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A boron trihalide mediated alkyne-aldehyde coupling reaction leading to stereodefined 1,3,5-triaryl-1,5-dihalo-1,4-pentadienes is described. The study led to the discovery of a direct substitution of hydroxyl groups by stereodefined alkenyl moieties using alkenylboron dihalides. During the investigation, it was also discovered that, at low temperatures, the reaction of BCl3 with alkynes produces monovinylboron dichloride rather than the reported divinylboron chloride. A modified reaction mechanism for the boron trichloride mediated alkyne-aldehyde coupling reaction is now proposed. The reaction temperature and mode of addition have been found to have dramatic affects on the stereochemistry of the diene products.

Several new organic transformations have been achieved utilizing boron halide reagents. Aryl aldehydes are conveniently converted to gem-dichloromethylbenzenes using boron trichloride. Aryl aldehydes are alkylated by alkylboron chlorides in a Grignard-like fashion to generate the corresponding arylalkanols or alkylboron chlorides. Aryl aldehydes react with divinylboron halides (generated via the haloboration of alkynes) to produce 1,5-di-halo-1,4-dienes in excellent yields.

Reactions of aryl aldehydes with styrenes in the presence of boron trichloride in CH2Cl2 at 0°C produce 1,3-dichloro-1,3-diarylpropanes in excellent yields. Reactions carried out using boron tribromide generate the corresponding 1,3-dibromo-1,3-diarylpropane in good yields.

The reductive bromination of aromatic aldehydes with isopinocampheyl boron dibromide in hexane at room temperature produces the corresponding benzyl bromides in excellent yields.

Reactions of aryl aldehydes with styrenes in the presence of boron trihalides produce 1,3-diaryl-1,3-dihalopropanes in excellent yields.

The reductive bromination of aromatic aldehydes with isopinocampheyl boron dibromide in hexane at room temperature produces the corresponding benzyl bromides in excellent yields.

Reactions of aryl aldehydes with styrenes in the presence of boron trichloride in CH2Cl2 at 0°C produce 1,3-dichloro-1,3-diarylpropanes in excellent yields. Reactions carried out using boron tribromide generate the corresponding 1,3-dibromo-1,3-diarylpropane in good yields.

The reactions of aryl aldehydes with 2 equiv of arylacetylenes in the presence of boron trichloride yield (E,Z)-1,3,5-triaryl-1,5-dichloro-1,4-pentadienes. Reactions carried out in the presence of ...

[reaction: see text] The reactions of aryl aldehydes with styrene derivatives, mediated by various boron Lewis acids, were investigated. 1,3-Dihalo-1,3-diarylpropanes were obtained in high yields with boron trihalides, while 3-chloro-1,3-diarylpropanols were obtained in good to excellent yields with phenylboron dichloride. Reactions involving nonenolizable aliphatic aldehydes, trans-cinnamaldehyde, and beta-substituted styrenes were also investigated for the first time.

[reaction: see text] A new halopropargylation of alkynes promoted by boron trihalides has been developed. Reactions of (Z)-2-halo-1-vinylboron dihalides (generated in situ via reaction of alkynes with boron trihalides) with lithium propargyloxides in CH(2)Cl(2) at room temperature produce the corresponding (Z)-1-halo-1,4-enynes in modest to good yields.

[reaction: see text]. The reactions of aryl aldehydes with 2 equiv of arylacetylenes in the presence of boron trichloride yield (E,Z)-1,3,5-triaryl-1,5-dichloro-1,4-pentadienes. Reactions carried out in the presence of boron tribromide generate the corresponding (Z,Z)-1,3,5-triaryl-1,5-dibromo-1,4-pentadienes.