Dehydrobromination of 3-bromotricyclo[5.3.1.03,8]undecane (3-bromo-4-homoisotwistane) and some addition reactions of tricyclo[5.3.1.03,8]-undec-2-ene. Stereochemistry of the formation and reaction of an anti-Bredt compound probed by deuterium substitution

Abstract

3-Bromotricyclo[5.3.1.0^3,8]undecane (2) is easily dehydrobrominated with sodium amide in refluxing toluene to give tricyclo[5.3.1.0^3,8]undec-2-ene (3), an anti-Bredt compound, in 52% yield. The olefin (3) obtained from the 2-exo-deuterio-derivative of the bromo-compound (2) was almost free from deuterium, indicating a predominant exo,syn-mechanism for dehydrobromination. Both acid-catalysed addition of water and oxymercuration of (3) proceeded highly regioselectively with exclusive formation of the 3-alcohol (4), whereas hydroboration of (3) proceeded with less regioselectivity. Use of deuteriated reagents for hydration and oxymercuration–demercuration of (3) and subsequent conversion of the product alcohol (4) into the olefin (3)via the bromide (2) revealed that water underwent addition with an almost pure exo,syn-mechanism, whereas mercury(II) acetate reacted with a mixed syn- and anti-mechanism. The predominant factor in determining the stereochemistry (complete exo,syn) of the acid-catalysed hydration of (3) is suggested to be electronic in nature, consisting of an uneven distribution of π-electrons between the exo- and endo-sides of the olefin (3).