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Timothy D Lash

Publications and source records attributed to Timothy D Lash.

27 records · Page 2Linked to original sources
PubMed2003

Conjugated macrocycles related to the porphyrins. 25. Proton NMR spectroscopic evidence for a preferred [18]annulene substructure in carbaporphyrins from the magnitude of selected 4J(H,H) CH=C-CH3 coupling constants.

Two new benzocarbaporphyrins with four or five alkyl substituents have been synthesized by the "3 + 1" MacDonald methodology. At lower temperatures, the proton NMR spectrum of the asymmetrically substituted carbaporphyrin 8 gave two NH resonances, while carbaporphyrin 9, which retains a plane of symmetry, gave only one resonance of this kind. As no additional peaks were seen for the remaining protons, these data strongly support the proposal that a single tautomer predominates in solution where the two NH protons flank the interior CH. Carbaporphyrin 8, which has a CH=CMe unit on the pyrrolic ring opposite the indene moiety, gave a long-range coupling constant of 4J(Me,H) = 1.3-1.4 Hz. On the other hand, the CH=CMe units of 9, which correspond to the pyrrole rings on each side of the carbocyclic moiety, gave 4J(Me,H) = 0.9-1.0 Hz. These values are in accord with those expected if the exterior carbon-carbon bonds of the pyrrole units next to the indene ring are part of a fully delocalized 18pi electron system, while the C=C bond of the remaining pyrrole ring retains substantial olefinic character.

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PubMed2002

Adaptation of the Rothemund reaction for carbaporphyrin synthesis: preparation of meso-tetraphenylazuliporphyrin and related benzocarbaporphyrins.

Electrophilic substitution of azulene has recently been shown to provide the means by which carbon-carbon bonds can be generated to form novel macrocyclic systems such as calixazulenes. These studies inspired us to develop a "one-pot" Rothemund-type synthesis of meso-tetraphenylazuliporphyrin. Azuliporphyrins, a group of cross-conjugated carbaporphyrinoids that exhibit intriguing chemistry and metallation properties, have previously only been available by multistep syntheses. In this work, azulene, pyrrole and benzaldehyde were shown to react in a 1:3:4 ratio in the presence of boron trifluoride etherate to give meso-tetraphenylazuliporphyrin 7a. The free base shows only a minor diatropic ring current, but addition of TFA generates the related dication which shows greatly enhanced diatropicity where the internal CH shifts from delta = +3.35 to -0.5 ppm. Addition of pyrrolidine to 7a gave rise to a carbaporphyrin adduct which showed a porphyrin-like UV/Vis spectrum and the internal CH shifted further upfield to give a resonance near delta = -5.7 ppm. Treatment of 7a with tertbutyl hydroperoxide in the presence of potassium hydroxide afforded a mixture of benzocarbaporphyrins 9a-c. These tetraphenylcarbaporphyrins were fully aromatic by NMR spectroscopy and gave typical porphyrin-type UV/Vis spectra with a strong Soret band near 446 nm. This new methodology makes these important porphyrin analogues readily available for further study.

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PubMed2002

Synthesis, spectroscopy and metallation of mixed carbaporphyrinoid systems.

Modified tripyrranes incorporating furan and thiophene rings were found to condense with benzene, pyridine and indene dialdehydes to give a series of novel porphyrin analogues, including thia- and oxa-carbaporphyrins; the latter readily forms nickel(II) and palladium(II) organometallic complexes.

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PubMed2002

Conjugated macrocycles related to the porphyrins. 21. Synthesis, spectroscopy, electrochemistry, and structural characterization of carbaporphyrins.

The "3 + 1" variant of the MacDonald condensation has been shown to be an excellent methodology for synthesizing carbaporphyrins. In particular, 1,3-indenedicarbaldehyde condenses with tripyrranes in the presence of TFA to give, following oxidation with DDQ, a series of benzocarbaporphyrins in excellent yields. Triformylcyclopentadienes also afford carbaporphyrin products, albeit in low yields ranging from 5 to 8%. These hybrid bridged annulene structures have porphyrin-like electronic absorption spectra with strong Soret bands near 420 nm and a series of Q-bands through the visible region. The proton NMR spectrum confirms the presence of a strong diamagnetic ring current, and the meso-protons show up at 10 ppm, while the internal CH is shielded to approximately -7 ppm. Carbaporphyrins undergo reversible protonation with TFA. Initial addition of acid affords a monocation, although mixtures of protonated species are observed in the presence of moderate concentrations of TFA. However, in the presence of 50% TFA a C-protonated dication is generated. The dications relocate the pi-delocalization pathway through the benzo moiety of benzocarbaporphyrins, and these therefore represent bridged benzo[18]annulenes, although they nevertheless retain powerful macrocyclic ring currents. Carbaporphyrins with fused acenaphthylene and phenanthrene rings have been prepared, and the former demonstrated significantly larger bathochromic shifts in UV-vis spectroscopy that parallel previous observations for acenaphthoporphyrins. A diphenyl-substituted benzocarbaporphyrin 19b was also characterized by X-ray crystallography, and these data show that the macrocycle is reasonably planar although the indene subunit is tilted out of the mean macrocyclic plane by 15.5 degrees. The structural data indicates that the preferred tautomer in the solid state has the two NH's flanking the pyrrolene unit in agreement with previous spectroscopic and theoretical studies. Cyclic voltammetry for carbaporphyrin 19a was more complex than for true porphyrins, showing five anodic waves and two quasi-reversible reductive couples.

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PubMed2002

Metabolism of analogues of coproporphyrinogen-III with modified side chains: implications for binding at the active site of coproporphyrinogen oxidase.

Porphyrinogens with modified propionate side chains bearing methyl substituents were found to be modest substrates for coproporphyrinogen oxidase; the results indicate that alteration of the substituents involved in secondary binding interactions has a comparable affect to modifying the side chain that undergoes degradation at the catalytic site.

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PubMed2002

Calix[4]azulene.

Azulene reacts with paraformaldehyde in the presence of florisil to give excellent yields of calix[4]azulene.

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