Aryl-aryl interactions as directing motifs in the stereodivergent iron-catalyzed hydrosilylation of internal alkynes

Article abstract of DOI:10.1039/c2cc31395b

The defined Fe hydride complex FeH(CO)(NO)(Ph₃P)₂ is highly active as a catalyst for selective hydrosilylation of internal alkynes to vinylsilanes. Depending on the silane employed either E- or Z-selective hydrosilylation products were formed in excellent yields and good to excellent stereoselectivities.

Full text of DOI:10.1039/c2cc31395b

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COMMUNICATION
Aryl–aryl interactions as directing motifs in the stereodivergent
iron-catalyzed hydrosilylation of internal alkynesw
Christian Belger and Bernd Plietker*
Received 24th February 2012, Accepted 16th April 2012
DOI: 10.1039/c2cc31395b
The defined Fe hydride complex FeH(CO)(NO)(Ph3P)2 is highly
active as a catalyst for selective hydrosilylation of internal alkynes
to vinylsilanes. Depending on the silane employed either E- or
Z-selective hydrosilylation products were formed in excellent
yields and good to excellent stereoselectivities.
Alkynes are versatile building blocks in organic syntheses and
allow for a plethora of transformations.1 Amongst them the
direct transformation of an alkyne into a vinylsilane—the
hydrosilylation—offers the chance to generate vinylanion
Fig. 1 Fe-catalyzed E- or Z-selective hydrosilylation of alkynes.
equivalents for various applications in organic synthesis or
materials science.2 Due to the stability of the Si–H bond this
type of reaction is usually achieved by means of metal catalysts.
However, although the metal catalyzed hydrosilylation of
terminal alkynes is well documented3–13 the corresponding
transformation of internal alkynes lacks behind.14–19 Whereas
the field of alkyne hydrosilylations has largely been dominated
by the use of noble transition metal catalysts defined Fe-,20
Co-21 or Ni-complexes22 have proven to be interesting alter-
natives to the established protocols. The low price and the high
reactivity of these complexes have kindled serious interest in
extending their application.
Table 1 Fe-catalyzed hydrosilylation of 1,2-diphenylacetylene 5—the
silane effecta
Entry
R1
R1
R1
Product
E : Zb
Yieldc [%]
1
2
3
4
5
6
7
Ph
H
H
H
H
H
Ph
Me
Ph
Et
6
7
8
1/>20
1/5
1/2
98
n-Hex
Ph
Ph
Ph
Vinyl
Et
94
Ph
Ph
Me
Me
Et
92d
—
Herein we report a stereodivergent iron-catalyzed hydro-
silylation of internal alkynes using a defined Fe-hydride
complex 1. Depending on the silane source various diarylalkynes
were transformed into the corresponding E- or Z-configured
vinylsilanes (Fig. 1).
9
—
10
11
12
>20/1
>20/1
4/1
97d
97d
90e
a
The reactions were performed on a 1 mmol-scale using 1 mol% 1,
0.5 equiv. NEt3 and 1.1 equiv. R1R2R3SiH at 40 1C in dry THF (1 mL)
Complex 1 is prepared in good yields from readily available
Fe(CO)5 (4) in a one-pot procedure following a slightly
modified literature procedure.23
b
for 12 h under a N2-atmosphere. Determined by 1H NMR-integration.
Isolated yields. d The reactions were performed using 2.5 mol% catalyst 1
c
e
at 60 1C. The reaction was performed using 5 mol% catalyst 1 at 60 1C.
This complex turned out to be highly active in the hydro-
silylation of 1,2-diphenylacetylene (5) which served as a model
reaction throughout the optimization study (Table 1).24
These studies indicated the hydrosilylation to be efficiently
performed using only 1 mol% of the Fe-hydride catalyst 1 in the
presence of 0.5 equiv. of NEt3 and 1.1 equiv. of PhSiH3 (2) in
THF at 40 1C. Although the exact role of the amine additive is
not clear at the current state of research the fact that in most
cases addition of 10 mol% of DMAP leads to comparable
results suggests that the additive might be involved in proton
transfer or ligand dissociation events. Surprisingly, under
these conditions the Z-configured vinyl silane Z-6, which is
obtained by a formal trans-addition of the Si–H-bond across the
CRC-triple bond, was obtained with excellent selectivity in
almost quantitative yield (entry 1, Table 1). The stereochemical
course of this transformation proved to be dependent on the
substitution of the silane source and was mainly directed by its
steric demand (entries 5 and 6, Table 1). Using densely substituted
silane 3 a total inversion of the stereochemistry was observed.
Institut fur Organische Chemie, Universitat Stuttgart,
¨
¨
Pfaffenwaldring 55, D-70569 Stuttgart, Germany.
E-mail: bernd.plietker@oc.uni-stuttgart.de; Tel: +49-711-68564283
w Electronic supplementary information (ESI) available: General
procedure for the preparation of all new compounds including full
characterization. See DOI: 10.1039/c2cc31395b
c
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Chem. Commun., 2012, 48, 5419–5421 5419

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Fig. 2 p-Bond isomerization in metal catalyzed hydrosilylations.
the primarily formed metallavinylsilane can undergo a p-bond
isomerization that is mainly driven by a favorable p–p-interaction
in the Z-configured vinylsilane between the b-aryl group of the
olefin and the phenyl group bound to the Si-atom (eqn (1),
Fig. 3). Employing the sterically hindered silane 3 such a
positive interaction in the resulting vinylsilane is prohibited
due to unfavorable steric repulsions (eqn (2), Fig. 3).
We finally set out to analyze the generality of the aryl–aryl-
directed stereoselective iron-catalyzed hydrosilylation of diaryl-
substituted alkynes (Table 2).
Various bis-aryl alkynes were hydrosilylated in good to
excellent yields and stereoselectivities. However, since a
mixture of regioisomers was obtained in the majority of cases
the crude reaction mixture was directly employed in a desilyla-
tion to give the corresponding olefins. The reaction proved to
possess a good level of functional group tolerance. Esters,
amides, ethers, alcohols, amines and nitriles were tolerated.
However, electron-withdrawing groups within the aromatic
moiety led to an erosion in p-bond selectivity employing
method A.
Scheme 1 Fe-catalyzed hydrosilylation of 1,2-disubstituted alkynes—
the aryl effect.
Changing the silane source from PhSiH3 to n-HexSiH3 led to a
decrease in the E–Z-selectivity (entry 2, Table 1).
In order to showcase the interplay of the silane source and
an alkyne substitution pattern a diaryl-, an aryl/alkyl- and a
dialkylacetylene were subjected to the standard reaction condi-
tions using either PhSiH3 (2) or PhMe(CH2QCH)SiH (3)
(Scheme 1). These experiments indicate that the substitution
pattern of the alkyne is the decisive regioselectivity directing
structural motif. The C–Si-bond is formed with almost exclusive
preference proximal to the aryl-group (eqn (2), Scheme 1).
However, the data also suggest that the stereoselective course
of the hydrosilylation is mainly directed by an interaction
between the silyl group and the second alkyne substituent.
Employing a diarylalkyne in the hydrosilylation using silane 2
led to almost exclusive formation of Z-configured vinylsilane
Z-6 (eqn (1), Scheme 1). The use of an arylalkylalkyne under
otherwise identical reaction conditions resulted in predominant
formation of the E-configured vinylsilane E-13 (eqn (2),
Scheme 1). Subjecting dialkylalkynes to the hydrosilylation
led to the formation of the E-configured products independent
of the silane source (eqn (3), Scheme 1).
The synthetic value of Fe-catalyzed hydrosilylation reported
in this manuscript was finally demonstrated by the stereo-
divergent synthesis of E- and Z-combrestatin A-4 27, a class of
microtubuli-stabilizing agents, that has attracted significant
attention within the past years (Scheme 2).26 Starting from readily
accessible alkyne 26 both natural products were obtained
The trans-addition of a silane across a CRC-triple bond is
usually believed to follow an Ojima–Crabtree-mechanism via a
carbanionic intermediate which is formed by electron-donation
from the metal center into the CQC-double bond of the metalla-
vinylsilane (Fig. 2).25 With regard to the results summarized in
Scheme 1 we propose the following mechanistic rationale for the
unexpected stereoselectivity. If the Fe-catalyzed hydrosilylation
using monosubstituted silane RSiH3 is regioselective with the
Si–C bond ending up proximal to the aryl group the formation
of a stable benzylic carbanion and the isomerization of the
metallavinylsilane appear to be favorable.
The stereoselectivity however is mainly directed by the distal
alkyne substituent. Using sterically less hindered silane PhSiH3 (2)
Fig. 3 p-Bond isomerization in metal catalyzed hydrosilylations.
c
5420 Chem. Commun., 2012, 48, 5419–5421
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Table 2 Scope and limitationsa
2005, 81, 54; (c) K. Itami, K. Mitsudo, A. Nishino and J.-I.
Yoshida, J. Org. Chem., 2002, 67, 2645.
4 Rhodium-catalyzed: (a) M. A. Brook and A. Neuy, J. Org. Chem.,
1990, 55, 3609; (b) A. Sato, H. Kinoshita, H. Shinokubo and
K. Oshima, Org. Lett., 2004, 6, 2217; (c) R. Takeuchi and N. Tanouchi,
J. Chem. Soc., Perkin Trans. 1, 1994, 2909; (d) R. Takeuchi, S. Nitta
and D. Watanabe, J. Org. Chem., 1996, 60, 3045.
5 Ruthenium-catalyzed: (a) Y. Na and S. Chang, Org. Lett., 2000,
2, 1887; (b) B. M. Trost and Z. T. Ball, J. Am. Chem. Soc., 2001,
123, 12726; (c) B. M. Trost and Z. T. Ball, J. Am. Chem. Soc., 2005,
127, 17644; (d) B. M. Trost, M. R. Machacek and Z. T. Ball,
Org. Lett., 2003, 5, 1895.
6 Osmium-catalyzed: R. D. Adams, J. E. Cortopassi and M. P.
Pompeo, Organometallics, 1992, 11, 2.
7 Organoactinide-catalyzed: A. K. Dash, J. Q. Wang and
M. S. Eisen, Organometallics, 1999, 18, 4724.
8 Iridium-catalyzed: R. S. Tanke and R. H. Crabtree, J. Am. Chem.
Soc., 1990, 112, 7984.
9 Lewis acid-catalyzed: N. Kato, Y. Tamura, T. Kashiwabara,
T. Sanji and M. Tanaka, Organometallics, 2010, 29, 5274.
10 Zinc-catalyzed: S. Nakamura, M. Uchiyama and T. Ohwada,
J. Am. Chem. Soc., 2004, 126, 11146.
E : Zb,c
Product (Method) (Alkane)
Yieldd [%]
Entry R1 R2
1
2
3
Ph p-MeOC6H4
Ph p-ClC6H4
17
18
19
17 : 1 (A) 96
1 : 20 (B) 93
7 : 1 (A)
1 : 10 (B) 95
93
4
5g
6
Ph p-MeO2CC6H4
Ph p-CH3C(O)C6H4
Ph p-HOCH2C6H4
Ph p-NCC6H4
1 : 1 (A)
50 (11)
1 : 10 (B) 76
3 : 1 (A)f 25 (4)
1 : 10 (B) 92
7e,g
8e
20
21
22
9e
9 : 1 (A)
1 : 9 (B)
3 : 1 (A)
76
95
71 (10)
10e
11g
12
13e
14e
15e
16e
23
24
1 : 12 (B) 73
20 : 1 (A) 92
1 : 20 (B) 89
14 : 1 (A) 90
1 : 20 (B) 82
Ph p-H2NC6H4
11 Nickel-catalyzed: T. Bartik, G. Nagy, P. Kvintovics and B. Happ,
J. Organomet. Chem., 1993, 453, 29.
12 Cobalt-catalyzed: L. D. Field and A. J. Ward, J. Organomet.
Chem., 2003, 681, 91.
Ph p-CH3C(O)NHC6H4 25
a
13 Palladium-catalyzed: D. Motoda, H. Shinokubo and K. Oshima,
Synlett, 2002, 1529.
14 For an overview of hydrosilylation see: B. M. Trost and T. B.
Zachary, Synthesis, 2005, 853.
The reactions were performed on a 1 mmol-scale using 1 mol% 1,
0.5 equiv. NEt3 in dry THF (1 mL) for 12 h under a N2-atmosphere.
b
Determined by 1H NMR-integration. c Method A: hydrosilylations were
performed under the standard conditions using 1 mol% 1, 2 (1.1 equiv.) at
a temperature of 40 1C. Desilylation: 1.5 equiv. of 1 M TBAF-solution in
7.5 mL THF at 0 1C for 2 h. Method B: hydrosilylations were performed
under the standard conditions using 2.5 mol% 1, 3 (1.1 equiv.) at a
temperature of 60 1C. Desilylation: addition of 1.5 mL THF, 1 mL MeOH
and 1 mL of 2 N NaOH-solution to the reaction mixture and MW for
15 Platinum-catalyzed: (a) M. Green, J. L. Spencer, F. Gordon,
A. Stone and C. A. Tsipis, J. Chem. Soc., Dalton Trans., 1977,
1525; (b) G. Stork and M. E. Jung, J. Am. Chem. Soc., 1974,
96, 3684; (c) H. Bock and H. Seidl, J. Organomet. Chem., 1968,
13, 87; (d) A. Hamze, O. Provot, M. Alami and J.-D. Brion, Org.
Lett., 2005, 7, 5625; (e) G. Berthon-Gelloz, J.-M. Schumers, G. De
d
10 min at 130 1C. Isolated yields. e 2.1 equiv. of the silane were used.
Bo and I. E. Marko, J. Org. Chem., 2008, 73, 4190; (f) D. A. Rooke
´
and E. M. Ferriera, Angew. Chem., Int. Ed., 2012, 51, 3225.
16 Rhodium-catalyzed: (a) M. Brockmann, H. T. Dieck and J. Klaus,
J. Organomet. Chem., 1986, 301, 209; (b) T. Sanada, T. Kato,
M. Mitani and A. Mori, Adv. Synth. Catal., 2006, 348, 51.
17 Ruthenium-catalyzed: (a) B. M. Trost, Z. T. Ball and T. Joege, Angew.
Chem., Int. Ed., 2003, 42, 3415 (Angew. Chem., 2003, 115, 3537);
(b) B. M. Trost and Z. T. Ball, J. Am. Chem. Soc., 2004, 126, 13942;
(c) B. M. Trost and A. Bertogg, Org. Lett., 2009, 11, 511.
f
g
The product was isolated as the alcohol. Partial overreaction to the
corresponding alkane was observed.
18 Lewis acid-catalyzed: (a) N. Asao, T. Sudo and Y. Yamamoto,
J. Org. Chem., 1996, 61, 7654; (b) K. Igawa, K. Tomooka and
Y. Kawasaki, Chem. Lett., 2011, 233.
19 Yttrium-catalyzed: G. A. Molander and W. H. Retsch, Organo-
metallics, 1995, 14, 4570.
20 (a) S. C. Bart, E. Lobkovsky and P. J. Chirik, J. Am. Chem. Soc., 2004,
126, 13794; (b) N. A. Kuzmina, E. T. Chukovskaya and R. K. Freidlina,
Bull. Acad. Sci. USSR Div. Chem. Sci. (Engl. Transl.), 1967, 1227; (c) S.
Enthaler, M. Haberberger and E. Irvan, Chem.–Asian J., 2011, 6, 1613.
21 L. Yong, K. Kirleis and H. Butenschon, Adv. Synth. Catal., 2006,
348, 833.
¨
Scheme 2 Stereodivergent synthesis of E- and Z-combrestatin A-4
27 (Reagents and conditions: 2.5 mol% 1, 2.1 equiv. 2, 0.5 equiv. NEt3,
60 1C were used for method A; 5 mol% 1, 2.1 equiv. 3, 0.5 equiv. NEt3,
80 1C were used for method B).
22 (a) M. J. Chaulagain, G. M. Mahandr and J. Montgomery, Tetrahedron
Lett., 2006, 62, 7560; (b) J. Berding, J. A. Van Paridon, V. H. S. Van
Rixel and E. Bouwman, Eur. J. Inorg. Chem., 2011, 2450; (c) K. Tamao,
M. Asahara and A. Kawachi, J. Organomet. Chem., 1996, 521, 325.
23 (a) W. Hieber and H. Beutner, Z. Anorg. Allg. Chem., 1963,
320, 101; (b) M. Cygler, F. R. Ahmed, A. Forgues and J. L. A.
Roustan, Inorg. Chem., 1983, 22, 1026.
using our hydrosilylation–desilylation strategy in good yields
and promising E–Z-selectivities. Interestingly protection of the
free phenolic OH-group was not necessary.
24 See ESIw for detailed information.
25 (a) I. Ojima, N. Clos, R. J. Donovan and P. Ingallina, Organometallics,
1990, 9, 3127; (b) C.-H. Jun and R. H. Crabtree, J. Organomet. Chem.,
1993, 447, 177.
26 (a) N. J. Lawrence, F. A. Ghani, L. A. Hepworth, J. A. Hadfield, A. T.
McGown and R. G. Pritchard, Synthesis, 1999, 1656; (b) R. Fuerst,
U. Rinner, I. Zupko, A. Berenyi and G. F. Ecker, Bioorg. Med. Chem.
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´
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5419–5421 5421