Rhodium-catalyzed addition of α-keto acid chlorides with terminal alkynes

Article abstract of DOI:10.1002/adsc.201100108

The addition reaction of α-keto acid chlorides with terminal alkynes proceeds in the presence of (acetylacetonato)dicarbonylrhodium used as catalyst to afford synthetically versatile (Z)-γ-chloro-α-oxo-β,γ- unsaturated ketones regio- and stereoselectively. Copyright

Full text of DOI:10.1002/adsc.201100108

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DOI: 10.1002/adsc.201100108
Rhodium-Catalyzed Addition of a-Keto Acid Chlorides with
Terminal Alkynes
Taigo Kashiwabaraa and Masato Tanakaa,*
a
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259-R1-13 Nagatsuta, Midori-ku, Yokohama 226-8503,
Japan
Fax : (+81)-45-924-5279; e-mail: m.tanaka@res.titech.ac.jp
Received: February 12, 2011; Published online: June 16, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100108.
Abstract: The addition reaction of a-keto acid
chlorides with terminal alkynes proceeds in the
presence of (acetylacetonato)dicarbonylrhodium
used as catalyst to afford synthetically versatile (Z)-
g-chloro-a-oxo-b,g-unsaturated ketones regio- and
stereoselectively.
In a representative reaction, a toluene (1.0 mL) so-
lution of phenylglyoxyl chloride (1a, 1 mmol), 1-
octyne
(2A,
0.5 mmol)
and
RhACTHNGUTERU(NNG acac)(CO)2
(0.025 mmol, 5 mol% relative to 2A) was heated at
608C for 24 h. Analysis of the resulting mixture by
GC showed 1a having been completely consumed to
yield (Z)-4-chloro-1-phenyl-3-decene-1,2-dione (Z)-
3aA in 70% yield with high regio- and stereoselectiv-
ity (Scheme 1; R1 =Ph, R2 =n-hexyl). An NOE ex-
periment verified the Z stereochemistry (cis addition).
Keywords: acylation; alkynes; homogeneous cataly-
sis; ketones; rhodium
1
The regiochemistry was also confirmed by H NMR
spectroscopy of the corresponding diol [(Z)-3aA-red]
obtained by reduction with LiAlH4. Besides (Z)-3aA,
Associated with the long known decarbonylation of by-products such as (Z)-4aA (trace), (Z)-5aA (2%),
acid chlorides,[1] addition reactions of aroyl chlorides and benzoyl chloride 6a (30% based on the 1a), ben-
to alkynes in the presence of rhodium complexes give zophenone (11% based on 1a), benzil (9% based on
products coming from formal addition of aryl chlor- 1a),
and
(Z,Z)-7,10-dichlorohexadeca-7,9-diene
ides.[2] We have reported, however, that acid chlorides, [(Z,Z)-7A; 9% based on 2A][8] were also found in the
substituted by electron-withdrawing groups, add to al- reaction mixture.
kynes without decarbonylation.[3] In our continued
Before the RhACTHUNGTRENNU(G acac)(CO)2 catalyst was determined
study on the reactivity of acid chlorides with rhodium to be the catalyst of choice, we ran a series of trial ex-
complexes, we have encountered mono- and dinuclear periments under the same conditions as those in the
a-keto acyl rhodium complexes being formed in the representative reaction (Table 1). Among b-diketona-
reaction of RhACHTUNGTRENNUNG
(acac)(CO)2 with a-keto acid chlor- to ligands ([R3COCHCOR4]À) screened (entries 1–5),
ides.[4] Although there are several papers reporting on
the generation of a-keto acyl complexes, special con-
ditions or procedures to circumvent the difficulty of
generation are required in most cases.[5] Accordingly,
since the publication of Sen and co-workers,[5e] we
have tacitly considered that a-keto acyl complexes
are usually thermally labile. With this tacit under-
standing in mind, our new findings prompted us to
scrutinize the possible addition of a-keto acid chlor-
ides to alkynes. This communication discloses that
such reactions proceed fairly well to furnish g-chloro-
a-oxo-b,g-unsaturated ketones (Scheme 1),[6] ana-
logues in the family of synthetically versatile b-chloro-
vinyl ketones.[3,7]
Scheme 1. Reaction of an a-keto acyl chloride with a termi-
nal alkyne.
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Table 1. Trial reactions of 1a with 2A under different conditions.[a]
Entry
Catalyst
Rh(acac)(CO)2
Rh(t-BuCOCHCO-t-Bu)(CO)2
Rh(PhCOCHCOPh)(CO)2
Rh(CF3COCHCOCF3)(CO)2
Rh(PhCOCHCOMe)(CO)2
Time [h]
Conversion [%][b]
Yield [%][b,c]
1a
2A
(Z)-3aA
4aA
(Z)-5aA
1[d]
2[d]
3[d]
4[d]
5[d]
6[d]
7[d]
8[d]
9[d,e]
10[d,f]
11
12
13
14
15
16
17
18
19
G
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
48
100
100
100
100
>99
100
100
100
100
100
100
100
100
100
100
100
100
100
100
28
94
70
73
28
trace
4
3
2
5
11
100
100
>99
100
99
100
99
93
73
96
67
60
complex mixture
45
31
29
21
63
48
52
29
13
9
10
16
trace
2
11
3
trace
2
1
9
0
1
A
U
A
U
E
0
trace
0
0
5
4
2
6
5
4
4
1
6
T
R
R
trace
2
trace
2
trace
trace
trace
trace
trace
trace
trace
7
A
U
N
R
N
N
G
E
N
G
68
58
51
49
A
U
A
U
76
A
U
85
20
RhCl(CO)
RhCl(CO)
U
nd[h]
85
4
21[i]
22[j]
23[k]
24[k]
100
100
81
3
14
0
1
1
38
15
1
Rh
Rh(acac)(CO)2
Rh(acac)(CO)2
A
100
67
12
34
48
N
N
100
100
trace
[a]
Reaction conditions: 1a (1.0 mmol), 2A (1.0 mmol), catalyst (5.0 mol% in terms of rhodium atom relative to 2A), toluene
(2.0 mL), 608C, 24 h.
Determined by GC by using n-tetradecane as an internal standard.
[b]
[c]
[d]
[e]
[f]
Based on 2A charged.
The quantity of 2A charged=0.5 mmol and the quantity of toluene=1.0 mL.
Run under atmospheric pressure of CO (balloon).
Run under 10 atm of CO.
[g]
[h]
[i]
In situ generated by treating Rh
Not determined.
Run at 1008C.
ACHTUNGTREN(NUNG acac)(CO)2 with the respective ligands (1.0 equiv.).
[j]
Run at 708C.
Run at 508C.
[k]
only [t-BuCOCHCO-t-Bu]À as ligand displayed nearly hoping to minimize the formation of miscellaneous
the same performance as the [MeCOCHCOMe]À by-products stemming from decarbonylation, resulted
(acac) ligand to give (Z)-3aA in 73% yield (vs. 70% in a decrease of catalytic activity as compared with
for acac ligand) and other b-diketonato ligands such entry 1; the yields of (Z)-3aA were 63% (CO balloon,
as [PhCOCHCOPh]À, [CF3COCHCOCF3]À and entry 9) and 48% (10 atm CO, entry 10). The decrease
[PhCOCHCOMe]À were much inferior. Other rhodi- is due most likely to coordination of extra CO ligand
[9]
um complexes were also active, albeit with a lower ef- to RhACTHUNTRGNEU(GN acac)(CO)2 or a detrimental effect against
ficiency; the yields of (Z)-3aA were 31% {for [RhCl- CO dissociation from Rh
(C2H4)2]2}, 29% {for [RhCl(cod)]2}, and 21% {for is envisioned to suppress the coordination of alkyne
[RhCl(CO)2]2} (entries 6–8). Attempted reactions run prior to insertion into the Rh Cl bond (vide infra).
in the presence of atmospheric or pressurized carbon Another set of trial experiments was run using 1a
monoxide under otherwise identical conditions, and 2A (1.0 mmol each) under otherwise identical
ACHTUNGTRENN(UGN acac)(CO)2. Either of these
A
ACHTUNGTRENNUNG
À
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Rhodium-Catalyzed Addition of a-Keto Acid Chlorides with Terminal Alkynes
conditions
(acac)(CO)L-type catalysts, Rh
best performance [52% yield of (Z)-3aA] and other 708C gave (Z)-3aA in a much lower yield (12%) to-
Rh(acac)(CO)L complexes have proved worse to gether with (Z)-5aA (15%) and a large quantity
reveal the following trends among ligands; PPh3 (39%) of 6a, while reactions at 508C were slow and
(entries 11–19).
Among
the
Rh- (entries 11, 22–24), 608C is the optimum temperature
A
ACHTUNGTRENN(NUG acac)(CO)2 gave the affording (Z)-3aA in 52% yield; another reaction at
ACHTUNGTRENNUNG
(29%)>P
(11%)ꢀP
N
ACHTUNGRTEN(NGNU p-An)3 (13%)>AsPh3 gave only 34 and 48% yields of (Z)-3aA even after 24
ACHTUNGTRENNUNG
complex [RhCl(CO)
active, resulted in a very messy mixture comprising vents in the Rh
(Z)-3aA (3%) and other compounds including (Z)- ene appears to be better performing as far as Rh-
and (E)-4aA, (Z)-5aA and 7A at both 60 and 1008C (acac)(CO)(PPh3)-catalyzed reactions are con-
(entries 20 and 21). As for the effect of the reaction cerned.[10]
temperature in Rh(acac)(CO)2-catalyzed reactions The catalysis by Rh
(PPh3)2], which was much less
Although we have not examined the effect of sol-
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
N
ACHTUNGTRENN(UGN acac)(CO)2 could be applied
readily to other terminal alkynes, yielding the corre-
sponding adducts in acceptable yields with excellent
regio- and stereoselectivities (Table 2).
Table 2. Reactions of 1a with various alkynes 2Y.[a]
Besides 1-octyne, sterically congested tert-butylace-
tylene also reacts smoothly. Functionalities such as
chloro and ester groups bound to aliphatic alkynes
are tolerated. Aromatic and heteroaromatic acety-
lenes also undergo the reaction smoothly. An elec-
tronic effect of the para substituents is evidently not
seen in this catalytic reaction. The reaction of trime-
thylsilylacetylene is somewhat messy and undergoes
rather extensive oligomerization, although the corre-
sponding adduct is obtained in a moderate yield. The
reactions of conjugated alkynes (methyl propiolate, 1-
ethynylcyclohexene) and internal alkynes (diphenyl-
Entry
2Y
R2 =
Yield [%][b]
1
2A
2B
2C
2D
2E
2F
2G
2H
2I
n-hexyl
t-Bu
70 (66)
68 (67)
69 (64)
75 (70)
63 (55)
65 (61)
66 (60)
81 (75)
53 (52)
2[c]
3
3-chloropropyl
3-(methoxycarbonyl)propyl
phenyl
p-methoxyphenyl
p-fluorophenyl
2-thienyl
4
5
6
7
AHCTUNGTREGaNNNU cetylene, dimethyl acetylenedicarboxylate, 4-octyne),
however, are not successful to give benzoyl chloride
as major product and oligo- or polymerization of the
alkynes also takes place.
8
9[c]
trimethylsilyl
[a]
Reaction conditions: 1a (1.0 mmol), 2Y (0.5 mmol), Rh-
(acac)(CO)2 (0.025 mmol), toluene (1 mL), 608C, 24 h, in
1,4-DiACTHNUTRGENUG(N ethynyl)benzene (2J) appears to undergo
ACHTUNGTRENNUNG
stepwise single and double addition selectively
(Scheme 2). Thus, when 2J was treated with 4 equiva-
lents of 1a, (Z)-3aJ-1 was formed in 65% NMR yield
(62% isolated yield; conversion of 2Jꢀ100%) nearly
as sole adduct. However, nearly the same reaction
using 8 equivalents of 1a furnished (Z,Z)-3aJ-2 in
a 20-mL Schlenk tube.
[b]
[c]
Determined by 1H NMR spectroscopy or GC using
1,1,2,2-tetrachloroethane or n-tetradecane, respectively,
as internal standard. Based on 2Y. The figures in paren-
theses are isolated yields.
Run in a 5-mL Schlenk tube.
Scheme 2. Reactions of 1,4-diACTHNUGTRNEUNG(ethynyl)benzene with phenylglyoxyl chloride.
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Table 3. Effects of the substituents on a-keto acid chlorides in the reaction with 2A.[a]
Entry
1x
R1 =
Yield [%][b]
(Z)-5xA
(Z)-3xA
6x[c] (mmol)[c]
1[d]
2
3
1a
1b
1c
1d
1e
1f
1g
1h
1i
phenyl
4-ClC6H4
4-CF3C6H4
C6F5
70 (66)
78 (70)
77 (72)
91 (88)
trace
2
0.30
trace
0.25
0.01
0.78
0.9
0.42
nd[e]
nd[e]
trace
nd[e]
0
43
18
nd[e]
0
5
6[d]
7
p-tolyl
p-anisyl
2-thienyl
2-furyl
t-Bu
0
8
9
10
74 (64)
83 (75)
21
0
[a]
Reaction conditions: 1x (1.0 mmol), 2A (0.5 mmol), RhACTHNUTRGNENG(U acac)(CO)2 (0.025 mmol), 608C, 24 h in toluene (1 mL).
[b]
1
Determined by H NMR spectroscopy or GC using 1,1,2,2-tetrachloroethane or n-tetradecane, respectively, as internal
standard. The figures in parentheses are isolated yields. Based on 2A.
Based on 1x charged and determined by GC using n-tetradecane as an internal standard.
A trace of 4xA was formed.
[c]
[d]
[e]
Not determined.
67% NMR yield (58% isolated yield). Although the
As for the initiation process generating intermedi-
selective stepwise addition is somewhat puzzling in ate 8, we have already reported that, with exceptions
view of the lack of electronic effects of the para sub- (vide infra), oxidative addition of an arylglyoxyl chlo-
stituents of aromatic alkynes on the reactivity, the se- ride with RhACHTUNTRGENUG(N acac)(CO)2 proceeds readily at room
lectivity suggests that (Z)-3aJ-1 is much less reactive temperature to generate arylglyoxylrhodium species
(Scheme 4).[4] Insertion of an alkyne into the Rh Cl
À
than the starting diyne 2J.
The reactions of other a-keto acid chlorides are af-
fected significantly by the nature of the substituents
(Table 3).
Table 3 shows the yield of 3 decreases in the order
of C6F5 >4-ClC6H4 >4-CF3C6H4 >Ph>p-Tolꢁp-An
(ꢀ0), indicating that arylglyoxyl chlorides having elec-
tron-withdrawing substituents undergo the reaction
smoothly, while those having electron-donating sub-
stituents fail to give the desired adducts. Heteroaryl-
glyoxyl chlorides such as 1g and 1h also participate in
this catalysis smoothly. On the other hand, the reac-
tions of alkyl or alkenyl a-keto acid chlorides are not
successful. Although the reaction of 3,3-dimethyl-2-
oxobutyryl chloride (1i) gave the corresponding prod-
uct 3 in 21% yield, pyruvoyl chloride gave a very
messy mixture. Likewise, the reactions of 3-methyl-3-
phenyl-2-oxobutyryl chloride and (Z)-4-phenyl-2-oxo-
3-butenoyl chloride did not form 3 at all. In the
former, the only event observed was the formation of
6 (84%) and in the latter 6 and (Z,Z)-7A were
formed in 51 and 32% yields, respectively.
Scheme 3. A plausible mechanism.
With our previous observations in mind, we pro-
pose the mechanism depicted in Scheme 3, which also
includes the processes leading to some of the by-prod-
ucts.
Scheme 4. Formation of arylglyoxylrhodium complexes.
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Rhodium-Catalyzed Addition of a-Keto Acid Chlorides with Terminal Alkynes
bond in 8[11] and subsequent C C reductive elimina-
In the present catalysis, other by-products such as
À
tion are also well supported by precedent publications benzil, benzophenone and 1,4-dichloro-1,3-butadiene
disclosing similar addition reactions of acid chlorides derivatives were also formed in small quantities. One
with alkynes[2,3] and chlorinative dimerization of al- can think of a bimolecular reductive elimination from
kynes with trichloroacetyl chloride.[8] To gain further or a disproportionation of intermediates 8, 9 and/or
supportive information, we ran a reaction of [Rh
Cl)(acac)(CO)(COCOPh)]2 with 2A {mole ratio of Indeed, p-chloro analogues of benzil and benzophe-
[Rh(m-Cl)(acac)(CO)(COCOPh)]2/2A=0.5} in ben- none were formed in the thermolysis of p-
zene-d6 for 24 h at 608C. The reaction gave (Z)-3aA ClC6H4COCORhCl2(CO)(PMe3)2, but not in the ther-
ACHTUNGTREN(NUNG m- 10 as candidate pathways to these by-products.
N
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
(38%), (Z,Z)-7,10-dichlorohexadeca-7,9-diene [(Z,Z)- molysis
7A; 9% based on 2A] and benzil (trace).
of
[RhACHTUNRGTNEG(NU m-Cl)ACHUTGTNRNEUG(N acac)(CO)ACHTNUGERTG(NUNN COCOACHTUNGTRENNUNG(p-
ClC6H4)]2.[4] The provenance of these by-products is
According to our previous paper, [RhACTHNUTRGENUGN(m-Cl)- uncertain at the present time.
(acac)(CO)
E
After the addition of a-keto acid chlorides with al-
mally stable at room temperature, but it does undergo kynes had been realized, it appeared intriguing to at-
decarbonylation and reductive elimination to give tempt the carbonylative addition of benzoyl choride
ArCOCl at higher temperatures, e.g., at 608C.[4] Com- 6a with 1-octyne 2A, since we have observed that car-
plex 8 generated under the present catalytic condi- bonylation of p-ClC6H4CORhCl2(CO)CAHTUNGTRENNUNG
tions is also envisioned to experience the same events ates p-ClC6H4COCORhCl2(CO)ACHTUNGTRENNUNG
to give 6 as a by-product. Indeed, all reactions starting low yield.[4] Indeed, a preliminary reaction of 6a with
with 1a–1g gave aroyl chlorides (6a–6g), although the 2A under CO atmosphere (50 atm) in the presence of
extent of their formation depended on the substituent Rh
on the aromatic ring as shown in Table 3 (vide infra). toluene gave (Z)-3aA in 21% GC yield, which en-
Since oxidative addition of 6 with Rh(acac)(CO)2 is couraged us to further studies, although the yield is
not a thermodynamically favored process,[4] once com- not satisfactory at this stage.
ACHUTGTNRNE(NUG acac)(CO)ACHTUNGTERN(NUGN AsPh3) (5 mol%) at 1008C for 12 h in
AHCTUNGTRENNUNG
pound 6 has been extruded from 9, regenaration of 9
The products prepared by the new procedure are
through the backward process can take place only quite useful to synthesize, for instance, heterocyclic
marginally.
compounds. The utility of the products can be readily
What merits further mechanistic consideration is exemplified with (Z)-3aA and (Z)-3aE. The former
the origin of the unsuccessful reactions. As is seen in cyclizes with hydroxylamine to afford isoxazole 12[7]
Table 3, arylglyoxyl chlorides having a more electron- and the latter with o-phenylenediamine to afford qui-
donating substituent, 1e and 1f, behaved differently noxaline derivative 13 (Scheme 5).
from those having an electro-neutral or more electro-
In conclusion, RhACHTUNGTERNNU(G acac)(CO)2 catalyzes the addi-
negative substituent, 1a–1d, in terms of the following tion of a-keto acid chlorides to terminal alkynes
three aspects. (i) They did not form 3 in a significant stereo- and regioselectively to give b-chlorovinyl a-di-
quantity, and (ii) the formation of 6 was more exten- ketones, which are useful synthetic intermediates. Fur-
sive. This reactivity trend agrees with our previous ob- ther investigations on catalyzed addition reactions of
servation that, unlike the reaction of 1a, for instance, acid chlorides, under carbonylative conditions in par-
the reaction of 1f with Rh
perature (in the absence of an alkyne) did not form
stable [Rh(m-Cl)(acac)(CO)(COCO(p-anisyl)]2, but
proceeded all the way to p-anisoyl chloride and Rh-
(acac)(CO)2, indicative of rapid decarbonylation from
ACHTUNGTRENN(UNG acac)(CO)2 at room tem- ticular, are in progress.
A
E
N
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
p-AnCOCORhCl to p-AnCORhCl and reductive
elimination.[4] (iii) Only 1e and 1f formed 5 in appre-
ciable quantities. As is mentioned in the introduction
section, we have reported that electron-withdrawing
substituents bound to acid chlorides suppress the de-
carbonylation from RCORhCl to RRhCl.[3,12] Inter-
mediate species 9e and 9f (R1 =p-Tol, p-An), howev-
Scheme 5. Examples of the utility of the products.
er, are substituted by electron-donating substituents. Experimental Section
Accordingly, the electronic nature is envisioned to, at
least partially, give an advantage to decarbonylation Typical Procedure for the Catalytic Addition
forming 10 in competition with the rapid reductive Reaction and Isolation of Products: Reaction of 1-
elimination forming 6. Conversion of 9 to 10 is made Octyne with Phenylglyoxyl Chloride
easier for 1e and 1f, as compared with 1a–1d, and
To a solution of Rh
ACHTUNGTERNN(UNG acac)(CO)2 (6.49 mg, 0.025 mmol) in
leads to the formation of 5 as Miura and co-workers
reported.[2]
toluene (1 mL) placed in a 20-mL Schlenk tube were added
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Taigo Kashiwabara and Masato Tanaka
COMMUNICATIONS
phenylglyoxyl chloride (1a; 169.2 mg, 1.0 mmol) and 1-
octyne (2A; 74 mL, 0.5 mmol) and the mixture was stirred at
608C for 24 h. After cooling to room temperature, n-tetrade-
cane (14.6 mg; internal standard for GC analysis) was added
to the resulting mixture. After GC analysis, which showed
the formation of (Z)-3aA in 70% yield, the mixture was
evaporated and 1,1,2,2-tetrachloroethane (9.9 mg; internal
standard for NMR analysis) was added. After NMR analy-
sis, the mixture was evaporated once again and the residue
was subjected to column chromatography (silica gel, hexane/
acetone=95/5) to furnish (Z)-3aA; yield: 66%; yellow oil.
1H NMR (300 MHz, CDCl3): d=7.97 (d, J=1.45 Hz, 2H, o-
Ph), 7.62 (t, J=1.90 Hz, 1H, p-Ph), 7.48 (d, J=1.45 Hz, 2H,
m-Ph), 6.70 (s, 2H, CH), 2.53 (t, J=6.27 Hz, 2H, C-5), 1.65
(quint, J=7.29 Hz, 2H, C-6), 1.36–1.27 (br-m, 6H, C-7, C-8,
C-9), 0.88 (t, J=6.78 Hz, 3H, C-10); 13C{1H} NMR (75 MHz,
CDCl3): d=192.0 (CO), 190.6 (CO), 154.9 (C-4), 134.4 (p-
C), 132.3 (ipso-C in Ph), 130.1 (o-C), 128.7 (m-C), 121.0 (C-
3), 41.6 (C-5), 31.3 (C-8), 28.2 (C-7), 27.2 (C-6), 22.4 (C-9),
13.9 (C-10); IR (neat): n=1726 (nCO), 1674 (nCO), 1597 cmÀ1
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[5] a) H. des Abbayes, J.-Y. Salaꢁn, Dalton Trans. 2003,
1041. For representative examples, see for Mn: b) J. B.
Sheridan, G. L. Geoffroy, A. L. Rheingold, J. Am.
Chem. Soc. 1987, 109, 1584; Fe: c) J. B. Sheridan, S.-H.
Han, G. L. Geoffroy, J. Am. Chem. Soc. 1987, 109,
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W. M. Vetter, R. R. Whittle, J. Am. Chem. Soc. 1987,
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Yamamoto, Organometallics 1984, 3, 692; Pt: g) Y.-J.
You, J.-T. Chen, M.-C. Cheng, Y. Wang, Inorg. Chem.
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was also documented, see ref.[3b]
(nC C); GC-MS (70 eV): m/z (% relative intensity)=278
=
([M]+, 5), 250 (26), 201 (15), 173 (16), 132 (62), 105 (100),
77 (34); HR-MS (EI): m/z=278.1069, calcd. for C16H19ClO2:
278.1074. An NOE experiment displayed 12.2% enhance-
ment of the vinylic proton signal (d=6.70 ppm) upon irradi-
ation at the allylic proton signal (2.53 ppm), indicative of cis
addition having taken place.
[6] For selected examples of synthetic applications of a-
oxo-b,g-unsaturated carbonyl compounds, see: a) H.
Sugimura, K. Yoshida, J. Org. Chem. 1993, 58, 4484;
b) E. Brown, G. Dujardin, M. Maudet, Tetrahedron
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Rev. 1966, 66, 161.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Re-
search on Priority Areas (No.18065008) from MEXT, Japan,
and by a research fellowship to T.K. from JSPS.
[8] The rhodium-catalyzed chlorinative dimerization of ter-
minal alkynes has been reported separately: T. Kashi-
wabara, K. Fuse, T. Muramatsu, M. Tanaka, J. Org.
Chem. 2009, 74, 9433.
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