Mechanistic studies of palladium(II)-catalyzed hydrosilation and dehydrogenative silation reactions

Article abstract of DOI:10.1021/ja962979n

The cationic Pd(II) complexes, [(phen)Pd(CH₃)(L)]⁺[BAr'₄]^- phen = 1,10-phenanthroline; L = Et₂O, Me₃SiC≡CSiMe₃; Ar' = 3,5-(CF₃)₂C₆H₃) catalyze the hydrosilation and dehydrogenative silation of olefins. Hydrosilation of ethylene, tert-butylethylene, 1-hexene, and cyclohexene by HSiR₃ (R = CH₂CH₃, C₆H₅) occurs in the presence of 1 mol% [(phen)Pd(CH₃)(L)]⁺[BAr'₄]^-. The reaction of tert-butylethylene with HSi(i-Pr)₃ in the presence of [(phen)Pd(CH₃)(L)]⁺[BAr'₄]^- yields neohexane and t-BuCH=CHSi(i-Pr)₃. Low-temperature NMR experiments revealed that the catalyst resting state for the silations of ethylene and alkyl-substituted olefins is [(phen)Pd(SiR₃)(η²-H₂C=CHR')]⁺[BAr'₄]^-. Evidence for rapid, reversible silyl migration at -70°C was observed by ¹H NMR spectroscopy. Deuterium labeling studies show that the intermediate Pd(II) alkyl complexes can isomerize via a series of β-hydride eliminations followed by reinsertions of olefin prior to reaction with DSiEt₃. Styrene undergoes both hydrosilation and dehydrogenative silation in the presence of [(phen)Pd(CH₃)(L)]⁺[BAr'₄]^- or [(phen)Pd(η³-CH(CH₃)C₆H₅)]⁺[BAr'₄]^- yielding ethylbenzene, R₃SiCH₂CH₂C₆H₅ and trans-R₃SiCH=CHPh (R = CH₂CH₃, CH(CH₃)₂). ¹H NMR spectroscopy revealed that the π-benzyl complexes [(phen)Pd(η³-CH(CH₂SiR₃)C₆H₅)]⁺[BAr'₄]^- and [(phen)Pd(η³-CH(CH₃)C₆H₅)]⁺[BAr'₄]^- are the catalyst resting states for the silation reactions of styrene.

Full text of DOI:10.1021/ja962979n

906
J. Am. Chem. Soc. 1997, 119, 906-917
Mechanistic Studies of Palladium(II)-Catalyzed Hydrosilation
and Dehydrogenative Silation Reactions
Anne M. LaPointe, Francis C. Rix, and Maurice Brookhart*
Contribution from the Department of Chemistry, UniVersity of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290
ReceiVed August 26, 1996X
Abstract: The cationic Pd(II) complexes, [(phen)Pd(CH3)(L)]+[BAr′4]- phen ) 1,10-phenanthroline; L ) Et2O,
Me3SiCtCSiMe3; Ar′ ) 3,5-(CF3)2C6H3) catalyze the hydrosilation and dehydrogenative silation of olefins.
Hydrosilation of ethylene, tert-butylethylene, 1-hexene, and cyclohexene by HSiR3 (R ) CH2CH3, C6H5) occurs in
the presence of 1 mol % [(phen)Pd(CH3)(L)]+[BAr′4]-. The reaction of tert-butylethylene with HSi(i-Pr)3 in the
presence of [(phen)Pd(CH3)(L)]+[BAr′4]- yields neohexane and t-BuCHdCHSi(i-Pr)3. Low-temperature NMR
experiments revealed that the catalyst resting state for the silations of ethylene and alkyl-substituted olefins is [(phen)-
1
Pd(SiR3)(η2-H2CdCHR′)]+[BAr′4]-. Evidence for rapid, reversible silyl migration at -70 °C was observed by H
NMR spectroscopy. Deuterium labeling studies show that the intermediate Pd(II) alkyl complexes can isomerize
via a series of â-hydride eliminations followed by reinsertions of olefin prior to reaction with DSiEt3. Styrene
undergoes both hydrosilation and dehydrogenative silation in the presence of [(phen)Pd(CH3)(L)]+[BAr′4]- or [(phen)-
Pd(η3-CH(CH3)C6H5)]+[BAr′4]- yielding ethylbenzene, R3SiCH2CH2C6H5 and trans-R3SiCHdCHPh (R ) CH2-
CH3, CH(CH3)2). 1H NMR spectroscopy revealed that the π-benzyl complexes [(phen)Pd(η3-CH(CH2SiR3)C6H5)]+-
[BAr′4]- and [(phen)Pd(η3-CH(CH3)C6H5)]+[BAr′4]- are the catalyst resting states for the silation reactions of styrene.
Hydrosilation of olefins (eq 1) can be achieved either through
hydrosilation.13-15 Because of the utility of vinylsilanes in
organic synthesis,16 the dehydrogenative silation reaction is of
interest. The Chalk-Harrod mechanism does not account for
dehydrogenative silation, and a number of groups have proposed
mechanisms based on silyl (rather than hydride) migration to
olefin.8,13,15,17-19
One such possible mechanism is shown in Scheme 2. Silyl
migration to olefin results in a â-silylalkyl intermediate, which
can then undergo two possible reactions. The first possibility
is â-hydride elimination to form an olefin-hydride complex.
If reinsertion of the olefin with opposite regiochemistry occurs,
isomerization of the alkyl complex results. Displacement of
the olefin from the metal results in dehydrogenative silation.
The second possibility is cleavage of the â-silylalkyl fragment
by reaction by HSiR3. This reaction may proceed via oxidative
addition of silane followed by reductive elimination of the
hydrosilated product or via a σ-bond metathesis reaction.
Efforts to detect intermediate species in many transition metal
catalyzed hydrosilation reactions have been hindered by ex-
tremely high reaction rates1 and heterogeneous systems.20 In
an earlier study from these laboratories, the cationic cobalt
a free radical process or with the use of a transition metal
catalyst, typically an electron-rich complex of a late transition
metal such as Co, Rh, Pd, or Pt.1,2 More recently, zirconocene
derivatives,3,4 lanthanide complexes,5-7 and electrophilic late
transition metal complexes8 have been reported to catalyze the
hydrosilation of olefins.
A variety of mechanisms have been proposed for this process.
A widely accepted mechanism was first proposed by Chalk and
Harrod (Scheme 1).9,10 The key feature in the Chalk-Harrod
mechanism is migratory insertion of an olefin hydride complex
followed by reductive coupling of the alkyl and silyl frag-
ments.9,10 If the intermediate alkyl complex undergoes revers-
ible â-hydride elimination and reinsertion with opposite regio-
chemistry, then the Chalk-Harrod mechanism provides an
explanation for the olefin isomerization and deuterium scram-
bling observed in many hydrosilation reactions.1
However, many catalysts form vinylsilanes and alkanes in
addition to the hydrosilation product (eq 2).1,11-14 In some
cases, dehydrogenative silation occurs more readily than
X Abstract published in AdVance ACS Abstracts, January 1, 1997.
(1) Ojima, I. In The Chemistry of Organic Silicon Compounds; Patai,
S., Rappoport, S., Eds.; Wiley: New York, 1989.
(11) Ojima, I.; Fuchikami, T.; Yatabe, M. J. Organomet. Chem. 1984,
260, 335.
(2) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Complexes; University Science
Books: Mill Valley, CA, 1987.
(3) Takahashi, T.; Hasegawa, M.; Suzuki, M.; Saburi, M.; Rousset, C.
J.; Fanwick, P. E.; Negishi, E.-I. J. Am. Chem. Soc. 1991, 113, 8564.
(4) Kesti, M. R.; Waymouth, R. M. Organometallics 1992, 11, 1095.
(5) Fu, P.-F.; Brard, L.; Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1995,
117, 7157.
(12) Onopchenko, A.; Sabourin, E. T.; Beach, D. L. J. Org. Chem. 1983,
48, 5101.
(13) Takeuchi, R.; Yasue, H. Organometallics 1996, 15, 2098.
(14) Christ, M. L.; Sabo-Etienne, S.; Chaudret, B. Organometallics 1995,
14, 1082.
(15) Tanke, R. S.; Crabtree, R. H. Organometallics 1991, 10, 415.
(16) Fleming, I.; Donogues, J.; Smithers, R. Org. React. 1989, 37, 57.
(17) Randolph, C. L.; Wrighton, M. S. J. Am. Chem. Soc. 1986, 108,
2266.
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289.
(6) Molander, G. A.; Julius, M. J. Org. Chem. 1992, 57, 6347.
(7) Sakakura, T.; Lautenschlager, H.-J.; Tanaka, M. J. Chem. Soc., Chem.
Commun. 1991, 40.
(8) Brookhart, M.; Grant, B. E. J. Am. Chem. Soc. 1993, 115, 2151.
(9) Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 87, 16.
(10) Harrod, J. F.; Chalk, A. J. J. Am. Chem. Soc. 1965, 87, 1133.
(19) Bergens, S. H.; Noheda, P.; Whelan, J.; Bosnich, B. J. Am. Chem.
Soc. 1992, 114, 2128.
(20) Lewis, L. N. J. Am. Chem. Soc. 1990, 112, 5998.
S0002-7863(96)02979-4 CCC: $14.00 © 1997 American Chemical Society

Palladium(II)-Catalyzed Hydrosilation
J. Am. Chem. Soc., Vol. 119, No. 5, 1997 907
Scheme 1. Chalk-Harrod Mechanism for the Hydrosilation
of Olefins
plexes, [(phen)Pd(CH3)(L)]+[BAr′4]- (phen ) 1,10-phenan-
throline; L ) Et2O (1a), Me3SiCtCSiMe3 (1b); Ar′ ) 3,5-
(CF3)2C6H3) are efficient precatalysts for the formation of
silicon-carbon bonds.
Hydrosilation and dehydrogenative silation of olefins are
achieved under mild conditions (25-85 °C, 0.1-2 mol % 1).
The mechanisms of the hydrosilation and dehydrgenative silation
reactions were studied by a variety of low-temperature NMR
experiments and deuterium labeling experiments. The results
of these studies are presented below.
Scheme 2. Silyl Migration Mechanism for Hydrosilation
and Dehydrogenative Silylation of Olefins
Results and Discussion
I. Survey of Silation Reactions. A. Silation of Ethylene
and Alkyl-Substituted Olefins. In the presence of a catalytic
amount of [(phen)Pd(CH3)(L)]+[BAr′4]-, hydrosilation of olefins
proceeds at 25 °C in CH2Cl2 (eq 4). The addition of HSiR3 to
R-olefins proceeds with a 1,2 regiochemistry. 1b is a more
convenient catalyst than the thermally unstable ether adduct 1a
since 1b is thermally stable, moderately air- and water-stable,
and stable at 25 °C in CH2Cl2. The results are summarized in
Table 1.
When the reaction was conducted in Et2O, lower yields were
obtained. Likewise, only 5% yield is obtained when [(phen)-
Pd(CH3)(NCCH3)(NCCH3)]+[BAr′4]- (1c) is used as the cata-
lyst. The low yields are due to inhibition of the silation reaction
rather than catalyst decomposition; NMR studies (see below)
suggest that coordinating ligands such as acetonitrile or ether
can compete with olefin or silane for the vacant coordination
site and thus inhibit the hydrosilation reaction.
No dehydrogenative silation is observed in the reactions of
ethylene, 1-hexene, or cyclohexene with HSiR3 (R ) Et, Ph).
Some dehydrogenative silation (5%) is observed in the reaction
of tert-butylethylene with HSiEt3 in the presence of 1a or 1b.
Efforts to increase the yield of t-BuCHdCHSiEt3 by increasing
the t-BuCHdCH2/HSiEt3 ratio were not successful.
complex [Cp*Co(P(OMe)3)(CH2CH2-µ-H)]+[BAr′4]- was found
to be an efficient precatalyst for the hydrosilation of 1-hexene.
Deuterium labeling studies and spectroscopic observation of
intermediate species in this system suggested that the reaction
proceeds by silyl migration to olefin followed by a series of
â-hydrogen elimination and reinsertion reactions. The terminal
cobalt alkyl complex [Cp*Co(CH2CH-µ-H(CH2)4SiEt3)(P(O-
Me)3)]+[BAr′4]- then reacts with HSiEt3 to form Et3Si(CH2)5-
CH3 (eq 3). Thus, silane adds with overall 1,6 regiochemistry.8
The reaction of ethylene with HSi(i-Pr)3 in the presence of 1
mol % 1a yields the hydrosilation product, i-Pr3SiEt (74% yield).
In contrast, the reaction of tert-butylethylene with 0.5 equiv of
HSi(i-Pr)3 in the presence of 1% 1a yields trans-t-BuCHdCHSi-
(21) Rix, F. C.; Brookhart, M. J. Am. Chem. Soc. 1995, 117, 1137.
(22) DiRenzo, G. M.; White, P. S.; Brookhart, M. J. Am. Chem. Soc.
1996, 18, 6225.
(23) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc.
1995, 117, 6414.
(24) Sen, A. Acc. Chem. Res. 1993, 26, 303.
(25) Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996,
118, 4746.
(26) Brookhart, M.; Rix, F. C.; DeSimone, J. M.; Barborak, J. C. J. Am.
Chem. Soc. 1992, 114, 5894.
(27) Drent, E.; van Broekhaven, J. A. M.; Doyle, M. J. Organomet. Chem.
1991, 417, 235.
(28) van Asselt, R.; Gielens, E. E. C. G.; Ru¨lke, R. E.; Vrieze, K.;
Elsevier, C. J. J. Am. Chem. Soc. 1994, 116, 997.
Cationic square-planar Pd(II) complexes have been found to
be efficient catalysts for the formation of carbon-carbon bonds,
including the dimerization21,22 and polymerization of R-olefins23
and the alternating co-polymerization of R-olefins with carbon
monoxide.24-28 We report here that the cationic Pd(II) com-

908 J. Am. Chem. Soc., Vol. 119, No. 5, 1997
LaPointe et al.
Table 1. Summary of Reaction Conditionsa,c and Yieldsb for the Reaction of Ethylene and Alkyl-Substituted Olefins with HSiR3 in the
Presence of 1 mol % [(phen)Pd(CH3)(L)]+[BAr′4]- (1a, L ) Et2O; 1b, L ) Me3CCSiMe3; 1c, L ) CH3CN)
HSiR3
R )
substrate ratio,
HSiR3/olefin
entry no.
olefin
ethylene
1-hexene
cyclohexene
t-BuCHdCH2
t-BuCHdCH2
t-BuCHdCH2
t-BuCHdCH2
cyclohexene
t-BuCHdCH2
ethylene
catalyst
solvent
product
yield,b %
1
2
3
4
5
6
7
8
9
Et
Et
Et
Et
Et
Et
Et
Ph
Ph
i-Pr
i-Pr
i-Pr
1a
1b
1b
1b
1b
1b
1c
1b
1b
1a
1a
1a
c
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Et2O
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
SiEt4
85
88
88
79
80
14
5
78
20e
75
82
94f
1.1
1.1
1.1
1.0
1.1
1.1
1.1
1.1
c
Et3Si(CH2)5CH3
Et3Si(Cy)
Et3SiCH2CH2-t-Bud
Et3SiCH2CH2-t-Bud
Et3SiCH2CH2-t-Bud
Et3SiCH2CH2-t-Bud
Ph3Si(Cy)
Ph3SiCH2CH2-t-Bu
Si(i-Pr)3Et
(i-Pr)3SiCHdCH-t-Bu
(i-Pr)3SiCHdCH-t-Bu
10
11
12
t-BuCHdCH2
t-BuCHdCH2
0.5
1.1
a 24 h, 25 °C. b Yields were calculated based on the limiting reagent. c C2H4 was bubbled through the solution until C2H4 uptake ceased.
d Et3SiCHdCH-t-Bu (5%) was also formed. e Recrystallized. f An equivalent amount of t-BuCH2CH3 was also produced.
Table 2. Reaction Conditionsa,c and Product Ratiosd for the
(i-Pr)3 (82% yield) (eq 5). The hydrosilation product, t-BuCH2-
Reaction of Styrene with HSiR3 (R ) Et, i-Pr) in the Presence of
1 mol % [(phen)Pd(η3-CH(CH3)C6H5)]+[BAr′4]- (7)
substrate
ratio,
R ) styrene/HSiR3 °C solvent
product ratio,
entry HSiR3
no.
T,
yield,b,d PhCH2CH2SiR3/
%
PhCHdCHSiR3
CH2Si(i-Pr)3, is not observed. When a slight excess of HSi(i-
Pr)3 (1.1 equiv) was used, no hydrosilation is observed.
B. Silation Reactions of Styrene. The reaction of styrene
with HSiEt3 in the presence of 1b or [(phen)Pd(η3-CH(CH3)-
C6H5)]+ (7)29 was investigated. The results are summarized in
Table 2. Both hydrosilation and dehydrogenative silation of
styrene occur, yielding a mixture of ethylbenzene, Et3SiCH2-
CH2Ph, and trans-Et3SiCHdCHC6H5 (eq 6). The product ratio
Et3SiCH2CH2Ph/trans-Et3SiCHdCHC6H5 is dependent on the
styrene/HSiEt3 ratio. Hydrosilation predominates when styrene/
HSiEt3 < 1 (entry nos. 1 and 5). Dehydrogenative silation
predominates when styrene/HSiEt3 g 1. At 85 °C, yields are
higher.
1
2
3
4
5
6
7
8
9
Et
Et
Et
Et
Et
Et
Et
Et
i-Pr
i-Pr
i-Pr
i-Pr
1:4
1:1
2:1
4:1
1:4
1:1
2:1
4:1
1:4
1:1
2:1
4:1
35 CH2Cl2
35 CH2Cl2
35 CH2Cl2
35 CH2Cl2
85 nonec
85 nonec
85 nonec
85 nonec
85 nonec
85 nonec
85 nonec
85 nonec
27
50
58
77
85
65
84
88
58
52
82
97
100:0
45:55
25:75
13:87
96:4
43:57
32:68
21:79
41:59
13:87
9:91
10
11
12
0:100
a 24 hours. b Yield ) (PhCH2CH2SiR3 + PhCHdCHSiR3). The
yield is calculated based on the limiting reagent. For the reactions of
equimolar amounts of HSiR3 and styrene, the isolated yield will be
<100% due to the conversion of styrene into ethylbenzene. c At 85
°C, 7 is soluble in neat HSiR3/styrene and no additional solvent was
used. d Ethylbenzene was removed in Vacuo during product isolation.
For this reason, the yield of ethylbenzene was not calculated but was
assumed to be equal to the yield of PhCHdCHSiR3.
Scheme 3. Possible Structures for 2
The reaction of HSi(i-Pr)3 with styrene in the presence of 1
mol % 7 was investigated. Higher temperatures (85 °C) are
required. Under identical conditions, the proportion of dehy-
drogenative silation observed is higher than in analogous
reactions of HSiEt3.
II. Silation of Ethylene and Alkyl-Substituted Olefins.
A. NMR Studies. The hydrosilation and dehydrogenative
silation reactions were investigated by low-temperature NMR
spectroscopic methods in order to gain mechanistic insight.
Complexes containing the [BAr′4]- anion frequently exhibit
enhanced stability and solubility and are well-suited for low-
temperature NMR studies.8,22,25,29,30
The reaction of the precatalyst 1a with 2 equiv of HSiR3 (R
) Et, Ph) at -78 °C in CD2Cl2 proceeds with loss of methane
and formation of a new complex with the general formation
[(phen)Pd(SiR3)2(H)]+[BAr′4]- (2a, R ) Et; 2b, R ) Ph) (eq
7).
In the presence of only 1 equiv of silane, a 1:1 mixture of 2
and 1a is obtained. In 2a the two SiEt3 fragments are equivalent
by 1H and 13C NMR spectroscopy at temperatures down to -95
°C. A singlet is observed at δ ) -9.99 (2a) or -12.13 (2b)
and is attributed to Si-H. At -95 °C no 29Si satellites are
observed, but the peak is quite broad and 29Si-H coupling
constants less than 40 Hz would not be detected. Based on the
spectroscopic evidence 2a and 2b are either Pd(IV) hydrides
with the general formula [(phen)Pd(SiR3)2H]+[BAr′4]- or a
rapidly fluxional Pd(II) species which contains as η1-silyl group
and a η2-silane (Scheme 3). A less likely possibility is that the
hydride is symmetrically bridged between the silyl groups, as
(29) Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996,
118, 2436.
(30) Brookhart, M.; Grant, B.; Volpe, A. F. Organometallics 1992, 11,
3920.

Palladium(II)-Catalyzed Hydrosilation
J. Am. Chem. Soc., Vol. 119, No. 5, 1997 909
shown in Scheme 3. The reaction of 1a with 2 equiv of HSi-
(i-Pr)3 in CD2Cl2 at -78 °C does not yield any well-defined
species, although CH4 evolution is observed. It is possible that
the larger steric requirements of Si(i-Pr)3 prevent formation of
a species with the general formula [(phen)Pd(Si(i-Pr)3)2(H)]+-
[BAr′4]-.
HSiEt3 is readily displaced from 2a by a variety of ligands,
allowing the formation of a variety of cationic Pd(II) silyl
complexes. Addition of triphenylphosphine to 2a results in
formation and isolation of [(phen)Pd(SiEt3)(PPh3)]+[BAr′4]-
(3a) (eq 8). When allyldiphenylphosphine is added to 2a at
(8)
Figure 1. A plot for kobs vs [HSiEt3] for the reaction of [(phen)Pd-
(SiEt3)(H2CdCH-t-Bu)]+ (4b) with HSiEt3 to form 2a and t-BuCH2-
CH2SiEt3 (-38 °C; [4b] ) 0.014 M; kobs ) 1.7 × 10-3 s-1 [HSiEt3]).
-78 °C, orange [(phen)Pd(SiEt3)(Ph2PC3H5)]+[BAr′4]- (3b) is
isolated. 1H and 31P NMR spectroscopy suggest that in 3b
allyldiphenylphosphine is coordinated to palladium via phos-
phorus rather than through the olefin. The isolable Pd silyl
complexes 3a,b are not suitable hydrosilation precatalysts; no
reaction of cyclohexene with HSiEt3 was observed in the
presence of 1 mol % 3a or 3b (CH2Cl2, 25 °C).
Addition of ethylene to a solution of 2a at -78 °C results in
displacement of HSiEt3 and formation of [(phen)Pd(SiEt3)(η2-
C2H4)]+[BAr′4]- (4a) (eq 9). No hydrosilation is observed at
-78 °C. In the presence of excess ethylene, exchange between
free and bound ethylene is observed at -78 °C and only a single
averaged signal can be observed (δ ) 5.0 ppm). In the absence
of excess ethylene, the coordinated ethylene ligand displays an
AA′BB′ pattern centered at 4.9 ppm. The olefinic doublets
collapse to a singlet as the temperature is raised. At the
and regenerate a Pd(II) silyl complex; this reaction is a key step
in the catalytic hydrosilation reaction (see below).
In the presence of 4a and excess HSiEt3 and ethylene (2-10
equiv), hydrosilation proceeds readily at -40 °C. During
catalysis, 4a is the only organometallic species observed by 1H
NMR spectroscopy. Upon complete consumption of ethylene,
2a is regenerated. Similar results were obtained in the reaction
of 4b with excess tert-butylethylene and HSiEt3. These results
establish that the silyl-olefin complexes [(phen)Pd(SiR3)(η2-
CH2CHR′)]+[BAr′4]- are the catalyst resting state for the silation
of olefins. This led us to propose a mechanism consisting of
silyl migration to olefin followed by reaction of the â-silylethyl
complex with HSiR3 to form the hydrosilation product and
regenerate [Pd]-SiR3 (eq 10).
coalescence temperature, Tc ) -70(1) °C, k ) 3.9 × 102 s-1
,
∆Gq ) 9.2 kcal/mol. The fluxional behavior of 4a is similar
to that observed for the cationic alkyl complexes [(phen)-
Pd(R)(C2H4)]+[BAr′4]- (R ) CH3, ∆Gq ) 9.2 (0.2) kcal/mol;
R ) C2H5, ∆Gq ) 9.2 (0.2) kcal/mol).21,25
Based on the rapid reaction of 1a with HSiEt3 and the rapid
formation of 4 from 2, it was anticipated that the rate-
determining step would be the silyl migration to olefin, and the
hydrosilation reaction would be zero-order in [HSiEt3]. CD2-
Cl2 solutions of 4a and 4b were monitored by 1H NMR
spectroscopy in the temperature range -60 to -20 °C. It was
found that in the absence of HSiEt3, complexes 4a,b are stable
in CD2Cl2 at temperatures up to ca. -20 °C. Neither the
â-silylalkyl complexes [(phen)Pd(CHR′CH2SiEt3)]+ (R′ ) H,
t-Bu) nor any other species attributable to â-silyl migration was
observed. This result suggested that â-silyl migration (4 h [Pd]-
R′′) may be rapid and reversible, favoring 4 in the catalytic
cycle. To examine this possibility, the kinetics of the reaction
of 4b with HSiEt3 under pseudo-first-order conditions (10-30
equiv of HSiEt3) was monitored at -38 °C. A first-order rate
dependence on [HSiEt3] was observed at all silane concentra-
tions. (kobs ) 1.7 × 10-3 M-1 s-1 [HSiEt3]) (Figure 1).
A crossover experiment was performed to determine whether
the silyl and olefin fragments in complexes 4 are coupled in
the hydrosilation product or if the bound olefin fragment is in
some way attacked by free HSiR3. 4b was generated in CD2-
Cl2 at -78 °C; care was taken not to introduce excess HSiEt3
Addition of tert-butylethylene to a CD2Cl2 solution of 2a at
-78 °C results in displacement of HSiEt3 and formation of
[(phen)Pd(SiEt3)(η2-CH2CH-t-Bu)]+[BAr′4]- (4b). At -78 °C,
no exchange is observed between free and bound t-butylethyl-
ene.
The reaction of 1a with 1 equiv of HSiEt3 followed by
addition of H2CdCHR′ (R′ ) H, t-Bu) at -78 °C yields 4a
and 4b, respectively. This method permits the generation of
silane-free solutions of 4a,b as well as the generation of tri-
(isopropyl)silyl analogues of 4; the reaction of 1a with 1 equiv
of HSi(i-Pr)3 at -78 °C followed by addition of tert-butyleth-
ylene results in formation of [(phen)Pd(Si(i-Pr)3)(η2-CH2CH-
t-Bu)]+[BAr′4]- (4c).
The rapid reaction of the precatalyst 1a with 2 equiv of HSiR3
to form 2 and the subsequent displacement of HSiR3 by olefin
to form 4 demonstrate how catalyst generation occurs in the
silation reactions. Additionally, the reaction of 1a with HSiR3
suggests that alkyl complexes of the general formula [(phen)-
PdR′′(L)]+ (R′′ ) alkyl) react readily with silane to form alkane

910 J. Am. Chem. Soc., Vol. 119, No. 5, 1997
LaPointe et al.
Scheme 4. Nondegenerate Silyl Scrambling and Olefin Displacement Reactions in [(phen)Pd(SiR3)(η2-CH2dCHCH2SiR′3)]+
or tert-butylethylene. HSiPh3 was added and the solution was
allowed to warm to 25 °C. The hydrosilation product was
Scheme 5. Reactions of R-Olefins with DSiEt3 in the
Presence of 1b
1
analyzed by H and 13C NMR spectroscopy and found to be
exclusively t-BuCH2CH2SiEt3 (eq 11). The results of this
experiment suggest that intramolecular silyl migration to olefin
occurs rather than an intermolecular reaction of bound tert-
butylethylene with HSiPh3.
The results of the kinetics and crossover experiments strongly
support rapid, reVersible silyl migration. To test the reversibility
of the silyl migration, a silyl scrambling experiment was
performed (Scheme 4). [(phen)Pd(SiEt3)(η2-H2CdCHCH2-
SiPh3)]+[BAr′4]- (5a) was generated in CD2Cl2 at -78 °C by
addition of allyltriphenylsilane to a solution of 2a. If silyl
migration to olefin occurs with a 2,1 regiochemistry (as is
suggested by the regiochemistry of the hydrosilated products),
the Pd(II) alkyl intermediate would have two nonequivalent
â-silyl substituents. If â-silyl elimination occurs it would be
nondegenerate and would result in formation of a mixture of
5a and a new complex, [(phen)Pd(SiPh3)(η2-H2CdCHCH2-
SiEt3)]+[BAr′4]-.
5a was generated at -78 °C and the NMR sample was
inserted into a precooled (-78 °C) NMR probe. Initially, ≈95%
5a was observed, but as the sample was allowed to warm to
-60 °C, a mixture of 5a (87%) and a new species 5b (13%)
At -70 °C, the equilibration of 5a and 5b occurred
competitively with the displacement of HSiR3 from 2. Accurate
rate constants for the interconversion of 5a and 5b could not
be obtained.
B. Deuterium Labeling Studies. Deuterium labeling studies
were conducted to determine whether â-hydride elimination and
reinsertion reactions occur in the â-silylalkyl intermediates prior
to cleavage by silane. The reactions of cyclohexene, tert-
butylethylene, and 1-hexene with DSiEt3 in the presence of 1b
were investigated. The catalyst and substrate concentrations
were similar to those used in the bulk hydrosilations. The
deuteriosilated products were isolated and analyzed by 13C NMR
spectroscopy. The results are shown in Scheme 5.
For cyclohexene, deuteriosilation occurs, with a 1,2 regio-
chemistry and <5% deuterium incorporation is observed at any
other position. These results are consistent with migratory
insertion of cyclohexene followed by reaction of the Pd
cyclohexyl intermediate with DSiEt3, as shown in Scheme 5.
In contrast, the deuterosilation reaction of tert-butylethylene
proceeds with a net 1,1 regiochemistry, and t-BuCH2CHDSiEt3
is the only product observed by 13C NMR spectroscopy. A
plausible mechanism is proposed consisting of migratory
insertion of tert-butylethylene, followed by â-hydride elimina-
1
whose H NMR spectrum closely resembled that of 5a was
formed. [(phen)Pd(SiPh3)(η2-H2CdCHCH2SiEt3)]+[BAr′4]- was
independently generated at -78 °C and its NMR spectrum was
identical with that of complex 5b. At -60 °C, 5b rapidly
equilibrated to a mixture of 5a (87%) and 5b (13%). For the
equilibrium 5a h 5b, K(eq) ) 0.15 at -60 °C. These
observations clearly establish that rapid and reversible silyl
migration is occurring at -60 °C.
After an hour at -60 °C, two new species were also observed
and identified as [(phen)Pd(SiEt3)(η2-H2CdCHCH2SiEt3)]+-
[BAr′4]- (5c) and [(phen)Pd(SiPh3)(η2-H2CdCHCH2SiPh3)]+-
[BAr′4]- (5d) (based on independent generation of these
complexes).
The observed preference for 5a may be due to preferential
binding of the more electron-rich silyl fragment SiEt3 to the
electrophilic Pd(II) cation. The eventual formation of 5c and
5d results from nondegenerate exchange of allylsilane. The
exchange may be explained by either an associative or dis-
sociative mechanism in the presence of traces of free allylsilane.
Mechanistic studies in related cationic Pd(II) systems suggest
that olefin displacement proceeds by an associative mecha-
nism.23,29

Palladium(II)-Catalyzed Hydrosilation
J. Am. Chem. Soc., Vol. 119, No. 5, 1997 911
Scheme 6. Proposed Mechanism for the
(phen)Pd+-Catalyzed Hydrosilation and Dehydrogenative
Silation of Alkyl-Substituted R-Olefins and Ethylene
([Pd] ) [(phen)Pd]+)
ruthenium silyl complex is formed via extrusion of ethylene
from a â-silylethyl intermediate (eq 12).31
The deuterium labeling studies suggest that [Pd]-R′′ (R′′ )
alkyl) can undergo rapid â-hydride elimination reactions.
Reinsertion of the bound olefin with opposite regiochemistry
results in isomerization of the Pd(II) alkyl intermediate. At -38
°C, the reaction of 4b with HSiEt3 proceeds fairly rapidly (kobs
) 1.6 × 10-3 M-1 s-1 [HSiEt3]); the deuterium labeling studies
suggest that isomerization of the alkyl fragment occurs prior to
reaction with HSiR3. The isomerization of the Pd(II) alkyl
intermediate must therefore be occurring quite rapidly even at
low temperatures. Similar evidence for rapid “chain-running”
reactions has been observed in Pd(II)- and Ni(II)-catalyzed
polymerization of R-olefins.23
The final step in the proposed catalytic cycle is the cleavage
of [Pd]-R′′ with HSiR3. The deuterium labeling studies suggest
that the regiochemistry of this reaction is dependent on the steric
demands of both HSiR3 and [Pd]-R′′; typically, the least
hindered isomer of [Pd]-R′′ reacts with HSiR3.
In the reaction of tert-butylethylene with HSi(i-Pr)3, both
possible isomers of [Pd]-R′′ have bulky substituents R to Pd
(eq 13); in this situation, the reaction of [Pd]-R′′ with HSi(i-
tion and reinsertion of t-BuCHCHSiEt3 with opposite regio-
chemistry and subsequent cleavage by DSiEt3.
The reaction of 1-hexene with DSiEt3 in the presence of 1b
yields Et3Si(C6H12D). Deuterium is incorporated at all positions
along the n-hexyl chain except at CH2SiEt3. This result suggests
that “chain-running” is facile. However, unlike the other
examples shown in Scheme 5, the reaction is not regioselective.
The deuterium labeling studies suggest that reversible â-hy-
dride elimination/olefin reinsertion reactions can occur prior to
eventual cleavage of the Pd alkyl intermediate with HSiEt3.
Steric factors appear to predominate in these systems; e.g., the
least congested alkyl intermediate is the most likely to react
with silane. For tert-butylethylene and cyclohexene the differ-
ence in reactivity of the R- and â-silylalkyl intermediates is large
enough that only the most accessible isomer reacts with HSiEt3.
For 1-hexene, there is not a significant steric difference between
the various alkyl intermediates of the general formula [(phen)-
PdCH(CnH2n+1)(C5-nH2(5-n))SiEt3]+ and thus the reaction of
1-hexene with DSiEt3 is not regiospecific.
C. Mechanism of Silation Reactions of Ethylene and
Alkyl-Substituted Olefins. A mechanistic scheme for the
(phen)Pd(II)+-catalyzed silation reactions of alkyl-substituted
olefins and ethylene can be proposed based on the results of
the low-temperature NMR studies, kinetics experiments, and
deuterium labeling studies. This mechanism is shown in
Scheme 6. The key features of this mechanism are (1) rapid,
reversible silyl migration to olefin, (2) isomerization of the
intermediate alkyl complexes through a series of â-hydride
elimination reactions and reinsertion of the bound olefin with
opposite regiochemistry, and (3) cleavage of the alkyl complex
by HSiR3.
Pr)3 is predicted to be disfavored, in agreement with the
experimental results. To test this hypothesis, a CD2Cl2 solution
of 4c was prepared at -78 °C and the solution was warmed to
-20 °C. Dissociation of trans-(i-Pr)3SiCHdCH-t-Bu occurs
both in the presence and absence of HSi(i-Pr)3. No hydrosilation
is observed. The results of this NMR experiment are consistent
with both the proposed mechanism and the results of the
catalytic silation of tert-butylethylene by HSi(i-Pr)3. The
dehydrogenative silation reaction will be discussed in greater
detail in the next section.
III. Silation Reactions of Styrene. The silation reactions
of styrene were investigated by low-temperature NMR spec-
troscopy and deuterium labeling studies in an attempt to gain a
mechanistic understanding of this system. To our knowledge,
no mechanistic studies on dehydrogenative silation have been
reported. We were particularly interested in determining why
dehydrogenative silation occurs much more readily for styrene
than for alkyl-substituted olefins such as cyclohexene or tert-
butylethylene.
A. NMR Studies. The reaction of 2a with styrene in CD2-
Cl2 at -78 °C yields a new complex, 6 (eq 14). 1H and 13C
NMR data for 6 are consistent with the formulation [(phen)-
Pd(η3-CH(CH2SiEt3)C6H5)]+[BAr′4]- based on comparisons
with the related, structurally characterized π-benzyl complex
[(phen)Pd(η3-CH(CH2CH3)C6H5)]+[BAr′4]- which is formed in
the reaction of 1a with styrene.29 In the absence of HSiEt3 or
Although there have been several examples of silyl migration
to olefins,18,19 including the cobalt-based hydrosilation catalyst
described above,8 there have been only a few well-defined
examples of â-silyl elimination reactions. Wrighton and co-
workers observed reversible insertion of ethylene into the Fe-
Si bond upon photolysis of (Cp)Fe(CO)2SiMe3 under ethylene.17
In an example of an irreversible â-silyl elimination reaction, a
(31) Wakatsuki, Y.; Yamazaki, H.; Nakano, M.; Yamamoto, Y. J. Chem.
Soc., Chem. Commun. 1991, 703.

912 J. Am. Chem. Soc., Vol. 119, No. 5, 1997
LaPointe et al.
styrene, CD2Cl2 solutions of 6 are stable for several days at 25
°C; however, when solutions of 6 were concentrated, decom-
position to palladium black and trans-Et3SiCHdCHC6H5 oc-
curred.
The η2-styrene complex, [(phen)Pd(SiEt3)(η2-H2CdCHC6H5)]+-
[BAr′4]-, was not observed in the reaction of 2a with styrene
at -78 °C. Thus, silyl migration is quite rapid even at low
temperatures. This result is in agreement with the results from
the silyl scrambling experiments described above. Following
silyl migration to styrene, isomerization of the initially formed
σ-benzyl intermediate to the more stable π-benzyl complex 6
occurs (eq 15). Qualitatively, the silyl migration to styrene is
Figure 2. Plots of kobs vs [HSiEt3] for the reaction of [(phen)Pd(η3-
CH(CH2SiEt3)C6H5)]+ (6) and [(phen)Pd(η3-CH(CH3)C6H5)]+ (7) ([6
or 7] ) 0.027 M) with HSiEt3 at 13 °C. (6 + HSiEt3 f 2a +
PhCH2CH2SiEt3; kobs ) 1.1 × 10-4 s-1 [HSiEt3]; R2 ) 0.995.) (7 +
HSiEt3 f 2a + PhCH2CH3; kobs ) 7.1 × 10-4 s-1 [HSiEt3]; R2 )
0.998.)
Scheme 7. Proposed Associative Benzyl Exchange
Mechanism for the Reaction of
[(phen)Pd(η3-CH(CH2SiR3)C6H5)]+ (6) and Styrene
([Pd] ) (phen)Pd+)
faster than the analogous methyl migration to styrene in [(phen)-
Pd(CH3)(η2-H2CdCHC6H5)]+[BAr′4]- (∆Hq ) 16.6 ( 1.3 kcal/
mol, ∆Sq ) -6.6 ( 5.4 eu).29
The reaction of 6 with excess HSiEt3 results in formation of
PhCH2CH2SiEt3 and regeneration of 2a. The rate was measured
under pseudo-first-order conditions (10-30 equiv of HSiEt3)
at 13 °C and found to be first order in HSiEt3 (kobs ) 1.1 ×
10-4 M-1 s-1 [HSiEt3] (Figure 2). The rate is much slower
than that observed for the reaction of 4b with HSiEt3 (-38 °C;
kobs ) 1.7 × 10-3 M-1 s-1 [HSiEt3]). Since the steric demands
of a tert-butyl group are actually greater than that of a phenyl
group, the low reactivity of 6 with HSiEt3 must be attributed to
the equilibrium between the σ- and π-benzyl isomers of 6, which
strongly favors the π-benzyl isomer.
In the presence of excess styrene at 25 °C, 6 reacts with
styrene to produce trans-Et3SiCHdCHC6H5 and [(phen)Pd(η3-
CH(CH3)C6H5)]+[BAr′4]- (7), (eq 16). The displacement of
The kinetics of the reaction of 6 with excess styrene were
measured at 13 °C. The results are shown in Figure 3. The
system approached saturation at higher styrene concentrations.
The saturation rate constant was obtained as the reciprocal of
the intercept of the plot of 1/kobs vs 1/[styrene] (ksat ) 1.1 ×
10-4 s-1). The rate dependence on [styrene] suggests that
displacement of trans-Et3SiCHdCHPh occurs via an associative
mechanism. Similar kinetic behavior was observed in the
reaction of [(phen)Pd(η3-CH(CH2CH3)C6H5)]+[BAr′4]- with
styrene.29 In a typical catalytic reaction (Table 2), [styrene]initial
> 2 M, so saturation behavior is assumed.
The π-benzyl complex 7 reacts with HSiEt3 to produce
ethylbenzene and 2a. The reaction of 7 with HSiEt3 was
monitored under pseudo-first-order conditions (0.027 M 7, 10-
30 equiv of HSiEt3) at 13 °C. The reaction of 7 with HSiEt3
was found to be first order in both 7 and HSiEt3 (kobs ) 7.1 ×
10-4 M-1 s-1 [HSiEt3]) (Figure 2).
trans-Et3SiCHdCHC6H5 is reversible, but the equilibrium lies
in favor of the less-crowded π-benzyl complex 7 (25 °C; Keq
) 26). In a related reaction, [(phen)Pd(η3-CH(CH2CH3)-
C6H5)]+[BAr′4]- reacts with excess styrene to produce 7 (25
°C; Keq ) 249) and trans-methylstyrene.29 Mechanistic studies
for this process suggest that the benzyl transfer reaction proceeds
via â-hydride elimination followed by associative displacement
of trans-CH3CHdCHPh by styrene. â-Hydride migration and
rapid isomerization produces the new π-benzyl complex 7.
Based on the kinetics experiments (see below) and the similari-
ties between the two systems, it is likely that the interconversion
of 6 and 7 proceeds through a similar series of reactions (Scheme
7).
1H NMR spectroscopy of a working catalyst solution revealed
the presence of both 6 and 7 in the presence of excess styrene
and HSiEt3. Thus, the π-benzyl complexes 6 and 7 are the
catalyst resting state for the silation reactions of styrene. 7 can
be recovered in >90% yield (based on 1b) from the reaction
mixture by precipitation with hexane upon completion of the
catalytic reaction.
B. Deuterium Labeling Studies. The hydrosilation and
dehydrogenative silation reactions of styrene with DSiEt3 were

Palladium(II)-Catalyzed Hydrosilation
J. Am. Chem. Soc., Vol. 119, No. 5, 1997 913
Scheme 8. Reactions of Styrene with DSiEt3 in the
Presence of 1b
investigated (Scheme 8). The reaction of a 3-fold molar excess
of DSiEt3 with styrene in the presence of 1% 1b yielded C6H5-
CHDCH2SiEt3. In contrast, the reaction of excess styrene with
DSiEt3 in the presence of 1% 1b yielded C6H5CH2CH2D as the
only deuterated product. No evidence for deuterium incorpora-
tion into trans-C6H5CHCHSiEt3 was observed. The reaction
of 7 with 4 equiv of DSiEt3 in CD2Cl2 yielded C6H5CH2CH2D.
Once again, steric factors appear to govern the regiochemistry
of cleavage by silane.
C. Mechanism of Hydrosilation and Dehydrogenative
Silation of Styrene. A mechanistic scheme can be proposed
based on the results of the NMR experiments and the deuterium
labeling studies. This mechanism is shown in Scheme 9. As
was the case with the hydrosilation, silyl migration to coordi-
nated olefin is a key feature in the silation reactions of styrene.
However, rapid isomerization to the π-benzyl complex 6 occurs,
so that the complexes [(phen)Pd(SiEt3)(η2-CH2CHPh)]+ and
[(phen)Pd(η1-CH(CH2SiEt3)(C6H5)], which are plausible inter-
mediates in the conversion of 2a to 6, are not observed.
The concentration-dependent rate constants for the conversion
of 6 to 7 and the reaction of 6 and 7 with HSiEt3 at 13 °C can
be compared directly (Scheme 10). The reaction of the π-benzyl
complexes 6 and 7 with excess HSiEt3 show a first-order
dependence on [HSiEt3] and no evidence of saturation kinetics
under the reaction conditions (Figure 2). In contrast, saturation
Figure 3. (A) A plot of kobs vs [styrene] for the reaction of
[(phen)Pd(η3-CH(CH2SiEt3)C6H5)]+ (6) ([6] ) 0.027 M) and styrene
yielding [(phen)Pd(η3-CH(CH3)C6H5)]+ (7) and trans-PhCHdCHSiEt3.
(B) A plot of 1/kobs vs 1/[styrene] for the conversion of 6 to 7. The ksat
was determined from the inverse of the intercept of the inverse plot
(ksat ) 1.1 × 10-4 s-1). See ref 29 for the derivation of the kinetic
expression for the benzyl exchange reaction.
behavior is observed in the conversion of 6 to 7. For the
conversion of 6 to 7, ksat ) 1.1 × 10-4 s-1. In a typical catalytic
reaction (Table 2), the initial styrene concentration >2 M and
saturation is assumed.
Scheme 9. Proposed Mechanism for the [(phen)Pd]+-Catalyzed Hydrosilation and Dehydrogenative Silation of Styrene ([Pd] )
(phen)Pd+)

914 J. Am. Chem. Soc., Vol. 119, No. 5, 1997
LaPointe et al.
Scheme 10. Comparison of the Concentration-Dependent
Rate Constants for the Rate-Determining Steps of the
Hydrosilation and Dehydrogenative Silation Reactions of
Styrene with HSiEt3 at 13 °C
suggests that â-silyl elimination may be a facile reaction of other
transition metal â-silyl(alkyl) complexes. The silyl migration
and elimination reactions are similar to hydride and alkyl
migration reactions and â-hydrogen and â-methyl abstraction
reactions, respectively.2
(3) The deuterium labeling studies and formation of vinyl
silanes suggest that in addition to the rapid, the degenerate
â-silyl migrations, â-hydrogen elimination and reinsertion
reactions are operative in this system. Both the â-silyl migra-
tions and the â-hydride migrations are operative at low
temperatures (<-40 °C).
(4) The eventual reaction of the Pd(II) alkyl intermediate
([Pd]-R ′′) with HSiR3 appears to be governed by steric factors,
with the most accessible isomer reacting the fastest. If
intermediate [Pd]-R′′ and/or the silane HSiR3 are too bulky,
the cleavage of [Pd]-R′′ by HSiR3 is disfavored and associative
displacement of vinylsilane by alkene can occur, resulting in
dehydrogenative silation. In this system, dehydrogenatiVe
silation results when the reaction with HSiR3 is inhibited, rather
than when associative displacement is promoted (e.g., by
addition of a coordinating ligands such as PPh3).
(5) The proposed mechanism of styrene hydrosilation and
dehydrogenative silation is similar to that proposed for the
silation reactions of ethylene and alkyl-substituted olefins. The
main difference is that in the silation reactions of styrene, the
catalyst resting states are the π-benzyl complexes 6 and 7 rather
than the η2-olefin complexes, [(phen)Pd(SiR3)(η2-H2CdCHR′)]+.
The stability of the π-benzyl complexes results in a slow reaction
with HSiR3. As a result of the low reactivity of 6 with HSiR3,
the associative displacement of vinylsilane by styrene becomes
competitive with cleavage by HSiR3, and significant amounts
of dehydrogenative silation are observed, despite the relatively
low steric demands of SiEt3 and phenyl groups.
(6) It should be emphasized that the catalysts used in this
study are highly electrophilic, cationic Pd(II) complexes. The
Pd(II)-catalyzed hydrosilation and dehydrogenative silation
reactions reported here may proceed by a different mechanism
than Pd(0)-catalyzed hydrosilation of olefins, in which dehy-
drogenative silation is not observed and hydrosilation of styrene
occurs with silation of the benzylic carbon.
The results reported here contribute to an understanding of
the factors that govern hydrosilation and dehydrogenative
silation reactions. Using the mechanistic information obtained
in this study, we are currently investigating the development
of new catalysts that are highly selective for dehydrogenative
silation.
As shown in Scheme 10, the displacement of trans-C6H5-
CHdCHSiEt3 from 6 results in dehydrogenative silation of
styrene, and the reaction of 6 with HSiEt3 results in hydrosi-
lation. The similarities in the rates of the two processes and
the fact that the rate of displacement of trans-C6H5CHdCHSiEt3
saturates at high styrene concentration suggest that the distribu-
tion of hydrosilation and dehydrogenative silation in the catalytic
reactions might be significantly altered by changing the relative
ratios of styrene and HSiEt3. This was observed in the catalytic
silations of styrene (Table 2), in agreement with the proposed
mechanism. No quantitative predictions of product distributions
were possible since the rates were determined at 13 °C in CD2-
Cl2 and the preparative scale silations were conducted at higher
temperature (35-85 °C).
The difference in the rate constants for the reactivity of the
π-benzyl complexes 6 and 7 with HSiEt3 may be due to steric
factors. If this is the case, then it suggests that the reaction of
the π-benzyl complexes 6 or 7 with a bulkier silane (such as
HSi(i-Pr)3) would be even slower. This was confirmed quali-
tatively by noting that no reaction was observed between 7 and
excess HSi(i-Pr)3 in CD2Cl2 at 25 °C. Since the rate of
associative displacement of styrene would be unlikely to change
as much, the use of bulkier silanes should favor the dehydro-
genative silation reaction rather than the hydrosilation reaction.
This was observed in the reactions of styrene with HSi(i-Pr)3
(Table 2). Similar results were recently observed in a Rh-
catalyzed dehydrogenative silation of styrene.13
Even when steric factors are taken into account, the silation
reactions of styrene proceed much more slowly than the silation
reactions of alkyl-substituted olefins. It is likely that this
difference results from the high stability of the π-benzyl
intermediates 6 and 7. The relatively low reactivity of 6 toward
HSiR3 allows an alternate reaction, the displacement of trans-
R3SiCHdCHPh by styrene, to occur at a similar rate. Thus,
dehydrogenative silation is competitive with hydrosilation in
this system.
Experimental Section
General. Unless otherwise noted, all reactions were conducted under
an atmosphere of dry, deoxygenated argon or nitrogen using standard
Schlenk techniques or in a Vacuum Atmospheres glovebox. Pentane,
hexane, ether, toluene, and tetrahydrofuran were distilled from sodium
benzophenone ketyl under a nitrogen atmosphere prior to use. Dichlo-
romethane was distilled from P4O10 under a nitrogen atmosphere. CD2-
Cl2 (CIL) was dried over CaH2 under argon and was degassed and
vacuum transferred. CDCl3 was used as received. (phen)PdMe2,29
H(OEt2)2BAr′4,30 [(phen)Pd(CH3)(OEt2)]+[BAr′4]- (1a),29 [(phen)Pd-
(CH3)(NCCH3)]+[BAr′4]- (1c),26 and [(phen)Pd(η3-CH(CH3)C6H5)]+-
[BAr′4]- (7)29 were prepared according to reported procedures. Eth-
ylene (polymer grade) was purchased from Matheson Gases. Unless
otherwise noted, silanes, chlorosilanes, and olefins were purchased from
Aldrich and used without further purification. Et3SiD was prepared
from Et3SiCl and LiAlD4 in Et2O.8 1H NMR of the product revealed
<5% H incorporation. Allyltriethylsilane was prepared from allyl-
magnesium bromide and Et3SiCl in THF.
Conclusions
Several conclusions can be drawn from the results of the
mechanistic studies and deuterium labeling studies.
(1) In the silation reactions of alkyl-substituted R-olefins and
ethylene, the catalyst resting state is the η2-olefin complex,
[(phen)Pd(SiR3)(η2-H2CdCHR′)]+. This is true for both hy-
drosilation and dehydrogenative silation.
1H and 13C chemical shifts were referenced to residual protio solvent
peaks and solvent 13C peaks, respectively. Coupling constants are
(2) Silyl migration to olefin is rapid and reVersible at -60
°C. The ease of â-silyl elimination reactions in this system

Palladium(II)-Catalyzed Hydrosilation
J. Am. Chem. Soc., Vol. 119, No. 5, 1997 915
reported in hertz; routine coupling constants are not listed. Elemental
analyses were performed by Oneida Laboratories.
was bubbled through the solution and the mixture was allowed to warm
to room temperature. After 20 min, a precipitate of Pd black had
formed and ethylene addition was stopped. Solvent was removed in
Atom labeling schemes for the phenanthroline and ((CF3)2C6H3)4B-
counterion resonances are as follows:
1
vacuo, yielding a colorless oil that was pure SiEt4 by H NMR (1.20
g, 85%).
Si(i-Pr)3(Et). Si(i-Pr)3(Et) was prepared in a procedure similar to
that used for SiEt4. The resulting colorless oil was pure by 1H and 13
C
NMR (yield 75%). 1H NMR (CDCl3, 20 °C) δ 1.0 (m, 24, i-Pr +
SiCH2CH3), 0.82 (q, 2, SiCH2CH3). 13C NMR (CDCl3, 20 °C) δ 18.0
(CHMe2), 10.0 (CHMe2), 7.33, 0.0 (SiEt). Anal. Calcd for C11H26Si:
C, 70.87; H, 14.06. Found: C, 70.66; H, 14.63.
SiPh3Cy. In a drybox, solid HSi(C6H5)3 (1.37 g, 5.27 mmol) and
1b (76 mg, 0.05 mmol) were loaded into a Schlenk flask. The solids
were then dissolved in 10 mL of CH2Cl2 at -78 °C and cyclohexene
(500 µL, 5.0 mmol) was added. The solution was warmed to room
temperature and allowed to stir overnight. Solvent was removed in
vacuo, yielding a gray solid. The solid was dissolved in 50 mL of hot
hexane and filtered. The volume of the filtrate was reduced to 20 mL,
at which point crystals began to form. Colorless crystals were collected
1
The H NMR resonances were assigned into groups of a, b, c, d or a,
a′, b, b′, c, c′, d, d′ according to their characteristic coupling patterns.
The 13C NMR resonances were assigned in pairs such as Ca or Ca′ and
Ca′ or Ca, based on their chemical shifts and 1JCH. The relation between
1
the H and 13C assignments and the stereochemistry with respect to
the ligands X and Y has not been ascertained.
1
and dried (1.37 g, 78%) H NMR (CDCl3, 20 °C) δ 7.3-7.5 (m, 15,
-
1H and 13C data attributed to the counterion BAr′4 (Ar′ )
SiPh3), 1.94 (br d, 2, CH2), 1.7 (br s, 2, CH2), 1.6 (t, 1, CHSiPh3),
1.4-1.1 (br m, 6, CH2). 13C NMR (CDCl3, 20 °C) δ 136.0, 134.6,
129.2, 122.7 (Ph), 28.2, 28.1, 26.8 (CH2), 24.1 (CHSiPh3).
3,5-(CF3)2C6H3) follow. These are consistent for all examined cationic
complexes and are not included in each compound characterized below.
1H NMR (CD2Cl2) δ 7.72 (s, 8, Ho), 7.56 (s, 4, Hp). 1H chemical shifts
are accurate to within (0.02 ppm. 13C NMR (CD2Cl2) δ 162.1 (q,
SiPh3(CH2CH2-t-Bu). Si(C6H5)3(CH2CH2-t-Bu) was prepared using
the same procedure described for Si(C6H5)3(C6H11). A colorless oil
was obtained which crystallized from hexane at -30 °C after 2 months
(recrystallized yield 20%). 1H NMR (CDCl3, 20 °C) δ 7.3-7.5 (m,
15, SiPh3), 1.55 (br s, 2, CH2-t-Bu), 1.32 (br s, 2, CH2SiPh3), 0.85 (s,
9, t-Bu). 13C NMR (CDCl3, 20 °C) δ 135.7, 135.4, 129.3, 127.8 (Ph),
37.6 (CH2-t-Bu), 31.3 (CMe3), 28.8 (CMe3), 7.5 (CH2SiPh3). Anal.
Calcd for C24H28Si: C, 83.66; H, 8.19. Found: C, 83.07; H, 7.92.
t-BuCHdCHSi(i-Pr)3. 1a was generated in situ at -78 °C from
phenPdMe2 (31 mg, 0.1 mmol) and [H(OEt2)2]+[BAr′4]- (101 mg, 0.1
mmol) in CH2Cl2. HSi(i-Pr)3 (2.0 mL, 10 mmol) and t-BuCHdCH2
(2.6 mL, 20 mmol) were added via syringe. The mixture was allowed
to warm to room temperature and stir for 24 h. CH2Cl2 and t-BuCH2-
CH3 were removed in vacuo. The resulting oil was dissolved in hexane
(10 mL) and filtered through a pipet of alumina (4 × 1 cm). Hexane
was removed in vacuo, leaving a colorless oil that was pure
t-BuCHdCHSi(i-Pr)3 by 13C NMR spectroscopy (1.97 g, 82%). 1H
NMR (CDCl3) δ 6.07 (d, 1, JH-H ) 19 Hz, CH-t-Bu), 5.37 (d, 1, JH-H
) 19 Hz, CHSi(i-Pr)3), 1.0 (br m, 30 total, Si(iPr)3 + t-Bu). 13C NMR
(CDCl3) δ 159.9 (JC-H ) 145 Hz, CH-t-Bu), 115.7 (JC-H ) 134 Hz,
CHSi(i-Pr)3), 35.4 (CMe3), 29.1 (CMe3), 18.6 (CH(Me)2), 10.9 (CH-
(Me)2). Anal. Calcd for C15H32Si: C, 74.91; H, 13.41. Found: C,
74.16; H, 13.24.
Reactions of Styrene. In a typical procedure, 1b or 7 (0.05 mmol)
was loaded into a flask that was fitted with a Teflon plug. For the
reactions conducted at 35 °C, CH2Cl2 (1 mL) was added to dissolve
the catalyst. For the reactions conducted at 85 °C, the catalyst was
soluble in styrene/silane and no solvent was added. Styrene and HSiR3
(R ) C2H5, CH(CH3)2) were then added and the reaction was allowed
to stir for 24 h. CH2Cl2, ethylbenzene, and unreacted styrene and HSiR3
were then removed in vacuo, leaving a pale yellow oil and a yellow
solid. The oil was dissolved in hexane and passed through a pipet full
of alumina (4 × 1 cm) to remove any remaining 7. Hexane was then
removed in vacuo, leaving a colorless oil that was a mixture of C6H5-
CH2CH2SiR3 and trans-C6H5CHdCHSiR3. 1H and 13C NMR data
matched reported values.13 Yields and product distribution are given
in Table 1. The hexane-insoluble yellow solid isolated from the reaction
mixture was washed with hexane and dried. 1H NMR data for the
solid matched that of 7.
2
JC-B ) 50 Hz, Ci), 135.2 (Co), 129.3 (q, JC-F ) 31 Hz, Cm), 125.0 (q,
JC-F ) 272 Hz, CF3), 117.8 (Cp). 13C NMR chemical shifts and coupling
constants are consistent to within (1 ppm and (2 Hz, respectively.
[(phen)Pd(CH3)(Me3SiCtCSiMe3)]+[BAr′4]- (1b). Solid (phen)-
Pd(CH3)2 (116 mg, 0.37 mmol) and [H(OEt2)2]+[BAr′4]- (365 mg, 0.37
mmol) were combined. The reaction flask was cooled to -30 °C and
Et2O (10 mL) and CH2Cl2 (5 mL) were added. The resulting slurry
was allowed to warm to 25 °C to dissolve solid (phen)Pd(CH3)2, and
then the solution was cooled to -30 °C. Bis(trimethylsilyl)acetylene
(85 µL, 0.37 mmol) was added and colorless microcrystals formed.
The mixture was allowed to warm to room temperature, and the solid
dissolved. The mixture was stirred for 1 h and then the volume was
reduced to 5 mL in Vacuo and cooled slowly to -78 °C. Colorless
needles formed and were washed with 10 mL of cold Et2O, collected,
and dried (yield 285 mg, 59%). 1H NMR (CD2Cl2, 20 °C) δ 8.95 (d,
1, phen Ha), 8.71 (d, 1, phen Ha), 8.59 (dd, 1, phen Hc), 8.54 (dd, 1,
phen Hc), 8.05 (s, 2, phen Hd), 8.00 (dd, 1, phen Hb), 7.92 (dd, 1, phen
Hb), 1.06 (s, 3, PdCH3), 0.32 (s, 18, Si(CH3)3). 13C NMR (CD2Cl2, 20
°C) δ 147.2, 145.2 (Ca, Ca′), 146.2, 143.2 (Ce, Ce′), 139.7, 138.4 (Cc,
Cc′), 129.9, 129.3 (Cf, Cf′), 126.8, 126.5 (Cd, Cd′), 124.6, 124.4 (Cb,
Cb′), 103.6 (Me3SiCtCSiMe3), 7.9 (PdCH3), -1.7 (SiMe3). Anal. Calcd
for C53H41N2BF24PdSi2: C, 47.67; H, 3.09; N, 2.10. Found: C, 47.83;
H, 3.04; N, 1.94.
Hydrosilation with HSiEt3. General Procedure. In a glovebox,
solid [(phen)Pd(CH3)(Me3SiCCSiMe3)]+[BAr′4]- (150 mg, 0.10 mmol)
was loaded into a Schlenk flask. The flask was removed from the
glovebox and 5 mL of CH2Cl2 was added. Olefin (10.0 mmol) was
added to the solution and HSiEt3 (1.80 mL, 11.2 mmol) was then added.
The yellow solution was stirred overnight. CH2Cl2, HSiEt3, and olefin
were removed in vacuo, leaving a clear oil and a precipitate of Pd black
and deactivated catalyst residue. The oil was dissolved in hexane (5
mL) and passed through alumina (4 × 1 cm). Hexane was removed
in vacuo, leaving a colorless oil which was pure by 1H NMR. Yields
and NMR data for the hydrosilation products are given below.
SiEt3Cy. Yield ) 88%. 1H NMR (CDCl3, 20 °C) δ 1.70 (m, 6),
1.17 (m, 4, CH2CHSiEt3), 0.92 (t, 9, SiCH2CH3, 0.7 (t, 1, CHSiEt3),
0.49 (q, 6, SiCH2CH3). 13C NMR (CDCl3, 20 °C) δ 28.4, 27.9, 27.2,
23.6 (C6H11), 7.7 (SiCH2CH3), 2.0 (SiCH2CH3).
SiEt3(CH2CH2-t-Bu). Yield ) 79%. 1H NMR (CDCl3, 20 °C) δ
1.1 (m, 2, CH2-t-Bu), 0.90 (t, 9, SiCH2CH3), 0.82 (s, 9, t-Bu) 0.5 (m,
2, CH2SiEt3), 0.47 (q, 6, SiCH2CH3). 13C NMR (CDCl3, 20 °C) δ 37.8
(CH2-t-Bu), 31.1 (CMe3), 28.8 (CMe3), 7.47 (SiCH2CH3), 5.05 (CH2-
SiEt3), 3.20 (SiCH2CH3).
[(phen)Pd(SiEt3)(HSiEt3)]+[BAr′4]- (2a). [(phen)Pd(CH3)(OEt2)]+-
[BAr′4]- (53 mg, 0.042 mmol) was loaded into a NMR tube. CD2Cl2
(700 µL) was added at -78 °C, the mixture was shaken to dissolve
the solid, and HSiEt3 (14 µL, 0.11 mmol) was added. The sample
was inserted into a precooled NMR probe. 1H NMR (CD2Cl2, -60
°C) δ 9.05 (d, 2, phen), 8.54 (d, 2, phen), 7.96 (br. s, 4, phen), 1.00
(m, 30, SiCH2CH3 + SiCH2CH3), -9.99 (s, 1, Si-H-Si). 13C NMR
(CD2Cl2, -60 °C) δ 151.8, 144.4, 140.3, 130.0, 127.6, 125.8 (phen)
9.2 (SiCH2CH3), 8.5 (SiCH2CH3). 1H and 13C NMR revealed that the
two silyl groups were equivalent at -80 °C.
SiEt3([CH2]5CH3). Yield ) 88%. NMR data matched reported
data.8
SiEt4. In a modification of the above procedure, 1a (0.1 mmol)
was generated from (phen)PdMe2 and [H(OEt2)2][BAr′4] at -78 °C in
10 mL of CH2Cl2. HSiEt3 (1.55 mL, 9.8 mmol) was added. Ethylene

916 J. Am. Chem. Soc., Vol. 119, No. 5, 1997
LaPointe et al.
[(phen)Pd(SiPh3)(HSiPh3)]+[BAr′4]- (2b). Solid [(phen)Pd(CH3)-
(OEt2)]+[BAr′4]- (30 mg, 0.024 mmol) and HSiPh3 (18 mg, 0.069
mmol) were combined in an NMR tube. CD2Cl2 (700 µL) was added
at -78 °C and the tube was shaken briefly to dissolve the solids. The
sample was inserted into a precooled NMR probe. 1H NMR (CD2Cl2,
-80 °C) δ 10.2 (br s, 2, phen Ha,a′) 8.2-7 (36, phen + SiPh3), -12.13
(s, 1, Si-H).
Table 3
[4a], M
[HSiEt3], M
vol of HSiEt3, µL
104 kobs, s-1
0.014
0.014
0.014
0.013
0.14
0.21
0.30
0.40
15.6
24
35
2.25
3.43
5.46
6.54
48
[(phen)Pd(SiEt3)(PPh3)]+[BAr′4]- (3a). Solid (phen)Pd(CH3)2 (64
mg, 0.20 mmol) and [H(OEt2)2]+[BAr′4]- (210 mg, 0.21 mmol) were
combined. CH2Cl2 (10 mL) was added at -78 °C and the mixture
was stirred until the solids had dissolved and methane evolution ceased.
HSiEt3 (65 µL, 0.40 mmol) was added to generate 2a. A solution of
PPh3 (62 mg, 0.20 mmol) in CH2Cl2 (3 mL) was added to the solution
of 2a at -78 °C. The mixture became orange and was stirred for 20
min. The volume was reduced to 3 mL and pentane (5 mL) was added.
An orange microcrystalline solid formed. The solid was collected,
washed with 5 mL of pentane and dried (145 mg, 48%). 1H NMR
(CD2Cl2) δ 7.2-9.0 (m, 23 total, phen + phenyl), 0.82 (t, 9, SiCH2CH3),
0.65 (q, 6, SiCH2CH3). 31P NMR (CD2Cl2) δ 37.8. Anal. Calcd for
C68H50N2BF24PPdSi: C, 53.47; H, 3.30; N, 1.83. Found: C, 53.03;
H, 2.89; N, 0.75.
probe. The concentration of 4b was monitored as a function of time;
the Ar′ Hpara resonance at 7.25 ppm was used as an internal standard.
Rate constants were obtained from a plot of ln [4b] vs time. Reaction
conditions and observed rate constants are shown in Table 3. The rate
dependence on [HSiEt3] was determined from the slope of the line
obtained from a plot of kobs vs [HSiEt3] (Figure 1) (kobs ) 1.7 × 10-3
M-1 s-1 [HSiEt3], R2 ) 0.979).
Reaction of 4b with HSiPh3. A CD2Cl2 solution (700 µL) of 4b
was generated from 1a (52 mg, 0.042 mmol), HSiEt3 (7 µL, 0.043
mmol), and tert-butylethylene (6 µL, 0.046 mmol) at -78 °C.
A
solution of HSiPh3 (24 mg, 0.09 mmol) in 300 µL of CD2Cl2 was then
added and the solution was allowed to warm to room temperature.
1
t-BuCH2CH2SiEt3 was observed by H and 13C NMR spectroscopy;
[(phen)Pd(SiEt3)(Ph2PCH2CHdCH2)]+[BAr′4]- (3b). 3b was
t-BuCH2CH2SiPh3 was not detected.
1
Observation of [(phen)Pd(SiEt3)(η2-H2CdCHCH2SiPh3)]+[BAr′4]-
(5a). 1a (33 mg, 0.027 mmol) was loaded into a 5-mm NMR tube
and dissolved in CD2Cl2 (400 µL) at -78 °C. HSiEt3 (8 µL, 0.05
mmol) was added and the mixture was shaken briefly to generate 2a.
Allyltriphenylsilane (10 mg, 0.033 mmol) was dissolved in 300 µL of
CD2Cl2 and added to the solution of 2a at -78 °C. The sample was
inserted into a precooled (-78 °C) NMR probe. 1H NMR (CD2Cl2,
-60 °C) δ 5.85 (br m, 1, H2CdCHCH2SiPh3), 4.22, 3.43 (d, 1 each,
H2CdCHCH2SiPh3), 2.20 (dd, CHaHbSiPh3), 1.62 (t, 1, CHaHbSiPh3).
Because of the equilibrium between 5a and 5b, the resonances
associated with SiPh3, phen, and SiEt3 are quite complicated and are
not listed here.
prepared in a manner similar to that described for 3a (yield 74%). H
NMR (CD2Cl2) δ 8.52, 8.50 (br s, 4 tot., phen + Ph), 7.99 (s, 2, phen),
7.1-7.8 (m, 12, phen + Ph), 5.93 (m, 1, CH2dCHCH2PPh2), 5.20 (dd,
1, CHaHbdCHCH2PPh2), 5.07 (dd, 1, CHaHbdCHCH2PPh2), 3.42 (m,
2, CH2PPh2), 0.95 (m, 15, SiEt3). 31P NMR (CD2Cl2) δ 29.3. Anal.
Calcd for C65H50N2BF24PPdSi: C, 52.35; H, 3.38; N, 1.87. Found:
C, 52.12; H, 3.20; N, 0.98.
[(phen)Pd(SiEt3)(η2-C2H4)]+[BAr′4]- (4a). [(phen)Pd(CH3)(OEt2)]+-
[BAr′4]- (10.2 mg, 0.08 mmol) was loaded into a NMR tube. CD2Cl2
(700 µL) was added at -78 °C, the mixture was shaken to dissolve
the solid, and HSiEt3 (2.0 µL, 0.015 mmol) was added. Ethylene (0.50
mL, 0.022 mmol) was added via syringe. The sample was inserted
into a precooled (-90 °C) NMR probe. 1H NMR (CD2Cl2, -90 °C)
δ 8.74, 8.37 (d, 1 each, phen Ha,a′), 8.65, 8.51 (d, 1 each, phen Hc,c′),
8.05, 7.88 (dd, 1 each, phen Hb,b′), 7.99 (s, 2, phen Hd,d′), 5.12, 4.70 (br
d, 2 each, C2H4), 0.88 (t, 9, SiCH2CH3), 0.55 (q, 6, SiCH2CH3). In the
presence of excess ethylene, exchange between free and bound ethylene
occurs and only a single averaged resonance is observed at 5.0 ppm.
[(phen)Pd(SiEt3)(η2-CH2dCH-t-Bu)]+[BAr′4]- (4b). [(phen)Pd-
(SiEt3)(η2-CH2dCH-t-Bu)]+[BAr′4]- was generated at -78 °C using
a procedure similar to that described for [(phen)Pd(SiEt3)(C2H4)]+[BAr′4]-.
1H NMR (CD2Cl2, -80 °C) δ 9.33, 9.25 (d, 1 each, phen Ha,a′), 8.50
(m, phen Hc,c′), 9.93 (phen Hb,b′ and Hd,d′), 4.17 (dd, 1, H2CdCH-t-
Bu), 3.63 (d, 1, HaHbCdCH-t-Bu), 3.12 (d, 1, HaHbCdCH-t-Bu), 1.02
(s, 9, t-Bu), 0.78 (t, 9, SiCH2CH3), 0.61 (q, 6, SiCH2CH3). 13C NMR
(CD2Cl2, -60 °C) δ 152.2, 151.5 (phen Ca,a′), 144.1, 143.2 (phen Ce,e′),
139.8, 139.5 (phen Cc,c′), 130.0, 129.7 (phen Cf,f′), 127.7, 127.5 (phen
Observation of [(phen)Pd(SiPh3)(η2-H2CdCHCH2SiEt3)]+[BAr′4]-
(5b). A solution of 5b in CD2Cl2 was prepared in a manner similar to
that described for 5a. 1H NMR (CD2Cl2, -60 °C) δ 4.8 (br m, 1,
H2CdCHCH2SiEt3), 4.05, 3.67 (d, 1 each, H2CdCHCH2SiEt3), 2.32
(dd, CHaHbSiEt3), 1.78 (t, 1, CHaHbSiEt3). Because of the equilibrium
between 5a and 5b, the resonances associated with SiPh3, phen, and
SiEt3 are quite complicated and are not listed here.
[(phen)Pd(SiEt3)(η2-H2CdCHCH2SiEt3)]+[BAr′4]- (5c). 1a (33
mg, 0.027 mmol) was loaded into a NMR tube. CD2Cl2 (700 µL) was
added at -78 °C, the mixture was shaken to dissolve the solid, and
HSiEt3 (4.2 µL, 0.036 mmol) was added. The mixture was shaken
briefly and allyltriethylsilane (5.4 µL, 0.27 mmol) was added via
syringe. The NMR sample was then inserted into a precooled (-78
°C) NMR probe and the spectra were acquired. 1H NMR (CD2Cl2,
-78 °C) δ 9.17 (d, 2, phen), 8.49 (d, 2, phen) 7.92 (s, 2, phen) 7.90
(m, 2, phen), 4.8 (br m, 1, H2CdCH2CH2SiEt3), 3.64, 3.36 (d, 1 each,
H2CdCH2CH2SiEt3), 1.74 (d, 1, H2CdCH2CHaHbSiEt3), 1.20 (t, 1,
H2CdCH2CHaHbSiEt3), 0.86 (t, 18, SiCH2CH3; note that the two SiEt3
resonances are apparently coincident), 0.63, 0.55 (q, 6 each, SiCH2-
CH3); 13C NMR (CD2Cl2) δ 150.7 (phen Ca,a′), 136.3 (phen Ce,e′), 139.4
(phen Cc,c′), 129.6 (phen Cf,f′), 125.4 (phen Cb,b′), 90.0 (H2CdCHCH2-
SiEt3), 61.8 (H2CdCHCH2SiEt3), 21.0 (H2CdCHCH2SiEt3), 8.3, 8.2
(SiCH2CH3), 6.9 (SiCH2CH3); note that the two SiEt3 resonances are
apparently coincident).
Cd,d′), 125.9, 125.85 (phen Cb,b′), 94.2 (H2CdCH-t-Bu), 55.0 (H2CdCH-
t-Bu), 35.8 (CMe3), 30.1 (CMe3), 15.5, 9.9 (SiEt3).
[(phen)Pd(Si(i-Pr)3)(η2-CH2dCH-t-Bu)]+[BAr′4]- (4c). In a 5-mm
NMR tube, 1a (52 mg, 0.042 mmol) was dissolved in CD2Cl2 (700
µL) at -78 °C. HSi(i-Pr)3 (8.5 µL, 0.042 mmol) was added and the
mixture was shaken once to mix. tert-Butylethylene (6 µL, 0.046
mmol) was added, yielding a yellow solution. The sample was inserted
into a precooled (-65 °C) NMR probe. 1H NMR (CD2Cl2, -65 °C)
δ 9.28, 9.00 (d, 1 each, phen Ha,a′) 8.55 (overlapping doublets, 2 total,
phen Hc,c′), 9.95 (s, 2, phen Hd,d′), 7.86 (m, 2, phen Hb,b′), 3.68 (d, 1,
HaHbdCH-t-Bu), 1.2 (br s, 30 total, i-Pr + t-Bu). Note: the peaks
associated with HaHbdCH-t-Bu and H2CdCH-t-Bu were not located
and were assumed to be obscured by the free ether peak at 3.4 ppm.
13C NMR (CD2Cl2, -65 °C) δ 154.3, 152.4 (Ca,a′) 146.2, 144.3 (Ce,e′),
139.5, 139.1 (Cc,c′), 130.2, 129.8 (Cf,f′), 127.4, 127.3 (Cd,d′), 126.1, 125.5
(Cb,b′), 72.0 (H2CdCH-t-Bu), 37.5 (H2CdCH-t-Bu), 30.2 (CMe3), 29.4
(CMe3), 18.3, 18.2 (CHMe2), 11.3 (CHMe2). Free t-BuCHdCHSi(i-
Pr)3 was observed when a solution of 4c was warmed to -20 °C.
Kinetic Study of the Reaction of 4b with HSiEt3. 1a (12 mg,
0.01 mmol) was loaded into a 5-mm NMR tube and dissolved in 700
µL of CD2Cl2 at -78 °C. HSiEt3 was added via syringe to generate
2a; t-BuCHdCH2 (1.2 µL, 0.009 mmol) was added and the NMR tube
was shaken briefly and inserted into a precooled (-38 ( 1 °C) NMR
[(phen)Pd(SiPh3)(η2-H2CdCHCH2SiPh3)]+[BAr′4]- (5d). Solid
HSiPh3 (15 mg, 0.05 mmol) and 1a (33 mg, 0.027 mmol) were loaded
into a NMR tube. The solids were dissolved in 400 µL of CD2Cl2 at
-78 °C. Allyltriphenylsilane (20 mg, 0.067 mmol) was dissolved in
400 µL of CD2Cl2 and added via syringe to the cold solution of
[(phen)Pd(SiPh3)(HSiPh3)]+[BAr′4]-. The NMR tube was shaken
briefly to mix the solution and then inserted into a precooled (-78 °C)
NMR probe. 1H NMR (CD2Cl2, -60 °C) δ 4.08, 3.42 (d, 1 each,
H2CdCHCH2SiPh3), 4.0 (br m, 1, H2CdCHCH2SiPh3), 3.08 (d, 1,
CHaHbSiPh3), 2.35 (t, 1, CHaHbSiPh3). The peaks associated with
phenyl, Ar′, and phen are not listed.
[(phen)Pd(η3-CH(CH2SiEt3)C6H5)]+[BAr′4]- (6). [(phen)Pd(CH3)-
(OEt2)]+[BAr′4]- (60 mg, 0.049 mmol) was loaded into a NMR tube.
CD2Cl2 (700 µL) was added at -78 °C. HSiEt3 (13 µL, 0.081 mmol)

Palladium(II)-Catalyzed Hydrosilation
J. Am. Chem. Soc., Vol. 119, No. 5, 1997 917
Table 4
Table 5
[6], M
[HSiEt3], M
vol of HSiEt3, µL
104kobs, s-1
[7], M
[HSiEt3], M
vol of HSiEt3, µL
104kobs, s-1
0.0263
0.0286
0.0273
0.49
0.87
1.18
60
98
141
0.53
1.05
1.30
0.0272
0.0267
0.026
0.287
0.414
0.560
0.742
34
50
70
95
2.12
3.07
4.12
5.18
0.0252
was added via syringe to the cold solution. The mixture was shaken
briefly to dissolve the solid and generate [(phen)Pd(SiEt3)(HSi-
Et3)]+[BAr′4]-. Styrene (6 µL, 0.0525 mmol) was added. 1H NMR
(CD2Cl2, -60 °C) δ 9.02, 8.56, 8.45, 8.41, 7.38, 7.22, 6.38 (d, 1 each,
phen + C6H5), 7.97 (dd, 1, phen), 7.93 (d, 2, phen), 7.56 (m, 2 phen),
7.29 (t, 1, phen), 4.11 (dd, 1, CHCH2SiEt3), 1.32 (m, 2, CH2SiEt3),
0.93 (t, 9, SiCH2CH3), 0.60 (q, 6, SiCH2CH3). 13C NMR (CD2Cl2,
-60 °C) δ 150.0, 149.9, 148.9, 147.7, 147.6, 146.5, 145.4, 144.0, 140.9,
140.8, 139.5, 139.2, 135.3, (phen + C6H5), 64.2 (CHCH2SiEt3), 14.6
(CHCH2SiEt3), 7.13 (SiCH2CH3), 2.64 (SiCH2CH3). Note: [(phen)Pd(η3-
CH(CH2SiEt3)(C6H5)]+[BAr′4]- is stable at 25 °C in CD2Cl2 solution,
but in the solid state it decomposes to form Pd black and (2-
triethylsilyl)styrene. For this reason it could not be isolated in
analytically pure form.
Kinetic Study of the Reaction of 6 with HSiEt3. 6 (0.02 mmol)
was generated in situ as described above. When the total volume of
HSiEt3 to be added was <100 µL, 700 µL of CD2Cl2 was added; when
the volume of HSiEt3 was >100 µL, 600 µL of CD2Cl2 was added.
Mesitylene (0.5 µL) was added as an internal standard and the solution
was cooled to -78 °C. HSiEt3 was added via a gas-tight syringe and
the sample was inserted into a precooled (13 ( 1 °C) NMR probe.
The concentration of 6 was monitored as a function of time. Rate
constants were obtained as the slope of the line -ln [6] vs time. Two
runs were performed at each HSiEt3 concentration and the rates were
averaged. The results are shown in Table 4 and in Figure 2. The rate
dependence on [HSiEt3] was determined from the slope of the line
obtained from a plot of kobs vs [HSiEt3] (kobs ) 1.1 × 10-4 M-1 s-1
[HSiEt3]).
Kinetic Study of the Reaction of 7 with HSiEt3. 7 (25 mg, 0.02
mmol) was loaded into a 5-mm NMR tube. Mesitylene (0.5 µL) was
added as an internal standard. When the total volume of HSiEt3 to be
added was <100 µL, 700 µL of CD2Cl2 was added; when the volume
of HSiEt3 was >100 µL, 600 µL of CD2Cl2 was added. The solution
was cooled to -78 °C and HSiEt3 was added via gas-tight syringe.
The sample was inserted into a precooled (13 ( 1 °C) NMR probe.
The concentration of 7 was monitored as a function of time. Rate
constants were obtained as the slope of the line -ln [7] vs time. Two
runs were performed at each HSiEt3 concentration and the rates were
averaged. The results are shown in Table 5 and in Figure 2. The rate
dependence on [HSiEt3] was determined from the slope of the line
obtained from a plot of kobs vs [HSiEt3] (kobs ) 7.1 × 10-4 M-1 s-1
[HSiEt3].)
Table 6
[6], M
[styrene], M
vol of styrene, µL
105kobs, s-1
0.0286
0.0266
0.025
1.25
1.75
2.18
100
150
200
7.14
8.33
8.62
the sample was inserted into a precooled (13 ( 1 °C) NMR probe.
The methine signals of 6 and 7 were monitored. Rate constants were
obtained as the slope of the line of -ln ([6]/[6]+[7]) vs time. Two
runs were performed at each styrene concentration and the rates were
averaged. The results are shown in Table 6 and in Figure 3. The
limiting rate constant (ksat) was obtained as the inverse of the intercept
of the plot of 1/kobs vs 1/[styrene]. ksat ) 1.1 × 10-4 s-1
.
Determination of the Equilibrium Constant for 6 + Styrene h
7 + trans-â-Triethylsilylstyrene (20 °C). A 5-mm NMR tube was
charged with 7 (21.5 mg, 0.0171 mmol), trans-â-triethylsilylstyrene
(45 mg, 0.204 mmol), and CD2Cl2 (700 µL). The tube was subjected
to three freeze-pump-thaw cycles and then flame-sealed under
vacuum. The 1H NMR spectrum was monitored until equilibrium was
reached (ca. 7 days). The equilibrium constant was calculated from
Keq ) [trans-â-triethylsilylstyrene][7]/[styrene][6]; the concentrations
were determined by the relative integrals. Keq ) 26.
Reactions of RCHdCH2 with DSiEt3: General Procedure. 1b
(15 mg, 0.01 mmol) was dissolved in 1 mL of CH2Cl2. Olefin (1 mmol)
and DSiEt3 (1.2 mmol) were added and the reaction was allowed to
stir overnight at 25 °C (1-hexene, cyclohexene, and tert-butylethylene).
When styrene was used as the substrate, the stoichiometry was varied
(styrene/DSiEt3 ) 0.33 and 2) and the reaction was stirred at 35 °C
overnight. CH2Cl2 was removed in vacuo and the resulting oil was
analyzed by 13C NMR spectroscopy. The results are shown below (only
the peaks showing D incorporation are listed).
Cyclohexene/DSiEt3. 13C NMR (CDCl3) δ 27.8 (t, JC-D ) 19,
CHDCHSiEt3).
t-BuCHdCH2/DSiEt3. 13C NMR (CDCl3) δ 4.78 (t, JC-D ) 17,
t-BuCH2CHDSiEt3).
1-Hexene/DSiEt3. 13C NMR (C6D6) δ 34.6, 32.6, 24.9, 15.0 (t).
Styrene (2 equiv)/DSiEt3. 13C NMR (CDCl3) δ 26.5 (t, JC-D ) 19
Hz, PhCH2CH2D).
Styrene (1 equiv) DSiEt3 (3 equiv). 13C NMR (CDCl3) δ 11.9 (t,
JC-D ) 19, PhCHDCH2SiEt3).
Kinetic Study of the Reaction of 6 with Styrene. 6 (0.02 mmol)
was generated in situ from 1a (25 mg, 0.02 mmol), HSiEt3 (3.5 µL,
0.022 mmol), and styrene (2.5 µL, 0.022 mmol) in CD2Cl2 (600 µL).
Mesitylene (0.5 µL) was added as an internal standard and the solution
was cooled to -78 °C. Styrene was added via gas-tight syringe and
Acknowledgment. We thank the National Institutes of
Health for funding (GM 28938). A.M.L. thanks the National
Science Foundation for a postdoctoral fellowship.
JA962979N