Platinum catalysis hydrosilylation

Hydrosilylation of allylic amines proceeds under platinum catalysis with the opposite regioselectivity to their oxygen analogs [133], The intermediate l-aza-2-silacyclobu-tanes resulting from a 4-ejco-trig addition are thermally stable and may be purified by distillation. Alternatively, direct oxidation using the Tamao conditions provides the corresponding 1,2-amino alcohols with excellent syn selectivity (Scheme 10-111). 

Scheme 3.7 Tandem double hydrosilylation reaction catalysed by a combination of platinum catalysis and chiral palladium catalysis followed by oxidation.

A proposed mechanism for hydrosilylation catalyzed by nickel-phosphine complexes, based on the Chalk-Harrod mechanism for platinum catalysis, incorporates an equilibrium between a o and a n complex. It is possible that four intermediates are formed a terminal or inner o or n complex (Scheme 22.7). It was observed that the internal-terminal ratio was consistent over a reaction period of 45 h, which suggests a similar stability of the formed inner and terminal o complexes (Table 22.17). In these o complexes the involved organic group can be regarded as carbanionic in... 

Metal chemical shifts have not found extensive use in relation to structural problems in catalysis. This is partially due to the relatively poor sensitivity of many (but not all) spin 1=1/2 metals. The most interesting exception concerns Pt, which is 33.7% abundant and possesses a relatively large magnetic moment. Platinum chemistry often serves as a model for the catalytically more useful palladium. Additionally, Pt NMR, has been used in connection with the hydrosilyla-tion and hydroformylation reactions. In the former area, Roy and Taylor [82] have prepared the catalysts Pt(SiCl2Me)2(l,5-COD) and [Pt()i-Cl)(SiCl2Me)(q -l,5-COD)]2 and used Pt methods (plus Si and NMR) to characterize these and related compounds. These represent the first stable alkene platinum silyl complexes and their reactions are thought to support the often-cited Chalk-Harrod hydrosilylation mechanism. 

Cyano compounds liquid crystals, 12, 278 in silver(III) complexes, 2, 241 Cyanocuprates, with copper, 2, 186 Cyano derivatives, a-arylation, 1, 361 Cyanosilanes, applications, 9, 322 Cyclic acetals, and Grignard reagent reactivity, 9, 53 Cyclic alkenes, asymmetric hydrosilylation, 10, 830 Cyclic alkynes, strained, with platinum, 8, 644 Cyclic allyl boronates, preparation, 9, 196 Cyclic allylic esters, alkylation, 11, 91 Cyclic amides, ring-opening polymerization, via lanthanide catalysis, 4, 145... 

While platinum and rhodium are predominantly used as efficient catalysts in the hydrosilylation and cobalt group complexes are used in the reactions of silicon compounds with carbon monooxide, in the last couple of years the chemistry of ruthenium complexes has progressed significantly and plays a crucial role in catalysis of these types of processes (e.g., dehydrogenative silylation, hydrosilylation and silylformylation of alkynes, carbonylation and carbocyclisation of silicon substrates). 

Although the platinum-catalyzed reactions are not as practical as palladium used for the same purpose, several types of unique discoveries still are noteworthy. Besides electrophihc C-H activation, the platinum complexes are efficient catalysts for hydrosilation see Hydrosilation Catalysis) that is important to silicon polymer industry. Platinum carbene catalysts are prepared from Karstedt catalyst (9) and imidazolium salts. The trials of model hydrosilylation reactions show very high yields of regioselective products (<1% impurity) with remarkable TON (see Turnover) (<30ppm catalyst load) (Scheme 81). ... 

All complexes have shown high catalytic activity, even at room temperature (in contrast to platinum catalysts). Hydrosilylation in the presence of phosphine-rhodium complexes occurred in air, because real catalyst (active intermediate) was formed after oxygenation and/or dissociation of phosphine, as reported previously [14]. The non-phosphine complexes 1 and 4 are also very efficient catalysts for the hydrosilylation of allyl glycidyl ether. Irrespective of the starting precursor, a tetracoordinated Rh-H species, responsible for catalysis, is generated under reaction conditions, as illustrated in Scheme 3. 

The silicon hydrides do not spontaneously add to alkenes either. However, the hydrosilation, or hydrosilylation reaction, of olefins is of significant utility in the preparation of alkyl-subtituted silanes with the use of either radical or transition metal catalysis. The preferred metal catalysts for hydrosilation are platinum complexes. Chloro-platinic acid will catalyze hydrosilations with halosilanes, alkylarylhalosilanes, alkoxy-silanes, and siloxanes that in many cases are quantitative under ambient conditions. Yields and conversions are generally poorer for alkyl,- and arylsilanes. Many other coordination complexes have been found to catalyze the hydrosilation reaction, and these can provide certain advantages, particularly in regiochemistry. Some typical hydrosilation reactions are shown in Table... 

Nickel exhibits lower catalytic activity than platinum and rhodium catalysts , and in many cases, phosphine-nickel complexes cause the disproportionation of chlorohy-drosilanes giving complex results. However, in some cases, the regioselectivity in the hydrosilylation of styrene with trichlorosilane catalyzed by nickel complexes is quite different from that achieved by platinum or rhodium catalysts. For instance, a-adduct is exclusively formed by the catalysis of [Ni(CO)(ir-C5H5)]2 ... 

The mechanism of catalytic hydrosilylation is not well understood. Study of these reactions is hampered by their complexity induction periods are often involved, reaction conditions such as the nature of the catalyst and reacting groups are critical factors, and side-reactions, such as alkene rearrangements, are common. A widely accepted mechanism for homogeneous catalysis by platinum complexes is based on the work of Chalk and Harrod (Scheme 10)5,8,262. With Speier s catalyst, it appears that initially, and probably during the induction period, silane reduces the platinum to a Pt(0) or Pt(II) complex that is the active catalytic species265. 

Catalysis of the hydrosilylation of allyl esters has been based on the platinum complexes used in the hydrosilylation of allyl acetate (160 °C, 6.75 h, 56% [12]), allyl acrylate (106-114 °C, 3h, 97 % [13]) and allyl methacrylate (40-50 °C, 3 h, 87 % [14]). We studied the catalytic activity of I in the hydrosilylation of allyl acetate and allyl butyrate to obtain products according to Eq. 5. The results are described in Table 3. 

Oxidation State. Homogeneous catalysis normally involves the metal in changes of oxidation state. It can therefore speed up reactions if the catalyst already contains the metal in an oxidation state involved in the catalytic cycle. However, these oxidation states are often lower than those of readily available compounds. Thus a reduction step is generally involved in the preparation of homogeneous catalysts. An example of this is the use of platinum catalysts for hydrosilylation. 

Traditionally, hydrosilylation reactions have been catalyzed using chloro-platinic acid. This is a readily available form of platinum. However, the platinum is in oxidation state IV. The catalysis of hydrosilylation is thought to proceed via steps such as the oxidative addition of HSiRs to platinum (3). Thus the catalyti-cally active form of platinum is in a lower oxidation state. There is an induction... 

One of the most confusing problems in the hydrosilylation catalysis by rhodium complexes is the influence of molecular oxygen as a cocatalyst. This is a general phenomenon and also occurs in the presence of other transition metal complexes, such as platinum and ruthenium, particularly those with CO and phosphine ligands. The concerted mechanism of hydrosilylation processes catalyzed by Wilkinson catalyst involves predissociation of the phosphine from the complex. In this respect, molecular oxygen functions as a promoter since the dissociation of phosphine occurs more readily from [RhCl(02)(PPh3)3] than from the... 

Early concepts regarding the extremely efficient hydrosilylation catalysis by chloroplatinic acid involved the reduction of the complex to colloidal platinum, which was believed to be the real catalyst. However, the reports by Lewis and co-workers (e.g., (131,132)) have shown that the processes leading to colloid formation can account for the high activity of a hydrosilylation catalyst based on the complexes of Pt(0) with olefins and dienes such as divinyltetramethyldisiloxane (Karstedt s catalyst) and cyclooctadiene. The reactivity order of platinum group... 

A proposed mechanism [9] for the hydrosilylation of olefins catalyzed by platinum(II) complexes (chloroplatinic acid is thought to be reduced to a plati-num(II) species in the early stages of the catalytic reaction) is similar to that for the rhodium(I) complex-catalyzed hydrogenation of olefins, which was advanced mostly by Wilkinson and his co-workers [10]. Besides the Speier s catalyst, it has been shown that tertiary phosphine complexes of nickel [11], palladium [12], platinum [13], and rhodium [14] are also effective as catalysts, and homogeneous catalysis by these Group VIII transition metal complexes is our present concern. In addition, as we will see later, hydrosilanes with chlorine, alkyl or aryl substituents on silicon show their characteristic reactivities in the metal complex-catalyzed hydrosilylation. Therefore, it seems appropriate to summarize here briefly recent advances in elucidation of the catalysis by metal complexes, including activation of silicon-hydrogen bonds. 

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