Highly enantioselective nickel-catalyzed hydrovinylation with chiral phosphoramidite ligands

Article abstract of DOI:10.1021/ja012099v

Readily available chiral phosphoramidites are a promising class of ligands for nickel-catalyzed asymmetric hydrovinylation of vinyl arenes. Cooperative effects are operative when ligands with more than one element of chirality are used. Choosing the prope

Full text of DOI:10.1021/ja012099v

Published on Web 01/08/2002
Highly Enantioselective Nickel-Catalyzed Hydrovinylation with Chiral
Phosphoramidite Ligands
Giancarlo Francio`,† Felice Faraone,‡ and Walter Leitner*,†
Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mu¨lheim an der Ruhr, Germany, and
Dipartimento Chimica Inorganica, Chimica Analytica e Chimica Fisica, UniVersita` degli Studi die Messina,
98166 Messina, Italy
Received September 4, 2001
The development of highly stereoselective carbon-carbon bond-
forming reactions continues to be one of the major challenges in
fine chemical synthesis.1 The nickel-catalyzed hydrovinylation is
a metal-mediated coupling reaction with a remarkable potential for
enantioselective synthesis.2,3 It comprises the formal addition of a
hydrogen atom and a vinyl group to a prochiral olefin and gives
access to high-value products in a very elegant and atom-efficient
way. Using ethene as the cheapest vinylic coupling partner, the
transformation results effectively in a chain elongation of two
carbon atoms simultaneously creating a stereogenic center in allylic
position (Scheme 1).
was investigated by the group of RajanBabu.10a Their results
suggested that the nature of the chloride-abstracting agent and the
presence of hemilabile donor groups had a synergistic influence
on the catalyst perfomance.10b Enantioselectivities of up to 80% ee
were obtained at substrate-to-nickel ratios of 70:1 under optimized
conditions.10b
In the search for new hydrovinylation catalysts that allow for
systematic modification of a readily accessible ligand framework,
we have turned our attention to chiral phosphoramidites.11 The
selection of the representative examples 5-7 was guided by the
rather heuristic design principle that an efficient ligand system for
asymmetric hydrovinylation should possess a P-N bond and
contain more than one element of chirality, one of them preferen-
tially being an atropisomeric unit. In addition, the set of ligands
should allow to assess the potential influence of additional donor
groups. The most significant results obtained with catalysts formed
from 5-7 and [Ni(allyl)Cl]2 and using NaBARF (BARF ) tetrakis-
[3,5-bis(trifluormethyl)phenyl]borate) as activator are summarized
in Table 1.12
Scheme 1
In contrast to many other enantioselective catalytic processes,4
ligand development for this particular reaction is still in its infancy.
The state of the art is defined by catalyst 4 containing a ligand that
The (Ra,RC) diastereomer of the new ligand 5 gave an active
catalytic system for the hydrovinylation of styrene 1a, but the optical
induction remained disappointingly low (entry 1). The C2 epimer
(Ra,SC)-5, however, provided reasonable enantioselectivity at mod-
erate conversion (entry 2). An even more dramatic difference in
activity and selectivity was observed for the two diastereomers of
the chloride-substituted ligand BINAPHOSQUIN (6).13 The (Ra,SC)
configuration in 6 led to an extremely active catalyst resulting
preferably in styrene dimerization and trimerization (entry 3). The
large tendency for oligomerization could not be suppressed even
at -78 °C or upon reducing the loading of (Ra,SC)-6/Ni. In contrast,
the other diastereomer (Ra,RC)-6 formed a less active catalyst,
yielding 2a very selectively with a remarkable ee of 87% (S) (entry
4). It is interesting to note that the two ligand systems 5 and 6 give
rise to products with the opposite preferred stereochemistry and
that the opposite configuration at C2 is required to obtain the highest
asymmetric induction.14
was found by serendipity in the pioneering investigations of Wilke
and co-workers.5 After activation with a suitable chloride-abstracting
reagent, this system provides high chemo- and stereoselectivities
in chlorinated solvents at very low temperatures3b and in compressed
CO26 or ionic liquid/CO2 systems7 around ambient temperature. The
scope of this “chiral pool”-derived ligand is, however, rather limited,
and only one enantiomer of the desired products 2a-d is readily
accessible. All attempts to optimize or simplify the ligand frame-
work of 4 have remained unsuccessful thus far.8
The only other promising class of Ni catalysts9 for asymmetric
hydrovinylation contains atropisomeric ligands such as MOP and
* Author for correspondence. E-mail: leitner@mpi-muelheim.mpg.de; fax:
+49-208-306 2993.
The results obtained with ligands 5-6 further substantiate the
cooperative effect of axial and central chirality and indicate also a
† Max-Planck Institut fu¨r Kohlenforschung.
‡ Universita` degli Studi die Messina.
9
736 VOL. 124, NO. 5, 2002 J. AM. CHEM. SOC.
10.1021/ja012099v CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Ni-Catalyzed Enantioselective Hydrovinylation of Vinyl Arenes 1a-d Using Chiral Phosphoramidites 5-7 as Ligands and NaBARF
as Activator12
selectivity [%]
T
[°C]
p (C H )
[bar]
t
[h]
conv.
[%]
ee (2)
[%]
2
4
entry
ligand
substrate
1/Ni
280
280
300
300
620
4600
5490
13270
1330
1330
2650
660
2
3
oligomersa
1
2
3
4
5
(Ra,RC)-5
(Ra,SC)-5
(Ra,SC)-6
(Ra,RC)-6
(Ra,SC,SC)-7
(Ra,SC,SC)-7
1a
1a
1a
1a
1a
1a
0
-30
44
49
12
1
2
2
2
4
4
16
2
0.3
4
5
99.7
13.5
100
33.1
100
89.2
82.7
100
99.5
28.4
83.4
100
85.4
93.3
<1
96.2
84.9
100
96.4
78.1
99.1
28.4
98.8
81.2
13.3
3.1
<1
0.4
4.3
<1
<1
9.1
0.9
<1
1.2
6.8
<1.5
3.7
100
3.7
8.1
<1
3.0
7.6 (R)
56.4 (R)
-
-32
-32
-70
-65
-65frt
0
12
87.2 (S)
94.8 (S)
91.1 (S)
91.4 (S)
76.2 (S)
89.7 (S)
67.7(S)
91.9 (S)
90.8 (S)
∼1
∼1
∼1
∼1
∼1
∼1
∼1
∼1
6a
6b
7
(Ra,SC,SC)-7
(Ra,SC,SC)-7
(Ra,SC,SC)-7
(Ra,SC,SC)-7
(Ra,SC,SC)-7
1a
1a
1b
1c
1d
12.7
<1
8b
9
-50
-70
-70
-30
<1
10
11
<1
4
12.0
a Oligomerization products of 1 and secondary hydrovinylation products of 2 and 3 are summarized as “oligomers”, see Supporting Information for
details. b NaAl[OC(CF3)2Ph]4 was used as activator.
strong influence of additional donor groups. The best result (87%
ee at -32 °C) was, however, achieved with ligand 6 containing
References
(1) (a) Applied Homogeneous Catalysis with Organometallic Compounds;
the weaker donor group at C8. We therefore extended our study to
Feringa’s ligand system 715 combining central and axial chirality
but lacking an obvious additional donor group.16
Cornils, B., Herrmann, W. A., Eds.; VCH: New York, 1996; Vols. 1 and
2. (b) Parshall, G. W. Homogeneous Catalysis; Wiley: New York, 1992.
(c) Trost, B. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 259. (d) Transition
Metals for Organic Synthesis: Building Blocks and Fine Chemicals;
Beller, M., Bolm, C., Eds.; Wiley: Weinheim, 1998; Vols. 1 and 2. (e)
ComprehensiVe Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A.,
Yamamoto, H., Eds.; Springer: Berlin, 1999; Vols. 1-3.
Employing the standard procedure under particularly mild
conditions, a Ni catalyst based on (Ra,SC,SC)-7 gave quantitative
conversion of 1a with 84.9% selectivity for the desired product 2a
and an excellent enantioselectivity of 94.8% (S) (entry 5). Moreover,
the catalyst system proved extremely efficient and remarkably robust
for the hydrovinylation of 1a. Almost 90% conversion and perfect
chemoselectivity was achieved within 4 h at -65 °C even at a
substrate-to-nickel ratio of 4600:1 (entry 6a). Further addition of
substrate to this reaction mixture led again to almost complete
conversion within 16 h (entry 6b) corresponding to a total turnover
number of ca. 8340. Chemo- and enantioselectivity remained
uniformly high under these conditions. At 0 °C, a styrene conversion
of 69% was reached at a substrate-to-nickel ratio of ca. 13000:1
within 30 min. These data correspond to an initial turnover
frequency approaching 18 000 h-1 even under the conservative
assumption that all Ni centers were available in active form.
Consumption of 1a was quantitative after a total reaction time of
2 h, but the reaction was slightly less selective at this temperature
(entry 7).
(2) (a) Bogdanovic´, B. AdV. Organomet. Chem. 1979, 17, 105. (b) Wilke, G.
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(3) For recent reviews see: (a) Jolly, P. W.; Wilke, G. In Applied
Homogeneous Catalysis with Organometallic Compounds; Cornils, B.,
Herrmann, W. A., Eds.; VCH: New York, 1996; Vol. 2, p 1024. (b)
RajanBabu, T. V.; Nomura, N.; Jin, J.; Radetich, B.; Park, H.; Nandi, M.
Chem. Eur. J. 1999, 5, 1963.
(4) Brunner, H.; Zettlmeier, W. Handbook of EnantioselectiVe Catalysis with
Transition Metal Compounds; VCH: Weinheim, 1993; Vol. II.
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50, 488.
(9) For selected examples of asymmetric hydrovinylation using palladium
catalysts see: (a) Bayersdo¨rfer, R.; Ganter, B.; Englert, U.; Keim, W.;
Vogt, D. J. Organomet. Chem. 1998, 552, 187. (b) Albert, J.; Cadena,
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(11) For selected recent examples highlighting the potential of this class of
ligands, see: (a) Feringa B. L. Acc. Chem. Res. 2000, 33, 346. (b) van
den Berg, M.; Minnaard, A. J.; Schudde, E. P.; van Esch, J.; de Vries, A.
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(12) The catalyst precursors were either preformed or formed in situ by addition
of ligands 5-7 to [Ni(allyl)Cl]2 in a 2:1 ratio. Substrates 1a-d were added
to their solutions in CH2Cl2, followed by activation of the catalyst
precursors with a slight excess of NaBARF at room temperature. The
resulting orange solutions were cooled to the desired temperature and
pressurized or purged with ethene under stirring for the reaction times
listed in Table 1. Conversion, product composition, and enantiomeric purity
of 2 were determined by GC and GC-MS techniques after quenching with
2 mL of concentrated aqueous ammonia and standard workup. Details of
the experimental and analytical procedures are given in the Supporting
Information.
A highly active and selective system was also formed from [Ni-
(allyl)Cl]2 and (Ra,SC,SC)-7 using NaAl[OC(CF3)2Ph]4 as activator
(entry 8).6 Other salts such as NaBF4 or LiNTf2 were considerably
less effective. A first screening of the substrate scope indicates that
electron-deficient vinyl arenes are hydrovinylated with activity and
selectivity similar to those for 1a (entry 10 and 11), whereas
electron-donating substituents such as in 1b lead to somewhat less
satisfactory results (entry 9).
Our results show that chiral phosphoramidites are the first
efficient and modular ligand system for highly enantioselective
hydrovinylation. The large potential for structural variation and the
straightforward synthesis of these ligands make them currently the
best lead structure for catalyst development in this field. Our
ongoing efforts in this area include the combination of their
molecular design with novel reaction and separation processes.6,7
(13) (a) Francio`, G.; Arena, C. G.; Faraone, F.; Graiff, C.; Lanfranchi, M.;
Tiripicchio, A. Eur. J. Inorg. Chem. 1999, 8, 1219. (b) Arena, C. G.;
Calabro` G.; Francio` G.; Faraone F. Tetrahedron: Asymmetry 2000, 11,
2387
(14) The assignment of the absolute configuration at C2 is based on X-ray
data of 6 and comparison of NMR data within the series of ligands 5-6
(cf. ref 11c). The 31P{1H} resonances of 5 are found at δ 146.9 (Ra,SC)
and δ 142.8 (Ra,RC).
Acknowledgment. This work was supported by the Max-
Planck-Society and the Fonds der Chemischen Industrie. We thank
Mr. Thomas Ku¨pper for his experimental collaboration.
(15) Feringa, B. L.; Pineschi, M.; Arnold, L. A.; Imbos, R.; de Vries, A. H.
M. Angew. Chem., Int. Ed. Engl. 1997, 36, 2620.
Supporting Information Available: Experimental details (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
(16) The potential influence of the phenyl groups in the amine part remains to
be addressed in future investigations.
JA012099V
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