Iridium complexes of chiral diamines containing carbon and nitrogen stereocentres: Synthesis, structure and evaluation as transfer hydrogenation catalysts

Article abstract of DOI:10.1039/b812699b

Novel Rh(iii) and Ir(iii) complexes of a chiral diamine ligand have been synthesised and structurally characterised. Both complexes were formed as a single diastereomeric species with a single configuration at nitrogen. Ir complexes of the chiral diamine

Full text of DOI:10.1039/b812699b

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Iridium complexes of chiral diamines containing carbon and nitrogen
stereocentres: synthesis, structure and evaluation as transfer
hydrogenation catalystsw
a
b
a
´
Jose A. Fuentes,* Marcia B. France, Alexandra M. Z. Slawin
and Matthew L. Clarkea
Received (in Durham, UK) 24th July 2008, Accepted 7th November 2008
First published as an Advance Article on the web 24th November 2008
DOI: 10.1039/b812699b
Novel Rh(III) and Ir(III) complexes of a chiral diamine ligand have been synthesised and
structurally characterised. Both complexes were formed as a single diastereomeric species with a
single configuration at nitrogen. Ir complexes of the chiral diamine were found to be active in the
asymmetric transfer hydrogenation of bulky ketones. Excellent conversions (up to 100%) and
moderate enantioselectivities (up to 60%) were obtained in the asymmetric reduction of
2,2-dimethylpropiophenone.
demonstrating the potential of this ligand class in asymmetric
Introduction
transfer hydrogenation of some bulky ketones that are
generally considered to be challenging substrates.
Chiral nitrogen ligands have many important applications in
asymmetric catalysis.1 A less studied class of nitrogen ligands
are those that have stereogenic nitrogen centres.2 Nitrogen
stereocentres are normally configurationally unstable, but upon
complexation to a metal, this rapid stereochemical inversion
process can be frozen. In the case of metal complexes that
contain other non-labile stereocentres, one configuration at
nitrogen can be favoured over the other. Thus chiral diamines
containing C stereocentres and N stereocentres can form
transition metal complexes with a single defined and stable
nitrogen stereochemistry. There is even an example where the
enantiomers of a complex containing only nitrogen stereo-
centres were resolved and studied.3 If the nitrogen stereocentres
freeze to a single configuration on co-ordination to the metal
centre and present a C2 symmetric or pseudo C2 symmetric
environment, it can be envisaged that such metal complexes
would be capable of high levels of enantiocontrol reminiscent
of the high stereocontrol possible with C2 symmetric P-chiral
diphosphines.4 The most readily available bidentate nitrogen
ligands that might display these characteristics are bis-secondary
amines derived from chiral diimines. One potential ligand that
attracted our attention was the known compound 1.5,6
Compound 1 has been reported as an intermediate en route
to the corresponding bis-primary amine but has not, to the
best of our knowledge, ever been used as a ligand. In this paper
we report the synthesis and structure of a rhodium and an
iridium complex of this ligand and present encouraging results
Results and discussion
Synthesis of complexes of ligand 1
Ligand 1 was prepared by addition of allyl magnesium
bromide to diimine 2 followed by reductive cyclisation with
n-butyl magnesium bromide and Cp2ZrCl2 (Scheme 1), as
described previously.6
The ligand forms complexes with rhodium and iridium
precursors. Reaction with [Ir(COD)Cl]2 or [Rh(COD)Cl]2
leads to a species active for the transfer hydrogenation of
ketones (see below); however, a stable complex could not be
isolated cleanly, even when the complex was first treated with
AgBF4 prior to addition of the ligand. On the other hand,
treatment of 1 with [RhCp*Cl2]2 followed by AgBF4 led to
clean formation of complex 3, isolated as a red solid in 87%
yield (Scheme 2).
a School of Chemistry, University of St. Andrews, EaSTCHEM,
St. Andrews, Fife, UK KY16 9ST. E-mail: jaf14@st-andrews.ac.uk;
Fax: +44 (0)1334 463808; Tel: +44 (0)1334 463859
b Department of Chemistry, Washington and Lee University,
Lexington, VA 24450, USA. E-mail: francem@wlu.edu;
Fax: +1 (0)540 458 8878; Tel: +1 (0)540 458 8879
w Electronic supplementary information (ESI) available: PDF file
containing bond lengths and distances for the X-ray structure
determination. CCDC reference number 708595. For ESI and
crystallographic data in CIF or other electronic format see
DOI: 10.1039/b812699b
Scheme
1
Conditions: (a) allylMgBr, THF, À78 1C to r.t.;
(b) n-BuMgBr (5 equiv.), Cp2ZrCl2 (20 mol%) Et2O, r.t.
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Transfer hydrogenation with complexes of ligand 1
Transfer hydrogenation of ketones is an extremely widely
studied reaction.9 Although pressure hydrogenation is
preferred at large scale, transfer hydrogenation can have a
niche for certain types of substrates that are incompatible with
hydrogen gas. In addition, at laboratory scale, the greater
convenience of transfer hydrogenation makes it the preferred
reaction. Many transfer hydrogenation catalysts are based on
ruthenium complexes,9,10 although there are also notable
examples of rhodium and iridium catalysts.9,11 We elected to
compare Ru, Rh and Ir precursors with ligand 1, with the aim
of understanding more about the ideal metal–ligand system for
the reaction and to discover a catalyst that could hydrogenate
challenging ketones such as bulky substrate 2,2-dimethyl-
propiophenone.12,13 The results obtained for the transfer
hydrogenation of acetophenone by various catalysts and
reaction conditions are shown in Table 1 (Scheme 3, R = Me).
When Rh was used with ligand 1, only traces of product
were obtained (entries 1 and 2). On the other hand, when
[Ru(C6H6)Cl2]2 was used as metal precursor, the desired
1-phenyl-ethanol was formed, but with low enantioselectivity
(32%) (entry 3). Increasing the temperature to 55 1C produced
an increase in the reaction rate to almost full conversion with
no detrimental effect on the ee (34%) (entry 4). The species
prepared in situ from 1 and [IrCp*Cl2]2 showed the same level
of enantioselectivity (entries 5 and 6), while the related isolated
complex 4 gave lower ee’s, although essentially full conversion
was obtained (entries 7 and 8). [Ir(COD)Cl]2 gave the best ee
(53%) and the highest conversion (92%) (entries 9–11) of the
metal precursors employed with ligand 1. Therefore, iridium
was the metal of choice to use in reactions with other more
challenging substrates.
Scheme 2 Synthesis of bidentate Rh and Ir complexes of ligand 1.
Similarly, [IrCp*Cl2]2 was converted into complex 4, which
was recrystallised by slow diffusion of hexane into a solution
of 4 in CHCl3 to give a 53% yield of pure yellow–orange
needles. Both complexes were characterised by optical
rotation, IR, 1H, 13C, 19F NMR spectroscopy and mass
spectrometry. The X-ray crystal structure determination
unambiguously confirmed the structure of 4 in Scheme 2
(Fig. 1). Upon complexation only a single stereoisomer
was observed in the solid state, and there are no signs of
diastereomers in solution. The nitrogen stereocentres are thus
frozen into a single S configuration upon co-ordination to the
metal centre. This homochirality at the N centres is in contrast
with the heterochirality observed in similar complexes. Jones
recently described two Rh(I) complexes of N,N-dibenzyl-
(1R,2R)-diaminocyclohexane and N,N-diferrocenyl-(1R,2R)-
diaminocyclohexane where the N atoms in both examples
acquired opposite absolute configurations and a syn relation-
ship between the nitrogen substituents.7 Relatively strong
heteronuclear hydrogen bonding interactions N(1)HÁ Á ÁF
À
between the amine group and the BF4
anion are
Table 1 Transfer hydrogenation of acetophenone using various
catalyst precursors and ligand 1
also observed, with an HÁ Á ÁF distance of 2.09 A. X-Ray
analysis of Rh complex 3 gave a structure similar to that of
Ir complex 4.8
KOtBu/
Entry Metal complex mol%
T/1C Conv./%a e.e./%b
1
2
3
4
5
6
7
8
9
[RhCp*Cl2]2
[Rh(COD)Cl]2 2.5
[Ru(C6H6)Cl2]2 2.5
[Ru(C6H6)Cl2]2
[IrCp*Cl2]2
[IrCp*Cl2]2
4
2.5
RT
RT
RT
55
55
55
55
55
RT
55
N/A
N/A
32
92
70
67
97
97
33
1
2
32 ((S)-(À))
34 ((S)-(À))
31 ((R)-(+))
29 ((R)-(+))
6 ((R)-(+))
14 ((S)-(À))
53 ((S)-(À))
49 ((S)-(À))
33 ((S)-(À))
5
2.5
5
2.5
5
2.5
2.5
5
4
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
10
11
92 (81)
92
55
% Conversion, determined by 1H NMR (% isolated yield following
b
chromatography on silica gel). Determined by HPLC on ChiralPak
a
AD or OD-H column. Absolute configuration and direction of optical
rotation are shown in parentheses.
Fig. 1 X-Ray structure of Ir complex 4. Key bond lengths (A):
Ir(1)–N(1)
2.4187(11).
= 2.179(3); Ir(1)–N(2) = 2.199(4); Ir(1)–Cl(1) =
Scheme 3 Transfer hydrogenation of ketones using ligand 1.
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Table 2 Transfer hydrogenation of various ketones using iridium complexes of ligand 1a
Entry
R
Metal complex
KOtBu/mol%
Conv./%b
Yield/%c
34
e.e./%d
1
2
3
4
5
6
7
8
9
i-Pr
i-Pr
i-Pr
i-Pr
Cy
Cy
Cy
Cy
t-Bu
t-Bu
t-Bu
t-Bu
[IrCp*Cl2]2
[IrCp*Cl2]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[IrCp*Cl2]2
[IrCp*Cl2]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[IrCp*Cl2]2
[IrCp*Cl2]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
2.5
5
2.5
5
2.5
5
2.5
5
2.5
5
2.5
5
19
22
61
53
17
15
80
78
18
22
100
100
17 ((S)-(À))
31 ((S)-(À))
53 ((S)-(À))
40 ((S)-(À))
26 ((S)-(À))
22 ((S)-(À))
58 ((S)-(À))
49 ((S)-(À))
26 ((S)-(À))
26 ((S)-(À))
60 ((S)-(À))
47 ((S)-(À))
54
57
10
11
12
81
74
a
b
1
c
All reactions were carried out at 55 1C. % Conversion, determined by H NMR. % Isolated yield following chromatography on silica gel,
eluting with CH2Cl2. Determined by HPLC on ChiralPak AD or OD-H column. Absolute configuration and direction of optical rotation are
shown in parentheses.
d
In the case of acetophenone, changing the metal precursor
from [Ir(COD)Cl]2 to [IrCp*Cl2]2 resulted in a change of the
configuration of the alcohol product from S to R, even though
the same ligand configuration was used (entries 5 and 10, 6
and 11). Such a reversal in the sense of asymmetric induction,
confirmed by both optical rotation and HPLC, was not
observed for other substrates (Table 2). Although this is relatively
unusual, the pseudo-octahedral Ir(III) complex presents a very
different structure to the square planar Ir complex. Such a
phenomenon has been observed in other systems.14 Surprisingly,
with complex 4, different configurations of the product were
obtained at different concentrations of KOtBu; however, in these
cases the enantioselectivity was low. In any case, the effect of
different catalyst systems on the absolute configuration remains
to be explored further with other substrates.
dried and degassed prior to use. Other reagents were
purchased commercially and used as received. Ligand 1 was
prepared as described previously.6 Solvents were removed by
rotary evaporation on a Heidolph labrota 4000. Flash column
chromatography was performed on Davisil silica gel
Fluorochem 60 A, particle size 35–70 mm. HPLC analysis
was determined on a Varian Prostar operated by Galaxie
workstation software equipped with ChiralPak AD or OD-H
columns. NMR spectra were recorded on Bruker Avance
300 and 400 instruments. Proton chemical shifts are referenced
to internal residual solvent protons. Proton signal multipli-
cities are given as s (singlet), d (doublet), t (triplet), q (quartet),
m (multiplet), br (broad) or a combination of the above. All
spectra were recorded at room temperature and the solvent for
a particular spectrum is given in parentheses. Carbon chemical
shifts are referenced to the carbon signal of the deuterated
solvents. Fluorine chemical shifts are reported relative to
CFCl3. Infrared spectra were recorded on a Perkin Elmer
Paragon 1000 Spectrum GX FT-IR system. Mass spectro-
metry was performed on a Waters Micromass GCT (time of
flight) fitted with lockspray for accurate mass (ESI) or GCT
(CI). Optical rotations were measured on a Perkin Elmer 341
polarimeter using a 1 ml cell with a 1 dm path length at 20 1C
using the sodium D-line.
The [Ir(COD)Cl]2–1 catalyst system (Scheme 3) was
screened with additional substrates at 55 1C, as shown in
Table 2. The lower concentration of base generally seems to
give higher enantioselectivities while having only a relatively
minor effect on the conversion. Interestingly, the challenging
substrates with bulkier substituents (R = Cy, t-Bu) gave the
highest enantioselectivities, with moderate to excellent conver-
sion and ee’s up to 60% (entry 11). The species prepared in situ
from 1 with [IrCp*Cl2]2, which is related to isolated complex 4,
was less effective than the species prepared from 1 with
[Ir(COD)Cl]2 (Table 2). While a stable species could not be
isolated from ligand 1 and [Ir(COD)Cl]2, the reasonably high
enantioselectivities obtained suggest that the active complex
presents a single well defined chiral environment.
Complex 3
A solution of ligand 1 (145 mg, 0.41 mmol) in methylene
chloride (2 mL) was added to a solution of [Rh(Cp*)Cl2]2
(127 mg, 0.20 mmol) in methylene chloride (2 mL) in a Schlenk
flask under a nitrogen atmosphere. The reaction mixture was
stirred at ambient temperature for 45 min, after which AgBF4
(79 mg, 0.41 mmol) and methylene chloride (1 mL) were
added, and the reaction mixture was stirred for an additional
30 min. The reaction mixture was filtered to remove AgCl, and
the filtrate was concentrated under vaccuum to give complex 3
as a red solid (253 mg, 87%). Mp 110–112 1C (decomp.); [a]D20
À131.3 (c 0.16, CHCl3); IR (CDCl3, cmÀ1) 3250, 3214, 3063,
2961, 1602, 1471, 1448, 1381, 1201 cmÀ1; 1H NMR (300 MHz,
CDCl3) d = 0.63 (3H, d, J 7.1 Hz, CH3), 0.70–1.37 (6H, m,
CH2CHCHCH2), 0.89 (3H, d, J 7.2 Hz, CH3), 1.59 (15H, s,
C5(CH3)5), 1.78 (3H, d, J 7.2 Hz, CH3), 1.85 (3H, d, J 7.2 Hz,
Conclusions
In conclusion, a new iridium complex of a C2 symmetric chiral
diamine has been synthesised. The complex formed was
homochiral at the N centres and was formed as a single
diastereomer. The complex exhibits good activity and
moderate enantioselectivity for the transfer hydrogenation of
some bulky ketones.
Experimental
All manipulations were carried out under an inert nitrogen
atmosphere using standard Schlenk techniques. Solvents were
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CH3), 2.69–2.80 (1H, m, CH), 3.23–3.45 (1H, m, CH), 4.63
(1H, d, J 10.9 Hz, NH), 4.73–4.81 (2H, m, 2ÂCH), 5.77 (1H, d,
J 10.7 Hz, NH), 7.16–7.45 (8H, m, Ar H), 7.57–7.60 (2H, m,
Ar H); 13C NMR (75 MHz, CDCl3) d = 9.5 (C5(CH3)5), 13.9
(CH3), 16.7 (CH3), 19.5 (CH3), 19.6 (CH3), 34.2 (CH), 34.5
(CH), 35.2 (CH2), 37.5 (CH2), 57.3 (CH), 58.3 (CH), 59.6
with full-matrix least-squares based on F2 by using SHELXTL.15
(Chloroform molecules are omitted in Fig. 1 for clarity.)
Typical transfer hydrogenation procedure
A
Schlenk tube was charged with the metal complex
(0.005 mmol complex, corresponding to 0.01 mmol Ir or Rh)
and put under nitrogen atmosphere. Isopropanol (1 mL) and a
solution of ligand 1 in isopropanol (0.066 M; 0.150 mL,
0.01 mmol) were added via syringe. The reaction mixture
was stirred at 80 1C for 30 min. After cooling to 55 1C,
additional isopropanol (3.3 or 3.1 mL) and a solution of
KOtBu (0.10 M; 0.25 mL, 0.025 mmol or 0.5 mL, 0.05 mmol)
were added followed by ketone (1 mmol). The reaction
mixture was stirred at 55 1C for 21 hours. After cooling to
room temperature, a small aliquot of the reaction mixture
was filtered through a plug of alumina, eluting with 30%
isopropanol–70% hexane and analysed by HPLC. The
remaining reaction mixture was diluted with HCl (aq),
extracted three times with CH2Cl2, dried over MgSO4 and
concentrated to a yellow oil, which was chromatographed on
silica gel eluting with CH2Cl2.
1
(CH), 62.4 (CH), 96.5 (d, JC,Rh 8.3 Hz, 5ÂC5Me5), 126.3
(Ar C), 126.5 (Ar C), 128.0 (Ar C), 128.4 (Ar C), 129.6 (Ar C),
129.7 (Ar C), 143.9 (Ar C), 144.4 (Ar C); 19F{1H} NMR
(282 MHz, CDCl3) d = À149.6 (s); MS (FAB) m/z: 623.3
(43%), 588.3 (5), 547.0 (4), 351.2 (100), 272.8 (14), 230.0 (15);
found (FAB) 623.2631 ([M À BF4]+), C34H49ClN2Rh requires
623.2634. X-Ray quality crystals were grown by slow diffusion
of hexane into a CHCl3 solution of 3.
Complex 4
A Schlenk flask under a nitrogen atmosphere was charged
with ligand 1 (40 mg, 0.114 mmol) and [Ir(Cp*)Cl2]2 (43 mg,
0.05 mmol). Methylene chloride (2 mL) was added, and the
reaction mixture was stirred at ambient temperature for
90 min. AgBF4 (26 mg, 0.13 mmol) was added, and the
reaction mixture was stirred for an additional 15 min. The
reaction mixture was filtered to remove AgCl, and the filtrate
was concentrated under vacuum to give complex 4 as a yellow
solid, which was recrystallised by slow diffusion of hexane into
a CHCl3 solution to yield X-ray quality crystals (46 mg, 53%).
[a]2D0 À42.8 (c 0.025, CHCl3); IR (KBr, cmÀ1) 3232, 3199, 3058,
3025, 2957, 2918, 1625, 1499, 1471, 1448, 1379, 1311, 1284
cmÀ1; 1H NMR (400 MHz, CDCl3) d = 0.68 (3H, d, J 7.3 Hz,
CH3), 0.77–1.33 (6H, m, CH2CHCHCH2), 0.92 (3H, d,
J 7.3 Hz, CH3), 1.53 (15H, s, C5(CH3)5), 1.79 (3H, d, J 7.4
Hz, CH3), 1.80 (3H, d, J 7.6 Hz, CH3), 3.02–3.12 (1H, m, CH),
3.31–3.40 (1H, m, CH), 4.77–4.87 (2H, m, 2ÂCH), 4.98 (1H, d,
J 11.2 Hz, NH), 6.22 (1H, d, J 10.8 Hz, NH), 7.23–7.58
(10H, m, Ar H); 13C NMR (100 MHz, CDCl3) d = 8.8
(C5(CH3)5), 13.2 (CH3), 16.3 (CH3), 19.1 (CH3), 19.2 (CH3),
34.1 (CH), 34.3 (CH2), 34.3 (CH), 36.8 (CH2), 57.9 (CH), 59.5
(CH), 67.7 (CH), 63.8 (CH), 87.8 (5ÂC5Me5), 125.9 (Ar C),
126.2 (Ar C), 127.8 (Ar C), 128.2 (Ar C), 129.3 (Ar C), 129.4
(Ar C), 142.8 (Ar C), 143.7 (Ar C); 19F{1H} NMR (376 MHz,
CDCl3) d = À149.4 (s); MS (ES) m/z: 713.3 (10%), 711.3 (5),
677.4 (100), 675.4 (50), 351.4 (40), 113.0 (5), 59.2 (4);
found (ES) 711.3178 ([M À BF4]+), C34H49Cl191IrN2 requires
711.3185.
1-Phenyl-ethanol
1H NMR (400 MHz, CDCl3) d = 1.43 (3H, d, J 6.5 Hz, CH3),
1.63 (1H, broad s, OH), 4.84 (1H, q, J 6.4, CHOH), 7.18–7.33
(5H, m, Ar H); 13C NMR (75 MHz, CDCl3) d = 25.19 (CH3),
70.46 (CHOH), 125.41 (Ar C), 127.51 (Ar C), 128.54 (Ar C),
145.83 (Ar C). Enantioselectivity determined by HPLC,
ChiralPak OD-H, 0.5 mL minÀ1, 95 : 5 hexane–2-propanol.
Retention times: 17.4 min (R)-(+)-enantiomer and 21.2 min
(S)-(À)-enantiomer.
2-Methyl-1-phenylpropan-1-ol
1H NMR (400 MHz, CDCl3) d = 0.73 (3H, d, J 6.8 Hz,
diastereotopic CH3), 0.93 (3H, d, J 6.6 Hz, diastereotopic
CH3), 1.63 (1H, broad s, OH), 1.89 (1H, octet, J 6.7, CHMe2),
4.30 (1H, d, J 6.9, CHOH), 7.16–7.21 (5H, m, Ar H); 13C
NMR (100 MHz, CDCl3) d = 18.25 (CH3), 19.03 (CH3), 35.29
(CHMe2), 80.09 (CHOH), 126.58 (Ar C), 127.45 (Ar C),
128.22 (Ar C), 143.70 (Ar C). Enantioselectivity determined
by HPLC, ChiralPak OD-H, 0.5 mL minÀ1, 98 : 2 hexane–
2-propanol. Retention times: 27.6 min (S)-(À)-enantiomer and
30.4 min (R)-(+)-enantiomer.
2,2-Dimethyl-1-phenylpropan-1-ol
X-Ray crystallography of (SN1,SN2,RC1,RC2,RC4,RC5
,
1H NMR (300 MHz, CDCl3) d = 0.83 (9H, s, 3ÂCH3), 1.97
(1H, broad s, OH), 4.28 (1H, s, CHOH), 7.19 (5H, m, Ar H);
13C NMR (100 MHz, CDCl3) d = 25.93 (CH3), 35.64 (CMe3),
82.42 (CHOH), 127.30 (Ar C), 127.57 (Ar C), 127.61 (Ar C),
142.20 (Ar C). Enantioselectivity determined by HPLC,
ChiralPak OD-H, 1 mL minÀ1, 99.5 : 0.5 hexane–2-propanol.
Retention times: 10.5 min (S)-(À)-enantiomer and 13.6 min
(R)-(+)-enantiomer.
S
C7,SC15)-(À)-4. C36H51BCl7F4N2Ir, M = 1038.95, ortho-
rhombic, space group P2(1)2(1)2(1), Z = 4, a = 13.5451(7),
b = 17.3548(10), c = 18.4163(10) A, a = 901, b = 901, g =
901, V = 4329.2(4) A3, rcalcd 1.594 mg mÀ3, m(MoKa) =
3.561 mmÀ1 (max./min. transmission 1.0000/0.7678),
T = 93(2) K. Of 28 479 measured data, 7939 were unique
(Rint = 0.0423) and 7663 observed [I 4 2s(I)] to give R =
0.0290, wR2 = 0.0570. Data were collected using a Rigaku
MM007 High brilliance RA generator (MoKa radiation,
confocal optics) and Saturn 70 CCD system. Intensities were
corrected for Lorentz-polarisation and for absorption. The struc-
tures were solved by direct methods. Hydrogen atoms bound to
carbon were idealised. Structural refinements were performed
Cyclohexyl(phenyl)methanol
1H NMR (300 MHz, CDCl3) d = 0.8–1.9 (11H, m, C6H11),
1.91 (1H, broad s, OH), 4.34 (1H, d, J 7.5, CHOH), 7.2–7.3
(5H, m, Ar H); 13C NMR (100 MHz, CDCl3) d = 26.02
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(CH2), 26.10 (CH2), 26.43 (CH2), 28.83 (CH2), 29.32 (CH2),
44.97 (CH), 79.42 (CHOH), 126.64 (Ar C), 127.43 (Ar C),
128.20 (Ar C), 143.63 (Ar C). Enantioselectivity determined
by HPLC, ChiralPak AD, 0.5 mL minÀ1, 98 : 2 hexane–
2-propanol. Retention times: 23.5 min (S)-(À)-enantiomer
and 26.5 min (R)-(+)-enantiomer.
6 Y. N. Belokon, J. Fuentes, M. North and J. W. Steed, Tetrahedron,
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8 J. A. Fuentes, A. M. Z. Slawin, M. B. France and M. L. Clarke,
unpublished results.
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Acknowledgements
The authors thank the EPSRC for an equipment grant,
and for the use of the National mass spectrometry service.
Mrs C. Horsborough and Mrs M. Smith are also thanked for
technical assistance. The authors thank the Donors of the
American Chemical Society Petroleum Research Fund and
Washington and Lee University (Glenn Grant) for support of
this work. The authors also wish to thank Johnson Matthey
for the loan of precious metal salts.
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