An Abnormally Unstable Hexagonal Macrocycle

Significance

The unusual instability of hexagonal diethynylbenzene macrocycle 2 was reported. While cross-coupling cyclization of linear tetramer 4 provided readily isolable and stable molecules 1 and 3, similar reaction conditions with hexamer 5 yielded 2 only in solution (as observed by ¹H NMR spectroscopy). Attempts to isolate this product only resulted in an intractable polymer material. The decomposition of 2 was monitored with HPLC, and the polymer was characterized as a mix of low molecular weight oligomers with LD-TOF. As compound 2 was predicted to be thermodynamically stable, its instability was assumed to be kinetic in nature, although the reason for this phenomenon remains unknown.