Richard G. Khoury

Regioselective syntheses and structural characterizations of 2,3-dibromo-and 2,3,7,8,12,13-hexabromo-5,10,15,20-tetraphenylporphyrins

Abstract The title dibromoporphyrins 4 5 and hexabromoporphyrins 11 12 are prepared from H2(2-NO2TPP) 1 and Cu(2-NO2TPP) 8, respectively. The β-nitro group confines the 18-π-annulene system of a tetraphenylporphyrin to its N22H-N24H aromatic delocalization pathway which induces the localization of an antipodal double bond on the porphyrin periphery and enhances its susceptibility to electrophilic attack. Dibromination of H2-NO2TPP) 1 occurs regioselectively affording the 12,13-dibromo-2- nitroporphyrin 2 which, upon Michael addition of NaBH4 and re-aromatization of the resulting nitrochlorin 3, provides an entry to 2,3-dibromoTPP 4 5 as well as an improved route to 2,3- dicyanoporphyrins 6 7 . Perbromination of Cu(2-NO2TPP) 8 and denitration of 9 gave, after demetalation, 2,3,7,8,12,13-hexabromoTPP 12. Both 4 and 12 are structurally characterized by X-ray crystallography.

SYNTHESIS AND CHEMISTRY OF NEW BENZOPORPHYRINS

Abstract Benzoporphyrins 5 and 6 are the major products obtained from the cycloaddition reactions of β-fused metallo-pyrroloporphyrins 1 and 2 with dimethyl acetylenedicarboxylate. In the presence of excess dienophile a bis-adduct is also obtained which undergoes retro-Diels-Alder reaction to produce 5 . Benzoporphyrin 5 was converted into the first reported β-fused benzochlorins 9–11 , and the free-base benzoporphyrin 12 was regioselectively brominated to afford 13 . Exhaustive bromination also yields hexabromobenzoporphyrin 14 .

Approaches to β-fused Porphyrinoporphyrins: Pyrrolo- and Dipyrromethaneporphyrins

Reaction of (2-nitro-5,10,15,20-tetraphenylporphyrinato)nickel(II) 3 or its copper(II) complex 4 with α-isocyanoacetic esters in the presence of 1,8-diazabicyclo[5.4.O]undec-7-ene accomplishes the first syntheses of β-fused pyrroloporphyrins (e.g. 5–10). The removal of the fused pyrrole ester protecting groups to give the di-α-free pyrroloporphyrins 2a and 13 is described, as well as some pyrrole-type chemistry. Unsymmetrical fused dipyrromethanoporphyrins (16–18) were synthesized by reacting 5-acetoxymethylpyrrole-2-carboxylates (14,15) with mono-α-free pyrroloporphyrins (5–8) in the presence of Montmorillonite clay. A symmetrical bis-porphyrinyl dipyrromethane 19 was also prepared by condensation of the β-fused methyl ester pyrroloporphyrin 7 with dimethoxymethane in ethanol and concentrated hydrochloric acid.

Metal ion-induced self assembly of open-chain tetrapyrrole derivatives: Double stranded dinuclear complexes from 10-oxo-5,15-biladienes

Abstract Syntheses of double stranded neutral species via the complexation of Zn, Cd, and Hg with 10-oxo-5,15-biladiene LH2 are described. Formation of the single stranded Cd complex indicates that the self assembly process which affords the double stranded system is not simply a consequence of relief of steric strain. The crystal structure of the cadmium(II) double stranded dimer 8 is described. Long contact distances between carbonyl spacers and Cd ions in the crystal structure of 8 complete the coordination sphere of each Cd, concomitantly forming two distorted trigonal bipyramidal metal centers. Fast coordination/decoordination phenomena involving the carbonyl spacers and the two metallic centers in the zinc(II), cadmium(II) and mercury(II) dimeric complexes 7,8 and 9, respectively, are investigated by using variable temperature 1H NMR. A bending/twisting process is proposed, and its associated energy barrier (40–50 kJ mol−1) is determined for the first time.

Functionalization of 2,3-Disubstituted-2,3-dihydro-5,10,15,20-tetraphenylporphyrins

Abstract A reduced pyrrole subunit directs electrophilic functionalizations of dihydroporphyrins to the antipodal pyrrole ring by confining the chromophore 18-π-electron delocalization pathway to its N(22)H-N(24)H tautomer. The 2,3-disubstituents inhibit oxidation, this being exemplified by the synthesis of perbrominated dodecasubstituted metallochlorins. Regiospecific nitration (using N2O4) of metal-free chlorins provides access to Michael acceptors such as 12-nitro-2,3-disubstituted chlorins which are used in the preparation of highly functionalized tetraaryl-bacteriochlorins by conjugate addition of carbon-centered nucleophiles.

Syntheses, characterization, and structural chemistry of biladien-ac-10-one and -bc-5-one metal complexes with 4N or (3N + O) co-ordination

Transition-metal(II) complexes have been formed between Cu, Co, Pd, Hg, Cd, Zn and Ni and biladien-bc-5-ones or -ac-10-ones. All show spectroscopic and analytical data in accordance with the formation of 1 : 1 complexes. Reaction of 5- and 10-oxobiladienes with CuII, CoII, PdII, ZnII, CdII and HgII always gave complexes in which the metal is co-ordinated to the four pyrrole nitrogens (4N co-ordination) and led to a cyclic, helical conformation of the tetrapyrrole ligand. Reaction of biladien-ac-10-ones with NiII also gave the 4N-co-ordinated derivative. However, reaction of biladien-bc-5-ones with NiII gave two compounds, one being the 4N-co-ordinated complex and other a side product in which the metal is co-ordinated to three pyrrole nitrogens of the conjugated system and to the meso-oxygen atom of the oxobiladiene [(3N + O) co-ordination]. The latter co-ordination type was found to occur only when the C(19) position became substituted by nucleophiles such as OH or OMe leading to steric repulsion of the fourth pyrrole ring away from the metal. Single-crystal structure analyses were made for 10-oxobiladien-ac complexes with 4N co-ordination of CuII, PdII, NiII or CoII and for 5-oxobiladien-bc complexes with 4N co-rdination of PdII, CoII or CuII. These showed tetrahedrally distorted co-ordination geometries in the MN4 unit. The molecular structures of two nickel complexes of 5-oxobiladien-bc derivatives with square-planar (3N + O) co-ordination have also been determined.

New chemistry of oxophlorins (oxyporphyrins) and their π-radicals

Abstract Syntheses of novel 15-substituted-oxophlorins via the MacDonald condensation of diformyl-dipyrroketones and 5-substituted-dipyrromethanes are described. The electronic and steric features of the 15-substituent enable facile control over the oxidation potential of the oxophlorins. Introduction of an electron-withdrawing group efficiently minimizes the formation of oxophlorin π-radicals. Stabilization of neutral π radicals is promoted by hyperconjugation with a 15-tert-butyl group. A sterically induced stabilization of a novel non-aromatic tautomer of oxophlorin, the so-called “iso-oxophlorin” is demonstrated. These species exist also as 15-iso-oxophlorins upon complexation to divalent metals. Radical formation, enhanced by mild oxidants such as K3FeCN6, yielded pure oligomers and stereospecific supramolecular arrays by radical dimerizations taking place at the 10- and 10′-positions.

Linear fused oligoporphyrins: potential molecular wires with enhanced electronic communication between bridged metal ions

Syntheses of nanometer scale, directly beta-fused oligoporphyrins sharing an extended π-system are described.

Rational approach to the synthesis of meso–meso (5,5′)linked bis-porphyrins

A novel approach for the synthesis of 5,5′-directly linked metal-free bis-porphyrins is reported; McMurry type coup-ling of dipyrroketones (e.g. 1) leads to 1,1,2,2-tetra(2-pyrrolyl)ethene 2, which after tetra-formylation (to give 3) and MacDonald macrocyclization with a 1,9-di-unsubstituted dihydrodipyrrin 4, affords the 5,5′-bis-porphyrin 5.

Bis-vinylogous corrole: The first expanded corrole

A dimeric structure that is held together by hydrogen bonds and is stable in solution is adopted by the hydrochloride salt of the expanded corrole shown. The synthesis, X-ray structure, and NMR and electronic absorption spectra of the new species all confirm the structural assignment in solution and in the solid state.