Inverse regioselectivity in the silylstannylation of alkynes and allenes: Copper-catalyzed three-component coupling with a silylborane and a tin alkoxide

Article abstract of DOI:10.1039/c5cc01856k

Silylstannylation of alkynes and allenes has been found to proceed by three-component coupling using a silylborane and a tin alkoxide in the presence of a Cu(i) catalyst. The regioselectivities are completely inverse to those of the conventional silylstannylation under palladium catalysis.

Full text of DOI:10.1039/c5cc01856k

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DOI: 10.1039/C5CC01856K
COMMUNICATION
Inverse regioselectivity in the silylstannylation of
alkynes and allenes: Copper-catalyzed three-
component coupling with a silylborane and a tin
alkoxide
Cite this: DOI:
10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
H. Yoshida,*a,b Y. Hayashi,a Y. Itoa and K. Takakia
DOI: 10.1039/x0xx00000x
www.rsc.org/
Silylstannylation of alkynes and allenes has been found to
proceed by three-component coupling using a silylborane and
a tin alkoxide in the presence of a Cu(I) catalyst. The
regioselectivities are completely inverse to those of the
conventional silylstannylations under palladium catalysis.
Sn
Si
R
R'3Si SnR'3
Pd catalyst
R
Si
•
R
R
Sn
Transition metal-catalyzed dual functionalization of a carbon–
carbon multiple bond of such unsaturated hydrocarbons as
alkynes and allenes with metallic elements (dimetallation)1 has
Scheme 1 Regioselectivity in Pd-catalyzed silylstannylation.
attracted considerable attention as
a
convenient and
Cu
B (Sn)
Sn
B (Sn)
R
R
Sn OR'
Cu B (Sn)
straightforward entry to synthetically potent vic-dimetallated
compounds of defined structure, whose carbon–metal bonds are
utilizable for construction of carbon framework2 and
introduction of functional groups. One of the most prevailing
dimetallations is silylstannylation,3 which has thus far been
shown to proceed through direct addition of a silicon–tin bond
of silylstannanes across unsaturated carbon linkages under
R
R
R
R
Scheme 2 Cu-catalyzed borylstannylation and distannylation.
borylstannylation5c-e of carbon–carbon multiple bonds take
place with unique reaction mode,6 where oxidation state of a
copper catalyst stays constant throughout the reaction, being in
marked contrast to the conventional oxidative addition–
insertion–reductive elimination sequence, which generally
involves two-electron redox of a transition metal catalyst.1,7
The key intermediates in these transformations are β-boryl(or
stannyl)organocopper species arising from insertion of
unsaturated hydrocarbons into boryl(or stannyl)copper species,
and capturing them by a tin alkoxide finally affords the
respective stannylated products as depicted in Scheme 2.5b-e
Thus, we envisaged that silylstannylation of unsaturated
hydrocarbons would also be feasible under the copper catalysis
by use of a suitable silylating reagent which allows facile
generation of silylcopper8 and β-silylorganocopper species.
palladium catalysis.
The characteristic feature of the
palladium-catalyzed silylstannylation is high level of
regioselectivity: the silyl addition commonly occurs at the
terminal carbon of terminal alkynes3a-f and at the central carbon
of allenes,3g-j irrespective of electronic and steric characters of
ligands employed and substituents on the carbon–carbon
multiple bonds (Scheme 1).4
Recently we have devoted our attention to exploitation of
potential copper catalysis toward the dimetallation of
unsaturated hydrocarbons, and have already disclosed that
diborylation,5a distannylation5b and three-component
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Table 1 Ligand and solvent effect on Cu-catalyzed silylstannylation of 1- Table 2 Cu-catalyzed silylstannylation of terminal alkynesa
octynea
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PhMe2Si SnBu3
DOI: 10.1039/C5CC01856K
Bu3SnOtBu (1.2 equiv)
P(tBu)3 (2 mol %)
PhMe2Si
SnBu3
R
Bu3SnOtBu (1.2 equiv)
Ligand (2 mol %)
CuCl (2 mol %)
2
CuCl (2 mol %)
nHex
+
:
PhMe2Si B(pin)
1.2
R
2a
MeCN, rt
+
:
PhMe2Si B(pin)
1.2
nHex
Bu3Sn
R
SiMe2Ph
1
solvent, rt
Bu3Sn
SiMe2Ph
1
1
1a
2'
O
nHex
2'a
B(pin) =
B
Entry
R
Time (h)
Yield (%)b
2:2’
95:5
94:6
93:7
97:3
90:10
94:6
91:9
O
1
2
3
4
5
6
7
8
nBu (1b)
3
1.5
7
76
68
65
64
61
56
47
43
39
75
59
61
64
nOct (1c)
Cypc (1d)
Me
Me
PR2
iBu (1e)
iAmyl (1f)
4
N
N
19
3.5
16
4
25.5
6.5
12.5
27
7
Me
Me
Cu
Cl
Me
Me
NC(CH2)3 (1g)
Br(CH2)2 (1h)
HO(CH2)2 (1i)
Et2NCH2 (1j)
THPOCH2 (1k)
MeOCH2 (1l)
1-Cyclohexenyl (1m)
Ph (1n)
JohnPhos: R = tBu
Cy-JohnPhos: R = Cy
IMesCuCl
97:3
Entry
1
2
3
Yield (%)b
2a:2’a
9
90:10
10:90
1:99
61:39
14:86
Ligand
P(tBu)3
PPh3
JohnPhos
Cy-JohnPhos
IMesd
P(tBu)3
P(tBu)3
P(tBu)3
Solvent
MeCN
MeCN
MeCN
MeCN
MeCN
Toluene
THF
Time (h)
10
11
12
13
6
3
23
27
3
86
88c
58c
54c
84c
69
93:7
80:20
86:14
85:15
75:25
99:1
4
5
a
General procedure:
1 (0.30 mmol), PhMe2Si–B(pin) (0.36 mmol),
Bu3SnOtBu (0.36 mmol), CuCl (6.0 µmol), P(tBu)3 (6.0 µmol), MeCN (1
6e
7f
8
4
3
63
79
99:1
90:10
mL). b Isolated yield. c Cyp = cyclopentyl.
DMF
1.5
a
Table 3 Cu-catalyzed silylstannylation of terminal allenesa
General procedure: 1a (0.30 mmol), PhMe2Si–B(pin) (0.36 mmol),
Bu3SnOtBu (0.36 mmol), CuCl (6.0 µmol), Ligand (6.0 µmol), solvent (1
b
c
d
e
mL). Isolated yield. NMR yield. IMesCuCl (2 mol%) was used.
A
SnBu3
hydrosilylation product, nHex(PhMe2Si)C=CH2 was formed in 6% NMR
f
yield.
A hydrosilylation product, nHex(PhMe2Si)C=CH2 was formed in
R
R
SiMe2Ph
(E)-4
Bu3SnOMe (1.2 equiv)
10% NMR yield.
Cl
IMesCuCl (2 mol %)
•
+
:
PhMe2Si B(pin)
1.2
R
SnBu3
MeCN, rt, 15 min
1
Herein we report that the silylstannylation of alkynes and
allenes facilely occurs by the copper-catalyzed three-
component coupling with
3
SiMe2Ph
(Z)-4
Cl
Cl
Me
Me
a
silylborane,9 and that the
N
N
Me
Me
Cl
Cu
Cl
regioselectivities become totally inverse to those of the
conventional silylstannylations3 in both cases.
Me
Me
IMesCuCl
The three-component silylstannylation was found to readily
occur to afford 2a and 2’a in 86% yield with regioselectivity
inverse to those of the previous Pd-catalyzed silylstannylation
(2a:2’a = 93:7), when the reaction of 1-octyne (1a), a
silylborane (PhMe2Si–B(pin), pin: pinacolato) and tributyltin
tert-butoxide10 was carried out in acetonitrile at room
temperature in the presence of CuCl–P(tBu)3 catalyst (Table 1,
entry 1). Although the silylstannylation products were also
formed with other monodentate phosphines (PPh3, JohnPhos
and Cy-JohnPhos) and N-heterocyclic carbene (IMes), the
yields and the regioselectivities were unsatisfactory (entries 2–
5). Acetonitrile has proven to be the solvent of choice: the
reaction in less polar solvents (toluene and THF) produced a
Entry
R
Yield (%)b
E:Z
1
2
3
4
5
Dodecyl (3a)
Ph(CH2)2 (3b)
Cy (3c)
nOct (3d)
TBSO(CH2)2 (3e)
87
86
77
71
93
78:22
77:23
87:13
91:9
75:25
O
MeN
NCH2
N
6
76
83:17
N
Me
O
(3f)
O
NCH2
7
63
56
79:21
75:25
O
(3g)
8
THPOCH2 (3h)
a
hydrosilylation product, nHex(PhMe2Si)C=CH2, as
a by-
General procedure:
3
(0.30 mmol), PhMe2Si–B(pin) (0.36 mmol),
b
product (entries 6 and 7), and the regioselectivity became lower Bu3SnOMe (0.36 mmol), ClIMesCuCl (6.0 µmol), MeCN (1 mL). Isolated
yield.
with DMF (entry 8).
Under the optimized reaction conditions, 1-hexyne (1b), 1-
decyne (1c) and branched aliphatic terminal alkynes (1d–1f)
could undergo the regioselective silylstannylation, where the
stannyl moieties were predominantly attached to the terminal
carbon of the alkynes (Table 2, entries 1–5). This unique
regioselectivity was also achievable with functionalized alkynes
bearing a cyano (1g), bromo (1h), hydroxy (1i) or amino (1j)
group (entries 6–9), and the results that these reactive moieties
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formation of 518 or 6, the latter of which has been demonstrated
PhMe2Si
SnBu3
to be kinetically favored in the stoichiometric reaction using a
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silylcopper species.8a On the other hanDdO, Ie: l1e0c.1t0ro39n/iCc5CdCir0e1c8t5in6gK
R
2
LCu–Cl
effect of a propargylic functional group (1k and 1l) or a phenyl
group (1n), which induces the addition of the copper moiety to
the internal carbon of the alkynes in step B,19 should become
dominant to provide 2’ as the major products.
SnBu3
Bu3SnOR'
Bu3SnCl
R
SiMe2Ph
4
LCu–OR'
PhMe2SiB(pin)
R'OB(pin)
In conclusion, we have demonstrated that the
regioselectivities of the silylstannylation of terminal alkynes
and allenes can totally be reversed depending upon the copper-
catalyzed three-component coupling using a silylborane and a
tin alkoxide, which leads to convenient and direct access to
diverse 2-silyl-1-stannyl-1-alkenes (from alkynes) and 1-silyl-
2-stannyl-2-alkenes (from allenes) of high synthetic utility.
Further studies on copper-catalyzed silylation reactions of
unsaturated carbon–carbon bonds as well as synthetic
application of the silylstannylation are in progress.
Bu3SnOR'
step C
step A
PhMe2Si
CuL
R
5
LCu–SiMe2Ph
CuL
R
SiMe2Ph
step B
6
R
•
Notes and references
Department of Applied Chemistry, Graduate School of Engineering,
R
a
Scheme 3 A plausible catalytic cycle for silylstannylation.
Hiroshima University, Higashi-Hiroshima 739-8527, Japan. Fax: +81-82-
424-5494; Tel: +81-82-424-7724; E-mail: yhiroto@hiroshima-u.ac.jp
remained intact demonstrate the high functional group
compatibility of the silylstannylation. In contrast, the stannyl
moiety was selectively introduced into the internal carbon of
THP-protected propargyl alcohol (1k) and propargyl ether (1l)
to provide 2’k and 2’l as the major product (entries 10 and 11),
and the reaction of enyne (1m) or phenylacetylene (1n) resulted
in low regioselectivity (entries 12 and 13).
The three-component silylstannylation of allenes was found
to also proceed smoothly with regioselectivity inverse to those
of the previous silylstannylation under palladium catalysis.
Thus, treatment of pentadeca-1,2-diene (3a) with a silylborane
and tributyltin methoxide11 in the presence of ClIMesCuCl
catalyst12 afforded an 87% yield of (E)- and (Z)-4a (ratio =
78:22), whose stannyl moiety was exclusively installed into the
b
ACT-C, Japan Science and Technology Agency, Higashi-Hiroshima
739-8527, Japan.
†
Electronic Supplementary Information (ESI) available: Experimental
procedures and characterization data. See DOI: 10.1039/c000000x/
1
For reviews, see: (a) I. Beletskaya and C. Moberg, Chem. Rev., 1999,
99, 3435; (b) M. Suginome and Y. Ito, Chem. Rev., 2000, 100, 3221;
(c) I. Beletskaya and C. Moberg, Chem. Rev., 2006, 106, 2320.
Metal-Catalyzed Cross-Coupling Reactions, ed. A. de Meijere and F.
Diederich, Wiley-VHC, Weinheim, 2004.
2
3
For representative silylstannylation of alkynes, see: (a) B. L. Chenard,
E. D. Laganis, F. Davidson and T. V. RajanBabu, J. Org. Chem.,
1985, 50, 3666; (b) T. N. Mitchell, H. Killing, R. Dicke and R.
Wickenkamp, J. Chem. Soc., Chem. Commun., 1985, 354; (c) T. N.
Mitchell, R. Wickenkamp, A. Amamria, R. Dicke and U. Schneider,
J. Org. Chem., 1987, 52, 4868; (d) I. Hemeon and R. D. Singer,
Chem. Commun., 2002, 1884; (e) M. Murakami, T. Matsuda, K.
Itami, S. Ashida and M. Terayama, Synthesis, 2004, 1522; (f) T. E.
Nielsen, S. Le Quement and D. Tanner, Synthesis, 2004, 1381; For
representative silylstannylation of allenes, see: (g) T. N. Mitchell and
U. Schneider, J. Organomet. Chem., 1991, 407, 319; (h) A. G. M.
Barrett and P. W. H. Wan, J. Org. Chem., 1996, 61, 8667; (i) S. Shin
and T. V. RajanBabu, J. Am. Chem. Soc., 2001, 123, 8416; (j) M.
Jeganmohan, M. Shanmugasundaram, K.-J. Chang and C.-H. Cheng,
Chem. Commun., 2002, 2552.
central carbon of the allene (Table 3, entry 1).
The
regioselective formation of silylstannylated products (4b–4d)
bearing allylsilane and alkenylstannane units was observed with
5-phenyl-penta-1,2-diene (3b), cyclohexylallene (3c) and
undeca-1,2-diene (3d) (entries 2–4), and furthermore
functionalized allenes possessing
a silyl ether (3e), a
theobromine (3f), a phthalimide (3g) or an acetal (3h) moiety
underwent the silylstannylation with a similar regioselectivity
to provide the respective products (4e–4h) without damaging
these functional groups (entries 5–8).
Generation of a silylcopper species, Cu–SiMe2Ph, via σ-
bond metathesis between a copper alkoxide and a silylborane
would trigger the silylstannylation (Scheme 3, step A).13 Then
an alkyne or an allene was inserted into the Cu–Si bond to give
a β-silylalkenylcopper species (5 or 6) (step B),14 which was
4
5
An alkoxyalkyne exceptionally accepts the silyl addition at the
internal carbon. See: M. Murakami, H. Amii, N. Takizawa and Y. Ito,
Organometallics, 1993, 12, 4223.
(a) H. Yoshida, S. Kawashima, Y. Takemoto, K. Okada, J. Ohshita
and K. Takaki, Angew. Chem. Int. Ed., 2012, 51, 235; (b) H. Yoshida,
A. Shinke and K. Takaki, Chem. Commun., 2013, 49, 11671; (c) Y.
Takemoto, H. Yoshida and K. Takaki, Chem. Eur. J., 2012, 18,
14841; (d) Y. Takemoto, H. Yoshida and K. Takaki, Synthesis, 2014,
subsequently trapped by
a tin alkoxide to furnish a
silylstannylation product with regenerating a copper alkoxide
(step C).15-17 The regiochemical outcome of the reaction with
an alkyne or an allene should be ascribable to the regioselective
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46, 3024; (e) H. Yoshida, Y. Takemoto and K. Takaki, Chem.
Commun., 2015, 51, 6297.
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6
We have also reported other borylation reactions of unsaturated
hydrocarbons under copper or silver catalysis. See: (a) H. Yoshida, I.
Kageyuki and K. Takaki, Org. Lett., 2013, 15, 952; (b) H. Yoshida, Y.
Takemoto and K. Takaki, Chem. Commun., 2014, 50, 8299; (c) I.
Kageyuki, H. Yoshida and K. Takaki, Synthesis, 2014, 46, 1924; (d)
H. Yoshida, Y. Takemoto and K. Takaki, Asian J. Org. Chem., 2014,
3, 1204; (e) H. Yoshida, I. Kageyuki and K. Takaki, Org. Lett., 2014,
16, 3512.
DOI: 10.1039/C5CC01856K
7
8
9
(a) M. Hada, Y. Tanaka, M. Ito, M. Murakami, H. Amii, Y. Ito and H.
Nakatsuji, J. Am. Chem. Soc., 1994, 116, 8754; (b) F. Ozawa, Y.
Sakamoto, T. Sagawa, R. Tanaka and H. Katayama, Chem. Lett.,
1999, 1307; (c) T. Sagawa, Y. Sakamoto, R. Tanaka, H. Katayama
and F. Ozawa, Organometallics, 2003, 22, 4433.
For reviews, see: (a) A. Barbero and F. J. Pulido, Acc. Chem. Res.,
2004, 37, 817; (b) F. J. Pulido and A. Barbero, Silyl and Stannyl
Derivatives of Organocopper Compounds, in The Chemistry of
Organocopper Compounds Part 2, ed. Z. Rappoport and I. Marek,
Wiley, Chichester, 2009, pp. 775–856.
For representative examples of copper-catalyzed silylation of
unsaturated hydrocarbons with a silylborane, see: (a) P. Wang, X.-L.
Yeo and T.-P. Loh, J. Am. Chem. Soc., 2011, 133, 1254; (b) T.
Fujihara, Y. Tani, K. Semba, J. Terao and Y. Tsuji, Angew. Chem. Int.
Ed., 2012, 51, 11487; (c) F. Meng, H. Jang and A. H. Hoveyda,
Chem. Eur. J., 2013, 19, 3204; (d) Y. Tani, T. Fujihara, J. Terao and
Y. Tsuji, J. Am. Chem. Soc., 2014, 136, 17706; (e) Y.-H. Xu, L.-H.
Wu, J. Wang and T.-P. Loh, Chem. Commun., 2014, 50, 7195; (f) J.
Rae, Y. C. Hu and D. J. Procter, Chem. Eur. J., 2014, 20, 13143.
10 The reaction with tributyltin methoxide resulted in lower
regioselectivity (2a:2’a = 89:11).
11 The use of tributyltin tert-butoxide resulted in lower yield. See ESI
for details.
12 For ligand effect on the silylstannylation of an allene, see ESI.
13 C. Kleeberg, M. S. Cheung, Z. Lin and T. B. Marder, J. Am. Chem.
Soc., 2011, 133, 19060.
14 For stoichiometric reactions of a silylcopper species with an alkyne
or an allene that produce a β-silylalkenylcopper species, see ref. 8a.
15 Generation of a silylcopper species (step A) and insertion of an
alkyne or an allene into the Cu–Si bond (step B) have been widely
accepted as fundamental elementary steps in the copper-catalyzed
silylation reactions of alkynes or allenes with a silylborane. See ref.
9.
16 We have already demonstrated that an alkenylcopper species is
readily captured with a tin alkoxide to give an alkenylstannane. See
ref. 5c.
17 Intermediacy of a silylstannane (PhMe2Si–SnBu3) in the present
silylstannylation could be ruled out, because the copper-catalyzed
reaction of 3d with PhMe2Si–SnBu3 did not produce 4d at all.
18 Regioselective formation of this alkenylcopper species was also
observed in the copper-catalyzed formal hydrosilylation of terminal
alkynes. See ref. 9a.
19 A similar regioselectivity was obtained in borylcupration of alkynes.
See: (a) H. R. Kim and J. Yun, Chem. Commun., 2011, 47, 2943; (b)
A. L. Moure, R. G. Arrayás, D. J. Cárdenas, I. Alonso and J. C.
Carretero, J. Am. Chem. Soc., 2012, 134, 7219.
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