António M. d'A. Rocha Gonsalves

State of the art in the development of biomimetic oxidation catalysts

Abstract Work during the last decade laid down the basis for biomimetic oxidation catalysis, which has become an established technique. Preliminary work relied on the use of metal complexes of 5,10,15,20-tetrakisphenylporphyrin (TPP), but these proved to be inadequate to perform the catalytic function. Further, derivatives of TPP with suitable and adequately positioned substituents, allowed the preparation of high performance catalysts, in terms of catalytic efficiency and stability, for some selected oxidations under specific reaction conditions and for a diversity of oxygen donors. The first oxidations were achieved using the ‘special’ oxygen donor iodosylbenzene, but cheaper and generally available oxidants such as sodium hypochlorite or hydrogen peroxide are presently used as the oxidants. Catalysts capable of promoting stereoselective catalysis are reported. Conditions for the efficient use of the catalysts so far reported require particular specificities and the catalytic activity is only shown, in the majority of the reported works, in the presence of selected species to act as axial ligands to the metal ion of the catalyst. Specific porphyrin structures were reported where the addition of a specific axial ligand can be avoided.

One-step synthesis of dipyrromethanes in water

In this communication we describe the first results of an efficient synthesis of β-free-dipyrromethanes in water. This new method affords very pure products in moderate to high yields, using a cheap, non-toxic, environment friendly solvent, in a one-step procedure from pyrrole and carbonyl compounds, needing little or no work up procedures.

Assignments of the paramagnetically shifted heme methyl nuclear magnetic resonance peaks of cyanometmyoglobin by selective deuteration

Abstract Three of the four paramagnetically shifted heme methyl nuclear magnetic resonance peaks of cyanometmyoglobin could be assigned by comparing the proton nuclear magnetic resonance spectra of myoglobins reconstituted from selectively deuterated hemes. These spectra indicate that the fourth methyl nuclear magnetic resonance peak has to be looked for outside the region −9 to −43 parts per million.

Synthesis of 2-halo-2H-azirines

Abstract α-Oxophosphonium ylides react with N -chlorosuccinimide, N -bromosuccinimide and N -iodosuccinimide in the presence of azidotrimethylsilane giving the corresponding haloazidoalkenes which were completely converted to the 2-halo-2 H -azirines on heating in heptane.

Metal-assisted reactions. Part 22. Synthesis of perhalogenated prophyrins and their use as oxidation catalysts

Abstract Meso-tetrakisarylporphyrins have been perchlorinated and perbrominated at peripheral (β)-positions of the pyrrole rings by high-yielding procedures. These halogenated porphyrins show enhanced catalytic activity towards oxidation of alkanes and alkenes but are not stable when hydrogen peroxide is used as the oxygen donor. The reason for this instability appears to lie in excessive homolytic dissociation of H 2 O 2 with consequent rapid attack at the porphyrin β-position.

Singlet oxygen in antimicrobial photodynamic therapy: Photosensitizer-dependent production and decay in E. coli

Several families of photosensitizers are currently being scrutinized for antimicrobial photodynamic therapy applications. Differences in physical and photochemical properties can lead to different localization patterns as well as differences in singlet oxygen production and decay when the photosensitizers are taken up by bacterial cells. We have examined the production and fate of singlet oxygen in Escherichia coli upon photosensitization with three structurally-different cationic photosensitizers, namely New Methylene Blue N (NMB), a member of the phenothiazine family, ACS268, a hydrophobic porphyrin with a single cationic alkyl chain, and zinc(II)-tetramethyltetrapyridinoporphyrazinium salt, a phthalocyanine-like photosensitizer with four positive charges on the macrocycle core. The kinetics of singlet oxygen production and decay indicate different localization for the three photosensitizers, whereby NMB appears to localize in an aqueous-like microenvironment, whereas ACS268 localizes in an oxygen-shielded site, highly reactive towards singlet oxygen. The tetracationic zinc(II) tetrapyridinoporphyrazine is extensively aggregated in the bacteria and fails to produce any detectable singlet oxygen.

On the mechanism of carboxylic acid co-catalyst assisted metalloporphyrin oxidations

The role of the carboxylic acid as co-catalyst in metalloporphyrin catalytic hydrogen peroxide oxidations is discussed, taking into account its dependence on the ratio relatively to the catalyst. The catalytic efficiency and stability of the catalyst in the presence of an excess of carboxylic acid suggests that a metallo-acylperoxo complex can be the effective oxidation intermediate.

Synthesis of isoquinolines by cycloaddition of arynes to 1,2,4-triazines

Abstract Benzyne has been generated from benzenediazonium-2-carboxylate in the presence of several 1,2,4-triazines 1 and isoquinolines 2 have been isolated in moderate yield. 1-Aminobenzotriazole was also used as the source of benzyne and isoquinolines were again isolated in moderate yield from these reactions. 4-Methylbenzyne, which was generated from 5-methylanthranilic acid, reacted unselectively with the triazines to give mixtures of 6- and 7-methylisoquinolines 3 and 4 On the other hand reactions of 3-methylbenzyne with the triazines 1d and 1e proceeded with high regioselectivity, giving only the 5-methylisoquinoline 5a and the 8-methylisoquinoline 6b, respectively.

Pyrrolidine-based amino alcohols: novel ligands for the enantioselective alkylation of benzaldehyde

A series of easily obtained pyrrolidine-based -amino alcohols derived from tartaric acid and primary amines was synthesized and used as chiral ligands in the enantioselective alkylation of benzaldehyde. Using diethylzinc, 1-phenyl-1-propanol was obtained with enantiomeric excesses of up to 80% when (3S,4S)-N-(1-naphthylmethyl)-3,4-dihydroxypyrrolidine was used. The nature of the N-substituent on the ligand, as well as the reaction temperature proved to significantly influence reaction product distribution as well as the enantiomeric excess of the chiral alcohol.

DIELS-ALDER REACTIONS OF 2-AZADIENES DERIVED FROM CYSTEINE AND SERINE METHYL ESTERS AND ALDEHYDES

Abstract The Diels-Alder reactions of N-benzylidenedehydroalanine methyl ester 1a with but-3-en-2-one and with other electron deficient dienophiles have been found to give new dihydro- and tetrahydropyridines. The cycloaddition reactions are regioselective but not stereoselective. Cycloaddition reactions between 1a and enamines have also been observed. The [4 + 2] cycloaddition reactions of other N-arylidenedehydroalanine methyl esters are also reported. Two new types of azadiene were prepared, namely N-(benzoylmethylene)dehydroalanine methyl ester 1e and N-(ethoxycarbonylmethylene)dehydroalanine methyl ester 1f. Their reactions with N-cyclohexen-1-ylpyrrolidine and with N-cyclopenten-1-ylpyrrolidine have led to the isolation of the dihydropyridine and pyridine esters.