Rhodium(I)-catalyzed carbonylative arylation of alkynes with arylboronic acids using formaldehyde as a carbonyl source

Article abstract of DOI:10.1055/s-0033-1341046

The rhodium(I)-catalyzed reaction of alkynes with arylboronic acids in the presence of formaldehyde resulted in a carbon monoxide gas-free carbonylative arylation to yield α,β-enones. The simultaneous loading of phosphine-ligated and phosphine-free rhodium(I) complexes is required for efficient catalysis, which catalyze the abstraction of a carbonyl moiety from formaldehyde (decarbonylation) and its subsequent introduction into the substrate (carbonylation), respectively. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1341046

LETTER
▌1155
lRetther odium(I)-Catalyzed Carbonylative Arylation of Alkynes with Arylboronic
Acids Using Formaldehyde as a Carbonyl Source
Carbonylative Arylation of Alkynes
Chuang Wang,a Tsumoru Morimoto,*a Hiroyuki Kanashiro,a Hiroki Tanimoto,a Yasuhiro Nishiyama,a
Kiyomi Kakiuchi,a Levent Artokb
a
Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), Ikoma, Nara 630-0192, Japan
Fax +81(743)726081; E-mail: morimoto@ms.naist.jp
b
Department of Chemistry, Faculty of Science, Izmir Institute of Technology, Urla 35430, Izmir, Turkey
Received: 27.01.2014; Accepted after revision: 28.02.2014
catalysis involving phosphine-ligated and phosphine-free
Abstract: The rhodium(I)-catalyzed reaction of alkynes with aryl-
rhodium catalysts, we found that the catalyst system is ap-
boronic acids in the presence of formaldehyde resulted in a carbon
plicable for another carbonylation reaction catalyzed by a
phosphine-free rhodium(I) complex. We report herein on
monoxide gas-free carbonylative arylation to yield α,β-enones. The
simultaneous loading of phosphine-ligated and phosphine-free rho-
dium(I) complexes is required for efficient catalysis, which catalyze the formaldehyde-substituted carbonylation reactions of
the abstraction of a carbonyl moiety from formaldehyde (decarbon-
ylation) and its subsequent introduction into the substrate (carbon-
ylation), respectively.
alkynes with arylboronic acids using such a catalyst sys-
tem, resulting in a carbonylative arylation to give enone
derivatives (Scheme 1, b). This can substitute for the con-
Key words: carbonylation, arylation, rhodium, formaldehyde, al-
kynes
ventional phosphine-free rhodium(I)-catalyzed reaction
and avoids the direct use of carbon monoxide.7
X
R
Transition-metal-catalyzed carbonylation represents an
essential type of reaction leading to the efficient prepara-
tion of a wide class of carbonyl-containing compounds.1
Considerable attempts have been made to simplify the ex-
perimental manipulation and to exploit novel carbonyla-
tive transformation, that is, the carbon monoxide gas-free
carbonylation methods using carbonyl donors,2 such as
formic acid,3a formates,3b formamides,3c chloroform,3d al-
cohols,3e acid halides,3f and metal carbonyls.3g We4 and
other groups5 also recently reported on the development
of a convenient carbonylation protocol using aldehydes as
a carbonyl source. In the methodology, the rhodium(I)-
catalyzed decarbonylation of an aldehyde is the essential
key process as a carbonyl donor. However, a catalyst that
is effective for the targeted carbonylation process is not
always consistent with that for the decarbonylation pro-
cess, which generally proceeds in the presence of a rhodi-
um(I)–phosphine complex.
+
+
(CH2O)n
R
B(OH)2
(a) X = Br
O
R
[RhCl(cod)]2/BINAP
(Rh/P = 5:2)
R
O
(b) X = H
[RhCl(cod)]2/BIPHEP
(Rh/P = 5:1)
R
R
Scheme 1 Phosphine-free and ligated dual Rh-catalyzed carbonyl-
ation reactions using formaldehyde as the CO source
Our previous reports revealed that a phosphine-free rhodi-
um(I) catalyst such as [RhCl(C2H4)2]2 is effective for the
carbonylative arylation of alkynes with arylboronic acids
using carbon monoxide.7 We examined the reaction of di-
phenylacetylene (1a), phenylboronic acid (2a), and para-
formaldehyde under catalytic conditions consisting of 5
mol% of [RhCl(cod)]2 and 2 mol% of BIPHEP,8 which
are similar to the catalytic conditions used in the above-
mentioned reaction.4d Consequently, the reaction resulted
in a carbon monoxide gas-free carbonylative arylation re-
action, with the production of enones 3aa in 46% yield as
a mixture of E- and Z-isomers in a ratio of 37:63, along
with a 9% yield of the hydroarylation product 4aa (Table
1, entry 3). The geometry of (E)- and (Z)-3aa was as-
signed by X-ray crystallographic analysis, and the ratio of
Among the examples we developed, the rhodium(I)-cata-
lyzed cyclocarbonylation reactions of alkynes with aryl-
boronic acids corresponds to such a situation (Scheme 1,
a).4d The original reaction, in which carbon monoxide is
used, is catalyzed by a phosphine-free rhodium(I) com-
plex.6 In this case, we achieved a similar transformation
using formaldehyde instead of carbon monoxide by con-
trolling the amount of added phosphine ligand so that rho-
dium complexes both with and without phosphine ligand
are present, thus permitting both decarbonylation and car-
bonylation to proceed in a single catalyst system. During
the study of the carbon monoxide gas-free carbonylation
1
the forms of 3aa was determined by H NMR analysis.
We subsequently examined different amounts of added
BIPHEP from 0 mol% to 10 mol% to 5 mol%
[RhCl(cod)]2 (Table 1, entries 1–5). The use of 1 mol% of
SYNLETT 2014, 25, 1155–1159
Advanced online publication: 27.03.2014
DOI: 10.1055/s-0033-1341046; Art ID: ST-2014-U0080-L
© Georg Thieme Verlag Stuttgart · New York
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6

1156
C. Wang et al.
LETTER
Table 2 Screening of Phosphine Ligands in Synthesis of 3aaa
BIPHEP resulted in the best result, 62% yield of 3aa. The
hydroarylation product 4aa was mainly obtained when
BIPHEP was absent (Table 1, entry 1). When 10 mol% of
BIPHEP, which accounts for all of the loaded rhodium,
was added, 3aa was produced in much lower yield (Table
1, entry 5). These results imply that [RhCl(BIPHEP)]2 and
[RhCl(cod)]2 are predominantly involved in the decarbon-
ylation and carbonylative arylation processes, respective-
ly.
Entry
Phosphine
Yield (%)b
3aa (E/Z)c
(E)-4aa
1
2
3
4
5
6
2 Ph3P
Xantphos
dppe
26 (37:63)
12 (35:65)
44 (37:63)
39 (35:65)
29 (37:63)
52 (46:54)
54
58
21
18
48
11
dppp
Table 1 Effect of BIPHEP in Rhodium(I)-Catalyzed Reaction of 1a
with 2a Using Paraformaldehyde as a CO Sourcea
dppf
[RhCl(cod)]2 (5 mol%)
BINAP
O
Ph
BIPHEP
PhB(OH)2
+
+
a Reaction conditions: 1 (1 mmol), 2 (2 mmol), paraformaldehyde (5
mmol), [RhCl(cod)]2 (5 mol%), phosphine as ligand (1 mol%) except
for Ph3P (2 mol%) in dioxane (1 mL) at 80 °C for 20 h.
b Isolated yield.
H
H
Ph
dioxane, 80 °C, 20 h
2a (2 equiv)
1a
(5 equiv)
O
Ph
Ph
c Determined by 1H NMR.
+
Ph
Ph
Ph
Ph
(E)-3aa
(E)-4aa
4ac, respectively (Table 3, entries 1 and 2). On the other
hand, the introduction of an electron-withdrawing group
(2d 4-Cl; 2e 4-F3C) decreased the yields of the carbonyl-
ated products 3ad (53%) and 3ae (45%), while the yields
of noncarbonylated products 4ad and 4ae were increased
(Table 3, entries 4 and 5). Generally, the more electron-
donating R group becomes, the more smoothly the migra-
tory insertion of the R group onto the carbonyl ligand
takes place to shift the equilibrium between acyl–metal
(RCOM) and alkyl–metal–carbonyl (RMCO, Scheme 2)
to the generation of the RCOM species, due to the thermo-
dynamic strength of the RM bond.9 In the present cases,
the former analogue would be involved in the formation
of the carbonylated products 3, and the latter in that of
noncarbonylated products 4. Thus, as the electron-donat-
ing strength of the substituent on the aromatic ring of
ArB(OH)2 decreases, the yields of enones 3 decrease and
those of 4 increase.
Entry
BIPHEP (mol%) Yield (%)b
3aa (E/Z)c
(E)-4aa
1
2
3
4
5
0
1
32 (39:61)
62 (32:68)
46 (37:63)
27 (35:65)
4 (57:43)
56
7
2
9
5
4
10
1
a Reaction conditions: 1a (1 mmol), 2a (2 mmol), paraformaldehyde
(5 mmol), [RhCl(cod)]2 (5 mol%) in dioxane (1 mL) at 80 °C for 20 h.
b Isolated yield.
c Determined by 1H NMR.
Results using various phosphines as a ligand to the rhodi-
um catalyst in the carbonylative arylation of 1a with 2a
are summarized in Table 2. With the addition of a mono-
dentate phosphine, such as triphenylphosphine, 3aa was
obtained in low yield of 26%, along with the hydroarylat-
ed product (E)-4aa in 54% yield (Table 2, entry 1).
Among other bidentate ligands tested (Table 2, entries 2–
6), BIPHEP gave the best result as shown in Table 1, entry
2. The standard catalytic conditions for the reaction were
determined to be 5 mol% of [RhCl(cod)]2 and 1 mol% of
BIPHEP.
O
Ar
O
Ph
Ar
Ph
R
M
CO
Ph
R
M
Ph
4
3
Scheme 2 Equilibrium between RCOM and RMCO
As the position of the substituent was changed from para
to meta and ortho, the yield of the carbonylative arylation
products 3ab, 3af, and 3ag decreased, and that of the hy-
droarylation products 4ab, 4af, and 4ag increased (Table
3, entries 1, 6, and 7). The position of the substituent has
almost no impact on the ratio of the E- and Z-somers of
3ab, 3af, and 3ag.
With the above optimized conditions in hand, we next in-
vestigated the effect of the electronic nature of the substit-
uent on the aromatic ring of arylboronic acids 2 on the
carbonylative arylation of diphenylacetylene (1a). Reac-
tions with arylboronic acids having an electron-donating
group at the para position of the aromatic ring, such as
4-methoxyphenylboronic acid (2b) and 4-methylphenyl-
boronic acid (2c), yielded the corresponding carbonylated
products 3ab and 3ac in 77% and 70% yields as a mixture
of E- and Z-isomers, along with a trace and a 2% yield of
the hydroarylated (noncarbonylated) products 4ab and
The applicability of other alkynes under the present car-
bonylation conditions was examined (Table 4). In most
cases, the present method leads to the efficient formation
of products even without the use of trifluoroacetic acid as
an additive, which is need for the reaction using carbon
monoxide.7 Reactions of diphenylacetylene derivatives
Synlett 2014, 25, 1155–1159
© Georg Thieme Verlag Stuttgart · New York

LETTER
Carbonylative Arylation of Alkynes
1157
Table 3 Reactions of 1a with Various 2 Using Paraformaldehyde as
Table 4 Reactions of Various Alkynes 1 with 2b Using Paraform-
a CO Sourcea
aldehyde as a CO Sourcea
[RhCl(cod)]2 (5 mol%)
BIPHEP (1 mol%)
[RhCl(cod)]2 (5 mol%)
BIPHEP (1 mol%)
R2
O
O
Ph
ArB(OH)2
+
+
ArB(OH)2
+
+
R1
H
H
H
H
Ph
2b (2 equiv)
Ar = 4-MeOC6H4
dioxane, 80 °C, 20 h
dioxane, 80 °C, 20 h
2 (2 equiv)
1a
(5 equiv)
(5 equiv)
1
O
O
O
Ar
Ph
+
Ar
R2
Ar
R1
+
Ar
Ph
Ph
R1
R2
Ph
(E)-3
(E)-4
(E)-3
(E)-3'
R1
R2
Yield (%)b
Entry
Ar 2
Yield (%)b
3 (E/Z)c
Entry
1
(E)-4
3 (E/Z)
1
2
3
4
5
6
7
4-MeOC6H4
4-MeC6H4
H
3ab 77 (26:74)
3ac 70 (27:73)
3aa 62 (32:68)
3ad 53 (35:65)
3ae 45 (41:59)
3af 61 (28:72)
3ag 23 (30:70)
4ab trace
4ac 2
1
1b
1c
1d
1e
4-MeOC6H4
4-F3CC6H4
n-Pr
3bb 60 (32:68)c
3cb 54 (25:75)d
3db 41 (93:7)d
2
4aa 7
3e
4
4-ClC6H4
4-F3CC6H4
3-MeOC6H4
2-MeOC6H4
4ad 9
Me
Ph
Ph
3eb 42 (80:20)c
3eb′ 15 (37:63)c
4ae 12
4af 2
5
1f
n-Bu
3fb 42 (75:25)c
3fb′ 22 (34:66)c
4ag 18
a Reaction conditions: 1 (1 mmol), 2 (2 mmol), paraformaldehyde (5
mmol), [RhCl(cod)]2 (5 mol%), BIPHEP (1 mol%) in dioxane (1 mL)
at 80 °C for 20 h.
a Reaction conditions: 1 (1 mmol), 2 (2 mmol), paraformaldehyde (5
mmol), [RhCl(cod)]2 (5 mol%), BIPHEP (1 mol%) in dioxane (1 mL)
at 80 °C for 20 h.
b Isolated yield.
b Isolated yield.
c Determined by 1H NMR.
c Determined by 1H NMR.
d Ratios of isolated products.
e 2 equiv of TFA were added.
1b and 1c with 4-MeOC6H4B(OH)2 (2b) gave the corre-
sponding enones 3bb and 3cb in 60% (E/Z = 32:68) and
54% (E/Z = 25:75) yields, respectively (Table 4, entries 1
and 2). An alkyl substituent (n-Pr) at the terminus of the
alkyne was tolerant in the present reaction, although the
addition of two equivalents of trifluoroacetic acid was re-
quired (Table 4, entry 3).10 For unsymmetrical alkynes
having phenyl and alkyl groups (1e and 1f), the reactions
proceeded somewhat regioselectively to give predomi-
nantly the enones 3eb and 3fb, respectively, in both of
which phenyl group is located at the β-position of the ar-
oyl groups (Table 4, entries 4 and 5).
A possible reaction pathway for the present reaction is
shown in Scheme 4. As in our previous report,4d the addi-
tion of BIPHEP, the amount of which cannot account for
all of the loaded rhodium metal, leads to the partial forma-
tion of a RhBIPHEP species, probably [RhCl(BIPHEP)]2,
along with the intact [RhCl(cod)]2. The former BIPHEP-
ligated rhodium(I) species mainly decarbonylates formal-
dehyde to generate the carbonyl unit and hydrogen, while
the latter predominantly catalyzes the actual carbonyl-
ation process. Thus, RhOH (A) generated in situ from
RhCl and H2O is transmetalated with PhB(OH)2 to gener-
[RhCl(cod)]2 (5 mol%)
BIPHEP (1 mol%)
In order to ascertain the origin of the hydrogen at the β-po-
sition of the enones 3aa, we carried out some deuterium-
labeling experiments. Initially, the reaction of diphenyl-
acetylene (1a) with phenylboronic acid (2a) in the pres-
ence of paraformaldehyde-d2 [(CD2O)n] was exposed to
the above reaction conditions to give 3aa as a mixture of
E- and Z-isomers in a ratio of 34:66. A 1H NMR analysis
revealed that less than 2% of deuterium was incorporated
into the β-position of the enones 3aa (Scheme 3). On the
other hand, when 2,4,6-triphenylboroxine [(PhBO)3] and
D2O, which should generate the deuterated phenylboronic
acid (PhB(OD)2) in situ, was used instead of phenylboron-
ic acid (2a), 76% and 87% D were incorporated into β-po-
sition of (E)- and (Z)-3aa.11 These results indicate that the
origin of the hydrogen is the acidic proton of phenyl-
boronic acid.
Ph
PhB(OH)2
+
+
(CD
2O)n
Ph
dioxane, 80 °C, 20 h
2a (2 equiv)
(5 equiv)
1a
O
D
3aa 57% (E/Z = 34:66)
Ph
Ph
E: <2% of D
Ph
Z: <2% of D
[RhCl(cod)]2 (5 mol%)
BIPHEP (1 mol%)
Ph
(PhBO)3
+
+
(CH2O)n
(5 equiv)
Ph
dioxane, 80 °C, 20 h
D2O (2 equiv)
(2/3 equiv)
1a
O
D
3aa 60% (E/Z = 33:67)
Ph
Ph
E: 76% of D
Ph
Z: 87% of D
Scheme 3 Deuterium-labelling experiments
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 1155–1159

1158
C. Wang et al.
LETTER
ate RhPh (B). The carbonyl moiety from the decarbonyl-
ation process is transferred to B, followed by the insertion
of the Rh–C bond in C to yield RhCOPh (D). The subse-
quent aroylrhodation to an alkyne 1 in a syn manner, fol-
lowed by the protonation of the formed vinylrhodium E,
produces the primary product (E)-3aa, along with the re-
generation of A. (E)-3aa isomerizes to give an equilibri-
um mixture of (E)- and (Z)-3aa under the rhodium-
catalytic conditions in the presence of an acidic arylboron-
ic acid.12,13
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
References and Notes
(1) (a) Kollár, L. Modern Carbonylation Methods; Wiley-VCH:
Weinheim, 2008. (b) Beller, M. Catalytic Carbonylation
Reactions; Springer: Berlin, 2006.
(2) For a review, see: Morimoto, T.; Kakiuchi, K. Angew. Chem.
Int. Ed. 2004, 43, 5580.
(3) For recent papers, see: (a) Brancour, C.; Fukuyama, T.;
Mukai, Y.; Skrydstrup, T.; Ryu, I. Org. Lett. 2013, 15, 2794.
(b) Ueda, T.; Konishi, H.; Manabe, K. Org. Lett. 2012, 14,
3100. (c) Ju, J.; Jeong, M.; Moon, J.; Jung, H. M.; Lee, S.
Org. Lett. 2007, 9, 4615. (d) Grushin, V. V.; Alper, H.
Organometallics 1993, 12, 3846. (e) Park, J. H.; Cho, Y.;
Chung, Y. K. Angew. Chem. Int. Ed. 2010, 49, 5138.
(f) Bjerglund, K.; Lindhardt, A. T.; Skrydstrup, T. J. Org.
Chem. 2013, 78, 6112. (g) Nordeman, P.; Odell, L. R.;
Larhed, M. J. Org. Chem. 2012, 77, 11393.
(4) For selected papers, see: (a) Ikeda, K.; Morimoto, T.;
Tsumagari, T.; Tanimoto, H.; Nishiyama, Y.; Kakiuchi, K.
Synlett 2012, 23, 393. (b) Fujioka, M.; Morimoto, T.;
Tsumagari, T.; Tanimoto, H.; Nishiyama, Y.; Kakiuchi, K.
J. Org. Chem. 2012, 77, 2911. (c) Makado, G.; Morimoto,
T.; Sugimoto, Y.; Tsutsumi, K.; Kagawa, N.; Kakiuchi, K.
Adv. Synth. Catal. 2010, 352, 299. (d) Morimoto, T.;
Yamasaki, K.; Hirano, A.; Tsutsumi, K.; Kagawa, N.;
Kakiuchi, K.; Harada, Y.; Fukumoto, Y.; Chatani, N.;
Nishioka, T. Org. Lett. 2009, 11, 1777.
O
Rh = BIPHEP-ligated Rh
Rh = BIPHEP-free Rh
H
H
Rh CO
Rh
– H2
ArB(OH)2
H2O
2
Rh Cl
Rh OH
Ar Rh
Ar Rh CO
– HCl
H
– B(OH)3
A
B
C
R
O
Rh
O
R
O
1
Ar
R
Ar
R
H2O
B(OH)n
Ar
Rh
R
R
(E)-3
E
D
(Z)-3
(5) For recent papers, see: (a) Lee, H. W.; Lee, L. N.; Chan, A.
S. C.; Kwong, F. Y. Eur. J. Org. Chem. 2008, 3403.
(b) Shibata, T.; Toshida, N.; Yamasaki, M.; Maekawa, S.;
Takagi, K. Tetrahedron 2005, 61, 9974. (c) Jeong, N.; Kim,
D. H.; Choi, J. H. Chem. Commun. 2004, 1134.
(6) Harada, Y.; Nakanishi, J.; Fujihara, H.; Tobisu, M.;
Fukumoto, Y.; Chatani, N. J. Am. Chem. Soc. 2007, 129,
5766.
(7) (a) Artok, L.; Kuş, M.; Aksın-Artok, Ö.; Dege, F. N.;
Özkılınç, F. Y. Tetrahedron 2009, 65, 9125. (b) Kuş, M.;
Artok, Ö. A.; Ziyanak, F.; Artok, L. Synlett 2008, 2587.
(8) Abbreviations: BIPHEP = 2,2′-bis(diphenylphosphino)-1,1′-
biphenyl; Xantphos = 9,9-dimethyl-4,5-bis(diphenyl-
phosphino)xanthene; dppe = 1,2-bis(diphenylphosphino)-
ethane; dppp = 1,3-bis(diphenylphosphino)propane; dppf =
1,1′-bis(diphenylphosphino)ferrocene; BINAP = 2,2′ =
bis(diphenylphosphino)-1,1′-binaphthyl.
Scheme 4 Possible reaction pathway
In conclusion, we report here on the rhodium(I)-catalyzed
carbonylative arylation of alkynes with various arylbo-
ronic acids in the presence of formaldehyde as a carbonyl
source, resulting in a carbon monoxide gas-free reaction,
affording enones. The addition of BIPHEP, the amount of
which must not account for all the total phosphine-free
rhodium complex [RhCl(cod)]2 leads to the partial forma-
tion of phosphine-ligated complex [RhCl(BIPHEP)]2
along with intact [RhCl(cod)]2, which are mainly involved
in the decarbonylation of formaldehyde to produce a car-
bonyl moiety and the subsequent carbonylative arylation
of alkynes with arylboronic acids using the resulting car-
bonyl moiety, respectively, leading to an efficient carbon-
ylative arylation to produce α,β-enones.
(9) (a) Fristrup, P.; Kreis, M.; Palmelund, A.; Norrby, P.-O.;
Madsen, R. J. Am. Chem. Soc. 2008, 130, 5206.
(b) Calderazzo, F. Angew. Chem., Int. Ed. Engl. 1977, 16,
299.
In a 10 mL screw-capped vial were placed [RhCl(cod)]2 (24.6 mg,
0.05 mmol), BIPHEP (5.3 mg, 0.01 mmol), alkyne 1 (1 mmol), ar-
ylboronic acid 2 (2 mmol), paraformaldehyde (150.2 mg, 5 mmol),
and 1,4-dioxane (1 mL). The mixture was degassed by three freeze-
pump-thaw cycles and then sealed under N2. The mixture was
stirred at 80 °C for 20 h, cooled to r.t. and then concentrated in vac-
uo. The residue was purified by flash chromatography on silica gel.
(10) The role of TFA is, at present, unclear. We postulate the
following two possibilities: (i) it promotes the generation of
free formaldehyde from paraformaldehyde, and (ii) it
accelerates the protonation of the vinylrhodium species E to
give the product.
(11) The fact that the deuteration of the β-position of the enones
3aa does not reach 99% is due to H–D exchange via 1,3- and
1,4-shift of the rhodium in the intermediate E to other Ph
rings at β-position: (a) Sasaki, K.; Nishimura, T.; Shintani,
R.; Kantchev, E. A. B.; Hayashi, T. Chem. Sci. 2012, 3,
1278. (b) Hayashi, T.; Inoue, K.; Taniguchi, N.; Ogasawara,
M. J. Am. Chem. Soc. 2001, 123, 9918; indeed, the H
integration of the aromatic ring is lower than that of 3aa
from the reaction of 1a with PhB(OH)2 using (CH2O)n.
(12) Even when C6F5CHO, which should not generate a RhH
species, was used as a carbonyl source, the reaction gave
Acknowledgment
This research was supported by a Grant-in-Aid for Scientific Re-
search on Innovative Areas ‘Molecular Activation Directed toward
Straightforward Synthesis’ from the Ministry of Education, Cultu-
re, Sports, Science and Technology, Japan. We also thank Ms. Yuri-
ko Nishiyama, Ms. Yoshiko Nishikawa, and Mr. Shouhei Katao for
assistance in obtaining HRMS and X-ray crystallographic data.
Synlett 2014, 25, 1155–1159
© Georg Thieme Verlag Stuttgart · New York

LETTER
unselectively a mixture of (E)- and (Z)-3aa in 2% yield (E/Z
Carbonylative Arylation of Alkynes
1159
present conditions. Moreover, the reactions of (E)- and (Z)-
3aa in the presence of a catalytic amount of
= 37:63). We have observed the similar result when carbon
monoxide was used as a carbonyl source (ref. 7).
RhH(CO)(PPh3)3 gave a E/Z mixture of 3aa quantitatively in
a similar ratio, E/Z = 36:64 and 34:66, respectively.
Therefore, we currently do not eliminate the possibility that
E/Z isomerization proceeds via the hydrorhodation of RhH,
generated from Rh and hydrogen (decarbonylation
counterpart), to the primary product (E)-enone and the
subsequent rotation of the C–C single bond, followed by the
β-H elimination to (Z)-enone. See the Supporting
Information.
(13) Each of the (E)- and (Z)-3aa isolated in pure form was
introduced into reactions of 4-methoxyphenylboronic acid
(2b) and alkyne 1a under the standard conditions. Each
reaction gave the product 3ab in almost the same yield. It is
noteworthy that 3aa was recovered in the same E/Z ratio
(E/Z = 32:68) in each case and that they are also the same as
that of the reaction in Table 1, entry 2. Thus, E/Z
isomerization of the products 3 takes place readily under the
© Georg Thieme Verlag Stuttgart · New York
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