Cobalt-Catalyzed Alkenylzincation of Unfunctionalized Alkynes

Article abstract of DOI:10.1002/anie.201508262

While transition metal catalyzed addition reactions of arylmetal reagents to unfunctionalized alkynes have been extensively developed in the last decade, analogous reactions using alkenylmetal reagents remain rare regardless of their potential utility for the synthesis of unsymmetrical 1,3-dienes. Reported herein is the development of a cobalt/diphosphine catalyst which promotes efficient and stereoselective addition of alkenylzinc reagents to unfunctionalized internal alkynes. The resulting dienylzinc species serve as versatile intermediates for further synthetic transformations.

Full text of DOI:10.1002/anie.201508262

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201508262
German Edition: DOI: 10.1002/ange.201508262
Homogeneous Catalysis
Cobalt-Catalyzed Alkenylzincation of Unfunctionalized Alkynes
Junliang Wu and Naohiko Yoshikai*
Abstract: While transition metal catalyzed addition reactions
of arylmetal reagents to unfunctionalized alkynes have been
extensively developed in the last decade, analogous reactions
using alkenylmetal reagents remain rare regardless of their
potential utility for the synthesis of unsymmetrical 1,3-dienes.
Reported herein is the development of a cobalt/diphosphine
catalyst which promotes efficient and stereoselective addition
of alkenylzinc reagents to unfunctionalized internal alkynes.
The resulting dienylzinc species serve as versatile intermediates
for further synthetic transformations.
1,4-cobalt migration.[14] These previous studies prompted us
to explore the ability of a cobalt/diphosphine complex to
catalyze the addition of alkenylzinc reagents to alkynes as
well as to participate in 1,4-cobalt migration in such systems.
The exploration commenced with a screening of cobalt
sources and ligands for the addition of the di(a-styryl)zinc
reagent 1a (prepared from 1.2 equiv of ZnCl2·TMEDA and
2.4 equiv of a-styrylmagnesium bromide) to 5-decyne (2a) in
toluene at 808C (Table 1). When using CoCl2 (10 mol%)
Table 1: Addition of the a-styrylzinc reagent 1a to 4-cctyne (2a).[a]
The addition of organometallic reagents to alkynes, com-
bined with electrophilic trapping of the resulting alkenylmetal
species, offers a useful method for the stereocontrolled
synthesis of multisubstituted alkenes.[1] Among such carbo-
metalation reactions, the reaction of arylmetal reagents with
unfunctionalized alkynes has gained particular attention as
a fundamentally challenging transformation, as well as an
attractive route to pharmaceutically relevant olefins such as
tamoxifen and its analogues.[2,3] Over the last decades,
nickel,[2a–c,4] manganese,[5] iron,[6] chromium,[7] and cobalt[8]
catalysts have been developed for arylmetalation of alkynes,
including dialkylacetylenes, using arylmagnesium, arylzinc, or
aryllithium reagents.[9] The success of these reactions may be
partly attributed to the reluctance of the product, that is,
alkenylmetal species, to undergo further addition to the
alkyne, and in turn implies a difficulty in achieving efficient
addition of preformed alkenylmetal reagents to unfunction-
alized alkynes. The difficulty of alkenylmetalation is also
expected from the apparent similarity of the starting alke-
nylmetal reagent and the resulting dienylmetal species, which
potentially causes oligomerization reactions. In fact, the
addition of alkenylmetal reagents to unfunctionalized alkynes
has rarely been achieved[6c,10–12] regardless of its potential
utility for the regio- and stereoselective synthesis of multi-
substituted 1,3-dienes. Herein we report that such alkenyl-
metalation can be achieved by the combination of a cobalt/
diphosphine catalyst and an alkenylzinc reagent. The reaction
allows facile preparation of synthetically versatile unsym-
metrical 1,3-diene building blocks.[13]
Entry
CoXn
Ligand
Yield [%][b]
1
2
3
4
5
6
7
8
CoCl2
CoCl2
CoCl2
CoCl2
CoCl2
CoF2
CoBr2
CoI2
Co(OAc)2
CoF2
None
Xantphos
30
33
43
21
27
83[c]
45
0
tBuXantphos
Cy-DPEphos
dtbpf
tBu-Xantphos
tBu-Xantphos
tBu-Xantphos
tBu-Xantphos
none
9
71
50
19
10
11[d]
CoF2
tBuXantphos
[a] Unless otherwise noted, the reaction was performed using 1a
(prepared from 0.3 mmol of ZnCl2·TMEDA and 0.6 mmol of a-styryl-
magnesium bromide, 1.2 equiv), 2a (0.25 mmol), CoXn (10 mol%),
ligand (10 mol%) in toluene. [b] Determined by GC using n-tridecane as
an internal standard. [c] Yield of isolated product. [d] THF was used as
the solvent. THF=tetrahyrofuran, TMEDA=N,N,N’,N’-tetramethyl-
ethylenediamine.
Recently, our group developed cobalt/diphosphine-cata-
lyzed addition reactions of arylzinc reagents to unfunction-
alized internal alkynes and norbornene derivatives involving
alone, the reaction afforded, upon protonation, the desired
1,3-diene 3aa in a modest yield of 30% (entry 1). Whereas the
addition of Xantphos as a supporting ligand did not have an
apparent effect on the catalytic activity (entry 2), a bulky and
electron-rich analogue of Xantphos, tBu-Xantphos, gave rise
to a noticeable improvement in the yield of 3aa (entry 3).
Other diphosphine ligands bearing bulky dialkylphosphino
groups did not show positive effects (entries 4 and 5), and
typical N-heterocyclic carbene ligands (e.g., IMes, IPr) were
much less effective (< 10% yield; data not shown). Subse-
quent screening using tBu-Xantphos as the ligand revealed
[*] Dr. J. Wu, 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
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.201508262.
336
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a significant influence of the cobalt precatalyst on the
We next explored the reactivity of a series of unfunction-
catalytic activity (entries 6–9). The use of CoF2 instead of
CoCl2 improved the yield of 3aa to 83% (entry 6), while the
reaction was completely shut down using CoI2 as the
precatalyst (entry 8). A control experiment using CoF2
alone also showed the superiority of CoF2 over CoCl2
(entry 1 versus entry 10), as well as the positive influence of
tBu-Xantphos (entry 6 versus entry 10). The use of THF as
the solvent resulted in a drastic decrease in the yield of 3aa
(entry 11). Note that the reaction using a mono(a-styryl)zinc
reagent or a-styrylmagnesium bromide instead of 1a did not
give any trace of 3aa.[15]
alized internal alkynes using 1a as the reaction partner
(Scheme 2). Not unexpectedly, 1a and 6-dodecyne (2b)
reacted smoothly to afford the desired product 3ab in
With the CoF2/tBu-Xantphos catalyst system in hand, we
explored the reaction of various alkenylzinc reagents with 2a
(Scheme 1). a-Styrylzinc reagents bearing various para and
Scheme 2. Scope with respect to the alkynes. The reaction was
performed according to the reaction conditions detailed in entry 6 of
Table 1.
Scheme 1. Scope with respect to the alkenylzinc reagents. The reaction
was performed according to the reaction conditions detailed in entry 6
of Table 1, using an alkenylzinc reagent prepared from a 1:2 mixture of
ZnCl2·TMEDA and the corresponding alkenylmagnesium bromide.
[a] Yield for a 5 mmol reaction is shown in the brackets. [b] 1-Phenyl-1-
butyne was used as the alkyne.
a good yield. The reaction of 1a with an unsymmetrical
dialkylalkyne, 4-methylpent-2-yne (2c), afforded the 1,3-
diene 3ac as a single regio- and stereoisomer, wherein the
À
C C bond formation took place at the less hindered
acetylenic carbon atom. This result is in a sharp contrast to
the poor regioselectivities in the cobalt-catalyzed addition of
arylzinc reagents to 2c,[8b, 14a] whereas the same sense of
regioselectivity was observed in a few examples of other
transition metal catalyzed arylmetalations.[6c,7] We speculate
that the steric hindrance around the a-carbon atom of 1a
outweighed the hindrance around the cobalt center, thus
resulting in the observed regioselectivity. Besides the dialkyl-
alkynes, a variety of aryl(alkyl)alkynes participated in the
reaction with 1a to regio- and stereoselectively afford the
products 3ad–ao in moderate to good yields with a tolerance
for functional groups such as chloro (3ai), amide (3am), and
ketone (3an) moieties. Following the typical regioselectivity
in carbometalation of aryl(alkyl)alkynes,[1] these reactions
meta substituents afforded the desired 1,3-dienes 3aa–fa in
moderate to good yields with exclusive syn stereoselectivity,
while the one bearing an ortho-methyl group failed to
undergo the addition reaction. The reaction of 1a and 2a
could be performed on a 5 mmol scale without significant
decrease in the yield of 3aa. b-Substituted a-styrylzinc
reagents such as (3,4-dihydronaphthalen-1-yl)zinc and (6,7-
dihydro-5H-benzo[7]annulen-9-yl)zinc reagents participated
in the reaction to afford the 1,2,3,4-tetrasubstituted 1,3-dienes
3ga and 3ha, respectively, in moderate yields. The cobalt
catalyst also promoted the addition of b-styrylzinc reagents to
2a, thus affording the desired tri- or tetrasubstituted dienes
3ia–ka in good yields. Alkenylzinc reagents without aryl
groups, such as cyclohexen-1-ylzinc, propen-2-ylzinc, and
propen-1-ylzinc reagents, were also amenable to the present
carbozincation reaction (3la, 3mb, and 3nb).
À
resulted in C C bond formation at the acetylenic carbon atom
bonded to the alkyl group, except that an acetal-appended C3
alkyl chain caused the opposite regioselectivity, presumably
through coordination of the oxygen atom to the cobalt center
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(see 3ao).[16] Unfortunately, the present catalytic system was
not effective for some nonpolar alkynes such as diphenyl-
acetylene (< 10% GC yield), 1-trimethylsilylprop-1-yne (no
adduct detected), and 1-trimethylsilyl-2-phenylacetylene
(< 15% GC yield, mixture of regio/stereoisomers). Also,
terminal alkynes such as phenylacetylene and 1-octyne failed
to give the desired adducts.
major isomer was isolated and identified as the expected
iododiene 3gaI. Likewise, b-styrylzinc reagent 1i and 2a gave
rise to four isomers (ratio = 100:39:4:3), among which the two
major isomers were isolated and identified as Z/E isomers
(4:1) of the expected iododiene 3iaI. We speculate that some
of the unknown minor isomer(s) detected in these reactions
correspond to regioisomeric iododienes such 3I’ arising from
1,4-cobalt migration. However, further characterization was
not possible because of their limited quantities. The reaction
of 1a and 1-phenyl-1-butyne (2e) afforded three isomers
(ratio = 100:66:24), where the two major isomers were
isolated and identified as Z/E isomers (5:3) of the expected
iododiene 3aeI. 1H NMR spectra of 3aeI and the original
product mixture suggested that the minor isomer corresponds
to a regioisomeric iododiene 3aeI’. Collectively, these iodina-
tion experiments show that dienylcobalt species may undergo
1,4-cobalt migration depending on the substituents, but only
to a small extent. Note also that the alkenylzincation/
iodination reactions could be performed on 5 mmol scale in
comparable yields (see the Supporting Information), thus
readily providing sufficient amounts of the iododienes for
further transformations (see below).
The present addition reaction is considered to proceed
through insertion of the alkyne into an alkenylcobalt species
A followed by transmetalation of the resulting dienylcobalt
species B and the alkenylzinc reagent 1 to afford a dienylzinc
species 3Zn while regenerating the alkenylcobalt species A
(Scheme 3a). Given our previous studies on arylzincation
involving 1,4-cobalt migration,[14] the dienylcobalt species B
may potentially rearrange to a regioisomeric dienylcobalt
species B’, thus giving rise to a regioisomeric dienylzinc
species 3Zn’. To confirm the formation of 3Zn as well as to
probe the possibility of 1,4-cobalt migration, we performed
iodinative quenching of the reactions of selected alkenylzinc
reagents and alkynes (Scheme 3b). The reaction of 1a with
2a, upon quenching with I2, afforded the regio- and stereo-
chemically pure 1-iodo-1,3-diene 3aaI in 70% yield without
detectable amounts of other isomers. In contrast, the reaction
of the (3,4-dihydronaphthalen-1-yl)zinc reagent 1g and 2a
produced three isomers in a ratio of 100:16:6 (judged from
GCMS analysis of the crude reaction mixture), where the
1,3-Dienes and 1-iodo-1,3-dienes prepared by the present
alkenylzincation reaction serve as versatile starting materials
for further synthetic transformations (Scheme 4). Diels–
Alder reaction of 3aa with maleic anhydride proceeded
Scheme 3. a) Proposed catalytic cycle. b) Alkenylzincation quenched
with I2. [a] Yield and/or stereoselectivity of the isolated products
(0.25 mmol scale). [b] Detected by GCMS analysis. [c] Detected and
assigned by GCMS and 1H NMR analysis.
Scheme 4. Transformations of 1,3-dienes and 1-iodo-1,3-dienes.
DMF=N,N-dimethylformamide, NMP=N-methylpyrrolidone.
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2992; d) N. F. McKinley, D. F. O’Shea, J. Org. Chem. 2006, 71,
smoothly in the presence of catalytic ZnBr2 (20 mol%) to
afford a bicyclic product 4 in a good yield, while exposure of
3aa alone to BF3·OEt2 (20 mol%) caused intramolecular
Friedel–Crafts reaction to furnish an indene derivative 5
(Scheme 4a). Besides standard palladium-catalyzed cross-
coupling reactions such as Sonogashira and Suzuki–Miyaura
couplings (Scheme 4b), 1-iodo-1,3-dienes proved to be appli-
cable to copper-catalyzed chalcogenative cyclization with
either elemental sulfur or selenium,[17] thus affording the
corresponding thiophenes 8 and selenophenes 9, respectively,
in excellent yields (Scheme 4c). It is worth noting that even
the stereochemically impure iododienes 3iaI and 3aeI (see
Scheme 3) were near quantitatively converted into the
corresponding chalcogenophene products.
In summary, we have developed a CoF2/tBu-Xantphos
catalytic system for the addition reaction of an alkenylzinc
reagent to an unactivated internal alkyne. The reaction allows
regio- and stereocontrolled synthesis of unsymmetrical multi-
substituted 1,3-dienes, which may be used as versatile
synthetic intermediates. Further synthetic exploration of the
present alkenylzincation is currently underway. Mechanistic
aspects of the reaction, including the positive influence of the
CoF2 precatalyst,[18] also deserve further investigation.
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Acknowledgements
This work was supported by Nanyang Technological Univer-
sity/Singapore Ministry of Education (Academic Research
Fund Tier 1: RG5/14) and JST, CREST.
Keywords: alkenes · alkynes · cobalt · homogeneous catalysis ·
organometallic reagents
How to cite: Angew. Chem. Int. Ed. 2016, 55, 336–340
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Received: September 4, 2015
Revised: October 21, 2015
Published online: November 11, 2015
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