Rhenium- and gold-catalyzed coupling of aromatic aldehydes with trimethyl(phenylethynyl)silane: Synthesis of diethynylmethanes

Article abstract of DOI:10.1002/anie.200700183

(Chemical Equation Presented) Very couply! Coupling reactions of propargyl or benzyl alcohols with allyl- or alkynylsilanes proceed efficiently using [{ReBr(CO)₃(thf)}₂] as catalyst. Additionally, diethynylmethane derivatives were obtained by the reaction of aromatic aldehydes with trimethyl(phenylethynyl)silane in the presence of both the rhenium complex and AuCl (see scheme).

Full text of DOI:10.1002/anie.200700183

Communications
DOI: 10.1002/anie.200700183
Cross-Coupling
Rhenium- and Gold-Catalyzed Coupling of Aromatic Aldehydes with
Trimethyl(phenylethynyl)silane: Synthesis of Diethynylmethanes**
Yoichiro Kuninobu,* Eri Ishii, and Kazuhiko Takai*
Table 1: Reactions of propargyl alcohols with silanes.[a]
Recently, synthetic reactions using a combination of two
metal complexes have received considerable attention
because they lead to products that cannot be synthesized
with only one of the catalysts.[1] We report herein on the
rhenium-[2,3] and gold-catalyzed[4] synthesis of diethynylme-
thanes[5,6] by reactions of aldehydes with trimethyl(phenyl-
ethynyl)silane.
Entry
1
Alcohol
Silane
Product
Yield [%][b]
2a
99 (>99)
As a preliminary investigation, reactions of propargyl or
benzyl alcohols with allyl- or alkynylsilanes were carried out
(Table 1).[7–9] Reaction of propargyl alcohol 1a with allyltri-
methylsilane (2a) in the presence of a rhenium catalyst,
[{ReBr(CO)3(thf)}2] (2.5 mol%), at 258C afforded enyne 3a
in quantitative yield (Table 1, entry 1). A propargyl alcohol
with an alkyl group at the terminal position of the acetylene
moiety, 1b, and another with dimethyl groups at the propargyl
position, 1c, also gave the corresponding enynes 3b and 3c
quantitatively (Table 1, entries 2and 3). Enyne 3d was
obtained in 69% yield by using a secondary propargyl alcohol
1d (Table 1, entry 4), a result that deserves special mention
because efficient transformations of secondary propargyl
alcohols bearing an alkyl group at the propargyl position with
organosilanes are usually difficult.[2b,7c] The reaction between
benzyl alcohol 4 and allylsilane 2a gave 4-phenylpent-1-ene
(5) in 95% yield (Table 1, entry 5). Reaction of propargyl
alcohols 1a or 1b with trimethyl(phenylethynyl)silane (2b)
afforded diethynylmethanes 6a and 6b in 76% and 67%
yields, respectively (Table 1, entries 6 and 7). However, the
reactions of propargyl alcohols 1c or 1d with 2b did not give
the corresponding coupling products, but 1,3-diphenylbut-1-
yne (7) was afforded in 56% yield by the treatment of benzyl
alcohol 4 with 2b (Table 1, entry 8).
2
2a
99 (>99)
3
2a
2a
2a
96 (99)
69 (72)
95 (98)
4[c]
5[d]
6
1a
2b
76 (79)
7[e]
1b
4
2b
2b
67 (73)
56 (57)
8[f]
As shown in entries 6 and 7 (Table 1), diethynylmethanes
(one kind of polyethynylmethane) were formed. Such poly-
[a] Silane (2.0 equiv). [b] Isolated yields (yields from 1H NMR analysis are
reported in parentheses). [c] 1,2-Dichloroethane was used as solvent,
808C, 24 h, 2a (4.0 equiv). [d] 1,2-Dichloromethane was used as solvent,
508C, 24 h. [e] 1,2-Dichloroethane was used as solvent, 808C, 24 h.
[f] 1,1,2-Trichloroethane was used as solvent, 1158C, 24 h, 2a
(4.0 equiv).
[*] Dr. Y. Kuninobu, E. Ishii, Prof. Dr. K. Takai
Division of Chemistry and Biochemistry
Okayama University
Tsushima, Okayama 700-8530 (Japan)
Fax: (+81)86-251-8094
E-mail: kuninobu@cc.okayama-u.ac.jp
ktakai@cc.okayama-u.ac.jp
ethynylmethanes are interesting compounds as high-carbon-
containing materials,[10] and several synthetic methods have
been reported. To construct the skeleton in one pot, we used
stable alkynylsilanes[11] as the alkynyl component.
Treatment of benzaldehyde (8a) with trimethyl(phenyl-
ethynyl)silane (2b) in the presence of a catalytic amount of
the rhenium complex, [{ReBr(CO)3(thf)}2] (2.5 mol%),
produced only a trace amount of the desired diethynyl-
methane 6a. However, when a gold salt, AuCl (5.0 mol%),
was added to the reaction mixture, the yield of 6a increased
[**] Financial support through a Grant-in-Aid for Scientific Research on
Priority Areas (no. 18037049, “Advanced Molecular Transformations
of Carbon Resources”) from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan, and a Grant-in-Aid for
Young Scientists (B) (no. 18750088) from the Japan Society for the
Promotion of Science is gratefully acknowledged. We thank Dr.
Masaichi Saito (Department of Chemistry, Saitama University) for
acquiring HR-MS spectra.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Angew. Chem. Int. Ed. 2007, 46, 3296 –3299

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Chemie
dramatically (72% yield). Because the reac-
tion did not occur in the presence of AuCl
only, we conclude that this combination of
catalysts is important to promote the reac-
tion.[12]
The yield of 6a increased slightly with use
of twice the amounts of rhenium and gold
catalysts (Table 2, entry 1). The yields of
diethynylmethanes 6c and 6d decreased
when using aromatic aldehydes that bear an
electron-donating group (Table 2, entries 2
and 3).[13] In contrast, an aromatic aldehyde
with an electron-withdrawing group gave
diethynylmethane 6e and propargyl alcohol
Scheme 1. Effect of either or both catalysts on the reaction of 8h with 2b. Yields from
1H NMR analysis are provided.
Table 2: Reactions of aldehydes with 2b.[a]
We examined the reaction with each catalyst. By using
[{ReBr(CO)3(thf)}2] or AuCl as the sole catalyst, propargyl
alcohol 1d was obtained in low yields. The results suggest that
both the rhenium and gold complexes are necessary for
promoting the formation of propargyl alcohol efficiently.
Next, the reaction between propargyl alcohol 1a and 2b
was carried out with [{ReBr(CO)3(thf)}2] and/or AuCl
(Scheme 2). These results show that the gold salt does not
disturb the addition.
Substitution of a hydroxy group at benzylic positions with
allylsilanes has been reported to occur with a combined
catalyst of [(dppm)ReOCl3] (dppm: bis(diphenylphosphino)-
methane) and NH4PF6 (or AgSbF6) in nitromethane at 658C
by Toste and Luzung.[2b] Compared with the results reported
therein, this reaction proceeds with a rhenium complex of
lower oxidation state and under milder reaction conditions.
Reaction of optically active propargyl alcohol 1b’ with
trimethyl(phenylethynyl)silane (2b) in the presence of
[{ReBr(CO)3(thf)}2] led to formation of racemic diethynyl-
methane 6b. This result indicates clearly that the formation of
diethynylmethanes proceeds through a propargyl cation
(Scheme 3).
Entry
Aldehyde
T [8C]
Product
Yield [%][b]
1[c]
2[d]
3[c]
4
8a: R=H
25
50
50
25
6a
6c
6d
6e
80
43
60
33
8b: R=p-MeO
8c: R=p-Me
8d: R=p-CF3
66
5[e]
6[e]
8e: R=p-Br
8 f: R=o-Me
50
50
6 f
6g
66
79
7[c]
50
6h
55
[a] Aldehyde (1.0 equiv), acetylene (4.0 equiv). [b] Isolated yield.
[c] [{ReBr(CO)3(thf)}2] (5.0 mol%), AuCl (10 mol%). [d] 24 h.
[e] [{ReBr(CO)3(thf)}2] (2.5 mol%), AuCl (10 mol%).
1e in 33% and 66% yields, respectively
(Table 2, entry 4). A bromine atom at the para
position of the aldehyde remained intact during
the reaction (Table 2, entry 5).[14] Diethynylme-
thane 6g was obtained in 79% yield by using
ortho-tolualdehyde (8 f) with no sign of steric
hindrance (Table 2, entry 6). 2-Naphthalenecar-
boxaldehyde (8g) afforded the corresponding
diethynylmethane 6h in 55% yield (Table 2,
entry 7). However, aliphatic aldehydes (see
below) and trimethyl(oct-1-ynyl)silane[15] gave
the corresponding diethynylmethanes in low
yields.
Scheme 2. Effect of either or both catalysts on the reaction of 1a with 2b. Yields
from 1H NMR analysis are provided.
To obtain information of the reaction mech-
anism, we carried out several reactions. The
reaction between decanal (8h) and trimethyl-
(phenylethynyl)silane (2b) in the presence of
[{ReBr(CO)3(thf)}2] gave the corresponding
diethynylmethane 6j and propargyl alcohol 1d
in 7% and 90% yields, respectively (Scheme 1).
Scheme 3.
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Scheme 4. Proposed mechanism for the formation of diethynylmethanes.
13498 – 13499; b) Y. Kuninobu, A. Kawata, K. Takai, Org. Lett.
2005, 7, 4823 – 4825; c) Y. Kuninobu, Y. Tokunaga, A. Kawata,
K. Takai, J. Am. Chem. Soc. 2006, 128, 202 – 209; d) Y. Kuninobu,
Y. Nishina, M. Shouho, K. Takai, Angew. Chem. 2006, 118, 2832 –
2834; Angew. Chem. Int. Ed. 2006, 45, 2766 – 2768; e) Y.
Kuninobu, A. Kawata, K. Takai, J. Am. Chem. Soc. 2006, 128,
11368 – 11369; f) Y. Kuninobu, Y. Nishina, C. Nakagawa, K.
Takai, J. Am. Chem. Soc. 2006, 128, 12376 – 12377.
The proposed reaction mechanism is shown in Scheme 4:
1) A combination of [{ReBr(CO)3(thf)}2] and AuCl acts as an
effective Lewis acid for promoting ethynylation of aldehydes
with trimethyl(phenylethynyl)silane; 2) the rhenium complex
promotes substitution of a hydroxy group by a phenylethynyl
group. In the reaction step, a propargyl cation is formed as an
intermediate.
In summary, we have demonstrated the rhenium-cata-
lyzed transformations of propargyl and benzyl alcohols using
organosilanes, as well as the rhenium- and gold-catalyzed
synthesis of diethynylmethanes. To the best of our knowledge,
the results shown here are the first example of the formation
of diethynylmethanes by coupling reactions between alde-
hydes and alkynylsilanes. This reaction proceeds via prop-
argyl alcohols as intermediates. A combination of the
rhenium and gold complexes promotes the first ethynylation
step, and the rhenium complex accelerates the second
ethynylation.
[4] For recent reviews of gold-catalyzed transformations, see:
a) A. S. K. Hashmi, Angew. Chem. 2005, 117, 7150 – 7154;
Angew. Chem. Int. Ed. 2005, 44, 6990 – 6993; b) A. S. K.
Hashmi, G. J. Hutchings, Angew. Chem. 2006, 118, 8064 – 8105;
Angew. Chem. Int. Ed. 2006, 45, 7896 – 7936.
[5] For previous reports on gallium trichloride promoted diethyny-
lation, see: a) R. Amemiya, K. Suwa, J. Toriyama, Y. Nishimura,
M. Yamaguchi, J. Am. Chem. Soc. 2005, 127, 8252 – 8253; b) R.
Amemiya, M. Yamaguchi, Eur. J. Org. Chem. 2005, 5145 – 5150;
c) R. Amemiya, Y. Miyake, M. Yamaguchi, Tetrahedron Lett.
2006, 47, 1797 – 1800. However, these reactions need stoichio-
metric amounts of gallium chloride and heating at 130–1708C.
[6] For copper(I)-catalyzed synthesis of 1,4-diynes by the reactions
of propargyl halides with trimethylsilylacetylenes, see: F.
Montel, R. Beaudegnies, J. Kessabi, B. Martin, E. Muller, S.
Wendeborn, P. M. Jung, Org. Lett. 2006, 8, 1905 – 1908.
[7] Metal-catalyzed couplings of propargyl alcohols with organo-
silanes are still rare.[2b] See also: a) M. Georgy, V. Boucard, J.-M.
Campagne, J. Am. Chem. Soc. 2005, 127, 14180 – 14181; b) Y.
Nishibayashi, A. Shinoda, Y. Miyake, H. Matsuzawa, M. Sato,
Angew. Chem. 2006, 118, 4953 – 4957; Angew. Chem. Int. Ed.
2006, 45, 4835 – 4839; c) Z.-p. Zhan, J.-l. Yu, H.-j. Liu, Y.-y. Cui,
R.-f. Yang, W.-z. Yang, J.-p. Li, J. Org. Chem. 2006, 71, 8298 –
8301; d) Z.-p. Zhan, W.-z. Yang, R.-f. Yang, J.-l. Yu, J.-p. Li, H.-j.
Liu, Chem. Commun. 2006, 3352– 3354.
Experimental Section
General procedure for the reaction of a propargyl alcohol with
organosilane:
trimethyl(phenylethynyl)silane
A
mixture of
a
propargyl alcohol (0.250 mmol),
(87.2mg, 0.500 mmol), and
[{ReBr(CO)3(thf)}2] (5.3 mg, 6.1 mmol) in dichloromethane (1.0 mL)
was stirred for 3 h at 258C. Then, the solvent was removed in vacuo
and the products were isolated by silica gel column chromatography.
General procedure for the reaction of an aldehyde with
trimethyl(2-phenylethynyl)silane:
A
mixture of an aldehyde
(0.250 mmol), trimethyl(phenylethynyl)silane (174 mg, 1.00 mmol),
[{ReBr(CO)3(thf)}2] (5.3 mg, 6.1 mmol), and AuCl (2.9 mg, 13 mmol)
in dichloromethane (1.0 mL) was stirred for 3 h at 258C. Then, the
solvent was removed in vacuo and the products were isolated by silica
gel column chromatography.
[8] For metal-catalyzed couplings of benzyl alcohols with organo-
silanes, see: a) M. Braun, W. Kotter, Angew. Chem. 2004, 116,
520 – 523; Angew. Chem. Int. Ed. 2004, 43, 514 – 517; b) M.
Yasuda, T. Saito, M. Ueba, A. Baba, Angew. Chem. 2004, 116,
1438 – 1440; Angew. Chem. Int. Ed. 2004, 43, 1414 – 1416; c) T.
Saito, M. Yasuda, A. Baba, Synlett 2005, 1737 – 1739; d) T. Saito,
Y. Nishimoto, M. Yasuda, A. Baba, J. Org. Chem. 2006, 71,
8516 – 8522.
Received: January 15, 2007
Published online: March 23, 2007
[9] For ruthenium-catalyzed carbon–carbon bond formation using
propargyl alcohols, see: a) Y. Nishibayashi, M. Yoshikawa, Y.
Inada, M. Hidai, S. Uemura, J. Am. Chem. Soc. 2002, 124,
11846 – 11847; b) Y. Nishibayashi, Y. Inada, M. Yoshikawa, M.
Hidai, S. Uemura, Angew. Chem. 2003, 115, 1533 – 1536; Angew.
Chem. Int. Ed. 2003, 42, 1495 – 1498.
[10] a) F. Diederich, Y. Rubin, Angew. Chem. 1992, 104, 1123 – 1146;
Angew. Chem. Int. Ed. Engl. 1992, 31, 1101 – 1123; b) F. Die-
derich, Nature 1994, 369, 199 – 207; c) U. H. F. Bunz, Angew.
Chem. 1994, 106, 1127 – 1131; Angew. Chem. Int. Ed. Engl. 1994,
33, 1073 – 1076; d) M. B. Nielsen, F. Diederich, Synlett 2002,
544 – 552.
[11] There have been some reports on reactions using alkynylsilanes
as an alkynyl component; see: a) L. Birkofer, A. Ritter, H.
Uhlenbrauck, Chem. Ber. 1963, 96, 3280 – 3288; b) A. B. Holmes,
C. L. D. Jennings-White, A. H. Schulthess, B. Akinde, D. R. M.
Walton, J. Chem. Soc. Chem. Commun. 1979, 840 – 842; c) I.
Keywords: aldehydes · cross-coupling · gold · rhenium
.
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complex with potential catalysts did not improve the yield of 6a:
AgOTf: 9%; CuCl: 8%; PdCl2: 11%; PtCl2: 19%.
[13] The structure of the diethynylmethane was determined by
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[12] We examined the catalytic activity of several catalyst combina-
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shown in parentheses). However, other combinations of the Re
[14] Aldehyde 8e was recovered in 34% yield.
[15] 9-Phenyl-7,10-heptadecadiyne (6i) was obtained in 23% yield.
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