Silyl sulphides

Sihcon is most important in the cycloaddition of alkynyl silyl sulphides to ynamines, the silyl shift giving the 4-silylcyclobut-2-enethione which, on S-methylation and desilylation, yields the cyclobutadiene (equation 10)18. 

Reductive cleavage of sulphides, selenides, and tellurides on Raney nickeP and cleavage by lithium in THE, leading to secondary or tertiary alkyl-lithium compounds, are useful synthetic operations novel variations are involved in the cleavage of dialkenyl sulphides with EtjSiH (giving low yields of silyl sulphides) and of alkenyl, aryl, and allyl sulphides and selenides with PPhj and... 

Studies of Equilibria and Shift Reagents.—N.m.r. studies of the exchange of halogen in boron trihalide adducts of trimethylphosphine, its oxide, and sulphide,49 and exchange of chloro- and methoxy-groups between methylphosphino and methyl-silyl or -germyl moieties,80 have been reported. The rates of ionization of phosphoranes... 

V,7V-Diethylcarbamoyl trimethylsilane has been prepared by the reaction of bis(trimethylsilyl) sulphide with bis(A,A-diethylcarbamoyl) mercury (Scheme 30)16. Silylation of the carbamoyl cuprate reagent derived from a lithium amide, by addition of copper(I) cyanide and subsequent exposure to carbon monoxide (1 atm), is also effective75,110. Poor to moderate yields of carbamoyl silanes may be isolated by treatment of lithium silylamides with carbon monoxide and methyl iodide, in a reaction sequence involving a nitrogen to carbon silyl shift in an intramolecular silylation (Scheme 31)111. 

Heating 4- and 5-bromoacyl silanes at 100 °C in a polar aprotic solvent induces cyclization through the enol forms to give 2-silyldihydrofurans and 2-silyldihydropyrans, respectively195. Similar transformation of 4- to 7-halothioacyl silanes, prepared from the corresponding haloacyl silanes by reaction with hydrogen sulphide, but induced by sodium hydroxide, gave the 2-silylated sulphur heterocycles in excellent yields (Scheme 84)196. Intermolecular enolate reactions of acyl silanes are also known (vide infra, Section IV.A.6). 

Aliphatic thioketones result tautomerically pure from the reaction of H2S with ketals in the presence of ZnCl2. Deprotonation with LDA gives the enethiolate which can be silylated to form the silyl vinyl sulphide. These are fairly stable to traces of water but react with MeOH to give the enethiol (equation 3)7. 

The hindered silyl chlorides (Ar X)3SiCl (X = S, Se) react with AgCICU to give a range of sulphides and selenides which are thought to result through the intermediacy of the silicenium ion which loses Ar X+ and (Ar X)2Si (X = S) or Ar X (X = Se). With the thio derivative, the products are the disulphide (5), sulphide (6) and arene (equation 13), while for the selenium chloride, diselenides and selenides dominate (equation 14)25. 

The photoelectron spectra of (Et3Si)2S and the silylalkyl sulphides (Et3Si(CH2)n)2S (n = 0-3) support a greater silyl donor influence for n = 0, 2 than for n = 1, 3, while the charge transfer spectra show a first ionization potential greatest for n = 036. 

It should be noted that several procedures are available for the preparation of l-(trimethylsilyloxy)cyclohexene. In an original report252 it was prepared by heating a dimethylformamide solution of cyclohexanone under reflux with chlorotrimethylsilane in the presence of triethylamine. This procedure is generally satisfactory for aldehydes and symmetrical ketones, or ketones which can enolise in one direction only. The method described in Expt 5.214 employs lithium sulphide as an additive to increase the silylating power of the chlorotrimethylsilane, so that the reaction proceeds at room temperature acetonitrile is used as the solvent.253... 

Alkylation of the sulphone (111) with Bu"Li-Me3SiCH2l gave the /8-silyl-sulphone (112) which with Bu NF gave ° 3-methylene-5a-cholestane (113). Alkenyl sulphoxides, such as (114), were reduced to the alkenyl sulphides, without double-bond isomerization, using EtNlgBr-Cul " and the terminal allene (115) was prepared by treatment of (114) with LTMP. Reaction of... 

The major product obtained by alkylation of the carbanion derived from 2-(dimethylamino)ethylmethylphenylphosphine oxide (66) is (67) rather than (68), while alkylation of the carbanion derived from ethylmethylphenylphosphine oxide (70) takes place exclusively at the methyl group.33 The former reaction provides a useful synthesis of appropriately alkylated vinylphosphine oxides (69) which are not otherwise easily prepared (Scheme 12). The N-silylated nitrile imines (72) have been prepared in solution by silylation of carbanions (71) of diazomethylphosphine sulphides.34 At room temperature compounds (72) rearrange to the isomeric diazo compounds (73). 

Desulphurisation of a diphosphine sulphide by heating with a metal is another good method (Figure 9.24), and in some circumstances P-N linkages can be replaced by P-P (6.664). Diphosphines are also formed from silyl phosphines (Figure 9.10) and by disproportionation of dimethylphosphi-nous fluoride (6.175) and of more complex phosphines. An uncommon reaction is the decomposition of mercury ter-butyl phosphide by ultraviolet radiation in benzene solution (6.665). At low temperatures some unsymmetrical derivatives can be formed (6.666). 

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