Metal-catalyzed hydrosilylation of alkenes and alkynes using dimethyl(pyridyl)silane

Article abstract of DOI:10.1021/jo0163389

Metal-catalyzed hydrosilylation of alkenes and alkynes using dimethyl(pyridyl)silane is described. The hydrosilylation of alkenes using dimethyl(2-pyridyl)silane (2-PyMe₂SiH) proceeded well in the presence of a catalytic amount of RhCl(PPh

Full text of DOI:10.1021/jo0163389

J . Org. Chem. 2002, 67, 2645-2652
2645
Meta l-Ca ta lyzed Hyd r osilyla tion of Alk en es a n d Alk yn es Usin g
Dim eth yl(p yr id yl)sila n e
Kenichiro Itami, Koichi Mitsudo, Akira Nishino, and J un-ichi Yoshida*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Yoshida, Kyoto 606-8501, J apan
yoshida@sbchem.kyoto-u.ac.jp
Received November 30, 2001
Metal-catalyzed hydrosilylation of alkenes and alkynes using dimethyl(pyridyl)silane is described.
The hydrosilylation of alkenes using dimethyl(2-pyridyl)silane (2-PyMe2SiH) proceeded well in the
presence of a catalytic amount of RhCl(PPh3)3 with virtually complete regioselectivity. By taking
advantage of the phase tag property of the 2-PyMe2Si group, hydrosilylation products were isolated
in greater than 95% purity by simple acid-base extraction. Strategic catalyst recovery was also
demonstrated. The hydrosilylation of alkynes using 2-PyMe2SiH proceeded with a Pt(CH2d
CHSiMe2)2O/P(t-Bu)3 catalyst to give alkenyldimethyl(2-pyridyl)silanes in good yield with high
regioselectivity. A reactivity comparison of 2-PyMe2SiH with other related hydrosilanes (3-PyMe2-
SiH, 4-PyMe2SiH, and PhMe2SiH) was also performed. In the rhodium-catalyzed reaction, the
reactivity order of hydrosilane was 2-PyMe2SiH . 3-PyMe2SiH, 4-PyMe2SiH, PhMe2SiH, indicating
a huge rate acceleration with 2-PyMe2SiH. In the platinum-catalyzed reaction, the reactivity order
of hydrosilane was PhMe2SiH, 3-PyMe2SiH . 4-PyMe2SiH > 2-PyMe2SiH, indicating a rate
deceleration with 2-PyMe2SiH and 4-PyMe2SiH. It seems that these reactivity differences stem
primarily from the governance of two different mechanisms (Chalk-Harrod and modified Chalk-
Harrod mechanisms). From the observed reactivity order, coordination and electronic effects of
dimethyl(pyridyl)silanes have been implicated.
In tr od u ction
ment of the metal-catalyzed cyclization/hydrosilylation
process stems from the ability to assemble complex
molecules from simple starting materials in a convergent
and atom-economical manner.4 Related carbonylative
silylation reactions have also been developed.5 Other
seminal advances have been realized in the development
of the general protocol for highly enantioselective hy-
drosilylation of alkenes, which has been an enduring
problem for nearly 30 years.6
The value of this hydrosilylation has been further
augmented by several protocols for converting the silyl
group to other functional groups.7 For example, certain
silyl groups can be oxidatively converted to a hydroxyl
group by hydrogen peroxide or peracids.8 In recent years,
the strategic use of the hydrosilylation/oxidation se-
quence has been well recognized as a powerful method
for the diastereo- and enantioselective synthesis of vari-
ous structurally diverse alcohols from structurally simple
starting materials.9
The transition metal-catalyzed hydrosilylation of car-
bon-carbon unsaturated molecules has proved to be an
extremely valuable tool for the carbon-silicon bond
formation by virtue of its high regio- and stereoselectivi-
ties.1 There has been extraordinary progress in this
hydrosilylation chemistry during the last two decades.
For example, various types of cyclization/hydrosilylation
are now viable when diene, enyne, and diyne are used
as substrates.2 Alternatively, intramolecular hydrosilyl-
ation of alkene can be used for the preparation of cyclic
compounds with control of the relative and/or absolute
stereochemistry.3 The growing interest in the develop-
(1) (a) Marciniec, B. Comprehensive Handbook on Hydrosilylation;
Pergamon Press: Oxford, 1992. (b) Hiyama, T.; Kusumoto, T. In
Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon Press: Oxford, 1991; Vol. 8, p 763. (c) Ojima, I. In The
Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z.,
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1992, 114, 6580. (d) Onozawa, S.; Sakakura, T.; Tanaka, M. Tetrahe-
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D. F. J . Am. Chem. Soc. 1998, 120, 6690. (h) Muraoka, T.; Matsuda,
I.; Itoh, K. Tetrahedron Lett. 1998, 39, 7325. (i) Widenhoefer, R. A.;
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Ito, Y. J . Am. Chem. Soc. 1986, 108, 6090. (b) Tamao, K.; Nakagawa,
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Lett. 1990, 31, 7333. (e) Bergens, S. H.; Noheda, P.; Whelan, J .;
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tallics 1991, 10, 38. (c) Chatani, N.; Fukumoto, Y.; Ida, T.; Murai, S.
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10.1021/jo0163389 CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/16/2002

2646 J . Org. Chem., Vol. 67, No. 8, 2002
Itami et al.
Sch em e 1
Ta ble 1. Effect of Solven t in th e Rh od iu m -Ca ta lyzed
Hyd r osilyla tion of 1-Octen e w ith 2-P yMe2SiHa
Although there are many factors governing the hy-
drosilylation, most of the works in this area were devoted
to a catalyst tuning for controlling the reactivity and
selectivity of the reaction.10 We envisioned that control
of the reactivity and selectivity might be achieved by
appending the secondary interaction moiety on the hy-
drosilane scaffold. During the course of a program
directed at the development of removable directing
groups, which control the metal-mediated and -catalyzed
processes by complex-induced proximity effects,11 we have
found that a dimethyl(2-pyridyl)silyl (2-PyMe2Si) group
works well for such a purpose by taking advantage of the
coordination of the pyridyl group to the metal.12-24 This
prompted us to investigate the behavior of dimethyl(2-
pyridyl)silane (2-PyMe2SiH) in the transition metal-
catalyzed hydrosilylation of alkene and alkyne. Since we
have already established that the resulting alkyl- and
alkenyl(2-pyridyl)silanes can be subjected to further
transformations such as oxidation,13,14,18-21 electrophilic
substitution,15,19 Hiyama-type cross-coupling reaction,17,23
carbomagnesation,20 and Heck-type reaction,16,23 an ef-
entry
solvent
toluene
MeOH
DMF
CH2Cl2
ClCH2CH2Cl
THF
CH3CN
CH3CH2CN
yield (%)b
1
2
3
4
5
6
7
8
50
0
42
71
78
82
88
84
a
All reactions were performed at room temperature for 30 min
under argon using 2-PyMe2SiH (0.5 mmol), 1-octene (1.5 mmol),
and RhCl(PPh3)3 (5 mol %) in the indicated solvent (1 mL).
Determined by capillary GC analysis using pentadecane as an
b
internal standard.
ficient procedure for the hydrosilylation of 2-PyMe2SiH
would be extremely worthwhile (Scheme 1). In this paper,
we report the full details of this study.
(9) (a) Uozumi, Y.; Hayashi, T. J . Am. Chem. Soc. 1991, 113, 9887.
(b) Fleming, I.; Barbero, A.; Walter, D. Chem. Rev. 1997, 97, 2063 and
references therein.
(10) For example: Hayashi, T.; Hirate, S.; Kitayama, K.; Tsuji, H.;
Torii, A.; Uozumi, Y. J . Org. Chem. 2001, 66, 1441.
(11) (a) Beak, P.; Meyers, A. I. Acc. Chem. Res. 1986, 19, 356. (b)
Snieckus, V. Chem. Rev. 1990, 90, 879. (c) Beak, P.; Basu, A.;
Gallagher, D. J .; Park, Y. S.; Thayumanavan, S. Acc. Chem. Res. 1996,
29, 552.
(12) Yoshida, J .; Itami, K.; Mitsudo, K.; Suga, S. Tetrahedron Lett.
1999, 40, 3403.
(13) Itami, K.; Mitsudo, K.; Yoshida, J . J . Org. Chem. 1999, 64, 8709.
(14) (a) Itami, K.; Mitsudo, K.; Yoshida, J . Tetrahedron Lett. 1999,
40, 5533. (b) Itami, K.; Mitsudo, K.; Yoshida, J . Tetrahedron Lett. 1999,
40, 5537. (c) Itami, K.; Kamei, T.; Mitsudo, K.; Nokami, T.; Yoshida,
J . J . Org. Chem. 2001, 66, 3970.
In addition, we have also found that the behavior of
2-PyMe2SiH is quite different from those of other related
hydrosilanes such as 3-PyMe2SiH, 4-PyMe2SiH, and
PhMe2SiH.25 The observation that the rate of hydrosilyl-
ation depends heavily on the choice of catalyst metal and
hydrosilane implies the unusual mechanism-dependent
acceleration and deceleration. These results are also
discussed in this paper.
Resu lts a n d Discu ssion
(15) (a) Itami, K.; Nokami, T.; Yoshida, J . Org. Lett. 2000, 2, 1299.
(b) Itami, K.; Nokami, T.; Yoshida, J . Tetrahedron 2001, 57, 5045.
(16) Itami, K.; Mitsudo, K.; Kamei, T.; Koike, T.; Nokami, T.;
Yoshida, J . J . Am. Chem. Soc. 2000, 122, 12013.
(17) Itami, K.; Nokami, T.; Yoshida, J . J . Am. Chem. Soc. 2001, 123,
5600.
(18) Itami, K.; Koike, T.; Yoshida, J . J . Am. Chem. Soc. 2001, 123,
6957.
(19) Itami, K.; Kamei, T.; Yoshida, J . J . Am. Chem. Soc. 2001, 123,
8773.
(20) Itami, K.; Mitsudo, K.; Yoshida, J . Angew. Chem., Int. Ed. 2001,
40, 2337.
(21) Itami, K.; Nokami, T.; Yoshida, J . Angew. Chem., Int. Ed. 2001,
40, 1074.
(22) Yoshida, J .; Itami, K. J . Synth. Org. Chem. J pn. 2001, 59, 1086.
(23) Itami, K.; Nokami, T.; Ishimura, Y.; Mitsudo, K.; Kamei, T.;
Yoshida, J . J . Am. Chem. Soc. 2001, 123, 11577.
(24) Itami, K.; Mitsudo, K.; Nokami, T.; Kamei, T.; Koike, T.;
Yoshida, J . J . Organomet. Chem., in press.
I. Ca t a lyt ic H yd r osilyla t ion of Alk en es Usin g
2-P yMe2SiH. 1. Rea ction Con d ition s. Preliminary
screening of the catalysts, including palladium(0), plati-
num(0), and rhodium(I) complexes, revealed that RhCl-
(PPh3)3 was most effective in terms of the catalytic
activity and regioselectivity for the hydrosilylation of
alkenes using 2-PyMe2SiH (1). For example, 1-octene can
be hydrosilylated at room temperature in toluene to
furnish 2a in 50% yield (Table 1, entry 1). With this
catalyst in hand, we have examined the effect of solvent
in the reaction using 1-octene as a model substrate (Table
(25) Itami, K.; Mitsudo, K.; Nishino, A.; Yoshida, J . Chem. Lett.
2001, 1088.

Metal-Catalyzed Hydrosilylation of Alkenes and Alkynes
J . Org. Chem., Vol. 67, No. 8, 2002 2647
Ta ble 2. Rh od iu m -Ca ta lyzed Hyd r osilyla tion of Alk en es w ith 2-P yMe2SiHa
a
All reactions were performed at room temperature under argon using 2-PyMe2SiH (0.5 mmol), alkene (1.5 mmol), and RhCl(PPh3)3
(5 mol %) in CH3CN (1 mL). Isolated yield. c Determined by GC and 1H NMR analysis.
b
1). The reaction proceeded with all the solvents examined
except MeOH (entry 2). Interestingly, the reaction was
completed within 30 min, regardless of the solvent
employed. It was found that the use of CH3CN resulted
in 88% yield of 2a with virtually complete regioselectivity
(entry 7).
of the 2-PyMe2Si group, all hydrosilylation products listed
in Table 2 were isolated by the simple acid-base “phase-
switching” technique. This phase tag property of the
2-PyMe2Si group enables the use of excess alkene (3 equiv
to 1 in this study) without having any difficulty at the
purification stage. Importantly, the purities of the prod-
ucts are over 95% in all cases examined. This easy
separation/purification protocol without using any chro-
matographic isolation technique is notable, and such
strategic purification in solution-phase synthesis should
become more and more important as an alternative to
the solid-phase synthesis.29
Moreover, it was possible to recover the catalyst from
the initial organic phase in the acid-base extraction
purification process as described in the previous paper,16
and the rhodium catalyst recovered in this manner could
be used in the second, third, and forth runs in the
hydrosilylation of 1 with 1-octene. When the reaction was
carried out at room temperature for 1 h, 2a was obtained
in 86, 67, 59, and 50% yields, respectively. Although
slight eroding of the catalytic activity was observed, these
demonstrations indicate that the recovery of a metal
catalyst does not necessarily have to rely on the polymer-
supported catalyst and clearly pave the way for the
strategic catalyst recovery in solution-phase synthesis.
2. Hyd r osilyla tion w ith Va r iou s Alk en es. Under
the standard set of reaction conditions (5 mol % of RhCl-
(PPh3)3 at room temperature in CH3CN), the hydrosilyl-
ation using 2-PyMe2SiH occurred with various terminal
alkenes (Table 2). It is possible to selectively hydrosilylate
a terminal carbon-carbon double bond in the presence
of an internal carbon-carbon double bond (entry 4). The
reaction proceeded well without affecting the other
functional groups such as ester and nitrile (entries 5 and
6). Unfortunately, internal alkene (2-octene, cyclohexene,
norbornene), 1,1-disubstituted alkene (methylenecyclo-
pentane), 1,3-diene (2-methyl-1,3-butadiene, 1,3-cyclo-
hexadiene), and heteroatom-substituted alkene (butyl
vinyl ether, 2,3-dihydrofuran, phenyl vinyl sulfide) were
not applicable to the reaction.
3. Str a tegic Sep a r a tion a n d Ca ta lyst Reu se Uti-
lizin g th e Acid -Ba se P h a se-Sw itch in g Tech n iqu e.
As we recently reported, the additional bonus of using a
2-PyMe2Si group is that it can also be utilized as a “phase
tag”,26-28 which enables easy purification of the prod-
uct.12,16,22 By taking advantage of the phase tag property
II. Ca t a lyt ic H yd r osilyla t ion of Alk yn es Usin g
2-P yMe2SiH. Having established the general protocol for
the hydrosilylation of alkenes using 2-PyMe2SiH (1), we
next examined the hydrosilylation of alkynes. The use
of RhCl(PPh3)3 resulted in a rapid consumption of 1 but
gave a complex mixture of unidentified products. Since
changing the solvent was found to be fruitless, catalyst
(26) Curran, D. P. Angew. Chem., Int. Ed. 1998, 37, 1174.
(27) Borman, S. Chem. Eng. News 2001, 79 (J une 4), 49.
(28) (a) Ramage, R.; Swenson, H. R.; Shaw, K. T. Tetrahedron Lett.
1998, 39, 8715. (b) Hay, A. M.; Hobbs-Dewitt, S.; MacDonald, A. A.;
Ramage, R. Tetrahedron Lett. 1998, 39, 8721. (c) Zhang, S.; Fukase,
K.; Kusumoto, S. Tetrahedron Lett. 1999, 40, 7479. (d) Wang, X.;
Parlow, J . J .; Proco, J . A., J r. Org. Lett. 2000, 2, 3509. (e) Ley, S. V.;
Massi, A.; Rodr´ıguez, F.; Horwell, D. C.; Lewthwaite, R. A.; Pritchard,
M. C.; Reid, A. M. Angew. Chem., Int. Ed. 2001, 40, 1053. (f) Bosanac,
T.; Yang, J .; Wilcox, C. S. Angew. Chem., Int. Ed. 2001, 40, 1875.
(29) (a) Do¨rwald, F. Z. Organic Synthesis on Solid Support; VCH:
Weinheim, Germany, 2000. (b) Terrett, N. K. Combinatorial Chemistry;
Oxford University Press: Oxford, 1998.

2648 J . Org. Chem., Vol. 67, No. 8, 2002
Itami et al.
Ta ble 3. P la tin u m -Ca ta lyzed Hyd r osilyla tion of
1-Octyn e w ith 2-P yMe2SiHa
III. Rea ctivity Com p a r ison of 2-P yMe2SiH w ith
Oth er Rela ted Hyd r osila n es. 1. Rh od iu m -Ca ta lyzed
Hydrosilylation of1-Octene with 2-P yMe2SiH,3-P yMe2-
SiH, 4-P yMe2SiH, a n d P h Me2SiH. Having observed
the high reactivity of 2-PyMe2SiH (1) in alkene hydrosi-
lylation, we subjected other structurally and electroni-
cally related hydrosilanes (3-PyMe2SiH, 4-PyMe2SiH, and
PhMe2SiH) to the rhodium-catalyzed alkene hydrosilyl-
ation to assess the reactivity value. Thus, we examined
the hydrosilylation of 1-octene with 2-PyMe2SiH, 3-PyMe2-
SiH, 4-PyMe2SiH, and PhMe2SiH in the presence of
RhCl(PPh3)3 (5 mol %) in CH3CN. The mixture was
stirred at 28 °C for from 15 min to 2 h, and the yield of
hydrosilylated product was determined by GC analysis
(Figure 1). The hydrosilylation using 2-PyMe2SiH was
completed within 30 min. On the other hand, hydrosilyl-
ation was extremely slow when using 3-PyMe2SiH,
4-PyMe2SiH, or PhMe2SiH.
2. Rh od iu m -Ca ta lyzed Hyd r osilyla tion of Vin yl-
t r im et h ylsila n e w it h 2-P yMe2SiH , 3-P yMe2SiH ,
4-P yMe2SiH, a n d P h Me2SiH. To see if the observed
reactivity trend (2-PyMe2SiH . 3-PyMe2SiH, 4-PyMe2-
SiH, PhMe2SiH) was general or not with regard to
alkene, we next investigated the use of vinyltrimethyl-
silane (Figure 2). Whereas the hydrosilylation using
2-PyMe2SiH was completed within 30 min to afford the
hydrosilylated product 2c in greater than 90% yield,
hydrosilylation using 3-PyMe2SiH, 4-PyMe2SiH, and
PhMe2SiH was extremely sluggish under identical condi-
tions. Thus, the reactivity trend (2-PyMe2SiH . 3-PyMe2-
SiH, 4-PyMe2SiH, PhMe2SiH) seems to be a universal
phenomenon for the rhodium-catalyzed alkene hydrosi-
lylation.
3. P latin u m -Catalyzed Hydr osilylation of 1-Octen e
w it h 2-P yMe2SiH , 3-P yMe2SiH , 4-P yMe2SiH , a n d
P h Me2SiH. Having established the unusual reactivity
trend in the rhodium-catalyzed alkene hydrosilylation,
we next examined platinum-catalyzed alkene hydrosilyl-
ation using 2-PyMe2SiH, 3-PyMe2SiH, 4-PyMe2SiH, and
PhMe2SiH. First, we examined the hydrosilylation of
1-octene with 2-PyMe2SiH, 3-PyMe2SiH, 4-PyMe2SiH,
and PhMe2SiH in the presence of Pt[(CH2dCHSiMe2)2O]
(1 mol %) and PPh3 (1 mol %) in EtCN. The mixture was
stirred at 80 °C for from 30 min to 2 h, and the yield of
hydrosilylated product was determined by GC analysis
(Figure 3). Quite interestingly, the reactivity trend was
completely different from that observed in the rhodium-
catalyzed reaction (Figure 1). Whereas the hydrosilyla-
tion using 3-PyMe2SiH and PhMe2SiH proceeded smooth-
ly, the hydrosilylation was much slower when using
2-PyMe2SiH and 4-PyMe2SiH (Figure 3).
4. P la tin u m -Ca ta lyzed Hyd r osilyla tion of Vin yl-
t r im et h ylsila n e w it h 2-P yMe2SiH , 3-P yMe2SiH ,
4-P yMe2SiH, a n d P h Me2SiH. To investigate in more
depth the reactivity difference of hydrosilanes in the
platinum-catalyzed alkene hydrosilylation, we next in-
vestigated the use of vinyltrimethylsilane (Figure 4). It
was found that the reaction using 4-PyMe2SiH was
slower than that using 3-PyMe2SiH or PhMe2SiH but
sufficiently faster than that using 2-PyMe2SiH. Exhibi-
tion of the reactivity difference between 2-PyMe2SiH and
4-PyMe2SiH may be due to the higher reactivity of
vinyltrimethylsilane toward hydrosilylation.
entry Pt (mol %) additive (mol %) conditions yield (%)b 3a :4a b
1
2
3
4
5
6
7
8
5
5
5
5
5
5
1
1
none
PPh3 (10)
P(C6H4F-p)3 (10) 40 °C, 24 h
PPh3 (5)
PPh3 (5)
PPh3 (5)
PPh3 (1)
P(t-Bu)3 (1)
rt, 24 h
40 °C, 24 h
16
62
61
70
86
92
89
89
84:16
86:14
85:15
85:15
84:16
84:16
99:1
40 °C, 24 h
70 °C, 24 h
100 °C, 1 h
100 °C, 3.5 h
100 °C, 15 h
a
All reactions were performed under argon using 2-PyMe2SiH
(0.5 mmol), 1-octyne (1.5 mmol), Pt(CH2dCHSiMe2)2O, and addi-
tive in toluene (1 mL). b Determined by capillary GC analysis using
pentadecane as an internal standard.
screening was again conducted. After many experiments,
platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane com-
plex [Pt(CH2dCHSiMe2)2O] was found to be a reasonable
catalyst precursor for the hydrosilylation in toluene
(Table 3, entry 1).30 On the basis of this auspicious result,
we have set out to investigate the effect of added ligand
and reaction temperature (Table 3). Addition of PPh3 (2
equiv to Pt) resulted in 62% yield of the desired hydrosi-
lylation products (3a and 4a ) at 40 °C with 84% regiose-
lectivity favoring the terminal silylated product 3a (entry
2). Electronic tuning of PPh3 did not improve the catalytic
activity (entry 3). Reducing the amount of added PPh3
(1 equiv to Pt) was found to be beneficial (entry 4).
Raising the reaction temperature was even more benefi-
cial without declining the regioselectivity (entries 5 and
6). In particular, the hydrosilylation products (3a and 4a )
were obtained in 92% yield after 1 h at 100 °C (entry 6).
These conditions permit the hydrosilylation at low cata-
lyst loadings without significant loss of the yield (entry
7). Moreover, we were delighted to find that the use of
electron-rich and bulky P(t-Bu)3 as a supporting ligand
resulted in the production of 3a with excellent regiose-
lectivity (99%).31
Under the standard set of reaction conditions (1 mol
% of Pt(CH2dCHSiMe2)2O and 1 mol % of P(t-Bu)3 at 100
°C in toluene), the hydrosilylation using 2-PyMe2SiH
occurred with various alkynes (Table 4). The regioselec-
tivity was again excellent for various terminal alkynes
(entries 2-5) when P(t-Bu)3 was used as a ligand. When
PPh3 was used as a ligand, poorer regioselectivity (72-
84%) was again observed. For the hydrosilylation of
symmetrical internal alkynes, where the regioselectivity
is no more an issue, we prefer to utilize PPh3 instead of
P(t-Bu)3 because it is easier to handle and cheaper
(entries 6 and 7).
(30) (a) Lewis, L. N. J . Am. Chem. Soc. 1990, 112, 5998. (b) Lewis,
L. N.; Sy, K. G.; Bryant, G. L., J r.; Donahue, P. E. Organometallics
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(31) (a) Chandra, G.; Lo, P. Y.; Hitchcock, P. B.; Lappert, M. F.
Organometallics 1987, 6, 191. (b) Denmark, S. E.; Wang, Z. Org. Lett.
2001, 3, 1073.
5. P la tin u m -Ca ta lyzed Hyd r osilyla tion of 1-Oc-
tyn e w ith 2-P yMe2SiH, 3-P yMe2SiH, 4-P yMe2SiH,
a n d P h Me2SiH. We next examined the platinum-

Metal-Catalyzed Hydrosilylation of Alkenes and Alkynes
J . Org. Chem., Vol. 67, No. 8, 2002 2649
Ta ble 4. P la tin u m -Ca ta lyzed Hyd r osilyla tion of Alk yn es Usin g 2-P yMe2SiH
a
Reactions were performed under argon using 2-PyMe2SiH (0.5 mmol), alkyne (1.0 mmol), Pt(CH2dCHSiMe2)2O (1 mol %), and P(t-
b
Bu)3 (1 mol %) in toluene (1.5 mL) at 100 °C. Reactions were performed under argon using 2-PyMe2SiH (0.5 mmol), alkyne (1.0 mmol),
Pt(CH2dCHSiMe2)2O (1 mol %), and PPh3 (1 mol %) in toluene (1 mL) at 100 °C. c Isolated yield. Determined by GC and 1H NMR
d
analysis.
F igu r e 2. Rh-catalyzed hydrosilylation of vinyltrimethylsi-
lane.
F igu r e 1. Rh-catalyzed hydrosilylation of 1-octene.
catalyzed hydrosilylation of 1-octyne with 2-PyMe2SiH,
3-PyMe2SiH, 4-PyMe2SiH, and PhMe2SiH in the presence
of Pt[(CH2dCHSiMe2)2O] (1 mol %) and PPh3 (1 mol %)
in EtCN. The mixture was stirred at 80 °C for from 30

2650 J . Org. Chem., Vol. 67, No. 8, 2002
Itami et al.
F igu r e 3. Pt-catalyzed hydrosilylation of 1-octene.
F igu r e 5. Pt-catalyzed hydrosilylation of 1-octyne.
Sch em e 2
be due to the higher reactivity of alkynes than that of
alkenes toward hydrosilylation.
IV. Mech a n ism a n d P ossible Exp la n a tion for th e
Ra te Acceler a tion a n d Deceler a tion . We have ob-
served unusual rate acceleration and deceleration in
hydrosilylation using 2-PyMe2SiH, 3-PyMe2SiH, 4-PyMe2-
SiH, and PhMe2SiH. We envisioned that these remark-
able reactivity differences might be closely related to the
mechanism of metal-catalyzed hydrosilylation.1 It has
been well documented that there are two mechanistically
different pathways for metal-catalyzed hydrosilylation.
The Chalk-Harrod mechanism is proposed for the plati-
num-catalyzed hydrosilylation of alkenes, in which alk-
ene is inserted into the Pt-H bond (generated by the
oxidative addition of Si-H to Pt) (Scheme 2).32 On the
other hand, a modified Chalk-Harrod mechanism is
proposed for the rhodium-catalyzed hydrosilylation, in
which alkene insertion takes place at the Rh-Si bond
(Scheme 2).33 Recent theoretical studies support the
F igu r e 4. Pt-catalyzed hydrosilylation of vinyltrimethylsi-
lane.
min to 2 h, and the yield of hydrosilylated product was
determined by GC analysis (Figure 5). Whereas the
hydrosilylation using 3-PyMe2SiH, 4-PyMe2SiH, and
PhMe2SiH proceeded smoothly and was completed within
30 min, the hydrosilylation was substantially slower
when using 2-PyMe2SiH (63% yield after 2 h). These
results are slightly different from those we observed in
the platinum-catalyzed hydrosilylation of 1-octene (the
rate deceleration was observed with 2-PyMe2SiH and
4-PyMe2SiH). We assume that the reactivity difference
between 2-PyMe2SiH and 4-PyMe2SiH in this case may
(32) Chalk, A. J .; Harrod, J . F. J . Am. Chem. Soc. 1965, 87, 16.
(33) (a) Schroeder, M. A.; Wrighton, M. S. J . Organomet. Chem.
1977, 128, 345. (b) Millan, A.; Towns, E.; Maitlis, P. M. J . Chem. Soc.,
Chem. Commun. 1981, 673. (c) Ojima, I.; Fuchikami, T.; Yatabe, M.
J . Organomet. Chem. 1984, 260, 335. (d) Randolph, C. L.; Wrighton,
M. S. J . Am. Chem. Soc. 1986, 108, 3366.

Metal-Catalyzed Hydrosilylation of Alkenes and Alkynes
J . Org. Chem., Vol. 67, No. 8, 2002 2651
Sch em e 3
Sch em e 4
due to the stronger electron-withdrawing ability of the
2-pyridyl group than that of the 4-pyridyl group.
Sakaki recently determined by theoretical calculations
that the platinum-catalyzed hydrosilylation proceeds
with the Chalk-Harrod mechanism, and the overall rate
is determined by the isomerization of the olefin-inserted
intermediate and subsequent reductive elimination.34
According to their report, the isomerization of the olefin-
inserted intermediate occurs readily when the silyl group
exhibits strong trans influence. Eaborn has reported that
the trans influence of the silyl group becomes weaker
when electron-withdrawing group is attached onto sili-
con.38 Thus, the 4-pyridyl group and, more efficiently, the
2-pyridyl group on silicon are expected to decelerate the
isomerization step (Scheme 4).
Electronic properties of silyl groups might have influ-
enced the reductive elimination step as well. Ozawa has
reported that the reductive elimination from the (alkyl)-
(silyl)platinum complex is slower when a electron-
withdrawing group is attached onto silicon.39 Sakaki also
observed the same trend in theoretical calculations.34
Therefore, one might expect that the reductive elimina-
tion is slower when 2- and 4-pyridyl groups, which are
better electron-withdrawing groups than the 3-pyridyl or
phenyl group, are attached onto silicon. However, since
there are many factors governing this elemental step,40
it is impossible to conclude any clear rationale at this
stage.
occurrence of these two reaction pathways.34,35 Alkyne
hydrosilylation has been proposed to occur via a mech-
anism similar to that of alkene hydrosilylation.36
1. P ossible Exp la n a tion for th e Rea ctivity Dif-
fer en ce of Hyd r osila n es in th e Rh od iu m -Ca ta lyzed
Hyd r osilyla tion . The mechanistic rationale for this
unusual metal-dependent acceleration and deceleration
is of special interest. In the case of rhodium-catalyzed
hydrosilylation, the electronic nature of 2-PyMe2SiH
cannot be a decisive factor for its high reactivity since
neither 3-PyMe2SiH, 4-PyMe2SiH, or PhMe2SiH rivals
2-PyMe2SiH in terms of reactivity (Figures 1 and 2). The
coordination of the pyridyl group on silicon might have
some accelerating effects at a certain stage of the reac-
tion. Previously,12 we presumed that the acceleration
might be attributed to the facile oxidative addition of
Si-H to Rh on the basis of the precoordination effect
(Scheme 3a).37 Alternatively, it is also plausible to
surmise that the pyridyl group on silicon coordinates to
the catalyst rhodium after the insertion of olefin, thereby
stabilizing the olefin-inserted intermediate (Scheme 3b).
This coordination might enhance the olefin insertion
process that has been theoretically uncovered as the rate-
determining step in the rhodium-catalyzed hydrosilyl-
ation of ethylene.35 It is noteworthy that the latter
coordination effect (Scheme 3b) can only be expected in
the rhodium-catalyzed reaction, which proceeds with the
modified Chalk-Harrod mechanism.33
Con clu sion
2. P ossible Exp la n a tion for th e Rea ctivity Dif-
fer en ce of Hyd r osila n es in th e P la tin u m -Ca ta lyzed
Hyd r osilyla tion . In the case of platinum-catalyzed
hydrosilylation, the electronic effect of the aryl group on
silicon should have played the key role since rate
deceleration was observed with 2-PyMe2SiH and 4-PyMe2-
SiH. It is well-known that 2- and 4-pyridyl groups are
better electron-withdrawing groups than 3-pyridyl and
phenyl groups because of a resonance-oriented reason.
Reactivity differences between 2-PyMe2SiH and 4-PyMe2-
SiH (4-PyMe2SiH > 2-PyMe2SiH) observed in the hy-
drosilylation of vinyltrimethylsilane and 1-octyne may be
In conclusion, we have established the efficient rhod-
ium- and platinum-catalyzed hydrosilylation of alkenes
and alkynes by utilizing 2-PyMe2SiH as a new silylating
agent. The phase tag property of the 2-PyMe2Si group
enabled the simple acid-base extraction as a product
purification method. Moreover, unusual metal- and hy-
drosilane-dependent acceleration and deceleration were
found during the reactivity comparison using 2-PyMe2-
SiH, 3-PyMe2SiH, 4-PyMe2SiH, and PhMe2SiH. It seems
that these reactivity differences stem primarily from the
governance of two different mechanisms (Chalk-Harrod
and modified Chalk-Harrod mechanisms). Coordination
of the 2-pyridyl group might have accelerated the rate-
determining olefin insertion process in the rhodium-
catalyzed reaction. The electron-withdrawing ability of
2- and 4-pyridyl groups might have decelerated the rate-
(34) (a) Sakaki, S.; Mizoe, N.; Sugimoto, M. Organometallics 1998,
17, 2510. (b) Sakaki, S.; Mizoe, N.; Sugimoto, M.; Musashi, Y. Coord.
Chem. Rev. 1999, 192, 933.
(35) Sakaki, S.; Sumimoto, M.; Fukuhara, M.; Sugimoto, M. Pre-
sented at the 46th Symposium on Organometallic Chemistry, J apan,
Osaka, 1999.
(36) Sugimoto, M.; Yamasaki, I.; Mizoe, N.; Anzai, M.; Sakaki, S.
Theor. Chem. Acc. 1999, 102, 377.
(37) (a) Auburn, M. J .; Holmes-Smith, R. D.; Stobart, S. R. J . Am.
Chem. Soc. 1984, 106, 1314. (b) Djurovich, P. I.; Safir, A. L.; Keder, N.
L.; Watts, R. J . Inorg. Chem. 1992, 31, 3195.
(38) Chatt, J .; Eaborn, C.; Ibekwe, S. D.; Kapoor, P. N. J . Chem.
Soc. A 1970, 1343.
(39) Ozawa, F. J . Organomet. Chem. 2000, 611, 332.
(40) Hasebe, K.; Kamite, J .; Mori, T.; Katayama, H.; Ozawa, F.
Organometallics 2000, 19, 2022.

2652 J . Org. Chem., Vol. 67, No. 8, 2002
Itami et al.
En tr y 1). To a solution of RhCl(PPh3)3 (23.1 mg, 5 mol %) in
CH3CN (1.0 mL) were added 1-octene (168 mg, 1.5 mmol) and
dimethyl(2-pyridyl)silane (1) (69 mg, 0.5 mmol) under argon.
After the mixture was stirred at room temperature for 30 min,
toluene (0.5 mL) was added to the mixture. This mixture was
extracted with 1 N aqueous HCl (6 × 5 mL). The combined
aqueous phase was neutralized by adding NaHCO3 and then
extracted with EtOAc (3 × 10 mL). Drying over Na2SO4 and
removal of the solvents under reduced pressure afforded 2a
(107 mg, 86%) as a colorless oil. The purity of 2a was
determined to be over 95% by capillary GC and 1H NMR
analysis.
determining isomerization and/or reductive elimination
steps in the platinum-catalyzed reaction. Although fur-
ther investigation is needed to settle the precise mech-
anism for metal- and hydrosilane-dependent acceleration
and deceleration, the present study clearly implies
alternative controlling factors in the metal-catalyzed
hydrosilylation chemistry.
Exp er im en ta l Section
Dim eth yl(2-p yr id yl)sila n e (1). To a solution of 2-bro-
mopyridine (40.4 g, 255.5 mmol) in Et2O (80 mL) was added
dropwise a hexane solution of butyllithium (255 mmol) at -78
°C under argon. The mixture was stirred at -78 °C for an
additional 2 h. The resultant solution of 2-pyridyllithium was
added to a solution of chlorodimethylsilane (23.2 g, 245.6
mmol) in Et2O (60 mL) at -78 °C. After being stirred at room
temperature for 1 h, the mixture was washed with H2O (4 ×
100 mL), and then the organic phase was dried over Na2SO4.
Removal of the solvent under reduced pressure and subsequent
distillation afforded the title compound in 86% purity as judged
by capillary GC. A second distillation afforded 1 (12.1 g, 36%)
as a colorless liquid in a pure form: bp 68-72 °C/20 mmHg;
1H NMR (300 MHz) δ 0.40 (d, J ) 3.9 Hz, 6 H), 4.46 (septet,
J ) 3.9 Hz, 1 H), 7.22 (ddd, J ) 7.5, 4.8, 1.2 Hz, 1 H), 7.54
(ddd, J ) 7.5, 1.8, 1.2 Hz, 1 H), 7.60 (td, J ) 7.5, 1.8 Hz, 1 H),
8.77 (ddd, J ) 4.8, 1.8, 1.2 Hz, 1 H); 13C NMR (75 MHz) δ
-4.8, 123.1, 129.8, 134.2, 150.4, 165.8; IR (neat) 2124, 1576,
1559, 1420, 1248 cm-1; HRMS (EI) m/z calcd for C7H11NSi
137.0661, found 137.0658.
Dim eth yl(3-p yr id yl)sila n e. By a procedure similar to that
used to prepare dimethyl(2-pyridyl)silane, the title compound
was obtained from 3-bromopyridine (78%) as a pale yellow
liquid: bp (bulb-to-bulb distillation) 120-150 °C/20 mmHg;
1H NMR (300 MHz) δ 0.38 (d, J ) 3.9 Hz, 6 H), 4.46 (septet,
J ) 3.9 Hz, 1 H), 7.30 (dd, J ) 7.5, 4.8 Hz, 1 H), 7.85 (dt, J )
7.5, 1.8 Hz, 1 H), 8.60 (dd, J ) 4.8, 1.8 Hz, 1 H), 8.71 (t, J )
1.8 Hz, 1 H); 13C NMR (75 MHz) δ -4.3, 123.3, 132.4, 141.7,
150.3, 154.5; IR (neat) 2124, 1574, 1557, 1395, 1252 cm-1. Anal.
Calcd for C7H11NSi: C, 61.25; H, 8.08; N, 10.20. Found: C,
61.05; H, 8.18; N, 10.00.
Typ ica l P r oced u r e for th e P la tin u m -Ca ta lyzed Hy-
d r osilyla tion of Alk yn es w ith 2-P yMe2SiH (Ta ble 4,
En tr y 1). To a solution of Pt(CH2dCHSiMe2)2O (0.005 mmol,
1 mol %, 0.1 M solution in poly(dimethylsiloxane)) and P(t-
Bu)3 (1.0 mg, 0.005 mmol, 1 mol %) in toluene (1.5 mL) were
added 1-octyne (110 mg, 1.0 mmol) and 1 (69 mg, 0.5 mmol)
under argon. After the mixture was stirred at 100 °C for 15 h,
toluene (0.5 mL) was added. This mixture was extracted with
1 N aqueous HCl (5 × 5 mL). The combined aqueous phase
was neutralized by adding NaHCO3 and then extracted with
EtOAc (3 × 10 mL). Drying over Na2SO4 and removal of the
solvents under reduced pressure afforded a mixture of 3a and
4a (110 mg, 89%) as a colorless oil. The ratio of 3a /4a was
1
determined to be 99:1 by capillary GC and H NMR analysis.
Regioisomers 3a and 4a were separated by gel permeation
chromatography.
P r oced u r e for th e Rea ctivity Com p a r ison Exp er i-
m en ts for Rh od iu m -Ca ta lyzed Rea ction s (F igu r es 1 a n d
2). To a solution of RhCl(PPh3)3 (23.1 mg, 5 mol %) in CH3CN
(1.0 mL) were added alkene (1.5 mmol) and hydrosilane (0.5
mmol) under argon. The mixture was stirred at 28 °C for from
15 min to 2 h, and the yield of hydrosilylated product was
determined by GC analysis.
P r oced u r e for th e Rea ctivity Com p a r ison Exp er i-
m en ts for P la tin u m -Ca ta lyzed Rea ction s (F igu r es 3-5).
To a solution of Pt(CH2dCHSiMe2)2O (0.005 mmol, 1 mol %,
0.1 M solution in poly(dimethylsiloxane)) and PPh3 (1.3 mg,
0.005 mmol, 1 mol %) in EtCN (1.0 mL) were added alkene or
alkyne (1.0 mmol) and hydrosilane (0.5 mmol) under argon.
The mixture was stirred at 80 °C for from 15 min to 2 h, and
the yield of hydrosilylated product was determined by GC
analysis.
Dim eth yl(4-p yr id yl)sila n e. By a procedure similar to that
used to prepare dimethyl(2-pyridyl)silane, the title compound
was obtained from 4-bromopyridine hydrochloride (78%) as a
pale yellow liquid: bp (bulb-to-bulb distillation) 120-150 °C/
20 mmHg; 1H NMR (300 MHz) δ 0.34 (d, J ) 3.9 Hz, 6 H),
4.38 (septet, J ) 3.9 Hz, 1 H), 7.38 (dd, J ) 4.5, 1.5 Hz, 2 H),
8.54 (dd, J ) 4.5, 1.5 Hz, 2 H), 13C NMR (75 MHz) δ -4.6,
Su p p or tin g In for m a tion Ava ila ble: Spectroscopic data
for all products, 13C NMR spectra of 2b, 2d , 3c, 4a , 4d , 4e,
(E)-1-dimethyl(3-pyridyl)silyl-1-octene, 2-dimethyl(3-pyridyl)-
silyl-1-octene, (E)-1-dimethyl(4-pyridyl)silyl-1-octene, and 2-di-
methyl(4-pyridyl)silyl-1-octene, and the procedure for the
consecutive use of recovered rhodium catalyst. This material
is available free of charge via the Internet at http://pubs.acs.org.
128.7, 147.3, 148.6; IR (neat) 2128, 1402, 1129, 880 cm-1
;
HRMS (EI) m/z calcd for C7H11NSi 137.0661, found 137.0660.
Anal. Calcd for C7H11NSi: C, 61.25; H, 8.08; N, 10.20. Found:
C, 61.05; H, 8.18; N, 10.00.
Typ ica l P r oced u r e for th e Rh od iu m -Ca ta lyzed Hy-
d r osilyla tion of Alk en es w ith 2-P yMe2SiH (Ta ble 2,
J O0163389