Novel phosphonium chloride-catalyzed dehydrohalogenative Si-C coupling reaction of alkyl halides with trichlorosilane [5]

Full text of DOI:10.1021/ja005814u

5584
J. Am. Chem. Soc. 2001, 123, 5584-5585
Table 1. Quaternary Organic Salt-Catalyzed Dehydrochlorinative
Coupling Reaction of Organic Chlorides (2) with 1aa
Novel Phosphonium Chloride-Catalyzed
Dehydrohalogenative Si-C Coupling Reaction of
Alkyl Halides with Trichlorosilane
conditions
temp time
products
2b
catalyst
Et3P
Bu3P
(°C)
(h)
(%)c 3
Yeon Seok Cho, Seung-Hyun Kang, Joon Soo Han,
Bok Ryul Yoo, and Il Nam Jung*
entry
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
2a (-)
2a (-)
150
150
150
150
2
2
2
6
2
3a (88), otherd,e
3a (88), otherd,f
3a (12), othersd,g
3a (11), othersd,h
3a (95)
Organosilicon Chemistry Laboratory
Korea Institute of Science & Technology
P.O. Box 131, Cheongryang, Seoul 130-650, Korea
2a (63) Ph3P
2a (68) tBu3P
2a (-)
2a (-)
2b (-)
2c (-)
2d (-)
2e (4)
2f (-)
2g (1)
2h (-)
2i (-)
2j (1)
(PhCH2)Bu3PCl 150
Bu4PCl
Bu4PCl
Bu4PCl
Bu4PCl
Bu4PCl
Bu4PCl
Bu4PCl
Bu4PCl
Bu4PCl
Bu4PCl
Bu4PCl
130
130
130
130
150
170
170
170
170
150
150
4
4
4
2
2
2
2
4
6
4
8
15
15
15
6
3a (95)
3b (93)
ReceiVed NoVember 22, 2000
ReVised Manuscript ReceiVed April 6, 2001
3c (96)
3d (72), otheri
3e (79)
Si-C bond forming reactions such as the direct synthesis,1
hydrosilylation,2 and general organometallic reactions3 are im-
portant methods for the preparation of various organosilicon
compounds. Another established Si-C bond forming reaction is
the coupling of activated organic chlorides with trichlorosilane
(1a) commonly known as the Benkeser reaction.4 Although this
reaction generally proceeds in good yields, the reaction conditions
require stoichiometric amounts of amine as an HCl scavenger.
Furthermore, this reaction cannot be applied to unactivated alkyl
chlorides such as butyl chloride,4b and all attempts to utilize
methyldichlorosilane (1b) in place of 1a as a coupling agent have
been unsuccessful. In this report we wish to communicate our
discovery of a novel high-yield approach to this reaction that
requires only catalytic amounts of phosphonium salt in place of
stoichiometric amounts of amine (eq 1). Moreover, the reaction
3f (94)
3g (94)
3h (92)
3i (93)
3j (95)
3k (91)
2k (4)
2a (-)
(PhCH2)Et3NCl 150
2b (11) (PhCH2)Et3NCl 150
3a (47), otherj
3b (49), otherk
3c (54), otherl
3f (-)
2c (6)
2f (100) Et4NCl
(PhCH2)Et3NCl 150
200
a The reactions were carried out using a 30:10:1 mole ratio of 1a to
2 to catalyst. b Unreacted 2 in parentheses. c The isolated yields unless
otherwise noted. d Yields determined by GLC using n-dodecane as an
internal standard. e (PhCH2)Et3PCl (10%) was obtained. (PhCH2)Bu3PCl
f
(10%). g (PhCH2)Ph3PCl (10%) and toluene (20%). h (PhCH2)tBu3PCl
(10%) and toluene (20%) as byproducts. i Cl3Si(CH2)3SiCl3 (4%).
j Toluene (50%). k p-Fluorotoluene (29%). l p-Methoxytoluene (33%).
(dichloromethyl)silanes for these reactions,7 we investigated the
selective reduction of polychlorinated methylsilanes with 1a in
the presence of catalytic amounts of group 10 transition metal
compounds.8 In our studies, we found that small amounts of
dehydrochlorinative Si-C coupling products of (chloromethyl)-
silane were obtained as byproduct when triphenylphosphine
(TPP)-metal complexes were used as catalysts.9 Speculating that
dissociated TPP was involved in this coupling reaction, we reacted
benzyl chloride (2a) with 1a at 150 °C in the presence of 10 mol
% of various triorganophosphines (Table 1, entries 1-4). Excess
amounts of 1a were employed to favor the formation of products.
As shown in Table 1, reaction of 1a with 2a in the presence
of triethylphosphine or tributylphosphine at 150 °C for 2 h (entries
1 and 2) gave in both cases the coupling product 3a in 88%
isolated yield and 10% of the corresponding benzyltrialkylphos-
phonium chlorides, a result of the coupling of the trialkylphos-
phines with 2a.10 However, 3a was obtained in only 12 or 11%
yields with toluene as the major product when TPP or tri(tert-
butyl)phosphine was employed, respectively (entries 3 and 4). It
works effectively with unactivated chlorides and can further be
extended to employ 1b in the coupling step. As a whole, this
chemistry will now enable the efficient, high-yield synthesis of a
wide range of functionalized organosilicon compounds that were
previously unavailable by simple means.
Recently, we have reported the successful extension of Roch-
ow’s direct synthesis reaction to include a variety of organic5
and organosilyl6 chloride compounds in place of chloromethane.
To prepare starting materials such as (chloromethyl)silanes and
(1) (a) Rochow, E. G. J. Am. Chem. Soc. 1945, 67, 963-965. (b) Rochow,
E. Chemistry of Silicones; Wiley: New York, 1951. (c) Petrov, A. D.; Mironov,
B. F.; Ponomarenko, V. A.; Chernyshev, E. A. Synthesis of Organosilicon
Monomers; Consultants Bureau: New York, 1964. (d) Lewis, K. M.;
Rethwisch, D. G., Eds. Catalyzed Direct Reactions of Silicon; Elsevier:
Amsterdam, 1993.
(2) (a) Lukevitts, E. Y.; Voronkov, M. G. Organic Insertion Reactions of
Group IV Elements; Consultants Bureau: New York, 1966. (b) Marciniec,
B., Ed. ComprehensiVe Handbook of Hydrosilylation; Pergamon Press: New
York, 1992; references therein. (c) Song, Y. S.; Yoo, B. R.; Lee, G.-H.; Jung,
I. N. Organometallics 1999, 18, 3109-3115.
(3) (a) Colvin, E. W. Silicon in Organic Synthesis; Butterworth: London,
1981. (b) Weber, W. P. Silicon Reagents for Organic Synthesis; Springer-
Verlag: New York, 1983 (c) Brook, M. A. Silicon in Organic, Organometallic,
and Polymer; Wiley: New York, 2000.
(4) (a) Benkeser, R. A.; Smith, W. E. J. Am. Chem. Soc. 1968, 90, 5307-
5309. (b) Benkeser, R. A.; Gaul, J. M.; Smith, W. E. J. Am. Chem. Soc. 1969,
91, 3666-3667. (c) Furuya, N.; Sukawa, T. J. Organomet. Chem. 1975, 96,
C1-C3. (d) Corriu, R. J. P.; Granier, M.; Lanneau, G. F. J. Organomet. Chem.
1998, 562, 79-88.
(5) (a) Yeon, S. H.; Lee, B. W.; Kim, S.-I.; Jung, I. N. Organometallics
1993, 12, 4887-4891. (b) Yeon, S. H.; Han, J. S.; Yoo, B. R.; Jung, I. N. J.
Organomet. Chem. 1996, 516, 91-95. (c) Han, J. S.; Yeon, S. H.; Yoo, B.
R.; Jung, I. N. Organometallics 1997, 16, 93-96.
(6) (a) Jung, I. N.; Yeon, S. H.; Han, J. S. Organometallics 1993, 12, 2360-
2362. (b) Lee, C. Y.; Han, J. S.; Oh, H. S.; Yoo, B. R.; Jung, I. N. Bull.
Korean Chem. Soc. 2000, 21, 1020-1024.
(7) (a) Krieble, R. H.; Elliott, J. R. J. Am. Chem. Soc. 1947, 67, 1810-
1812. (b) Runge, F.; Zimmermann, W. Chem. Ber. 1954, 87, 282-287; Chem.
Abstr. 1955, 49, 6088a.
(8) Cho, Y. S.; Han, J. S.; Yoo, B. R.; Kang, S. O.; Jung, I. N.
Organometallics 1998, 17, 570-573.
(9) The 1:5 reaction of (trimethylsilyl)methyl chloride (2j) with 1 in the
presence of tetrakis(triphenylphosphine)palladium under the same conditions
of the reductive hydrodechlorination previously reported8 gave [(trimethylsilyl)-
methyl]trichlorosilane (3j) in 1% yield, based on 2j used, as a byproduct. In
a later study we found that (silylmethyl)triphenylphosphonium chloride, formed
from the reaction of 2j with triphenylphosphine dissociated from the palladium
complex, catalyzed this reaction, even though the coupling product 3j was
obtained in low yield.
(10) (a) Arbuzov, B. A. Pure Appl. Chem. 1964, 9, 307-335 (b) Barton,
S. D.; Ollis, W. D., Eds. ComprehensiVe Organic Chemistry; Pergamon
Press: New York, 1979; Vol. 2, p 1127.
10.1021/ja005814u CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/21/2001

Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 23, 2001 5585
is not presently clear whether toluene was produced by a direct
reduction of 2a by 1a or by the desilylation of 3a by HCl.11
However, these reactions at least demonstrate that sterically bulky
catalysts are less active and enhance the reduction or desilylation
reaction. The presence of the phosphonium salts in the reactions
also suggested to us that the trialkylphosphines might not be the
catalysts but rather the phosphonium salts. Accordingly, the same
coupling reactions using tetrabutylphosphonium chloride (TBPC)
or benzyltributylphosphonium chloride as a catalyst gave 3a in
near quantitative yields within 4 h (entries 5 and 6), confirming
that the quaternary phosphonium chlorides were indeed the
catalysts for the Si-C coupling reaction.
Scheme 1
Once we had determined that phosphonium salts were the
catalysts for this reaction, we examined the reactions of various
organic chlorides with 1a.12 The reactions of para-substituted
benzyl chlorides 2a-c at the slightly lower temperature of 130
°C gave products 3a-c in 93-95% isolated yields (entries 6-8).
Allyl chloride (2d) reacted with 1a to give 3d in moderate yield
(72%) along with 1,3-bis(trichlorosilyl)propane (4, 4%) as the
byproduct (entry 9). 4 was likely obtained by the hydrosilylation
of 3d with 1a.13 The reaction with crotyl chloride (2e) at 130 °C
gave 3e in good yield (79%) without any hydrosilylation product
(entry 10). Of particular significance is the extension of the
coupling reaction to unactivated n-alkyl chlorides; simple chlo-
rides [CnH2n+1Cl: n ) 6 (2f), 8 (2g), 12 (2h), 18 (2i)] and
ω-(trimethylsilyl)alkyl chlorides [alkyl ) methyl (2j), propyl
(2k)] were reacted with 1a at 170 °C. In 2-6 h reactions, products
3f-i were obtained in excellent yields (92-94%) (entries 11-
14), and in 4-8 h reactions, products 3j and 3k were obtained in
91 and 95% yields, respectively. We found that in general the
coupling reactivity of the n-alkyl chlorides decreased as the chain
length of the alkyl group increased. In the case of ω-(trimethyl-
silyl)alkyl chlorides, the reactivity of 2j at 150 °C was higher
than that of 2k, possibly indicating that the electron-donating
effect of the silyl group facilitated the Si-C coupling reaction.
Bromoorganic compounds could also be utilized in this coupling
reaction, although side products were more evident. As an
example, 1a was reacted with benzyl bromide for 10 h at 130 °C
in the presence of TBPC catalyst. The coupling product 3a was
obtained in 63% yield with a 37% recovery of toluene as a
byproduct. Compounds such as bromotrichlorosilane and bro-
modichlorosilane, resulting from bromine-chlorine exchange,
were also obtained in this reaction.
Compared to the phosphonium salts, quaternary organoammo-
nium chlorides were found to be less effective catalysts for the
coupling reaction. Reactions of benzyl chlorides 2a-c with 1a
in the presence of 10% benzyltriethylammonium chloride using
the phosphonium salts-catalyzed reaction conditions gave products
3a, 3b, and 3c in 47, 49, and 54% yields and the corresponding
methylbenzene compounds in 50, 29, and 33% yields at 150 °C
for 15 h, respectively (entries 17-19). Reactions with nonacti-
vated alkyl chlorides 2f-k using quaternary ammonium chloride
catalysts gave no coupling products up to a reaction temperature
of 200 °C but instead gave the reductive dealkylation products
of quaternary ammonium chloride (entry 20).
Although requiring slightly higher temperatures and reaction
times, reactions with 1b in place of 1a give the corresponding
coupled products in respectable yields. In the presence of TBPC
catalyst, reaction of 1b with 2a for 10 h at 130 °C gave benzyl-
(methyl)dichlorosilane in 50% yield and 50% toluene as a
byproduct. In a similar reaction with 2f for 12 h at 200 °C, 91%
of 2f was consumed to give hexyl(methyl)dichlorosilane (66%)
with smaller amounts of unidentified products.
The mechanism proposed by Benkeser4b is not appropriate for
our system. On the basis of our results, we propose a plausible
mechanism for the TBPC-catalyzed dehydrohalogenative Si-C
coupling reaction as shown in Scheme 1. In this mechanism, the
TBPC interacts with 1a to form a pentacoordinated hydrido-
tetrachlorosilane anion I14 which loses hydrogen chloride upon
heating to give the tetraalkylphosphonium cation/trichlorosilyl
anion pair II. Intermediate II subsequently attacks the alkyl
chloride to produce the Si-C coupling product and regenerate
the TBPC catalyst.15,16
In conclusion, we describe a novel catalytic approach to the
coupling of both activated and nonactivated alkyl halides with
both trichlorosilane and methyldichlorosilane. A wide variety of
silyl-functionalized compounds may now be prepared easily and
in good yields. The extension of the coupling reaction to other
alkyl halides such as secondary and tertiary alkyl halides is
currently being investigated.
(11) The protodesilylation of activated alkylsilanes such as allylsilanes and
benzylsilanes is well-known. (a) Jenkins, P. R.; Gut, R.; Wetter, H.;
Eschenmoser, A. HelV. Chim. Acta 1979, 62, 1922-1931. (b) Fleming, I.;
Marchi, D.; Patel, K. J. Chem. Soc., Perkin Trans. 1 1981, 2518-2519. (c)
Yeon, S. H.; Lee, B. W.; Yoo, B. R.; Suk, M.-Y.; Jung, I. N. Organometallics
1995, 14, 2361-2365. (d) Yoo, B. R.; Kim, J. H.; Lee, H.-J.; Lee, K.-B.;
Jung, I. N. J. Organomet. Chem. 2000, 605, 239-245.
(12) As a representative reaction, the reaction of 2a with 1 is described as
follows: A 25 mL dried stainless steel bomb equipped with a valve was
charged with TBPC (0.22 g, 0.75 mmol), 2a (0.95 g, 7.5 mmol), and 1 (3.41
g, 25.2 mmol) under a dry nitrogen atmosphere. After the valve was closed,
the reactor was kept in an oil bath at 130 °C for 4 h. The reaction mixture
was fractionally distilled to give 3a (1.61 g, 7.1 mmol) in 95% yield. TBPC
remained as a residue in the bomb after distillation, as confirmed by the
analysis of NMR data.
(13) The Bu4PCl-catalyzed hydrosilylation of 3d with 1 at 130 °C for 2 h
gave product 4 in low yield (2%).
(14) See reviews for pentacoordinated organosilicon intermediates: (a)
Prakash, G. K. S.; Yudin, A. K. Chem. ReV. 1997, 97, 757-786. (b) Corriu,
R. J.; Colin, Y. J. Chemistry of Organosilicon Compounds, Patai, S.,
Rapporport, Z., Eds.; Wiley & Sons: New York; 1989; Vol. 2, p 1241.
(15) (a) Tandura, S. N.; Voronkov, M. G.; Alekseev, N. V. Top. Curr.
Chem. 1986, 131, 99-189. (b) Patai, S.; Rapporport, Z., Eds. The Chemistry
of Organosilicon Compounds; Wiley & Sons: New York, 1989; p 279.
(16) (a) Nozakura, S.; Konetsune, S. Bull. Chem. Soc. Jpn. 1956, 29, 332-
326. (b) Nozakura, S. Bull. Chem. Soc. Jpn. 1956, 29, 660-663. (c) Nozakura,
S. Bull. Chem. Soc. Jpn. 1956, 29, 784-789.
Acknowledgment. This research was supported financially by the
Ministry of Science and Technology of Korea (National Research
Laboratory Project). We thank Prof. D. Son of Southern Methodist
University, Dallas, Texas, for his help and discussions in the preparation
of this paper.
JA005814U