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| Author(s): |
Artur Franz Keppler
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Conjunto das Químicas (IQ e FCF) (CQ/DBDCQ) |
| Defense date: | 2009-05-15 |
| Examining board members: |
João Valdir Comasseto;
Leandro Helgueira Andrade;
Reinaldo Camino Bazito;
Antonio Luiz Braga;
Rodrigo Luiz Oliveira Rodrigues Cunha
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| Advisor: | João Valdir Comasseto |
| Abstract | |
The first experimental evidence for the formation of an organic tellurium-centered radical (RTe center dot, 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-nitrosobenzene-sulfonate) as the spin trap. The radical adducts (DBNBS/TeBu 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. The mechanism of the addition reaction of TeCl4 to alkynes was indirectly established by the detection of TeCl3 using EPR spin trapping, ESI-MS and ESI-MS/MS characterization. Carbon centered radicals were detected during the lithium butylchalcogenolate induced Michael-aldol tandem sequence. Reaction of the lithium alkylchalcogenolates with activated alkenes and aldehydes gives the corresponding aldol adducts. The selenium-containing products give MoritaBaylisHillman adducts after the oxidation/elimination of the selenoxide. The whole sequence can be performed in a one-pot procedure. (AU) | |