Hydrovinylation of Norbornene. Ligand-Dependent Selectivity and Asymmetric Variations

Article abstract of DOI:10.1021/ol0356284

(Equation presented) Norbornene undergoes Ni-catalyzed (1-2 mol% allylnickel bromide/phosphine/NaBARF or AgSbF₆, 1 bar ethylene, -50°C) hydrovinylation (>97% yield), giving either a 1:1 or a 2:1 (norbornene/ethylene) adduct depending on the size of the phosphine. Use of binaphthol-derived phosphoramidite ligand results in up to 80% ee for the 1:1 adduct. The course of the reaction is highly dependent on the ligand (size and configuration of the appendages) and the counteranion present.

Full text of DOI:10.1021/ol0356284

ORGANIC
LETTERS
2
003
Vol. 5, No. 23
345-4348
Hydrovinylation of Norbornene.
Ligand-Dependent Selectivity and
Asymmetric Variations
4
Ramaiah Kumareswaran, Malay Nandi, and T. V. RajanBabu*
Department of Chemistry, 100 W. 18th AVenue, The Ohio State UniVersity,
Columbus, Ohio 43210
rajanbabu.1@osu.edu
Received August 27, 2003
ABSTRACT
6
Norbornene undergoes Ni-catalyzed (1−2 mol% allylnickel bromide/phosphine/NaBARF or AgSbF , 1 bar ethylene, −50 °C) hydrovinylation
(>97% yield), giving either a 1:1 or a 2:1 (norbornene/ethylene) adduct depending on the size of the phosphine. Use of binaphthol-derived
phosphoramidite ligand results in up to 80% ee for the 1:1 adduct. The course of the reaction is highly dependent on the ligand (size and
configuration of the appendages) and the counteranion present.
Among asymmetric carbon-carbon bond-forming reactions,
only Nozaki’s Cu-catalyzed cyclopropanation of styrene
pressure in THF in the presence of [Ni(2,4,6-Me
3
C
6
H
2
)(CH
3
-
1
4
CN)(phosphane) ]BF . Quantitative conversion of nor-
2 4
with ethyl diazoacetate precedes the Ni-catalyzed asymmetric
hydrovinylation reaction reported by the Wilke group. In
bornene with up to 95% selectivity for the exo-2-vinylnor-
bornane has been reported, even though ethylene polymeriza-
tion, which ensues after the depletion of norbornene, is likely
to be a problem in the isolation of pure 2. Likewise,
2
one of the earliest examples of this reaction, it was found
that with a catalyst system made up of π-allylnickel chloride,
triethyldialuminum trichloride, and (-)-dimenthyl(2-propyl)-
phosphane (4), norbornene underwent heterodimerization
with ethylene to give exo-(+)-2-vinylnorbornane (2) along
with other side-products, including 3 (Z + E) and lower
oligomers (eq 1). Subsequently, (R,R)-azaphospholene ligand
hydrovinylation using (PCy
3
)
2
(CO)RuHCl/HBF
4
2
‚OEt gave
a mixture of 2 and a poorly characterized oligomeric product
5
in a ratio of 3:2.
Recognizing the potential of this reaction for the prepara-
6
tion of various [2.2.1]-bicycloheptane derivatives, including
5
was reported to give a much cleaner reaction in CH
2 2
Cl at
heteroatom analogues (eq 2), we have reinvestigated this
reaction under the protocols recently developed in our
laboratories for the hydrovinylation of vinylarene deriva-
-
65 °C with up to 87% yield and 53% ee for exo-(-)-2-
3
vinylnorbornane. However, the lengthy synthesis and the
3
a
rigorous structural requirements for high selectivity make
this ligand one of limited scope. Hydrovinylation of nor-
bornene can also be carried out under 15 bar ethylene
(3) (a) Jolly, P. W.; Wilke, G. Hydrovinylation. In Applied Homogeneous
Catalysis with Organometallic Compounds; Cornils, B., Herrmann, W. A.,
Eds.; VCH: New York, 1996; Vol. 2, pp 1024-1048. (b) Wilke, G.;
Monkiewicz, J.; Kuhn, H. Preparation of Optically ActiVe Azaphospholenes
and Their Use in Catalysis for Asymmetric Codimerization of Olefins; U.S.
Patent 4912274, 1990; Chem. Abstr. 1991, 114, 43172.
(4) Muller, G.; Ordinas, J. I. J. Mol. Catal., A: Chem. 1997, 125, 97.
(5) Yi, C. S.; He, Z.; Lee, D. W. Organometallics 2001, 20, 802.
(
1) Nozaki, H.; Moriuti, S.; Takaya, H.; Noyori, R. Tetrahedron Lett.
966, 5239.
2) Bogdanovi c´ , B.; Henc, B.; L o¨ sler, A.; Meister, B.; Pauling, H.; Wilke,
G. Angew. Chem., Int. Ed. Engl. 1973, 12, 954.
1
(
1
0.1021/ol0356284 CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/18/2003

7
tives. In this paper we report that the hydrovinylation of
norbornene can be achieved in nearly quantitatiVe yield with
exquisite selectivity for either a 1:1 or a 1:2 (ethylene to
norbornene) adduct depending on the nature of the phosphine
employed.
Scheme 1. Ligand Dependence of Hydrovinylation of
Norbornene
observed. A minor component (<2%) has not been identified.
1
13
As suggested by the H and especially the C NMR spectra,
one of the two possible diastereomers of the exo,exo-dimer
8
is formed in excess of 97% yield. The identity of the major
product as a single diastereomer was confirmed by conver-
sion of the olefin 6 into an alcohol 7 by hydroboration and,
subsequently, to the corresponding Mosher ester 8 with (R)-
6 6
MTPA chloride (Scheme 2). Fluorine-19 (C D ) NMR of
In addition, we also document the viability of a number of
tunable ligands for the asymmetric hydrovinylation of
norbornene, including one that gives the highest selectivity
reported to date. This asymmetric-catalyzed reaction is highly
dependent on the nature of the ligand and the counteranion
employed. The details of these studies are reported in this
paper.
Scheme 2. Stereochemical Homogeneity of 6
Ligand-Dependent Selectivity in the Heterodimeriza-
tion Reaction. Norbornene, upon treatment with ethylene
in the presence of a catalyst derived from allylnickel bromide,
tricyclohexylphosphine, and AgOTf, undergoes an excep-
tionally clean reaction to give a 1:1 heterodimer 2 (Scheme
8
1
). At -70 °C, less than 0.5% of the 2:1 adduct 6 is
observed in the GC or NMR. No other oligomeric product
or the isomerization product (e.g., 3) can be detected under
these conditions.
In sharp contrast, the same reaction, when carried out with
triphenylphosphine as the ligand, gives almost exclusively
a 2:1 adduct, 6 (Scheme 1). This reaction is best carried out
at -55 °C using 0.7 mol % of the nickel catalyst. Under
these conditions, less than 1% of the 1:1 heterodimer 2 is
the Mosher ester shows only two peaks (δ 70.425 and
7
0.450) in a ratio of 1:1 in the expected range for the CF
3
groups, thereby confirming the homogeneity of the 2:1
adduct. These signals were also used to determine the ees
of the adducts (see later). The two diastereomeric Mosher
esters result from the enantiomers of 6. The absolute
configuration of the dimer, tentatively assigned as (1R*,1′S*,
2S*,2′S*,3S*,4S*,4′R*)-, has not been confirmed. The choice
(
6) For other asymmetric transformations of norbornene, see the fol-
is based on the well-known propensity of exo-addition of
various reagents to norbornene and the relative stabilities
of the two possible exo-diastereomers (6a and 6b) as inferred
lowing. Hydrocyanation: Hodgson, M.; Parker, D.; Taylor, R. J.; Ferguson,
G. Organometallics 1988, 7, 1761. Baker, M. J.; Pringle, P. G. J. Chem.
Soc., Chem. Commun. 1991, 1292. Horiuchi, T.; Shirakawa, E.; Nozaki,
K.; Takaya, H. Tetrahedron: Asymmetry 1997, 8, 57. Hydrosilylation:
Hayashi, T. Acta Chem. Scand. 1996, 50, 259. Hydroalkenylation: Ozawa,
F.; Kabatake, Y.; Kubo, A.; Hayashi, T. J. Chem. Soc., Chem. Commun.
6
from calculations at HF/6-31G* and B3LYP/6-31G* levels
9
(
Figure 1). The minor exo,exo-diastereomer (the unidentified
1
994, 1323. For hydrovinylation of norbornadiene, see ref 2 and: Pillai, S.
M.; Tembe, G. L.; Ravindranathan, M. J. Mol. Catal. 1993, 84, 77.
7) (a) Nomura, N.; Jin, J.; Park, H.; RajanBabu, T. V. J. Am. Chem.
component?) is approximately 6 kcal/mol less stable than
the major one.
(
Soc. 1998, 120, 459. (b) Nandi, M.; Jin, J.; RajanBabu, T. V. J. Am. Chem.
Soc. 1999, 121, 9899. (c) RajanBabu, T. V.; Nomura, N.; Jin, J.; Radetich,
B.; Park, H.; Nandi, M. Chem. Eu. J. 1999, 5, 1963. (d) Jin, J.; RajanBabu,
T. V. Tetrahedron 2000, 56, 2145. (e) Park, H.; RajanBabu, T. V. J. Am.
Chem. Soc. 2002, 124, 734. (f) RajanBabu, T. V.; Nomura, N.; Jin, J.; Nandi,
M.; Park, H.; Sun, X. J. Org. Chem. 2003, 68, 8431-8446. (g) For a recent
review, see RajanBabu, T. V. Chem. ReV. 2003, 103, 2845.
This remarkable difference in selectivity is presumably
related to the relative reactivities of the two olefins and the
size of the two phosphines. It is conceivable that the more
+
reactive norbornene undergoes fast initial [PNi-H] -
(
8) See Supporting Information for gas chromatograms and NMR spectra
(9) We thank Professor Christopher Hadad of our department for the
B3LYP calculations.
of key compounds, including those of the crude products.
4346
Org. Lett., Vol. 5, No. 23, 2003

Figure 1. Calculated Relative Energies of the 2:1 Adducts 6a and
b.
6
catalyzed dimerization10 when a small phosphine11 (Ph
P,
3
cone angle 145°) is used (Scheme 3). The initially formed
Figure 2. Ligands for Hydrovinylation of Norbornene.
Scheme 3. Ligand Dependence in Norbornene
Hydrovinylation
where only a few ligand/metal combinations have been
3
a,7f
successful.
We had previously reported that nickel(II)
complexes of selected triarylphosphine and phospholane
ligands with an appropriately placed hemilabile group could
serve as efficient catalysts for the hydrovinylation of
7
a-c
vinylarenes.
Leitner et al. has reported that phosphora-
midites derived from binaphthol are excellent ligands for
1
2
hydrovinylation of styrene derivatives. We have tested
several of these ligands (Figure 2, 11-15) for asymmetric
hydrovinylation of norbornene. A prototypical example of
the reaction conditions is shown in eq 3 and, a summary of
1
3
the results is shown in Table 1. The enantiomeric excess
Table 1. Asymmetric Hydrovinylation of Norbornenea
entry
ligand
additive
2 (%)b
6 (%)b
% eec
1
1
1
1
2
2
2
3
4
a
b
c
d
a
b
c
11a
11a
11a
11b
12 (Ra,Sc,Sc)
12 (Ra,Sc,Sc)
12 (Ra,Sc,Sc)
12 (Ra,Rc,Rc)
13 (Ra,Sc,Sc)
13 (Sa,Sc,Sc)
14a
NaBARF
AgSbF6
AgNTf2
AgSbF6
NaBARF
AgOTf
AgSbF6
NaBARF
AgSbF6
NaBARF
AgSbF6
NaBARF
AgSbF6
AgOTf
69d
0
0
0
0
0
0
99
0
0
0
44
50
50
46
80
>99
>99
>99
>99
d
20
0
34
<2%
0
ae
e
σ-complex 10, for stereoelectronic reasons (the only H cis
to Ni is at the bridgehead position), cannot undergo â-hydride
elimination and hence reacts further with the smaller of the
two olefins, ethylene, eventually giving the 2:1 adduct 6a.
With a bulky phosphine (e.g., tricyclohexylphosphine, cone
angle 180°), the coordination sphere is too crowded to
accommodate two norbornene molecules. Heterodimerization
between norbornene and ethylene ensues, giving the 1:1
adduct 2.
4b
0
40
d
d
5
5
5
6
a
b
c
15
60
0
14a
14b
15
40
0
11
88
33
a
See eq 3 for typical procedure. Results of at least two experiments are
b
given in each case. By GC and NMR, isolated yields vary because of the
volatility of 2. c By NMR (see text). d Rest was starting material. e Polymers
(
∼100%)
Asymmetric Hydrovinylation. Asymmetric heterodimer-
ization of ethylene and other olefins is a demanding reaction
and the absolute configuration of the 1:1 adduct 2 were
determined by conversion to known mandelic acid derivatives
(
10) For a discussion of the mechanism of hydrovinylation, see ref 7f.
(11) (a) Tolman, C. A. Chem. ReV. 1977, 77, 313. (b) White, D.; Coville,
(12) Franci o´ , G.; Faraone, F.; Leitner, W. J. Am. Chem. Soc. 2002, 124,
736.
N. J. AdV. Organomet. Chem. 1994, 36, 95.
Org. Lett., Vol. 5, No. 23, 2003
4347

(
entries 2-4) depends on both the counteranion and the
nature of the secondary amine appendage. Whereas the
(R ,S ,S )-isomer is a good ligand (entry 2a), the correspond-
ing (R ,R ,R )-diastereomer gives less than 2% of the product
entry 3). For the ligand (R ,S ,S )-12, the counteranion
Scheme 4. Absolute Configuration of 2 and Configuration of
the (S)-Mandelate Esters
a
c
c
a
c
c
(
a
c
c
determines whether the 1:1 or 1:2 adduct is produced. With
NaBARF, only 1:1 adduct is produced (entry 2a), whereas
6
AgSbF (which we have successfully used in place of
7e
NaBARF in some early hydrovinylation experiements ) now
gives exclusively the 2:1 adduct 6 in nearly quantitatiVe yield
(entry 2c)! Phosphramidites derived from (2S,4S)-diethylpyr-
rolidine and either (R)-binaphthol or (S)-binaphthol gave only
polymeric materials (entry 4). Phospholane ligand 14a with
a benzyloxymethyl hemilabile side-chain gives a mixture of
1
:1 and 1:2 adducts (entry 5). Increasing the size of the
phospholane substituents (e.g., 14b) turns off the catalytic
activity. Simple 2,5-dimethylphospholane gives mostly the
2
:1 adduct (entry 6). A modest enantioselectivity of 33%
has been observed for this product as determined by the
Mosher ester method (Scheme 2). As we have documented
before, the use of AgOTf as an additive is important for
ligands such as 15 with no hemilabile side-chain. Chelating
ligands inhibit the reaction under the typical conditions
reported here.
In conclusion, we report a remarkable ligand-dependent
selectivity in the Ni-catalyzed heterodimerization of nor-
bornene and ethylene. We also show the viability of several
classes of tunable ligands for this exacting reaction, one of
them giving up to 80% ee for the 1:1 exo-adduct. We believe
that this prototypical reaction between a strained olefin and
ethylene may have broader applications in the hydroviny-
lation of other similar substrates. Such studies are in progress.
as shown in Scheme 4. Ozonolysis of 2 followed by oxidation
of the resulting aldehyde (16) gave norbonane-2-carboxylic
acid 17, the enantiomers of which were converted into esters
of (S)-methyl mandelate (18 and 19) by the standard
procedure using DCC. The absolute configuration of these
7
b
1
4
diastereomers had been fully established by Parker. For
the product 2 obtained by reaction shown in eq 3, the C2-
hydrogen of the derived major methyl mandelate ester
1
appears at δ 2.55 (reported 2.54) in the H NMR (C
D
6 6
)
spectrum, whereas it appears at δ 2.88 (reported 2.87) in
the minor diastereomer. The benzylic hydrogens of the major
and minor isomers appear at δ 6.093 (reported 6.077) and
6.082 (reported 6.061), respectively. Accordingly, the major
enantiomer of the 2-vinylnorbornane can be assigned the
configuration (1R,2S,4S). The sense of optical rotation (-)
is also consistent with this assignment.3 On the basis of
the integral values of these signals, the enantiomeric excess
for 2 was determined as 80%. The results of other hydrovi-
nylation reactions are shown in Table 1. As expected,
phosphines with large cone angles (11a,b and 12) give
exclusively the 1:1 adduct in nearly quantitative yield and
modest enantioselectivity (entries 1, 2). Note the use of
highly dissociated counteranions in these reaction.
Acknowledgment. The authors wish to thank Drs. J. Jin
and H. Park for their initial contributions in this area.
Financial assistance by U.S. National Science Foundation
,15
(CHE-0308378) and the donors of the Petroleum Research
Fund, administered by the American Chemical Society
(36617-AC1), is gratefully acknowledged.
Supporting Information Available: Full experimental
details of various hydrovinylation reactions and spectroscopic
and chromatographic data for characterization of compounds
listed. This material is available free of charge via the Internet
at http://pubs.acs.org.
OL0356284
(13) See Supporting Information for NMR spectra from which ees were
determined.
(14) Hodgson, M.; Parker, D.; Taylor, R. J.; Ferguson, G. Organome-
tallics 1988, 7, 1761.
(15) For assignments of absolute configuration of 2-vinylnorbornenes,
see: (a) Hayashi, T.; Kanehira, K.; Hioki, T.; Kumada, M. Tetrahedron
Lett. 1981, 22, 137. (b) K o¨ hler, J.; Deege, A.; Schomburg, G. Chro-
matographia 1984, 18, 119. See also, ref 2.
No trace of the 2:1 adduct 6 is observed under these
conditions. The selectivity with the phosphoramidite ligands
4348
Org. Lett., Vol. 5, No. 23, 2003