A practical and catalytic reductive olefin coupling

Article abstract of DOI:10.1021/ja4117632

A redox-economic method for the direct coupling of olefins that uses an inexpensive iron catalyst and a silane reducing agent is reported. Thus, unactivated olefins can be joined directly to electron-deficient olefins in both intra- and intermolecular settings to generate hindered bicyclic systems, vicinal quaternary centers, and even cyclopropanes in good yield. The reaction is not sensitive to oxygen or moisture and has been performed on gram-scale. Most importantly, it allows access to many compounds that would be difficult or perhaps impossible to access using other methods.

Full text of DOI:10.1021/ja4117632

Communication
pubs.acs.org/JACS
A Practical and Catalytic Reductive Olefin Coupling
Julian C. Lo, Yuki Yabe, and Phil S. Baran*
Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, United States
S
* Supporting Information
ABSTRACT: A redox-economic method for the direct
coupling of olefins that uses an inexpensive iron catalyst
and a silane reducing agent is reported. Thus, unactivated
olefins can be joined directly to electron-deficient olefins
in both intra- and intermolecular settings to generate
hindered bicyclic systems, vicinal quaternary centers, and
even cyclopropanes in good yield. The reaction is not
sensitive to oxygen or moisture and has been performed
on gram-scale. Most importantly, it allows access to many
compounds that would be difficult or perhaps impossible
to access using other methods.
wo-phase terpene total synthesis has been demonstrated to
be a useful strategy to access steroids, diterpenes, and
T
sesquiterpenes.1 The approach consists of a cyclase phase, where
a lowly oxidized terpene skeleton is rapidly constructed from
inexpensive starting materials, and an oxidase phase, where a
series of site-selective oxidations furnishes the desired natural
product. Our laboratory is interested in pursuing such a strategy
toward the ent-kaurane family of diterpenoids,1d whose members
have been utilized in traditional Chinese medicine for centuries
as they possess anticancer and antibacterial activities.2 Analysis of
an archetypical ent-kaurane, rosthorin A (1, Figure 1A), through
the two-phase paradigm led to 2 as a proposed cyclase-phase end
point. The C4−C5 motif in 2 appears in numerous terpenes and
is biosynthetically derived from a polyene cyclization where C4
and C5 formally are electrophilic and nucleophilic positions,
respectively.3 Polarity reversal4 of this disconnection led to 3 and
inspired a method for directly adding an olefin, one of the most
ubiquitous functionalities in organic chemistry, to an enone.
Here we describe the development of a simple catalytic method
for forging this type of C−C bond and demonstrate its scope in
both intra- and intermolecular modes, even in highly sterically
demanding contexts.
Figure 1. (A) Retrosynthesis of rosthorin A. (B) Examples of C−C
bond formation that directly utilize alkynes and olefins. (C) Recent
hydrofunctionalizations of unactivated olefins inspire reductive olefin
coupling. (D) Optimization of reaction conditions. aRatios by GC/MS.
The disconnection leading 2 to 3 is desirable from the
standpoint of redox economy.5 Some inspirational and relevant
examples are depicted in Figure 1B, where alkynes and olefins are
used directly (i.e., without functional group interconversions) to
form C−C bonds through advances in Ni catalysis,6 metathesis,7
reductive aldol reactions,8 and hydrogenative C−C bond
formation.9 Of particular relevance to us are the powerful olefin
functionalization methods of Boger,10 Carreira,11 Mukaiyama,12
and others13−15 which showed that unactivated olefins can be
converted to putative radical species that can be intercepted by
heteroatom acceptors (Figure 1C). Only three categories of
C−C bond formation using these methods are reported to date:
hydrocyanation with TsCN,10b,11f hydroaldoximation and hydro-
cyanooximation with phenyl sulfonyl oximes,11i and cyclization
of 1,6-dienes.10b,15 With these promising precedents, initial
studies toward a reductive cyclization16 were carried out.
When a simplified variant of proposed intermediate 3, enone
4, was subjected to Boger’s Fe2(ox)3·6H2O and NaBH4
conditions,10 the cis-fused decalin 7 was obtained along with
both unreacted starting material and a small amount of allylic
alcohol 5 (Figure 1D, entry 1). Although the trans-decalin was
Received: November 19, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja4117632 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
reductant led to the recovery of starting material (entry 6),
whereas PhSiH3 led to full conversion to the desired 7 (entry 7).
When the amount of Fe(acac)3 was decreased to 30 mol%, 7 was
still formed exclusively (entry 8) with 60% isolated yield (Table
1), demonstrating that this reductive olefin coupling could be run
in a catalytic fashion. Although not essential, the use of ethylene
glycol as a cosolvent in entry 8 prevented the formation of
PhSi(OEt)3, a byproduct of the reaction that complicated
purification.
Table 1. Scope of the Intramolecular Reductive Coupling
Other substrates were also found to be amenable to
intramolecular coupling (Table 1), with the reaction succeeding
in more sterically demanding contexts, such as the formation of
vicinal all-carbon quaternary centers.18 Thus, cyclizing benzyl
geranate (8), N-benzylgeranamide (10), and citral (12) to
cyclopentanes 9, 11, and 13 proceeded smoothly in 76, 93, and
79% yield, respectively. As the formation of 9 demonstrates,
decreasing the amount of Fe(acac)3 to 20 mol% and using DCE
as cosolvent instead of EtOH still resulted in cyclization. The
cyclizations were found to be insensitive to oxygen and moisture
and were generally complete in <1 h.
Highly strained ring systems could also be formed, as
evidenced by the cyclization of the skipped diene α-ionone
(14) to generate cyclopropane 15 in 97% yield. This cyclization
could also be conducted easily on gram-scale, albeit with a small
reduction in yield (81%). Although cyclopropanes have been
previously formed by irradiating skipped dienes in specialized
cases,19 to our knowledge, this represents the first general
method for such a transformation. To our surprise, even
(+)-nootkatone (16) was cyclized in 64% yield to give highly
congested bicyclo[2.2.1]heptane 17, in which one of the newly
formed adjacent quaternary centers is neopentyl. Such ring
systems would be difficult to construct otherwise.
The applicability of this method to intermolecular settings was
next examined by evaluating the reaction of a series of “donor”
olefins with 3 equiv of methyl vinyl ketone (MVK) as an
“acceptor” (Table 2). Upon exposure to the standard conditions,
1-methylcyclohexene (18) proved to be a competent donor, with
the corresponding adduct 19 isolated in 66% yield. Although
homocoupling of the participating olefins was anticipated, such
a
b
Used 30 mol% Fe(acac)3 in EtOH/(CH2OH)2 (5:1). Used 20
c
mol% Fe(acac)3 in DCE/(CH2OH)2 (5:1). Run on gram-scale with
2.0 equiv PhSiH3. Used 1.5 equiv PhSiH3.
d
originally desired, the cis-decalin still provided an opportunity to
develop a general C−C bond-forming reaction. Thus,
optimization of the formation of cis-decalin 7 was pursued.
Since NaBH4 would limit the functional group tolerance of the
reaction, milder reducing agents were explored. NaBH(OAc)3
eliminated the formation of 5 but resulted in the undesired
formation of 6, presumably from a competitive vinylogous Prins
cyclization where Fe2(ox)3·6H2O simply acts as a Lewis acid17
(entry 2). This byproduct was exclusively formed when
(TMS)3SiH and Et3SiH were used as the reducing agent (entries
3 and 4), whereas PhSiH3 produced cis-decalin 7 as the major
product along with a small amount of 6 (entry 5).
Switching the Fe(III) salt to Fe(acac)3 gave a monophasic
reaction medium.10a Under these conditions, use of Et3SiH as the
Table 2. Donor Scope of the Intermolecular Reductive Coupling
a
b
Used 2.5 equiv PhSiH3. Used 100 mol% Fe(acac)3.
B
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Journal of the American Chemical Society
Communication
with acrylonitrile on gram-scale to provide nitrile 41 with only a
minor reduction in yield. Phenyl vinyl sulfone also participated in
the reaction, although it required a catalyst loading of 40 mol%
and gave moderate yields of 42. The reaction proceeded more
smoothly using doubly activated acceptors, with dimethyl
fumarate providing diester 43 in 90% yield.
Table 3. Acceptor Scope of the Intermolecular Reductive
Coupling
Cyclic acceptors generally gave slightly lower yields, with 2-
furanone providing adduct 44 in 61% yield. Cyclic enones could
also be used, with 2-cyclopentenone furnishing 45 and 46 in 65%
and 60% yield, respectively. Substitution at the α-position of the
acceptor olefin was also tolerated, although the α-methylated
cyclopentanone 47 was obtained in a diminished 38% yield as a
2:1 mixture of diastereomers. Interestingly, the use of 2-
cyclohexenone as an acceptor led to a significantly lower yield
of 48 as compared to 45, even when a stoichiometric amount of
Fe(acac)3 was used. Finally, a heteroaromatic acceptor, acridine,
could be employed to generate 49 as a proof of principle for
applying this chemistry in reductive Minisci-type functionaliza-
tion.22
The mechanism of this reductive olefin coupling was probed
through a variety of experiments (Figure 2). Fe(acac)3 was found
to be necessary for the coupling, as its omission resulted in only
recovery of starting material (Figure 2A). Interestingly, running
the reaction under oxygen-free conditions in a Schlenk tube fully
cyclized α-ionone (14) to cyclopropane 15, indicating that
oxygen was not responsible for reoxidizing the iron catalyst.
a
b
Used 1.0 equiv donor and 3.0 equiv acceptor. Used 40 mol%
c
d
e
Fe(acac)3. Run on gram-scale. Used DCE/(CH2OH)2 (1:1). Used
100 mol% Fe(acac)3. Used 1.0 equiv donor and 1.1 equiv acceptor.
f
products were not observed. Furthermore, the newly forged
bond was formed at the more substituted side of the donor olefin,
in a fashion complementary to the hydroboration/radical
conjugate addition developed by Renaud20 and the hydro-
zirconation/conjugate addition sequence developed by Wipf.21
Both of those methods result in the opposite regioselectivity,
where bond formation occurs at the least substituted side of the
donor olefin.
Tolerated functional groups include TBS ethers and tert-butyl
carbamates, as demonstrated by the formation of 21 and 23 in 73
and 62% yield, respectively. The use of styrene (24) as a donor
led to ketone 25 as the major product in 87% yield. Additionally,
the methodology tolerated heteroaromatic functionalities,
although in modest yields, with 3-vinylpyridine (26) and N-
Boc-3-prenylindole (28) giving 27 and 29 in 42 and 34% yield,
respectively.
A competitive process that was occasionally observed was the
reduction of the donor olefin to its saturated counterpart. Thus,
coupling 1-dodecene (30) to MVK resulted in the isolation of
dodecane (not shown) in 30% yield along with the desired
ketone 31 in 40% yield. The sterically encumbered estrone 3-
methyl ether derivative 33, which bears two vicinal all-carbon
quaternary centers, was isolated in 54% yield as a single
diastereomer, with the reduced derivative of 32 (not shown) also
isolated in 17% yield. However, MVK could be added to
(+)-sclareolide derivative 34 from its least hindered face to
produce 35 in 56% yield without reduction of the starting
material.
The scope of the acceptor olefins was next probed, using 1-
methylcyclohexene (18) or TBS ether 20 as the donor (Table 3).
In addition to MVK, methyl acrylate, N,N-dimethylacrylamide,
and acrylonitrile all served as efficient acceptors, providing 36−
41 in yields of 64−81%. Notably, TBS ether 20 can be coupled
Figure 2. (A) Control experiments. (B) Deuterium labeling studies. (C)
Proposed mechanism of the reductive olefin coupling.
C
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Journal of the American Chemical Society
Communication
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cyclopropane 50, bearing a sole deuterium atom adjacent to the
ketone (Figure 2B). The lack of deuterium incorporation on the
cyclohexane ring in both instances suggests that the H that adds
across the unactivated olefin originates from PhSiH3. Addition-
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A mechanistic scenario consistent with these results is outlined
in Figure 2C. Donor olefin 53 would abstract a hydrogen radical
from Fe hydride23 52derived from Fe(III) species 51 and
24
PhSiH3 to generate reduced Fe species 55 and tertiary radical
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Conjugate addition of 54 into Michael acceptor 56,25 followed by
single-electron transfer with 55, would provide intermediate 58
and regenerate 51, which would re-enter the catalytic cycle.
Protonation of 58 would give the coupled product 59.
In conclusion, a practical method for the reductive coupling of
olefins that utilizes a readily available and inexpensive Fe source
as a catalyst has been developed. This reaction is operationally
simple and has been run on gram-scale. Furthermore, it builds
molecular complexity rapidly, with most reactions reaching
completion in <1 h. The coupling works in both intra- and
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systems, cyclopropanes, and vicinal quaternary centers.26 The
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terpene synthesis.
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and analytical data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
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AUTHOR INFORMATION
Corresponding Author
pbaran@scripps.edu
■
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Notes
The authors declare no competing financial interest.
(18) Quaternary Stereocenters: Challenges and Solutions for Organic
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ACKNOWLEDGMENTS
■
Financial support for this work was provided by NIH/NIGMS
(GM-097444), NSF (predoctoral fellowship to J.C.L.), and JSPS
(postdoctoral fellowship to Y.Y.).
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