Hydrovinylation of 1,3-dienes: A new protocol, an asymmetric variation, and a potential solution to the exocyclic side chain stereochemistry problem

Article abstract of DOI:10.1021/ja0561338

Cyclic and acyclic 1,3-dienes undergo efficient (substrate/catalyst = 72) heterodimerization with ethylene in the presence of a Ni catalyst prepared from [o-(PhCH₂O)]C₆H₄PPh₂, [(allyl)₂NiBr]₂, and Na₊ BAr₄^- (Ar = 3,5-bis-trifluromethylphenyl), giving 1,2-addition products. Yields up to 99% can be realized for several 1-vinylcycloalkenes and 1-substituted 1,3-butadienes. Phospholanes with suitably placed hemilabile ligating groups and phosphoramidites derived from binaphthol are excellent ligands for an asymmetric variation of this reaction, the latter giving 99% yield and >95% ee's for selected substrates. An example of how an exocyclic chiral center can be used to install other stereo-centers in the ring (e. g., the carbon to which the side chain is attached) is also provided. These discoveries open an expeditious entry into several biologically relevant classes of compounds, especially those carrying tetrahydronaphthalene and related heterocyclic moieties. Copyright

Full text of DOI:10.1021/ja0561338

Published on Web 12/07/2005
Hydrovinylation of 1,3-Dienes: A New Protocol, an Asymmetric Variation, and
a Potential Solution to the Exocyclic Side Chain Stereochemistry Problem
Aibin Zhang and T. V. RajanBabu*
Department of Chemistry, 100 W. 18th AVenue, The Ohio State UniVersity, Columbus, Ohio 43210
Received September 6, 2005; E-mail: rajanbabu.1@osu.edu
Table 1. Hydrovinylation of 1,3-Dienes (eq 2)a
Although asymmetric hydrovinylation of 1,3-cyclooctadiene (eq
1), is one of the earliest reported metal-catalyzed asymmetric C-C
bond-forming reactions,1 no satisfactory solution to the problem
of hydrovinylation of 1,3-dienes has emerged.2 Both the Wilke con-
ditions,3 using an azaphospholene ligand 1, and the use of a catalyst
from aminophosphine-phosphinite/Ni(COD)2/Et2AlCl,4 reported
for 1,3-cyclohexadiene, are limited either by the esoteric nature of
the azaphospholene ligand, which permits no structural simplifica-
tions,5 and/or by the constraints imposed by the need for a strong
Lewis acid such as Et2AlCl. The isomerization of the product 1,4-
diene at higher conversion is one of the serious limitations of a
recently reported nonasymmetric Ru-catalyzed reaction.6
In previous work we developed highly practical asymmetric
hydrovinylation of vinylarenes by employing monodentate phos-
pholane or diarylphosphinite ligands carrying hemilabile groups.2a,7
Since then Leitner has reported8 the use of binaphthol-derived
phosphoramidite ligands9 for the same reaction, and we reported
their use in asymmetric hydrovinylation of norbornene.10a,b An
interaction between a hemilabile group on the ligand and the
cationic Ni center and a synergistic relation between such a group
and a highly dissociated counteranion such as [1,3-(CF3)2 C6H3)4B]-
or SbF6- appear to be crucial for the success of these reactions.7b,c
We recently discovered that hydrovinylation of 1,3-dienes also
benefit from such effects, and with the right choice of ligands,
exquisite selectivity for this reaction can be realized. An asymmetric
version of this reaction would be especially attractive for 1-vinyl-
cycloalkenes, since the product 1,4-dienes would allow control of
absolute and relative configurations of the side chains and of other
stereogenic centers on the ring, a common feature in many important
natural products,11 including D-ring-functionalized steroids, serru-
latanes, and psuedopterosins12 (Scheme 1).
a See Supporting Information for details. b Mixture of two diastereomers
d
(∼2:1). c Rest isomerized plus 1,4-adducts. ∼20% 1,4-adduct.
Our studies started with an examination of hydrovinylation of
cyclohexa-1,3-diene (2) and 4-tert-butyl-1-vinylcyclohexene (3)
using the procedure7a,b we had successfully employed for the
hydrovinylation of vinylarenes [Ph3P/[(allyl)NiBr]2/AgOTf, 0.07
equiv Ni, low temp, CH2Cl2, 1 atm ethylene]. It soon became
apparent that, under these conditions, 1,3-dienes were much less
reactive compared to the vinylarenes, and higher temperatures (∼25
°C) were needed for the reaction. We decided to explore new
protocols for this potentially useful reaction by systematically
examining the use of the hemilabile ligand effects7a-c using 3 as a
substrate. These studies revealed that the best ligand for this reaction
was 2-benzyloxyphenyldiphenylphosphine (4a).13 Thus, 0.14 mol
% of a catalyst generated from 4a, allyl-nickel bromide dimer, and
Na+{[1,3-(CF3)2C6H3]4B}- effects the reaction of 3 with ethylene
(1 atm) to give a quantitative yield of the product 6, as a mixture
of two diastereomers (eq 2 and Table 1, entry 2). This product is
Scheme 1. Controlling Exocyclic and Ring Junction
Configurations through Hydrovinylation
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10.1021/ja0561338 CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 2. Asymmetric Hydrovinylation of 1,3-dienes (eq 3)a
formed with exquisite regioselectivity (1,2-addtion at the less
hindered olefin). The racemic chiral olefin 3 gave a nearly ∼2:1
mixture of diastereomers. The results of hydrovinylation of other
typical dienes are shown in Table 1.13 In general, excellent yields
(>97%) and selectivities (>95%) are observed for the hydroviny-
lation of both cyclic and acyclic dienes (entries 1, 2, 5-9) under 1
atm of ethylene. Lack of selectivity is seen only for 1-vinylcyclo-
hexene (7, entry 3) and the 1-vinylcyclopentene 9 (entry 4, prepared
by enyne metathesis), which gave a mixture of 1,2- and 1,4- addition
products.
a See eq 3 for typical procedure and Supporting Information for
experimental details.
The most promising results with model compounds, suitable for
a study of the exocyclic stereochemistry problem, are shown in eq
3 and Table 2. Hydrovinylation of 11, 13, and 15, under our
Acknowledgment. This paper is dedicated to Professor David
Hart to honor his numerous scientific contributions and outstanding
service to chemical education. Financial assistance for this research
by U.S. National Science Foundation (CHE-0308378) and the ACS-
PRF (36617-AC1) is gratefully acknowledged.
Supporting Information Available: Full experimental details of
various hydrovinylation reactions, spectroscopic and chromatographic
data for characterization of compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
standard conditions (eq 3) using the phospholane (L1)7e or the
phosphoramidite ligand (L2) gave exceptionally high yields and
regio- and enantioselectivities for these cyclic dienes.13 Acyclic
diene 17 under these conditions gave nearly racemic product.
The absolute configuration of the 14 was established by its
conversion to the fully aromatic product of known14 configuration
(eq 4a). The configurations of 12 and 16 were assigned by analogy.
References
(1) Bogdanovic´, B.; Henc, B.; Meister, B.; Pauling, H.; Wilke, G. Angew.
Chem., Int. Ed. 1972, 11, 1023.
(2) For recent reviews of hydrovinylation reaction, see: (a) RajanBabu, T.
V. Chem. ReV. 2003, 103, 2845. (b) Jolly, P. W.; Wilke, G. Hydroviny-
lation. In Applied Homogeneous Catalysis with Organometallic Com-
pounds; Cornils, B., Herrmann, W. A., Eds.; VCH: New York, 1996;
Vol. 2, p 1024.
(3) Wilke, G.; Monkiewicz, J.; Kuhn, H. U.S. Patent, 4,912,274, 1990; Chem.
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1998, 120, 459. (b) RajanBabu, T. V.; Nomura, N.; Jin, J.; Nandi, M.;
Park, H.; Sun, X. J. Org. Chem. 2003, 68, 8431. (c) Nandi, M.; Jin, N.;
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RajanBabu, T. V. J. Am. Chem. Soc. 2002, 124, 734. (e) Zhang, A.;
RajanBabu, T. V. Org. Lett. 2004, 6, 1515.
(8) Francio`, G.; Faraone, F.; Leitner, W. J. Am. Chem. Soc. 2002, 124, 736.
(9) Arnold, L. A.; Imbos, R.; Mandoli, A.; de Vries, A. H. M.; Naasz, R.;
Feringa, B. L. Tetrahedron 2000, 56, 2865.
(10) (a) Kumareswaran, R.; Nandi, M.; RajanBabu, T. V. Org. Lett. 2003, 5,
4345. (b) Park, H.; Kumareswaran, R.; RajanBabu, T. V. Tetrahedron
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(11) For a leading reference, discussion, and citations of earlier work, see:
Guevel, A.-C.; Hart, D. J. J. Org. Chem. 1996, 61, 465 and references
therein.
(12) For recent references: (a) Rodr´ıguez, I. I.; Rodr´ıguez, A. D. J. Nat. Prod.
2003, 66, 855 and references therein. (b) Tanaka, J.-i.; Ogawa, N.; Liang,
J.; Higa, T. Tetrahedron 1993, 49, 811. Synthetic studies: erogorgiaene:
(c) Cesati, R. R., III; de Armas, J.; Hoveyda, A. H. J. Am. Chem. Soc.
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The sense of asymmetric induction is consistent with the model
we proposed for asymmetric hydrovinylation of vinylarenes.7e The
ee’s were unambiguously established by GC, HPLC, and/or Mosher
ester13 methods.
A number of different strategies including hydroboration and
directed hydrogenation can be envisioned for controlling the
configuration of the ring carbon to which the side chain is attached.
One example is shown in eq 4b. The hydroboration (9-BBN, H2O2)
of 14 followed by directed hydrogenation using Crabtree’s catalyst,
([(COD)(Cy3P)Ir(py)]+PF6-), gives a reduced product (21, dr 30:
1) with very high stereoselectivity.13
(13) See Supporting Information for full experimental details, analytical data,
characterization, and assignments of configurations of 14, 21, and 22.
(14) Menicagli, R.; Piccolo, O.; Lardicci, L.; Wis, M. L. Tetrahedron 1979,
35, 1301.
Further expansion of the scope of the asymmetric reaction and
its applications for the synthesis of natural products will be reported
in due course.
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