Medium-sized cyclophanes--Part 85: benzylation by 8-(bromomethyl)[2.2]metacyclophanes. Through-space electronic interactions of [2.2]metacyclophane benzyl cations

[TiCl.sub.4]-mediated Friedel-Crafts benzylation of benzene with 8-(bromomethyl)[2.2]metacyclophanes to afford 8-benzyl[2.2] metacyclophanes is described. Substituent effect through space on the rate of the benzylation of benzene with a bromomethyl group attached on the opposite aromatic ring was first found in this investigation. Interestingly, the introduction of the substituents at the internal position 16 tends to promote the present benzylation reaction rate 1.8-3.8 times. It was found that the benzyl cation intermediate was stabilized by the the direct through-space cation-πt interaction among the opposite benzene ring in the benzylation of [2.2]metacyclophane systems. Key words: cyclophanes, Friedel-Crafts benzylation, benzyl cation intermediate, through-space electronic interaction. On decrit la reaction de benzylation du benzene par le 8-bromomethyl)[2,2]metacyclophanes par la meethode de Friedel-Crafts catalysee par le [TiCl.sub.4]. Dans ce travail, on a mis en evidence pour la premiere fois, un effet de substituant qui se fait sentir a travers l'espace sur la vitesse de benzylation du benzene en raison d'un groupe bromomethyle attacheee sur le noyau aromatique oppose. Il est interessant de noter que l'introduction de substituants a la position interne 16 tendent a acceterer les vitesses de reaction de benzylation par des facteurs allant de 1,8 a 3,8. On a trouvee que le cation benzylique intermeediaire est stabilise; par une interaction cation-π directe a travers l'espace du noyau benzeenique oppose dans la benzylation de systemes [2,2] meetacyclophanes. Mots-cles: cyclophanes, benzylation suivant Friedel-Crafts, cation benzylique intermeediaire, interaction electronique a travers l'espace.
Introduction The geometry of the [2.2]metacyclophane ([2.2]MCP) skeleton is known in detail from an X-ray structural analysis. (2,3) The crystal consists of discrete molecules, each of which has a center [...]