Synthesis of cyclopentenols and cyclopentenones via nickel-catalyzed reductive cycloaddition

Article abstract of DOI:10.1021/ja206722t

Strategies for the reductive cycloaddition of enals or enoates with alkynes have been developed. The enal-alkyne cycloaddition directly affords cyclopentenols, whereas the enoate-alkyne cycloaddition affords the analogous cyclopentenones. The mechanism of

Full text of DOI:10.1021/ja206722t

ARTICLE
pubs.acs.org/JACS
Synthesis of Cyclopentenols and Cyclopentenones via
Nickel-Catalyzed Reductive Cycloaddition
Aireal D. Jenkins, Ananda Herath, Minsoo Song, and John Montgomery*
930 North University Avenue, Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055, United States
S Supporting Information
b
ABSTRACT: Strategies for the reductive cycloaddition of enals or
enoates with alkynes have been developed. The enalꢀalkyne cycload-
dition directly affords cyclopentenols, whereas the enoateꢀalkyne
cycloaddition affords the analogous cyclopentenones. The mechanism
of these processes likely involves formation and protonation of a
metallacyclic intermediate. The general strategy provides a straightfor-
ward entry to five-membered ring products from simple, stable π-systems.
Scheme 1. Redox Description of Cycloaddition Modes
’ INTRODUCTION
The assembly of five-membered rings by cycloaddition has
been the subject of considerable study in many different contexts.
It has traditionally been accomplished by the use of two-
component cycloadditions involving 1,3-dipolar species,1 vinyl
carbenoid species,2 or multifunctional cyclopropanes.3 Addition-
ally, a number of multicomponent cycloadditions typified by the
PausonꢀKhand reaction provide attractive and versatile cycload-
dition entries to five-membered ring products.4 Alternatively,
compared with more specialized or reactive reagents, the routine
use of only simple, stable π-systems such as alkenes, alkynes,
dienes, or unsaturated carbonyls in a two-component cycloaddi-
tion would present advantages, including ease of access to the
requisite reagents and the ability to advance the cycloaddition
precursors through more complex linear synthetic sequences. In
a generic example involving substrates 1 and 2, cycloaddition
where the terminal atoms participate in bond formation would
afford a six-membered ring product 3 by hetero-DielsꢀAlder
cycloaddition (Scheme 1).5 The direct formation of a five-mem-
bered ring cycloadduct from these precursors would require forma-
tion of a biradical species or a more complex fused ring bicyclic
product. This complexity in accessing five-membered ring products
by cycloaddition of simple unsaturated systems has rendered the use
of 1,3-dipolar species, functionalized cyclopropanes, and vinylcarbe-
noids exceptionally useful in five-membered ring synthesis.
The inability to form a stable nonradical five-membered ring
product by cycloaddition of precursors such as 1 and 2 can be
avoided if the atomic connectivity is rearranged during the
cycloaddition. For example, the cycloaddition of allyl silanes as
a three-atom component directly affords five-membered ring
products by a process that involves a 1,2-silicon shift during the
ring-forming process.6 Similarly, a number of organocatalytic
processes have been reported to form five-membered ring
products by cascade sequences that involve a 1,2-hydrogen
shift.7 Many other classes of five-membered ring cycloadditions
share the feature of a 1,2-shift accompanying the cycloaddition,
and this strategy provides a conceptual solution to the limitations
cited above in the assembly of five-membered ring products from
simple π-components.
An alternate and distinct strategy for allowing the assembly of
five-membered rings by two-component processes involves a
change in substrate oxidation state during the cycloaddition. For
example, using the substrates 1 and 2 highlighted above, a net two-
electron reduction during cycloaddition would allow a stable five-
membered product 4 to be formed without requiring any change of
the original atomic connectivity established in the starting reagents
(Scheme 1). We report herein the development of a general strategy
for effecting reductive cycloadditions of this type utilizing a nickel-
catalyzed operation. Building upon our initial communication of
[3 + 2]-reductive cycloadditions of enals and alkynes to afford
cyclopentenols,8 we now describe a more detailed analysis of the
strategy and document a related sequence involving [3 + 2]-
reductive cycloaddition of enoates and alkynes as a new entry to
cyclopentenones. Including important recent advances from Cheng
that documented the cobalt-catalyzed [3 + 2] reductive cycloaddi-
tion of enones and allenes,9 from Sato that documented intramo-
lecular titanium-mediated [3 + 2] reductive cycloadditions of
enoates and alkynes,10 from Krische that illustrated a three-
component reductive cycloaddition,11 as well as other classes of pro-
cesses that generate reactive vinyl dianion synthetic equivalents,12
Received: July 19, 2011
Published: August 07, 2011
r
2011 American Chemical Society
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Scheme 2. Envisioned Pathway for Catalytic Reductive
Cycloadditions of Enals and Alkynes
Table 2. Scope of Intramolecular Reductive Cycloadditions
of Enals and Alkynes
entry
R1
R2
X
yield (%)
1
2
3
H
H
H
Ph
Me
Ph
CH2
CH2
O
87
78
44
Table 3. Enal Variation in Intermolecular Reductive
Cycloadditions of Enals and Alkynes
Table 1. Ligand Selection in Intramolecular Reductive
Cycloadditions of Enals and Alkynes
entry
ligand (mol %)
yield (%)
1
2
3
4
5
6
tmedaa (10)
dppeb (10)
DPEphosc (10)
dppfd (10)
PPh3 (20)
77
47
87
82
24
50
PBu3 (20)
a tmeda, tetramethyl(ethylene)diamine. b dppe, 1,2-bis(diphenylphosphino)
c
d
ethane. DPEphos, bis(2-diphenylphosphinophenyl)ether. dppf, 1,10-bis-
(diphenylphosphino)ferrocene.
this work begins to establish the generality of reductive cycloaddi-
tion entries for five-membered ring synthesis.13
’ RESULTS
EnalꢀAlkyne Reductive Cycloadditions. In the course of
studying nickel-catalyzed cyclizations of alkynyl enals, our lab
previously documented the preparation of a nickel metallacycle
derived from the oxidative cyclization of Ni(0) with the enal and
alkyne units.14 A broad range of processes involving protonation
and alkylation of the nickel enolate motif were described in a
series of publications,14 but the development of an efficient
catalytic process with substoichiometric loadings of nickel
proved elusive. In analogy to demonstrations in stoichiometric
processes, the mechanism of the desired catalytic reductive
cycloaddition involving the conversion of enal 5 and alkyne 6
into cyclopentenol 10 was envisioned to involve formation of
metallacycle 7, protonation to afford intermediate 8, and then
vinyl nickel addition to the carbonyl to afford nickel alkoxide 9
(Scheme 2). A requirement for achieving a substoichiometric
catalytic version of this transformation is the in situ reduction of
the nickel(II) alkoxide 9 to release the desired cyclopentenol
product 10 and regenerate the active Ni(0) catalyst.
proved ineffective, the combination of a Ni(COD)2-derived
(COD = 1,5-cyclooctadiene) complex, an organoborane redu-
cing agent, and a cosolvent system involving an alcohol compo-
nent ultimately provided an effective solution.15 To explore the
potential for a catalytic reductive cycloaddition, simple alkynyl
enal 11a was first examined in the production of bicyclooctenol
12a. As illustrated (Table 1), a number of monodentate and
bidentate ligands were effective in promoting the organoborane-
mediated procedure when a THF/methanol cosolvent system
was employed.
On the basis of the high chemical yields achieved with a nickel
DPEphos (bis(2-diphenylphosphinophenyl)ether) catalyst (Table 1,
entry 3), a few examples were explored under these conditions. As
illustrated (Table 2), bicyclooctenol products 12 were efficiently
obtained with substrates that possessed aryl and alkyl-substituted
alkynes (entries 1 and 2). Additionally, an oxygen heterocycle could
be accessed in moderate yield (entry 3). While these experiments
duplicated the types of compounds that had previously been accessed
A key hurdle was the need to have a Ni(0) active catalyst, a
Brønsted acid or electrophile, and an effective reducing agent all
present in the reaction medium. While many potential solutions
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Table 4. Alkyne Variation in Intermolecular Reductive
Cycloadditions of Enals and Alkynes
Table 5. Comparison of Enoate Derivatives in Reductive
Cycloadditions of Enoates and Alkynes
β-disubstituted enal with an internal alkyne. In some instances,
acyclic reductive coupling processes were observed as minor
components of the reaction mixture. This was particularly pre-
valent with internal alkynes undergoing addition to enals with an
R-substituent, as illustrated (Table 4, entry 5), where 60% yield of
the acyclic reductive coupling product 16 was observed. This acyclic
pathway also dominates when couplings of enones and alkynes are
attempted, and a full description of the acyclic addition pathway in
this context has been reported elsewhere.16,17
Given the superiority of DPE-Phos-promoted intramolecular
versions, the PBu3 procedure was not extensively studied in
tethered systems. However, two examples involving the formation
of a triquinane framework 18 from substrate 17 were established
using the optimized PBu3-promoted procedure (eq 1).
in stoichiometric procedures,14 we quickly turned our attention to
intermolecular variants. As seen with the stoichiometric procedures,
catalytic intermolecular processes were completely ineffective with
the Ni(0)-DPEphos catalyst.
Upon reexamining the ligands from Table 1 in intermolecular
processes, we found that monodentate phosphines, while less
effective than DPEphos in intramolecular processes, were sig-
nificantly more effective than bidentate ligands in intermolecular
processes. Using an optimized procedure with Ni(COD)2, PBu3,
Et3B, and THF:MeOH (1:8), a broad range of intermolecular
reductive cycloadditions involving the conversion of enals 13 and
phenyl propyne into cyclopentenols 15 were accessible (Table 3).
Enals witha singlesubstituentat the R- or β-positionwereeffective
participants, as examples with aromatic or aliphatic substituents at
the β-position or a methyl substituent at the R-position proceeded
in good to excellent yield (Table 3, entries 1ꢀ3). Additionally,
acyclic and cyclic enals with a trisubstituted alkene underwent
effective cycloaddition (Table 3, entries 4ꢀ6), with the latter case
providing access to bicyclic products.
Significant variation of the alkyne is also possible, as demon-
strated with the use of silyl alkynes (Table 4, entries 1ꢀ3), terminal
alkynes (Table 4, entry 4), and internal alkynes with two aromatic
(Table 4, entries 5 and 6) or two aliphatic (Table 4, entry 7)
substituents. Embedded within these examples are further illustra-
tions of the involvement of R-substituted and β-substituted enals.
A single example with a β,β-disubstituted enal is provided (Table 4,
entry 4); however, this substrate class is only effective with terminal
alkynes due to the substantial steric demand of coupling a β,
EnoateꢀAlkyne Reductive Cycloadditions. While the cy-
clopentenol products described above are useful building blocks
for many applications, the corresponding cyclopentenones are
also broadly useful in a variety of synthetic transformations.
Rather than preparing the starting enals 13 for reductive
cycloaddition, and then reoxidizing the cyclopentenol product
15 into a desired cyclopentenone, we realized that the direct
conversion of a more stable and readily accessible enoate
derivative into a cyclopentenone product would be considerably
more efficient. With an eye toward developing this transforma-
tion, we examined a number of enoate derivatives 19ꢀ22 in
Ni(0)-PBu3-promoted reductive cycloadditions with phenyl
propyne (Table 5). While simple methyl enoates 19 were
unreactive in the reductive cycloaddition procedure (entry 1),
the use of more reactive phenyl enoates 20, unsaturated acyl
oxazolidinones 21, and unsaturated N-acylpyrroles 2218 partici-
pated in effective coupling procedures to afford the desired
cyclopentenone 23a or acyclic reductive coupling products 24
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Table 6. Ligand Selection in Reductive Cycloadditions of
Enoates and Alkynes
Table 7. Scope of Reductive Cycloadditions of Enoates and
Alkynes
entry
R1
R2
R3
R4
% yield (regiosel)
entry
ligand (mol %)
% yield
1
2
H
Me
H
Hex
Me
Oct
Oct
Me
Me
Me
Oct
Hex
Ph
H
87 (--)
1
2
3
4
5
PBu3 (20 mol %)
PPh3 (20 mol %)
PCy3 (20 mol %)
IMes (10 mol %)
IPr (10 mol %)
44
35
81
87
46
H
69 (93:7)
67 (90:10)
66 (99:1)a
77 (86:14)
19 (64:36)b
97 (95:5)
63 (97:3)c
68 (87:13)c
89d
3
H
H
4
H
H
H
5
H
Me
H
Ph
Ph
Ph
H
6
Me
H
7
Ph
H
(entries 2ꢀ4). While N-acylpyrroles participated in very efficient
reductive couplings with alkynes to afford acyclic product 24
(entry 4), the unsaturated acyl oxazolidinone and phenyl enoate
participated in efficient reductive cycloaddition to produce the
desired cyclopentone 23a while expelling the acyl substituent
(entries 2 and 3). On the basis of the slightly higher chemical yield
and simpler access to phenyl enoates 20 compared with acyl
oxazolidinones 21, phenyl enoates were selected for further study.
Employing 10 mol % Ni(COD)2 and Et3B in THF:MeOH
(50:1), a number of ligands were examined in reductive cycloaddi-
tions of phenyl enoate 20a and alkyne 14b to produce cyclopen-
tenone 23b (Table 6). As depicted, PCy3 and the N-heterocyclic
carbene IMes emerged as the optimum ligands for promoting the
transformation. Further study with other substrate classes illu-
strated the best generality with IMes, and this N-heterocyclic
carbene was therefore the major focus of our further studies.
A series of examples of the Ni(0)-IMes-promoted reductive
cycloaddition to produce cyclopentenones 23 were illustrated under
optimized conditions (Table 7). The process is efficient with simple
unsubstituted enoates using either internal or terminal alkynes
(entries 2ꢀ4). While regioselectivities with the terminal alkyne
were good using IMes, improved regioselectivity was seen with the
bulkier ligand IPr (entry 4), as anticipated from our recent studies of
ligand size-controlled regioselection in another reaction class.19
Although yields were comparable between IMes and IPr for this
particular substrate combination, other examples were generally
lower yielding with IPr. Additional examples (entries 5ꢀ8) illustrate
that a variety of R- and β-substituted enoates undergo this
cycloaddition. While cycloadditions of R-substituted enoates are
effective with terminal alkynes (entry 8), the major pathway of this
enoate with an internal alkyne leads to production of the acyclic
reductive coupling product (entry 6). The mechanistic implication
of this finding is discussed below. Additionally, a number of
functional groups, including free hydroxyls, secondary amines, and
simple esters, are tolerated under the reductive cycloaddition
conditions (entries 9ꢀ12).
8
Me
H
9
Pr
Ph
Ph
Pr
(CH2)2OH
Me
Ph
Ph
Ph
10
11
12
H
(CH2)3OH
H
(CH2)3NHBn
78d
H
(CH2)2CO2Me
78 (92:8)
a Ligand = IPr. b The major product is the acyclic reductive coupling
product 25 (63% yield, see Scheme 4 for discussion). c PCy3 (20 mol %)
was used as ligand in this example. d Yield is of the single regioisomer
shown. The minor regioisomer is unstable, and its yield was not
quantified.
no reports of related catalytic or intermolecular [3 + 2] alkylative
cycloadditions have appeared.
Although processes of this type have not yet been generalized
or optimized, a preliminary example to document the feasibility of
catalytic alkylative [3 + 2] cycloadditions is now illustrated (eq 2).
Upon treating a mixture of phenyl enoate 20a, phenyl propyne,
and benzaldehyde with triethylborane and a catalyst derived from
Ni(COD)2/PBu3 in toluene, a 58% yield of two diastereomers of
product 26, epimeric at the secondary hydroxyl, was obtained.
Aprotic solvents become necessary in the aldol variant of this
procedure, to avoid the rapid enolate protonation that leads to
reductive cycloaddition products such as those illustrated in
Table 7. Further study of the scope and generality of alkylative
cycloadditions of this type is in progress.
’ DISCUSSION
As described above, earlier developments in stoichiometric
reductive cycloadditions of enals and alkynes presented a rational
mechanistic framework for the [3 + 2] reductive cycloaddition of
enals and alkynes.14 On the basis of this analogy, a likely mechan-
istic pathway for the organoborane-mediated catalytic reductive
cycloaddition of enals 13 and alkynes 14 to cyclopentenols 15
involves oxidative cycloaddition of complex 27a to metallacycle
28a (Scheme 3). Triethylborane is known to undergo partial
methanolysis to Et2B(OMe),20 which is therefore depicted in
reactions conducted in cosolvent systems involving methanol.
Catalytic Alkylative [3 + 2] Cycloadditions. All of the above
methods rely on a methanol-mediated protonation, likely of a nickel
or boron enolate species. The derivatization of these intermediate
enolate species through aldol functionalization or alkylation would
represent an important advance for the catalytic methods reported
herein, although considerable additional challenges in both reactivity
and chemoselectivity are presented by advances of this type. Earlier
reports of intramolecular stoichiometric processes described a range
of alkylation reactions for the isolated metallacyclic species,10,14 but
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Scheme 3. Mechanism of Catalytic Reductive
Cycloadditions
Scheme 4. RoleofSubstrateSterics inCyclizationvsCoupling
phenoxide. However, the fact that the phenyl ester is maintained
in the product in select hindered examples suggests that this is not
the case. As seen in one example (from Table 7, entry 6), acyclic
product 25 is generated by the major reaction pathway, and the
desired cyclopentenone 23c is seen only as a minor product
(Scheme 4). We attribute this outcome to the increased steric
hindrance in intermediate 31, which likely impedes the addition
of the vinyl unit to the complexed ester. Observation of this
product illustrates that the ester enolate generated in this example
does not eliminate phenoxide under catalytically relevant condi-
tionsin the presenceofthe methanol solventcomponent. Notably,
by removal of either the enoate R-substituent (product 23a
Table 7, entry 5) or the acetylenic substituent (product 23d
Table 7, entry 8), five-membered ring closure efficiently occurs,
and acyclic products are not observed (Scheme 4). Furthermore,
while this study largely focuses on the reaction of phenyl enoates, it
is worth noting that unsaturated acyl oxazolidinones, which also
undergo efficient reductive cycloadditions (Table 5), have been
demonstrated to undergo many metallacycle-based transforma-
tions without extrusion of the oxazolidinone unit.24
The preliminary alkylative cycloaddition example involving aldol
addition (product 26, eq 2) may involve a pathway similar to that
depicted in Scheme 3 for reductive cycloadditions of enoates. In this
instance with an aprotic solvent, Et3B is likely the active reducing
agent rather than (MeO)BEt2. Rather than protonation of metalla-
cycle 28b by methanol as seen in reductive cycloadditions, the
enolate motif of 28b likely undergoes aldol addition instead but
otherwise follows the illustrated sequence analogous to that shown
for product 23. Given the slower rate of aldol addition compared with
proton transfer, alternate pathways such as that involving phenoxide
extrusion from intermediate 28b may be likely in cases involving aldol
addition. Future studies of the scope and mechanism of alkylative
versions of the [3 + 2] cycloadditions will be reported in due course.
Complexation of the borane in intermediate 27a likely accelerates
the rate of the oxidative cyclization. In the presence of methanol,
fast monoprotonation of the enolate unit of 28a would afford vinyl
nickelspecies 29a, and directadditionof thevinyl nickel unittothe
borane-complexed aldehyde of 29a would afford boron alkoxide
30a with release of a Ni(II) species. The borane then serves as the
reducing agent to regenerate the active Ni(0) catalyst.
The precise nature of the interaction of the organoborane in
species 27a is unclear. While a simple Lewis acidic interaction of
the borane with the aldehyde carbonyl is depicted for simplicity,
computational studies suggested a more complex interaction in
similar organozinc-mediated oxidative cyclizations when no
phosphine or N-heterocyclic carbene ligands were employed.21
The role of organoborane Lewis acidity was recently studied in
other classes of oxidative cyclization processes.22 Furthermore,
several possibilities exist for the structure of the initially formed
alkoxide product. Mechanisms can be envisioned that generate
either boron alkoxide 30a as shown or alternatively a nickel
alkoxide during addition of the vinyl nickel unit to the aldehyde in
structure 29a. However, it is important to note that, in the absence
of triethylborane, no five-membered ring cyclization is observed
when enals are employed. Instead, an internal redox reaction
predominates in the absence of borane, leading to acyclic products
where the aldehyde is converted to a methyl ester during the
coupling event.23 This result suggests that borane complexation to
the aldehyde in intermediate 29a is a necessary step in promoting
ring closure to cyclopentenol products.
On the basis of the above analysis for enalꢀalkyne cycloaddi-
tions, we propose that cycloadditions involving the conversion of
enoates 20 and alkynes 14 into cyclopentenones 23 proceed via a
similar pathway (Scheme 3). Formation of metallacycle 28b
followed by ester enolate protonation and then vinyl nickel addition
to the borane-complexed ester 29b would afford borane hemiacetal
30b. Collapse of this tetrahedral species with phenoxide loss would
then lead to the cyclopentenone product 23, and the borane would
again serve to regenerate the active Ni(0) catalyst.
’ CONCLUSIONS
In summary, a series of [3 + 2] cycloaddition reactions that
proceed by formal reductive cycloaddition has been illustrated.
The processes involve addition of an enal or enoate to an alkyne.
By altering the starting material oxidation state, the procedure
Given the general lability of ester enolates, it is possible that the
enolate motif of metallacycle 28b undergoes extrusion of
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provides either cyclopentenol or cyclopentenone products.
Intermolecular procedures provide monocyclic products,
whereas the corresponding intramolecular versions provide
bicyclic or tricyclic compounds. A mechanistic pathway involving
the formation of metallacyclic enolates followed by rapid pro-
tonation and carbonyl addition is most consistent with the results
obtained. In contrast to well-established cycloaddition proce-
dures for assembly of five-membered rings, the use of simple
π-components as starting reagents is made possible by the net
two-electron reduction that occurs during cyclizations. We
envision that many classes of odd-numbered ring cycloadditions
may become possible by strategies of this type.
(7) (a) For a review of phosphine-based organocatalytic [3 + 2]
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(8) A portion of this work has previously been described: Herath, A.;
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(9) Chang, H. T.; Jayanth, T. T.; Cheng, C. H. J. Am. Chem. Soc. 2007,
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that involves ring formation via cycloaddition of two reagents with a net
two-electron reduction. For examples of reductive cycloadditions:
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’ ASSOCIATED CONTENT
S
Supporting Information. Detailed experimental proce-
b
dures, characterization data, and copies of NMR spectra. This
material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
jmontg@umich.edu
Author Contributions
The manuscript was written through contributions of all authors.
All authors have given approval to the final version of the
manuscript.
’ ACKNOWLEDGMENT
The U.S. National Science Foundation is acknowledged for
support of this work (grant CHE-1012270).
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