Chiral 1,2-bis(phosphetano)ethanes

Article abstract of DOI:10.1016/S0022-328X(00)00910-4

Optically pure 1,2-bis(phosphetano)ethanes 3 (BPE-4) have been prepared from 1,2-bis(phosphino)ethane and the cyclic sulfates of symmetrical anti-1,3-diols. Diphosphine 3c (R = cyclohexyl) is an easily accessible, air-stable chiral ligand. Its suitability to the ruthenium-catalysed hydrogenation of functionalised ketones has been examined by using several catalyst precursors. Significant enantiomeric excesses were obtained. A ruthenium complex containing two coordinated diphosphines 3c was characterised by X-ray diffraction studies.

Full text of DOI:10.1016/S0022-328X(00)00910-4

Journal of Organometallic Chemistry 624 (2001) 162–166
www.elsevier.nl/locate/jorganchem
Chiral 1,2-bis(phosphetano)ethanes
Angela Marinetti a,*, Se´bastien Jus a, Jean-Pierre Geneˆt a, Louis Ricard b
a Laboratoire de Synthe`se Se´lecti6e Organique et Produits Naturels, ENSCP 11, rue Pierre et Marie Curie, F 75231 Paris Cedex 05, France
b Laboratoire He´te´roe´le´ments et Coordination, Ecole Polytechnique, F 91128 Palaiseau Cedex, France
Received 9 October 2000; accepted 15 November 2000
Abstract
Optically pure 1,2-bis(phosphetano)ethanes 3 (BPE-4) have been prepared from 1,2-bis(phosphino)ethane and the cyclic sulfates
of symmetrical anti-1,3-diols. Diphosphine 3c (R=cyclohexyl) is an easily accessible, air-stable chiral ligand. Its suitability to the
ruthenium-catalysed hydrogenation of functionalised ketones has been examined by using several catalyst precursors. Significant
enantiomeric excesses were obtained. A ruthenium complex containing two coordinated diphosphines 3c was characterised by
X-ray diffraction studies. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Phosphetanes; Asymmetric hydrogenation; Ruthenium
tween the two classes of ligands. However, it appears
that the presence of the four-membered phosphetane
ring in diphosphines 1 and 2 brings about peculiar
properties and specific application fields with respect to
the analogous phospholane derivatives. Thus, for in-
stance, ligands 2 display better enantioselectivities than
the corresponding phospholanes in rhodium-catalysed
hydrogenations of itaconic acid derivatives. By con-
trast, DuPHOS ligands perform better than the corre-
sponding phosphetanes in the rhodium-catalysed
hydrogenation of the few model dehydroaminoacid
derivatives tested to date. In this reaction, however, an
intriguing behaviour of phosphetane-based ligands has
been observed [5] when either the 1,2-bis(pho-
sphetano)benzenes 1 or the l,2-bis(phosphetano)ethanes
3 are used as chiral auxiliaries: an unusual, strong
increase of the enantioselectivity at higher hydrogen
pressure is noticed. Mechanistic implications are cur-
rently under study in our group.
1. Introduction
C2-Symmetric bidentate ligands bearing phosphetane
units are currently outlined as a new class of chiral
auxiliaries for asymmetric catalysis. Notably, optically
pure 1,2-bis(phosphetano)benzenes, 1 (CnrPHOS) [1]
and 1,1%-bis(phosphetano)ferrocenes, 2 [2], are easily
accessible ligands which display significant enantioselec-
tivity in ruthenium and rhodium-catalysed hydrogena-
tions, respectively. Albeit the search for their catalytic
applications is at an early stage, their high potential
cannot be denied in view of the reported preliminary
results of catalytic tests [1,2].
The origin of the divergent behaviours of pho-
sphetanes and phospholanes with overall similar struc-
tures, should be related to restricted flexibility, specific
geometrical features and electronic effects of the con-
strained four-membered ring. Full rational is not yet
available. However, it appears that chiral phosphetanes
are interesting targets for both applied purposes and
fundamental studies. In this context, we wish to present
herein a new family of phosphetane ligands, the ethyli-
Clearly, the design of phosphetanes 1 and 2 has been
drawn by their structural analogy to the well-known,
phospholane-based DuPHOS [3] and bis(phospho-
lano)ferrocene ligands [4]. The very restricted informa-
tion available to date on phosphetanes as catalytic
auxiliaries does not allow exhaustive comparison be-
* Corresponding author.
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00910-4

A. Marinetti et al. / Journal of Organometallic Chemistry 624 (2001) 162–166
163
Table 1
Hydrogenation reactions promoted by ruthenium–3c complexes a
Entry
Ru-catalyst
Substrate
Conditions
e.e. (config)
1
2
3
4
5
6
7
8
Cl2Ru(PPh3)3
(COD)Ru(2-Me-allyl)2+2HBr
[(C6H6)RuCl2]2
[(C6H6)RuCl2]2+HCl
[(C6H6)RuCl2]2+NH4PF6
(COD)Ru(2-Me-allyl)2+2HBr
[(C6H6)RuCl2]2
(COD)Ru(2-Me-allyl)2+2HBr
[(C6H6)RuCl2]2
MeCOCH2CO2Me
50 bars, 80°C
80 bars, 80°C
10 bars, 80°C
10bars, 80°C
10 bars, 80°C
80 bars, 80°C
10 bars, 80°C
80 bars, 80°C
10 bars, 80°C
80 bars, 80°C
10 bars, 80°C
80 bars, 80°C
l5(S) b
87(R)
91(R)
73(R)
92(R)
EtCOCH2CO2Et
PhCOCH2CO2Et
iPrCOCH2CO2Et
MeCOCH2COMe
91(R) b
90(R)
80(S) b
90(S)
9
10
11
12
(COD)Ru(2-Me-allyl)2+2HBr
[(C6H6)RuCl2]2+NH4PF6
(COD)Ru(2-Me-allyl)2+2HBr
95(S) b
92(S)
98(R,R) c
a Conditions: 1 mmol substrate, solvent MeOH or EtOH (1.5 ml), catalysts formed in situ from ruthenium complexes (1% molar amount) and
ligand 3c (Cy-BPE-4) (1:1 ratio to Ru). Reaction times of 20 h (non-optimised) led to complete conversions.
b Enantiomeric excesses were determined by GC on a Lipodex A column.
c Diastereomeric excess=95%; the enantiomeric excess was determined by 1H-NMR or GC analysis of the Mosher diester prepared from the
crude diol and (S)-MPTA-Cl.
dene bridged [6] l,2-bis(phosphetano)ethanes 3 (BPE-4).
The experimental procedure for their synthesis as well
as several data on their coordinating properties and
catalytic behaviour in ruthenium promoted hydrogena-
tions are reported hereafter.
carbons, as their air stable borane complexes 4, in
reasonably good yields (40–65%). The best yields are
obtained for the cyclohexyl-substituted phosphine. Af-
ter purification of 4, the borane protecting group is
removed by heating at 60°C for several hours with an
excess DABCO. Longer reaction times and harder con-
ditions are needed for the decomplexation of 3 com-
pared to the analogous reactions on the
bis(phosphetano)benzenes 1, owing probably to their
higher
basicity.
The
l,2-bis(2,4-dimethylphos-
phetano)ethane 3a (Me-BPE-4) is an extremely air sen-
1
sitive compound. It has been characterised only by H-
and 31P-NMR spectroscopy in the crude reaction mix-
ture of the deboronation reaction. The sterically hin-
dered phosphines 3b (i-Pr-BPE-4) and 3c are less
sensitive compounds that can be purified by column
chromatography. Phosphetane 3c (Cy-BPE-4) can be
handled in air in the solid state, and oxidises very
slowly in solution. It is then a well-suited representative
substrate for initial studies on the coordination chem-
istry and catalytic behaviour of the new diphosphines 3.
Phosphetane 3c has been evaluated in the ruthenium-
catalysed hydrogenations of model b-ketoesters and
1,3-diketones, as shown in Table 1. Several catalyst
precursors have been tested in the hydrogenation of
methyl acetoacetate (entries 1–5): the catalyst formed
in situ from (COD)Ru(2-Me-allyl)2 and ligand 3c by
addition of two equivalents of HBr (entry 2) [7] gave
complete conversion and high enantiomeric excesses
when the reaction was performed in hard conditions (80
bars, 80°C, 20 h). Under milder conditions both con-
versions and e.e. decreased significantly, and somewhat
inconsistent results were obtained.
2. Results and discussion
The preparation of the 1,2-bis(phosphetano)ethanes 3
follows the well-established method based on the nucle-
ophilic cyclisation of a lithiated primary phosphine with
the cyclic sulfate of an optically pure 1,3-diol. Starting
materials are the commercially available 1,2-diphosphi-
noethane (Strem Chemicals, Inc.) and chiral anti-1,3-di-
ols, easily obtained by enantioselective ruthenium-
catalysed hydrogenation of the corresponding symmet-
rical diketones.1
(1)
The procedure in Eq. (1) gives access to 1,2-bis(pho-
sphetano)ethanes, symmetrically substituted on the a-
With the catalyst formed by mixing Cl2Ru(PPh3)3
and ligand 3c, according to the procedure already suc-
1 See Ref. [1b] and references therein.

164
A. Marinetti et al. / Journal of Organometallic Chemistry 624 (2001) 162–166
cessfully applied to the reduction of b-dicarbonyls [8],
the hydroxybutanoate was obtained in very low e.e.,
with the S-configurated product as the major enan-
tiomer (entry 1). The highest catalytic activities, associ-
ated with high enantioselectivity, were attained by using
[(benzene)RuCl2]2 as the catalyst precursor [9]. Neither
acid additives (the beneficial effect of acid additives in
analogous hydrogenations has been reported in Ref.
[10]) (aq. HCl, entry 4), nor addition of NH4PF6 (coun-
ter-ion exchange, entry 5) did not improve substantially
the catalytic properties. The few hydrogenations of
other b-ketoesters and 1,3-diketones (entries 6–12),
which have been tested to date, show analogous trends
and high enantioselectivities.
The catalyst precursors formed from the three ruthe-
nium complexes above and ligand 3c, have been exam-
ined by 31P-NMR spectroscopy. A 1:1 mixture of
RuCl2(PPh3)3 and phosphetane 3c in CD2Cl2 after 2 h
at room temperature, shows the presence of a large
amount (50%) of free 3c and three ruthenium
complexes.
One of them displays an ABC spectrum at l 128.2
(dd, J=11 and 27 Hz), 115.2 (dd, J=11 and 32 Hz),
42.9 (dd, J=27 and 32 Hz), which suggests coordina-
tion of 3c and triphenylphosphine to the same ruthe-
nium centre. However, the 31P-NMR spectrum is not
fully consistent with the formation of the expected
Cl2Ru(PPh3)(3c) complex, which should give a large
2JPꢀP coupling between PPh3 and the phosphorus atom
of the bidentate ligand which occupies the trans coordi-
nation site [8], unless the complex adopts an fac coordi-
nation mode with three cis phosphorus atoms. The
second complex shows an AB spectrum at l=50.7 and
50.0 ppm (JAB=39 Hz) which could be assigned to the
dimeric [(3c)RuCl2]2 complex. A third complex, 5, is
formed (l31P=86 ppm) which becomes the main
product on heating or when an excess 3c is added.
Complex 5 (trans-Cl2Ru((S,S)-3c)2) is a poorly soluble
compound which has been isolated by crystallisation
and characterised by X-ray diffraction studies. ORTEP
drawing of 5 is shown in Fig. 1.
The phosphetane ring is bent, with a flapping angle
of 15.2°. The four phosphorus atoms occupy equatorial
positions in the distorted octahedral ruthenium com-
plex. The P(CH2)2P framework adopts a twist d confor-
mation which closely resembles that of the ethano
bridge in the cationic (COD)Rh((R,R)-Me-BPE)+SbF6−
complex [3]. The d conformation minimises sterical
interactions by moving the syn-cyclohexyl group to an
axial position, back away from the ruthenium centre.
The observed twist conformation contrasts with the
geometry of the analogous chiral ruthenium–chiraphos
and ruthenium–bnpe complexes [11] in which the
chelate rings adopt envelope conformation in the solid
state (conformational mobility is expected to be facile
in solution). The envelope conformation induces loss of
the C2 symmetric arrangement of the phenyl rings of
the chiraphos and bnpe ligands, giving a nearly cen-
trosymmetric molecule, while the twist conformation of
complex 5 retains the C2 symmetry of the molecule. The
isolated complex 5 is catalytically inactive in the hydro-
genation of methyl acetoacetate at 10 bars, 80°C.
31P-NMR analysis of the catalyst formed in situ from
(COD)Ru(2-Me-allyl)2 and 3c by addition of HBr
(Table 1, entry 2), shows formation of a major compo-
nent with l31P at 82 ppm. This sparingly soluble com-
plex can be isolated by precipitation from
acetone–ether mixtures. Analytical data for this com-
pound suggest formation of the Br2Ru(3c)2 complex,
the bromo-analogue of 5. This complex is also catalyti-
cally inactive in the reduction of b-ketoesters. Thus, it
seems that the observed catalytic activity comes from
minor components of the mixture which cannot be
detected by 31P-NMR.2 This could explain the low and
somewhat erratic catalytic activity.
Fig. 1. ORTEP drawing of the ruthenium complex 5, Cl2Ru(3c)2.
Selected bond distances: RuꢀP(1) 2.3443(7), RuꢀP(2) 2.3522(8),
RuꢀCl(1) 2.4325(6), P(1)ꢀC(1) 1.889(3), C(1)ꢀC(2) 1.540(4), C(2)ꢀC(3)
1.559(4), P(1)ꢀC(3) 1.867(3), P(1)ꢀC(4) 1.852(3), C(4)ꢀC(5) 1.529(4).
Selected bond angles: C(1)ꢀP(1)ꢀC(3) 77.8(1), C(1)ꢀP(1)ꢀC(4)
2 Alternatively, the catalytically active species could be formed
under the hydrogenation conditions, upon heating the mixture at
80°C.
102.7(1),
C(3)ꢀP(1)ꢀC(4)
110.8(1),
C(4)ꢀP(1)ꢀRu
107.1(1),
P(1)ꢀRuꢀP(2) 83.11(3), P(1)ꢀRuꢀP(4) 97.31(3).

A. Marinetti et al. / Journal of Organometallic Chemistry 624 (2001) 162–166
165
Finally, the catalyst precursor used in entry 3 of
Table 1 was checked by 31P-NMR. The reaction be-
tween 3c and the dimeric [(benzene)RuCl2)]2 complex at
room temperature is slow.
procedure [1b]. The cyclic sulfates have been prepared
according to Ref. [1b].
3.1. Synthesis of the 1,2-bis(phosphetano)ethane
bis-borane complexes 4
The synthesis of 4c is given hereafter as a representa-
tive example.
A solution of l,2-bis(phosphino)ethane (0.30 g, 3.2
mmol) in THF (40 ml) was cooled to −78°C and
n-BuLi (2.4 M solution in hexane, 2.8 ml, 6.7 mmol.)
was added. After warming to room temperature, the
yellow solution was stirred for 1.5 h and added then to
After 3 h some free ligands are still present, together
with equal amounts of the Cl2Ru(3c)2 complex 5 and
two other species whose structures are tentatively as-
signed as the non-chelated complexes (ben-
zene)RuCl2(3c) (l31P at 68.7 and 23.7 ppm, JPꢀP=27
Hz) and [(benzene)RuCl2]2(3c) (l31P=66.2 ppm). Ad-
dition of NH4PF6, which should favour cationic
chelated species through Cl−/PF6− exchange, did not
change the product ratios in the reaction mixture
significantly.
a
solution of (R,R)-l,3-dicyclohexyl-l,3-propanediol
cyclic sulfate (2.0 g, 6.6 mmol) in THF (200 ml) at
−78°C. The reaction mixture was allowed to warm to
room temperature and stirred for about 1 h. After
cooling to −78°C, s-BuLi (1.3 M solution, 5.2 ml, 6.7
mmol) was added. The reaction mixture was warmed
up to room temperature and stirred for 3 h. Then,
BH3·SMe2 complex (1 ml, 10 mmol) was added. After
hydrolysis, the crude mixture was concentrated under
vacuum. Ether was added, the organic phase was
washed with water and dried over MgSO4. The colour-
less solution was evaporated and the residue chro-
matographed on alumina with cyclohexane–ether 98:2
as eluent. The 1,2-bis((S,S)-2,4-dicyclohexylpho-
sphetano)ethane bis(borane) complex was obtained as a
colourless solid (1.1 g, 65% yield) and recrystallised
from an ether–pentane mixture. 31P-NMR (C6D6) l
57.7; 13C-NMR (C6D) l 17.4 (pseudo-t, PCH2), 25.8,
25.9, 26.2, 26.3, 26.4, 26.6 (CH2), 28.6 (pseudo-t, CH2),
30.5 (pseudo-t, CH2), 31.0 (pseudo-t, CH2), 32.6 (CH2),
In summary, the NMR data above point out that
any of the catalyst precursors examined to date afford
a
single, catalytically active ruthenium complex.
Diphosphine 3c displays a slow complexation rate as
well as propensity to form the dihalide complexes
X2Ru(3c)2 (X=halide). From previous studies [11] it
appears that dihalide-bis-phosphine ruthenium com-
plexes analogous to 5 are especially favoured in the case
of diphosphines giving five-membered chelate rings,
while they cannot be prepared with, for instance,
Josiphos or Binap ligands which form six- and seven-
membered chelate rings, respectively. The main draw-
back of the easy formation of 5 is a consequent
decrease of the catalytic activity. Thus, albeit the enan-
tiomeric excesses obtained, for instance, with the [(ben-
zene)RuCl2]2 precursor are rather satisfying, we guess
that the catalytic performances of the 1,2-bis(pho-
sphetano)ethanes 3 could be improved by the choice of
a more suitable catalyst precursor. Further studies are
in progress.
1
33.3 (CH2), 38.2 (CH), 38.6 (d, JCꢀP=36.1 Hz, CH),
1
39.0 (t, JCꢀP=2.6 Hz, CH), 39.5 (d, JCꢀP=38.0 Hz,
CH) ppm. Mass spectrum (EI): m/e 529 (M+−H,
40%), 516 (M+−BH3, 40%), 310 (85%), 82 (100%).
[h]D= +39 (c=0.5, CH2Cl2). Anal. Calc. for
C32H62B2P2: C, 72.46; H, 11.78. Found, C, 72.55; H,
11.86.
Following the same procedure, the 1,2-bis-((S,S)-2,4-
diisopropylphosphetano)ethane borane complex 4b was
obtained as a colourless solid (0.25 g, 31% yield) and
recrystallised from an ether–pentane mixture. 31P-
NMR (C6D6) l 56.6; 1H-NMR (C6D6) l 0.54 (d,
3. Experimental section
All reactions were performed under argon in dry
solvents. NMR spectra were recorded either on a
3
3JHꢀH=6.4 Hz, 2Me), 0.72 (d, JHꢀH=6.5 Hz, 2Me),
1
3
3
Bruker AM 200 (at 200.13 MHz for H and 50.32 MHz
0.88 (d, JHꢀH=6.4 Hz, 2Me), 1.07 (d, JHꢀH=6.5 Hz,
2Me), 1.2–2.5 (m, 16H); 13C-NMR (C6D6) l 17.0
(pseudo-t, PCH2), 20.0 (pseudo-t, Me), 20.4 (pseudo-t,
Me), 22.1 (2Me), 29.1 (CH), 29.5 (CH), 30.0 (pseudo-t,
for 31P) or on a Bruker 400 (at 400.13 MHz for H,
1
100.61 MHz for 13C and 161.97 MHz for 31P)
spectrometers.
1
1
(S,S)-2,4-Pentanediol,
(R,R)-2,6-dimethyl-3,5-hep-
CH2), 39.7 (d, JCꢀP=36.2 Hz, CH), 40.8 (d, JCꢀP=
tanediol and (R,R)-1,3-dicyclohexyl-1,3-propanediol
have been prepared via ruthenium/(S)-MeO-Biphep-
catalysed hydrogenations of 2,4-pentanedione, 2,6-
dimethyl-3,5-heptanedione and 1,3-dicyclohexyl-1,3-
propanedione, respectively, according to the published
38.1 Hz, CH) ppm. Mass spectrum (EI): m/e 342
(M+−2BH3, 5%), 314 (70%), 245 (80%), 190 (100%).
[h]D= +62 (c=0.5, CH2Cl2). Anal. Calc. for
C20H46B2P2: C, 64.9; H, 12.53. Found, C, 64.79; H,
12.42.

166
A. Marinetti et al. / Journal of Organometallic Chemistry 624 (2001) 162–166
3
3
Following the experimental procedure above, the 1,2-
l 0.96 (t, J=7.3 Hz, 2Me), 1.29 (dd, J=16.5 and 7.4
bis((S,S)-2,4-dimethylphosphetano)ethane borane com-
plex 4a was obtained as a colourless solid (0.26 g, 32%
yield) after purification by column chromatography
(cyclohexane–ether 90:10 as eluent). Complex 4a was
recrystallised from an ether–pentane mixture. 31P-
Hz, 2Me) ppm.
3.3. Synthesis of the ruthenium complex 5
Two equivalents of the bis(phosphetano)ethane 3c
(50 mg, 0.1 mmol) were added to a solution of the
Cl2Ru(PPh3)2 complex (43 mg, 0.05 mmol) in CH2Cl2
(1.5 ml) at room temperature. Complex 5 was formed
quantitatively after 1 h at room temperature. It was
isolated as a yellow solid after addition of pentane to
the crude reaction mixture. Complex 5 was character-
1
NMR (CDCl3) l 57.7 (JPꢀB=47 Hz); H-NMR (400.13
3
3
MHz, CDCl3) l 1.24 (dd, JHꢀH=7.4 Hz, JHꢀP=15.4
3
3
Hz, 2Me), 1.29 (dd, JHꢀH=7.1 Hz, JHꢀP=18.4 Hz,
2Me), 2.0–2.1 (m, 4H), 2.3 (m, 4H), 2.4–2.5 (m, 2H),
2.6–2.7 (m, 2H); 13C-NMR (100.61 MHz, CDCl3) l
14.3 (Me), 15.5 (Me), 16.0 (pseudo-t, CH2), 26.3 (d,
1JCꢀP=38.9 Hz, CH), 27.0 (d, JCꢀP=38.5 Hz, CH),
1
1
ised by H- and 31P-NMR spectroscopy. Its structure
35.8 (pseudo-t, CH2) ppm. Mass spectrum (EI): m/e
230 (M+−2BH3, 5%), 202 (80%), 160 (100%). [h]D=
−23 (c=1, CH2Cl2). Anal. Calc. for C12H30B2P2: C,
55.88; H, 11.72. Found, C, 55.64; H, 11.87.
was established by X-ray diffraction studies.
References
3.2. Remo6al of the bis(phosphetano)ethanes 3 from
their borane complexes
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General procedure. The 1,2-bis(phosphetano)ethane
bis(borane) complexes 4b or 4c (0.40 mmol) were re-
acted with 1,4-diazabicyclo[2.2.2]octane (160 mg, 1.4
mmol) in benzene (3 ml) at 60°C for 6 h. The reaction
mixture was diluted with benzene (5 ml) and directly
chromatographed under argon on a short alumina
column with cyclohexane–ether 99:1 as eluent.
1,2-Bis((S,S)-2,4-diisopropylphosphetano)ethane, 3b,
was obtained in quantitative yield as a colourless oil.
1
31P-NMR (C6D6) l 22; H-NMR (400.13 MHz, C6D6)
3
3
l 0.71 (d, JHꢀH=6.4 Hz, 2Me), 0.86 (d, JHꢀH=6.5
Hz, 2Me), 0.96 (d, 3JHꢀH=6.5 Hz, 2Me), 1.15 (d,
3JHꢀH=6.6 Hz, 2Me), 1.6–2.2 (m, 12H), 2.4–2.5 (m,
2H) 2.6–2.7 (m, 2H); 13C-NMR (100.6 MHz, C6D6) l
1
18.8 (Me), 19.2 (d JCꢀP=10.5 Hz, PCH2), 19.9 (Me),
20.2 (Me), 20.4 (t, JCꢀP=5.8 Hz, Me), 29.4 (CH), 30.4
(t, JCꢀP=8.0 Hz, CH), 33.2 (CH2), 34.6 (CH), 36.2
(CH) ppm.
1,2-Bis((S,S)-2,4-dicyclohexylphosphetano)ethane,
3c, was obtained in 87% yield as a colourless solid.
31P-NMR (C6D6) l 23.3; 13C-NMR (62.9 MHz, C6D6,
selected data) l 20.9 (d, 1JCꢀP=11.0 Hz, PCH2), 34.3 (t,
JCꢀP=3.3 Hz, CH), 36.5 (t, JCꢀP=3.7 Hz, CH), 40.1
(CH), 40.9 (t, JCꢀP=7.5 Hz, CH) ppm.. Mass spectrum
(EI): m/e 502 (M+, 5%), 474(10%), 237 (PC15H26, 20%),
311(20%), 81 (100%). [h]D= +266 (c=0.5, CH2Cl2).
Anal. Calc. for C32H56P2: C, 76.45; H, 11.23. Found: C,
76.35; H, 11.37.
1,2-Bis((R,R)-2,4-dimethylphosphetano)ethane, 3a,
has been displaced from its borane complex by heating
with DABCO (two equivalents) for 6 h in C6D6. It has
been characterised by NMR in the crude mixture:
31P-NMR (C6D6) l 26.3; 1H-NMR (400.13 MHz, C6D6)
[10] M. Kitamura, N. Yoshimura, R. Kanda, Noyori, Tetrahedron
55 (1999) 8769.
[11] R.M. Stoop, C. Bauer, P. Setz, M. Wo¨rle, T.Y.H. Wong, A.
Mezzetti, Organometallics 18 (1999) 5691.
.