Direct Oxidations of meso-Tetrakis(pentafluorophenyl)porphyrin: Porphotrilactones and Entry into a Nonbiological Porphyrin Degradation Pathway.

The direct oxidations of meso-tetrakis(pentafluorophenyl)porphyrin using cetyltrimethylammonium permanganate (CTAP), RuCl3/Oxone/base or Ag+/oxalic acid each generate distinctive product mixtures that may contain, inter alia, porpho-mono-, di-, and trilactones. The CTAP and RuCl3/Oxone/base oxidations also generate a unique open chain tripyrrin derived from the degradation of a porpholactone oxazolone moiety. Thus, its formation and structure are distinctly different from all biological or nearly all other nonbiological biliverdin-like linear porphyrinoid degradation products that are derived from ring cleavages between the pyrrolic building blocks.

Syntheses and Aromaticity Parameters of Hexahydroxypyrrocorphin, Porphotrilactones, and Their Oxidation State Intermediates.

Triple OsO4-mediated dihydroxylation of meso-tetrakis(pentafluorophenyl)porphyrin formed a non-aromatic hexahydroxypyrrocorphin as a single stereo-isomer. A one-step oxidative conversion of all three diol functionalities to lactone moieties generated three out of the four possible porphotrilactone regioisomers that were spectroscopically and structurally characterized. This conversion recovered most of the porphyrinic macrocycle aromatic ring current, as seen in their 1H NMR spectra and modeled using DFT computations. Stepwise OsO4-mediated dihydroxylations of porpho-mono- and -di-lactones generated intermediate oxidation state compounds between the pyrrole-three pyrroline macrocycle of the pyrrocorphin and the pyrrole-three oxazolone chromophore of the trilactones. The aromaticity of these chromophores was reduced with increasing number of oxazolone to pyrroline replacements, showing the importance for the presence of three lactone moieties for the retention of the macrocycle aromaticity in the tris-ß,ß'-modified macrocycles. This work first describes hexahydoxypyrrocorphins, porphotrislactones, and the oxidation state intermediates between them; furthers the understanding of the roles of ß-lactone moieties in the expression of porphyrinic macrocycle aromaticity; and generally broadens access to chemically stable pyrrocorphins and pyrrocorphin analogues.

Bis(catecholato-κ2 O,O')bis-(dimethyl sulfoxide-κO)titanium(IV).

Bis(benzene-1,2-diolato-κ2 O,O')bis-(dimethyl sulfoxide-κO)titanium(IV), [Ti(C6H4O2)2(C2H6OS)2], crystallizes with two crystallographically independent mol-ecules in the space group P21/c emulating ortho-rhom-bic Pbca symmetry (ß = 90.0445 (9)°]. The two mol-ecules are related by pseudo-glide symmetry, broken by modulation of each one catecholate and dimethyl sulfoxide (DMSO) ligand. Twinning by pseudomerohedry was observed [twin ratio 0.5499 (7):0.4401 (7)]. Complex 3 was obtained by heating of diprotonated titanium tris-catecholate precursor 2H in DMSO, by formal displacement of a catechol mol-ecule by two DMSO mol-ecules. Complex 3 is just the second heteroleptic, mono-nuclear, neutral bis-catecholate complex with TiO6 metal coordination, the only other one being its bis-DMF analogue 6. The two mol-ecules of 3 exhibit a distorted octa-hedral geometry. The geometry and distortions from ideal symmetry of 3 are discussed and compared to 6 and to cationic tris-catecholate titanium complexes.

Origins of the Electronic Modulations of Bacterio- and Isobacteriodilactone Regioisomers.

Advances in the utilization of porphyrinoids for photomedicine, catalysis, and artificial photosynthesis require a fundamental understanding of the relationships between their molecular connectivity and resulting electronic structures. Herein, we analyze how the replacement of two pyrrolic Cß═Cß bonds of a porphyrin by two lactone (O═C-O) moieties modulates the ground-state thermodynamic stability and electronic structure of the resulting five possible pyrrole-modified porphyrin isomers. We made these determinations based on density functional theory (DFT) and time-dependent DFT computations of the optical spectra of all regioisomers. We also analyzed the computed magnetically induced currents of their aromatic π-systems. All regioisomers adopt the tautomeric state that maximizes aromaticity, whether or not transannular steric strains are incurred. In all isomers, the O═Cß-Oß bonds were found to support a macrocycle diatropic ring current. We attributed this to the delocalization of nonbonding electrons from the ring oxa- and oxo-atoms into the macrocycle. As a consequence of this delocalization, the dilactone regioisomers are as-or even more-aromatic than their hydroporphyrin congeners. The electronic structures follow different trends for the bacteriochlorin- and isobacteriochlorin-type isomers. The presence of either oxo- or oxa-oxygens conjugated with the macrocyclic π-system was found to be the minimal structural requirement for the regioisomers to exhibit distinct electronic properties. Our computational methods and mechanistic insights provide a basis for the systematic exploration of the physicochemical properties of porphyrinoids as a function of the number, relative orientation, and degree of macrocycle-π-conjugation of ß-substituents, in general, and for dilactone-based porphyrinic chromophores, in particular.

Bacterio- and Isobacteriodilactones by Stepwise or Direct Oxidations of meso-Tetrakis(pentafluorophenyl)porphyrin.

Porpholactones are porphyrinoids in which one or more ß,ß'-bonds of the parent chromophore were replaced by lactone moieties. Accessible to varying degrees by direct and nonselective oxidations of porphyrins, the rational syntheses of all five dilactone isomers along stepwise, controlled, and high-yielding routes via porphyrin → tetrahydroxyisobacteriochlorin metal complexes → isobacteriochlorindilactone metal complexes or porphyrin → tetrahydroxybacteriochlorin → bacteriochlorindilactone (and related) pathways, respectively, are described. A major benefit of these complementary routes over established methods is the simplicity of the isolation of the dilactones because of the reduced number of side products formed. In an alternative approach we report the direct and selective conversion of free base meso-tetrakis(pentafluorophenyl)porphyrin to all isomers of free base isobacteriodilactones using the oxidant cetyltrimethylN+MnO4-. The solid-state structures of some of the isomers and their precursors are reported, providing data on the conformational modulation induced by the derivatizations. We also rationalize computationally their differing thermodynamic stability and electronic properties. In making new efficient routes toward these dilactone isomers available, we enable the further study of this diverse class of porphyrinoids.

The limited extent of the electronic modulation of chlorins and bacteriochlorins through chromene-annulation.

Optical data (UV-vis absorption and fluorescence emission spectra, including fluorescence yields and lifetimes) and electrochemical measurements are used to quantify the modulation of the electronic properties of meso-tetrakis(pentafluorophenyl)-chlorin diol and -bacteriochlorin tetraols upon intramolecular chromene-annulation, including the investigation of regio- and stereoisomers. The small modulations of the frontier orbitals of the porphyrinoids are rationalized using DFT computations and can be traced to small electronic effects due to the co-planarized meso-aryl groups in combination with conformational effects.

Oxidations of chromene-annulated chlorins.

A chromene-annulated chlorin is a meso-pentafluorophenyl-2,3-dihydroxychlorin in which one of the hydroxy groups is linked to the ortho-position of a flanking meso-aryl group, thereby forming an annulated chromene moiety; the second hydroxy group is unprotected. This report illustrates the oxidation chemistry of the chromene-annulated chlorin under a number of different oxidation conditions. In some reactions, chromene annulation is retained and the non-protected alcohol is oxidized, generating chlorin-like chromophores carrying pyrroline ß-carbons in oxidation states between those found in typical porphyrins and chlorins. Other products have lost the chromene moiety. The reactivity of the pyrroline carbon with respect to undergoing radical chemistry is shown with the formation of a number of products following a Swern oxidation, including the formation of two directly linked pyrroline-pyrroline and pyrroline-pyrrole chlorin dimers. This work contributes to the further understanding of the chemistry of chlorins. Selected compounds possess functionalities at their periphery susceptible to nucleophilic attack by MeOH, suggesting their use as chemosensors.

Chromene-Annulated Bacteriochlorins.

Syntheses and optical properties of mono- and bis-chromene-annulated bacteriochlorins are described. Known monochromene-annulated meso-(pentafluorophenyl)chlorin is susceptible to a regioselective OsO4-mediated dihydroxylation, generating two monochromene-annulated trihydroxybacteriochlorin stereoisomers: either the newly introduced vic-cis-diol functionality is on the same side as the vic-cis-diol moiety the chromene-annulation was based on or on the opposite side. Treatment of the two isomers with heat or base generates different sets of bis-chromene-annulated bacteriochlorin stereo- and regioisomers. Detailed 1D and 2D (1)H and (19)F NMR spectroscopic investigations allowed the characterization of the isomers that formed. The regioselectivity of the second annulation reaction was rationalized computationally on steric grounds. The bacteriochlorin-type optical spectra of the mono- and bis-chromene-annulated bacteriochlorins are modulated as a result of the annulation, with each isomer possessing a unique spectrum, attributed to the effects the regiochemically distinct annulations have on the conformation of the chromophore. The formation of a bis-chromene-annulated chlorin from the bacteriochlorins is also described, including its X-ray crystal structure, revealing some details of the metrics of the chromene-annulated moiety. The vic-diol functionality of monochromene-annulated trihydroxybacteriochlorins is also susceptible to oxidation and ring-expansion reactions, generating chromene-annulated pyrrole-modified chlorins incorporating oxazolone and morpholine moieties. The work expands the body of work on the synthesis and optical fine-tuning of meso-aryl-substituted bacteriochlorins.