Stereo specific halogenation

Bis (pyridine) iodine (I) fluoroboratelfluoroboric acid Regio- and stereo-specific halogenative ring closures... 

Some of the applications of the organometallic compounds of lanthanides are as catalysts for (i) stereo specific polymerization of diolefins and in particular to obtain high yields of 1,4-ci.v-polybutadiene and 1,4-cw-polyisoprene and copolymer of the two monomers. The order of effectiveness of the rare earths as catalysts is Nd > Ce, Pr < Sm, Eu. The nature of halogen of the Lewis acid affecting the catalytic activity is in the order Br > Cl > I > F. Detailed work on the activity of cerium octanoate-AlR3-halide showed stereo specificity with cerium as the primary regulator. Cerium is thought to form jr-allyl or 7r-crotyl complexes with butadiene. 

The halogen-Li exchanges between Me3SiCX=CHR (XIII) (R = n-Bu, cyclo-CgHii or t-Bu X = Br or I) and n-BuLi proceeds stereoselectively rather than stereo- specifically . Reactions of (Z)-XIII (X = Br) proceed at — 95°C in THF-Et20-CgHi4 reactions of (Z)-XIII (X = I) occur in Et O at —70°C. The more hindered (E) isomers react slower. 

Zweifel s group has developed two complementary syntheses of 1,1-dihalo-alkenes this permits mixed dihaloalkenes to be prepared. Thus, the hydro-boration of 1-halo-l-alkynes (117) followed by oxidation gives the borinic alkenes (118), which are converted into the bromohaloalkenes (119) stereo-specifically. Alternatively, tra 5-halogenation of ( )-haloalkenylsilanes (120) followed by a/iti-desilicohalogenation gives the ( )-dihaloalkenes (121) [similarly. 

A cycloaddition reaction of the halogen-substituted nitrone (289) with olefins in the presence of AgBp4 provides reactive iminium salts in a regio- and stereo-specific manner. These react with various nucleophiles to give 1,2-oxazine derivatives as shown. In a study of the reactions of P-enamino-esters with carbonyl compounds, WamholF et have isolated a series of acyclic and... 

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