Which isomer is it, 1,2,5,6- or 1,4,5,8-tetrasubstituted cycloocta-1,3,5,7-tetraene? Synthesis of symmetrically tetrasubstituted cycloocta-1,3,5,7-tetraene derivatives

Abstract

Ni(0) catalyzed cyclotetramerization [(2 + 2 + 2 + 2) cycloaddition] of propargyl alcohol gave D₂ symmetric 1,4,5,8-tetrakis(hydroxymethyl)cycloocta-1,3,5,7-tetraene (1) as the major product contrary to what was reported earlier to be the corresponding C_2v symmetric isomer, namely 1,2,5,6-tetrakis(hydroxymethyl)cycloocta-1,3,5,7-tetraene. The structure of 1 has been unambiguously assigned on the basis of single crystal XRD data which showed a very interesting “densely” intermolecularly hydrogen bonded solid state supramolecular structure. Tetrabromide (3) prepared from tetra-alcohol (1) reacted with a variety of nucleophiles, namely carbon, nitrogen, sulfur and phosphorus nucleophiles, to give the corresponding derivatives in good yields. The tetraphosphonate derivative (9) underwent the Wittig–Horner reaction with aromatic aldehydes to give the corresponding tetra-styryl derivatives (10a–i). The reaction of tetrabromide with aniline, benzyl amine and R(+)-α-methylbenzyl amine gave the corresponding C_2v symmetric pyrroline fused 1,2,5,6-tetra-substituted cycloocta-1,3,5,7-tetraene derivatives, while all the other derivatives belonged to D₂ symmetric 1,4,5,8-tetra-substituted cycloocta-1,3,5,7-tetraene. Quantum mechanical calculations support the above conclusion based on the relative stabilities of the two isomers. The fusion of a 5-membered pyrroline ring with the tub-shaped cyclooctatetraene (COT) ring leads to flattening of the tub (shallow tub) making it easier for the ring inversion of COT through a planar intermediate to take place readily resulting in the formation of C_2v symmetric pyrroline fused COT derivatives.