Synthetic routes to 1,3-divinyl cyclobutanes

Abstract

The chemistry of the cyclobutadiene metal complexes of the last decade is reviewed. Emphasis is given to the organic reactions involving the cyclobutadiene ligand. Dichoro, dimethyl, and diphenyl ketenes were used in [2+21-cycloaddition reactions with 1,3-dienes, which included 1-vinyl cyclohexene, 2-triethylsilylbuta-l,3-diene and 1- and 2- trimethylsilyloxybuta-1,3-dienes. Thecycloadditions were rarely found to be regiospecific and nearly always low yielding. The produced cyclobutanones were very often unstable. 2,2-Diphenyl-3-(1'-triethylsilyl-1'-ethenyl) cyclobutanone was prepared but it was found not to produce the expected 1,3-divinyl cyclobutan-l-ol when treated with vinylmagnesium bromide. 2,2-Dimethyl and 2,2-diphenyl-3-ethoxy cyclobutanones were prepared and their reactions with vinylmagnesium bromide and methyl lithium were investigated. Ethyl 4,4-diphenylbut-3-enoate was formed when 2,2-diphenyl-3-ethoxy cyclobutanone was treated with vinylmagnesium bromide. As well as the above ester, the same cyclobutanone gave 5,5-diphenylpent4-en-2-one when treated with methyl lithium. 2,2-Dimethyl3-ethoxy cyclobutanone gave the expected alcohols when reacted with the same two reagents. 1-(11-Phenyl-2'-propenyl)-2,2-dimethyl-3-vinyl cyclobutane was synthesised starting from a-pinene. The best route was found to involve the preparation of pinonic aldehyde by ozonolysis of q-pinene. The aldehyde was reduced to pinonic alcohol which was then converted to pinonic iodide by treatment with triphenylphosphine and iodine. The iodide was dehydrohalogenated by a new method, described in chapter six of the thesis, to givel-acetyl-2,2-dimethy1-3-vinyl cyclobutane. A subsequent Wittig reaction gave the desired divinyl cyclobutane. Finally, the development of a new methodology is described for the dehydrohalogenation of primary bromides and iodides. It involves a novel, facile synthesis of tetrakis(triphenylphosphine) nickel. The primary halide is mixed with 1,8-diazabicyclo[5.4.0lundec-7-ene and the mixture added to the low valent nickel reagent. The alkene was formed in good yield after twelve hours of stirring at room temperature under argon. The reaction was applied to a few halides and it was found to be compatible with alcohol, ketone, ester, and ether groups.