Phosphine-silane systems

Early in the study of silicon atom recoil chemistry, ethylene was used as a scavenger to diflFerentiate between silyl radicals, SiHs, and silylene, SiH2, as the intermediate responsible for the formation of silane and disilane in phosphine-silane systems (21). 

Efforts to tune the reactivity of rhodium catalysts by altering structure, solvent, and other factors have been pursued.49,493 50 Although there is (justifiably) much attention given to catalysts which provide /raor-addition processes, it is probably underappreciated that appropriate rhodium complexes, especially cationic phosphine complexes, can be very good and reliable catalysts for the formation of ( )-/3-silane products from a air-addition process. The possibilities and range of substrate tolerance are demonstrated by the two examples in Scheme 9. A very bulky tertiary propargylic alcohol as well as a simple linear alkyne provide excellent access to the CE)-/3-vinylsilane products.4 a 1 In order to achieve clean air-addition, cationic complexes have provided consistent results, since vinylmetal isomerization becomes less competitive for a cationic intermediate. Thus, halide-free systems with... 

Repeating the sequence shown in Figure 18.8 another five or six times (when congestion at the periphery occurs) is not an easy task without the introduction of Si-O-Si links and very few silane dendrimers of such a high generation have been prepared. The reaction is much more facile when HSi(CH3)Cl2 is used. Equipped with phosphine complexes either in the core or at the periphery the dendrimeric systems are used as catalysts [12], Controlling the state of platinum seems to be a key problem. 

A variety of 1,6-diyne substrates have been investigated for activity in the HSiR3/CO system. A ruthenium complex, Ru3(CO)12, with a tertiary phosphine additive catalyzes the formation of good yields of catechol products in which one molecule of diyne, two molecules of CO, and one or two molecules of silane are incorporated, as seen in Eq. (39).108... 

In related chemistry, the borane/silane catalytic system can cleanly dealkylate phosphonic and phosphinic esters RPO(OR,)2 or R2PO(OR/) to give silyl esters with trialkyl silanes.259 If more active silane reagents like Ph2SiH2 or PhSiH3 are employed, catalytic reduction to primary or secondary phosphines is observed. Mechanistic experiments strongly support a silane activation pathway for this chemistry. 

Diethyl (trimethylsilyl) phosphine has been prepared by the reaction of lithium diethylphosphide with chlorotrimethyl-silane in ether solution.4 The lithium diethylphosphide may be prepared by the reaction of an ether solution of phenyllithium with diethylphosphine.6 However, the dialkylphosphines are most conveniently prepared by the reduction of the corresponding tetraalkyldiphosphine disulfides with lithium tetrahydro-aluminate in ether.6 7 An alternative method for the preparation of dimethyl(trimethylsilyl)phosphine which eliminates the handling of the volatile dimethylphosphine involves the preparation of lithium dimethylphosphide from tetramethyldiphosphine. The latter is prepared by the reduction of tetramethyldiphosphine disulfide8 with tributylphosphine.9 The reaction of chlorotrimethylsilane with lithium dimethylphosphide is most conveniently carried out in a vacuum system without solvent at -78°. 

Current developments in combinatorial chemistry and rapid screening techniques promote new systems in the field of polymer-supported catalysts (cf. Section 3.1.3) [279]. A drawback for this technology is still the leaching tendency of the catalyst which is clearly shown in recycling experiments (cf. Section 3.1.1.3) [280]. Polycondensation of organo-functionalized silanes and polysiloxanes leads to covalent support of catalyst on the surface, also known as the sol-gel process [281-283]. Van Leeuwen and co-workers reported a phosphine ligand with a xanthene backbone which was functionalized with a propyltrialkoxysilane... 

A different example of triphasic catalysis for the Heck, Stille and Suzuki reactions relied on a three-phase microemulsion/sol-gel transport system. Gelation of an z-octyl(triethoxy)silane, tetramethoxysilane and Pd(OAc)2 mixture in a H2O/CH2CI2 system led to a hydrophobicitized sol-gel matrix that entrapped a phosphine-free Pd(ii) precatalyst. The immobilized precatalyst was added to a preformed microemulsion obtained by mixing the hydrophobic components of a cross coupling reaction with water, sodium dodecyl sulfate and a co-surfactant, typically zz-propanol or butanol. This immobilized palladium catalyst was leach proof and easily recyclable. 

Preparation of Phosphines by Reduction. - The reduction of phosphine oxides in the final stage of phosphine synthesis remains a common strategy, with silane reagents often being used. Trichlorosilane has found application in the synthesis of the axially chiral systems (83), 73 (84), 7 (85), 75 the chiral... 

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