Reactivity of Silole within a Core-Modified Porphyrin Environment: Synthesis of 21-Silaphlorin and its Conversion to Carbacorrole

摘要

Condensation of 1,1-dimethyl-3,4-diphenyl-2,5-bis(p-tolylhydroxymethyl) silole with pyrrole and p-tolyl- ACHTUNGTRENUNGaldehyde did not form the expected 21,21-dimethyl-2,3-diphenyl-5,10,15,20-tetraACHTUNGTRENUNG(p-tolyl)-21-silaporphyrin, but rather its reduced derivative, 21-silaphlorin, which contains a tetrahedrally hybridised C5 carbon atom. Attempts to trap 21-silaporphyrin resulted in the serendipitous discovery of a unique transformation of 21-silaphlorin into a non-aromatic isomer of 2,3-diphenyl-5,10,15,21-tetraACHTUNGTRENUNG(p-tolyl)-carbacorrole (iso-carbacorrole). This novel carbaporphyrinoid contains a cyclopentadiene ring embedded in a tripyrrolic framework. This transformation of 21-silaphlorin to iso-carbacorrole, carried out under oxidative conditions, involves extrusion of dimethylsilylene accompanied by migration of the Cmeso-(p-tolyl) unit to create a cyclopentadiene ring directly linked to the adjacent pyrrole through a tetrahedral carbon atom. Insertion of silver or copper ions into isocarbacorrole gave two structurally related organometallic complexes of true carbacorrole in which the metalACHTUNGTRENUNG(III) ions are bound by three pyrrolic nitrogen atoms and a tetrahedrally hybridised C21 atom of the cyclopentadiene moiety. In the presence of oxygen, the silverACHTUNGTRENUNG(III) carbacorrole undergoes internal oxidation to 21-oxacorrole. The structure of silverACHTUNGTRENUNG(III) carbacorrole was determined by X-ray crystallography. The C21 atom was found to have a tetrahedral geometry. The Ag-C(sp3) (2.046(5)A) bond length is similar to that in silverACHTUNGTRENUNG(III) carbaporphyrinoids in which a trigonal carbon atom coordinates to the metal ion. Density functional theory was applied to model the molecular and electronic structure of 21-silaphlorin and feasible isomers of carbacorrole. The total energies (kcalmol-1 vs. iso-carbacorrole), calculated at the B3LYP/6- 31G**//B3LYP/6-31G* level for carbacorrole, iso-carbacorrole, vacataporphyrin and cyclobutadienephlorin, demonstrate the energetic preference for iso-carbacorrole.