Phthalocyanines (Pcs) have been the subject of intensive study in a variety of academic and commercial fields. For example, they have industrial applications as dyes and pigments, photoconducting reagents in photocopiers, catalysts for removing sulfur from crude oil, deodorants, in the photodynamic therapy of cancer, and as germicides. [1] However, Pc analogues which contain seven-membered carbon rings instead of benzene rings have so far not been reported. [2] Herein we report the first example of this kind of Pc analogue, namely, compound 1. This compound contains four fused cycloheptatriene (CHT) rings on the periphery of the parent tetraazaporphyrin skeleton. Cycloheptatriene is well known for being able to convert between nonaromatic CHT and the aromatic tropylium cation through a redox reaction during the elimination/recombination of a hydride ion (H À ). [3] Accordingly, we considered that it might be possible to adjust the size of the p-conjugation system in 1 by dehydrogenation/hydrogenation of the CHT Scheme 3. A plausible mechanism for the palladium-catalyzed [32] cycloaddition of alkylidenecyclopropanes 1 with aldehydes 2.
In conclusion, we are now in a position to synthesize various types of exo-methylene ± tetrahydrofuran derivatives through the palladium-catalyzed [32] cycloaddition between methylenecyclopropanes and aldehydes. The present atom-economical reaction may be potentially useful for constructing biologically important tetrahydrofuran skeletons.
Experimental Section
3 a: Compounds 2 a (1.5 mmol) and 1 a (0.5 mmol) were added to a mixture of [Pd(PPh 3 ) 4 ] (11.5 mg, 0.01 mmol) and tributylphosphane oxide (4.4 mg, 0.02 mmol) under argon in a pressure vial. The reaction mixture was heated at 120 8C for 5 h, and then filtered through a silica-gel column using ethyl acetate as an eluent. The product was isolated by passing it through a silicagel column, and purified by medium-pressure liquid column chromatography (silica gel) to afford the cycloadduct 3 a.