Halides continued vinyl

Corresponding replacement of halide from an unconjugated vinylic site has also been reported. The use of the sodium salts of dialkyl phosphites in tetrahydrofuran at low temperature has been found to provide the vinylic phosphonate in good yield,72 (Figure 6.22) and triisopropyl phosphite serves similarly to replace a fluoride of trifluoroiodoethene (Figure 6.23).91 The reaction proceeds ste-reospecifically to replace the fluoride cis to the iodide, and in continuing reaction the iodide is replaced. 

Cross coupling between an aryl halide and an activated alkyl halide, catalysed by the nickel system, is achieved by controlling the rate of addition of the alkyl halide to the reaction mixture. When the aryl halide is present in excess, it reacts preferentially with the Ni(o) intermediate whereas the Ni(l) intermediate reacts more rapidly with an activated alkyl halide. Thus continuous slow addition of the alkyl halide to the electrochemical cell already charged with the aryl halide ensures that the alkyl-aryl coupled compound becomes the major product. Activated alkyl halides include benzyl chloride, a-chloroketones, a-chloroesters and amides, a-chloro-nitriles and vinyl chlorides [202, 203, 204], Asymmetric induction during the coupling step occurs with over 90 % distereomeric excess from reactions with amides such as 62, derived from enantiomerically pure (-)-ephedrine, even when 62 is a mixture of diastereoisomcrs prepared from a racemic a-chloroacid. Metiha-nolysis of the amide product affords the chiral ester 63 and chiral ephedrine is recoverable [205]. 

Photoinduced free-radical halogenation is very useful for the functionalization of electron-deficient alkenes such as vinyl halides. Among halides, the addition of bromine is by far the most useful reaction, as shown in Scheme 3.25. In the first example, a polyfluoroalkene was brominated in a high yield by using a 300 W light bulb as the light source. Of note, the synthesis of l,2,4-tribromo-l,l,2-trifluorobutane (38) was carried out under solvent-free conditions in a near-quantitative yield and on a multigram scale (Scheme 3.25a) [67]. l,2-Dibromo-l,l,2-trichloroethane (39) was likewise obtained in about 250 g amounts by the solar lamp irradiation of neat trichloroethylene to which bromine was continuously added (Scheme 3.25b) [68],... 

The application of the ketone synthesis has continued. Phenylethynyl and vinylethynyl ketones have been synthesized by the reaction of appropriate acid halides with phenylethynylzinc chloride 521, 522) and vinyl-ethynylzinc chloride 552), respectively. 

Miscellaneous Methods of Preparing Phosphines. Palladium-catalysed formation of phosphorus-carbon bonds continues to be developed as a useful route to organophosphines. The reactions of primary or secondary phosphines with aryl- or vinyl-halides in the presence of a palladium catalyst, usually palladium acetate or a zerovalent palladium-phosphine complex, have been used in the synthesis of steroidal phosphines, e.g., (128), the cationic diphosphine system... 

Nucleophilic attack at other atoms. The chemistry of phosphine-borane adducts has continued to generate interest. Simple borane adducts of primary vinyl-, ethynyl- and allenyl-phosphines have been prepared and studied by a range of spectroscopic and theoretical techniques. The same group has also shown that attachment of the BH3 unit to a variety of primary phosphines results in a substantial increase in the intrinsic acidity of the system in the gas-phase. Group III halide adducts of the type Bu 2PH EX3 (E = B, Al, Ga or In X = Cl or Br) are accessible from the reactions of the secondary phosphine with the trihalides at room temperature. The solid state structure and reactivity of these adducts was also reported. Treatment of l,8-bis(diphenylphosphino)naphthalene with the borane-dimethylsulfide complex in ether solvents affords a simple monoborane adduct of the diphosphine irrespective of the molar ratio of the... 

At least for aryl iodides, the application of phosphine complexes of palladium still continues to be practised in complex preparations where mild conditions are desirable to secure high selectivity. In these cases, halide scavenger additives are often used to enable the polar pathway, because phosphine ligands under mild conditions block the coordination places need for alkene binding. Owing to the low reactivity of those systems, high loadings of precatalyst are only tolerable in complex molecule synthesis. For example, this approach enabled clean vinylation of vinylic boronate 63 to afford complex diene 64 (62 64, Scheme 2.13) [62]. 

Osborn s group has continued its studies on whether alkyl halides add oxidatively to iridium(I) compounds by a radical chain or an S 2 mechanism by using more reactive metal complexes such as [Ir(PMe3)2(CO)Cl]. For simple alkyl (methyl excepted), vinyl, and aryl halides and a-halo-esters, evidence based on the effect of radical initiators and inhibitors, structure-reactivity relationships, the trapping of radicals by acrylonitrile, and the loss of stereospecificity at the reacting carbon atom all indicate a radical chain process, perhaps as in equations (14) and (15) ... 

Vinyl silanes continue to attract attention as intermediates for the stereoselective synthesis of olefins. Zweifel and Lewis now describe the stereoselective synthesis of both -and Z-(l-halogenoalk-l-enyl)silanes (15) from alk-l-ynyl-silanes, and show how they may then be processed to dialkyl-substituted vinyl-silanes, alkenyl halides, and trisubstituted olefins (Scheme 12). The E-l-halogenoalkenyltrimethylsilanes are readily prepared in high isomeric purity by treatment of the dialkylhydroalumination adduct (14) with N-chlorosuccinimide,... 

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