Proof of concept for the synthesis of [3.4.3.4] metacyclophane using a porphyrin as a scaffold

Abstract

Several endo- and exo- cyclophanes of various annulus sizes have been synthesized in the past without the benefit of a template and yields were considered moderate to good. The yields of the exo- cyclophanes were better than the endo- due to “self templating”. Wishing to improve yields of the endo- cyclophanes, we have incorporated into its synthetic scheme a porphyrin as a template to direct the macrocycle’s formation. Incorporating a [2+2] synthetic scheme, half of the cyclophane is synthesized by reacting two equivalents of 2-bromoanisole and one equivalent of 1,3-alkyldiol bistosylate through a high yield Grignard reaction coupled by our copper catalyst. The para positions of the bis-anisole are then functionalized to protect the para position from a Claisen rearrangement later in the scheme and also to allow flexibility for elaboration once finished. The bis-phenol is deprotected from the bis-anisole. The newly exposed phenols are allylated and rearranged to give the bis-ortho elaborated bis-phenol. Using a tetra-alpha-5,10,15,20-tetrakis(2-aminophenyl)porphyrin as the basic scaffold, its amino groups were transformed into isocyanates. In situ, the isocyanates were reacted with our ortho-elaborated bis-phenol to rigidly secure two sides of the cyclophane to the porphyrin by way of the newly formed carbamate groups. The bisortho-olefins are then metathesised using Grubbs’ second generation catalyst furnishing the cyclophane ring.