A rhodium-catalyzed tandem alkyne dimerization/1,4-addition reaction

Article abstract of DOI:10.1002/adsc.201300743

A convenient three-component coupling reaction for the construction of conjugated enynes using rhodium catalysis is reported. Dimerization of a monosubstituted alkyne followed by trapping of the vinyl metal intermediate with an electron-deficient alkene, such as methyl vinyl ketone, provided moderate to good yields of these enynes. The use of the hindered electron-rich tris(ortho-tolyl)phosphine as a ligand for the rhodium catalyst provided the best conversions to these complex products. Copyright

Full text of DOI:10.1002/adsc.201300743

UPDATES
DOI: 10.1002/adsc.201300743
A Rhodium-Catalyzed Tandem Alkyne Dimerization/
1,4-Addition Reaction
Ronald V. Lerum,a Christopher M. Russo,a Jocellie E. Marquez,a
and John D. Chisholma,*
a
Department of Chemistry, Syracuse University, 1-014 Center for Science and Technology, Syracuse, NY 13244, USA
Fax : (+1)-315-443-4070; phone: (+1)-315-443-6894; e-mail: jdchisho@syr.edu
Received: August 16, 2013; Revised: October 2, 2013; Published online: November 13, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300743.
Abstract: A convenient three-component coupling
reaction for the construction of conjugated enynes
using rhodium catalysis is reported. Dimerization of
a monosubstituted alkyne followed by trapping of
the vinyl metal intermediate with an electron-defi-
cient alkene, such as methyl vinyl ketone, provided
moderate to good yields of these enynes. The use of
the hindered electron-rich tris(ortho-tolyl)phos-
phine as a ligand for the rhodium catalyst provided
the best conversions to these complex products.
process[8] where triisopropylsilylacetylene was added
to an alkynyl ester using rhodium catalysis, with the
resultant vinylrhodium species being trapped by an
electron-deficient alkene to form complex enyne
products similar to 3a. Also relevant are reports by
Sato and Ikeda on the use of a nickel-catalyzed
tandem coupling reaction of alkynylstannanes, alk-
AHCTUNGTRENNUNG
ynes and enones to form similar enynes.[9] A distin-
guishing feature of the nickel-catalyzed process is the
requirement for activation of an alkyne as an alkynyl-
stannane, which is not necessary for the rhodium-cata-
lyzed reactions. As enynes similar to 3a have found
uses in transition metal-catalyzed cycloadditions[10]
and in materials science,[11] the reaction was further
examined to optimize for the dimerization/addition
product and determine the substrate scope.
Keywords: 1,4-addition; alkynes; enynes; multicom-
ponent reactions; rhodium
Initial attempts to influence the product distribu-
One of the most challenging goals of synthetic tion of the reaction focused on varying the phosphine
chemistry is the development of methods to elaborate ligand (Table 1). The use of tris(ortho-tolyl)phosphine
simple and readily available starting materials into (TOTP) significantly increased the yield of the dime-
significantly more complex products in a single step.
Pursuant to this goal much effort has been devoted to
the development of multicomponent reactions.[1] In-
terest in this area has been spurred in recent years by
the use of such transformations in the synthesis of di-
verse chemical libraries, which are often used in
screening efforts to develop new pharmaceuticals and
small molecule probes of biological systems.[2]
Transition metals have also been shown to catalyze
multicomponent reactions.[3] During the course of our
studies on the rhodium-catalyzed 1,4-addition of al-
kynes to a,b-unsaturated ketones,[4] we serendipitous-
ly isolated a distinct p-conjugated enyne adduct that
was comprised of two alkyne units and one equivalent
of methyl vinyl ketone (MVK) (compound 3a,
Scheme 1). The dimerization of alkynes is a well pre-
cedented reaction,[5] often used in organic synthesis.[6]
The use of the alkyne dimerization reaction as a start-
ing point for a multicomponent coupling, however, is
Scheme 1. Isolation of an enyne side product in the Rh-cata-
far more rare.[7] Recently Tanaka highlighted a similar lyzed 1,4-addition reaction.
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Ronald V. Lerum et al.
UPDATES
Table 1. Phosphine ligand screen.
Entry
Ligand
2a[a]
3a[a]
1
2
3
4
5
6
7
8
tris(ortho-methoxyphenyl)phosphine (TOMP)
tris(ortho-tolyl)phosphine (TOTP)
tris(para-methoxyphenyl)phosphine[b]
tris(2,4,6-trimethoxyphenyl)phosphine[b]
triphenylphosphine
72
24
14
19
26
8
16
28
15
0
11
76
23
0
16
7
10
17
44
0
tris(2,4,6-trimethylphenyl)phosphine
tris(pentafluorophenyl)phosphine
tris(para-trifluoromethylphenyl)phosphine
tris(para-fluorophenyl)phosphine
1,2-bis(diphenylphosphino)benzene[c]
none
9
10
11
12
13
14
17
23
34
8
5
36
8
tris(ortho-tolyl)phosphine (TOTP)[d]
tris(ortho-tolyl)phosphine (TOTP)[e]
tris(ortho-tolyl)phosphine (TOTP)[f]
trace
[a]
Isolated yields [%].
[b]
[c]
[d]
[e]
[f]
Baylis–Hillman product 4 was also isolated.
Ether 5 was isolated.
Rh(acac)(ethylene)2 was used as catalyst.
[Rh(COD)Cl]2 (2.5 mol%) was used as catalyst.
[Rh(Cl)(CO)2]2 (2.5 mol%) was used as catalyst.
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
rization/addition product while suppressing the forma- the Baylis–Hillman product 4 (Figure 1). Both rhodi-
tion of the 1,4-addition product (entry 2). The alkyne um complexes[12] and phosphines[13] have been report-
starting material could be completely accounted for in ed to dimerize enones, forming 1,5-diketones like 4,
the two products when using the ligand TOTP. No ap- so either the phosphine or the rhodium complex
parent trends as a result of the phosphineꢁs nature formed with these phosphines may be responsible for
(electron-rich, electron-deficient, size of cone angle, this mode of reactivity. The use of 1,2-bis(diphenyl-
mono- or bidentate) could be identified. Additional phosphino)benzene as a ligand (entry 10) failed to
experiments suggest that the active catalyst was gen- give either alkyne 2 or enyne 3 but instead gave the
erated in situ from both the rhodium complex O-Michael addition product 5 (Figure 1). This reac-
Rh
ment with only the rhodium pre-catalyst present gave ligand dependence is notable as in her report rhodium
both 1,4-addition and dimerization/addition products salts [including Rh(acac)(CO)2] were used as catalysts
ACHTUNGTRENNUNG
(acac)(CO)2 and the phosphine. A control experi- tion is precedented from the work of Louie,[14] but the
AHCTUNGTRENNUNG
in 17% and 5% yields, respectively (entry 11). When in the presence of added Na2CO3 without added phos-
no rhodium precatalyst was applied and just the phos- phine ligands.
phine was used, the alkyne starting material 1 was
isolated quantitatively. Other rhodium sources dimerization/addition product, the substrate scope of
{such as Rh(acac)(ethylene)2, [Rh(COD)Cl]2 and the reaction was investigated. Outlined in Table 2 are
Having optimized the phosphine ligand to favor the
A
R
ACHTUNGTRENNUNG
[Rh(Cl)(CO)2]2} were less effective. Based on this the comparative yields of the 1,4-addition adduct 2
study, both the rhodium pre-catalyst and TOTP are
required for the reaction to perform efficiently.
In some cases products other than alkyne 2 and
enyne 3 were detected during the phosphine screen-
ing. The use of the electron-rich phosphines tris(para-
methoxyphenyl)phosphine (Table 1, entry 2) and
tris(2,4,6-trimethoxyphenyl)phosphine (entry 3) led to
a competing reaction, dimerizing the MVK to form screening.
Figure 1. Side products observed during the phosphine
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A Rhodium-Catalyzed Tandem Alkyne Dimerization/1,4-Addition Reaction
Table 2. Variation of alkynes in the dimerization/addition re-
action.
Entry
R Group
2[a]
3[a]
1
2
3
4
5
6
7
8
9
N
24 (2a)
30 (2b)
30 (2c)
0 (2d)
3 (2e)
20 (2f)
29 (2g)
24 (2h)
0 (2i)
76 (3a)
51 (3b)
63 (3c)
21 (3d)
29 (3e)
67 (3f)
38 (3g)
47 (3h)
0 (3i)
Figure 2. HMBC correlations (a) and (b) and NOESY cor-
relations (c) on enyne 3e.
10
TIPS
75 (2j)
0 (3j)
[a]
Isolated yields [%].
H-5 and H-10. The stereochemistry of the other
enyne products was assigned in analogy to these find-
ings.
versus the enyne product 3 using various alkynes. Al-
In contrast to other alkynes, the reaction of 2-
kynes containing primary alcohols, esters, and phtha- methyl-3-butyn-2-ol 6 with methyl vinyl ketone af-
lamide-protected amines were found to participate in forded two different alkyne products, 7 and 8
the dimerization/addition reaction (Table 2). These (Table 3). Alkyne 7 appeared to be the result of an
substrates afforded modest to satisfactory yields of di- alkyne dimerization/addition reaction that was fol-
merization/addition product 3. Propargyl alcohol itself lowed by loss of acetone and 1,4-addition of the re-
was a poor substrate (entry 4), perhaps ionizing to an sulting alkyne to a second equivalent of MVK.
inert rhodium allenylidene complex by dehydration. Alkyne 8 was identified as a symmetrical alkyne
Similar organometallic structures have been isolated adduct of 2-methyl-3-butyn-2-ol 6 and MVK. At-
by Werner.[15] Phenylacetylene (entry 9) yielded only tempts were made to determine if the yields of the
an inseparable mixture of products that did not alkyne products 7 and 8 were dependent on the ratio
appear to contain either alkyne 2 or enyne 3. The use of alkyne to MVK (Table 3). Since both the alkyne
of triisopropylsilylacetylene led only to the 1,4-addi- and the MVK must compete for a coordination site
tion adduct (entry 10). This selectivity is dictated by on the transition metal catalyst, increasing the
the presence of the bulky TIPS group, which inhibits amount of MVK was expected to provide an im-
the dimerization reaction. Hayashi observed a similar proved ratio favoring product 7. When equal ratios of
reactivity pattern in the 1,4-addition reactions of al- both starting materials were used, a small amount of
kynes.[16]
(17%) product 7 was isolated along with product 8 in
While the gross structure of the dimerization/addi- 54% yield (entry 1). Use of 2.5 equiv. of MVK gave
tion product was clear, the stereochemistry of the tri- 50% of 7 and 41% of 8 (entry 2, a 55:45 ratio of 7:8).
substituted alkene was as yet undetermined. NMR Use of a large excess of MVK resulted in a further
studies were therefore undertaken with enyne 3e to improvement in the ratio of 7:8 (71:29), although the
determine the alkene stereochemistry. After the as- yield of product 7 was only 53% compared to 22% of
signment of all protons and carbons of 3e using product 8 (entry 3). The requirement for a large
COSY and HSQC experiments, an HMBC was per- excess of enone is likely due to the MVK being an in-
formed which confirmed the connectivity of the struc- ferior ligand for the rhodium relative to the alkyne.
ture. Specifically, the HMBC spectrum confirmed the
dimerization of the alkyne component to form an sp
Studies were also performed to determine the com-
patibility of other electron-deficient alkenes with the
À
sp2 bond, as indicated by correlations of H-10 to C-5, reaction conditions (Table 4). Reactions with ethyl
C-6, C-7 and C-9 and of H-5 to C-3, C-4, C-7 and C- vinyl ketone, phenyl vinyl ketone and acryloyloxazoli-
10 (Figure 2). The Z stereochemistry of the alkene dinone[17] proceeded to give the respective dimer ad-
was also established by NMR, using a NOESY experi- dition products in more modest yields when compared
ment, which showed a significant correlation between to MVK. The greater amount of the 1,4-addition
Adv. Synth. Catal. 2013, 355, 3485 – 3491
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Ronald V. Lerum et al.
UPDATES
Table 3. Reaction of 2-methyl-3-butyn-2-ol 6.
Entry
Equiv MVK
7[a]
8[a]
Ratio 7:8
1
2
3
1
3.5
10
17
50
53
54
41
22
24:76
55:45
71:29
[a]
Isolated yields [%].
Table 4. Variation of electron-deficient alkenes in the dimerization/addition reaction.
Entry
R1
R2
R3
2[a]
3[a]
1
2
3
Me
Et
Ph
H
H
H
H
H
H
24 (2a)
34 (2k)
36 (2l)
76 (3a)
46 (3k)
45 (3l)
4
H
H
H
H
H
H
44 (2m)
18 (2m)
8 (2m)
45 (3m)
23 (3m)
16 (3m)
5[b]
6[c]
5
6
7
8
OEt
NMe2
Ph
H
H
Me
H
H
H
H
Bu
0 (2n)
0 (2o)
0 (2p)
0 (2q)
0 (3n)
0 (3o)
0 (3p)
0 (3q)
Me
[a]
Isolated yields [%].
2.2 equiv. of alkene were used.
1.1 equiv. of alkene were used.
[b]
[c]
products 2k, 2l and 2m formed may be due to the strates a greater concentration of alkene may com-
lower volatility of the alkenes in these cases. With pete more effectively for the rhodium with the
MVK some loss by evaporation may occur, leading to alkyne, leading to a more significant amount of the
more dimerization/addition, but with less volatile sub- alkyne product 2. Attempts were made to probe this
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A Rhodium-Catalyzed Tandem Alkyne Dimerization/1,4-Addition Reaction
Figure 3. Proposed mechanism.
hypothesis using a smaller excess of acryloyloxazolidi-
Loss of the acetylacetonate ligand from 17 and co-
none, as in this case volatility should not be an issue ordination of a second equivalent of alkyne and
(acryloyloxazolidinone is a solid with a melting point a phosphine provides intermediate 18, which then un-
of 80–818C). Using a smaller excess of alkene did dergoes a regioselective head-to-tail dimerization of
provide a product ratio favoring product 3, but the the alkynes to provide vinylrhodium 19. A selective
conversion was significantly lower, with ~25% start- syn addition across the alkyne to generate 19 accounts
ing alkyne being recovered in both entries 5 and 6 for the observed alkene geometry in product 3. The
(Table 4). Ethyl acrylate and dimethylacrylamide vinylrhodium then adds to MVK forming the oxa-p-
proved to be inert under these conditions. Any substi- allyl rhodium complex 20. Coordination of another
À
tution on the alkene also caused the reaction to shut equivalent of alkyne and insertion into the alkyne C
down, and provided no dimerization/addition or 1,4- H bond then provides complex 21, which through re-
addition product.
ductive elimination releases the enyne product 3 and
Taking into account the stereochemistry of the di- returns rhodium complex 18 to initiate another cata-
merization/addition product (Figure 2) and the need lytic cycle. The formation of 1,4-addition product 2
for a phosphine ligand to promote the reaction we can be explained by addition of MVK to complex 17
propose the following reaction mechanism for the di- instead of the alkyne. The alkyne could then add di-
merization/addition reaction (Figure 3). Rhodium rectly to the MVK, providing the oxa-p-allyl rhodium
complex 15 is generated in situ by the displacement of complex 24. Insertion of the rhodium into the alkyne
À
the CO ligands with a phosphine and chelation of the C H bond followed by protonation of the rhodium
alkyne. Oxidative addition of the alkyne C H bond enolate leads to complex 26, which can the release
À
and protonation of the acetylacetonate ligand then ketone 2, regenerating complex 23 and beginning an-
provides complex 17.
other cycle. Complexes 23 and 18 may also equilibrate
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Ronald V. Lerum et al.
UPDATES
Scheme 2. Deuterium labeling studies.
through ligand exchange, so the initial formation of tandem reaction, which is selective for a Z trisubsti-
23 or 18 may not determine the dominant pathway. tuted alkene. A number of alkynes (including some
Deuterium-labeled terminal alkyne 27 was subject- with unprotected alcohols) and electron-deficient al-
ed to the dimerization/addition reaction conditions to kenes were compatible with the reaction conditions.
gain additional insight into the mechanism of the re- This multicomponent reaction provides rapid access
action (Scheme 2). With alkyne 27 minimal deuterium to p-conjugated alkynes, which are important inter-
incorporation (15%) was observed at C-3 while 100% mediates in transition metal-catalyzed cycloadditions
incorporation was observed at C-5. The low incorpo- and are also useful in molecular electronics.
ration of deuterium at C-3 was initially puzzling, but
we hypothesized that the deuterium on the alkyne
may exchange with adventitious water or the free ace- Experimental Section
tylacetonate ligand leading to lowered deuterium in-
corporation in the product. When the reaction was re- General Procedure for the Dimerization/1,4-Addition
peated with the addition of D2O (10 equiv. with re- Reaction of Alkynes and Electron Deficient Alkenes
1
spect to alkyne) the H NMR indicated quantitative
To
dicarbonylacetylacetonatorhodium(I)
(0.021 mmol,
inclusion of a single deuterium at the C-3 position.
Given that the deuterium could be incorporated from
a reductive elimination or a protonation of a rhodium
allyl intermediate, alkyne 30 was also subjected to the
above experiment with excess of D2O. The enyne
product was obtained with 100% d-incorporation (rel-
ative to one proton) at C-3 and with 80% d-incorpo-
ration at the olefinic position (C-5). Deuterium incor-
porated at the olefinic position when using alkyne 30
5.4 mg) and the tris(ortho-tolylphosphine) (0.08 mmol,
26 mg) in benzene (1.1 mL) was added a solution of 10-un-
decyn-1-ol (0.42 mmol, 73 mg) and methyl vinyl ketone
(1.47 mmol, 121 mL) dissolved in benzene (1.0 mL). The re-
action was stirred for 24 h at 808C then pre-absorbed onto
silica gel and purified by silica gel flash chromatography
(gradient elution of 40%–80% ethyl acetate:hexanes) to
give alkyne 1,4-addition product 2a (yield: 24 mg, 24%) and
dimerization/addition product 3a (yield: 64 mg, 76%).
À
suggests that the C H insertion of the rhodium com-
plex into the terminal alkyne is reversible and that
this proton exchanges rapidly with the excess D2O.
In summary, a tandem rhodium-catalyzed dimeriza-
tion/1,4-addition of terminal alkynes and enones is re-
ported. This methodology provides a convenient entry
into highly functionalized enyne systems. Two carbon-
Acknowledgements
Acknowledgement is made to the Donors of the American
Chemical Society Petroleum Research Fund and to Syracuse
carbon bonds are formed in a single step in this University for partial support of this research. J.E.M. thanks
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A Rhodium-Catalyzed Tandem Alkyne Dimerization/1,4-Addition Reaction
the Syracuse University NSF-REU program for summer sup-
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