Cobalt-Catalyzed Annulation of Salicylaldehydes and Alkynes to Form Chromones and 4-Chromanones

Article abstract of DOI:10.1002/anie.201510999

A unique cobalt(I)-diphosphine catalytic system has been identified for the coupling of salicylaldehyde (SA) and an internal alkyne affording a dehydrogenative annulation product (chromone) or a reductive annulation product (4-chromanone) depending on the alkyne substituents. Distinct from related rhodium(I)- and rhodium(III)-catalyzed reactions of SA and alkynes, these annulation reactions feature aldehyde C-H oxidative addition of SA and subsequent hydrometalation of the C=O bond of another SA molecule as common key steps. The reductive annulation to 4-chromanones also involves the action of Zn as a stoichiometric reductant. In addition to these mechanistic features, the Co^I catalysis described herein is complementary to the Rh^I- and Rh^III-catalyzed reactions of SA and internal alkynes, particularly in the context of chromone synthesis.

Full text of DOI:10.1002/anie.201510999

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International Edition: DOI: 10.1002/anie.201510999
German Edition: DOI: 10.1002/ange.201510999
Cobalt Catalysis
Cobalt-Catalyzed Annulation of Salicylaldehydes and Alkynes to Form
Chromones and 4-Chromanones
Junfeng Yang and Naohiko Yoshikai*
Abstract: A unique cobalt(I)–diphosphine catalytic system has
been identified for the coupling of salicylaldehyde (SA) and an
internal alkyne affording a dehydrogenative annulation prod-
uct (chromone) or a reductive annulation product (4-chroma-
none) depending on the alkyne substituents. Distinct from
related rhodium(I)- and rhodium(III)-catalyzed reactions of
SA and alkynes, these annulation reactions feature aldehyde
À
C H oxidative addition of SA and subsequent hydrometala-
=
tion of the C O bond of another SA molecule as common key
steps. The reductive annulation to 4-chromanones also involves
the action of Zn as a stoichiometric reductant. In addition to
these mechanistic features, the CoI catalysis described herein is
complementary to the RhI- and RhIII-catalyzed reactions of SA
and internal alkynes, particularly in the context of chromone
synthesis.
T
he catalytic coupling of aldehydes and unsaturated hydro-
carbons by means of transition-metal-mediated aldehyde
À
C H activation is an atom-efficient synthetic approach to
ketones.[1] Such reactions often employ chelating aldehydes to
À
facilitate the C H activation and to prevent decarbonylation
of the resulting acyl metal species. Among various chelating
aldehydes, salicylaldehyde (SA) is particularly attractive
because of its ready availability as well as the frequent
occurrence of ortho-hydroxy or alkoxy arylcarbonyl motifs in
natural products and bioactive substances. Indeed, hydro-
acylation and related acylation reactions of SA
have been developed using alkynes,[2–5] alkenes,[2b, 6,7] and
allenes[2b, 8] as the reaction partners, most extensively using
rhodium catalysts. For example, Miura and co-workers
demonstrated two distinct reaction manifolds for the coupling
of SA and an internal alkyne under RhI or RhIII catalysis
(Scheme 1a). A RhI diphosphine catalyst promotes a standard
Scheme 1. Transition-metal-catalyzed reactions of salicylaldehyde and
alkyne.
We have found that cobalt(I)–diphosphine catalysts can
serve as viable alternatives to rhodium(I)–diphosphine cata-
lysts for the intramolecular hydroacylation of 2-acylbenz-
aldehydes and 2-alkenylbenzaldehydes.[9,10] We have also
developed another cobalt(I)–diphosphine catalyst as an
alternative to the Wilkinson catalyst for the intermolecular
olefin hydroacylation using a chelating aldimine.[11–13] Along
with this line of research, we became interested in the ability
of a CoI catalyst to effect activation and transformation of SA.
We report here that cobalt(I)–dcype catalytic systems
À
hydroacylation process through C H oxidative addition,
À
À
alkyne insertion into the Rh H bond, and C C reductive
elimination to afford an a,b-unsaturated ketone.[2a,b] On the
other hand, a RhIII catalyst, in the presence of a CuII species as
oxidant, effects oxidative annulation through deprotonative
À
À
C H metalation, alkyne insertion into the Rh C bond, and
[3]
À
C O reductive elimination to afford a chromone derivative.
(dcype = 1,2-bis(dicyclohexylphosphino)ethane)
promote
annulation reactions of SA and internal alkynes to afford
chromone or 4-chromanone derivatives depending on the
alkyne substituents (Scheme 1b). Both the reactions com-
monly feature the expense of another molecule of SA as an
acceptor of the aldehyde hydrogen presumably through a
[*] Dr. J. Yang, Prof. N. Yoshikai
Division of Chemistry and Biological Chemistry
School of Physical and Mathematical Sciences
Nanyang Technological University, Singapore 637371 (Singapore)
E-mail: nyoshikai@ntu.edu.sg
À
=
C H oxidative addition/C O hydrometalation sequence,
while the latter reaction involves the action of Zn as
a stoichiometric reductant. In addition to these mechanisti-
cally distinct features in comparison to RhI catalysis, the
CoI catalysis is complementary to the RhIII catalysis as
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201510999.
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a method for the synthesis of chromones, which represent
core structures of flavonoids, isoflavonoids, and related
bioactive compounds.[14]
With the similarities between CoI and RhI catalysts in
previous hydroacylation reactions in mind, we initially
intended to study the hydroacylation reaction using SA (1a)
and an alkyne such as 1-phenyl-2-trimethylsilylacetylene (2a)
in the presence of a CoI catalyst. Contrary to this initial
intention, we did not obtain the expected hydroacylation
product in any of our screening experiments. Instead,
a
catalyst generated from CoBr2 (10 mol%), dcype
(10 mol%), and Zn (50 mol%) promoted the reaction of
excess 1a (0.3 mmol) and 2a (0.1 mmol) in DMSO at 808C
(conditions A), affording a chromone derivative 3aa in 82%
yield, along with a substantial amount (0.079 mmol) of 2-
hydroxybenzyl alcohol (4a; Table 1, entry 1). The presence of
Table 1: Effect of reaction conditions on the dehydrogenative annulation
of salicylaldehyde (1a) and alkyne 2a.[a]
Entry
Deviation from conditions A[b]
Yield of 3aa [%][c]
1
2
3
4
none
82
11
6
0
0
Scheme 2. Scope of the dehydrogenative annulation reaction of salicy-
laldehydes with silylacetylenes and dialkylacetylenes (0.3 mmol scale).
dppe instead of dcype
dppp instead of dcype
dppf instead of dcype
dippf instead of dcype
PCy3 instead of dcype
Mn instead of Zn
In instead of Zn
THF instead of DMSO
MeCN instead of DMSO
5
In addition to the 1-aryl-2-silylacetylenes, silylacetylenes
bearing alkenyl and alkyl groups also underwent regioselec-
tive dehydrogenative annulation to afford the chromone
derivatives 3am–3ao. The catalytic system could also be
applied to dialkylalkynes such as 4-octyne (see 3ap). Reac-
tions of SAs substituted at the 3-, 4-, or 5-position with 2a
proceeded smoothly, thus affording the corresponding chro-
mones 3ba–3ia in moderate to good yields. In contrast, 6-
methoxysalicylaldehyde failed to participate in the dehydro-
genative annulation with 2a, presumably as a result of steric
repulsion between the methoxy and the formyl groups,
which would interfere with the formation of a metallacycle
intermediate (see below).
6[d]
7
0
41
16
8
8
9
10
0
[a] The reaction was performed on a 0.1 mmol scale. [b] Abbreviations:
dppe=1,2-bis(diphenylphosphino)ethane; dppp=1,3-bis(diphenyl-
phosphino)propane; dppf=1,1’-bis(diphenylphosphino)ferrrocene;
dippf=1,1’-bis(diisopropylphosphino)ferrocene. [c] Determined by GC
using n-tridecane as an internal standard. [d] 20 mol% of PCy3 was used.
product 4a apparently indicated that 1a acted not only as
a reactant but also as a hydrogen acceptor (see below), and
indeed the yield of 3aa dropped significantly when 1a was
used as a limiting reagent.[15] The reaction is highly dependent
on the ligand. Thus, the yield of 3aa dropped significantly
using other typical diphosphine or monophosphine ligands
(entries 2–6). The use of Mn or In as an alternative reductant
resulted in a lower yield of 3aa (entries 7,8). The reaction
became sluggish in other solvents, such as THF and MeCN
(Table 1, entries 9,10).
With the Co–dcype catalytic system in hand, we explored
the scope of the dehydrogenative annulation (Scheme 2). A
variety of 1-aryl-2-silylacetylenes participated in the reaction
with 1a to regioselectively afford the corresponding 2-aryl-3-
silylchromones 3aa–3al, with tolerance of functional groups
including methoxy, hydroxy, fluoro, ester, and ketone groups.
During exploration of the scope of alkynes in the
dehydrogenative annulation, we found that the reaction of
1a and 1-phenyl-1-propyne (2q) under conditions A regiose-
lectively produced
a 4-chromanone derivative 5aq in
a modest yield of 32% with only a trace amount of the
expected chromone derivative 3aq (Scheme 3a). Upon
further modification of the reaction conditions, the yield of
5aq could be improved to 73% using the same precatalysts
along with a catalytic amount of Na2CO3 (20 mol%) and
a superstoichiometric amount of Zn (200 mol%; Scheme 3b,
conditions B), where 3aq was obtained in 7% yield. This
reductive annulation reaction was applicable to other aryl-
alkylacetylenes and diphenylacetylene, affording the corre-
sponding 4-chromanone derivatives 5ar–5at. The exclusive
regioselectivity detected with the arylalkylacetylenes is nota-
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Scheme 4. Deuterium-labeling experiments.
Scheme 3. Reductive annulation of salicylaldehyde with arylalkyl-
acetylene and diarylacetylene.
propose possible catalytic cycles A and B for the dehydrogen-
ative and the reductive annulation reactions, respectively
(Scheme 5). In catalytic cycle A, a CoI species generated by
reduction of the CoBr2–dcype precatalyst and 1a form
a cobalt(I)–aryloxide I, which then undergoes intramolecular
ble in comparison with imperfect regioselectivities detected in
the RhI- and RhIII-catalyzed reactions using such alkynes.[2c,3]
It should also be noted that the reaction of silylacetylene 2a
under conditions B did not produce the corresponding
4-chromanone at all, but afforded only 3aa.
To gain insight into the reaction pathways of the
dehydrogenative and reductive annulation reactions, we
performed deuterium-labeling experiments (Scheme 4). The
reaction between SA deuterated at the formyl carbon ([D]-
1a) and 2a under conditions A afforded the expected product
3aa as well as the 2-hydroxybenzyl alcohol with almost
complete deuteration of the benzylic position ([D2]-4a),
demonstrating the transfer of the aldehydic deuterium atom
of [D]-1a to another molecule of [D]-1a (Scheme 4a). The
reaction of [D]-1a and 2q under conditions B also afforded
[D2]-4a, while no deuterium incorporation into the annula-
tion product 5aq was
À
aldehyde C H oxidative addition to produce an acyl-
(hydrido)cobalt(III) species II. Unlike the RhI-catalyzed
hydroacylation using SA,[2] species II does not directly react
with any alkynes. Instead, II undergoes insertion of another
À
molecule of 1a into the Co H bond to give an
acyl(benzyloxy)cobalt(III) species III. Migratory insertion
of the alkyne 2a into the cobalt(III)–acyl bond of III is
À
followed by C O reductive elimination to furnish the product
3aa along with a cobalt(I)–benzyloxide V. The reaction of V
with 1a finally affords 2-hydroxybenzyl alcohol as the
coproduct and thus completes a catalytic turnover.
The major difference between catalytic cycle B from
cycle A is the involvement of Zn as a stoichiometric reduc-
detected (Scheme 4b). Fur-
thermore, the reductive
annulation of 1a and 2q in
the presence of D2O
resulted in substantial deu-
terium incorporation into
both the 2- and 3-positions
of 5aq (Scheme 4c). The
latter two results exclude
a
hydroacylation–oxy-
Michael addition cascade,
which may operate under
RhI-catalyzed
conditions
(see Scheme 1a),[2b,c] as
a mechanism for the reduc-
tive annulation.
On the basis of the
above results, we tentatively Scheme 5. Proposed catalytic cycles for the dehydrogenative and reductive annulation reactions.
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tant. While cycle B shares the key species I–III with cycle A,
carbonylation,[17] and annulation with benzyne,[18] thus afford-
CoIII species III is reduced by Zn into an acylcobalt(I) species
VI bearing a zinc alkoxide moiety. Subsequent migratory
insertion of 2q into the cobalt(I)–acyl bond and protonation
of the resulting alkenylcobalt(I) species VII with 1a afford an
a,b-unsaturated ketone VIII while regenerating the species I.
Note that these reduction, migratory insertion, and proto-
nation steps may take place in different orders (see Schem-
es S1 and S2 in the Supporting Information), although we
consider that the alkyne-dependent reaction outcomes can be
better explained by the proposed mechanistic model. The zinc
alkoxide moiety of VIII then undergoes intramolecular oxy-
Michael addition. The resulting zinc enolate IX would be
protonated by a coexisting proton source, such as 1a or
adventitious water, to give the product 5aq along with zinc
alkoxide species such as X, which would make the reaction
mixture basic. Note that the basic nature of the reaction
mixture of reductive annulation was supported by electro-
philic trapping experiments using benzyl bromide (see the
Supporting Information).
ing the corresponding products 7–9, respectively, in good
yields.
In summary, we have investigated the use of a unique
cobalt(I)–diphosphine catalyst in the coupling of SA and
internal alkynes.[19,20] Unlike the conventional RhI-catalyzed
hydroacylation using the same reactants, the CoI catalyst
effects dehydrogenative or reductive annulation through
À
=
C H oxidative addition followed by C O hydrometalation
to afford chromone or 4-chromanone derivatives, respec-
tively. The product selectivity primarily depends on the
alkyne substituents, although its origin remains unclear at this
time. Further studies on CoI-catalyzed reactions through
À
aldehyde C H activation are underway.
Acknowledgements
This work was supported by the Singapore Ministry of
Education (RG5/14), Nanyang Technological University,
and the JST, CREST.
We speculate that the unique reaction pathway of the
CoI catalysis detailed herein compared with the RhI-catalyzed
alkyne hydroacylation may be ascribed to the more polarized
Keywords: aldehydes · alkynes · C-H functionalization · cobalt ·
heterocycles
III
[16]
nature of the CoIII H bond than of a Rh H bond. Thus,
À
À
the CoIII H moiety of intermediate II would serve as a better
hydride donor to the C O bond of 1a, thus opening
À
How to cite: Angew. Chem. Int. Ed. 2016, 55, 2870–2874
Angew. Chem. 2016, 128, 2920–2924
=
the reaction channel to the acyl(benzyloxy)cobalt(III)
intermediate III.
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Received: November 26, 2015
Published online: January 25, 2016
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