Hydrogen Hexachloroplatinate(IV)1

H2PtCl6.6H2O

[16941-12-1]  · Cl6H14O6Pt  · Hydrogen Hexachloroplatinate(IV)  · (MW 517.92)

(catalyst for hydrosilylation, hydrogenation, carbonylation, coupling, and oxidation)

Alternate Names: CPA; chloroplatinic acid; hexachloroplatinic acid; Speier's catalyst.

Physical Data: mp 60 °C, d 2.431 g cm-3.

Solubility: sol water, alcohol, ether.

Form Supplied in: brownish-yellow crystalline mass; very deliquescent; widely available. Drying: compound decomposes on heating.

Handling, Storage, and Precautions: harmful if inhaled, swallowed, or absorbed through skin. The compound may be carcinogenic. Use in a fume hood.

Hydrosilylations.

Hydrosilylation is a process in which one or more silicon-hydrogen bonds add to a substrate.1 The reaction typically involves the addition of silanes to alkenes, carbon-heteroatom double bonds, and alkynes (eq 1).

This reaction can be catalyzed by a variety of metals and metal complexes. One of the most useful catalysts for hydrosilylation is CPA. A dilute solution of CPA in isopropanol is typically used as the catalyst (Speier's catalyst).2 CPA also serves as the starting material for a very active form of homogeneous platinum(0) catalyst known as the Karstedt catalyst.3 The major features of hydrosilylations are: an induction period; moderate to good yields; facility of the reaction on the nature of the silane, substrate, and the catalytic species; isomerization of the alkene substrate; and applications to a wide variety of substrates. A generally accepted mechanism for hydrosilylation has been proposed by Chalk and Harrod.4 It involves the coordination of the metal to the alkene followed by oxidative addition of the silane to the metal to give a PtIV intermediate. Rearrangement gives s-bonded PtIV and subsequent reductive elimination gives the hydrosilylated product and PtII species.

Hydrosilylation reactions of electron-rich alkenes with activated silanes such as halo- or alkoxysilanes are simple. Terminal alkenes react with Trichlorosilane under CPA catalysis to furnish high yields of the linear trihalosilane (eq 2).5

Hydrosilylations of internal alkenes with trichlorosilane and CPA as the catalyst proceed in a different manner. In these reactions, both bond migration and hydrosilylation occur. For example, reaction of 2-pentene with trichlorosilane for 30 min at 130 °C using CPA as the catalyst furnishes the terminal silane (eq 3).5

Styrene undergoes regioselective hydrosilylation with trichlorosilane in the presence of CPA and Triphenylphosphine, providing the b-adduct selectively (eq 4).6 The regioselectivity depends on the additive as well as the catalyst. For example, reaction of styrene with trichlorosilane and Tetracarbonylnickel as the catalyst furnished the a-adduct as the sole product. Rhodium catalysts have also been evaluated in hydrosilylations of styrene with various silanes.7

Reactions of terminal alkenes with dichlorosilane provides the monoalkyldichlorosilanes (eq 5).8 In this reaction, the terminal double bond undergoes hydrosilylation preferentially. Hydrosilylation of terminal alkenes with dichlorosilane using triphenylphosphine complexes of Group 8-10 metals (Ni, Ru, Rh, Pd, Pt) gives the monoalkyldichlorosilanes in excellent yields.9

Internal alkenes produce a mixture of hydrosilylated products upon treatment with dichlorosilane and CPA (eq 6).5 In this reaction, products from bond migration followed by hydrosilylation were not observed.

Hydrosilylation of alkenes with chiral silanes and CPA catalysis proceeds with retention of configuration at the silicon center (eq 7).10 Similarly, Si-H to Si-D exchanges can also be carried out with retention of configuration using CPA catalysis.

1,1,3,3-Tetramethyldisiloxane, a compound with a preexisting Si-O-Si linkage, undergoes hydrosilylation with terminal alkenes and CPA catalysis to furnish silylated products in moderate yields (eq 8).11

Molecular oxygen and CPA can effectively catalyze the addition of unreactive monoalkyl silanes to terminal alkenes to furnish tetrasubstituted silicon compounds in high yields (eq 9).12 In the absence of oxygen, these reactions do not go to completion since it is believed that the oxygen reactivates the CPA for hydrosilylation.

Intramolecular hydrosilylations of alkenylsilanes with CPA catalysis gives a mixture of regioisomeric cyclic silanes (eq 10).13 The yields and ratio of products depends on the ring size, while the product distribution has been explained using the Chalk and Harrod mechanism for the hydrosilylation reaction. Vinylsilanes and allylsilanes gave no cyclized product.

Hydrosilylations using silane have not been extensively investigated. One example involves the hydrosilylation of 1,5-hexadiene with silane and CPA (pretreated with oxygen) catalysis and subsequent intramolecular hydrosilylation of the intermediate to provide a silaheptane as shown in eq 11.14 Pt(PPh3)4 was a better catalyst than CPA for this reaction.

Hydrosilylations of alkenes containing a variety of functional groups have also been explored. The success of these reactions depends on the nature of the functional groups. A variety of functional groups survive the hydrosilylation conditions. These include epoxides, nitro groups, esters, carbamates, and acetals.1d

Epoxy organosilanes are very useful materials and are extensively used as silane-coupling agents. These compounds can be conveniently synthesized by the hydrosilylation of epoxyalkenes with a variety of alkoxysilanes in the presence of CPA (eq 12).15

Unsaturated sulfides undergo hydrosilylation with triethyl- and triethoxysilanes.16 These reactions are generally nonselective and proceed in only moderate yields (eq 13). The selectivity and yields depend upon the nature of the substrate, hydrosilylating agent, and catalyst. In the case of diallyl sulfide, hydrosilylation using Triethylsilane and CPA produces the monosilylated product and a byproduct arising from cleavage of the sulfur-carbon bond (eq 14).

Allylamine undergoes hydrosilylation with Triethoxysilane and CPA as the catalyst (eq 15).17 The effect of additives such as triphenylphosphine and carboxylic acids have been evaluated with respect to regioselectivity. Hydrosilylation of secondary and tertiary allylamines have also been evaluated. With secondary allyl amines, the regioselectivity of the silane addition depends upon the size of the N-substituent, and the g-isomer is formed predominantly. In the case of tertiary allyl amines, the g-isomer is the sole adduct.18

Acetylene undergoes hydrosilylation with trialkylsilanes in the presence of CPA to provide a mixture of mono- and diadducts (eq 16).19 A convenient laboratory method for hydrosilylation of acetylene with a variety of silanes to provide synthetically useful vinylsilanes has been reported by Watanabe.20 Although CPA gives good selectivity and high yields with trichlorosilane, ruthenium, rhodium, and platinum triphenylphosphine complexes were found to be better suited for these reactions.

Hydrosilylation of monosubstituted alkynes may also be catalyzed with CPA. Generally the reaction proceeds through cis addition of the silane and gives a mixture of a- and cis and trans b-adducts, with the latter product predominating.21 A comparison of CPA to Karstedt's catalyst in the hydrosilylation of monosubstituted alkynes has been reported.22 Maricinec and co-workers have investigated the hydrosilylation of a variety of monosubstituted alkynes with (±)-a-naphthylphenylmethylsilane (eq 17).23 Several other platinum catalysts were evaluated in this reaction with tetrakis(triphenylphosphine)platinum being as effective as CPA. In a similar fashion, hydrogermylation of monosubstituted alkynes with CPA catalysis gave the trans-germyl alkenes as the major product.24 Hydrogermylations with chiral germanes and CPA catalysis proceed with retention of configuration.25

Voronkov and Sushchinskaya26 have shown that the hydrosilylation of phenylacetylene with triethylsilane and CPA catalysis can be markedly influenced by the addition of aluminum or germanium chlorides. For example, the reaction in the presence of Aluminum Chloride (CPA:AlCl3 = 1:200) gave 93% of the hydrosilylated products, while in the absence of AlCl3 the yield was only 24% (eq 18).

Doyle and co-workers have reported an alternate mode of selectivity of silane addition to monosubstituted alkynes, wherein allylsilanes are formed as the major products, along with minor amounts of the vinylsilanes (eq 19).27 The reaction involves the slow addition of alkyne to a solution of silane and CPA catalyst. The allylsilanes are formed in good yields. Use of rhodium(II) perfluorobutyrate in place of CPA gave higher yields of the allylsilane. Addition of silane to a solution of alkyne and CPA gave the normal vinylsilane product.

Acetylene and monosubstituted alkynes undergo dehydrocondensative hydrosilylation in the presence of catalytic CPA and iodine (eq 20).28 This method provides a convenient route to silylalkynes. The effect of various halide additives on the product distribution and yields has been examined. Elemental iodine gave good yields of the dehydrocondensation product and these reactions were found to be temperature dependent. Trimethylsilane can be used instead of triethylsilane with similar product distributions. However, the best yields of the dehydrocondensation product were obtained using trialkylsilyl iodides as the reactant. The role of solvents in these reactions has also been reported. Hydrosilylation of phenylacetylene with transition metal complexes containing a Si-H bond proceeds with CPA catalysis giving a cis-trans mixture of products in high yields.29

Hydrosilylation and hydrogermylation of disubstituted alkynes30 using CPA as the catalyst have been explored en route to substituted furans (eq 21).31 Similarly, alkynylsilanes can be hydrosilylated in a highly regioselective manner to give 1,2-disilaalkenes (eq 22).32

Intramolecular hydrosilylations of silicon-substituted silaalkynes have been achieved using CPA as the catalyst (eq 23).33 Similar to the results obtained for intramolecular hydrosilylations of alkenes, the product distribution depends on the size of the ring formed.

Hydrosilylations of dienes generally provide three 1:1 adducts arising from 1,2- and 1,4-addition. Isoprene, a simple diene, reacts with a variety of silanes in the presence of catalytic CPA to furnish hydrosilylated products. For example, the reaction of isoprene with trichlorosilane and catalytic CPA gave the 1,4-adduct, with addition occurring across the least hindered double bond (eq 24).34 However, trimethylsilane gave both 1,2- and 1,4-adducts with addition occurring at both ends of the diene.

1,4-Bis(trimethylsilyl)-1,3-butadiyne, a stable alternative to the highly reactive 1,3-butadiyne, undergoes hydrosilylation with CPA catalysis and a variety of silanes (eq 25).35 The product distribution depends on the nature of the silane. The more reactive dimethylchlorosilane gave the 1:2 adduct (12) in moderate yields, while reactions with the less reactive and bulkier triisopropylsilane furnished the 1:1 adduct (14) in higher yields. Reaction with triethylsilane was unusual in that the product was an allene (13) arising from 1,4-addition of a second silane molecule to the intermediate 1:1 adduct (14).

Hydrosilylations of enynes occur at the more reactive alkyne end. An example of enyne hydrosilylation with methyldichlorosilane is shown in eq 26.36 The reaction requires a small amount of either Dibenzoyl Peroxide or trichlorosilane as a cocatalyst and is believed to proceed by a radical pathway. An example of a highly chemoselective hydrosilylation has been reported by Voronkov (eq 27).37 The reactions occur at the alkyne terminus in the presence of an alcohol and alkene.

Hydrosilylations of a,b-unsaturated esters, ketones, and nitriles with CPA catalysis have also been examined. The course of these reactions depends on the nature of the silane and the electron-withdrawing substituent. Hydrosilylation of a series of a,b-unsaturated esters using CPA catalysis was evaluated by Yoshii and co-workers (eq 28).38 The reactions were generally nonselective and gave all three possible 1,2- and 1,4-addition products. When Wilkinson's catalyst (Chlorotris(triphenylphosphine)rhodium(I)) was used in place of CPA, the 1,4-addition products, silylketene acetals, were obtained as the major products.

Hydrosilylation of enones proceeds by a 1,4-addition of the silane to produce the enol silyl ethers, from which the reduced compounds are obtained through hydrolysis.39 An application of this methodology is illustrated in the synthesis of a precursor to purpnigenin (eq 29).40 Of the many silanes examined in this reaction, diethylethoxysilane gave the highest yields and least number of side reactions. Hydrosilylations of a,b-unsaturated carbonyl compounds proceeds well with a variety of silanes, generally with good regioselectivity when Wilkinson's catalyst is used.41

Hydrosilylation of acrylonitrile with trichlorosilane using CPA catalysis does not produce any hydrosilylated products.42 However, hydrosilylation of a,b-unsaturated nitriles proceeds well when Wilkinson's catalyst is used.43

Hydrosilylation Followed by Oxidation.

Tamao and co-workers have developed elegant methodologies for the conversion of the silyl intermediates obtained in hydrosilylation of alkenes and alkynes to a variety of functional groups. The key transformations in these methodologies include the oxidation of a C-Si bond to a hydroxy group with retention of configuration,44 the conversion of a C-Si bond to a C-Br bond,45 and conversion of a C-Si bond to a C-C bond.46

The hydrosilylation-oxidation sequence can be effectively used for the transformations of terminal alkenes to the corresponding anti-Markovnikov alcohols (eq 30).44 The silane used in these experiments, methyldiethoxysilane, is commercially available and air stable. Thus the hydrosilylations can be carried out without solvent under air using CPA catalysis. Hydrogen Peroxide oxidation proceeds smoothly only if there is at least one alkoxy group on the silicon. The reactivity of the carbon attached to silicon can be controlled by the nature of the alkoxy group.

The hydrosilylation-oxidation sequence can also be effectively used for the conversion of an alkyne to a ketone. An application of this methodology in the synthesis of 5-decanone is shown in eq 31.44 The reaction involves the hydrosilylation of an alkyne to a vinylsilane using CPA catalysis followed by hydrogen peroxide oxidation to the decanone. The procedure can be carried out without isolating the intermediate vinylsilane.

The hydrosilylation-oxidation sequence is a convenient route for the preparation of a-hydroxy ketones from alkynes (eq 32).47 The process involves hydrosilylation of an alkyne to a vinylsilane, epoxidation of the vinylsilane using m-Chloroperbenzoic Acid, and final oxidative cleavage using hydrogen peroxide.

Regioselective functionalization of unsymmetrical internal alkynes can be effected through the intramolecular hydrosilylation of homopropargyl alcohols (eq 33).45 The hydrosilylation proceeds selectively through a 5-exo-dig ring closure to provide the vinylsilane (15). This intermediate can then be oxidized with hydrogen peroxide to give the b-hydroxy ketone in high yields. Alternatively, treatment of (15) with bromine provides the vinyl bromide in a highly selective manner. Vinylsilane (15) undergoes transition metal-catalyzed cross-coupling reactions with alkenyl and aryl halides to provide a convenient route to homoallyl alcohols (eq 34).46

The hydrosilylation-oxidation sequence is an attractive route to the regio- and stereoselective synthesis of the 1,3-diol skeleton. Intramolecular hydrosilylation of allyl and homoallyl alcohols containing di- or trisubstituted internal alkenes can be achieved through CPA catalysis. The reactions proceed with high regio- and stereoselectivity. The intermediate cyclosiloxane undergoes oxidative C-Si bond cleavage with retention of configuration to furnish the 1,3-diol product (eq 35).48

Hydrogenations and Carbonylations.

A complex prepared from CPA and Tin(II) Chloride dihydrate functions as a useful homogeneous hydrogenation catalyst. Ethylene can be quantitatively hydrogenated to ethane at 1 atm using this catalyst system (eq 36).49 Similarly, a 1:1 mixture of acetylene and hydrogen gave ethane and ethylene in a 3:1 ratio. In these reactions, neither CPA nor tin dichloride catalyzed the hydrogenations individually. The homogeneous hydrogenation of polyalkenes using CPA/tin dichloride dihydrate catalyst has been evaluated. The hydrogenation of methyl linolenate using this catalytic system under 500 psi hydrogen gave diene products selectively.50

Terminal alkenes under relatively mild reaction conditions can be converted to a mixture of linear and branched esters by using CO and methanol with a CPA-SnCl2 couple as the catalyst (eq 37).51 Use of water in place of methanol produced the corresponding carboxylic acids in similar ratios. Internal alkenes gave a mixture of products in very low yields under these carbonylation conditions.

Methyl acetate can be hydrocarbonylated under a carbon monoxide-hydrogen atmosphere using a mixed Rhodium(III) Chloride-CPA catalyst to furnish ethylidene diacetate (eq 38).52 The reaction gave higher yields with rhodium trichloride-Palladium(II) Acetate as the catalyst.

Miscellaneous.

Tetraalkylsilanes undergo redistributions selectively in the presence of trichlorosilane and CPA. Generally, the smallest alkyl group exchanges with the chloro group on the silane (eq 39).53

Allyl chloride and allyl bromide react with less hindered silanes under CPA catalysis to provide hydrosilylation products, along with minor amounts of silyl halides. Reactions of bulky silanes with excess allyl bromide in the presence of CPA furnishes the silyl bromide as the major or sole product (eq 40).54

Arenes react with CPA to give the s-aryl complexes of PtIV.55 Regioselectivity of the metallation with monosubstituted benzenes is quite poor, giving both m- and p-substituted complexes. These complexes then undergo further reaction to produce mixtures of chlorinated arenes and substituted biphenyls (eq 41).

Chlorination of alkanes and arenes in the presence of stoichiometric CPA in refluxing trifluoroacetic acid has been reported (eq 42).56 These reactions furnish chlorinated arenes or n-chloroalkanes and require PtII as a cocatalyst.

Cyclopropanes react with CPA to form platinacyclobutanes (eq 43).57 With substituted cyclopropanes, Ziese's dimer provides the best results. Alternatively, cyclopropanes form mixtures of pyrylium ions in very low yields upon treatment with acetic anhydride and catalytic CPA.58

2-Allyloxypyridines may be rearranged to N-allylpyridones when using CPA (eq 44).59 The reaction is carried out by heating the pyridine at 125 °C for 36 h without solvent. The reaction can also be catalyzed by Pt(PPh3)4.

Alcohols can be converted to carbonyl compounds through a two-electron oxidation process using molecular oxygen, CPA, and visible light.60 The reaction is catalytic in PtIV and requires CuII cocatalysis.


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Mukund P. Sibi

North Dakota State University, Fargo, ND, USA



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