Highly enantioselective Rh-catalyzed hydrogenations with heterocombinations of pentafluorobenzyl- and methoxybenzyl-derived binaphthyl phosphites

Article abstract of DOI:10.1016/j.tetlet.2007.11.201

The Rh-catalyzed hydrogenations of dimethyl itaconate and methyl acetamido acrylate using selected heterocombinations of pentafluorobenzyl- and methoxybenzyl-derived binaphthyl phosphites proved to be highly enantioselective (ee 93-99%). In these selected

Full text of DOI:10.1016/j.tetlet.2007.11.201

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 755–759
Highly enantioselective Rh-catalyzed hydrogenations
with heterocombinations of pentafluorobenzyl- and
methoxybenzyl-derived binaphthyl phosphites
a
b,
a,
*
Benita Lynikaite , Jan Cvengros , Umberto Piarulli *, Cesare Gennari
ˇ a
´
a
`
Dipartimento di Chimica Organica e Industriale, Centro di Eccellenza C.I.S.I., Universita degli Studi di Milano, Via G. Venezian, 21, 20133 Milano, Italy
b
`
Dipartimento di Scienze Chimiche e Ambientali, Universita degli Studi dell’Insubria, Via Valleggio, 11, 22100 Como, Italy
Received 20 November 2007; accepted 30 November 2007
Available online 4 December 2007
Abstract
The Rh-catalyzed hydrogenations of dimethyl itaconate and methyl acetamido acrylate using selected heterocombinations of penta-
fluorobenzyl- and methoxybenzyl-derived binaphthyl phosphites proved to be highly enantioselective (ee 93–99%). In these selected cases
the Rh-heterocomplexes, which were formed in a statistical amount (ca. 50% by 31P NMR), turned out to be more active and selective
than the two homocomplexes.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Phosphites; Asymmetric hydrogenation; Rhodium; Enantioselective catalysis
In recent years, monodentate phosphorus ligands have
held the stage in asymmetric catalysis.1 In addition to their
outstanding activity and selectivity, comparable or even
superior to those of bidentate ligands, their convenient, fast
and practical synthesis from commercially available mate-
rials underlines their potential for industrial applications.2
Furthermore, the simple preparation of the phosphite,3
phosphonite4 and phosphoramidite5 functional groups,
which are the most frequently represented in monodentate
ligands, allows for a facile variation of their design.
(preorganization and rigidity) with the economy and effi-
ciency of monodentate ligands.6
The non-covalent assembly of monodentate ligands
reported so far in the literature relies mostly on coordinative
bonding of nitrogen to zinc7 or hydrogen bonding.8 More
recently, ionic interactions9 and the formation of inclusion
complexes10 were exploited to bring monodentate ligands
into close proximity. We felt intrigued by these approaches
and decided to investigate other non-covalent interactions,
that is, the perfluroarene-arene p–p interactions.
Still, the presence of a bidentate ligand is sometimes an
inevitable requirement for a successful (stereo)chemical
result. Supramolecular bidentate ligands, based on the
self-assembly of monodentate ligands possessing comple-
mentary functionalities, thus represent a clever solution
which combines the important features of bidentate ligands
Perfluroarene-arene p–p interactions were observed for
the first time in 1960.11 Their relevance in the solid state
has been confirmed several times since then,12 and they have
found widespread application in supramolecular design and
crystal engineering.13 The contribution of perfluoroarene-
arene p–p interactions to the efficient photodimerization
of olefins in the solid state was also investigated.14 On the
contrary, only a few papers report a quantitative descrip-
tion of the perfluoroarene-arene p–p interactions in solu-
tion,15 and a few applications supposedly exploited them
in solution-phase synthetic processes.16
*
Corresponding authors. Tel.: +39 025031 4091; fax: +39 025031 4072
(C.G.); tel.: +39 031238 6444; fax: +39 031238 6449 (U.P.).
E-mail addresses: umberto.piarulli@uninsubria.it (U. Piarulli), cesare.
gennari@unimi.it (C. Gennari).
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.11.201

756
B. Lynikaite et al. / Tetrahedron Letters 49 (2008) 755–759
In this Letter we report the synthesis of a series of novel
of the electron-poor phosphites (R)-9 and (S)-10, contain-
ing the pentafluorobenzyl moiety.
We thus synthesized a small library of eight electron-rich
phosphites (including six BINOL and two octahydro-
BINOL derivatives) and two electron-poor pentafluoro-
benzyl binaphthyl phosphites (Fig. 1).
perfluroarene- and methoxyarene-derived phosphites,
which were used to investigate (by 31P NMR) the possible
formation of supramolecular bidentate ligands in the
Rh-complexes via a non-covalent assembly induced by the
perfluroarene–methoxyarene p–p interactions. Homo- and
hetero-combinations of these ligands were used in enantio-
selective Rh-catalyzed hydrogenations of dimethyl itaco-
nate and methyl acetamido acrylate.
Two strategies were applied to the synthesis of the
designed binaphthyl phosphites (Scheme 1). The commer-
cially available methoxy substituted phenols were refluxed
with PCl3 to give the corresponding methoxy substituted-
phenyl dichlorophosphites (Route A). Subsequent reaction
with BINOL in the presence of Et3N furnished the elec-
tron-rich phosphites (R)-1–(R)-3 (Fig. 1), which were
obtained in moderate yields as pure compounds after
chromatographic purification.
In the case of methoxy substituted benzylalcohols, an
alternative approach (Route B) was followed. BINOL was
thus treated with PCl3 in the presence of a catalytic amount
of NMP17 to give the corresponding binaphthyl chloro-
phosphite, which was then reacted with benzylalcohols in
the presence of Et3N to afford phosphites (R)-4–(R)-8
(Fig. 1). The same protocol was used also for the synthesis
With the ligand library in our hands, we proceeded with
the 31P NMR investigations of perfluoroarene–methoxya-
rene p–p interactions in the rhodium complexes. At first,
the 31P NMR spectra of a single ligand in the presence of
Rh(acac)(eth)2 (L:Rh ratio = 2:1) were recorded revealing
the fast formation of the corresponding homocomplexes
L2Rh(acac) (Fig. 2). Then two different ligands (one elec-
tron-rich and the other electron-poor) were mixed in the
presence of the Rh source (ratio 1:1:1) and the formation
of two homocomplexes and one heterocomplex was
observed. The NMR spectra were recorded in a broad range
of solvents (CDCl3, THF-d8, CD3OD, i-PrOH/CD3OD,
CD2Cl2, toluene-d8, CCl4). In all these solvents, the observed
ratio was essentially statistical within the experimental error
(RhLALA:RhLBLB:RhLALB = 1:1:2), which means that
there is no detectable preference for the formation of the
heterocomplex due to the methoxyarene–perfluoroarene
interactions. In crystals featuring arene–perfluoroarene
interactions, the packing energy is dispersion-dominated
and coulombic terms are selective rather than quantitatively
predominant.12f This means that these interactions are likely
to be very weak in solution.15b,c
Although we did not observe any relevant p–p-interac-
tion between electron-rich and electron-poor ligands, we
screened the library, using both homo- and heterocombina-
tions of the ligands, in the Rh-catalyzed asymmetric hydro-
genation of prochiral alkenes. Typically, the substrate was
added to a stirred mixture of Rh(cod)BF4 (1 mol %) and
ligand [2 mol % (1:1 ratio in the case of heterocombina-
tions)] in dichloromethane and the mixture was hydro-
genated for 24 h at room temperature and pressure.
In the Rh-catalyzed asymmetric hydrogenation of
dimethyl itaconate, the use of homocombinations of
1. PCl3
OH
2. (R)-BINOL, Et3N
O
R
P O
Route A
O
R
R
O
1. PCl3, NMP
2.
OH
OH
OH
, Et3N
R
P O
Route B
O
Scheme 1. Synthesis of benzylalcohol- and phenol-derived binaphthyl
phosphites.
MeO
OMe
OMe
MeO
MeO OMe
OMe
OMe
OMe
MeO
MeO
OMe
O
O
O
O
O
P O
O
P O
P O
P O
P O
O
O
O
O
(R)-1
(39%)
(R)-2
(41%)
(R)-3
(17%)
(R)-4
(37%)
(R)-5
(64%)
MeO
MeO
O
MeO
MeO
OMe
OMe
OMe
F
F
F
F
F
F
F
O
O
O
O
F
O
P O
O
P O
P O
P O
P O
F
F
O
O
O
(R)-6
(74%)
(R)-7
(36%)
(R)-8
(45%)
(R)-9
(67%)
(S)-10
(64%)
Fig. 1. Library of phosphite ligands (isolated yields are reported in the brackets).

B. Lynikaite et al. / Tetrahedron Letters 49 (2008) 755–759
757
Fig. 2. Studies of perfluoroarene–alkoxyarene p–p interactions in the rhodium complexes by 31P NMR spectroscopy.
monodentate ligands gave the reaction product in high
yields (98–100%, Table 1). Methoxyphenol derived ligands
(R)-1, (R)-2 and (R)-3 gave the hydrogenated product in
high enantiomeric excesses (P97% ee, entries 1–3). A prod-
uct of lower enantiomeric purity was obtained with the
methoxybenzylalcohol derived ligands (R)-4–(R)-6 (70–
84% ee, entries 4–6). Only modest ee’s were obtained with
the octahydro-BINOL derivatives (R)-7 and (R)-8 (50–54%
ee, entries 7 and 8). The pentafluorobenzylalcohol derived
phosphite (R)-9 [or (S)-10] afforded the product in 92%
ee (entries 9 and 10).
for (R)-4, 92% ee for (R)-9 and 99% ee for the heterocom-
bination of (R)-4 and (R)-9, entry 17].
A similar trend was observed in the Rh(I)-catalyzed
asymmetric hydrogenation of methyl acetamido acrylate
with selected ligands (Table 2). Complete conversions and
modest enantiomeric excesses (74–88%) were achieved in
all cases. The heterocombination of ligands (R)-6 and
(R)-9 was the only case where a significantly higher enan-
tiomeric excess (93%, entry 13) was obtained in comparison
to the corresponding homocombinations.
In conclusion, we prepared a library of electron-rich
phosphites (including BINOL and octahydro-BINOL
derivatives) and one electron-poor pentafluorobenzyl
binaphthyl phosphite. Although no preferential formation
of the Rh-heterocomplexes was observed by 31P NMR
The ligand heterocombinations were then studied, and
also in these cases a complete conversion was observed in
most of the reactions. Enhanced activity and selectivity
were observed combining ligands (R)-4 and (R)-9 [84% ee
Table 1
Rh(I)-catalyzed asymmetric hydrogenations of dimethyl itaconate
Rh(cod)2BF4 / Ligand(s)
H2 (1 atm), DCM, rt, 24 h
COOCH3
COOCH3
H3COOC
H3COOC
*
Homocombinations
Conv. (%)
Heterocombinations
Conv. (%)
Entry
Ligand
ee (%) (abs. conf.)
Entry
Ligands
ee (%) (abs. conf.)
1
2
3
4
5
6
7
8
9
(R)-1
(R)-2
(R)-3
(R)-4
(R)-5
(R)-6
(R)-7
(R)-8
(R)-9
(S)-10
98
100
100
100
100
100
100
100
100
100
97 (R)
99 (R)
99 (R)
84 (R)
78 (R)
70 (R)
54 (R)
50 (R)
92 (R)
92 (S)
11
12
13
14
15
16
17
18
19
20
21
22
23
24
(R)-1/(R)-9
(R)-1/(S)-10
(R)-2/(R)-9
(R)-2/(S)-10
(R)-3/(R)-9
(R)-3/(S)-10
(R)-4/(R)-9
(R)-4/(S)-10
(R)-5/(R)-9
(R)-5/(S)-10
(R)-6/(R)-9
(R)-6/(S)-10
(R)-7/(R)-9
(R)-8/(R)-9
100
83
96 (R)
52 (R)
98 (R)
48 (R)
64 (R)
14 (R)
99 (R)
86 (R)
70 (R)
46 (R)
76 (R)
24 (R)
52 (R)
68 (R)
100
100
100
100
100
100
100
100
100
100
100
64
10

758
B. Lynikaite et al. / Tetrahedron Letters 49 (2008) 755–759
Table 2
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Rh(I)-catalyzed asymmetric hydrogenation of methyl acetamido acrylate
H
H
Rh(cod)2BF4 / Ligand(s)
H2 (1 atm), DCM, rt, 24 h
O
N
COOCH3
O
N
COOCH3
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Homocombinations
Heterocombinations
Conv. ee (%)
Entry Ligand Conv. ee (%) Entry Ligands
(%)
(abs.
(%)
(abs.
conf.)
conf.)
1
2
3
4
5
6
7
(R)-1
(R)-2
(R)-3
(R)-4
(R)-5
(R)-6
(R)-9
100
100
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100
100
100
100
86 (S)
86 (S)
8
9
(R)-1/(R)-9 100
(R)-2/(R)-9 100
(R)-3/(R)-9 100
(R)-4/(R)-9 100
(R)-5/(R)-9 100
(R)-6/(R)-9 100
84 (S)
86 (S)
86 (S)
86 (S)
88 (S)
93 (S)
82 (S) 10
88 (S) 11
84 (S) 12
80 (S) 13
74 (S)
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(the perfluoroarene–methoxyarene p–p interactions are
possibly too weak in solution), these ligands were tested
in the Rh-catalyzed asymmetric hydrogenations of
prochiral olefins (dimethyl itaconate and methyl acetamido
acrylate) with good results. In two cases, the heterocombi-
nation of one methoxybenzyl-derived binaphthyl phosphite
[(R)-4 or (R)-6] and the electron-poor pentafluorobenzyl-
derived binaphthyl phosphite [(R)-9] led to very high
enantiomeric excesses, significantly higher than those
obtained with the corresponding homocombinations; while
both homocomplexes (RhLALA and RhLBLB) were still
present (ca. 50%) in the precatalyst mixture, the hetero-
complex (RhLALB, ca. 50%) turned out to be more active
and selective, as already reported in many other
precedents.18
`
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van Leeuwen, P. W. N. M. Chem. Eur. J. 2007, 13, 3424–3430.
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Acknowledgements
We thank the European Commission [RTN Network
(R)Evolutionary Catalysis MRTN-CT-2006-035866] for
financial support and for a predoctoral fellowship (to B.
Lynikaite) and a postdoctoral fellowship (to J. Cvengros).
C. Gennari gratefully acknowledges Merck Research Lab-
oratories for the Merck’s Academic Development Program
Award. We also wish to thank Professor Franco Cozzi
(Milan University) for reading the manuscript.
12. For recent examples, see: (a) Gdaniec, M.; Jankowski, W.; Milewska,
M. J.; Polonski, T. Angew. Chem., Int. Ed. 2003, 42, 3903–3906; (b)
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