Regioselective nickel-catalyzed reductive couplings of enones and allenes

Article abstract of DOI:10.1002/anie.201004740

Alkenes made easy: In a complement to coupling processes of terminal alkynes, the reductive coupling of enones and allenes provides access to conjugate addition products that possess a 1,1-disubstituted alkene (see scheme; cod=1,5-cyclooctadiene). The sol

Full text of DOI:10.1002/anie.201004740

Communications
DOI: 10.1002/anie.201004740
Synthetic Methods
Regioselective Nickel-Catalyzed Reductive Couplings of Enones and
Allenes**
Wei Li, Nan Chen, and John Montgomery*
Catalytic reductive coupling processes have been developed
in numerous contexts as a strategy for the regio- and
stereoselective installation of alkenes.[1] The direct coupling
of a polar p system with a relatively nonpolar p system is
often a successful strategy for selectively accomplishing
synthetically desirable heterocouplings while avoiding unde-
sired homocoupling of either reagent. For example, reductive
couplings of aldehydes, enones, enals, or imines paired with
alkynes, alkenes, or allenes have been extensively developed
in recent years. Processes of this type are often highly
effective in controlling both the position and stereochemistry
of di-, tri-, and tetrasubstituted alkenes within polyfunctional
molecules. Among these methods, the preparation of g,d-
unsaturated carbonyl groups by nickel- and cobalt-catalyzed
processes has been the subject of considerable study. The
catalytic strategies developed for synthesis of g,d-unsaturated
carbonyl groups by the coupling of two p-containing compo-
nents include enone–alkyne additions in work reported by our
group[2,3] and by Cheng and co-workers,[4] and enone–alkene
additions in work reported by Jamison and co-workers,[5] and
Ogoshi et al.[6] (Scheme 1). Processes of this type provide an
important counterpart to organocuprate technology and
hydrometallative processes, while having the advantage of
not requiring the stoichiometric preparation of an alkenyl
metal species.
Stereodefined trisubstituted alkenes 1, trans-disubstituted
alkenes 2, and monosubstituted alkenes 3 may be readily
prepared by the above-mentioned enone–alkyne and enone–
alkene coupling processes. However, a general strategy for
the preparation of 1,1-disubstituted alkenes 4 by these
methods has not been developed.[7,8] Since terminal alkyne
reductive coupling strategies and hydrometallative processes
typically favor formation of the conjugate addition product 2
with a 1,2-disubstituted alkene, the 1,1-disubstituted alkene 4
is considerably more difficult to prepare.[9,10] As depicted
below (Scheme 2), regiocontrolled reductive coupling of an
Scheme 2. New entry into g,d-unsaturated ketones with a 1,1-disubsti-
tuted alkene.
enone and allene could potentially provide efficient access to
product 4, although issues of regioselectivity and chemo-
selectivity often render intermolecular processes involving
allenes difficult to optimize into efficient processes.[11] Cata-
lytic couplings of enones with allenes have been developed as
efficient entries into 1,3-dienes,[12] cyclopentanols,[13] and d,e-
unsaturated carbonyl compounds;[14] however, the direct
catalytic reductive coupling of enones and allenes has not
been previously described. Herein, we describe that highly
regioselective catalytic reductive coupling of enones and
terminal allenes provides an effective and general solution to
the preparation of g,d-unsaturated carbonyls that possess the
1,1-disubstituted alkene substructure (4).
Our efforts to develop the desired reductive coupling of
enones and allenes began with a broad screen of ligands and
reducing agents in the coupling of the enone 5a and allene 6a,
using [Ni(cod)2] (10 mol%) as the precatalyst. The use of
Et3B as a reducing agent with monodentate phosphines led to
inefficient couplings in THF (Table 1, entry 1), but the use of
a methanol/THF mixed solvent system afforded efficient
couplings to provide a 41:59 mixture of regioisomers 4a and
1a (Table 1, entry 2). Notably, since regioisomer 1 may be
obtained by reductive couplings of internal alkynes with
enones (Scheme 1), we quickly focused on efforts to optimize
the formation of isomer 4 given the lack of efficient methods
for preparing this structural motif. In addition to PPh3, other
monodentate phosphines such as PCy3 and P(o-tol)3 were
Scheme 1. Previously reported entries into g,d-unsaturated ketones.
[*] W. Li, N. Chen, Prof. J. Montgomery
Department of Chemistry, University of Michigan
Ann Arbor, MI 48109-1055 (USA)
Fax: (+1)734-763-1106
E-mail: jmontg@umich.edu
Homepage: http://www.umich.edu/~jmgroup
[**] We acknowledge support from the National Science Foundation
(CHE-0718250). N.C. acknowledges support from NSF REU grant
CHE-0754639.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004740.
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Table 1: Optimization of the enone–allene reductive couplings.
primary focus for gaining access to the 1,1-disubstituted
alkene products, 1,3-disubstituted cyclic and acyclic allenes
were also effective participants in preparing mixtures of
stereoisomeric trisubstituted alkene products in which the Z
alkene stereochemistry is favored (Table 2, entries 13 and 14).
The high levels of regioselectivity and stereoselectivity
observed in this report are noteworthy, given the complexities
with these issues that are often seen with allene addition
processes. Presuming that the processes involve an initial
oxidative cyclization to a metallacyclic intermediate,[15] six
different metallacycles (7—12) could potentially be formed
with different regiochemical arrangements or with different
stereochemistry of the exocyclic alkene (Scheme 3). Addi-
tionally, metallacycles 9 and 12 could each be formed as a
mixture of diastereomers. Of these possibilities, 7–9 are
Entry Ligand
Solvent
THF
THF/MeOH (1:8) Et3B
THF/MeOH (1:8) Et3B
Reductant Yield [%] (4a/1a)[a]
1
2
3
4
5
6
7
8
9
10
PPh3
PPh3
PCy3
Et3B
39 (43:57)
73 (41:59)
71 (31:69)
63 (31:69)
77 (93:7)
84 (93:7)
58 (62:38)
27 (81:19)[b]
<10
P(o-tol)3 THF/MeOH (1:8) Et3B
PPh3
PPh3
PPh3
PCy3
IMes
IPr
THF
toluene
THF/MeOH (1:8) Et3SiH
toluene
THF
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
THF
<10
À
derived from C C bond formation at the allene central
carbon atom, as required for formation of the observed
[a] Product ratios were determined for crude reaction mixtures by using
GC and NMR analyses. Yield refers to combined yield of all product
isomers recovered after chromatographic purification. Unless otherwise
noted, compound 1a was obtained as the E isomer. [b] Compound 1a
was obtained as a 59:41 ratio of E and Z isomers. IMes=N,N’-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene, IPr=N,N’-bis(2,6-(diisopropyl)phe-
nyl)imidazol-2-ylidene.
À
products. Metallacycles 10–12 would be derived from C C
bond formation at an allene terminal carbon atom, and are
therefore inconsistent with the products obtained.
À
The regioselectivity of the initial C C bond-forming step
in other classes of reductive couplings of allenes has been
shown to strongly depend upon the nature and Lewis acidity
of the reducing agent.[16–19] Processes selective for addition to
the internal or the terminal carbon atoms of allenes have both
been demonstrated in various nickel-catalyzed coupling
processes. For example, the extensive developments from
Ng and Jamison involving allene–aldehyde reductive coupling
reactions,[16] as well as the related allene–CO2 coupling
processes developed by Mori and co-workers,[17] both involve
effective with Et3B in methanol/THF to obtain similar
mixtures of products 4a and 1a (Table 1, entries 3 and 4).
We then examined the use of Et3SiH as a reducing agent, and
couplings with PPh3 as a ligand in THF were effective and
highly selective for the desired product 4a (Table 1, entry 5).
Results were comparable with Et3SiH and PPh3 in toluene
(Table 1, entry 6). However this reducing agent/ligand com-
bination in MeOH/THF afforded a 2:1 mixture of 4a and 1a
in moderate yield (Table 1, entry 7). Couplings with Et3SiH
and PCy3 in toluene were low yielding (Table 1, entry 8).
Additionally, couplings with Et3SiH and either of the N-
heterocyclic carbenes IMes and IPr in THF provided low
yields of 4a.
On the basis of the above experiments, we opted to
additionally explore the formation of 1,1-disubstituted alkene
products of general structure 4 using the optimized reaction
conditions (Et3SiH and [Ni(cod)2] (10 mol%), PPh3 (20
mol%), toluene, 508C; Table 1, entry 6). Under these reac-
tion conditions, reductive couplings of methyl vinyl ketone
were efficient with a range of monosubstituted allenes
(Table 2), including aliphatic (Table 2, entry 1) and aromatic
(Table 2, entry 2) allenes, those possessing silyloxy (Table 2,
entry 3), acetoxy (Table 2, entry 4), or hydroxy functionality
(Table 2, entry 5) in a remote position, or an alkoxy functional
group directly attached to the allene (Table 2, entry 6). All
cases were selective for the desired 1,1-disubstituted alkene
product, with the exception of a hydroxy-containing allene,
À
C C bond formation at the allene central carbon atom. In
contrast, the inter- and intramolecular nickel-catalyzed alde-
hyde–allene coupling reactions involving organozinc reagents
reported independently by Kang and Yoon, and by our group,
exclusively involved addition to the terminal carbon atom of
the allene.[18] Extensive work from Krische and co-workers
has demonstrated couplings at an allene terminal carbon
atom through transfer hydrogenative couplings of allenes
with alcohols.[19] Alkylative cyclizations of allenes with
electron-deficient alkenes were reported in our synthetic
approach to kainic acid, where intramolecular additions were
observed exclusively at the proximal terminal carbon atom of
the allene.[14] As seen with the allene–aldehyde reductive
couplings involving silane or borane reducing agents noted
above, the coupling reactions developed in this work exclu-
sively involve addition at the allene central carbon atom,
which is consistent with the involvement of metallacycles 7–9.
The observed regioselectivity of enone–allene reductive
couplings using silane reagents in THF or toluene is most
readily rationalized by the involvement of metallacycle 9.
Cases where metallacycle 7 would be destabilized by allylic1,3
strain derived from substitution of R3 and R4 (i.e. Table 2,
entries 8–11) are highly selective for the desired terminal
methylene isomer 4. Selective formation of metallacycle 9 and
subsequent s-bond metathesis of the Ni–O and Si–H bonds
would afford the intermediate 13 (Scheme 4). Direct reduc-
tive elimination of this intermediate without allylic rearrange-
ment would afford the observed product 4 after hydrolysis of
the enol silane 14. In contrast, we propose that reactions
which predominantly formed
a regioisomeric product
(Table 2, entry 5). The interesting mechanistic basis for
differing regioselectivity in the presence of hydroxy function-
ality (either in the substrate or solvent) is discussed below.
Variation in the enone substrate is also tolerated, as
evidenced by the efficient participation of a-alkyl-substituted
and b-alkyl- or b-aryl-substituted enones (Table 2, entries 7–
12). Additionally, although monosubstituted allenes were our
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Table 2: Scope of the enone–allene reductive couplings.
Entry
Enone
Allene
Product
Entry
8
Enone
Allene
Product
yield [%] (isomer ratio)[a]
yield [%] (isomer ratio)[a]
1
6a
5a
5a
6b
6c
73 (78:15:7)
50 (91:9)
5c
5d
51 (>95:5)
90 (>95:5)
2
3
4
5
9
6a
6b
6e
6a
5a
5a
5a
10
11
12
6d
6e
85 (>95:5)
5e
5e
72 (93:5:2)
70 (90:5:5)
68 (79:21)
6 f
85 (78:13:9)[b]
63 (79:21)[c]
5 f
78 (>95:5)
6
7
5a
13
14
5a
6g
6h
6i
59 (78:22)
5a
5b
6a
71 (>95:5)
78 (87:13)
[a] Major product is depicted in the table. Minor isomers are E and Z isomers of structures analogous to compound 1a unless otherwise noted.
Product ratios were determined for the crude reaction mixtures by using GC and NMR analyses. Yield refers to combined yield of all product isomers
recovered after chromatographic purification. [b] Major isomer is depicted. Minor isomers are the Z isomer of the product shown (13%) and the
terminal methylene isomer (9%). [c] Yield refers to the major isomer in this example. The minor isomer detected by GCMS analysis was not isolated.
Cy =cyclohexyl, Bn=benzyl, TBS=tert-butyldimethylsilyl.
conducted in a methanol/THF mixed solvent system proceed
by protonation of the metallacyclic intermediate before
hydride (from Et3SiH) or ethyl (from Et3B) transfer oc-
curs.[2e,h] In this event, the species 15 would be produced by
methanol addition to metallacycle 9. Unlike species 13, which
can undergo reductive elimination faster than allylic isomer-
ization occurs, the species 15 and 16 are likely longer lived and
can interconvert via p-allyl complex 17. Species 18 and 19 may
similarly undergo interconversion faster than b-hydride
elimination occurs. Thus, pathways that involve either protic
solvents, protic reagents, or triethylborane likely proceed
through pathways that present opportunities for regiochem-
ical erosion that are not available to the optimized procedure
using triethylsilane in aprotic media. The formation of
mixtures of 4 and 20 for reactions conducted in methanol
(Table 1, entries 2–4 and 7), with unprotected hydroxy
Scheme 3. Possible metallacycles in enone–allene couplings.
functionality (Table 2, entry 5), or with triethylborane in
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aprotic media (Table 1, entry 1) are consistent with this
rationale.
In conclusion, an efficient and highly regioselective
catalytic reductive coupling of enones and allenes has been
developed. With terminal allenes, this process selectively
produces g,d-unsaturated carbonyl products with a 1,1-
disubstituted alkene, which are not generally available by
other reductive coupling procedures. The process provides an
alternative to the stoichiometric generation of alkenyl
cuprate reagents, which are somewhat cumbersome to gen-
erate for the alkene 1,1-disubstitution pattern targeted by this
approach.
[7] A single example of the catalytic addition of 1-octene to a b-
unsubstituted enone was previously reported by Jamison and co-
workers (reference [5]). The reaction afforded enol silanes with
the 1,1- and 1,2-disubstitution pattern as a 4:1 mixture of
regioisomers 4 and 2. Aromatic alkynes cleanly provided the 1,2-
disubstituted isomer 2. As this procedure was largely developed
for ethylene additions to generate product 3, the generality with
alkyl-substituted olefins for obtaining the 1,1-disubstitution
pattern (4) across a range of substrates was not established in
this report. The report from Ogoshi et al. (reference [6])
illustrated that, without silyl triflate promoters and using b-
substituted enones, the 1,2-disubstituted isomer 2 was exclu-
sively obtained, irrespective of the electronic nature of the
alkene.
[8] Jamison has extensively developed the synthesis of 1,1-disub-
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Experimental Section
General procedure for enone–allene reductive coupling: Toluene
(1 mL) was added to a mixture of [Ni(cod)2] (0.03 mmol) and
triphenylphosphine (PPh3; 0.06 mmol) at RT. After stirring for 5–
10 min at RT, triethylsilane (0.6 mmol) and enone (0.3 mmol) were
sequentially added by syringe to the dark red mixture. The allene
(0.45 mmol) in toluene (4 mL) was then added using a syringe pump
(1 mLhÀ1) while the reaction mixture was heated at 508C. The
reaction mixture was stirred at 508C until TLC analysis indicated the
disappearance of the enone. The reaction mixture was concentrated,
and then diluted with ethyl acetate. A solution of tetrabutylammo-
nium fluoride (nBu4NF; 0.6 mmol) in THF was added to the reaction
mixture and stirred until TLC analysis indicated the disappearance of
the enol silane product. The reaction mixture was washed with brine,
dried over magnesium sulfate, filtered, concentrated, and then the
residue was purified by column chromatography on silica gel.
Received: July 30, 2010
Published online: October 7, 2010
Keywords: allenes · conjugate addition · nickel ·
.
reductive coupling · regioselectivity
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