Search

Artificial photosynthesis enables the conversion and storage of solar energy into chemical energy, producing substances with high energy content. In this sense, the oxidation of water can provide the H+ ions and electrons needed for the energy conversion and storage processes. Since 2005, it has been known that single-site coordination compounds can act as water oxidation catalysts (WOC). Improvement of the catalytic activity, however, has occurred mainly by the choice of the redox-active metal matching with a series of compatible ligands, more specifically, paying attention to the electronic characteristics of the organic framework of the first coordination sphere. Recently, the use of dangling bases dramatically increased the catalytic activity of new species as WOC, taking advantage of what is called a second coordination sphere. With this assistance, some compounds were shown to reach turnover frequencies (TOF) of 104 s-1, while compounds with the first coordination sphere commonly exhibit TOF ca. 10-1 s-1. In this manuscript, we discuss the concept, together with a number of examples, of the use of controlled interactions between the first and second coordination spheres that have been wielded to improve the performance of ruthenium-centered complexes as WOC in water oxidation reactions.

The mechanism of the addition reaction of TeCl 4 to alkynes was indirectly established by the detection of TeCl 3 centered radicals using EPR spin trapping, ESI-MS and ESI-MS/MS characterization.

The use of whole cells of micro-organisms to bring about the biotransformation of an organic compound offers a number of advantages, but problems caused by enzymatic promiscuity may be encountered upon with substrates bearing more than one functional group. A one-pot screening method, in which whole fungal cells were incubated with a mixture of 4-methylcyclohexanone 1 and phenyl methyl sulfide 2 , has been employed to determine the chemoselectivity of various biocatalysts. The hyphomycetes, Aspergillus terreus CCT 3320 and A. terreus URM 3571, catalysed the oxidation of 2 accompanied by the reduction of 1 to 4-methylcyclohexanol 1a and, for strain A. terreus CCT 3320, the Baeyer-Villiger oxidation of 1 . The Basidiomycetes, Trametes versicolor CCB 202, Pycnoporus sanguineus CCB 501 and Trichaptum byssogenum CCB 203, catalysed the oxidation of 2 and the reduction 1 , but no Baeyer–Villiger reaction products were detected. In contrast, Trametes rigida CCB 285 catalysed the biotransformation of 1 to 1a , exclusively, in the absence of any detectable sulfide oxidation reactions. The chemoselective reduction of (±)-2-(phenylthio)cyclohexanone 3 by T. rigida CCB 285 afforded exclusively the (+)- cis -(1 R , 2 S) and (+)- trans -(1 S ,2 S ) diastereoisomers of 2-(phenylthio)cyclohexan-1-ol 3a in moderate yields (13% and 27%, respectively) and high enantiomeric excesses (>98%). Chemoselective screening for the reduction of a ketone and/or the oxidation of a sulfide group in one pot by whole cells of micro-organisms represents an attractive technique with applications in the development of synthesis of complex molecule bearing different functional groups.

The first experimental evidence for the formation of an organic tellurium-centered radical (RTe•, R = Ph or nBu groups) during the hydrotelluration of alkenes and alkynes is provided. The radicals were detected by electronic paramagnetic ressonance (EPR), using DBNBS (3,5-dibromo-4-nitrosobenzenesulfonate) as the spin trap. The radical adducts (DBNBS/•TenBu and DBNBS/•TePh) presented an EPR spectrum characterized by a triplet of triplets due to one nitrogen and two equivalent hydrogen atoms (aN = 21.6 G and aH = 0.7 G) and a g value of 2.0060. The presence of tellurium radical adducts was confirmed by isotopic substitution (125Te) and by electrospray and chemical ionization mass spectrometry and MS/MS analysis. The products showed isotopic patterns expected for compounds containing Br and Te. These results provide evidence for organic tellurium-centered radical formation and suggest that the hydrotelluration reactions occur by a free radical mechanism.