enantioselectivity of that major product was about the same.
However, the corresponding reduction of the 1,3-diene in entry 3
gave a poor yield of the product and inferior stereoselectivities.
Entries 4–6 of Table 1 feature (E,E)-1,4-diaryl-2,3-dimethylbuta-
1,3-dienes (type 2). The benzenoid systems give quantitative
conversions of the starting material, but moderate yields of the
product. The mass balance is largely accounted for by formation
of the double bond migration products 2. Curiously, hydrogena-
tion of the furan-substituted system in entry 6 is not complicated in
the same way. All three substrates of this class are marginally
selective for the ent-products, and these formed in excellent
enantiomeric excesses.
Table 2 Hydrogenations of dienes with Crabtree’s catalyst 5
Conv.a Yielda
(%) (%)
Entry Diene
1
ent:mesoa
30
9
1.0:11
2
96
16
1.0:2.0
(E,E)-1,4-Diaryl-1,4-dimethyl-1,3-dienes tend to be the best-
behaved substrates in the series of dienes examined (entries 7–9).
The benzenoid systems give high ent/meso ratios, and the optically
active product forms in high ee’s. However, the diene with two
furan groups is again anomalous (entry 9) insofar as both aspects
of the stereoselectivity were less.
3
91
36
1.0:1.1
Collectively, the data in Table 1 indicate several trends. Dienes
with 1,1-disubstituted alkene functionalities can be hydrogenated
with moderate enantioselectivities, though there are some unpre-
dictable variations with molecular structure. (E,E)-1,4-Diaryl-2,3-
dimethylbuta-1,3-diene substrates tend to be better behaved, and
give higher enantioselectivities, though double bond migration to
unreactive tetrasubstituted alkenes can reduce the yields. However,
the more hindered o-tolyl system 3 was a poor substrate only
giving 8% yield of the reduction product under the same
conditions. Side products from migration reactions were not
observed for the furan-substituted system in entry 6, unlike the
benzenoid analogs in entries 4 and 5, and that may be indicative of
a coordinating effect and/or the enhanced nucleophilicity of the
p-bonds. Those same parameters may be the origin of the
relatively poor enantioselectivity observed for the furan-substituted
system in entry 9 relative to the other (E,E)-1,4-dimethyl-1,3-
dienes. Related to this, another observation provides circumstan-
tial evidence that coordination effects are important. Thus, the
analogous thiophene-substituted alkene 4 was a poor substrate
giving only 20% conversion (de/ee not determined).
4b
0
0
—
5b
47
37
2.2:1.0
a
b
Determined by GC using a b- or a c-CD column.7 2 mol%
catalyst.
This is a comprehensive account of all the asymmetric
hydrogenations of aryl-substituted dienes that we have studied
so far. Even substrates that gave relatively poor results were
mentioned, to give a balanced impression of the scope and
limitations of the reaction. Good ee’s can be obtained for two of
the three substrate types studied, and excellent ent/meso diaster-
eoselectivities were obtained for the type 3 dienes. The conversions
were high, much better than for Crabtree’s catalyst under similar
conditions, though some material was diverted to the relatively
unreactive tetrasubstituted alkenes 2 in the particular case of the
type 2 alkenes. The next step in this project will be to investigate
hydrogenations of dienes and trienes that give more useful chirons
for natural product syntheses.
Crabtree’s Ir(py)(PCy3)(COD)PF6 5 is the most widely recog-
nized homogeneous catalyst for hydrogenations of unfunctiona-
lized, highly hindered alkenes.9 A selection of dienes were also
hydrogenated with Crabtree’s catalyst, for comparison (Table 2),
but only after some controls were performed. It was shown that
the conversions were less when a 50 atm H2 pressure (as used in the
asymmetric hydrogenations) was used. Moreover, since Pfaltz’s10
and our groups5 have observed that the BARF counter-ion has a
beneficial influence on conversions in these Ir-mediated reactions,
we performed experiments with Crabtree’s catalyst in the presence
of 5 mol% of NaBARF; however, the outcome was marginally
worse (data not shown).
Financial support for this work was provided by The Robert
Welch Foundation. We thank Dr Shane Stichy and the TAMU/
LBMS-Applications Laboratory for MS support, and NMR
Laboratory at Texas A&M University, supported by a grant from
the National Science Foundation (DBI-9970232) and the Texas
A&M University System.
Table 2 shows that the conversions with Crabtree’s catalyst tend
to be less than those observed in the corresponding asymmetric
reactions (Table 1). Moreover, the yields tend to be significantly
lower than the conversions reflecting decomposition of Crabtree’s
catalyst before the reactions were complete, leading to partial
reduction products. Catalyst 1 is more robust in the hydrogena-
tions of these substrates. None of the ent/meso selectivities for
Crabtree’s catalyst are high, and all but one of the reactions
favored the meso product.
Xiuhua Cui, James W. Ogle and Kevin Burgess*
Department of Chemistry, Texas A&M University, Box 30012, College
Station, TX 77842-3012, USA. E-mail: burgess@tamu.edu;
Fax: +1 979 845 8839; Tel: +1 979 845 4345
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 672–674 | 673
Cui, Xiuhua
