Towards the catalytic asymmetric bromination of alkenes

This thesis relates our efforts towards the development of a general method for the catalytic asymmetric bromination of alkenes. Previous work within the group had reported the synthesis and the use of 2,6-di-[(4R,5R)-4,5-diphenyl-4,5-dihydro-1H-imidazol-2yl]-iodobenzene to catalyse the bromolactonisation of (±)-2-cyclopentene-1-acetic acid with some asymmetric induction observed. The reaction was initially postulated to proceed via formation of a hypervalent N-I(III)-Br bond in the catalyst, which by placing the electrophilic bromine in a chiral environment would allow for its selective delivery to the alkene substrate. Initial work aimed to acquire a better understanding of that reaction system and some mechanistic studies were undertaken. A different outcome to the one expected is reported and it was concluded that during the bromolactonisation reaction, no hypervalent iodine species was forming in situ and that instead a potential kinetic resolution via α-salt formation and diastereoselective halolactonisation mechanism was operating. These findings necessitated a redesign of the catalyst and the synthesis of various bis-amidine analogues is described. An unexpected rearrangement is also unveiled and its mechanism discussed. The new catalysts were screened in our asymmetric brominating system, using different alkene substrates, and the results are reported and discussed. Subsequent studies focused on elaborating a general method for the asymmetric dibromination of alkenes, where Br+ would be delivered by a stoichiometric chiral promoter. Screening of various alkene substrates to identify a suitable candidate is reported and discussed. The asymmetric dibromination of the chosen alkene is described and further investigations in order to elucidate the reasons for the lack of enantioselectivity observed are reported. Finally, attempts to synthesise Ts-DPEN following a similar route as previously developed within the group for the synthesis of DPEN is reported as a side chapter. Various attempts at the final benzoyl cleavage are described and discussed

German Unification Influence on its Economy

German reunification was unprecedented. A marriage of differences, combining biasedchoices and high rigidities, has postponed the adjustment in economic and financial circles. Since then, Germany reabsorbs the imbalances introduced and alternates between remission and relapse at the discretion of the global economy

DMTTF-CA revisited: temperature-induced valence and structural instability

We report a detailed spectroscopic investigation of temperature-induced valence and structural instability of the mixed-stack organic charge-transfer (CT) crystal 4,4'-dimethyltetrathiafulvalene-chloranil (DMTTF-CA). DMTTF-CA is a derivative of tetrathiafulvalene-chloranil (TTF-CA), the first CT crystal exhibiting the neutral-ionic transition by lowering temperature. We confirm that DMTTF-CA undergoes a continuous variation of the ionicity on going from room temperature down to $\sim$ 20 K, but remains on the neutral side throughout. The stack dimerization and cell doubling, occurring at 65 K, appear to be the driving forces of the transition and of the valence instability. In a small temperature interval just below the phase transition we detect the coexistence of molecular species with slightly different ionicities. The Peierls mode(s) precursors of the stack dimerization are identified.Comment: 8 pages, 6 figures, Phys. Rev. B forma

Biomineralization in plants as a long-term carbon sink

Carbon sequestration in the global carbon cycle is almost always attributed to organic carbon storage alone, while soil mineral carbon is generally neglected. However, due to the longer residence time of mineral carbon in soils (102-106years), if stored in large quantities it represents a potentially more efficient sink. The aim of this study is to estimate the mineral carbon accumulation due to the tropical iroko tree (Milicia excelsa) in Ivory Coast. The iroko tree has the ability to accumulate mineral carbon as calcium carbonate (CaCO3) in ferralitic soils, where CaCO3 is not expected to precipitate. An estimate of this accumulation was made by titrating carbonate from two characteristic soil profiles in the iroko environment and by identifying calcium (Ca) sources. The system is considered as a net carbon sink because carbonate accumulation involves only atmospheric CO2 and Ca from Ca-carbonate-free sources. Around one ton of mineral carbon was found in and around an 80-year-old iroko stump, proving the existence of a mineral carbon sink related to the iroko ecosystem. Conservation of iroko trees and the many other biomineralizing plant species is crucial to the maintenance of this mineral carbon sin

Anomalous dispersion of optical phonons at the neutral-ionic transition: Evidence from diffuse X-ray scattering

Diffuse X-ray data for mixed stack organic charge-transfer crystals approaching the neutral-ionic phase transition can be quantitatively explained as due to the softening of the optical phonon branch. The interpretation is fully consistent with vibrational spectra, and underlines the importance of electron-phonon coupling in low-dimensional systems with delocalized electrons.Comment: 4 pages, 4 figure