Highly active and selective platinum(0)-carbene complexes. Efficient, catalytic hydrosilylation of functionalised olefins

Article abstract of DOI:10.1002/adsc.200404048

Readily available N-heterocyclic platinum-carbene complexes 1 are highly efficient catalysts for the regioselective hydrosilylation of alkenes. These novel organometallics tolerate a wide range of functional and protecting groups, can be stored for prolonged periods of time and are particularly active (TON > 10⁶).

Full text of DOI:10.1002/adsc.200404048

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Highly Active and Selective Platinum(0)-Carbene Complexes.
Efficient, Catalytic Hydrosilylation of Functionalised Olefins
a,
a
a
a
Istv a´ n E. Mark o´ , * S e´ bastien St e´ rin, Olivier Buisine, Guillaume Berthon,
a
b
b
Guillaume Michaud, Bernard Tinant, Jean-Paul Declercq
Universit e´ catholique de Louvain, D e´ partement de Chimie, B aˆ timent Lavoisier, Place Louis Pasteur 1, 1348
Louvain-la-Neuve, Belgium
a
Fax: þ32 (10) 47 27 88, e-mail: marko@chim.ucl.ac.be
b
Universit e´ catholique de Louvain, Unit e´ de Chimie Structurale, Place Louis Pasteur 1, 1348 Louvain-la-Neuve, Belgium
Received: February 17, 2004; Revised: August 20, 2004; Accepted: August 23, 2004
th
This article is dedicated fondly to Joe Richmond on the occasion of his 60 birthday.
Abstract: Readily available N-heterocyclic platinum-
carbene complexes 1 are highly efficient catalysts for
the regioselective hydrosilylation of alkenes. These
novel organometallics tolerate a wide range of func-
tional and protecting groups, can be stored for pro-
longed periods of time and are particularly active
6
(
TON>10 ).
Keywords: alkenes; N-heterocyclic carbenes; homo-
geneous catalysis; hydrosilylation; platinum
The hydrosilylation reaction, the addition of an SiÀH
unit onto a carbon-carbon double or triple bond to gen- Figure 1. The hydrosilylation of alkenes and alkynes.
erate an alkyl- or a vinylsilane, is of paramount impor-
[
1]
tance for the silicon industry. This key transformation,
[
2]
[8]
which is highly atom-economical, is at the heart of the ways generated under these conditions. Moreover, the
production of silicon polymers, the basic materials for extreme reactivity of 8 precludes its application in the
the manufacture of diverse commodities such as lubri- case of substrates bearing sensitive functionalities,
cating oils, resins, pressure-sensitive adhesives, liquid in- such as epoxides. To overcome some of these undesired
[
3]
jection moulding products and paper release coatings.
side-reactions, recent research efforts have been aimed
[
9]
The hydrosilylation of alkenes can be initiated in numer- at the synthesis of new Pt(0) complexes.
[4]
ous ways. Among these, the use of metal complexes
In a previous communication, we have reported that a
has been the most successful, with platinum derivatives novel class of platinum(0) carbene complexes 10 dis-
[
5]
occupying a cardinal position.
played good reactivity and excellent selectivity in the
[10]
[
6]
The Karstedt complex 8 and the Speier system (H2 hydrosilylation of the model alkene, 1-octene.
PtCl /i-PrOH) are the most commonly employed in- this article, we report the preparation of a variety of
In
[7]
6
dustrial catalysts. However, over the years, Speierꢀs cat- such platinum derivatives and the results of our study
alyst has been gradually replaced by the more active and on the hydrosilylation of a range of functionalised al-
selective Karstedt derivative. Unfortunately, hydrosily- kenes catalysed by these organometallics. In addition,
lation of olefins catalysed by complex 8 leads, besides some insights into the mechanism of this transformation
the desired adduct 3, to a mixture of by-products among will be discussed.
which the isomerised alkene 4, the saturated derivative
These novel platinum(0) complexes 10 are readily
5
, the secondary alkylsilane 6 and the vinylsilane 7 are prepared by the reaction of N-heterocyclic carbenes,
the major components (Figure 1).
generated from the corresponding imidazolium salts 9
[11]
Furthermore, colloidal platinum species, allegedly re- and NaH or KOBu-t, with Karstedt catalyst 8. Exclu-
sponsible for some of the observed side-reactions, are al- sive displacement of the internal, more labile, bridging
Adv. Synth. Catal. 2004, 346, 1429–1434
DOI: 10.1002/adsc.200404048
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Istv a´ n E Mark o´ et al.
Table 1. Preparation of the platinum(0) carbene catalysts.
[a]
Entry
R
Product
10a
Yield [%]
[
b]
1
2
80
Figure 2. Chem 3D views of the X-ray diffraction analyses of
1
0c and 10b.
10b
85–90
85–90
As depicted in Figure 2, the platinum occupies the
3
4
10c
10d
centre of a trigonal planar arrangement and the carbene
ligand is nearly perfectly orthogonal to the plane de-
fined by the metal and its substituents. The DVTMS li-
gand folds around the platinum centre into a chair-like
six-membered ring array. A selection of pertinent crys-
tallographic data are displayed in Ref.
These platinum(0) carbene complexes were initially
tested in our model system using 1-octene as the sub-
[
b]
70
5
6
10e
10f
85–90
75–80
[
12]
strate, 30 ppm of 10, MDꢀM [Me SiOSi(H)(Me)O-
3
SiMe , 12] as the silylating agent, in xylene, at 728C.
The results of these experiments are collected in Fig-
ure 3.
3
[
[
a]
b]
All yields are for isolated, pure compounds.
Performed only once.
As can be seen from Figure 3, the conversion of 1-oc-
divinyltetramethylsiloxane (DVTMS) ligand occurs, af- tene 11 to 1-silyloctane 13 is excellent in all cases, reach-
fording organometallics 10 in high yield and purity (Ta- ing 80–90% after a few hours. Some catalysts – especial-
ble 1).
ly 10e and 10f – exhibit a significant induction period,
These complexes are beautifully crystalline solids, in- which appears to depend upon the size of the carbene
sensitive to air and moisture. They can be stored for pro- substituents. The cyclohexyl derivative 10b, particularly
longed periods of time without noticeable loss in activity easy to prepare and to handle, displays a good reaction
when protected from light. The Chem 3D views of the X- profile with minimum induction period. It was thus se-
ray diffraction analyses of the cyclohexyl 10b and lected for further studies with functionalised olefins.
t-butyl 10c derivatives are displayed in Figure 2.
Some pertinent results of the hydrosilylation of repre-
Figure 3. Model hydrosilylation with various platinum(0) carbene catalysts.
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Efficient, Catalytic Hydrosilylation of Functionalised Olefins
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Table 2. Hydrosilylation of functionalised alkenes catalysed by 10b.
[a, b]
Entry
1
R
Product
Yield [%]
92
2
3
81
96
4
5
90
92
[c]
6
78
7
8
80
–
[d]
–
[a]
[b]
[c]
[d]
All yields are for isolated, pure compounds. Unless otherwise mentioned, all the conversions are quantitative.
In all cases, the use of Karstedt catalyst 8 leads to a mixture of products and to the formation of colloidal platinum species.
The reaction was stopped after 80% conversion.
No reaction was observed and the starting materials were recovered unchanged.
[
14]
sentative alkenes, catalysed by 10b are collected in Ta- yields. Finally, complete chemoselectivity in favour
ble 2. of the hydrosilylation of the terminal alkene is observed
As illustrated in Table 2, catalytic hydrosilylation of in the presence of a ketone or an ester function (En-
all the terminal olefins proceeds smoothly and leads in tries 6 and 7).
excellent yields to the desired addition products (En-
It is interesting to note that most of these substrates
tries 1–7). In all cases, complete regiocontrol is ob- (Entries 1–3, 6 and 7) are excellent chelating ligands,
served and only the anti-Markovnikov adduct is ob- binding strongly to a variety of metals and impeding
tained. Internal alkenes are totally inert under these their catalytic activity. Such an inhibition was usually
conditions and the starting material is recovered quanti- not observed in the hydrosilylations catalysed by the car-
tatively (Entry 8). The reaction tolerates a variety of bene complex 10b. One exception is allylacetone (En-
functionalities and protecting groups. Thus, hydrosilyla- try 6), which slows down the rate of the reaction though
[
15]
tion of olefins containing a tetrahydropyranyl ether (En- it never completely represses it.
try 1) or a t-butyl dimethylsilyl ether (Entry 2) affords It is worth mentioning that hydrosilylations catalysed by
the desired adducts in high yields. It is noteworthy that 10b afford crude products that are typically>95% pure.
free alcohol functions are unaffected under these condi- Only traces (<2%) of internal alkenes can be detected
tions (Entry 3). No trace of the corresponding silyl ether in the unpurified reaction mixtures and no colloidal plati-
[
13]
could be detected in the crude reaction mixture. In num speciesareformed. Evenmoreimportantisthe obser-
stark contrast to Karstedt catalyst 8, which leads to sig- vation that the catalytically active species is still competent
nificant decomposition of epoxide-containing sub- at the end of the reaction. Indeed, addition of a second and
strates, the carbene complex 10b tolerates the presence third portion of alkene 14 and silane 12 leads, with equal
of this sensitive functionality (Entries 4 and 5) and the efficiency, to the desired adduct 15. It is noteworthy that,
hydrosilylated adducts are obtained in excellent after several days at ambient temperature, the platinum-
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Figure 4. An efficient Tamao–KumadaÀFleming oxidation.
Figure 5. Proposed mechanism for the platinum-carbene catalysed hydrosilylation
carbene catalyst is still active and turnover numbers be-
Insertion of the SiÀH bond into the olefinic linkage
6
yond 10 have been obtained. In sharp contrast, the use leads tothe platinum(II) derivative 18 which, after reduc-
of Karstedt catalyst 8 leads, in all cases, to numerous tive elimination of 15, affords the platinum(0) species 19,
side-products and to the generation of colloidal platinum, probablystabilisedbycoordinationtooneortwoalkenes.
[16]
with concomitant loss of activity.
Interestingly, the reaction displays saturation kinetics.
In order to broaden the scope of this hydrosilylation Whilst increasing the concentration of the alkene leads
protocol, adduct 13 was treated with KHF /H O under initially to an enhanced reaction rate, the presence of a
2
2
2
[
17]
the usual Tamao–KumadaÀFleming conditions. Grat- large excess of olefin eventually results in a partial inhib-
ifyingly, a smooth reaction occurred, leading to 1-octa- ition of the hydrosilylation reaction, probably arising
nol 16 in essentially quantitative yields (Figure 4).
fromthesaturationofthecoordinationsphereoftheplat-
Thus, the platinum-carbene-catalysed hydrosilylation inum complex 19. Addition of the silane then regenerates
of terminal alkenes, coupled with a Tamao–Fleming ox- the active complex 17 and a new catalytic cycle ensues.
idative protocol, offers an efficient, cheap and highly re- The variation in the induction period observed with dif-
gio- and chemo-selective alternative to the more classi- ferent precatalysts 10, reflects the respective difficulty
[
18]
cal hydroboration of olefins.
in hydrosilylating the chelating DVTMS ligand and cor-
Although the mechanism of the hydrosilylation reac- relates with the steric encumbrance provided by the N-
tion catalysed by the platinum-carbene complex 10b is heterocyclic carbene substituents.
not yet fully elucidated, preliminary kinetic studies have
already revealed some interesting facets of the reactivity inum(0)-carbene complexes 10 efficiently catalyse the re-
of these novel organometallic derivatives (Figure 5). gio- and chemoselective hydrosilylation of a range of
In summary, we have shown that readily prepared plat-
Carbene complexes 10 are precatalysts that initially functionalised terminal alkenes. These novel precatalysts
undergo hydrosilylation/dissociation of the DVTMS are stable towards air and moisture and can be stored for
substituent. The departure of this ligand, the slowest extended periods of timewithout loss of activity. Coupled
step of the whole process, requires the presence of the al- with a Tamao–Fleming oxidation, this hydrosilylation
kene 14 and the silane 12 and generates the transient in- procedure provides an efficient and somewhat more eco-
termediate – or transition state – 17. The rate of the hy- logically friendly alternative to the hydroboration of ole-
drosilylation reaction appears to be first order in the al- fins. The synthetic utility of this reaction and its intimate
kene 14 and in the silane 12, implying that both partners mechanism are currently being actively investigated.
are involved at the same time in the “rate-determining
step” of the catalytic cycle. Interestingly, incubation of
complex 10b with excess alkene does not promote the Experimental Section
exchange between the DVTMS ligand and the olefin.
In contrast, treatment of 10b with excess silane gener-
ates the corresponding N-heterocyclic platinum(II) silyl
General Considerations
hydride dimer. This dimer is, however, a less efficient All reactions were conducted under argon unless otherwise
[19]
catalyst than 10b.
stated. All solvents were freshly distilled. All chemicals were
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Efficient, Catalytic Hydrosilylation of Functionalised Olefins
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used as received without prior purification. Divinyltetrame- Acknowledgements
thyldisiloxane (DVTMS) and Karstedtꢀs catalyst (15% weight
in Pt) were generously donated by Rhodia Silicones S. A. Imi-
dazolium salts were prepared according to literature proce-
Financial support of this work by Rhodia Silicone, Rhodia CST
and the Universit e´ catholique de Louvain is gratefully acknowl-
edged. I. E. M. is also indebted to Rhodia for receiving the 2002
Rhodia Outstanding Award.
[11]1
13
dures. H and C NMR spectra were recorded using a Varian
195
Gemini 200 MHz. Pt NMR spectra were recorded on Bruker
00 MHz and externally referenced to H PtCl .
5
2
6
References and Notes
General Procedure for the Synthesis of the Pt(0)-
Carbene Complexes (Table 1, Entries 1–6)
[1] a) B. Marciniec, J. Gulinski, W. Urbaniak, Z. W. Kornet-
ka, in: Comprehensive Handbook on Hydrosilylation,
Synthesis of platinum-carbene complex 10b (Table 1, Entry
(
Ed.: B. Marciniec), Pergamon, Oxford, 1992; b) V. B.
2
(
9
): In an oven-dried 3-necked round-bottomed flask
50 mL), the dicyclohexylimidazolium tetrafluoroborate salt
b (573 mg, 1.79 mmol) and Karstedtꢀs catalyst 8 (15.8%
weight of Pt, 2.01 g, 1.63 mmol in Pt) were suspended in
mL of distilled toluene. To this suspension, t-BuOK
274 mg, 2.45 mmol) was added all at once. The formation of
Pukhnarevitch, E. Lukevics, L. I. Kopylova, M. Voron-
kov, Perspectives of Hydrosilylation, Institute of Organic
Synthesis, Riga, Latvia, 1992; c) I. Ojima, in: The Chem-
istry of Organic Silicon Compounds, (Eds.: S. Patai, Z.
Rapoport), Wiley-Interscience, New York, 1989, Vol. 2,
chap. 25, p. 1479; for some recent examples of plati-
num-catalysed hydrosilylation of alkenes, see: d) Y. Ya-
mamoto, T. Ohno, K. Itoh, Organometallics 2003, 22,
2267; e) F. Wang, D. C. Neckers, J. Organomet. Chem.
2003, 665, 1 and references cited therein.
5
(
a fine KBF suspension was observed immediately. The reac-
4
tion was stirred for 2 hours. The reaction mixture was filtered
on a pad of celite that was rinsed with toluene. A clear, light or-
ange solution was obtained. The solvents were removed under
vacuum. At this point, the complex begins to precipitate in the
remaining DVTMS, as a white microcrystalline solid. The solid
was filtered and washed with cold isopropanol to afford a fine
[2] R. A. Sheldon, Pure Appl. Chem. 2000, 72, 1233 and ref-
erences cited therein.
1
white powder; typical yields of 10b: 85–90%. H NMR
[
3] L. N. Lewis, J. Stein, Y. Gao, R. E. Colborn, G. Hutchins,
eq
ax
(
CDCl ): d¼ À0.25 (s, 6H, SiCH ), 0.34 (s, 6H, SiCH3 ),
3
3
Platinum Met. Rev. 1997, 41, 66.
1
4
.14–2.20 [m, 6Hþ16H, CH (cyclohexyl)þSiCH¼CH ],
2
2
4,5
[4] D. A. Armitage, in: Comprehensive Organometallic
Chemistry, (Eds.: G. Wilkinson, F. G. A. Stone, E. W.
Abel), Pergamon, Oxford, 1982, Vol. 2, p. 117.
4
,25 (m, 2H, NCH), 7.02 (t, J ¼8.75 Hz, 2H, im-H );
Pt-H
13
eq
3
ax
3
C NMR (75 MHz, CDCl ): d¼ À1.9 (SiCH ), 1.4 (SiCH ),
3
2
5.3 (CH ), 33.5 (SiCH¼CH ), 33.9 (NCHCH ), 40.3
2
2
2
[
[
5] J. W. Grate, S. N. Kaganove, Polym. News 1999, 24, 149.
6] a) B. D. Karstedt, (General Electric), US Patent 3 715
4
,5
2
(
SiCH¼CH ), 58.4 (NCH), 117.2 (im-C ), 180.0 (im-C );
2
195
Pt NMR (CDCl ): d¼ À5338; IR (film): n¼2933 (w), 2856
3
3
34, 1973; b) P. B. Hitchcock, M. F. Lappert, N. J. W.
(
(
m), 1450 (m), 1423 (m), 1410 (m), 1296 (m), 1237 (m), 1195
À1
Warhurst, Angew. Chem. Int. Ed. Engl. 1991, 30, 438.
7] J. L. Speier, Adv. Organomet. Chem. 1979, 17, 407.
m), 1170, 993 (w), 896 (m), 860 (m), 837 (m), 779 (m) cm
;
[
MS (APCI): m/z (%)¼585 (88), 586.1 (100), 587 (93), 588
þ
[8] J. Stein, L. N. Lewis, Y. Gao, R. A. Scott, J. Am. Chem.
Soc. 1999, 121, 3693; the presence of colloidal platinum
species is easily detected by ultraviolet-visible spectro-
scopy and is commonly associated with the appearance
of a yellow colouration of the reaction mixture.
(
30), 589 (30) [MÀC H ] , 453 (100), 454 (97) [CyN-
2
3
þ
þ
HCÀPtÀC H ] , 233 (100), 234 (20) [CyNHC] .
2
3
[
9] For some recent contributions on the preparation and
use of related Pt(0) and Pt(0)-carbene complexes, see:
a) P. Steffanut, J. A. Osborn, A. DeCian, J. Fisher,
Chem. Eur. J. 1998, 4, 2008; b) A. K. Roy, R. B. Taylor,
J. Am. Chem. Soc. 2002, 124, 9510; c) M. A. Duin,
N. D. Clement, K. J. Cavell, C. J. Elsevier, Chem. Com-
mun. 2003, 400; d) J. W. Sprengers, M. d. Greef, M. A.
Duin, C. J. Elsevier, Eur. J. Inorg. Chem. 2003, 3811;
e) J. W. Sprengers, M. J. Mars, M. A. Duin, K. J. Cavell,
C. J. Elsevier, J. Organomet. Chem. 2003, 679, 149;
f) J. W. Sprengers, M. J. Agerbeek, C. J. Elsevier, Orga-
nometallics 2004, 23, 3117.
General Procedure for the Hydrosilylation of
Functionalized Alkenes (Table 2, Entries 1–8)
Hydrosilylation of 1,2-epoxy-7-octene (Table 2, Entry 4): To
a solution of 1,2-epoxy-7-octene (504 mg, 4 mmol) and silane
1
9
1
2 (896 mg, 4 mmol) in 5.5 mL of xylene, at 728C, was added
0 mL (0.0002 mmol) of a xylene solution of carbene complex
0b (prepared by dissolving 27.5 mg of 10b in 20 mL of xylene).
The reaction mixture was heated at 728C for 210 min (the reac-
tion was monitored by GC). The cold, colourless solution was
filtered through a short pad of charcoal and the solvent was re-
moved under vacuum. The crude product was purified by chro-
matography on silica gel (eluent: petroleum ether/5% Et N),
3
1
[10] I. E. Mark o´ , S. St e´ rin, O. Buisine, G. Mignani, P. Bran-
affording the pure adduct; yield: 1.25 g (90%); H NMR
lard, B. Tinant, J.-P. Declercq, Science 2002, 298, 204.
(
CDCl , 300 MHz): d¼0.01 (s, 3H), 0.09 (s, 18H), 0.43–0.48
3
[
11] a) A. J. Arduengo, R. L. Harlow, M. Kline, J. Am. Chem.
Soc. 1991, 113, 361; b) J. Huang, S. P. Nolan, J. Am.
Chem. Soc. 1999, 121, 9889; c) V. P. W. Bꢂhm, W. A.
Herrmann, Angew. Chem. Int. Ed. Engl. 2000, 39, for
an excellent review on the chemistry of N-heterocyclic
(
m, 2H), 1.32–1.58 (m, 10H), 2.48 (dd, J ¼3 Hz, J ¼5.4 Hz,
1
2
1
3
1
5
5
H), 2.76 (brt, 1H), 2.89–2.95 (m, 1H); C NMR (CDCl ,
3
0 MHz): d¼ À0.2, 2.0, 17.6, 23.0, 26.0, 29.2, 32.6, 33.2, 47.2,
þ.
þ
2.4; MS (CI): m/z¼349 (M þ1), 333 (M ÀCH ), 221
3
þ
[SiMe(OSiMe ) ].
3
2
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Istv a´ n E Mark o´ et al.
carbenes, see: D. Bourissou, O. Guerret, F. P. Gabbai, G.
Bertrand, Chem. Rev. 2000, 100, 39.
isomerised alkene. With Karstedt catalyst 8, yellow solu-
tions, including colloidal platinum species, are generated.
The purity of the crude reaction mixture varies between
72–92% and the amount of isomerised alkene oscillates
between 5–11%. In the case of epoxide-containing sub-
strates, extensive decomposition is observed.
[
12] Selected X-ray crystallographic data: d PtÀC ¼2.133 ꢃ,
1
Vi
d’ PtÀC ¼2.123 ꢃ, d1C –C ¼1.436 ꢃ, d PtÀC ¼
1
Vi
Vi
Vi
2
Vi
2
.139 ꢃ,
d’ PtÀC ¼2.115 ꢃ;
d C –C ¼1.420 ꢃ,
2
Vi
2
Vi
Vi
0
dPtÀCarb¼2.027 ꢃ, torsion C –PtÀCcarbÀN¼98 .
Vi
[
[
13] Hydrosilylation of alcohol-containing substrates using
the Karstedt catalyst results in significant formation of
the corresponding silyl ethers (up to 40%).
[17] a) K. Tamao, N. Ishida, T. Tanaka, M. Kumada, Organo-
metallics 1983, 2, 1694; b) K. Tamao, T. Kakui, M. Akita,
T. Iwahara, R. Kanatani, J. Yoshida, M. Kumada, Tetra-
hedron 1983, 39, 983; c) I. Fleming, R. Henning, H. Plant,
J. Chem. Soc. Chem. Commun. 1984, 29.
[18] a) H. C. Brown, E. Negishi, M. Zaidlewicz, in: Compre-
hensive Organometallic Chemistry, (Eds.: G. Wilkinson,
F. G. A. Stone, E. W. Abel), Pergamon, Oxford, 1982,
Vol. 7, p. 111; b) D. S. Matteson, Synthesis 1989, 973.
[19] For the preparation and structure determination of the
analogous phosphine-containing dimers, see: M. Ciriano,
M. Green, J. A. K. Howard, J. Proud, J. L. Spencer,
G. A. F. Stone, C. A. Tsipis, J. Chem. Soc. Dalton Trans.
1978, 801; for the hydrosilylation of alkenes using these
dimers, see: M. Green, J. L. Spencer, G. A. F. Stone,
C. A. Tsipis, J. Chem. Soc. Dalton Trans. 1977, 1519.
14] The high reactivity of the Karstedt catalyst precludes its
use in the case of epoxide-bearing substrates. For exam-
ple, more than 50% of vinyl cyclohexyl epoxide (Table 2,
Entry 5) is decomposed in the presence of this complex.
15] While most hydrosilylations are complete within 2–4 h,
the reaction of allylacetone with 10b, under identical
conditions, requires up to 24 h to reach 80% conversion.
In the case of the platinum-carbene complex 10b, the cat-
alyst is still active and prolonged reaction time leads to
full conversion.
[
[
16] All the hydrosilylations performed using the carbene
complex 10b afford crude products which are at least
9
5% pure and which typically contain less than 2% of
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