 (99),
(4H, dd, J 10.3, 4.2, CH
6
H), 2.83 (4H, dd, J 10.3, 6.1,
73 (58), 59 (20).
CHH), 2.37 (12H, s, Me), NH not observed; lC (100.6
6
MHz; CD3OD) 172.9 (CꢁO), 143.0 (C), 137.7 (C), 136.4
(C), 129.1 (ArC), 129.0 (ArC), 128.0 (ArC), 126.6
(ArC), 126.3 (ArC), 57.4 (CH), 38.5 (CH2), 20.0 (Me);
m/z (FAB) 1479 (MH+, 32%), 1388 (25), 1204 (31), 946
(26), 727 (30), 705 (100), 533 (35).
4.5.3. Methyl 2-(tri-isopropylsilyl)phenylacetate 2c. To a
stirred solution of methyl 2-diazophenylacetate (100
mg, 0.57 mmol) in dry dichloromethane (2 mL) under a
nitrogen atmosphere was added tri-isopropylsilane (99
mg, 0.63 mmol) followed by a chiral dirhodium(II)
catalyst (2 mol%). After stirring for 30 min the solvent
was removed under reduced pressure to yield a green
oil. This crude product was then subjected to flash silica
gel chromatography using light petroleum and ethyl
acetate (19:1) as eluant to yield the title compound as a
colourless oil; (Found: M+, 306.2019. C18H30O2Si
requires 306.2015); wmax (film)/cm−1 3058, 2948, 1728,
1604, 1453, 1147, 782; lH (400 MHz; CDCl3) 7.42 (2H,
m, ArH), 7.25 (2H, m, ArH), 7.18 (1H, m, ArH), 3.70
(1H, s, CH), 3.66 (3H, s, OMe), 1.19 (3H, h, J 7.1,
4.4.1. Dirhodium(II) tetrakis(N-(4-tert-butylphenylsul-
fonyl)-(S)-leucinate) L4/6. Prepared as described
previously.35
4.4.2. Dirhodium(II) tetrakis(N-(4-dodecylphenylsul-
fonyl)-(S)-leucinate) L4/7. Prepared as described
previously.35
4.4.3. Dirhodium(II) tetrakis(N-(4-toluenesulfonyl)-(S)-
tert-leucinate) L4/8. Prepared as described previously.35
CH6 Me2), 1.07 (9H, d, J 7.3, Me), 1.00 (9H, d, J 7.4,
Me); lC (100.6 MHz; CDCl3) 174.0 (CꢁO), 136.8 (C),
129.5 (ArC), 128.0 (ArC), 125.7 (ArC), 51.4 (OMe),
4.5. Silane insertions: conventional method
41.5 (CH), 18.6 (Me), 18.5 (Me), 11.6 (C6 HMe2); m/z
306 (M+, 28%), 263 (12), 156 (35), 145 (100), 118 (49),
103 (11), 89 (51), 75 (52), 59 (93).
4.5.1. Methyl 2-(dimethylphenylsilyl)phenylacetate 2a.
To a stirred solution of methyl 2-diazophenylacetate
(100 mg, 0.57 mmol) in dry dichloromethane (2 mL)
4.5.4. 2-(Tri-isopropylsilyl)-2-phenylethanol 17. To a
stirred solution of methyl 2-(tri-isopropylsilyl)-
phenylacetate 2c (0.53 g, 1.72 mmol) in dry ether (20
mL) under a nitrogen atmosphere at −78°C was added
di-isobutylaluminium hydride (1 M solution in toluene)
(3.44 mL, 3.44 mmol) dropwise. After allowing the
reaction mixture to stir for 30 min it was then allowed
to warm to room temperature where aqueous hydro-
chloric acid (1 M) (20 mL) was added. After stirring for
a further 30 min the organic layer was removed and the
aqueous phase was then extracted with ether (3×50
mL). The organic extracts, including the removed
organic layer, were combined, washed with saturated
brine (50 mL), dried (MgSO4) and the solvent was then
removed under reduced pressure to yield a pale brown
oil. This crude product was then subjected to flash silica
gel chromatography using light petroleum and ethyl
acetate (4:1) as eluant to yield a the title compound as a
colourless oil (0.43 g, 90%), [h]2D2=+15.0 (c 1.0, CHCl3);
under
a
nitrogen
atmosphere
was
added
dimethylphenylsilane (85 mg, 0.63 mmol) followed by a
chiral dirhodium(II) catalyst (2 mol%). After stirring
for 30 min the solvent was removed under reduced
pressure to yield a green oil. This crude product was
then subjected to flash silica gel chromatography using
light petroleum and ethyl acetate (19:1) as eluant to
yield the title compound as a colourless oil; (Found: M+,
284.1235. C17H20O2Si requires 284.1233); wmax (film)/
cm−1 3025, 2951, 1719, 1599, 1453, 1149, 736; lH (250
MHz; CDCl3) 7.37 (5H, m, ArH), 7.17 (5H, m, ArH),
3.60 (1H, s, CH), 3.54 (3H, s, OMe), 0.35 (3H, s, Me),
0.32 (3H, s, Me); lC (62.9 MHz; CDCl3) 173.1 (CꢁO),
136.0 (C), 135.5 (C), 134.0 (ArC), 129.6 (ArC), 128.4
(ArC), 128.0 (ArC), 127.7 (ArC), 125.7 (ArC), 51.3
(OMe), 46.1 (CH), −4.1 (Me), −4.5 (Me); m/z 284 (M+,
13%), 151 (10), 135 (60), 118 (100), 105 (8), 90 (14), 77
(4), 43 (10).

R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
813
(Found: M+, 278.2068. C17H30OSi requires 278.2066);
wmax (film)/cm−1 3383, 3025, 2944, 1599, 1464, 1078,
765; lH (400 MHz; CDCl3) 7.28 (2H, m, ArH), 7.23
(2H, m ArH), 7.15 (1H, m, ArH), 4.26 (1H, dd, J 11.9,
(CH), 26.6 (CHMe2), 26.5 (CHMe2), 24.3 (C
6
HMe2),
23.2 (CH2); m/z 348 (M+, 42%), 291 (9), 257 (11), 218
(14), 193 (17), 173 (43), 143 (79), 118 (100), 101 (25), 75
(35), 59 (28).
11.5, CH
3.6, CH), 1.13 (3H, h, J 7.6, CH
6.9, Me), 0.99 (9H, d, J 7.1, Me), OH not observed; lC
(100.6 MHz; CDCl3) 141.0 (C), 129.0 (ArC), 128.6
(ArC), 125.5 (ArC), 64.2 (CH2), 38.9 (CH), 18.8 (Me),
18.7 (Me), 11.0 (C
219 (25), 157 (100), 131 (74), 104 (69), 91 (38), 75 (58),
61 (52).
6
H), 4.02 (1H, m, CHH
6
), 2.78 (1H, dd, J 11.9,
Me2), 1.04 (9H, d, J
6
4.5.7. Methyl 2-(tris(trimethylsilyl)silyl)phenylacetate 2e.
To a stirred solution of methyl 2-diazophenylacetate
(100 mg, 0.57 mmol) in dry dichloromethane (2 mL)
under
a
nitrogen
atmosphere
was
added
6
HMe2); m/z 278 (M+, 1%), 261 (23),
tris(trimethylsilyl)silane (155 mg, 0.63 mmol) followed
by a chiral dirhodium(II) carboxylate catalyst (2
mol%). After stirring for 30 min the solvent was
removed under reduced pressure to yield a green oil.
This crude product was then subjected to flash silica gel
chromatography using light petroleum and ethyl ace-
tate (19:1) as eluant to yield a colourless solid. Recrys-
tallisation from ethyl acetate and light petroleum
yielded the title compound as a colourless solid, mp
145–146°C; (Found: C, 54.8; H, 9.4. C18H36O2Si4
requires C, 54.5; H, 9.1%); (Found: M+, 396.1794.
C18H36O2Si4 requires 396.1792); wmax (film)/cm−1 3054,
2951, 1720, 1598, 1453, 1151, 739; lH (400 MHz;
CDCl3) 7.33 (2H, m, ArH), 7.26 (2H, m, ArH), 7.17
(1H, m, ArH), 3.68 (1H, s, CH), 3.65 (3H, s, OMe),
0.13 (27H, s, Me); lC (100.6 MHz; CDCl3) 174.6
(CꢁO), 139.1 (C), 129.3 (ArC), 128.3 (ArC), 125.9
(ArC), 51.5 (OMe), 38.3 (CH), 1.2 (Me); m/z 396 (M+,
1%), 381 (4), 263 (98), 205 (32), 175 (9), 147 (12), 117
(18), 83 (100), 73 (43), 51 (19).
4.5.5. (2-Phenyl-2-tri-isopropylsilyl)ethyl 4-bromoben-
zoate 18. To a stirred solution of 2-(tri-isopropylsilyl)-2-
phenylethanol 17 (100 mg, 0.36 mmol) in dry
dichloromethane (4 mL) under a nitrogen atmosphere
was added 4-bromobenzoylchloride (86 mg, 0.39 mmol)
followed by triethylamine (40 mg, 0.39 mmol) drop-
wise. After stirring at room temperature overnight the
solvent was then removed under reduced pressure to
yield a pale yellow solid. This crude product was then
subjected to flash silica gel chromatography using light
petroleum and ethyl acetate (19:1) as eluant to yield the
title compound as a colourless solid (146 mg, 88%), mp
78–79°C; [h]2D3=−18.0 (c 1.0, CHCl3); (Found: C, 62.8;
H, 7.3. C24H33BrO2Si requires C, 62.5; H, 7.2%);
(Found: M+, 460.1430. C24H33BrO2Si requires
460.1433); wmax (CHCl3)/cm−1 3028, 2946, 1716, 1591,
1451, 1082, 757; lH (400 MHz; CDCl3) 7.64 (2H, d, J
6.7, ArH), 7.46 (2H, d, J 6.8, ArH), 7.23 (4H, m, ArH),
7.12 (1H, m, ArH), 4.85 (2H, m, CH2), 3.01 (1H, dd, J
4.5.8. Methyl 2-(1,1,1,3,5,5,5-heptamethyltrisiloxyl)-
phenylethanote 2f. To a stirred solution of methyl 2-dia-
zophenylacetate (100 mg, 0.57 mmol) in dry
dichloromethane (2 mL) under a nitrogen atmosphere
was added 1,1,1,3,5,5,5-heptamethylsiloxane (139 mg,
0.63 mmol) followed by a chiral dirhodium(II) carboxyl-
ate catalyst (2 mol%). After stirring for 30 min the
solvent was removed under reduced pressure to yield a
green oil. This crude product was then subjected to
flash silica gel chromatography using light petroleum
and ethyl acetate (19:1) as eluant to yield the title
compound as a colourless oil; (Found: M+, 370.1456.
C16H30O4Si3 requires 370.1452); wmax (film)/cm−1 3027,
2958, 1729, 1598, 1454, 1149, 756; lH (400 MHz;
CDCl3) 7.35 (2H, m, ArH), 7.24 (2H, m, ArH), 7.18
(1H, m, ArH), 3.68 (1H, s, OMe), 3.41 (1H, s, CH),
0.10 (3H, s, Me), 0.03 (18H, s, SiMe3); lC (100.6 MHz;
CDCl3) 172.5 (CꢁO), 135.8 (C), 128.9 (ArC), 128.0
(ArC), 125.7 (ArC), 51.4 (OMe), 47.5 (CH), 1.5 (Me),
−1.6 (SiMe3); m/z 370 (M+, 15%), 237 (34), 221 (31),
207 (15), 118 (94), 84 (100), 83 (62), 73 (33), 51 (16).
9.7, 6.6, CH), 1.23 (3H, h, J 7.4, CH6 Me2), 1.08 (9H, d,
J 7.3, Me), 1.04 (9H, d, J 7.3, Me); lC (100.6 MHz;
CDCl3) 166.0 (CꢁO), 141.0 (C), 131.5 (ArC), 130.9
(ArC), 129.4 (C), 128.5 (ArC), 128.3 (ArC), 127.7 (C),
125.2 (ArC), 67.1 (CH2), 34.5 (CH), 18.8 (Me), 18.7
(Me), 11.1 (C6
HMe2); m/z 460 (M+, 1%), 356 (16), 274
(25), 131 (54), 91 (34), 77 (24), 54 (12). HPLC condi-
tions: column: Chiralpak AD; retention times (min):
4.84, 6.97; solvent ratio: hexane:2-propanol (99:1); flow
rate (mL/min): 1.0.
4.5.6. Methyl 2-(tri-isobutylsilyl)phenylacetate 2d. To a
stirred solution of methyl 2-diazophenylacetate (100
mg, 0.57 mmol) in dry dichloromethane (2 mL) under a
nitrogen atmosphere was added tri-isobutylsilane (125
mg, 0.63 mmol) followed by a chiral dirhodium(II)
carboxylate catalyst (2 mol%). After stirring for 30 min
the solvent was removed under reduced pressure to
yield a green oil. This crude product was then subjected
to flash silica gel chromatography using light petroleum
and ethyl acetate (19:1) as eluant to yield the title
compound as a colourless oil; (Found: M+, 348.2491.
C21H36O2Si requires 348.2485); wmax (film)/cm−1 3056,
2954, 1725, 1598, 1464, 1147, 771; lH (400 MHz;
CDCl3) 7.32 (2H, m, ArH), 7.26 (2H, m, ArH), 7.18
(1H, m, ArH), 3.67 (3H, s, OMe), 3.55 (1H, s, CH),
4.5.9.
Dihydro-4,4-dimethyl-3-(dimethylphenylsilyl)-2-
furanone 20a. To a stirred solution of dihydro-3-diazo-
4,4-dimethyl-2-furanone (100 mg, 0.71 mmol) in dry
dichloromethane (2 mL) under a nitrogen atmosphere
was added dimethylphenylsilane (107 mg, 0.78 mmol)
followed by dirhodium(II) acetate (2 mol%). After stir-
ring for 30 min the solvent was removed under reduced
pressure to yield a green oil. This crude product was
then subjected to flash silica gel chromatography using
light petroleum and ethyl acetate (9:1) as eluant to yield
a colourless solid. Recrystallisation from ethyl acetate
1.77 (1H, h, J 6.6, CH
CHMe2), 0.90 (9H, d, J 6.6, CHMe2), 0.67 (3H, dd, J
15.1, 6.6, CHH), 0.62 (3H, dd, J 15.1, 6.6, CHH); lC
(100.6 MHz; CDCl3) 173.7 (CꢁO), 136.9 (C), 128.8
(ArC), 128.1 (ArC), 125.6 (ArC), 51.3 (OMe), 44.4
6 Me2), 0.91 (9H, d, J 6.6,
6
6

814
R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
and light petroleum yielded the title compound as a
colourless solid (134 mg, 76%), mp 50–51°C; (Found: C,
67.7; H, 8.3. C14H20O2Si requires C, 67.7; H, 8.1%);
(Found: M+, 248.1233. C14H20O2Si requires 248.1233);
wmax (CHCl3)/cm−1 3049, 2960, 1751, 1598, 1456, 1167,
757; lH (400 MHz; CDCl3) 7.58 (2H, m, ArH), 7.39 (3H,
Methyl
2-(tert-butyldimethylsilyl)phenylacetate
2b:
column: Chiralpak AD; retention times (min): 8.41, 9.01;
solvent ratio: hexane:2-propanol (199:1); flow rate (mL/
min): 0.5, or column: S,S)-WHELK-O1; retention times
(min): 4.38, 5.50; solvent ratio: heptane:ethyl acetate
(19:1); flow rate (mL/min): 1.0.
m, ArH), 3.73 (1H, d, J 8.5, CH
6 H), 3.62 (1H, d, J 8.5,
CHH), 2.09 (1H, s, CH), 1.04 (3H, s, CMe2), 1.03 (3H,
6
Methyl 2-(tri-isopropylsilyl)phenylacetate 2c: column:
Chiralpak AD; retention times (min): 8.58, 9.08; solvent
ratio: hexane:2-propanol (199:1); flow rate (mL/min):
0.5, or column: (S,S)-WHELK-O1; retention times
(min): 4.16, 5.21; solvent ratio: heptane:ethyl acetate
(19:1); flow rate (mL/min): 1.0.
s, CMe2), 0.52 (6H, s, Me); lC (100.6 MHz; CDCl3) 179.0
(CꢁO), 136.4 (C), 134.0 (ArC), 129.7 (ArC), 128.0 (ArC),
79.1 (CH2), 44.2 (CH), 40.6 (C), 28.6 (CMe2), 24.3
(CMe2), −2.0 (Me), −2.9 (Me); m/z 248 (M+, 1%), 233
(76), 161 (13), 135 (100), 105 (8), 91 (5).
Methyl 2-(tri-isobutylsilyl)phenylacetate 2d: column:
(S,S)-WHELK-O1; retention times (min): 5.23, 6.34;
solvent ratio: heptane:ethyl acetate (49:1); flow rate
(mL/min): 1.0.
4.5.10. Dihydro-3-(tert-butyldimethylsilyl)-4,4-dimethyl-
2-furanone 20b. To a stirred solution of dihydro-3-diazo-
4,4-dimethyl-2-furanone (100 mg, 0.71 mmol) in dry
dichloromethane (2 mL) under a nitrogen atmosphere
was added tert-butyldimethylsilane (91 mg, 0.78 mmol)
followed by dirhodium(II) acetate (2 mol%). After stir-
ring for 30 min the solvent was removed under reduced
pressure to yield a green oil. This crude product was then
subjected to flash silica gel chromatography using light
petroleum and ethyl acetate (9:1) as eluant to yield the
title compound as a colourless oil (84 mg, 52%); (Found:
M+, 228.1545. C12H24O2Si requires 228.1546); wmax (film)/
cm−1 2958, 1758, 1467, 1124; lH (400 MHz; CDCl3) 3.89
Methyl
2-(tris(trimethylsilyl)silyl)phenylacetate
2e:
column: (S,S)-WHELK-O1; retention times (min): 4.57,
5.64; solvent ratio: heptane:ethyl acetate (49:1); flow rate
(mL/min): 1.0.
Methyl
2-(1,1,1,3,5,5,5-heptamethyltrisiloxyl)phenyl-
ethanote 2f: column: (S,S)-WHELK-O1; retention times
(min): 4.72, 5.76; solvent ratio: heptane:ethyl acetate
(49:1); flow rate (mL/min): 1.0.
(1H, d, J 8.4, CH6 H), 3.81 (1H, d, J 8.4, CHH6 ), 1.92 (1H,
s, CH), 1.20 (6H, s, CMe2), 0.97 (9H, s, CMe3), 0.19 (3H,
s, Me), 0.17 (3H, s, Me); lC (100.6 MHz; CDCl3) 178.8
(CꢁO), 79.6 (CH2), 41.0 (C), 40.8 (CH), 27.3 (CMe2), 27.1
4.7.1. 2-(Dimethylphenylsilyl)-2-phenylethanol 21. To a
stirred solution of methyl 2-(dimethylphenylsilyl)-
phenylacetate (0.98 g, 3.45 mmol) in dry ether (40 mL)
under a nitrogen atmosphere at −78°C was added di-
isobutylaluminium hydride (1 M solution in toluene)
(6.90 mL, 6.90 mmol) dropwise. After allowing the
reaction mixture to stir for 30 min it was then allowed
to warm to room temperature where aqueous hydrochlo-
ric acid (1 M) (40 mL) was added. After stirring for a
further 30 min the organic phase was removed and the
aqueous phase was then extracted with ether (3×100 mL).
The organic extracts, including the removed organic
phase, were combined, washed with saturated brine (100
mL), dried (MgSO4) and the solvent was then removed
under reduced pressure to yield a pale brown oil. This
crude product was then subjected to flash silica gel
chromatography using light petroleum and ethyl acetate
(4:1) as eluant to yield a colourless solid. Recrystallisa-
tion from ethyl acetate and light petroleum yielded the
title compound as a colourless solid (0.80 g, 91%), mp
56–57°C; (Found: C, 74.6; H, 7.9. C16H20OSi requires C,
74.9; H, 7.9%); (Found: M+, 256.1290. C16H20OSi
requires 256.1283); wmax (CHCl3)/cm−1 3500–3100, 3024,
2956, 1599, 1450, 1112, 736; lH (400 MHz; CDCl3) 7.38
(5H, m, ArH), 7.24 (2H, m, ArH), 7.16 (1H, m, ArH),
(CMe3), 24.2 (CMe2), 17.3 (C6 Me3), −4.1 (Me), −5.2 (Me);
m/z 228 (M+, 1%), 213 (25), 171 (100), 129 (18), 99 (31),
75 (30), 59 (10).
4.6. Silane insertions: parallel synthesis method
To methyl 2-diazophenylacetate (10 mg, 0.057 mmol) in
dry dichloromethane (0.1 mL) was added a silane (0.062
mmol) in dichloromethane (0.1 mL) followed by a chiral
dirhodium(II) carboxylate catalyst (2 mol%). After the
solvent and any unreacted silane were removed, this
crude product was then subjected to flash silica gel
chromatography through a BondElut® column with
hexane and ethyl acetate as eluant, to yield the desired
SiꢀH insertion product, which was subjected to auto-
mated HPLC analysis to determine the enantioselectivity
of the reaction.
4.7. Determination of enantiomeric excess
The enantiomeric purity of the (silyl)phenylacetates 2
was determined by compariosn with the racemate by
HPLC on a chiral stationary phase; the conditions for
each product are summarised below.
7.02 (2H, m, ArH), 4.09 (1H, dd, J 11.2, 11.1, CH
6 H), 3.95
(1H, dd, J 11.2, 4.5, CHH), 2.66 (1H, dd, J 11.1, 4.5, CH),
6
0.28 (3H, s, Me), 0.24 (3H, s, Me), OH not observed; lC
(100.6 MHz; CDCl3) 139.9 (C), 136.7 (C), 134.0 (ArC),
129.3 (ArC), 128.5 (ArC), 128.1 (ArC), 127.8 (ArC),
125.4 (ArC), 63.2 (CH2), 41.6 (CH), −3.7 (Me), −4.9
(Me); m/z 256 (M+, 1%), 223 (17), 155 (20), 137 (45), 104
(100), 91 (35), 71 (41), 57 (78).
Methyl 2-(dimethylphenylsilyl)phenylacetate 2a: column:
Chiralpak AD; retention times (min): 11.62, 12.69; sol-
vent ratio: hexane:2-propanol (199:1); flow rate (mL/
min): 0.5, or column: (S,S)-WHELK-O1; retention times
(min): 6.11, 7.30; solvent ratio: heptane:ethyl acetate
(19:1); flow rate (mL/min): 1.0.

R. T. Buck et al. / Tetrahedron: Asymmetry 14 (2003) 791–816
815
4.7.2. Phenyl-1,2-ethanediol. To a stirred solution of
2-(dimethylphenylsilyl)-2-phenylethanol (0.77 g, 3.00
mmol) in peracetic acid (18 mL of a 32% solution in
acetic acid) and acetic anhydride (2.5 mL) was added
mercuric acetate (1.65 g, 5.15 mmol). After stirring for
2 h the solvent was removed under reduced pressure
and ether (100 mL) was added to the residue which was
filtered and the solvent was then removed under
reduced pressure to yield a pale grey solid. This crude
product was then subjected to flash silica gel chro-
matography using light petroleum and ethyl acetate
(3:7) as eluant to yield a colourless solid. Recrystallisa-
tion from ethyl acetate and light petroleum yielded the
title compound as colourless needles (0.17 g, 42%), mp
64–65°C (lit.,41 mp 64°C); wmax (CHCl3)/cm−1 3500–
3100, 3032, 1602, 1448, 1192, 739; lH (400 MHz;
CDCl3) 7.31 (5H, m, ArH), 4.78 (1H, dd, J 8.1, 3.6,
10. Doyle, M. P.; Yan, M. Tetrahedron Lett. 2002, 43, 5929–
5931.
11. Buck, R. T.; Doyle, M. P.; Drysdale, M. J.; Ferris, L.;
Forbes, D. C.; Haigh, D.; Moody, C. J.; Pearson, N. D.;
Zhou, Q.-L. Tetrahedron Lett. 1996, 37, 7631–7634.
12. Buck, R. T.; Coe, D. M.; Drysdale, M. J.; Moody, C. J.;
Pearson, N. D. Tetrahedron Lett. 1998, 39, 7181–7184.
13. Bagheri, V. B.; Doyle, M. P.; Taunton, J.; Claxton, E. E.
J. Org. Chem. 1988, 53, 6158–6160.
14. (a) Rijkens, F.; Janssen, M. J.; Drenth, W.; VanDerKerk,
G. J. M. J. Organometal. Chem. 1964, 2, 347–356; (b)
Watanabe, H.; Kakano, T.; Araki, K.; Matsumoto, H.;
Nagai, Y. J. Organomet. Chem. 1974, 69, 389–396; (c)
Baikov, V. E.; Danilkina, L. P.; Oglobin, K. A. J. Gen.
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CH), 3.75 (1H, dd, J 11.3, 3.6, CH
6 H), 3.66 (1H, dd, J
11.3, 8.1, CHH), 3.03 (1H, bs, exch D2O, OH), 2.61
6
(1H, bs, exch D2O, OH); lC (100.6 MHz; CDCl3) 140.5
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68.0 (CH2).
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We thank GlaxoSmithKline for support, partly through
the Asymmetric Synthesis Link Programme, and partly
through the CASE Scheme; we also thank the EPSRC
for support, the EPSRC Mass Spectrometry Centre at
Swansea for mass spectra, and Dr Darko Butina for
help with the cluster analysis.
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