Copper-catalyzed silacarboxylation of internal alkynes by employing carbon dioxide and silylboranes

Article abstract of DOI:10.1002/anie.201207148

Silalactones: The copper-catalyzed reaction of internal alkynes with (dimethylphenylsilyl)pinacolborane (Me₂PhSi-B(pin)) as the silicon source and CO₂ at atmospheric pressure afforded silalactones selectively in good to high yields. The silalactones can be used as substrates for the Hiyama cross-coupling reaction as demonstrated for one substrate. Copyright

Full text of DOI:10.1002/anie.201207148

Angewandte
Chemie
DOI: 10.1002/anie.201207148
Homogeneous Catalysis
Copper-Catalyzed Silacarboxylation of Internal Alkynes by Employing
Carbon Dioxide and Silylboranes**
Tetsuaki Fujihara, Yosuke Tani, Kazuhiko Semba, Jun Terao, and Yasushi Tsuji*
Carbon dioxide (CO2) is a nontoxic, abundant, and renewable
carbon source.[1] The utilization of this environmentally
friendly raw material in carbon–carbon bond-forming reac-
tions is one of the most important challenges in homogeneous
transition metal catalysis.[2] To date, two types of catalytic
Besides these reactions, catalytic heterocarboxylation, in
which the heteroatom functionality and CO2 are simultane-
ously and catalytically incorporated into unsaturated sub-
strates, is extremely useful, since the reaction will provide
a valuable synthetic route employing CO2 for the formation
of highly functionalized carboxylic acid derivatives. For the
silacarboxylation of alkynes, the only precedent is the
stoichiometric reaction of 1-hexyne (one example) reported
by Fleming et al., who carried out the reaction of 1-hexyne
with a stoichiometric amount of (Me2PhSi)2CuLi·LiCN fol-
lowed by trapping of the resulting Cu species with CO2
(Scheme 2a).[6] Herein, we report the first catalytic silacar-
boxylation of internal alkynes employing CO2 and silylborane
in the presence of a copper catalyst (Scheme 2b).[7] The
reaction afforded silalactone products regioselectively in
good to high yields.
À
transformations using CO2 with C C bond formation have
been investigated intensively: a) the substitution of Ar–Y
with CO2 (Scheme 1a) and b) the hydrocarboxylation of C–C
À
Scheme 1. Typical transformations using CO2 with C C bond forma-
tions.
unsaturates (Scheme 1b). The former reactions involve the
carboxylations of organozinc[3a,b] and organoborane[3c–f] com-
[3g-j]
pounds, C H bonds,
and bromoarenes[3k] (Scheme 1a).
À
Recently, we reported the Ni-catalyzed carboxylation of
chloroarenes with CO2 in the presence of Mn powder under
ambient conditions.[4a] As for the hydrocarboxylation
(Scheme 1b), the reactions of dienes,[5a,b] alkenes,[5c] and
alkynes[5d] have been reported. However, in all these cases,
highly reactive and pyrophoric Et3Al[5a,b] or Et2Zn[5a,c,d] must
be used as a hydride source. We recently reported the
hydrocarboxylation of alkynes, by employing stable and easy-
to-handle hydrosilanes as the hydride source.[4b]
Scheme 2. Silacarboxylation of alkynes.
First, the silacarboxylation of 1-phenyl-1-propyne (1a)
was carried out using the readily available Me2PhSi-B(pin)[8,9]
as the silicon source in the presence of a copper catalyst in
octane at 1008C under an atmospheric pressure of CO2
(Table 1).[10] A mixture of CuCl and PCy3 (P/Cu = 1) was
employed as the catalyst, and silalactones[11] were afforded in
97% yield with high regioselectivity (2a/2a’ = 97:3, Table 1,
entry 1). PtBu3 is also an efficient ligand and the products
were obtained regioselectively in high yield (Table 1, entry 2).
On the other hand, PBu3, PPh3, Xantphos, rac-BINAP, and
DPPBz were not effective ligands, affording 2a and 2a’ in low
yields (Table 1, entries 3–7). [CuCl(IMes)][12a] bearing IMes,
an N-heterocyclic carbene (NHC) ligand, gave 2 in high yield,
but with slightly lower regioselectivity (2a/2a’ = 88:12;
Table 1, entry 8). [CuCl(IPr)],[12b] bearing the sterically
demanding IPr ligand, gave a considerably lower yield
[*] Prof. Dr. T. Fujihara, Y. Tani, K. Semba, Prof. Dr. J. Terao,
Prof. Dr. Y. Tsuji
Department of Energy and Hydrocarbon Chemistry
Graduate School of Engineering, Kyoto University
Kyoto 615-8510 (Japan)
E-mail: ytsuji@scl.kyoto-u.ac.jp
Homepage: http://twww.ehcc.kyoto-u.ac.jp/
[**] This work was supported by Grant-in-Aid for Scientific Research on
Innovative Areas (“Organic synthesis based on reaction integra-
tion” and “Molecular activation directed toward straightforward
synthesis”) from MEXT (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207148.
Angew. Chem. Int. Ed. 2012, 51, 11487 –11490
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
11487

.
Angewandte
Communications
Table 1: Copper-catalyzed silacarboxylation of 1a with CO2 employing
Table 2: Copper-catalyzed silacarboxylation of internal alkynes.[a]
Me2PhSi-B(pin).[a]
Entry Substrate
Yield of 2+2’ [%][b]
(Ratio of 2/2’)[c]
Yield of isolated
major isomer: 2 [%]
Entry
Cu catalyst
Ligand
Yield of
Ratio of
2a+2a’ [%][b]
2a/2a’[c]
1
2
3
4
5
6
7
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
PCy3
PtBu3
PBu3
PPh3
Xantphos
rac-BINAP
DPPBz
97
90
15
28
0
7
0
97:3
97:3
90:10
74:26
–
1
2
3
4
1b: R=Me
1c: R=OMe
1d: R=Br
93 (95:5)
91 (94:6)
97 (96:4)
82 (98:2)
2b: 85
2c: 84
2d: 64
2e: 80
–
–
1e: R=CO2Et
8
9
10
11[f]
12[g]
[CuCl(IMes)]
[CuCl(IPr)]
[CuCl(PCy3)]2
[CuCl(PCy3)]2
[CuCl(PCy3)]2
96
88:12
77:23
96:4
95:5
94:6
18
99 (81)[e]
[d]
[d]
[d]
5
98
84
1 f
86 (87:13)
2 f: 70
[a] Reaction conditions; 1-phenyl-1-propyne (1a, 0.50 mmol), copper
catalyst (0.015 mmol, 3.0 mol%), ligand (0.015 mmol, 3.0 mol%),
NaOtBu (0.060 mmol, 12 mol%), Me2PhSi-B(pin) (0.60 mmol,
1.2 equiv), under CO2 (1 atm) in octane (0.50 mL), at 1008C for 20 h.
[b] Total yield of 2a and 2a’ determined by GC analysis using tetradecane
as an internal standard. [c] Determined by GC. [d] [CuCl(PCy3)]2
(0.0125 mmol, 2.5 mol%) was used as the catalyst. [e] Yield of isolated
2a. [f] Toluene was used as the solvent. [g] 1,4-Dioxane was used as the
solvent.
2 f’: 11
6
1g
1h
1i
84 (92:8)
2g: 76
2h: 77
2i: 81
(Table 1, entry 9). [CuCl(PCy3)]2,[12c] prepared from CuCl and
PCy3, was the best catalyst, giving 2 in quantitative yield while
maintaining high regioselectivity (Table 1, entry 10). Through
simple filtration and washing with hexane, the major regio-
isomer 2a was isolated in 81% yield in pure form (Table 1,
entry 10). The structure of 2a was confirmed by X-ray
crystallographic analysis[10] (Figure S1 in the Supporting
Information) as well as 2D NMR spectroscopy measure-
ments. In toluene or 1,4-dioxane as the solvent, the products
were obtained in 98% and 84% yields, respectively (Table 1,
entries 11 and 12), while acetonitrile and N,N-dimethylform-
amide did not afford the product at all.
The silacarboxylation of various alkynes (1b–i) was
carried out (Table 2). The reactions of 1-aryl-1-propynes
bearing both electron-rich and electron-poor substituents on
the aryl ring gave the corresponding silalactones (Table 2,
entries 1–4). The GC and GC–MS analyses of the crude
products indicated that these reactions proceeded regioselec-
tively, and major regioisomers (2b–e) were isolated in good to
high yields. Gratifyingly, the bromo (Table 2, entry 3) and
ester (entry 4) functionalities remained intact under these
reaction conditions. 1-Phenyl-1-hexyne (1 f) afforded the
products in 86% total yield (by GC) with slightly lower
regioselectivity (2 f/2 f’ = 87:13). Pure 2 f and 2 f’ were isolated
from the reaction mixture by silica gel column chromatog-
raphy in 70% and 11% yields, respectively (Table 2, entry 5).
The conjugated enyne 1g as the substrate gave 2g and 2g’ in
84% total yield (2g/2g’ = 92:8), from which the major isomer
2g was isolated in 76% yield in pure form (Table 2, entry 6).
7[d]
92
8[e]
88
[a] Reaction conditions; alkyne (1, 0.50 mmol), [CuCl(PCy3)]2
(0.0125 mmol, 2.5 mol%), NaOtBu (0.060 mmol, 12 mol%),
Me2PhSi-B(pin) (0.60 mmol, 1.2 equiv), under CO2 (1 atm) in octane
(0.50 mL), at 1008C, for 16 h. [b] Total yield of 2 and 2’ determined by GC
analysis using tetradecane as an internal standard. [c] Determined by
GC. [d] For 60 h. [e] At 908C, for 2 h.
Diphenylacetylene (1h) also afforded the corresponding
product 2h in good yield with a longer reaction time
(Table 2, entry 7). The reaction of 5-decyne (1i) proceeded
smoothly at 908C for two hours and 2i was isolated in 81%
yield (Table 2, entry 8).
Et3Si-B(pin)[13] (in place of Me2PhSi-B(pin)) was reacted
with 1a under the same reaction conditions as for entry 10 in
Table 1 [Eq. (1)]. However, the reaction of Et3Si-B(pin) was
not as clean as that with Me2PhSi-B(pin), and some unidenti-
fied byproducts were formed. In the reaction, the correspond-
ing silalactone 2a-Et was afforded in only trace yield
11488 www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 11487 –11490

Angewandte
Chemie
(determined by GC–MS). Instead, a mixture of two b-silyl-
a,b-unsaturated carboxylic acids (3a-Et/3a-Et’ = 96:4) was
isolated in 64% yield [Eq. (1)]. Disilanes such as Me2PhSi-
SiPhMe2 could not replace Me2PhSi-B(pin) as the silicon
source in the present silacarboxylation reaction, since neither
silalactones (2) nor b-silyl-a,b-unsaturated carboxylic acids
(3) were detected in the reaction mixture. Thus, the readily
and commercially available Me2PhSi-B(pin) is the best silicon
source for this reaction.[14] Regarding the alkynes, terminal
alkynes could not be used as the substrates.
Scheme 4. Plausible reaction mechanism.
the phenylcopper species D by extrusion of the silalactone 2
(step c). As shown in Equation (1), Et3Si-B(pin) afforded the
corresponding silalactone 2a-Et in only trace yield. Thus, the
À
phenyl moiety on the Si atom is indispensable for efficient Si
C bond cleavage.[18] Finally, s-bond metathesis of D with
silylborane provides PhB(pin), and the silylcopper species A
is regenerated (step d). In fact, PhB(pin) was found (86%
yield) in the reaction mixture in entry 10 of Table 1.
The silalactones are good substrates for the Hiyama cross-
coupling reaction.[11b] Indeed, in the reaction of 2a with 4-
bromobenzaldehyde in the presence of [Pd2(dba)3] as a cata-
lyst, the trisubstituted a,b-unsaturated carboxylic acid 5a was
isolated in 76% yield [Eq. (2)].
Scheme 3. Control experiments relevant to the mechanism.
Some control experiments were carried out (Scheme 3) to
provide an insight into the silacarboxylation mechanism.
There is some possibility that the reaction could proceed
stepwise through two sequential known reactions, that is, the
silaboration of an alkyne[15] followed by the carboxylation of
the resulting vinylboronic ester.[3c–f] Thus, we first prepared
a possible intermediate 4i[15a] through the silaboration of 1i
with Me2PhSi-B(pin). Then the reaction of 4i with CO2 was
carried out under the same reaction conditions as for entry 8
in Table 2. As a result, 4i was not converted at all and no 2i
was provided (Scheme 3a). Furthermore, 4i was not detected
at all in the reaction mixture at a low conversion of 1i
(reaction time: 15 min; conversion of 1i: 19%; yield of 2i:
12%) under otherwise the same reaction conditions as for
entry 8 in Table 2 (Scheme 3b). Thus, in the present catalytic
system, the silacarboxylation must proceed in one step, and
not through silaboration of the alkyne followed by carbox-
ylation.
In conclusion, the first catalytic silacarboxylation of an
alkyne has been achieved by utilizing silylborane as the silicon
source under atmospheric pressure of CO2 to afford the
silalactone selectively in high yield. Further studies on
applications and the reaction mechanism are now in progress.
Received: September 4, 2012
Published online: October 9, 2012
With consideration of these observations, a possible
catalytic cycle for the silacarboxylation of alkynes with CO2
is proposed (Scheme 4). First, the reaction of the alkoxocop-
per species with silylborane affords the silylcopper complex
A.[16,17] Syn addition of A to the alkyne initiates the catalytic
cycle and affords the b-silylalkenylcopper intermediate B
Keywords: alkynes · carboxylation · copper ·
.
homogeneous catalysis · silylborane
À
(step a). Then, insertion of CO2 into the Cu C(vinyl) bond
[1] a) M. Aresta in Carbon Dioxide as Chemical Feedstock, Wiley-
VCH, Weinheim, 2010; b) T. Sakakura, J.-C. Choi, H. Yasuda,
Chem. Rev. 2007, 107, 2365 – 2387.
occurs,[4b] providing the corresponding copper carboxylate
species C (step b). Intramolecular cyclization via C’ affords
Angew. Chem. Int. Ed. 2012, 51, 11487 –11490
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 11489

.
Angewandte
Communications
[2] For recent reviews, see: a) Y. Tsuji, T. Fujihara, Chem. Commun.
2012, 48, 9956 – 9964; b) K. Huang, C.-L. Sun, Z.-J. Shi, Chem.
Soc. Rev. 2011, 40, 2435 – 2452; c) M. Cokoja, C. Bruckmeier, B.
Rieger, W. A. Herrmann, F. E. Kꢀhn, Angew. Chem. 2011, 123,
8662 – 8690; Angew. Chem. Int. Ed. 2011, 50, 8510 – 8537.
[3] a) C. S. Yeung, V. M. Dong, J. Am. Chem. Soc. 2008, 130, 7826 –
7827; b) H. Ochiai, M. Jang, K. Hirano, H. Yorimitsu, K.
Oshima, Org. Lett. 2008, 10, 2681 – 2683; c) K. Ukai, M. Aoki, J.
Takaya, N. Iwasawa, J. Am. Chem. Soc. 2006, 128, 8706 – 8707;
d) T. Ohishi, M. Nishiura, Z. Hou, Angew. Chem. 2008, 120,
5876 – 5879; Angew. Chem. Int. Ed. 2008, 47, 5792 – 5795; e) J.
Takaya, S. Tadami, K. Ukai, N. Iwasawa, Org. Lett. 2008, 10,
2697 – 2700; f) X. Zhang, W.-Z. Zhang, L.-L. Shi, C.-X. Guo, L.-
L. Zhang, X.-B. Lu, Chem. Commun. 2012, 48, 6292 – 6294;
g) I. I. F. Boogaerts, S. P. Nolan, J. Am. Chem. Soc. 2010, 132,
8858 – 8859; h) L. Zhang, J. Cheng, T. Ohishi, Z. Hou, Angew.
Chem. 2010, 122, 8852 – 8855; Angew. Chem. Int. Ed. 2010, 49,
8670 – 8673; i) I. I. F. Boogaerts, G. C. Fortman, M. R. L. Furst,
C. S. J. Cazin, S. P. Nolan, Angew. Chem. 2010, 122, 8856 – 8859;
Angew. Chem. Int. Ed. 2010, 49, 8674 – 8677; j) H. Mizuno, J.
Takaya, N. Iwasawa, J. Am. Chem. Soc. 2011, 133, 1251 – 1253;
k) A. Correa, R. Martꢁn, J. Am. Chem. Soc. 2009, 131, 15974 –
15975.
[4] a) T. Fujihara, K. Nogi, T. Xu, J. Terao, Y. Tsuji, J. Am. Chem.
Soc. 2012, 134, 9106 – 9109; b) T. Fujihara, T. Xu, K. Semba, J.
Terao, Y. Tsuji, Angew. Chem. 2011, 123, 543 – 547; Angew.
Chem. Int. Ed. 2011, 50, 523 – 527.
[5] a) J. Takaya, N. Iwasawa, J. Am. Chem. Soc. 2008, 130, 15254 –
15255; b) J. Takaya, K. Sasano, N. Iwasawa, Org. Lett. 2011, 13,
1698 – 1701; c) C. M. Williams, J. B. Johnson, T. Rovis, J. Am.
Chem. Soc. 2008, 130, 14936 – 14937; d) S. Li, W. Yuan, S. Ma,
Angew. Chem. 2011, 123, 2626 – 2630; Angew. Chem. Int. Ed.
2011, 50, 2578 – 2582.
[6] a) I. Fleming, F. Roessler, J. Chem. Soc. Chem. Commun. 1980,
276 – 277; b) I. Fleming, T. W. Newton, F. Roessler, J. Chem. Soc.
Perkin Trans. 1 1981, 2527 – 2532.
[7] During the preparation of this manuscript, Hou et al. reported
the copper-catalyzed boracarboxylation of alkynes using a dibor-
ane compound. L. Zhang, J. Cheng, B. Carry, Z. Hou, J. Am.
Chem. Soc. 2012, 134, 14314 – 14317.
[8] Abbreviations: Me2PhSi-B(pin): (dimethylphenylsilyl)pinacol-
borane, PCy3: tricyclohexylphosphine, Xantphos: 4,5-bis(diphe-
nylphosphino)-9,9-dimethylxanthene, rac-BINAP: 2,2’-bis(di-
phenylphosphino)-1,1’-binaphthyl, DPPBz: 1,2-diphenylphos-
phinobenzene, IMes: 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-
ylidene, IPr: 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene,
dba: dibenzylideneacetone, 1,5-diphenyl-1,4-pentadien-3-one.
[9] a) For a review on the utility of Me2PhSi-B(pin)in organic
synthesis, see: T. Ohmura, M. Suginome, Bull. Chem. Soc. Jpn.
2009, 82, 29 – 49, and references therein; b) Commercially
available from Aldrich (No. 683434).
[10] See the Supporting Information for details.
[11] Silalactones could be prepared in two steps through the reaction
of acylsilanes with ynolates[11a] followed by Si C bond cleavage
À
with iodine under reflux in CCl4 for 18–50 h.[11b] a) M. Shindo, K.
Matsumoto, S. Mori, K. Shishido, J. Am. Chem. Soc. 2002, 124,
6840 – 6841; b) M. Shindo, K. Matsumoto, K. Shishido, Angew.
Chem. 2004, 116, 106 – 108; Angew. Chem. Int. Ed. 2004, 43, 104 –
106.
[12] a) S. Okamoto, S. Tominaga, N. Saino, K. Kase, K. Shimoda, J.
Organomet. Chem. 2005, 690, 6001 – 6007; b) H. Kaur, F. K.
Zinn, E. D. Stevens, S. P. Nolan, Organometallics 2004, 23, 1157 –
1160; c) G. A. Bowmaker, S. E. Boyd, J. V. Hanna, R. D. Hart,
P. C. Healy, B. W. Skelton, A. H. White, J. Chem. Soc. Dalton
Trans. 2002, 2722 – 2730.
[13] T. A. Boebel, J. F. Hartwig, Organometallics 2008, 27, 6013 –
6019.
[14] Me2PhSi-B(pin)was used as a silicon source also in the copper-
catalyzed hydrosilylation of alkynes,[14a] the copper-catalyzed
conjugate silyl addition to unsaturated carbonyls,[14b] and the
copper-catalyzed synthesis of 2-silylindoles.[14c] a) P. Wang, X.-L.
Yeo, T.-P. Loh, J. Am. Chem. Soc. 2011, 133, 1254 – 1256; b) K.-s.
Lee, A. H. Hoveyda, J. Am. Chem. Soc. 2010, 132, 2898 – 2900;
c) M. Tobisu, K. Fujihara, K. Koh, N. Chatani, J. Org. Chem.
2010, 75, 4841 – 4847.
[15] a) M. Suginome, T. Matsuda, H. Nakamura, Y. Ito, Tetrahedron
1999, 55, 8787 – 8800; b) T. Ohmura, K. Oshima, M. Suginome,
Chem. Commun. 2008, 1416 – 1418; c) T. Ohmura, K. Oshima, H.
Taniguchi, M. Suginome, J. Am. Chem. Soc. 2010, 132, 12194 –
12196.
[16] [(IPr)CuSiMe2Ph] was isolated in 89% yield by the reaction of
Me2PhSi-B(pin) with [(IPr)CuOtBu], and fully characterized
including single-crystal X-ray structural determination: C.
Kleeberg, M. S. Cheung, Z. Lin, T. B. Marder, J. Am. Chem.
Soc. 2011, 133, 19060 – 19063.
[17] As for formation of silylcopper species and their catalytic use in
silylation reactions of unsaturated carbon linkages, see a) H. Ito,
T. Ishizuka, J. Tateiwa, M. Sonoda, A. Hosomi, J. Am. Chem.
Soc. 1998, 120, 11196 – 11197; b) L. Iannazzo, G. A. Molander,
Eur. J. Org. Chem. 2012, 4923 – 4926.
À
[18] For recent examples for catalytic cleavage of Si C bond, see:
a) Y. Liang, W. Geng, J. Wei, Z. Xi, Angew. Chem. 2012, 124,
1970 – 1973; Angew. Chem. Int. Ed. 2012, 51, 1934 – 1937; b) M.
Tobisu, M. Onoe, Y. Kita, N. Chatani, J. Am. Chem. Soc. 2009,
131, 7506 – 7507.
11490 www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 11487 –11490