Enantiodifferentiating photoisomerization of (Z)-cyclooctene sensitized by chiral C2-symmetric phosphoramide

Article abstract of DOI:10.1071/CH01020

Optically active C₂ symmetric phosphoramides were used as chiral sensitizers for enantiodifferentiating photoisomerization of a cyclooctene. The use of chiral aromatic amides and phosphoryl esters as chiral sensitizers fro the reaction was also

Full text of DOI:10.1071/CH01020

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Aust. J. Chem. 2001, 54, 113–115
Enantiodifferentiating Photoisomerization of (Z)-Cyclooctene
Sensitized by Chiral C -Symmetric Phosphoramide
2
A
B
Min Shi and Yoshihisa Inoue
A
Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, China.
E-mail: mshi@pub.sioc.ac.cn
Inoue Photochirogenesis Project, ERATO, JST, 4-6-3 Kamishinden,
Toyonaka 565–0085, Japan.
B
The optically active C -symmetric phosphoramides (2) and (3) were employed as chiral sensitizers for the
2
geometrical photoisomerization of (Z)-cyclooctene (1Z) in pentane at varying temperatures. At low temperature
(
–56°C), the achieved enantiomeric excesses (e.e.) for (1Z) are 20 and 28%, respectively.
Manuscript received: 14 March 2001.
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Introduction
Results and Discussion
Asymmetric photosensitization, which enables catalytic
asymmetric synthesis in the excited state, has attracted much
attention for more than three decades.[1,2] However, in spite
of the considerable efforts devoted to photochemical studies
The novel chiral sensitizers (2) and (3) were prepared from
the reaction of diphenylphosphinic chloride and
diphenylthiophosphoryl chloride with (1R,2R)-(–)-
cyclohexane-1,2-diamine
in
the
presence
of
employing various chiral sensitizers and prochiral
diisopropylethylamine in dichloromethane. After the usual
workup and purification by silica-gel column
chromatography the phosphoramides (2) and (3) were
obtained as colourless solids in over 90% yields (Scheme 1).
Their structures were established by spectroscopic analysis
and high-resolution mass spectroscopy. These chiral
sensitizers were directly used for the enantiodifferentiating
isomerizations of (Z)-cyclooctene (1Z) under various
irradiation conditions (Scheme 2). The major photochemical
reaction observed was the E-to-Z isomerization of
cyclooctene. Thus, the phosphoramides, possessing
substrates,[
1,2]
only
a
limited number of enantio-
[3–8]
differentiating photosensitizing systems
have been
known to give better enantiomeric excesses of product than
the original e.e. (5.0%) reported by Hammond and Cole in
9]
1
965.[ Of the successful systems in this extremely difficult
research area, the photosensitized enantiodifferentiating
[3–5]
isomerizations of (Z)-cyclooctene (1Z)
and (Z,Z)-
[6]
cycloocta-1,3-diene (1ZZ) are of particular interest to us,
since the sensitization with chiral aromatic esters gives
exceptionally high enantiomeric excesses up to 64% and
shows unique temperature effects that lead to unprecedented
temperature switching behaviour of the product chirality.
Since a wide variety of optically active alcohols are
available from the chiral pool, we have hitherto employed
aromatic carboxylic esters as chiral sensitizers, in which
various naturally occurring and synthesized optically active
alcohols were incorporated in the ester moiety as chiral
[
3–6,8]
auxiliaries.
We have also explored novel chiral aromatic
amides[
10]
and chiral phosphoryl esters
[11]
as chiral
sensitizers for this reaction. In order to extend the category
of chiral sensitizers which can be used to achieve
enantiodifferentiating photosensitization, we wish to report
the chiral phosphoramides (2) and (3) as novel chiral
sensitizers for the enantiodifferentiating isomerization of
(Z)-cyclooctene (1Z).
©
CSIRO 2001
10.1071/CH01020
0004-9425/01/020113

1
14
M. Shi and Y. Inoue
isoelectronic structures with the phosphoryl esters, were
shown to function as effective sensitizers for the geometrical
photoisomerization of simple alkenes, although the E-to-Z
ratios obtained were smaller in general than those obtained
with the phosphoryl esters.
Table 1. Enantiodifferentiating photoisomerization of (Z)-
cyclooctene (1Z) sensitized by the chiral phosphoramides (2)
and (3) in pentane
Sens* Temp. Time E-to-Z Conversion
Yield
(%)
E.e.
(%)
Fig. 1. Reciprocal yield of (1E) against reciprocal concentration of
(
°C)
(h)
ratio
(%)
(
1Z) for the Z-to-E photoisomerization of (1Z) with the chiral
sensitizer.
(
2)
25
1
2
3
4
8
0.024
0.036
0.086
0.015
0.085
0.090
0.12
17
23
33
19
20
22
24
1.6
2.9
6.6
1.2
6.9
8.1
8.2
8.3
20.3
20.4
20.0
20.0
enantiomeric excesses of 10 and 28% at 25 and –56°C,
respectively. These standard deviations obtained are well
within the experimental error (±1.0%) in the determination
by chiral gas chromatography (GC). However, appreciable
deviation to slightly higher enantiomeric excesses are
observed upon irradiation at –56°C.
–56
1
1
2
6
8.3
10.2
(3)
25
56
1
2
3
4
8
2
6
0.024
0.036
0.086
0.015
0.085
0.090
0.12
17
23
33
19
20
22
24
1.6
2.9
6.6
1.2
6.9
9.7
10.2
10.3
27.3
28.4
28.0
28.0
Analogous temperature dependence has been already
–
reported in the enantiodifferentiating isomerizations of (Z)-
cyclooctene (1Z) sensitized by aromatic esters,[
3–6]
although
1
1
8.3
10.2
no temperature switching of the product chirality was
observed in the present sensitization with (2) and (3). Hence,
we first disclosed that chiral phosphoramides (2) and (3) can
be used for the enantiodifferentiating isomerizations of (Z)-
cyclooctene (1Z). The chiral sensitizers for achieving
enantiodifferentiating isomerizations of (Z)-cyclooctene
The (E)-product was isolated from the photolysate
through the selective extraction of (1E) with aqueous silver
nitrate in >96% chemical yield.
[
12–15]
Gas chromatographic
analyses indicated that the isolated (1E) contained a small
amount (<2%) of (1Z). Enantiomeric excesses of isomer
(
1Z) can be chiral aromatic esters, amides, phosphoryl esters,
and phosphoramides. In general, the phosphoramides
reported in this paper give the highest enantiomeric excesses
under the same conditions.
(
1E) were determined by gas chromatographic analysis on a
chiral capillary column. The effects of the irradiation period,
or conversion, upon the product’s yield and e.e. were first
examined. As can be seen from Table 1, the E-to-Z ratio
increases with increasing irradiation time, ultimately giving
the photostationary-state E-to-Z ratio upon prolonged
irradiation. In contrast, the product’s e.e. is not appreciably
affected by the irradiation time, at least at the initial stages.
Thus, the sensitization with (2) gives practically constant
enantiomeric excesses of 8 and 20% at 25 and –56°C,
respectively. In addition, the sensitization with (3) gives
To confirm the excited state involved, the photosensitiza-
–3
tion with (2) (1.0 mmol dm ) was carried out using various
concentrations of (1Z) in a merry-go-round apparatus. A
Stern–Volmer treatment of the product yield, in which the re-
ciprocal yield of (1E) is plotted against the reciprocal con-
centration of (1Z), afforded a good straight line as shown in
Fig. 1. It means that only one excited state is involved in the
photoisomerization of (1Z). Unfortunately, (2) and (3) are
not fluorescent at all, and thus we cannot obtain the rate con-

Enantiodifferentiating Photoisomerization of (Z)-Cyclooctene
115
stants from the fluorescence quenching to decide whether a
singlet or triplet excited state is involved in this photochem-
dichloromethane was added diphenylphosphinic chloride (757 mg, 3.20
mmol) at –30°C. After being stirred for 6 h, the reaction mixture was
washed with 3% aq. HCl, water, 10% Na2CO3 and brine and the product
[3–6]
ical reaction. But based on our previous results,
we be-
was extracted into ether. The extract was dried over MgSO , and then
4
lieve that this photo-induced enantiodifferentiating
isomerization may proceed via an exciplex intermediate. A
plausible reaction mechanism is elucidated in Scheme 3.
In conclusion, although the enantiomeric excesses ob-
tained are low, and no definitive conclusions on the enantio-
differentiation mechanism can be drawn at the present stage,
it should be emphasized that the enantiodifferentiating
isomerizations of (Z)-cyclooctene (1Z) can be effected by us-
ing chiral phosphoramide sensitizers, and the use of a steri-
cally, well designed, photosensitizer is expected to achieve
higher enantiomeric excesses in the future.
evaporated under reduced pressure. The residue was purified by silica-
gel column chromatography to give compound (2) (740 mg, 90%), m.p.
196–198°C, [α]D +40.5 (c, 1.0 in CHCl3) [Found: C, 70.2; H, 6.3; N,
+
5
5
.4%; m/z, 514.1961 (M ). C H N O P requires C, 70.0; H, 6.3; N,
30 32
2
2 2
+
.4%; M , 514.1939]. Ultraviolet (UV) spectrum (MeCN) λ
262 nm
max
(
ε 2407). δ (CDCl ): 1.0–1.20, 2H, m, CH ; 1.22–1.49, 2H, m, CH ;
H 3 2 2
1
.50–1.72, 2H, m, CH ; 1.80–2.10, 2H, m; 2.90–3.10, 2H, m; 4.40–
2
4.60, 2H, m; 7.30–7.40, 4H, m, Ar; 7.40–7.60, 8H, m, Ar; 7.70–7.90,
4H, m, Ar; 7.95–8.12, 4H, m, Ar. Electron-impact mass spectrum
+
(
EIMS): m/z (%) 514 (M , 5.4), 313 (50), 297 (40), 270 (45), 201 (100).
Preparation of Chiral Thiophosphoramide (3)
This compound was prepared in the same manner as that described
above. Thus, to a solution of (1R,2R)-(–)-cyclohexane-1,2-diamine
(158 mg, 1.38 mmol) and diisopropylethylamine (536 mg, 4.15 mmol)
in dichloromethane was added diphenylthiophosphinic chloride (702
mg, 2.78 mmol); this gave thiophosphoramide (3) as a colourless solid
(693 mg, 92%), m.p. 84–85°C, [α]D +18.9 (c, 1.3 in CHCl3) [Found: C,
Experimental
Optical rotations were determined in a solution of CHCl by using a
3
Perkin–Elmer 241 MC digital polarimeter; [α] values are given in
D
–
1
2
1
units of 10 deg cm /g. H nuclear magnetic resonance (NMR) spectra
were determined for solutions in CDCl3 with tetramethylsilane as
internal standard on a Bruker AMX-300 spectrometer; values for the
coupling constant (J) are in Hertz (Hz). High-resolution mass spectra
were recorded on a Finnigan MA+ instrument. Organic solvents used
were dried by standard methods when necessary. All solid compounds
reported in this paper gave satisfactory CHN microanalyses with an
Italian Carlo–Erba 1106 analyser. Commercially obtained reagents
were used without further purification. The reaction was monitored by
thin-layer chromatography (TLC) with Huanghai 60F254 silica gel
coated plates. Flash column chromatography was carried out with 300–
+
65.9; H, 5.8; N, 5.2%; m/z, 546.1499 (M ). C30H32N2P2S2 requires C,
+
65.9; H, 5.9; N, 5.1%; M , 546.1482]. δH (CDCl3) 0.80–1.72, 6H, m,
CH2; 1.80–2.00, 2H, m; 3.20–3.45, 2H, m; 3.90–4.20, 2H, m; 7.20–
7.60, 12H, m, Ar; 7.70–7.90, 4H, m, Ar; 7.95–8.12, 4H, m, Ar. EIMS:
+
m/z (%) 547 (MH , 5.4), 513 (5.4), 328 (59.7), 313 (90.3), 217 (100).
Acknowledgments
We thank the State Key Project of Basic Research (Project
4
00 mesh silica gel at increased pressure.
9
73; No. G2000048007) and the National Natural Science
Gas chromatographic analyses of the geometrical isomers of (1Z) in
Foundation of China for financial support. We also thank the
Inoue Photochirogenesis Project (ERATO, JST) for chemical
reagents.
photolysed solutions were performed on a 50-m capillary column
Shimadzu CBP-20) at 65°C with a Shimadzu 14A instrument. The
enantiomeric excesses of (1E), isolated through selective extraction
(
with 20% aqueous silver nitrate,[
chromatography over a 60-m chiral capillary column (Superlco β-DEX
20) at 70°C, by means of a Shimadzu 14A instrument.
3–6,12–15]
were determined by gas
1
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–
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[
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5
1
[
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3
(25°C) or propan-2-ol (–25 to –78°C) bath. A pentane solution (3 cm )
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–3
containing (1Z) (5 mmol dm ), the optically active sensitizer (1 mmol
–
3
–3
dm ) and cycloheptane (5 mmol dm ; added as an internal standard)
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7
[
[
8
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Preparation of Chiral Phosphoramide (2)
To a solution of (1R,2R)-(–)-cyclohexane-1,2-diamine (182 mg, 1.60
[15] E. Vedejs, K. A. Snable, P. L. Fuchs, J. Org. Chem. 1973, 38,
mmol) and diisopropylethylamine (724 mg, 5.60 mmol) in
1178.