Table 2 Epoxidation of trans-hex-2-en-1-ol with tBHP catalysed by l -(+)-polyester 3 and Ti(OPri)4
Molar ratio
4; Ti:tartrate
Reaction Epoxide
Isolated
yield (%)c
Ligand
T/°C time/ha
yield (%)b
Work-upd
Eee (%)
DMT
3a
3b
3c
100:5:6
230
220
220
220
220
220
220
220
220
220
220
3
3
3
3
7
3
3
3
3
3
3
91g
51
63
22
92
65
21
80
75
73
74
44
50
63
59
58
60
46
61
61
42
80
A
B
B
B
B
B
B
B
B
B
B
!98g
8
100:17:20
100:17:20
100:5:10
100:17:20
100:12:12
100:5:10
100:10:30
100:17:20
100:17:20
100:17:20
55
79
79
41
52
77
64
47
68
3c
3d
3d
3d
3d
3e
3f
a From addition of 4. b From GC analysis after 3 h. c After additional 12 h in freezer, work-up and Kugelrohr distillation. d A = ferrous sulfate, tartaric acid
e
(ref. 15); B = non-acidic aqueous workshop (ref. 15). (2S,3S)-(2)-Propyloxiranemethanol determined via chiral HPLC (Chiralcel OB, hexane–PriOH.
f DMT = l -(+)-dimethyl tartrate. g Chemical yield = 85%, ee = 94%; Sharpless using l -(+)-diethyltartrate (ref. 15).
O
may well therefore depend also on the ability of these
i
C3H7
OH
C3H7
OH
backbones to assume a conformation which allows maximum
opportunity for the formation of such complexes. In this respect,
in due course, investigation of poly(tartrate ester)s may well
provide useful structural information on the active species
themselves.
4
5
Scheme 3 Reagents and conditions: i, poly(tartrate ester) (10–30 mol%),
Ti(OPri)4 (5–17 mol%), ButOOH (2 equiv.), 4 Å molecular sieves, CH2Cl2,
220 °C, 3 h
At the moment we are further optimising these reactions and
pursuing the above structural aspects. We are also making
crosslinked poly(tartrate ester) gels and immobilising linear
poly(tartrate ester)s on supports such as poly-
(styrenedivinylbenzene) in an attempt to produce highly
practical re-usable heterogeneous Sharpless epoxidation cata-
lysts.
We acknowledge funding from the Neste Oy Foundation and
the Finnish cultural Foundation. We also appreciate the
molecular weight determinations carried out by RAPRA.
rotations are comparable with those reported14 for dimethyl
22
l -(+)-tartrate {[a]D + 21 (c 2.5, H2O)} and for diethyl
l -(+)-tartrate {[a]D20 + 8.5 (neat)}.
Polyesters 3a–f were used as ligands in the epoxidation of 4
with Ti(OPri)4–tBHP as shown in Scheme 3. Powdered
activated 4 Å molecular sieves, polymeric ligand and Ti(OPri)4
were first mixed in CH2Cl2 at 220 °C for 1 h, the tBHP was
added and the mixture stirred for a further 1 h at 220 °C before
4 in CH2Cl2 was added. The reaction was then left to proceed
for 3 h at ca. 220 °C, when the GC yield was determined. Each
reaction mixture was then stored overnight in a freezer before
work-up and isolation of pure 5 via Kugelrohr distillation.
Enantiomeric excess was determined by chiral HPLC using a
Chiralcel OB column and hexane–isopropyl alcohol (97.5:2.5)
as eluent. The results are summarised in Table 2. Generally
reactions are fast even at 220 °C with GC yields (3 h) in the
range 20–90% depending upon the polymer ligand 3 used. In
some instances isolated yields are higher than GC yields
because the reactions continue in the freezer overnight. Isolated
yields, however, are by no means optimised; pure 5 was isolated
simply for optical rotation measurements and considerable
scope remains for optimisation. Enantiomeric excesses gen-
erally fall in the range 40–80% with a significant dependence on
the poly(tartrate ester) employed. Polymer (3a) is very poor,
and polymer (3c) the most effective. The origin of the ‘polymer
effect’ is not clear and may well involve a contribution from
solubility and molecular weight variation, and from conforma-
tional factors. Polymers 3a and 3b are rather insoluble in
CH2Cl2, as is 3c. However, the latter does form a CH2Cl2
soluble complex with Ti(OPri)4. This led us to prepare 3d which
is itself soluble in CH2Cl2; however, 3d gave poorer levels of
induction than 3c under comparable conditions so that solubility
alone is not the only factor. Solubility limitations have made
molecular weight determination difficult, but two different
References
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––
–– ––
batches of 3d yielded Mw = 4150 and 4210, with Mw/MN = 1.7
(gel permeation chromatography; polystyrene standards in
THF). Compound 3d is therefore oligomeric rather than
polymeric, and further work is underway to investigate this
parameter. It seems most likely that the active species with
simple dialkyl tartrate–Ti(OPri)4 are rather complicated, poss-
ibly 2:2 complexes.16 The effectiveness of poly(tartrate ester)s
14 Aldrich Handbook of Fine Chemicals, 1994–1995.
15 Y. Gao, R. M. Hanson, J. M, Klunder, S. Y. Ko, H. Masumune and
K. B. Sharpless, J. Am. Chem. Soc., 1987, 109, 5765.
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Received, 10th October 1996; Com. 6/06954A
124
Chem. Commun., 1997