Platinum compounds silyl complexes

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. 

A mechanism for catalysis by platinum compounds was proposed in 1965 by Chalk58) and has since been supported by increasing knowledge about silyl-metal systems and by the direct detection of Pt-Si211) and Rh-Si61,18s) complexes in the reaction mixtures. The suggested mechanism requires olefin coordination to the Pt(II) species (in the case of H2PtCl6 formed by reduction by the silicon hydride), oxidative addition of the silane, formation of an intermediate in which silicon and alkyl are both bonded to the platinum center, and reductive elimination of alkylsilane, probably assisted by coordination of more olefin ... 

The chemistry of silicon, germanium, and tin transition metal compounds has been the subject of several reviews (12, 180). Optically active silyl ligands have been introduced in a transition metal complex by reaction of chiral functional organosilanes. However chiral silyl ligands containing complexes are limited to a few metal centers we shall discuss in turn iron, cobalt, platinum, and manganese complexes. 

Nucleophilic attack on the central allyl carbon atom of (T) -allyl) palladium and platinum compounds was employed in the synthesis of cyclopropanes from allylic electrophiles and silyl enolates. Treatment with base of (r) -allyl) palladium and platinum complexes bearing a methoxymethoxy group at the 2-position afforded the corresponding oxodimethylenemethane complexes in contrast to the formation of 2-hydroxysubstitutcd-(Ti5-allyl) complexes which was observed under acidic conditions. 

Next, the activation pathway promoted by the silane was investigated. While the reaction between (ICy)Pt(dvtms) 15c and bis(trimethylsilyloxy)methylsilane (3) produced smoothly the dimer 25 (see Scheme 5.7), treating the second-generation derivative 45 with MD M (3) generated a complex mixture upon attempted purification. Replacing MD M by phenyldimethylsilane led to the bis-silyl complex 55 (Scheme 5.16) [39]. This compound is the first purely tricoordi-nated platinum(II) complex ever isolated. We are currently investigating the full scope and application of this intermediate in hydrosilylation. 

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

Several groups have screened a variety of transition metal complexes for activity in the double silylation system, but only compounds of nickel, palladium, and platinum appear to be viable catalysts. The key factor appears to be the involvement of a M(0) species, although certain M(II) complexes can also be used, presumably with in situ reduction to M(0). Generalizations regarding the activity of the different transition metal complexes are difficult, as many variables exist in each system. However, the most active complexes seem to combine palladium metal centers with dba, small basic phosphine, or isocyanate ligands. 

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). 

Platinum-silylenes are important to numerous transformations involving organosilicon compounds. Base-stabihzed Fisher-type cationic silylene complex of platinnm(II) may be prepared by anion abstraction from a silyl group, as shown in Scheme 51. The counter anion is cracial... 

The double silylation of unsaturated organic compounds catalyzed by group 10 metals is a convenient synthetic route to disilacyclic compounds. Nickel and platinum complexes, in particular, are excellent catalysts for the transformation of disilanes. Cyclic bis(silyl)metal complexes2,3 have been implicated as key intermediates in the metal-catalyzed double silylation of alkynes, alkenes, and aldehydes however, the intermediates have not been isolated due to their instability. We now describe (i) the isolation of the reactive intermediates cyclic bis(silyl)metal compounds (1) with bulky o-carborane unit 4 (ii) the generation of a new class of heterocyclic compounds (4-5) by the stoichiometric reaction of the intermediates with a variety of substrates such as an alkyne, dione, and nitrile 4 and (iii) the facile double silylation of alkenes and alkynes (10,12-14) catalyzed by the intermediate under mild conditions.5... 

Of the new Pt catalysts reported since 1990 platinum complexes with new ligands and activators are noteworthy. Cyclodextrin complexes of platinum (as host-guest complexes) have been employed as hydrosilylation catalysts active at elevated temperature after releasing the guest compound [40]. Some other organic compounds have recently been used as activators (ligands) of Pt complexes, e. g., unsaturated secondary and tertiary alcohols and silylated unsaturated alcohols [41], alkadiynes, cyclooctadiene [42], and vinylnorbomene as well as quinones and methylnaphthoquinones [43]. 

Ishikawa found the Ni(PEt3)4-catalyzed silylation of aromatic compounds with 3,4-benzo-1,1,2,2-tetraethyl-1,2-disilacyclobutene, providing the l-(di-ethylarylsilyl)-2-(diethylsilyl)benzene in high yields (Eq. 44) [89]. In the case of the reaction of mesitylene, interestingly, the sp3 C-H bond adds to the disilacy-clobutene, albeit in low yield (28% yield) [89]. Platinum(O) complexes are also applicable to this silylation reaction as the catalyst [90]. 

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