Dehydrobromination of Secondary and Tertiary Alkyl and Cycloalkyl Bromides with 1,8-Diazabicycloundec-7-ene. Synthetic Applications

Article abstract of DOI:10.1021/jo00349a023

The alkene proportions from the dehydrohalogenation of eight secondary and six tertiary aliphatic bromides, four 1-bromo-1-alkylcycloalkanes, three dibromobutanes, and 2-hexyl chloride and iodide with the hindered amidine 1,8-diazabicyclo<5.4.0>undec-7-ene (DBU) have been determined.For secondary bromides, the ratios of the more substituted alkene (Saytzeff product) to less substituted alkene (Hofmann product) are on the order of 9:1; for tertiary bromides, the corresponding ratios are 2-3:1 for aliphatic systems and >6:1 for cycloalkyl systems.The presence of a tertiary or benzylic β-H favors the Saytzeff product.Chain branching or elongation of the tertiary aliphatic bromides has only a small effect upon the regioselectivity.Activation of the β-H with a bromine atom considerably reduces the regioselectivity.For 2-halohexanes, the proportion of Saytzeff product increases in the order chloride < bromide < iodide, while the trans/cis ratio increases in the same order from 4.9 to 9.Reduced regio- and stereoselectivities were observed when 2-hexyl p-toluenesulfonate was treated with DBU.The thermolysis of the same compound gave a mixture of 1-, 2-, and 3-hexene