Extensive isomerization of alkenes using a bifunctional catalyst: An alkene zipper

Article abstract of DOI:10.1021/ja073457i

Catalyzed movement of alkene double bonds up to 30 positions has been accomplished using a catalyst featuring a cationic CpRu fragment and bifunctional imidazolylphosphine. The basic nitrogen of the latter is thought to deprotonate coordinated alkene intermediates reversibly, facilitating isomerization between terminal and (E)-alkenes and accelerating conversions by factors of up to 1 × 10⁴. Copyright

Full text of DOI:10.1021/ja073457i

Published on Web 07/18/2007
Extensive Isomerization of Alkenes Using a Bifunctional Catalyst: An Alkene
Zipper
Douglas B. Grotjahn,* Casey R. Larsen, Jeffery L. Gustafson, Reji Nair, and Abhinandini Sharma
Department of Chemistry and Biochemistry, 5500 Campanile DriVe, San Diego State UniVersity,
San Diego, California 92182-1030
Received May 15, 2007; E-mail: grotjahn@chemistry.sdsu.edu
Redox isomerizations are examples of atom-economical pro-
cesses in which one site in an organic substrate is oxidized with
Scheme 1. Reactants and Products
1
concomitant reduction of another site. One especially well-studied
example is the conversion of allylic alcohols to aldehydes or
ketones,2 which involves movement of the alkene double bond
,3
4-7
over two positions (eq 1, n ) 2). Far fewer catalysts exist for the
movement of a more remote double bond (n > 2). For example,
8
+
Kirchner et al. reported that, although [CpRu(PR
1a-1c) were improved catalysts for allylic alcohol isomerization
relative to CpRu(PR Cl, it failed in the case of 3-buten-1-ol (n )
) or alkenes devoid of alcohol functionality. The apparent record
for alkene isomerization of any kind is over 20 positions on the
hydrocarbon CH (CH 19CHdCH(CH by stoichiometric
amounts of the reagent Cp Zr(H)(Cl). The apparent record for
catalyzed double bond movement is on 9-decene-1-ol (nine posi-
3 3 2
)(CH CN) ]
(
3 2
)
3
3
2
)
2 3
)19CH
9
2
10,11
tions, n ) 9) using Fe
3
(CO)12
.
However, 30 mol % was required,
which means that nearly a mole of metal was used per mole of
alkenol.
pyrid-2-yl substituents on P as well as t-Bu, i-Pr, Ph, and Me groups,
the phosphine ligand1 in 2 emerged as a superior choice for its
ability to promote isomerization of both substrates.
4,15
+
-
To prepare 2, [CpRu(CH
3
CN)
3
] PF
6
in acetone was allowed
to react with the required ligand, forming a mixture of 1d and
chelate 2.16 This mixture could be used but for purposes of catalyst
characterization and further testing as described in this paper was
converted to >90% pure 2 by repeated coevaporation of added
acetone with the acetonitrile liberated. Complex 2 showed diagnostic
NMR data consistent with a chiral structure and four unique,
diastereotopic methyl groups.
Because pent-4-en-1-ol (13a) represented a more challenging
class of substrate, its isomerization to pentanal (14a) was optimized
with respect to solvent, catalyst amount, and reaction temperature.
At the 2 mol % level, heating at 70 °C was advised, whereas by
using 5 mol %, reactions could be completed at 25 °C within 1 d.
Acetone emerged as the solvent of choice, although dichlo-
romethane could also be used. The effects of a substrate hydroxyl
group were apparent by noting that under the optimized conditions,
isomerization of 1-pentene to (E)-2-pentene required only minutes
at room temperature using 2 mol % catalyst (Scheme 1).
Substrate scope was explored. Table 1 shows results which are
in most cases unoptimized for a particular substrate but are designed
to give an idea of the capabilities and limitations of the catalyst. In
general, full isomerization of substrates with polar groups (OH,
amide) required heating at 70 °C, whereas partial isomerization of
these compounds or full isomerization of nonpolar substrates could
be done at 25 °C. Most dramatically, alkenol isomerization over 9,
22, and even 30 positions could be achieved (entries 9-12). The
silyl ether 11 (entry 8) gave enol ether (E)-12 in which only traces
Here we highlight the discovery and development of a novel
catalyst (2) whose notable and attractive features include the
following: (1) ability to isomerize a variety of heterofunctionalized
alkene derivatives, (2) formation of (E)-isomer products of high
purity, (3) useful activity in many cases at low loadings (2-5 mol
%), and (4) a heterocyclic ligand which appears to act not merely
as a hemilabile group but rather as an internal base. Finally, we
show that bifunctional catalyst 2 is truly an alkene zipper12 by
reporting the catalyzed moVement of an alkene double bond oVer
30 positions along an alkyl chain (eq 1, n ) 30).
Initial screening experiments were conducted on the alkenes
-pentene and pent-4-en-1-ol, representing two different classes of
1
substrate, one lacking any polar or protic group and the other
featuring the alkene and OH separated by three carbons (eq 1, n )
13
4
). Such substrates were inert to 1a-1c. From these preliminary
studies, which included phosphines with both imidazol-2-yl and
9592
9
J. AM. CHEM. SOC. 2007, 129, 9592-9593
10.1021/ja073457i CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Scope and Limitations Using 2a
mixtures consisting of almost exclusively these latter isomers. The
mechanistic hypothesis in Scheme 2 helps rationalize the unusual
ability of 2 to convert geraniol to (E)-isogeraniol rather than to the
aldehyde, because formation of the enol isomer would require the
catalyst to form an allyl intermediate like 16-Z. Finally, we note
that, since isomerization of the longer substrates presumably occurs
by a random walk of the double bond up and down the chain en
route to the most stable isomer, the observed movement of the
double bond is actually just a lower estimate of the number of times
the catalyst acts on a substrate.
2
temp
C)
yield
(%)
entry
reactant
product
(%)
(
°
time
1
2
3
4
5
6
7
8
9
1-pentene
3
5
7
(E)-2-pentene
(E, E)-4
(E)-6
8
10
14a
14b
(E)-12
14c
14d
14e
2
2
2
2
2
2
2
5
5
25
25
25
25
25
70
70
70
70
70
70
70
15 min
40 min
4 h
26 h
4 d
1 h
1 h
4 h
4 h
95
96
75
b
70:30c
d
9
61
95
97
90
84
97
e
13a
13b
11
13c
13d
13e
13f
In summary, catalyst 2 is unusual in its activity (in many cases
10
11
12
5
20
30
4 h
3.6 d
3 d
2-5 mol % is sufficient), selectivity, and unprecedented ability to
b
91
81
b
move unhindered double bonds over many positions. Ongoing
studies are designed to elucidate further the mechanism of action
and the capabilities of this bifunctional catalyst design, and these
will be reported in due course.
14f
a
1
Acetone-d6 solvent, yields determined by H NMR and internal standard
b
c
unless otherwise indicated. Isolated. Equilibrium ratio of 7 and 8, also
reached from either side within 2-4 h using 2 mol % of 2 at 70 °C.
Unreacted 9 (29%) and an unidentified isomer (10%) also present.
d
Acknowledgment. The NSF is thanked for support of this and
related work, and Dr. LeRoy Lafferty is thanked for assisting with
NMR experiments.
e
Isomerization could also be completed using 5 mol % of 2 for 1 d at
2
5 °C.
Scheme 2. Mechanistic Hypothesis
Supporting Information Available: Details of compound prepara-
tion, characterization, and use of the catalysts. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
(
(
(
(
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(
1
(
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(
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(
ether17-19
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1, pp 199-209.
entry under neutral conditions to a little-explored class of com-
pounds which have typically been made as mixtures using strong
base.20
(
(
8) R u¨ ba, E.; Simanko, W.; Mauthner, K.; Soldouzi, K. M.; Slugovc, C.;
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nistic hypothesis advanced in Scheme 2. Rather surprisingly,
bubbling ethylene (∼25 equiv) into an acetone solution of 2 leads
to formation of free acetonitrile and an ethylene complex similar
to 15, rather than a chelate-opened species. Therefore, we propose
that the first step of the catalytic cycle is exchange of acetonitrile
for alkene, forming 15. The role of the basic nitrogen of the
imidazole could then be deprotonation at an allylic position, forming
(10) Iranpoor, N.; Mottaghinejad, E. J. Organomet. Chem. 1992, 423, 399-
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(
11) In eq 1, isomerization to the enol requires a movement over n-1 positions,
all of which can be reasonably attributed to catalysis by metal. The final
enol-keto tautomerization may or may not involve metal catalyst but for
simplicity is included in the total count of n.
(12) The term “zipper” has also been applied to alkyne isomerization by
superstoichiometric amounts of strong bases: Brown, C. A.; Yamashita,
A. J. Am. Chem. Soc. 1975, 97, 891-892. Bifunctional reactivity may be
involved, but salts of simple amines also work: Abrams, S. R.; Nucciarone,
D. D.; Steck, W. F. Can. J. Chem. 1983, 61, 1073-1076.
(
(
(
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4
1
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5
(
16) See Supporting Information for details.
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(
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(
(
20) Taskinen, E.; Virtanen, R. J. Org. Chem. 1977, 42, 1443-1449.
21) Placement of the base in the catalyst is crucial: adding 1-methyl-4-tert-
butylimidazole to the 1-pentene isomerization by 1e made the reaction
even slower.
(
1
5 to allyl complex 16) or intermediates (e.g., 16) leading to (Z)-
alkene are sufficiently high in energy to leave the system shuttling
between terminal and (E)-alkene(s), creating “semi-thermodynamic”
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