Ylid Chemistry by A. William Johnson

By A. William Johnson

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The major limitation of the method is that ligand exchange is liable to occur, the carbanion group R displacing either the vinyl or a phenyl group from phosphorus, ultimately leading to the generation of another ylid. This alternate reaction accounted for the 6 5 3 2 38 3. 23] ITΝ ο CHO CH — CH— P(C H ) Θ Θ 6 5 + 3 (C H ) PO e xxvm 5 3 presence of the methylenecyclohexane along with the propylidenecyclohexane reported by Seyferth and outlined above ( 5 0 ) . This limitation is likely to render the cyclization method of Schweizer applicable only to oxygen or nitrogen bases since most carbon bases would be nucleophilic enough to carry out ligand exchange.

In spite of the fact that azines from a variety of diazo com­ pounds and tertiary phosphines were prepared by these workers they were unable to isolate any other ylids by an analogous pyrolysis. Occa­ sionally they obtained some triphenylphosphine, an indication of cleavage of the azine. Seyferth et ah (32) proposed that the elimination of nitrogen in the single case reported above may have resulted from a reversal of the azine-formation reaction followed by decomposition of the diphenyldiazomethane into diphenylcarbene, the latter then being trapped by triphenylphosphine.

52] 2 + · CHBr 2 Br© Carbon tetrachloride and triphenylphosphine were found to react at 65° to afford a solution which appeared to contain triphenylphosphinedichloride and dichloromethylenetriphenylphosphorane (114, Rabinowitz and Marcus). The presence of these two substances was detected by reaction of the solution with benzaldehyde, the former producing benzal chloride and the latter affording β,β-dichlorostyrene. There was 56 3. PHOSPHONIUM YLIDS no indication that this reaction involved radicals so the authors proposed that the phosphine initially attacked halogen.

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