Carbosilane dendrimers peripherally functionalized with P-stereogenic diphosphine ligands and related rhodium complexes

Article abstract of DOI:10.1016/j.jorganchem.2009.02.002

The first carbosilane dendrimer functionalized with P-stereogenic diphosphine ligands was prepared along with its cationic rhodium derivative. A mononuclear rhodium model compound was also synthesized. Both species were used as catalysts in the hydrogenation of dimethylitaconate and the results compared with those obtained with the related rhodium-containing P-stereogenic monophosphine dendrimers.

Full text of DOI:10.1016/j.jorganchem.2009.02.002

Journal of Organometallic Chemistry 694 (2009) 1938–1942
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Carbosilane dendrimers peripherally functionalized with P-stereogenic
diphosphine ligands and related rhodium complexes
*
*
L.I. Rodríguez , O. Rossell , M. Seco, G. Muller
Departament de Química Inorgànica, Universitat de Barcelona, Martí i Franquès, 1-11, 08028 Barcelona, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 January 2009
Received in revised form 2 February 2009
Accepted 3 February 2009
Available online 13 February 2009
The first carbosilane dendrimer functionalized with P-stereogenic diphosphine ligands was prepared
along with its cationic rhodium derivative. A mononuclear rhodium model compound was also synthe-
sized. Both species were used as catalysts in the hydrogenation of dimethylitaconate and the results com-
pared with those obtained with the related rhodium-containing P-stereogenic monophosphine
dendrimers.
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
Dendrimer
Chirality
Rhodium
Catalysis
1. Introduction
Here we report efficient synthesis of the first P-stereogenic
diphosphine-functionalized dendrimer, the corresponding rho-
The synthesis of efficient, catalytically active species is one of
the most studied areas in the chemistry of dendrimers, and in par-
ticular in the field of asymmetric dendrimer catalysis [1]. Several
strategies have been envisaged for inducing chirality in dendri-
mers. One of them involves the attachment of a chiral phosphine
onto the surface of the dendrimer. Following this method, we have
recently described the first examples of carbosilane dendrimers
peripherally functionalized with P-stereogenic monophosphines
2, 3, and the corresponding model compound 1 [2] (Chart 1).
These supramolecules are easily able to incorporate different
dium complex and preliminary results of its catalytic behaviour.
A mononuclear metal model was also prepared for comparison.
2. Results and discussion
2.1. Synthesis of diphosphine-functionalized dendrimers
One way to graft phosphine ligands onto a carbosilane dendri-
mer is to attack dendrimers containing peripheral SiR2Cl units with
the lithium salt LiCH2PR2 [6]. If the arms of the dendrimer are func-
tionalized with SiRCl2 groups, then the resulting species will dis-
play diphosphine ligands on the surface.
Our first attempts at obtaining dendrimers functionalized with
chiral diphosphines consisted of making dendrimer 4 react with
LiP–BH3 (Scheme 1). However, the results were not satisfactory
due to the incomplete functionalization of all the arms, probably
because of strong steric congestion in the molecular system.
In order to overcome this problem, we decided to use compound
5 as a starting dendrimer. Compound 5 contains –Si(CH3)2CH2CH2–
spacers, which were expected to reduce the steric hindrance. In this
case, we were able to isolate the dendrimer 6 (Scheme 1).
Dendrimer 6 was obtained along with P–BH3. After deprotec-
tion with morpholine, 7 and the excess of free monophosphine P
were separated by column chromatography (Chart 2).
metal fragments, such as PdCl(g
3-2-MeC3H4), RhCl(cod) or
RuCl2(p-cymene), to afford species with potential catalytic behav-
iour. Thus, the palladium complexes proved to be excellent cata-
lysts in the asymmetric hydrovinylation of styrene, both in
organic solvents [2] and in scCO2 [3], in terms of selectivity and
enantiomeric excess of 3-phenyl-1-butene. However, the rhodium
compounds were less effective in the hydrogenation of dimethyli-
taconate [4]. With the goal of improving the catalytic results of this
latter process, we decided to synthesize analogous carbosilane
dendrimers with peripheral chiral P-stereogenic diphosphines, in-
stead of monophosphines. This idea was drawn from the literature,
which shows that, in general, diphosphine ligands are better cata-
lysts, in the presence of metal salts, than the corresponding mono-
phosphines in processes such as hydrogenation and others [5].
Following the same strategy, the model compound 8 was pre-
pared from commercial Me2SiCl2. Compound 8 appeared in all at-
tempts mixed with the compound resulting from the substitution
* Corresponding authors.
E-mail address: oriol.rossell@qi.ub.es (O. Rossell).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.02.002

L.I. Rodríguez et al. / Journal of Organometallic Chemistry 694 (2009) 1938–1942
1939
Me
Cl
Me
Me
Me
Cl
Me
Si
Rh
Si
Rh
Si
P
P
R
Ph
R
Ph
4
1
2
R =
Me
Cl
Me
Me
Si
Me
Si
Rh
Si
Si
P
Me
R
Ph
2
4
3
Chart 1.
of only one chloride ligand. Column chromatography allowed us to
separate the products. However, in spite of numerous attempts, the
excess of P–BH3 could not be eliminated, either in its protected
form or after treatment with morpholine. Thus, 9 was obtained
with a 56% yield, and contained about 15% P (Chart 2).
2.2. Synthesis and characterization of cationic rhodium complexes
The synthesis of cationic complexes of 7 and 9 requires the prior
preparation of a thf solution of the appropriate rhodium (I) salt ob-
tained by reaction of [Rh(l-Cl)(g
4-cod)]2 with silver triflate. AgOTf
The presence of a C2 symmetry element justifies that 9 shows
only one signal (d = ꢀ29.2 ppm) in the 31P NMR spectrum. For
the same reason, the methyl groups are homotopic and conse-
quently they appear in the 1H NMR spectrum as a singlet
acts as a halide abstractor. After filtering the AgCl, the solution was
added to a solution of 7 or 9 and stirred for several minutes. Com-
pound 10 precipitated as an orange solid, whereas the complex
[Rh(cod)P2]OTf that was also formed remained in solution, allowing
the obtention of 10 in pure form. Metallodendrimer 11 was more
soluble and was only isolated after the addition of diethyl ether
to the mixture. In both cases high yields were obtained (Chart 3).
The 31P NMR spectrum of 10 contains one signal (d = 22.3 ppm,
1JPRh = 145.6 Hz) shifted downfield in relation to the free ligand 9
(d = ꢀ29.2 ppm), as expected. That of 11 shows two doublets, as-
signed to the two different phosphorous nuclei in the four diphos-
phine units. The 1H NMR spectrum of 10 displays two broad signals
derived from the four protons in the CH groups of the diene li-
gands, since two protons are homotopic with respect to the other
two. An identical situation is observed for the protons in the
CH2P unit. One singlet is seen (d = ꢀ1.10 ppm) and attributed to
the equivalent methyl groups. Interestingly, the four CH protons
from the diene in each arm are different from those in the metal-
lodendrimer 11, due to the lack of axial symmetry in the diphos-
phine fragment, although only two broad signals were actually
seen in the spectrum.
(d = ꢀ0.47 ppm) and as
a
triplet in the 13C NMR spectrum
3
(d = ꢀ0.7 ppm, JCP = 4.5 Hz). However, the protons bound to the
same methylenic carbon are diastereotopic with respect to each
other, but they are homotopic in relation to those of the remaining
CH2P group. This situation results in an AA0BB0 spin system. In the
1H NMR spectrum these protons appear as a pseudoquadruplet,
2
due to the overlapping of two pseudotriplets (2JHH ꢁ JHP). Since
the starting phosphine, P–BH3, displays S configuration, and
assuming that no inversion in the stereogenic centre takes place
during the synthesis of 8 or the borane deprotection process, the
two P-sterogenic centres of the diphosphine in the model com-
pound should have the same configuration, that is, they give rise
to the isomer l. In the hypothetical formation of the isomer
meso-(R, S) (isomer u), the symmetry plane generated in this iso-
mer would make both methyl groups diastereotopic and conse-
quently, potentially distinguishable in the NMR spectrum. For 9,
no evidence of formation of the isomer meso-(R, S) was detected,
which, according to Mezzeti and coworkers [7], indicates that the
diastereomers ratio l:u is at least 99:1.
Degradation of 11 in solution precluded us from taking its 13C
NMR spectrum. For 10, the most interesting features were the
two bands assigned to the CH groups of the diene, in accordance
with the previous discussion. The 19F NMR spectra of both com-
plexes showed a unique signal at ꢀ79 ppm, corroborating the pres-
ence of the triflate ion. ESI(+) mass spectrometry confirmed the
identity of the two compounds.
The mass spectrum for 10 contained the molecular peak along
with [MꢀOTf]+ and [MꢀOTfꢀcod]+ fragments. In that for 11, peaks
corresponding to [Mꢀ3OTf]3+ and [Mꢀ4OTf]4+ units were identi-
fied, as well as the successive loss of the ligand cyclooctadiene. It
should be noted that for the last fragments, the separation among
consecutive peaks was 1/3 or 1/4, respectively, of the massic unity
(amu) confirming the charge of the ion.
The 31P NMR spectrum of 7 reveals two very close singlets
(d = ꢀ29.4 and ꢀ29.7 ppm) due to the lack of the C2 symmetry axis.
Although the four diphosphine groups in the dendrimer are equiv-
alent to each other, each diphosphine is formed of two P-stereo-
genic groups. So, in spite of showing the same S configuration,
they do not have identical environments. The same explanation
is valid for the other nuclei. For example, the four protons in the
Si{CH2P}2 are now different and they appear as a multiplet
(d = 1.04–0.92 ppm) in the 1H NMR spectrum.
The MALDI-TOF spectrum of 7 and 9 unambiguously confirmed
the formation of the new species and showed the molecular peak
with no traces of incomplete functionalization.

1940
L.I. Rodríguez et al. / Journal of Organometallic Chemistry 694 (2009) 1938–1942
Scheme 1.
ized with chiral monophosphines showed that the rhodocom-
plexes 1, and were active in terms of conversion,
R
Ph
R
Ph
2
3
P
P
P
P
Me
Me
Me
Si
although the enantiomeric excesses produced were zero in all
cases [4]. In this paper we have explored the catalytic behav-
iour of 10 and 11 in the same process in order to compare their
catalytic results with those reported for chiral monophosphines.
Dichloromethane was the solvent used, since the model com-
pound was insoluble in thf. The conversion was monitored by
GC and the results are listed in Table 1. For comparison, we
have included results obtained previously with neutral rhodium
dendrimers.
Me
Si
Si
Si
Me
R
Ph
R
Ph
4
7
9
Chart 2.
Table 1 shows that the conversion decreases in going from the
neutral to the cationic derivatives. However, for the latter, the
enantioselectivity is no longer zero, though it is still low. Mezzetti
et al. have described similar results with other diphosphines [8]. A
dendrimeric positive effect in terms of activity was also observed.
Thus, the metallodendrimer is approximately three times more ac-
tive than the model compound. We assume that in the model com-
pound the formation of species like [Rh(diphosphine)2]+ would
block access to the active centre. However, this mechanism is
2.3. Hydrogenation of dimethylitaconate
Asymmetric hydrogenation of dimethylitaconate under mild
conditions has previously been achieved by using rhodium or
ruthenium complexes generated in situ via reaction of metal
complexes with monodentate or bidentate chiral phosphines.
Our previous results that made use of dendrimers functional-

L.I. Rodríguez et al. / Journal of Organometallic Chemistry 694 (2009) 1938–1942
1941
4
R
Ph
Rh
R
Ph
R
P
P
Me
Me
Me
Me
Si
Rh
Si
Si
(OTf)
OTf
4
Si
P
P
Me
R
Ph
Ph
4
10
11
Chart 3.
Table 1
Hydrogenation of dimethylitaconate using catalytic precursors 1, 2, 3, 10 and 11.
4.2. Synthesis 7
Catalytic precursor
t (h)
Conversion (%)
TOF (hꢀ1
)
e.e.
The phosphine–borane P–BH3 (0.949 g, 3.271 mmol) was dis-
solved in 20 ml of thf and cooled to ꢀ78 °C. Sec-butyllithium
(2.40 ml of 1.3 M cyclohexane/hexane solution, 3.120 mmol) was
added slowly. After stirring the violet mixture for 2 h, a precooled
solution of dendrimer 5 (0.280 g, 0.297 mmol) in thf was added.
The temperature was maintained at ꢀ78 °C for 3 h and after that
the mixture was left stirring for 14 h, slowly reaching room tem-
perature. Afterwards, 20 ml of HCl 0.5 M was added and the thf
was removed in vacuo. The remaining aqueous suspension was ex-
tracted with dichloromethane (3 ꢂ 10 ml) and the combined or-
ganic portions dried with anhydrous sodium sulphate. After
evaporating the CH2Cl2 to dryness, the crude product was dis-
solved in morpholine (20 g, 20 ml) and stirred for 14 h at room
temperature. Morpholine was then removed and the crude product
was passed through a short column of alumina with toluene as the
eluent. A mixture of dendrimer 7 and the free phosphine P was ob-
tained after evaporating the toluene. The product was purified by
flash chromatography under N2 in a silica gel column eluting with
hexane:thf 10:2. The target compound was finally obtained as a
white foam. Yield: 0.250 g (29%). 31P{1H} NMR (101.3 MHz, CDCl3,
298 K), d (ppm): ꢀ29.4 (s), ꢀ29.7 (s). 1H NMR (400.1 MHz, CDCl3,
298 K), d (ppm): 7.34–7.06 (m, Ar, 112H), 1.04–0.92 (m, CH2P,
16H), 0.34–0.24 (m, 1CH2Si, 8H), 0.20–0.14 (m, 2CH2Si, 8H), 0.06–
(ꢀ0.18) (m, 3CH2Si + 4CH2Si, 16H), ꢀ0.24 (s, 1CH3Si, 12H), ꢀ0.26
(s, 1’CH3Si, 12H), ꢀ0.51 (s, 2CH3Si, 12H). 13C{1H} NMR (100.6 MHz,
CDCl3, 298 K), d (ppm): 147.9–127.1 (m, Ar), 13.2–12.6 (m, CH2P),
1
1
2
2
2
2
2
61.2
98.1
94.4
68.6
13.7
46.7
306
245
236
171
7
ꢃ0
ꢃ0
2
3
10
11
ꢃ0
ꢃ0
7.6 (S)
7.8 (S)
23
Catalytic conditions for the neutral complexes: [Rh]/substrate 1:500; T = 20 °C;
10 bar H2(g); 20 mL thf. Catalytic conditions for cationic complexes: [Rh]/substrate
1:100; T = 20 °C; 10 bar H2(g); 20 mL CH2Cl2.
much less probable in 11 because of the greater volume of the
dendrimer.
3. Conclusion
The first dendrimer containing P-sterogenic diphosphines has
been synthesized along with the model compound 9. The reaction
of these species with the dinuclear rhodium complex [Rh(l -
-Cl)(g4
cod)]2 in the presence of silver triflate yielded the corresponding
metallated species, which were tested as catalysts in the hydroge-
nation of dimethylitaconate. The complexes proved to be catalyti-
cally active and, in terms of enantiomeric excesses, both species
improved on previous results involving rhodium dendrimers func-
tionalized with chiral monophosphines.
3
8.0 (pt, JCP ꢁ 3 Hz, 4CH2Si), 7.2 (s, 2CH2Si), 6.5 (s, 3CH2Si), 2.9 (s,
3
1CH2Si), ꢀ3.2 (pt, JCP = 4.9 Hz, 2CH3Si),ꢀ4.3 (s, 1CH3Si), ꢀ4.3 (s,
4. Experimental
1’CH3Si). 29Si{1H} NMR (49.7 MHz, CDCl3, 298 K), d (ppm): 9.13 (s,
Si0), 5.84 (s, Si1), 3.98 (m, Si2). MS (MALDI-TOF, DBH, m/z):
2858.5 (2858.1 calculated) [M+H]+.
4.1. General data
All manipulations were performed under purified nitrogen
using standard Schlenk techniques. All solvents were distilled from
appropriate drying agents. 1H, 13C{1H}, 31P{1H}, 19F{1H} and
29Si{1H} were obtained using Bruker DXR 250, Varian Unity 300
and Varian Mercury 400 spectrometers. Two-dimensional NMR
spectra were recorded on a Varian Mercury 400 spectrometer.
Chemical shifts are reported in ppm relative to external standards
(SiMe4 for 1H, 13C and 29Si, CF3COOH for 19F and 85% H3PO4 for 31P)
and coupling constants are given in Hz. MS ESI(+) spectra were re-
corded using an LC/MSD-TOF (Agilent Technologies) spectrometer.
MS MALDI-TOF spectra were recorded with a Voyager DE-RP
(Perseptive Biosystems) spectrometer using DBH (2,5-dihydroxy-
benzoic acid). Conversions and enantiomeric excesses were deter-
4.3. Synthesis 9
The phosphine–borane, P–BH3, (0.477 g, 1.644 mmol) was dis-
solved in 20 ml of thf and cooled to ꢀ78 °C. Sec-butyllithium
(1.235 ml of 1.3 M cyclohexane/hexane solution, 1.605 mmol)
was added slowly. After stirring the violet mixture for 2 h, Me2SiCl2
was added (0.095 ml, 1.064 g/ml, 0.783 mmol). The temperature
was maintained at ꢀ78 °C for 3 h and after that the mixture was
left stirring for 14 h, slowly reaching room temperature. After-
wards, 20 ml of HCl 0.5 M was added and the thf was removed in
vacuo. The remaining aqueous suspension was extracted with
dichloromethane (3 ꢂ 10 ml) and the combined organic portions
dried with anhydrous sodium sulphate. The monosubstituted
product was removed by flash chromatography (SiO2, CH2Cl2).
The mixture formed by the excess of P–BH3 and the model com-
pound was dissolved in morpholine (15 g, 15 ml) and stirred for
14 h at room temperature. Morpholine was then removed and
the crude product was passed through a short column of alumina
mined by GC using
chromatograph (30-m Chiraldex DM column) with an FID detector.
[Rh( -Cl)(
4-cod)]2, P–BH3, and the starting chlorocarbosilane
a Hewlett-Packard 5890 Series II gas
l
g
dendrimers were prepared as previously described [4]. Other re-
agents were used as received from commercial suppliers.

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L.I. Rodríguez et al. / Journal of Organometallic Chemistry 694 (2009) 1938–1942
0
with toluene as the eluent to eliminate the borane–morpholine ad-
duct. The product was obtained as a white foam. Yield: 0.290 g
(56%) (15% mol impure with P). 31P{1H} NMR (101.3 MHz, CDCl3,
298 K), d (ppm): ꢀ29.2 (s). 1H NMR (250.1 MHz, CDCl3, 298 K), d
(ppm): 7.37–7.10 (m, Ar, 28H), 1.01 (pq, J = 16.3 Hz, 4H), ꢀ0.47 (s,
CH3Si, 6H). 13C{1H} NMR (100.6 MHz, CDCl3, 298 K), d (ppm):
(ꢀ1.00) (m, CH2Si, 32H), ꢀ0.37 (s, 1CH3Si + 1 CH3Si, 24H), ꢀ1.10
(s, 2CH3Si, 12H). 19F{1H} NMR (376.4 MHz, CDCl3, 298 K),
d
(ppm): ꢀ78.2 (s, OTf). MS (ESI(+), m/z): 1283.3 (1283.3 calculated)
[Mꢀ3OTf]3+, 1247.3 (1247.3 calculated) [Mꢀ3OTfꢀcod]3+, 925.3
(925.3 calculated) [Mꢀ4OTf]4+
,
898.2 (898.2 calculated)
[Mꢀ4OTfꢀcod]4+
,
871.2 (871.2 calculated) [Mꢀ4OTfꢀ2cod]4+
,
1
3
148.0–127.1 (m, Ar), 14.9 (dd, JCP = 30.1 Hz, JCP = 4.5 Hz, CH2P),
844.2 (844.2 calculated) [Mꢀ4OTfꢀ3cod]4+, 817.2 (817.2 calcu-
3
ꢀ0.7 (t, JCP = 4.5 Hz, CH3Si). MS (MALDI-TOF, DBH, m/z): 608.2
lated) [Mꢀ4OTfꢀ4cod]4+
.
(608.2 calculated) [M]+Å.
4.6. Catalytic reactions
4.4. Synthesis 10
Hydrogenation reactions were performed in a stainless-steel
autoclave fitted with an external jacket connected to an isobutanol
bath. The temperature was controlled using a thermostat to
0.5 °C. Internal temperature was monitored using a thermopar
coupled to a digital recorder, whereas the internal pressure was
continuously measured as a function of time with a Linseis L-200
recorder.
[Rh(l-Cl)(g
4-cod)]2 (0.117 g, 0.243 mmol) was dissolved in
10 ml of thf and AgCF3SO3 was added (0.137 g, 0.486 mmol). The
mixture was stirred at room temperature for 1 h in the dark and
the AgCl that formed was removed by filtration through Celite.
The solution was added to a solution of model compound 9
(0.290 g of the impure mixture) in 10 ml of thf and the resulting
mixture was stirred for 30 min. The orange precipitate was filtered
off, washed three times with thf and recrystallized in CH2Cl2/
Dimethyl itaconate (10ꢀ3 mol) and the precursor complex
(10ꢀ5 mol [Rh]) were dissolved in 10 ml of CH2Cl2. The resulting
solution was immediately placed in the autoclave, which had pre-
viously been purged by successive vacuum/nitrogen cycles and
maintained at 20 °C. Hydrogen was admitted until a pressure of
10 bar was reached. After the time indicated for each reaction,
the autoclave was slowly depressurized and the quantitative distri-
bution of products and their e.e. were determined by GC analysis.
diethyl ether. Yield: 0.301 g (70%). 31P{1H} NMR (101.3 MHz,
1
CD2Cl2, 298 K),
d
(ppm): 22.3 (d, JPRh = 145.6 Hz). 1H NMR
(400.1 MHz, CD2Cl2, 298 K), d (ppm): 7.86–6.56 (m, Ar, 28H), 5.19
(s (br), CH, 2H), 4.00 (s (br), CH, 2H), 2.96–2.56 (m, CH2, 4H), 2.15
(m, CH2, 4H), 1.31 (s (br), CH2P, 2H), 0.56 (m, CH2P, 2H), ꢀ1.10 (s,
CH3Si, 6H). 13C{1H} NMR (100.6 MHz, CD2Cl2, 298 K), d (ppm):
145.6–119.7 (m, Ar + OTf), 100.9 (s (br), CH), 98.4 (s (br), CH),
34.7 (s (br), CH2), 27.1 (s (br), CH2), 13.6 (s (br), CH2P), 0.2 (s (br),
CH3Si). 19F{1H} NMR (376.4 MHz, CD2Cl2, 298 K), d (ppm): ꢀ79.3
(s, OTf). MS (ESI(+), m/z): 819.2 (819.2 calculated) [MꢀOTf]+,
711.1 (711.1 calculated) [MꢀOTfꢀcod]+.
Acknowledgement
Financial support for this research was provided by the DGICYT
(Project CTQ2006-02362/BQU).
4.5. Synthesis 11
References
[Rh(l-Cl)(g
4-cod)]2 (0.052 g, 0.105 mmol) was dissolved in
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10 ml of thf and AgCF3SO3 was added (0.054 g, 0.211 mmol). The
mixture was stirred at room temperature for 1 h in the dark and
the AgCl formed was removed by filtration through Celite. The
solution was added to a solution of the dendrimer 7 (0.151 g,
0.053 mmol) in 10 ml of thf and stirred for 30 min. The resulting
orange solution was concentrated to a small volume and diethyl
ether was added. The orange solid was filtered off, washed twice
with Et2O and dried. Yield: 0.179 g (79%). 31P{1H} NMR
[4] L.I. Rodríguez, O. Rossell, M. Seco, G. Muller, J. Organomet. Chem. 692 (2007)
851.
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Organometallics 18 (1999) 5691.
1
(101.3 MHz, CDCl3, 298 K), d (ppm): 21.7 (d, JPRh = 144.7 Hz),
1
21.5 (d, JPRh = 144.9 Hz). 1H NMR (400.1 MHz, CDCl3, 298 K), d
(ppm): 8.04–6.43 (m, Ar, 112H), 5.15 (s (br), CH, 8H), 3.97 (s (br),
CH, 8H), 2.93–2.57 (m, CH2, 16H), 2.16 (s (br), CH2, 16H), 1.37 (s
(br), CH2P, 8H), 0.57 (m, CH2P, 4H), 0.43 (m, CH2P, 4H), 0.29–
[8] F. Maienza, F. Santoro, F. Spindler, C. Malan, A. Mezzetti, Tetrahedron:
Asymmetry 13 (2002) 1817.