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The Porphyrinogen−Porphodimethene Relationship Leading to Novel Synthetic Methodologies Focused on the Modification and Functionalization of the Porphyrinogen and Porphodimethene Skeletons

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journal contribution
posted on 18.05.2000, 00:00 by Lucia Bonomo, Euro Solari, Rosario Scopelliti, Carlo Floriani, Nazzareno Re
The general synthetic methods presented in this paper make available, on a preparative scale, unprecedented porphyrinogen-derived skeletons, including their functionalization at the meso positions. The stepwise dealkylation of meso-octaalkylporphyrinogen R8N4H4 [R = Et, 1; R = Bun, 2] was chemically, mechanistically, and structurally followed until the formation of porphomethene and porphodimethene derivatives 513, obtained with a sequential use of SnCl4. In particular, the porphodimethene derivative [(Et6N4)SnCl2], 9, was reductively transmetalated using Li metal to Et6N4Li2, 14, subsequently hydrolyzed to Et6N4H2, 15. The porphodimethene−nickel complex [(Et6N4)Ni], 16, was used for studying the reactivity and the ligand modification of the porphodimethene skeleton. The reactivity of 16 toward nucleophiles led to otherwise inaccessible meso-substituted-meso-functionalized porphyrinogens [(Et6N4R2)NiLi2], [R = H, 18; R = Bun, 19; R = CH2CN, 20], thus exemplifying a general methodology to meso-functionalized porphyrinogens. In addition, when [NMe2]- was used as the nucleophile, 16 was converted into mono- and bis-vinylideneporphyrinogen derivatives [{Et4(CHMe)N4}NiLi], 21, and [{Et5(CHMe)2N4}NiLi2], 22, through the intermediacy of meso-(dimethylamino)-porphyrinogens undergoing an α-H elimination from the meso positions. Such intermediates were isolated and characterized in the stepwise reaction of 14 with LiNMe2 leading to [{Et6(NMe2)2N4}Li4], 23, and [{Et5(NMe2)(CHMe)N4}Li4], 25. Both compounds, as a function of the reaction solvent, undergo the thermal elimination of HNMe2 with the formation of [{Et4(CHMe)2N4}Li4], 24, which is then protonated to [{Et4(CHMe)2N4}H4], 27. Transmetalation from 23 to 24 can be used as the methodology for the synthesis of a remarkable variety of meso-substituted and functionalized porphyrinogen complexes. The deprotonation of 16 is reversible, therefore 22 and 23 can be protonated back to their starting materials. We took advantage of the nucleophilicity of the vinylidene carbon in 21 and 22 for establishing a general synthetic method to produce meso-functionalized porphodimethenes. This approach was exemplified with the alkylation and the benzoylation of 22 and 21 leading to [{Et4Pri2N4}Ni], 28, [Et4{CH(Me)(PhCO)}2N4Ni], 29, and [Et5{CH(Me)(PhCO)}N4Ni], 30, respectively. Complex 21 displays a bifunctional behavior, as shown by the formation of 30, whereas in the reaction with LiBu, led to [{Et5(Bun)(CHMe)N4}NiLi2], 31.