Acetylene-cumulene macrocycle

The first example of a stretched porphycene was reported in 1990 by Vogel and coworkers. - It was the acetylene-cumulene macrocycle 4.115 ([22]tetra-dehydroporphyrin-(2.2.2.2)) prepared via the McMurry coupling of the diformyl dipyrryldimethyne derivative 4.113. As proved true for porphycene, the presumed intermediate species 4.114 could not be isolated. Instead it was allowed to undergo spontaneous oxidation during workup to afford the aromatic macrocycle 4.115 in 18% yield (Scheme 4.3.1). 

The structure and electronic character of this acetylene-cumulene macrocycle was assigned on the basis of spectroscopic analyses. For instance, the H NMR and NMR spectra of 4.132 were found to resemble those of the smaller [22]tetradehydroporphyrin-(2.2.2.2) 4.115. This was taken as prima-facie evidence that a delocalized 26 7c-electron pathway is available to this molecule, and that the system is thus both aromatic and cumulenic in nature. The UV-vis absorbance spectrum of 4.132 was also found to resemble closely that of 4.115. Compared to this control, however, a significant bathochromic shift in the Soret band ca. 50 nm) was noted. This is as expected given the larger vr-electron conjugation pathway available to 4.132. 

Expanded porphycene 80 containing triple bonds inserted between the 2 and 2 positions of the 2,2 -biyrrole subunits (1990ACI1385,1990ACI1387) is a 22it-electron aromatic macrocycle that displayed an acetylene-cumulene bonding motif, a feature in common with Sondheimer-Nakagawa [4n+2]dehydroannulenes. 

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