Rhodium-catalyzed linear codimerization and cycloaddition of ketenes with alkynes

Article abstract of DOI:10.3390/molecules15064189

A novel rhodium-catalyzed linear codimerization of alkyl phenyl ketenes with internal alkynes to dienones and a novel synthesis of furans by an unusual cycloaddition of diaryl ketenes with internal alkynes have been developed. These reactions proceed smoothly with the same rhodium catalyst, RhCl(PPh ₃)₃, and are highly dependent on the structure and reactivity of the starting ketenes.

Full text of DOI:10.3390/molecules15064189

Molecules 2010, 15, 4189-4200; doi:10.3390/molecules15064189
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Rhodium-Catalyzed Linear Codimerization and Cycloaddition
of Ketenes with Alkynes
Teruyuki Kondo 1,2,*, Masatsugu Niimi 2, Yuki Yoshida 2, Kenji Wada 2, Take-aki Mitsudo 2,
Yu Kimura 1 and Akio Toshimitsu 3
1
Advanced Biomedical Engineering Research Unit, Kyoto University, Katsura, Nishikyo-ku, Kyoto
615-8510, Japan; E-Mail: ykimura@scl.kyoto-u.ac.jp (Y.K.)
2
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto
University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan;
E-Mails: Masatsugu_Niimi@jsr.co.jp (M.N.); yoshida.y@ky3.ecs.kyoto-u.ac.jp (Y.Y.);
wadaken@scl.kyoto-u.ac.jp (K.W.); mitsudo@tech.email.ne.jp (T.M.)
3
Office of Society-Academia Collaboration for Innovation, Kyoto University, Yoshida-Honmachi,
Sakyo-ku, Kyoto 606-8501, Japan; E-Mail: akiot@saci.kyoto-u.ac.jp (A.T.)
* Author to whom correspondence should be addressed; E-Mail: teruyuki@scl.kyoto-u.ac.jp;
Tel.: +81-75-383-7055; Fax: +81-75-383-2805.
Received: 11 May 2010; in revised form: 25 May 2010 / Accepted: 7 June 2010 /
Published: 9 June 2010
Abstract: A novel rhodium-catalyzed linear codimerization of alkyl phenyl ketenes with
internal alkynes to dienones and a novel synthesis of furans by an unusual cycloaddition of
diaryl ketenes with internal alkynes have been developed. These reactions proceed
smoothly with the same rhodium catalyst, RhCl(PPh3)3, and are highly dependent on the
structure and reactivity of the starting ketenes.
Keywords: rhodium; catalyst; ketene; alkyne; codimerization; cycloaddition
1. Introduction
Ketenes are very important intermediates in the field of organic synthesis [1-3], and much attention
has been focused on the ketene-metal complexes [4]. In general, ketenes coordinate to transition-metal
complexes in two ways: 1) coordination through a C=C bond in ketenes [5], and 2) coordination

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through a C=O bond in ketenes [6-10]. If these coordination modes can be controlled through the
selection of ketenes in transition-metal catalysis, completely different methods for the construction of
novel organic molecules could be developed according to the structure and reactivity of ketenes using
the same transition-metal catalyst.
We have previously developed a ruthenium-catalyzed synthesis of pyranopyrandiones by the ring-
opening carbonylation of cyclopropenones [11] and a novel synthesis of 2-pyranones by the
ruthenium- or rhodium catalyzed ring-opening dimerization of cyclobutenones [12], as well as a
rhodium-catalyzed synthesis of 2-substituted phenols from cyclobutenones and alkenes via cleavage of
a carbon-carbon bond [13]. We propose that (η4-bisketene)- and (η4-vinylketene)metal complexes are
important key intermediates in these reactions; however, there are still few examples of transition-
metal complex-catalyzed transformations of ketenes themselves [14-20]. Thus, we focused our
attention on the development of novel reactions of ketenes with unsaturated compounds in the
presence of ruthenium or rhodium catalysts, and recently developed rhodium-catalyzed
decarbonylative coupling reactions of diphenyl ketene with 2-norbornenes and electron-deficient
alkenes [21]. Then, the reactions of ketenes with alkynes were investigated in the presence of several
transition-metal catalysts. After many trials, we developed the novel RhCl(PPh3)3-catalyzed linear
codimerization of alkyl phenyl ketenes with internal alkynes and a novel synthesis of furans by the
unusual RhCl(PPh3)3-catalyzed cycloaddition of diaryl ketenes with internal alkynes. In these reactions,
the catalyst is the same but the products are completely different, depending on the structure and
reactivity of the starting ketenes.
2. Results and Discussion
Treatment of alkyl phenyl ketenes 1a-c with internal alkynes 2 in the presence of 5 mol %
RhCl(PPh3)3 in mesitylene at 120 ºC for 12 h under an argon atmosphere gave the corresponding
dienones 3 in high yield with high stereoselectivity (Scheme 1). For example, RhCl(PPh3)3-catalyzed
reaction of ethyl phenyl ketene (1a) with 3-hexyne (2a) gave only (2Z, 5E)-5-ethyl-3-phenylocta-2,5-
dien-4-one (3a) in 92% yield, and no stereoisomers were obtained at all.
Scheme 1. Rhodium-catalyzed linear codimerization of ketenes with alkynes to dienones.
First, the catalytic activities of several transition-metal complexes were examined in the reaction of
1a with 2a, and the results are summarized in Table 1. Among the catalysts examined, RhCl(PPh3)3
.
(3a, 92%) showed the highest catalytic activity. RhCl(CO)(PPh3)2 (3a, 46%) and RhCl3 3H2O (3a,
26%) also showed moderate catalytic activity; however, other rhodium complexes, such as
RhH(PPh3)4 and RhH(CO)(PPh3)3, as well as ruthenium complexes, such as RuCl2(PPh3)3,
[RuCl2(CO)3]2, and RuH2(PPh3)4, and an iridium complex, IrCl(CO)(PPh3)2, were totally ineffective,

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whereas Pd(PPh3)4 showed slight catalytic activity (3a, 29%). An attempt to reduce the amount of
RhCl(PPh3)3 catalyst from 5.0 mol % to 2.0 mol % resulted in vain (Entry 2).
Table 1. Catalytic activity of several transition-metal complexes in the reaction of 1a with 2a to 3a.
Et
O
O
Catalyst (0.050 mmol)
+
Et
Mesitylene (1.0 mL)
120 °C, 12 h
Ph
Ph Et
Et
1a
1.0 mmol
2a
3.0 mmol
3a
Entry
Catalyst
Yield of 3a (%) a Entry
Catalyst
Yield of 3a (%) a
1
RhCl(PPh3)3
92
7
RuCl2(PPh3)3
1
2 b
RhCl(PPh3)3
5
8 c
[RuCl2(CO)3]2
RuH2(PPh3)4
0
0
3
RhCl(CO)(PPh3)2
46
9
4
5
6
RhCl3·3H2O
RhH(PPh3)4
26
2
10
11
Pd(PPh3)4
29
0
IrCl(CO)(PPh3)2
RhH(CO)(PPh3)3
0
a GLC yield; b RhCl(PPh3)3 (2.0 mol %, 0.020 mmol) for 40 h; c [RuCl2(CO)3]2 (0.025 mmol).
Table 2. RhCl(PPh3)3-catalyzed linear codimerization of alkyl phenyl ketenes 1a-c with
internal alkynes 2a,b to dienones 3a-d.a
Entry
Ketene
Alkyne
Product
Isolated Yield (%)
O
1
74 (92) b
Et
2a
1a
Ph Et
O
3a
3b
nC5H11
nC5H11
2
3
1a
68
52
nC5H11
nC5H11
2b
2a
Ph
O
O
Et
Et
Ph
1b
1c
Ph Et
O
3c
4
2a
40
Ph Et
3d
a
Ketene (1.0 mmol), alkyne (3.0 mmol), RhCl(PPh3)3 (0.050 mmol), and mesitylene
(1.0 mL) at 120 ºC for 12 h under an argon atmosphere; b GLC yield.
The results obtained in the reaction of several alkyl phenyl ketenes 1a-c with alkynes 2a and b under
the optimized reaction conditions are summarized in Table 2. Ethyl phenyl ketene (1a) reacted with

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6-dodecyne (2b) to give the corresponding dienone 3b in an isolated yield of 68% (Entry 2). As for
ketenes, cycloalkyl phenyl ketenes, such as 1b and 1c, also reacted with 2a to give the corresponding
dienones, 3c and 3d, in isolated yields of 52% and 40%, respectively (Entries 3 and 4). Unfortunately,
when terminal alkynes, such as phenylacetylene and 1-hexyne, were used in RhCl(PPh3)3-catalyzed
reaction with ethyl phenyl ketene (1a), the corresponding dienones were obtained in low yield (below
10%) together with various byproducts, probably due to the formation of a (vinylidene)Rh species.
In sharp contrast, treatment of diaryl ketenes 1d and e instead of alkyl phenyl ketenes 1a-c with
internal alkynes 2 in the presence of the same RhCl(PPh3)3 catalyst (5 mol %) in mesitylene at 120 ºC
for 12 h under an argon atmosphere gave unusual cycloadducts, the furans 4, instead of dienones 3, in
good to high yields (Scheme 2). The structure of furan 4a was confirmed by 13C Inadequate NMR
measurement (see Experimental, Figure 2).
Scheme 2. Rhodium-catalyzed cycloaddition of ketenes with alkynes to furans.
The catalytic activities of several transition-metal complexes were also examined in the reaction of
diphenyl ketene (1d) with 3-hexyne (2a), and the results are summarized in Table 3. Among the
catalysts examined, only RhCl(PPh3)3 showed catalytic activity (4a, 74%). Other rhodium, ruthenium,
iridium and palladium complexes were totally ineffective in the present reaction.
Table 3. Catalytic activity of several trasition-metal complexes in the reaction of 1d with 2a to 4a.
Entry
Catalyst
RhCl(PPh3)3
Yield of 4a (%) a
Entry
Catalyst
Yield of 4a (%) a
1
2
74
2
7
8
[Cp*RuCl2]2
RuCl2(PPh3)3
0
0
RhCl(CO)(PPh3)2
3
4
RhCl3·3H2O
[RhCl(cod)]2
0
0
9
[RuCl2(CO)3]2
0
0
10
IrCl(CO)(PPh3)2
5
6
[RhCl(C2H4)2]2
RhH(PPh3)4
0
0
11
Pd(PPh3)4
0
a GLC yield.

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6-Dodecyne (2b), as well as 4-octyne (2c) and 5-decyne (2d), reacted with 1d to give the
corresponding furans 4b-d in moderate yields (Entries 2-4 in Table 4). As for ketenes, di(4-chloro-
phenyl) ketene (1e) also reacted with 2a to give the corresponding furan 4e in an isolated yield of
51% (Entry 5).
Table 4. RhCl(PPh3)3-catalyzed unusual cycloaddition of diaryl ketenes 1d,e with internal
alkynes 2a-d to furans 4a-e.a
Entry
Ketene
Alkyne
Product
Isolated Yield (%)
1
64 (74) b
2a
1d
4a
nC5H11
nC5H11
nC5H11
2
1d
43
52
Ph
Ph
Ph
nBu
2b
2c
O
O
Ph
4b
4c
nPr
nPr
nPr
3
4
1d
1d
Et
Ph
nBu
nBu
nBu
43
2d
2a
nPr
O
Ph
4d
Et
51 (70)b
O
5
4-ClC6H4
4-ClC6H4
4-ClC6H4
4-ClC6H4
O
1e
4e
a
Ketene (1.0 mmol), alkyne (3.0 mmol), RhCl(PPh3)3 (0.050 mmol), and mesitylene (1.0 mL) at
120 ºC for 12 h under an argon atmosphere; b GLC yield.
While the reaction mechanism is not yet clear, the possible mechanisms are illustrated in Schemes 3
and 4. Scheme 3 shows a possible mechanism of the reaction of alkyl phenyl ketenes 1a-c with internal
alkynes 2 to give dienones 3. We now believe that the initial step is the coordination of alkyl phenyl
ketenes 1 to an active rhodium center through a C=C bond in ketenes. Oxidative cyclization of alkyl
phenyl ketenes 1a-c with alkynes 2 would give rhodacyclopentenone intermediates I [5].
Stereoselective β-hydrogen elimination, followed by reductive elimination, would give the
corresponding dienones 3 stereoselectively. In addition, we now consider that a catalytically active
.
species is a Rh(I) bearing a chloro ligand, and RhCl3 3H2O would be reduced to a Rh(I)-Cl species by
crystal water to show some catalytic activity (Entry 4 in Table 1).

Molecules 2010, 15
Scheme 3. A possible mechanism of linear codimerization of alkyl phenyl ketenes 1a-c
4194
with internal alkynes 2 to give dienones 3.
Scheme 4. A possible mechanism of the synthesis of furans 4 by unusual cycloaddition of
diaryl ketenes 1d and e with internal alkynes 2.
On the other hand, a possible mechanism for the reaction of diaryl ketenes 1d and e with internal
alkynes 2 to furans 4 is shown in Scheme 4. In the synthesis of furans 4, the reaction starts from the
coordination of diaryl ketenes to an active rhodium center through a C=O bond in ketenes (not through
a C=C bond in ketenes). Oxidative cyclization of diaryl ketenes 1d, and e with alkynes 2 gives an
oxametallacycle intermediate II [22-25]. β-Hydrogen elimination and insertion of an allenyl group in

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an intermediate III into a Rh-H bond, followed by reductive elimination/isomerization, would give the
desired furans 4.
3. Experimental
3.1. General
GLC analyses were carried out on a Shimadzu GC-18A gas chromatograph equipped with a glass
column (2.8 mm i.d. × 3 m) packed with Silicone OV-17 (2% on Chromosorb W(AW-DMCS),
60–80 mesh). Recycling preparative HPLC was performed with an LC-918 (Japan Analytical Industry
1
Co. Ltd.) equipped with JAIGEL-1H and 2H columns (GPC) using CHCl3 as an eluent. H-NMR
spectra were recorded at 300 or 400 MHz, and 13C-NMR spectra were recorded at 75 or 100 MHz.
Samples were analyzed in CDCl3, and the chemical shift values are expressed relative to Me4Si as an
internal standard. IR spectra were obtained on a Nicolet Impact 410 spectrometer. Elemental analyses
were performed at the Microanalytical Center of Kyoto University.
3.2. Materials
Ketenes were synthesized as described in the literature [26,27]. Alkynes were obtained
commercially and purified before use by standard procedures. RhCl3.·3H2O, [RhCl(cod)]2, [Cp*RhCl2]2,
[RuCl2(CO)3]2, IrCl(CO)(PPh3)2, and Pd(PPh3)4 were obtained commercially and used without further
purification. RhCl(PPh3)3 [28], RhCl(CO)(PPh3)2 [29], RhH(PPh3)4 [30], RhH(CO)(PPh3)3 [31],
[RhCl(C2H4)2]2 [32], RuCl2(PPh3)3 [33], and RuH2(PPh3)4 [34] were prepared as described in the
literature.
3.3. General procedure for the rhodium-catalyzed reaction of ketenes with alkynes to give dienones
and furans
A mixture of ketene 1 (1.0 mmol), alkyne 2 (3.0 mmol), RhCl(PPh3)3 (0.050 mmol), and mesitylene
(1.0 mL) was placed in a two-necked 20-mL Pyrex flask equipped with a magnetic stirring bar and a
reflux condenser under a flow of argon. The reaction was carried out at 120 ºC for 12 h with stirring.
After the reaction mixture was cooled, the products were analyzed by GLC and isolated by Kugelrohr
distillation followed by recycling preparative HPLC.
(2Z,5E)-5-Ethyl-3-phenylocta-2,5-dien-4-one (3a). Yellow liquid; b.p. 130 ºC (3.0 mmHg, Kugelrohr);
1
IR (cm-1) 1652 (CO); H-NMR (CDCl3, 300 MHz): δ 0.98 (t, 3H, J = 7.52 Hz, C8-H), 1.03 (t, 3H,
J = 7.52 Hz, 5-CH2CH3), 1.72 (d, 3H, J = 7.16 Hz, C1-H), 2.24 (dq, 2H, J = 7.52 Hz, C7-H), 2.42 (q,
2H, J =7.52 Hz, 5-CH2CH3), 6.15 (q, 1H, J = 7.16 Hz, C2-H), 6.64 (t, 1H, J = 7.52 Hz, C6-H), 7.19–
7.37 (m, 5H, 3-phenyl-H); 13C-NMR (CDCl3, 75 MHz): δ 13.09 (5-CH2CH3), 13.79 (C8), 15.33 (C1),
18.12 (5-CH2CH3), 22.22 (C7), 125.06 (C2), 125.55 (3-phenyl), 127.25 (3-phenyl), 128.48 (3-phenyl),
129.19 (C3 or C5), 142.85 (3-phenyl-C1), 145.41 (C3 or C5), 150.28 (C6), 200.61 (C4); MS (EI) m/z
228 (M+). Anal. Calcd. for C16H20O: C 84.36, H 9.00. Found: C 84.14, H 8.83. A nuclear Overhauser
enhancement (NOE) study was undertaken to determine the stereochemistry of dienone 3a. Irradiation

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of olefinic CH at δ 6.15 ppm gave a 6.7% NOE of the phenyl group at δ 7.26–7.28 ppm, while
irradiation of CH2 in ethyl group at δ 2.24 ppm showed 5.2% NOE with CH3 in the other ethyl group
at δ 1.03 ppm. The stereochemistry of 3a was therefore assigned to 2Z and 5E (Figure 1). The same
method was used to determine the stereochemistry of 3b-d.
Figure 1. The NOE study of 3a.
O
H
5.2%
Ph
6.7%
(2Z,5E)-5-Pentyl-3-phenylundeca-2,5-dien-4-one (3b). Colorless liquid; b.p. 140 ºC (1.0 mmHg,
Kugelrohr); IR (cm-1): 1652 (CO) ; 1H-NMR (CDCl3, 400 MHz): δ = 0.84 (t, 3H, C11-H), 0.89 (t, 3H,
5-CH2(CH2)3CH3), 1.17–1.35(m, 12H, 5-CH2(CH2)3CH3/C8-H/C9-H/C10-H), 1.72 (d, 3H, J = 7.16 Hz,
C1-H), 2.21 (dt, 2H, C7-H), 2.37 (t, 2H, 5-CH2(CH2)3CH3), 6.14 (q, 1H, J = 7.16 Hz, C2-H), 6.66 (t,
1H, C6-H), 7.18-7.31 (m, 5H, 3-phenyl-H); 13C-NMR (CDCl3, 75 MHz): δ 13.93, 14.04, 15.38 (C1),
22.40, 22.51, 24.97, 28.09, 28.61, 29.03, 31.89, 44.87, 125.09 (C2), 125.63 (3-phenyl), 127.14 (3-
phenyl), 128.61(3-phenyl), 134.58 (C3 or C5), 141.94 (3-phenyl-1), 144.80 (C3 or C5), 149.70 (C6),
200.62 (C4); MS (EI) m/z 312 (M+).
(3E)-1-Cyclopentylidene-3-ethyl-1-phenylhex-3-en-2-one (3c). Yellow liquid; b.p. 140 oC (2.0 mmHg,
Kugelrohr); IR (cm-1) 1650 (CO); 1H-NMR (CDCl3, 400 MHz): δ 0.96 (t, 3H, J =7.52 Hz, C6-H), 0.98
(t, 3H, J = 7.52 Hz, 3-CH2CH3), 1.61-1.71 (m, 4H, 1-cyclopentylidene-H), 2.21 (q, 2H, J = 7.52 Hz, 1-
cyclopentylidene-H), 2.30–2.46 (m, 6H, 1-cyclopentylidene-H/C5-H/3-CH2CH3), 6.63 (t, 1H,
J = 7.52 Hz, C4-H), 7.17-7.33 (m, 5H, 1-phenyl-H); 13C-NMR (CDCl3, 100 MHz): δ 13.31 (3-
CH2CH3), 13.77 (C6), 18.51 (3-CH2CH3), 22.19 (C5), 26.34 (1-cyclopentylidene), 26.38 (1-
cyclopentylidene), 32.18 (1-cyclopentylidene), 32.39 (1-cyclopentylidene), 126.50 (1-phenyl), 127.82
(1-phenyl), 128.06 (1-phenyl), 132.91 (1-cyclopentylidene), 138.53 (C1 or C3), 142.08 (1-phenyl-1),
146.26 (C1 or C3), 148.48 (C4), 200.16(C2); MS (EI) m/z 268 (M+).
o
(3E)-1-Cyclohexylidene-3-ethyl-1-phenylhex-3-en-2-one (3d). Yellow liquid; b.p. 160 C (8.0 mmHg,
1
Kugelrohr); IR (cm-1) 1641 (CO); H-NMR (CDCl3, 400 MHz): δ 0.91 (t, 3H, C6-H), 1.04 (t, 3H, 3-
CH2CH3), 1.60 (br, 6H, 1-cyclohexylidene-H), 2.13 (br, 4H, 1-cyclohexylidene-H), 2.23 (br, 2H, C5-
H), 2.31 (q, 2H, J = 7.32 Hz, 3-CH2CH3), 6.78 (t, 1H, J = 7.32 Hz, C4-H), 7.21–7.31 (m, 5H, 1-
13
phenyl-H); C-NMR (CDCl3, 100 MHz): δ 13.62 (3-CH2CH3), 13.79 (C6), 18.46 (3-CH2CH3), 22.35
(C5), 26.54 (1-cyclohexylidene), 27.96 (1-cyclohexylidene), 28.36 (1-cyclohexylidene), 30.90 (1-
cyclohexylidene), 32.88 (1-cyclohexylidene), 126.61 (1-phenyl), 128.77 (1-phenyl), 129.03 (1-phenyl),
133.32 (1-cyclohexylidene), 137.20 (C1 or C3), 140.50 (C1 or C3), 142.81 (1-phenyl-1), 148.96 (C4),
200.32(C2); MS (EI) m/z 282 (M+).

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2-(Diphenylmethyl)-3-ethyl-5-methylfuran (4a). Yellow liquid; b.p. 135–145 ºC (0.1 mmHg,
Kugelrohr); 1H-NMR (CDCl3, 400 MHz): δ 1.05 (t, 3H, J = 7.57 Hz, 3-CH2CH3), 2.20 (s, 3H, 5-CH3),
2.29 (q, 2H, J = 7.49 Hz, 3-CH2CH3), 5.40 (s, 1H, 2-CHPh2), 5.85 (s, 1H, C4-H), 7.18–7.29 (m, 10H,
phenyl-H); 13C-NMR (CDCl3, 100 MHz): δ 13.85 (5-CH3), 15.31 (3-CH2CH3), 18.24 (3-CH2CH3),
48.43 (2-CHPh2), 107.14 (C4), 123.09 (C3), 126.16 (phenyl), 128.24 (phenyl), 128.73 (phenyl), 142.30
(phenyl-1), 147.46 (C2), 150.22 (C5); MS (EI) m/z 276 (M+). Anal. Calcd for C20H20O: C 86.92, H
7.29. Found: C 87.02, H 7.29. The relationship of the substituted group is confirmed by 13C Inadequate
NMR measurement (Figure 2).
2-(Diphenylmethyl)-5-n-butyl-3-n-pentylfuran (4b). Yellow liquid; b.p. 170 ºC (0.1 mmHg, Kugelrohr);
1H-NMR (CDCl3, 400 MHz): δ 0.82 (t, 3H, J = 6.84 Hz), 0.89 (t, 3H, J = 7.32 Hz), 1.22 (m, 6H, 3-
(CH2)2(CH2)2CH3/5-(CH2)2CH2CH3), 1.43 (m, 2H, 3-CH2CH2(CH2)2CH3), 1.56 (m, 2H, 5-
CH2CH2CH2CH3), 2.27 (dt, 2H, J = 2.12 Hz, 7.69 Hz, 3-CH2(CH2)3CH3), 2.53 (t, 2H, J = 7.33 Hz, 5-
CH2(CH2)2CH3), 5.37 (s, 1H, 2-CHPh2), 5.82 (s, 1H, C4-H), 7.18–7.29 (m, 10H, phenyl-H); 13C-NMR
(CDCl3, 100 MHz): δ 13.99, 14.16, 22.36, 22.60, 24.97 (3-CH2(CH2)3CH3), 27.86 (5-CH2(CH2)2CH3),
30.20 (5-CH2CH2CH2CH3), 30.34 (3-CH2CH2(CH2)2CH3), 31.04, 48.38 (2-CHPh2), 106.54 (C4),
121.41 (C3), 126.09 (phenyl), 128.01 (phenyl), 128.73 (phenyl), 142.48 (phenyl-1), 147.54 (C2),
154.63 (C5); MS (EI) m/z 360 (M+).
Figure 2. 13C Inadequate NMR spectrum of 4a.
2-(Diphenylmethyl)-5-ethyl-3-n-propylfuran (4c). Yellow liquid; b.p. 140–150 ºC (0.1 mmHg,
1
Kugelrohr); H-NMR (CDCl3, 400 MHz): δ 0.85 (t, 3H, J = 7.33 Hz, 3-CH2CH2CH3), 1.17 (t, 3H,
J = 7.33 Hz, 5-CH2CH3), 1.47 (sixtet, 2H, J = 7.42 Hz, 3-CH2CH2CH3), 2.26 (t, 2H, J = 7.57 Hz, 3-
CH2CH2CH3), 2.56 (q, 2H, J = 7.53 Hz, 5-CH2CH3), 5.38 (s, 1H, 2-CHPh2), 5.83 (s, 1H, C4-H),
7.10–7.29 (m, 10H, phenyl-H); 13C-NMR (CDCl3, 100 MHz): δ 12.14 (5-CH2CH3), 14.06 (3-
CH2CH2CH3), 21.46 (5-CH2CH3), 23.79 (3-CH2CH2CH3), 27.08 (3-CH2CH2CH3), 48.36 (2-CHPh2),

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105.86 (C4), 121.32 (C3), 126.18 (phenyl), 128.11 (phenyl), 128.82 (phenyl), 142.56 (phenyl-1),
147.81 (C2), 156.00 (C5); MS (EI) m/z 304 (M+).
2-(Diphenylmethyl)-3-n-butyl-5-n-propylfuran (4d). Yellow liquid; b.p. 150–160 ºC (0.1 mmHg,
1
Kugelrohr); H-NMR (CDCl3, 300 MHz): δ 0.84 (t, 3H, J = 7.25 Hz, 3-CH2(CH2)2CH3), 0.91 (t, 3H,
J = 7.52 Hz, 5-(CH2)2CH3), 1.24 (m, 4H, 3-CH2(CH2)2CH3), 1.59 (sixtet, 2H, J = 7.38 Hz, 5-
CH2CH2CH3), 2.27 (t, 2H, J = 7.52 Hz, 3-CH2(CH2)2CH3), 2.50 (t, 2H, J = 7.43 Hz, 5-CH2CH2CH3),
5.37 (s, 1H, 2-CHPh2), 5.83 (s, 1H, C4-H), 7.17-7.27 (m, 10H, phenyl-H); 13C-NMR (CDCl3,
75 MHz): δ 13.71, 13.89, 21.39 (5-CH2CH2CH3), 22.41, 24.58 (3-CH2(CH2)2CH3), 30.10 (5-
CH2CH2CH3), 32.72, 48.36 (2-CHPh2), 106.79 (C4), 121.52 (C3), 126.23 (phenyl), 128.16 (phenyl),
128.88 (phenyl), 142.67 (phenyl-1), 147.77 (C2), 154.67 (C5); MS (EI) m/z 332 (M+). Anal. Calcd for
C24H26O: C 86.70, H 8.43. Found: C 86.43, H 8.67.
2-[Bis(4-chloropheny)methyl]-3-ethyl-5-methylfuran (4e). Yellow liquid; b.p. 150 ºC (3.0 mmHg,
Kugelrohr); 1H-NMR (CDCl3, 300 MHz): δ 1.05 (t, 3H, J = 7.57 Hz, 3-CH2CH3), 2.20 (s, 3H, 5-CH3),
2.28 (q, 2H, J = 7.49 Hz, 3-CH2CH3), 5.31 (s, 1H, 2-CH(p-ClPh)2), 5.86 (s, 1H, C4-H), 7.06–7.31 (m,
13
8H, phenyl-H); C-NMR (CDCl3, 75 MHz): δ 13.63 (5-CH3), 15.10 (3-CH2CH3), 18.06 (3-CH2CH3),
47.08 (2-CH(p-ClPh)2), 107.35 (C4), 128.47 (C3), 130.13 (phenyl), 132.03 (phenyl), 132.38 (phenyl),
140.56 (phenyl-1), 146.54 (C2), 150.87 (C5); MS (EI) m/z 344 (M+).
4. Conclusions
In conclusion, we have developed a novel rhodium-catalyzed cross-coupling reaction of ketenes
with alkynes. The different coordination modes of ketenes to rhodium, which highly depend on the
structure and reactivity of the starting ketenes, realized the selective formation of totally different
products, dienones and furans in the presence of the same rhodium catalyst, RhCl(PPh3)3. Both
reactions proceed via characteristic rhodacyclic intermediates.
Acknowledgements
This work was supported in part by a Grant-in-Aid for Scientific Research (B) (No. 20350048) from
the Ministry of Education, Culture, Sports, Science and Technology, Japan. T.K. acknowledges
financial support from Yazaki Memorial Foundation for Science and Technology. This research was
conducted in part at the Advanced Research Institute (the Research Project of Engineering for
Sustainable Environment), Katsura-Int’tech Center, Graduate School of Engineering, Kyoto University.
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Sample Availability: Samples of the compounds 3a-d and 4a-e are available from the authors.
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