Silylene phosphine

The coordinated silylenes in both the iron and the chromium compounds can be photolytically activated Photolysis of the complexes in the presence of triphenylphosphine gives the trans-silylene-phosphine complex, which in a second step is transformed into the trnns-bisphosphine compound by excess phosphine. If the silylenes are not trapped, polysilanes are isolated in almost quantitative... 

Few silaphosphenes (phosphasilenes, silylene phosphines, silylidene phosphanes) have as yet been isolated. They are formed as intermediate species in reactions of type (9.251). First evidence was obtained in 1979 with the reaction (9.252) [49]. A stable compound is formed in Equation 9.251 when large protecting groups are present. 

Starting from (OC)5MnSiR2H (R = Me, Ph, Cl), the p-silylene complex 70 is accessible via the oxidative addition of the Si —H bond to Pt(C2H4.)(PPh3)2 and Pt(PPh3)4, respectively. Structure 70 can be functionalized by displacement of the phosphine ligands alcoholysis and hydrolysis of the compound 70 leads to silicon-free complexes [175]. 

According to Scheme 1, the resulting phosphine-stabilized silylene nickel complex 11 is transformed to the planar Ni(II) complex 12, releasing unstable silylenes (other fragments than Me2Si also can be obtained similarity). Thus complexes like 11 may be regarded as a depot form for silylenes. 

Like silylenes and carbenes germylenes are isolobal with phosphines and may function as ligands to transition metal complexes. (Adapted from Bazlnet et ah, 2001)... 

Finally, atomic and molecular proton affinities (PAs) have also been evaluated for various functionals for ammonia, water, acetylene, silane, phosphine, silylene, hydrochloric acid, and molecular hydrogen. For G2 and G3 theories, the mean unsigned error in PAs is 1.1 and... 

Products of the type (24) also result from enolizable ketones without the formation of silyl enol ethers if the reaction is carried out in the presence of tertiary phosphines. The proposed mechanism involves the betaine R3P—SiMe2 as the silylene transfer agent. In preventing a 1,3-hydrogen migration, the phosphine may well induce dimerization prior to oxasilacyclopropane formation. The dioxadisilacyclohexane (24) can be reduced with LiAIHU to give dimethylsilyl-substituted carbinols, so the reaction is of synthetic value (Scheme 34) (78JA7074). 

In a number of ways the reactions of stable silylenes resemble those of phosphines, R3P, to which they are isolobal analogs. Examples are provided by the reactions of 59 with covalent azides. Phosphines are known to react with azides to give phosphineimines, Ph3P=NR. In similar fashion, 59 reacted with triphenylmethyl azide in THF to give the silanimine 72 as its THF complex (equation 109)148. This reaction provides a new method for synthesizing compounds containing Si=N double bonds, which have previously been made by salt elimination reactions375. 

Silylene 59 also behaves somewhat like a phosphine in its interactions with metal carbonyls98,149-376. Typical reactions involve substitution of silylene for CO, to give a silylene-metal complex. Three examples are shown in Scheme 20, and the structure of the nickel complex 75 is displayed in Figure 7149. This complex is both the first silylene-nickel complex, and the first example of a bis-silylene-metal complex free of stabilization by Lewis base donors. 

In the case of platinum, the Pt(0) complex Pt(85)3(PPh3) was isolated from the reaction of Pt(PPh3)4 and silylene 85 <2001JOM209, 2003JOM321>. The only characterized complex of coin metals with l,3,2(A2)diazasiloles is the tetrahedral Cu(l) complex 111, which was obtained by phosphine displacement reaction from CuI(PPh3)3. 

The experimental data demonstrate that in their transition metal chemistry silylenes 83-85 are able to replace carbonyl, tertiary phosphine or alkene ligands from a metal and it suggests that the silylene behaves as a strong cr-donor and a weak 7i-acceptor. Therefore, it behaves more like the isolobal PR3 ligand than CO. Differences in reactivity between the two silylenes 83 and 85 are due to the different steric requirements. The sterically more flexible ligand is silylene 85. 

Scheme 7.15. Effect of phosphine ligand on rate of silylene transfer.

Quantum-chemical studies showed that the ability to form complexes with Lewis bases decreases on going from silylenes to stannylenes and increases in the following series of n-donor agents amines < phosphines < arsines < stibines189. This series somewhat differs from that proposed based on experimental data188. Calculations were successfully used to predict the absorption maxima shifts on complexation of silylenes with amines190. 

The reaction of hydrosilylmanganese complexes with ethylenebis(phos-phine) platinum or tetrakis(phosphine)platinum offers a convenient method for the synthesis of silylene-bridged heterometallic complexes [Eq. (18)].45... 

The recoil-atom reaction with phosphine-ethylene mixture (38) produced a product identified as 31SiH3CH2CH2PH2. This finding was rationalized by a mechanism involving addition of silylene to the 71-bond of ethylene ... 

Reports of nucleophilic attack at atoms other than those above have also appeared. A kinetic study has compared the effectiveness of tertiary phosphines and phosphite esters in the catalysis of the cleavage of the silicon-silicon bond of methylchlorodisilanes, providing evidence for the involvement of a stabilised silylene intermediate. The anion of the y-phosphino-p-diketimine (197) has been shown to react with arsenic trichloride to give the phosphino-arsino-p-diketimine (198), in which there is a coordinative link from phosphorus to arsenic. ... 

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