Platinum silyls

The preparation of various platinum-silyl complexes, including a platinum(II), platinum(IV), and mixed-valence platinum(II)-(IV) species, has been described.59 The treatment of one equivalent of 1,2-disilylbenzene with [Pt(PEt3)3] in toluene solution at 0°C affords [Pt(C6H4Si2H4)-(PEt3)2] (25) as a white powder the use of excess 1,2-disilylbenzene in the reaction leads to the formation of various platinum(IV) derivatives. 

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. 

Recently, a new class of platinum silyl complexes was reported (89-91). The complexes of type 49, formed from LPtfQH (L = bulky phosphine) and trialkylsilanes, are extremely efficient hydrosilation catalysts. Upon heating, 49 is converted into 50 via an ill-defined a-elimination process. In all examples of redistributions, the presence of at least one hydro-... 

Although it is now almost fifty years since Speier and his colleagues first announced the chloroplatinic acid-catalyzed hydrosilation of olefins, we are still far from complete control of the chemistry. A particular problem is the suppression of double bond migration. A solution of this problem will require a more detailed understanding of the factors affecting the relative rates of P-hydride elimination from an alkyl group and of the reductive elimination of Si-H from a platinum silyl hydride complex. Another factor which is poorly understood is suppression of the irreversible reduction of the platinum catalyst to Pt° metal. Both of these problems can greatly increase costs of production of certain products. 

Arii H, Takahashi M, Takahashi H, Mochida K, Kawashima T (2012) Formation of unique platinum-silyl complexes by the reactimis of a zero-valent platinum complex with various... 

A great number of articles related to the mechanism of this reaction has been published. It can be considered as certain that the silanes react with the platinum center by an oxidative addition to the metal with formation of a silylplatinum hydride and subsequent transfer of the silyl group to the coordinated alkene. 

SN displacement reactions, 27-9 a-Selenocyclohexanones, 77 Senecioyl chloride, 33 Silmagnesiation, platinum-catalysed, 8 Silametallation of terminal alkynes, 7-9 Silver acetylide, 49 Silver trifluoroacctate, 42,127 Silyl cuprates, 7... 

In platinum chemistry, cleavage of bis(silyl) compounds by HCl may be used [Eqs. (58), (59)] to generate monosilyl complexes 63). 

Silyl(pinacol)borane (88) also adds to terminal alkenes in the presence of a coordinate unsaturated platinum complex (Scheme 1-31) [132]. The reaction selectively provides 1,2-adducts (97) for vinylarenes, but aliphatic alkenes are accompanied by some 1,1-adducts (98). The formation of two products can be rationalized by the mechanism proceeding through the insertion of alkene into the B-Pt bond giving 99 or 100. The reductive elimination of 97 occurs very smoothly, but a fast P-hydride elimination from the secondary alkyl-platinum species (100) leads to isomerization to the terminal carbon. 

Mono- and bis(silyl)platinum(II) complexes are believed to play important catalytic roles in hydrosilylation, dehydrocoupling, and double silylation reactions with disilanes and hydrosilanes. A stable, mono(silyl)platinum(II) complex has been prepared by the oxidative addition reaction of the sterically hindered, primary arylsilane 2,6-Mes2C6H3SiH3 (Mes = 2,4,6-trimethylbenzene) to the platinum(O) species [Pt(PPr3)3] in hexane solution at room temperature.133 The colorless product m-[PLl 1(2,6-Mes2C6II3(11 )2Si)(PPr3)2] (21) was isolated as the OPPr3 adduct, and its... 

Chiral bis-phosphine acylplatinum complex 210 with a strong acid such as TfOH serves as an effective enantio-selective catalyst for aldol-type reactions of aldehydes with ketene silyl acetals (Equation (127)).486 The presence of water and oxygen in the catalyst preparation step is required to obtain the highly enantioselective catalyst. The intermediacy of a C-bound platinum enolate was suggested by IR and 31P NMR spectroscopies. 

Intermolecular bis-silylation of unactivated alkenes has been achieved initially with a zerovalent platinum catalyst such as Pt(PPh3)4 (Equations (30) and (31)).101 1,2-Difluorotetramethyldisilane undergoes addition to ethylene and norbornene in the presence of Pt(PPh3)4 catalyst at 150 °G to give the corresponding adducts in 95% and 26% yields, respectively. For the addition of 1,2-diphenyltetramethyldisilane to ethylene, Pt(PMe3)4 (33% yield) was found to be more active than Pt(PPh3)4 (4% yield). 

Although no efficient bis-silylation reaction for aldehydes and ketones with acyclic disilanes has been established, highly strained cyclic disilanes add to C=0 bond in a 1,2-fashion in the presence of nickel and platinum catalysts (Equations (44) and (45)).83 128... 

In relation to the mechanistic proposal, an interesting reactivity of (boryl)(silyl)platinum(n) complex has been reported.223 The complex is prepared by the reaction of silylborane with Pt(cod)2 complex via oxidative addition (Scheme 46). The (boryl)(silyl)platinum complex undergoes insertion of alkynes at the B-Pt bond to give (/3-borylalkenyl)(silyl)platinum(n) complex in high yield. Importantly, the insertion takes place regioselectively, with Pt-G bond formation at the internal. -carbon atom. This result may indicate that the boron-transition metal bond is more prone to undergo insertion of unsaturated molecules. 

Addition of silylpinacolborane to alkenes has been achieved with platinum catalysts (Equation (83)).225 The reaction proceeds regioselectively to provide products in which the silyl groups are attached to the terminal carbon atoms contrary to the silaboration of alkynes. Although no regioisomers are detected, 1-boryl-l-silylalkenes are formed as major byproducts. 

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