Acceleration of the substitution of silanes with Grignard reagents by using either LiCl or YCl3/MeLi

Article abstract of DOI:10.1002/anie.201003174

Getting up to speed: Both LiCl and the YCl₃/MeLi catalyst system have an acceleration effect upon the substitution of silanes using Grignard reagents (see scheme). The method provides access to benzyl-, allyl-, and arylsilanes in good yields from the starting silanes.

Full text of DOI:10.1002/anie.201003174

Communications
DOI: 10.1002/anie.201003174
Synthetic Methods
Acceleration of the Substitution of Silanes with Grignard Reagents by
Using either LiCl or YCl3/MeLi**
Naoki Hirone, Hiroaki Sanjiki, Ryoichi Tanaka, Takeshi Hata, and Hirokazu Urabe*
Silanes are the primary source for a variety of organosilicon
compounds and frequently appear as intermediates in organic
synthesis.[1] Attachment of a carbon chain to silanes is one of
the most fundamental derivatizations, but thus far one-step
methods are limited and rely mostly on the hydrosilylation of
olefins and acetylenes.[1b,c,2] If the direct substitution of the
hydride of a silane using an organometallic compound, such
as a Grignard reagent, is viable, then this would be an
alternative method for the above process; however, this
approach has not been adopted because of the low leaving
ability of the hydride towards substitution.[3] Herein we show
that the practical substitution of silanes with Grignard
reagents is possible in the presence of LiCl[4] as formulated
in Equation (1).[5]
substitution, depending on its quantities, to give 1 in good
yield [Eqs. (3) and (4)].
Additional examples are shown in Equations (5) and (6).
Whereas methyl(phenyl)silane was not alkylated with ben-
zylmagnesium bromide at room temperature, its alkylation
The impressive effect of the lithium salt upon the
substitution of a silane is highlighted below. In 1959,
Gilman et al. reported that triphenylsilane and benzyl
Grignard reagent in boiling THF gave triphenyl(benzyl)silane
(1) in 53% yield after 4 days.[3a] This is consistent with our
observation that a similar reaction for a shortened period of
8 hours yielded only a small amount of 1 [Eq. (2)]. However,
the addition of LiCl to this system nicely accelerated the
readily proceeded in the presence of LiCl to give 2 in
excellent yield [Eq. (5); hereafter yields in parentheses refer
to those determined by 1H NMR analysis using trichloro-
ethylene as an internal standard]. The quantity of LiCl could
be reduced to 5 mol% without considerable decrease in the
product yield. Although diphenylsilane is more reactive
towards Grignard reagents, the LiCl-induced acceleration
itself is still notable when the reaction is carried out at a low
temperature [Eq. (6)]. This fast alkylation finds application
even at low temperature in the double benzylation of
phenylsilane with excess Grignard reagent, and results in
delivering the trisubstituted silane 3 [Eq. (7)]. The acceler-
ation was also notable for the alkylsilane 4, which after
reacting gave 5 [Eq. (8)], even though the silane 4 is
intrinsically less reactive than its aryl counterpart, PhSiH3.[6]
[*] N. Hirone, H. Sanjiki, Dr. R. Tanaka, Dr. T. Hata, Prof. Dr. H. Urabe
Department of Biomolecular Engineering, Graduate School of
Bioscience and Biotechnology, Tokyo Institute of Technology
4259-B-59 Nagatsuta-cho, Midori-ku
Yokohama, Kanagawa 226-8501 (Japan)
Fax: (+81)45-924-5849
E-mail: hurabe@bio.titech.ac.jp
Homepage: http://www.urabe-lab.bio.titech.ac.jp
[**] This work was supported by The Kurata Memorial Hitachi Science
and Technology Foundation and a Grant-in-Aid for Scientific
Research on Priority Area (No.16073208) from the Ministry of
Education, Culture, Sports, Science, and Technology (Japan).
The above acceleration is also valid for allyl or aryl
Grignard reagents. The results of allylation are shown in
Table 1, wherein using LiCl in either a stoichiometric amount
or an amount as low as 5 mol% resulted in good product
yields.[7] The absence of LiCl resulted in poor product yields.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201003174.
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Table 1: The LiCl acceleration of the allylation of silanes.
Entry RnSiH4Àn
Allyl-MgCl LiCl
T
t
[h]
Yield [%][a]
[equiv]
[equiv] [8C]
1
2
3
4
5
6
7
8
Ph3SiH
2
2
2
2
1
1
1
1
1
1
1
1
1
0.3
0.05
none
1
reflux
4
4
4
4
1
1
1
1
4
4
4
4
93 (96)
(70)
(64)
(16)
81
80
86
20
92 (quant.)
(81)
(75)
(13)
reflux
reflux
reflux
RT
RT
RT
The LiCl-induced acceleration described above is not
limited to Grignard reagents. For example, the benzyltitanium
reagent 7, prepared in situ from equimolar amounts of
PhCH2MgCl and Ti(OiPr)4,[9] is almost inert to PhSiH3, but
the addition of LiCl to this mixture promoted the reaction to
give 8 in a better yield [Eq. (10)].
PhMeSiH2
Ph2SiH2
0.3
0.05
none
1
0.3
0.05
none
RT
9
À78
10
11
12
À78
À78
À78
[a] Yields of isolated products. Yields in parentheses were determined by
1H NMR analysis using an internal standard (trichloroethylene).
In general, the results shown in Table 1 are similar to those of
the benzylation reactions depicted in Equations (2)–(6). The
advantage of using LiCl was also observed for the arylation of
silanes (Tables 2 and 3).[8]
The practical advantage of LiCl-promoted arylation lies in
the reaction of substrates that are intrinsically unreactive at or
above room temperature (Table 2 and Table 3, entries 3–5). A
more interesting case is shown in Equation (9), wherein the
introduction of a mesityl group to PhSiH3 gives 6 in an
excellent yield.
Another aspect of the lithium effect is that it can be used
with other metal catalysis, and this is illustrated by the reagent
system of YCl3 and MeLi.[10] Although PhSiH3 and 2-allyl-1-
naphthyl Grignard reagent gave only a small amount of 9,
even under forcing conditions, the presence of YCl3/MeLi [11]
promoted the substitution to give the transient product 9 in
greater than 65% yield; and 9 eventually underwent yttrium-
catalyzed intramolecular hydrosilylation[12] to give cyclic
silane 10 in good overall yield [Eq. (11)]. Obviously, the
metathesis between MeLi and YCl3 generated LiCl in situ as
well as an active yttrium catalyst. Similarly, the YCl3/MeLi
catalyst enabled the preparation of 12 from PhSiH3 and 11 in
one pot [Eq. (12)].[13,14]
Table 2: The LiCl acceleration of the arylation of Ph(Me)SiH2.
Entry
Ar (equiv)
t [h]
Yield [%][a]
LiCl
No LiCl
1
2
3
Ph (1.25)
p-FC6H4 (1)
o-(MeO)C6H4
(1.25)
2
8
4
57 (63)
67 (70)
81 (96)
(32)
(18)
(47)
[a] Yields of isolated product are based on silane. Yields in parentheses
were determined by 1H NMR analysis using an internal standard
(trichloroethylene).
Table 3: The LiCl acceleration of the arylation of PhSiH3.
Entry Ar
LiCl
T [8C] t [h]
Yield [%][a]
[equiv]
LiCl
No LiCl
1
2
3
4
5
Ph
1
1
1
1
À60
À20
0
0
0
1
2
2
2
2
72 (75)
84 (93)
81 (80)
85 (98)
(7)
53
(2)
(27)
o-(MeO)C6H4
1-naphthyl
o-MeC6H4
o-MeC6H4
0.3
97 (quant.) (27)
[a] Yields of isolated product are based on silane. Yields in parentheses
are those determined by 1H NMR analysis using an internal standard
(trichloroethylene).
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Communications
[5] It is interesting to note that metal catalysts still find merit in the
substitution of chloride of chlorosilanes with Grignard reagents,
even though chlorosilanes are far more reactive than silanes.
With Cu: a) A. Shirahata, Tetrahedron Lett. 1989, 30, 6393 –
6394. With Ag: b) K. Murakami, K. Hirano, H. Yorimitsu, K.
Oshima, Angew. Chem. 2008, 120, 5917 – 5919; Angew. Chem.
Int. Ed. 2008, 47, 5833 – 5835. With Zn: c) K. Murakami, H.
Yorimitsu, K. Oshima, J. Org. Chem. 2009, 74, 1415 – 1417. See
also: with Sm: d) Z. Li, X. Cao, G. Lai, J. Liu, Y. Ni, J. Wu, H.
Qiu, J. Organomet. Chem. 2006, 691, 4740 – 4746.
[6] (tert-Alkyl)silanes such as (tBu)2SiH2 and (tBu)Ph2SiH did not
react with benzyl Grignard reagent even under the LiCl catalysis.
[7] Nickel-catalyzed allylation of trialkylsilanes with Grignard
reagent was described in the following literature, which men-
tions 1) that other metals such as Co, Fe, Cu, Zr, and Ti are less
or not active, 2) that benzylation and arylation are not suscep-
tible to the acceleration, and 3) there are no applications starting
with mono- or disubstituted silanes. R. J. P. Corriu, J. P. R.
Massꢀ, B. Meunier, J. Organomet. Chem. 1973, 55, 73 – 84.
[8] In our hands, the acceleration effect of LiCl was not apparent in
the addition of alkyl Grignard reagents such as BuMgBr and
iPrMgCl.
In summary, substitution of silanes with Grignard reagents
is accelerated by the presence of either stoichiometric
amounts or catalytic amounts of LiCl. The new cooperative
effect of lithium and yttrium derived from the combination of
YCl3 and MeLi has led to the one-pot substitution/intra-
molecular hydrosilylation sequence of a silane and o-allylaryl
Grignard reagents.
Experimental Section
=
Preparation of Ph(Me)HSi(CH2CH CH2) with 0.05 equiv LiCl
(Table 1, entry 7): Allylmagnesium chloride (2.0m in THF,
0.500 mL, 1.00 mmol) and
1.00 mmol) were added to
methylphenylsilane (0.139 mL,
suspension of LiCl (2.1 mg,
a
0.050 mmol) in 1.0 mL of THF at room temperature under argon.
After the reaction mixture had been stirred at room temperature for
1 h, the reaction was terminated by the addition of an aqueous
solution of NH4Cl (0.5 mL). The resulting heterogeneous mixture was
filtered through Celite, which was rinsed with diethyl ether. The
organic phase was dried over Na2SO4 and concentrated in vacuo to
give a crude oil, which was chromatographed on silica gel (eluent:
hexanes) to afford the title compound (139 mg, 86%) as a colorless
oil. The product was fully characterized by 1H and 13C NMR, IR, and
elemental analyses.
[9] For an alternative and direct generation of benzyltitanium
reagents of the similar composition, see: a) R. Tanaka, Y.
Nakano, D. Suzuki, H. Urabe, F. Sato, J. Am. Chem. Soc. 2002,
124, 9682 – 9683; b) F. Sato, H. Urabe in Titanium and Zirconium
in Organic Synthesis (Ed.: I. Marek), Wiley-VCH, Weinheim,
2002, pp. 319 – 354. For a review on organotitanium reagents,
see: c) M. T. Reetz in Organometallics in Synthesis (Ed.: M.
Schlosser), Wiley, Chichester, 1994, pp. 195 – 282.
Received: May 26, 2010
Revised: July 26, 2010
Published online: September 6, 2010
[10] For a recently reported Y-mediated reaction, see: R. Tanaka, H.
Sanjiki, H. Urabe, J. Am. Chem. Soc. 2008, 130, 2904 – 2905.
[11] The undesirable transfer of the Me group to the silane could be
suppressed by adjusting the YCl3/MeLi ratio from 1:1 to 1:0.6
and its quantity from stoichiometric to catalytic (0.3 equiv)
amounts.
[12] For yttrium-catalyzed intermolecular hydrosilylation of olefins,
see: a) G. A. Molander, E. D. Dowdy, B. C. Noll, Organometal-
lics 1998, 17, 3754 – 3758; b) G. A. Molander, E. E. Knight, J.
Org. Chem. 1998, 63, 7009 – 7012, and references therein. In
contrast to the better designed Y catalysts utilized in the above
precedents, the simpler YCl3/MeLi combination is not a good
catalyst for intermolecular hydrosilylation of olefins, as an
attempted silylation of 1-decene with phenylsilane with this
catalyst afforded no more than a trace amount of decyl-
(phenyl)silane. This result indicates that the reactions of
Equations (11) and (12) should consist of the first substitution
of silane with Grignard reagent, followed by intramolecular
hydrosilylation, as described in the text.
[13] Whereas intramolecular hydrosilylation of similar (4-pentenyl)-
silanes under Pt, Rh, or Co catalysis preferentially affords 5-
membered products through 5-exo-trig cyclization, the 6-endo-
trig fashion under Y catalysis as can be seen in Equations (11)
and (12) has not been reported; a) H. Sakurai, T. Hirose, A.
Hosomi, J. Organomet. Chem. 1975, 86, 197 – 203; b) J. V.
Swisher, H.-H. Chen, J. Organomet. Chem. 1974, 69, 83 – 91.
For a relevant 5-endo-trig cyclization of (3-butenyl)silane, see:
c) G. A. Molander, C. P. Corrette, Organometallics 1998, 17,
5504 – 5512.
[14] For syntheses and utility of cyclic silanes of this class, see: a) M.
Oestreich, Synlett 2007, 1629 – 1643; b) H. F. T. Klare, M.
Oestreich, Angew. Chem. 2007, 119, 9496 – 9499; Angew.
Chem. Int. Ed. 2007, 46, 9335 – 9338; c) M. Oestreich, U. K.
Schmid, G. Auer, M. Keller, Synthesis 2003, 2725 – 2739; d) H.
Gilman, O. L. Marrs, J. Org. Chem. 1965, 30, 325 – 328; e) H.
Gilman, O. L. Marrs, J. Org. Chem. 1964, 29, 3175 – 3179.
Keywords: lithium · magnesium · silanes · synthetic methods ·
yttrium
.
[1] For preparation and reactions of organosilicon compounds
including benzyl-, allyl-, and arylsilanes, see: a) I. Fleming in
Comprehensive Organic Chemistry, Vol. 3 (Eds.: D. Barton,
W. D. Ollis), Pergamon Press, Oxford, 1979, pp. 541 – 686;
b) E. W. Colvin, Silicon in Organic Synthesis, Butterworth,
London, 1981; c) W. P. Weber, Silicon Reagents for Organic
Synthesis, Springer, Berlin, 1983; d) I. Fleming, J. Dunoguꢀs, R.
Smithers in Organic Reactions, Vol. 37 (Ed.: A. S. Kende), Wiley,
New York, 1989, pp. 57 – 575; e) Chem. Rev. (Ed.: J. Michl),
1995, 95, 1135 – 1673.
[2] a) K. Yamamoto, T. Hayashi in Transition Metals for Organic
Synthesis, Vol. 2 (Eds.: M. Beller, C. Bolm), Wiley-VCH,
Weinheim, 2004, pp. 167 – 181; b) T. Hiyama, T. Kusumoto in
Comprehensive Organic Synthesis, Vol. 8 (Eds.: B. M. Trost, I.
Fleming), Pergamon, Oxford, 1991, pp. 763 – 792.
[3] a) H. Gilman, E. A. Zuech, J. Am. Chem. Soc. 1959, 81, 5925 –
5928; b) H. Gilman, E. A. Zuech, J. Am. Chem. Soc. 1957, 79,
4560 – 4561; c) H. Gilman, H. W. Melvin, Jr., J. Am. Chem. Soc.
1949, 71, 4050 – 4051. For a review on substitution of silanes with
organometallic reagents, see: d) I. Fleming in Comprehensive
Organic Chemistry, Vol. 3 (Eds.: D. Barton, W. D. Ollis),
Pergamon Press, Oxford, 1979, p. 563.
[4] For recent examples of favorable LiCl effects to organometallic
reactions, see: a) L. Gupta, A. C. Hoepker, K. J. Singh, D. B.
Collum, J. Org. Chem. 2009, 74, 2231–2233; b) D. A. Kummer,
W. J. Chain, M. R. Morales, O. Quiroga, A. G. Myers, J. Am.
Chem. Soc. 2008, 130, 13231–13233; c) H. Ren, G. Dunet, P.
Mayer, P. Knochel, J. Am. Chem. Soc. 2007, 129, 5376–5377;
d) M. Hatano, S. Suzuki, K. Ishihara, J. Am. Chem. Soc. 2006, 128,
9998–9999; e) A. Krasovskiy, F. Kopp, P. Knochel, Angew. Chem.
2006, 118, 511–515; Angew. Chem. Int. Ed. 2006, 45, 497–500;
f) A. Krasovskiy, V. Krasovskaya, P. Knochel, Angew. Chem. 2006,
118, 3024–3027; Angew. Chem. Int. Ed. 2006, 45, 2958–2961.
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