Palladium or platinum complex catalysed reactions of carbonyl and imine compounds with disilanes

Article abstract of DOI:10.1039/b207987a

The transition metal catalysed reactions of benzaldehydes and benzylideneamines with disilanes have been investigated. Palladium phosphine complexes catalyse the double silylation of the C=O bond in benzaldehydes and the C=N bond in benzylideneamines with

Full text of DOI:10.1039/b207987a

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Palladium or platinum complex catalysed reactions of carbonyl
and imine compounds with disilanes†
Neil A. Williams,*a Yuko Uchimarub and Masato Tanaka*b,c
a School of Chemical and Pharmaceutical Sciences, Kingston University,
Kingston upon Thames, UK KT1 2EE
b National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5,
Ibaraki 305-8565, Japan
c Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku,
Yokohama 226-8503, Japan
Received 14th August 2002, Accepted 13th November 2002
First published as an Advance Article on the web 12th December 2002
The transition metal catalysed reactions of benzaldehydes and benzylideneamines with disilanes have been
investigated. Palladium phosphine complexes catalyse the double silylation of the C᎐O bond in benzaldehydes and
᎐
the C᎐N bond in benzylideneamines with 1,2-difluoro-1,1,2,2-tetramethyldisilane to yield α-(fluorodimethylsilyl)-α-
᎐
(fluorodimethylsiloxy)toluene and N-methyl-N-(fluorodimethylsilyl)-α-(fluorodimethylsilyl)benzylamine respectively.
When less activated disilanes such as 1,2-dichloro- and 1,2-dimethoxy-1,1,2,2-tetramethyldisilane were employed, the
palladium phosphine complexes were less active and selective, resulting in extensive side reactions inclusive of 1,2-
disiloxy-1,2-diphenylethane formation. The reaction of benzophenone with the difluorodisilane formed 2,2-dimethyl-
4,4,5,5-tetraphenyl-1,3-dioxa-2-silacyclopentane without affording the corresponding simple double silylation
product. The formation of side products such as 1,2-disiloxy-1,2-diphenylethane in the reaction of benzaldehyde
and 2,2-dimethyl-4,4,5,5-tetraphenyl-1,3-dioxa-2-silacyclopentane in the reaction of benzophenone appears to
suggest intermediacy of radical and silylene species. Tris(dibenzylideneacetone)diplatinum–etpo (etpo = 4-ethyl-1-
phospha-2,6,7-trioxabicyclo[2.2.2]octane catalyst system was more active for unactivated disilanes, catalysing
double silylation of benzaldehydes with hexamethyldisilane. The same catalyst system was found to catalyse the
ortho silylation of benzylideneamines with disilanes via intramolecular C–H activation; both mono- and bis-silylated
products were obtained. Reaction rates and product distributions are rationalised in terms of the steric and electronic
properties of the disilanes, substrates and the catalyst used.
attractive synthetic strategy and is an area of great current
Introduction
interest. ortho-Functionalisation of aromatic imines has also
Organosilicon reagents have a diverse range of applications in
been reported recently.15
organic synthesis.1 Furthermore, they have important utilities
in material applications as monomers in synthesis of silicon-
containing functional polymers2 and as coupling agents
between organic polymers and inorganic surface modifiers.3
The transition metal catalysed double silylation of unsatur-
ated hydrocarbons has been developed into a convenient and
efficient method for synthesising organosilicon compounds.4,5
Unsaturated carbon heteroatom linkages however have received
much less attention.6–8 Previously, we discovered the platinum
complex catalysed dehydrogenative double silylation of alde-
hydes and ketones with o-bis(dimethylsilyl)benzene.9 This
prompted us to investigate the reactivity of disilanes towards
Results
Palladium or platinum complex catalysed double silylation of
benzaldehyde with disilanes
Table 1 summarises the results of the reaction of disilanes with
benzaldehyde (Scheme 1).
(1) Reaction of benzaldehyde with 1,2-difluorotetramethyl-
disilane. Screening of catalysts has revealed that Pd(PPh3)4 is
the catalyst of choice as far as 1,2-difluoro-1,1,2,2-tetramethyl-
disilane 1a is concerned. Thus, the reaction at 120 ЊC for 5 h
C᎐O and C᎐N bonds in the presence of a palladium or
᎐
᎐
platinum catalyst. To our knowledge, the only examples of
transition metal catalysed double silylation of aldehydes with
disilanes have involved the highly strained cyclic 3,4-benzo-
1,1,2,2-tetraethyl-1,2-disilacyclobut-3-ene and 3,4-bis(alkyl-
idene)-1,2-disilacyclobutanes.10 The double silylation of
imines has never been documented. The present paper
reports successful double silylations of C᎐O and C᎐N bonds,
᎐
᎐
inclusive of the unexpected o-silylation of imines via C–H
activation11 observed when the attempted double silylation
reaction with hexamethyldisilane or 1,2-diphenyl-1,1,2,2-
tetramethyldisilane was run in the presence of a catalyst com-
prising Pt2(dba)3 (dba = dibenzylideneacetone) and P(OCH2)3-
CEt (10/3 equivalents relative to Pt; this catalyst system will
be abbreviated as the Pt–etpo system). Functionalisation of
organic materials via catalytic C–H activation,12,13 including
silylation of aromatics with hydrosilanes,14 is an extremely
† Electronic supplementary information (ESI) available: characteriz-
ation data of the silicon-containing products obtained in the catalytic
reactions. See http://www.rsc.org/suppdata/dt/b2/b207987a/
Scheme 1
D a l t o n T r a n s . , 2 0 0 3 , 2 3 6 – 2 4 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
236

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Table 1 Catalytic performance for double silylation of benzaldehydea
Conversion
of 2a (%)
Yield
Yield
Entry
Disilane
Catalyst
t/h
T /ЊC
of 3 (%)b
of 4 (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13e
14
15
16
17
18
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1d
1d
1e
1e
1f
Pd(PPh3)4
Pd(PPh3)2Cl2
Pd(PEt3)2Cl2
Pd(PMe3)2Cl2
Pd(dppb)Cl2
Pd(dppf )Cl2
Pd(dba)2 ϩ 2PPh3
Ni(PEt3)4
Pd(PPh3)4
5
5
5
5
5
5
5
5
5
120
120
120
120
120
120
120
120
120
120
120
160
160
160
160
120
160
160
100
90
95
100
70
100
40
100
60
90
25
100
90
3a 97
3a 88
3a 85
3a 86
3a 35c
3a 30c
3a 38
3a 20c
3bd ∼12
3cd ∼20
3d 9
—
—
—
—
—
—
—
—
—
—
4d 5
4d 35
4d 10
4ed —
—
—
—
—
Pd(PPh3)4
5
5
½ Pt2(dba)3 ϩ5/3P(OCH2)3CEt
½ Pt2(dba)3 ϩ5/3P(OCH2)3CEt
½ Pt2(dba)3 ϩ5/3P(OCH2)3CEt
½ Pt2(dba)3 ϩ5/3P(OCH2)3CEt
Pt(PPh3)4
½ Pt2(dba)3 ϩ5/3P(OCH2)3CEt
½ Pt2(dba)3 ϩ5/3P(OCH2)3CEt
Pt(PPh3)4
20
120
20
20
20
20
20
3d 50
3d 78
3e 75
3e 7
100
7
5
100
95
3f 5
1f
3f 90
1f
3f 80
a Reaction conditions: benzaldehyde (0.5 mmol), disilane (0.5 mmol), catalyst 2 mol%, decane (30 µl) as internal standard, toluene (0.1 ml), under
nitrogen. b Yields determined by GLC and based on the quantity of the aldehyde charged. c Many products were observed by GLC analysis.
d Compound identified by GC/MS, but not isolated. e Reaction conditions: benzaldehyde (0.5 mmol), disilane (1.0 mmol).
gave the double silylated product α-(fluorodimethylsilyl)-α-
(fluorodimethylsiloxy)toluene 3a in 97% yield (entry 1). The
next best set of catalysts are the bis(monodentate phosphine)-
palladium() complexes (entries 2–4). The lower conversions,
compared to the palladium() complex, may be attributed to
the necessity of activation by reduction of palladium() to
palladium() in order to generate the proposed active catalyst
species or to the undesired interaction of the active pallad-
ium species with chlorofluorodimethylsilane generated in the
reduction process (vide infra). Bidentate phosphine complexes
Pd(dppb)Cl2 [dppb = 1,4-bis(diphenylphosphino)butane] and
conversion is envisioned to be associated with lower concen-
tration of active species as compared with the reaction using
Pd(PPh3)4.
Use of Pd(dba)2 or Pd(PhCN)2Cl2 without addition of any
phosphine ligand was totally inactive. Pt(PPh3)2Cl2, Pt(PEt3)2-
Cl2, Pt2(dba)3 and Ni(PEt3)2Cl2 were also completely inactive.
Ni(PEt3)4 catalysed the reaction (entry 8), but very many
peaks were observed in GC analysis, 2,2,4,4-tetramethyl-6,7-di-
phenyl-1,3,5-dioxa-2,4-disilacycloheptane being the major
product (27%) and the selectivity for 3a (20% yield) was much
lower than that with the palladium-monophosphine systems.
Pt(PPh3)4 catalysed reaction under similar conditions was even
less selective.
Pd(dppf )Cl2 [dppf
= 1,1Ј-bis(diphenylphosphino)ferrocene]
have been reported to exert high performance for the double
silylation of 4-phenyl-3-buten-2-one with PhCl2SiSiMe3.5g In
the present study however, these complexes were far less select-
ive (entries 5–6). Undesired side products (Scheme 2), tenta-
(2) Reaction of benzaldehyde with other disilanes. The
reaction of benzaldehyde with 1,2-dimethoxy-1,1,2,2-tetra-
methyldisilane 1b (entry 9) and 1,2-dichloro-1,1,2,2-tetramethyl-
disilane 1c (entry 10) was much less selective and gave only
small yields of the anticipated double silylated product. A
range of side products, inclusive of those similar to Scheme 2,
appeared to have been formed, as judged on the basis of the
GC-MS analysis.
In the reaction with hexamethyldisilane 1d, Pd(PPh3)4,
Pd(PEt3)2Cl2 and Pd(PMe3)2Cl2 did not show any catalytic
activity, although these complexes have been revealed to be
effective in the reaction of 1d with α-diketones and an α-keto
ester.6 The Pd(dba)2 ϩ 2P(OCH2)3CEt system that exhibited
high performance in the double silylation of acetylenes with
1d5d was also inactive, even when the reaction was executed in
the absence of any solvent. The analogous Pt–etpo system,
which is effective in the double silylation of α-diketones,6
catalysed double silylation with 1d at 120 ЊC, albeit slowly
(entry 11). The platinum catalyst system is more thermally
stable than the palladium systems, allowing the reaction to be
performed at 160 ЊC; the reaction was complete in 20 h. How-
ever the selectivity for 3d was poor (50% yield) and a consider-
able amount of a double silylated, dimerisation product, 1,2-
bis(trimethylsiloxy)-1,2-diphenylethane 4d (35% yield) was also
formed (entry 12). The selectivity for 3d could be improved by
using two equivalents (relative to benzaldehyde) of the disilane
1d; this gave a yield of 78% of 3d and 10% of 4d (entry 13). The
improvement may be rationalised in terms of disilane-assisted
reductive elimination (vide infra).
Scheme 2
tively identified from GC and GC–MS analyses, include:
PhCH(SiMe2SiMe2F)(OSiMe2F) m/z (EI, 70 eV) 318 (Mϩ),
[PhCH(OSiMe2F)]2 m/z 351 (Mϩ–Me), {PhCH(OSiMe2Si-
Me2F)}2 m/z (EI, 20 eV) 482 (Mϩ), 2,2-dimethyl-4,5-diphenyl-
1,3-dioxa-2-silacyclopentane m/z (EI, 70 eV) 270 (Mϩ) and
2,2,4,4-tetramethyl-6,7-diphenyl-1,3,5-dioxa-2,4-disilacy-
cloheptane m/z (EI, 70 eV) 344 (Mϩ). The formation of these
compounds may be partially rationalised in terms of the gener-
ation of silylene species, dimerisation of radicals produced by
Pd–C bond homolysis (vide infra), and redistribution reaction
of starting and resulting silane compounds.
Given that a bis(phosphine)palladium() species is the active
species, Pd(dba)2 ϩ two equivalents of a phosphine ligand is
expected to provide a good catalyst system. However, the con-
version of benzaldehyde with the Pd(dba)2 ϩ 2PPh3 system
(entry 7) was much lower than that with Pd(PPh3)2Cl2 or
Pd(PPh3)4. Since dibenzylideneacetone is known to be a
better ligand than PPh3 to low ligated Pd() species,16 the low
The reaction with 1,2-diphenyl-1,1,2,2-tetramethyldisilane 1e
was also efficiently catalysed by the Pt–etpo system at 160 ЊC,
D a l t o n T r a n s . , 2 0 0 3 , 2 3 6 – 2 4 3
237

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Table 2 Pd(PPh3)4 catalysed double silylation of aromatic aldehydes with 1aa
Entry
Aldehyde
t/h
Conversion of 2 (%)b
Yield of 3 (%)b
1
2
3
4
5
6
7
8
9
2a Ar = Ph
1
5
1
10
1
10
1
2.5
2.5
10
46
100
23
100
29
100
60
100
65
44
97
21
92
26
90
57
93
45
58
2g Ar = p-MeO–Ph
2h Ar = p-Me–Ph
2i Ar = p-F–Ph
2j Ar = thienyl
10
100
a Reaction conditions: aldehyde (0.5 mmol), F(SiMe2)2F (0.5 mmol), Pd(PPh3)4 2 mol%, decane (30 µl) as internal standard and toluene (0.1 ml),
120 ЊC, under N2 (g). b Conversions and yields determined by GLC and based on the quantity of the aldehyde charged.
affording 3e in 75% yield (entry 14). Pt(PPh3)4 gave only a trace
of 3e (entry 15), but Pt2(dba)3 was inactive. Pd(PPh3)4,
Pd(PEt2)2Cl2 and Pd(dba)2–2P(OCH2)3CEt were also inactive at
120 ЊC.
The reaction with a cyclic disilane 1,1,2,2-tetramethyl-1,2-
disilacyclopentane 1f (entries 16–18) was catalysed by the Pt–
etpo system to give the double silylation product 3f in a high
yield at 160 ЊC. Unlike the reaction with 1e however, Pt(PPh3)4
also displayed high activity. This is presumably associated with
the ring strain in the disilacyclopentane significantly weakening
the Si–Si bond and the relatively small steric hindrance around
the Si–Si bond as compared with 1e.
As is anticipated on the basis of the electronic effect of the
para substituent of the aromatic aldehydes, aliphatic aldehydes
exhibited very low reactivity with 1a under the same conditions.
Even when the reaction of n-hexanal was continued for 40 h,
only 10% of the aldehyde was consumed; several products were
found by GC analysis to have been formed.
Pd(PPh3)4-catalysed reaction of benzophenone with 1a
The Pd(PPh3)4 catalysed reaction of benzophenone with 1a
differs considerably from that of benzaldehyde (Scheme 4). The
Pd(PPh3)4-catalysed reaction of various aldehydes with 1a
A variety of p-substituted benzaldehydes reacted smoothly with
1a catalysed by Pd(PPh3)4 to give the double silylated products
(Scheme 3, Table 2). Regardless of the nature of the p-substitu-
Scheme 4
major product formed after heating at 120 ЊC for 100 h was
2,2-dimethyl-4,4,5,5-tetraphenyl-1,3-dioxa-2-silacyclopentane
1
5 (36% H NMR yield); the formation of SiMe2F2 was also
evident by the NMR analysis, but there was no trace of the
corresponding simple double silylated adduct. No reaction was
observed when 1b was employed instead of 1a.
Scheme 3
The double silylation of the C᎐N bond in N-benzylidenemethyl-
amine
᎐
ents, the selectivity did not vary significantly and was generally
greater than 90%. Accordingly, if the reaction was continued
sufficiently long (>2.5 h), the products could be obtained in
excellent yields.
As the combination of 1,2-difluoro-1,1,2,2-tetramethyldisilane
1a and Pd(PPh3)4 was most effective in the catalysed double
silylation of benzaldehydes, the same procedure was applied to
N-benzylidenemethylamine 6a (Table 3, Scheme 5). On heating
On the other hand, the reactivity of the aromatic aldehydes
was dependent on the nature of substituents. To compare the
reactivity, the reaction at 120 ЊC was discontinued after 1 h. The
results illustrate that p-fluorobenzaldehyde 2i reacted at a sig-
nificantly faster rate than 2a, whereas p-methoxybenzaldehyde
2g and p-methylbenzaldehyde 2h reacted at an appreciably
slower rate (entries 1, 3, 5, 7). To compare the reactivities of
2g and 2h further a 1 : 1 mixture of these two aldehydes was
subjected to a reaction that was discontinued after 2 h (other-
wise the same conditions). The result revealed 48% conversion
of 2h and 34% conversion of 2g. Thus benzaldehydes with elec-
tron withdrawing para substituents react at a faster rate than
those with para donor substituents. The results suggest that the
aldehyde is involved in the rate-determining step as far as
1,2-difluorodisilane is concerned.
Scheme 5
a toluene solution of 1a (0.5 mmol) and 6a (0.5 mmol) with
Pd(PPh3)4 (0.01 mmol) in a sealed tube at 120 ЊC for 5 h, double
silylation of the C᎐N bond proceeded quantitatively to give
᎐
2-Thiophenecarboaldehyde also reacted with 1a smoothly
but somewhat slowly to furnish the corresponding double
silylated product (entries 9–10). The selectively was less
satisfactory.
N-methyl-N-(fluorodimethylsilyl)-α-(fluorodimethylsilyl)-
benzylamine 7a (entry 1).
Palladium() phosphine complexes were also efficacious
towardsthedoublesilylation;bothPd(PPh3)2Cl2 andPd(PMe3)2-
D a l t o n T r a n s . , 2 0 0 3 , 2 3 6 – 2 4 3
238

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Table 3 Palladium catalysed double silylation of benzylideneaminesa
Entry
Imine
Catalyst
t/h
T /ЊC
Conversion of 6 (%)b
Yield of 7 (%)b
1
2
3
4
5
6
7
8
6a
6a
6a
6a
6a
6a
6b
6b
Pd(PPh3)4
5
5
5
120
120
120
120
60
60
120
120
100
100
100
76
68
42
7a 99
7a 98
7a 98
7a 70
7a 66
7a 40
7b 82
7b 62
Pd(PPh3)2Cl2
Pd(PMe3)2Cl2
Pd(dba)2 ϩ 2PPh3
Pd(PMe3)2Cl2
Pd(PPh3)4
Pd(PMe3)2Cl2
Pd(PPh3)4
5
70
70
20
20
85
65
a Reaction conditions: benzylideneamine (0.5 mmol), F(SiMe2)2F (0.5 mmol), catalyst 2 mol%, decane (30 µl) as internal standard, toluene (0.1 ml),
under nitrogen. b Conversions and yields determined by GLC and based on the quantity of the imine charged.
Cl2 gave 7a in 98% yield under the standard conditions (entries
2, 3). A combination of Pd(dba)2 and two equivalents of PPh3
was less active giving 7a in 70% yield under identical conditions
(entry 4) and phosphine free complexes Pd(PhCN)2Cl2 and
Pd(dba)2 were totally inactive. The double silylation of benzyl-
idenemethylamine proceeded even 60 ЊC, albeit more slowly
(entries 5, 6).
N-benzylidenebutylamine 6b also undergoes selective double
silylation with 1a in the presence of Pd(PMe3)2Cl2 or Pd(PPh3)4
to give the double silylated product 7b (entries 7, 8). N-Benzyl-
idenebenzylamine 6c gave a mixture of products that could not
be separated or characterised. Other disilanes such as 1c and 1d
were completely unreactive towards 6a in the presence of
Pd(PPh3)4.
silyl)benzylidene]methylamine 9a in 54% yield based on the di-
silane charged (yield of 9a based on the imine is 27%; entry 1).
1H NMR analysis of a separate experiment run in a sealed
NMR tube revealed that a small amount of trimethylsilane
(δ 0.02, d, 3J(H–H) = 3.6 Hz) was formed as a co-product
although its quantitative analysis was not possible in this par-
ticular case due to its gaseous nature. Hydrosilylation products
that could arise from trimethylsilane and the starting and result-
ing imines were not detected in the reaction mixture. Selectivity
towards 8a could be enhanced by employing an excess of
the imine (entry 2) and conversely a higher yield of 9a was
achieved with excess hexamethyldisilane (entry 3).
The reaction also proceeded with other aldimines. N-Benzyl-
idenebutylamine 6b slowly reacted to give the mono-silylated
8b in only 19% yield even after 120 h (entry 4). Approximately
3% of 9b was also identified by GC and GC/MS. N-Benzyl-
idenebenzylamine 6c was even less reactive under the same con-
ditions: GC and GC/MS analyses indicated that mono-silylated
product 8c was formed in approximately 3% yield.
In the reaction between 1,2-diphenyl-1,1,2,2-tetramethyl-
disilane 1e and 6a, mono-silylated product 8d was formed in 15%
yield while only a slight trace of the bis-ortho-silylated product
9d was detected by GC/MS (entry 7). GC analysis indicated
that an equimolar amount of phenyldimethylsilane was pro-
duced. The hydrosilane does not appear to play further role; in
an attempted hydrosilylation of 6a with phenyldimethylsilane
in the presence of the Pt–etpo system, 6a was completely
recovered unchanged after heating at 160 ЊC for 48 h.
The platinum catalysed ortho-silylation of benzylideneamines
with disilanes
In our investigations on the double silylation of benzaldehyde
we observed that whereas Pd(PPh3)4 was very effective in con-
junction with 1a, no reaction was observed with unactivated
disilanes such as 1d. However the Pt–etpo system did catalyse
the double silylation with 1d. This prompted us to try the same
catalyst for the reaction between 6a and 1d.
Remarkably, the Pt–etpo system catalysed the regioselective
ortho-silylation of 6a with 1d, giving the mono- and bis-
silylated products in high yields (Scheme 6, Table 4). For
The lower yields of 8 and 9 in the reactions involving 6b, 6c
and 1e can presumably be attributed to steric hindrance.
In the study to determine the mechanism involved in the pro-
duction of the ortho-silylated products, reactions were per-
formed with para-substituted aromatic imines (entries 8∼10). If
the mechanism is to be considered in terms of electrophilic
C–H activation, the usual reaction mode for palladium() and
platinum() species, the results are opposite to what would be
anticipated; the electron-donating methyl group (6e) retarded
the reaction, whereas the withdrawing fluorine substituent (6f )
accelerated it, to realise a combined yield of nearly 90% for the
mono- and bis-silylated products, under identical conditions.
Discussion
Mechanism for the reaction between disilanes and benzaldehyde
The formation of the double silylated adduct and others can be
explained by the mechanism illustrated in Scheme 7, which is
analogous, in part, to that postulated for the double silylation
of other carbon unsaturated molecules.4,5 The catalytic cycle
comprises of oxidative addition of the Si–Si bond to the
active catalyst species, insertion of the carbonyl group into the
silicon–metal bond to form an α-siloxybenzyl–metal species 11
and the reductive elimination. Depending on the structure of
the precursor complex, generation of the active species must
precede the catalysis.
Scheme 6
instance, heating a toluene solution of 6a (0.5 mmol) and 1d
(0.5 mmol) in the presence of the Pt–etpo catalyst system (0.01
mmol Pt) at 160 ЊC for 120 h gave N-[(2-trimethylsilyl)benzyl-
idene]methylamine 8a in 38% yield and N-[2,6-bis(trimethyl-
D a l t o n T r a n s . , 2 0 0 3 , 2 3 6 – 2 4 3
239

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Table 4 Platinum catalysed ortho-silylation of benzylideneaminesa
Imine-to-disilane
ratio
Imine
conversion (%)
Reaction
time/h
Yield
Yield
Entry
Imine
Disilane
of 8 (%)b
of 9 (%)b
1
2
3
4
5
6
7
8
9
6a
6a
6a
6b
6c
6f
6a
6a
6e
6f
1d
1d
1d
1d
1d
1d
1e
1d
1d
1d
1 : 1
4 : 1
1 : 4
1 : 1
1 : 1
1 : 1
1 : 1
1 : 2
1 : 2
1 : 2
64
—
—
—
15
71
—
69
51
90
120
120
120
120
120
120
120
20
8a 38
8a 82d
8a 23
8b 19
8ce ∼3
8f 42
9a 27c
9a 9d
9a 52
9be ∼3
—
9f 29
9d trace
9a 38
9e 20
9f 66
8d 15f
8a 31
8e 29
8f 23
20
20
10
a Reaction conditions: benzylideneamine (0.5 mmol), disilane (0.5 mmol), Pt2(dba)3 (0.005 mmol), P(OCH2)3CEt (0.0167 mmol), decane (20 µl) as
internal standard, toluene (0.1 ml), 160 ЊC, under nitrogen. b Yields determined by GLC and unless stated differently, are based on the amount of
imine charged. c 54% yield based on disilane. d Yields based on disilane charged. e GC and GC-MS analyses indicated an equimolar amount of
PhMe2SiH was produced. f Identified by GC/MS but not isolated.
oxidative addition to form more stable bis(silyl)metal species.
This means that intermediate 10 is smoothly generated for the
reaction of 1a to proceed briskly. The next insertion process
into silyl–metal species has also been documented for late
transition metal complexes.20 The insertion of carbonyl com-
pounds into silicon–manganese bonds affording α-siloxybenzyl-
manganese complex has also been demonstrated.21 The last
elemental process to conclude the catalytic cycle is the reductive
elimination from the (silyl)(alkyl)metal intermediate 11 [path
(i)],22 a common process involved in all the other double
silylation reactions catalysed by metal complexes. Thus, the
observations made in the palladium-catalysed catalytic double
silylation reactions with 1a are mostly in consistent with these
precedents, which are accommodated in Scheme 7. A minor
aspect that is apparently inconsistent is the effect of the sub-
stituent bound to the aldehyde. In the insertion reaction with
the Mn–SiMe3 complex, the reactivity trend is reported to be
p-NMe2 > p-OMe > p-H.21 On the other hand, the reactivity
trend was p-F > p-H > p-Me > p-OMe in our palladium
catalysed reaction with 1a. We speculate that the fluorine sub-
stituent bound to the silicon effectively reduces the intrinsic
electro-positive nature of the silyl group, which in turn
enhances the electrophilic reactivity towards the carbonyl oxy-
gen. In addition, more electron-deficient carbonyl compounds
may interact more strongly with the palladium centre.
On the other hand, the Pt–etpo catalysed double silylation
with unactivated 1d appears somewhat different in terms of the
formation of 4 from the palladium-catalysed reaction with 1a.
The intermediate 11, leading to product 3 if reductive elimin-
ation follows, can react in an alternative direction [path (ii)],
which is the homolysis of the metal–carbon bond to generate a
benzylic radical (12);23 subsequent homo-coupling of 12 would
give the side product 4d. Gladysz et al.21 have reported for their
Mn–Si systems that heating Me3SiO(Ph)CHMn(CO)5 indeed
gave 4d in high yields, presumably via homolysis of the metal–
carbon bond and coupling of the benzylic radicals.
The quantity of 4d formed in the catalysis could be sup-
pressed when a two fold excess of 1d was used. We have recently
found that formal high valent species can be generated when
silyl ligands are bound to the metal centre. If we assume a high
valent species or σ-bond metathesis in the interaction of a di-
silane molecule with the intermediate 11, the disilane-assisted
reductive elimination [path (iii)] can be envisioned to take place,
which rationalises the increased selectivity for 3d achieved in the
presence of excess 1d.
Scheme 7
The active species is thought to be a bis(phosphine)-
palladium() complex; if a dichloropalladium() complex is
used as precursor, reduction to palladium() is envisioned to be
effected by interaction with a disilane molecule. Indeed, treat-
ment of Pd(PPh3)2Cl2 with octamethyl-1,2-disilacyclobutane
forms 1,4-dichloro-1,4-disilaoctamethylbutane.17 The reduced
metal species, which is supposed to carry the double silylation
catalysis, is reported to be reactive towards chlorosilanes.18
Accordingly, the chlorosilane generated in the reduction of the
precursor complex may interact under the catalytic conditions
with the catalytically active species, resulting catalytic activity
being lowered as compared with its intrinsic activity observed
in the absence of the chlorosilane. The observation that the
catalysis by the species generated from the palladium()
complex precursors was less active than that where preformed
palladium() complex was used is presumably associated with
this “poisoning” by the chlorosilane.
The mechanism of the double silylation of imine is presum-
ably similar to the aldehyde reaction and will not be discussed.
A number of bis(silyl)-palladium and -platinum species have
been synthesised and their roles in the double silylation of
unsaturated compounds is well documented.19 In general, di-
silanes substituted by more electro-negative groups like fluorine
readily react with low valent transition metal complexes via
Mechanism for the reaction between disilane and benzophenone
The mechanistic key feature of the reaction of benzophenone
lies in the formal involvement of silylene species, which is
D a l t o n T r a n s . , 2 0 0 3 , 2 3 6 – 2 4 3
240

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palladium is also well known.29 Accordingly one can imagine a
mechanism that involves ortho-metallation of an imine with a
Pt(SiMe3)2 species generating a Pt() species, reductive elimin-
ation of a hydrosilane and further reductive elimination of a
silylated imine, as depicted in Scheme 9.
Scheme 8
evident in the structure of 5. The formation of 5 is best
explained as illustrated in Scheme 8, which comprises of
silylene and oxasilacyclopropane intermediates 14–16. Since the
reactivity of benzophenone was low as compared with alde-
hyde, the reaction was run at a relatively high temperature.
Under the conditions, the intermediate 13 is likely to undergo
α-migration of the fluorine attached to the silicon to the plat-
inum centre, generating silylene species 14. Subsequent reduc-
tive elimination from 14 would give Me2SiF2. The difluorosilane
was actually detected in the reaction mixture by NMR spectro-
scopy. There are a number of reports of silylene species being
generated from transition metal silyl species.24 For instance,
Rich noted the formation 2,2-dimethyl-4,5-diphenyl-1,3-dioxa-
2-silacyclopentene in the palladium catalysed reaction of benzil
with 1b, presumably via silylene formation.24e One of us also
encountered similar silylene generation and its reaction with
acetylenes forming 1,4-disilacyclohexadienes.24d
Scheme 9
In the closely related stoichiometric reaction reported by
Puddephatt et al.29 Me2Pt() sulfide complexes react with
aromatic imines via a platinum() species to give ortho-metal-
lated MePt() complexes with extrusion of a methane molecule.
Oxidative addition of the C–H bond as opposed to electrophilic
substitution was proposed for this C–H cleavage mechanism,
which was supported by the observation that electron-with-
drawing fluorine substituents promoted this transformation. In
our ortho-silylation, hydrosilane was extruded and the p-fluor-
ine substitutent in the imine molecule facilitated the reaction.
Thus the proposed catalytic cycle illustrated in Scheme 9 is in
full agreement with the Pudephatt’s observations.
Evidence of the generation of oxasilacyclopropane species
from the reaction of a silylene species with benzophenone has
appeared in the literature.25 It has also been postulated that the
reaction between di(t-butyl)silylene and ketones goes through
oxasilacyclopropane intermediates before forming silyl enol
ethers.26 In addition, a crystalline oxasilacyclopropane has
been isolated from the reaction of photochemically generated
dimesitylsilylene with 1,1,3,3-tetramethyl-2-indanone.27 The
addition reactions of thermally stable bis(amino)silylenes to the
Reductive elimination followed by decomplexation of the
mono-silylated imine regenerates the active catalyst species,
which re-enters the catalytic cycle to undertake the second
ortho-silylation at the oЈ-position to form the bis-silylated
imine. Alternatively, analogous to Murai’s postulated mechan-
ism13 one can envision the second silylation occurring whilst the
imine remains co-ordinated to the platinum.
The remaining observation that merits further consideration
is the para substituent effect. There are two possibilities for
ortho C–H activation: electrophilic substitution by an electron
deficient platinum and oxidative addition of the C–H bond to
an electron rich metal centre. In related reactions, such as the
ruthenium catalysed ortho-alkylation of aromatic ketones and
imines with alkenes, oxidative addition of aryl C–H bond to an
electron-rich ruthenium() centre is the generally accepted
mechanism for C–H activation. However, an alternative mech-
anism consisting of the addition of the ruthenium species at the
ortho-position, followed by a 1,2-hydrogen migration to give a
ruthenium hydride intermediate has been suggested by Murai.
Murai observed that the ortho-alkylation of aromatic ketones
was accelerated by electron donating groups at the para-
position.30 This is the reactivity trend that would be expected if
electrophilic substitution was involved and hence lends cre-
dence to the latter mechanism. In the Pt catalysed ortho-silyl-
ation of benzylideneamines we observed the opposite trend;
electron-withdrawing groups at the para-position increased the
rate of reaction, which suggests that oxidative addition of C–H
bond is involved. Recently, Jun et al. have reported that elec-
tron-withdrawing substituents enhanced the rate of rhodium
catalysed ortho-alkylation of para-substituted ketimines.15a This
C᎐O bond reported by Lappert and co-workers also lend
᎐
support.28
At the moment we are unable to exclude the possibility that
intermediate 14 reacts with benzophenone, forming 16 and a
fluoro(fluorodimethylsilyl)platinum intermediate, the latter of
which then reductively eliminates difluorodimethylsilane.
As for the final process forming the product 5, there are also
precedents, in which oxasilacyclopropane species react with
another ketone molecule, giving 1,3-dioxa-2-silacyclopentane
compounds.25b
Alternatively, complex 15 may interact as such with benzo-
phenone to furnish intermediate 17, which undergoes insertion
of benzophenone at the Pt–Si bond. Reductive elimination
from the resulting species can afford product 5.
Reaction mechanism for ortho-silylation of benzylideneamines
As already mentioned, the oxidative addition of disilanes to
platinum yielding bis(silyl)platinum species19 and the catalytic
activity of the Pt catalyst system for the double silylation with
the unactivated hexamethyldisilane have been demonstrated.5f
Cyclometallation of benzylideneamines with platinum and
D a l t o n T r a n s . , 2 0 0 3 , 2 3 6 – 2 4 3
241

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was interpreted as supporting oxidative addition of the
ortho-C–H bond to the electron rich Rh() centre. In the Pt
catalysed ortho-silylation of benzylideneamines C–H activation
is effected most likely by (Me3Si)2Pt() species. Generally,
aromatic C–H activation with platinum() species has been
assumed to involve electrophilic substitution. Numerous
examples indicate that this supposition is reasonable for species
like [PtCl4]2Ϫ as the platinum is considerably electron deficient.31
This is not necessarily true for (Me3Si)2Pt bis(phosphine)
species, where the silyl groups are electron-donating in nature.
The platinum centre is envisioned to be much more electron rich
than the formal oxidation state; this is supported by theoretical
calculations.32 In addition, intermolecular aromatic C–H
activation is observed with Ta–Si complexes, but not with the
analogous tantalum methyl species.33 This is rationalised in
terms of the superior electron-donation from silyl groups facili-
tating oxidation addition of C–H bond to electron rich metal
centre. In our own work, it seemed apparent that a bis(silyl)-
platinum species was the active complex in the coupling of an
aryl C–H with o-bis(dimethylsilyl)benzene.14b The feasibility of
oxidative addition to bis(silyl)Pt() and Pd() species has been
further exemplified by the isolation of unusually stable Pt()
and Pd() species.34 There are other examples of oxidative
addition of C–H to platinum() species in the literature.35
Accordingly, the substituent effect in the present ortho-silyl-
ation catalysis, which is apparently unusual for Pt() species,
can be attributed to oxidative addition with the “electron-rich”
(Me3Si)2Pt() species. An alternative possibility that we are
unable to rigorously exclude is the ortho-metallation taking
place between Pt() species and the imine before interaction
with a disilane molecule.
Experimental
General comments
All manipulations were performed under a nitrogen atmos-
phere. 1H (300 MHz), 13C (75 MHz) and 29Si (59.7 MHz) NMR
spectra were recorded in C6D6 on a Bruker ARX 300 (300
MHz) machine. The spectra were referenced internally using
the signal from the residual protio-solvent (1H) or the signals of
the solvent (13C). Infrared spectra were measured on a JASCO
FT/IR 5000 spectrometer. Mass spectra (EI) were recorded on
Shimadzu QP-5000 and JEOL JMS-DX303 spectrometers.
Solvents were dried over sodium wire and distilled under nitro-
gen. Other organic liquids were dried over 4 Å molecular sieves
and distilled before use. The following starting materials were
prepared by literature methods: Pd(PPh3)4,37 Pd(PMe3)2Cl2,38
Pd(PEt3)2Cl2,39 Pd(PPh3)2Cl2,40 Pd(dppf )Cl2,41 Pd(dppb)Cl2,42
Pd(dba)2,43 Pt2(dba)3,44 Pt(PPh)
,
Pt(PPh3)2Cl2,46 Pt(CH ᎐
᎐
2
45
4
CH2)( PPh3)2,47 Ni(PEt3)2Cl2,48 Ni(PEt3)4,49 1,2-difluoro-1,1,2,2-
tetramethyldisilane50 and 1,1,2,2-tetramethyl-1,2-disilacyclo-
pentane.51
Representative procedure for the double silylation: the reaction of
benzaldehyde with 1f
A
mixture of 1,1,2,2-tetramethyl-1,2-disilacyclopentane 1f
(0.5 mmol, 71 µl), benzaldehyde 2a (0.5 mmol, 51 µl), Pt2(dba)3
0.005 mmol; 2 mol% Pt relative to 1f ), P(OCH2)3CEt (0.0166
mmol), decane (30 µl) as an internal GC standard and toluene
(100 µl) were sealed in a reaction tube under an atmosphere of
nitrogen. The tube was heated at 160 ЊC for 20 h. GLC analysis
(OV-17 column) showed 3f was formed in 90% yield. The prod-
uct was obtained by Kugelrohr distillation at 85 ЊC (1 mm Hg)
in 81% yield. Analytically pure samples of compounds contain-
ing fluorine were obtained by preparative GC.
The factor that differentiates double silylation and ortho-
silylation of aromatic imines
General procedure for the ortho-silylation: the reaction of benzyl-
idenemethylamine 6a with hexamethyldisilane 1d
The double silylation of aromatic imines proceeds when 1a was
used in the presence of palladium catalyst. On the other hand,
the ortho-silylation takes place with 1d, catalysed by the Pt–etpo
system. As already mentioned, the fluorosilyl group ligated to
the metal centre is envisioned to be sufficiently electrophilic due
to the fluorine. In addition, it is less sterically demanding than
Me3Si. These structural features of the Me2FSi ligand allow
insertion of the imine linkage to the silicon–palladium bond at
a relatively low temperature, leading to the double silylation. In
the Pt–etpo catalysed reaction, the Me3Si ligand in the postu-
lated intermediate is less electrophilic and sterically more
demanding, which hampers the insertion. Only when the reac-
tion was run at a higher temperature (160 ЊC), a reaction can
proceed, but in a different direction, leading to the ortho-silyl-
ation. The high performance of the Pt–etpo system in the pres-
ent catalysis as well as in other double silylation reactions with
1d is also envisaged to be associated with the steric factor of
etpo, which is a quite small-sized ligand due to its cage struc-
ture. Since the aldimine substrate is a trisubstituted unsaturated
compound, its double bond is rather congested and the inser-
tion into the silicon–metal bond is not possible, prohibiting the
double silylation with 1d to occur. However, aldehydes, disub-
stituted unsaturated compounds, did undergo double silylation,
albeit reluctantly. It is interesting to note that acetophenone,
more congested than aldehydes, when treated with 1a at 160 ЊC
for 120 h in the presence of the Pt–etpo system, gave o-tri-
methylsilylacetophenone (∼4% yield) m/z (EI, 70 eV) 192 (Mϩ)
and o,oЈ-bis(trimethylsilyl)acetophenone (∼5% yield) m/z (EI,
70 eV) 264 (Mϩ).36 Thus, both electronic and steric factors play
important roles in differentiation of the reaction pathways. In
conclusion it can be seen that in the palladium and platinum
catalysed reaction of disilanes with unsaturated carbon–heter-
oatom systems the reaction pathway is very sensitive to the
nature of the metal, ligand, disilane and the organic substrate
employed.
A mixture of 6a (0.5 mmol, 61 µl), 1d (0.5 mmol, 102 µl),
Pt2(dba)3 (0.005 mmol, 5.35mg), P(OCH2)3CEt (0.0167 mmol,
2.65 mg), decane (20 µl) as an internal standard and toluene
(100 µl) were sealed in a glass reaction tube under an atmos-
phere of nitrogen. The tube was heated at 160 ЊC for 120 h.
GLC analysis (OV-17 column) showed 8a and 9a were formed
in 38 and 27% yields respectively, based on imine charged. The
yield of 9a corresponds to 54% yield based on disilane charged.
Acknowledgements
We acknowledge a research fellowship from Japan Science and
Technology Agency (NAW).
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D a l t o n T r a n s . , 2 0 0 3 , 2 3 6 – 2 4 3
243