Stereocontrol in nickel mediated syntheses of cyclobutabenzenes. The selective preparation of cis and trans derivatives of 7,8-dibromocyclobutabenzene.
In: Tetrahedron (Tetrahedron), Jg. 54 (1998) ; Nr. 7, S. 1207-1220
Zeitschriftenaufsatz / Fach: Chemie
Most a,a,a',a'-tetrabromo-o-xylenes that are substituted at the ortho and meta positions yield, upon applying nickel mediated cyclization conditions, the resp. trans-7,8-dibromocyclobutabenzenes derivs. as major products. However, when the ortho substituent is an ester or an amide, a good stereochem. control can be achieved by running the reactions with or without the presence of phosphines. The bromine atom at the 7 position can be substituted specifically with inversion by methoxy, yielding the 7-Br-8-OMe cyclobutabenzene, that can be reacted in a stereospecific Diels-Alder reaction to form the cis-bromo-methoxy-tetrahydronaphthalene deriv.