Trajectories of nutrients concentrations and ratios in the French coastal ecosystems: 20 years of changes in relation with large-scale and local drivers.

Along with their important diversity, coastal ecosystems receive various amounts of nutrients, principally arising from the continent and from the related human activities (mainly industrial and agricultural activities). During the 20th century, nutrients loads have increased following the increase of both the global population and need of services. Alongside, climate change including temperature increase or atmospheric circulation change has occurred. These processes, Ecosystem state changes are hard to monitor and predict. To study the long-term changes of nutrients concentrations in coastal ecosystems, eleven French coastal ecosystems were studied over 20 years as they encompass large climatic and land pressures, representative of temperate ecosystems, over a rather small geographical area. Both univariate (time series decomposition) and multivariate (relationships between ecosystems and drivers) statistical analyses were used to determine ecosystem trajectories as well as typologies of ecosystem trajectories. It appeared that most of the French coastal ecosystems exhibited trajectories towards a decrease in nutrients concentrations. Differences in trajectories mainly depended on continental and human influences, as well as on climatic regimes. One single ecosystem exhibited very different trajectories, the Arcachon Bay with an increase in nutrients concentrations. Ecosystem trajectories based on ordination techniques were proven to be useful tools to monitor ecosystem changes. This study highlighted the importance of local environments and the need to couple uni- and multi-ecosystem studies. Although the studied ecosystems were influenced by both local and large-scale climate, by anthropogenic activities loads, and that their trajectories were mostly similar based on their continental influence, non-negligible variations resulted from their internal functioning.

Asymmetric allylic substitution catalyzed by C1-symmetrical complexes of molybdenum: structural requirements of the ligand and the stereochemical course of the reaction.

Application of new chiral ligands (R)-(-)-12 a and (S)-(+)-12 c (VALDY), derived from amino acids, to the title reaction, involving cinnamyl (linear) and isocinnamyl (branched) type substrates (4 and 5 --> 6), led to excellent regio- and enantioselectivities (>30:1, < or =98 % ee), showing that ligands with a single chiral center are capable of high asymmetric induction. The structural requirements of the ligand and the mechanism are discussed. The application of single enantiomers of deuterium-labeled substrates (both linear 38 c and branched 37 c) and analysis of the products (41-43) by (2)H{(1)H} NMR spectroscopy in a chiral liquid crystal matrix allowed the stereochemical pathways of the reaction to be distinguished. With ligand (S)-(+)-12 c, the matched enantiomer of branched substrate was found to be (S)-5, which was converted into (R)-6 with very high regio- and stereoselectivity via a process that involves net retention of stereochemistry. The mismatched enantiomer of the branched substrate was found to be (R)-5, which was also converted into (R)-6, that is, with apparent net inversion, but at a lower rate and with lower overall enantioselectivity. This latter feature, which may be termed a "memory effect", reduced the global enantioselectivity in the reaction of the racemic substrate (+/-)-5. The stereochemical pathway of the mismatched manifold has been shown also to be one of net retention, the apparent inversion occurring through equilibration via an Mo-allyl intermediate prior to nucleophilic attack. Incomplete equilibration leads to the memory effect and thus to lower enantioselectivity. Analysis of the mismatched manifold over the course of the reaction revealed that the memory effect is progressively attenuated with the nascent global selectivity increasing substantially as the reaction proceeds. The origin of this effect is suggested to be the depletion of CO sources in the reaction mixture, which attenuates turnover rate and thus facilitates greater equilibrium. The linear substrate was also converted into the branched product with net syn stereochemistry, as shown by isotopic labeling. An analogous process operates in the generation of small quantities of linear product from branched substrate.

Mechanistic studies of the molybdenum-catalyzed asymmetric alkylation reaction.

Enantiomerically enriched, deuterated branched carbonates (Z)-(S)-PhCH(OCO(2)Me)-CH = CHD (1-D), (Z)-(R)-PhCH(OCO(2)Me)CH = CHD (2-D), and linear carbonate (E)-(S)-PhCH = CHCHD(OCO(2)Me) (3-D) were used as probes in the Mo-catalyzed asymmetric allylic alkylation with sodium dimethyl malonate, catalyzed by ligand-complex 11 derived from the mixed benzamide/picolinamide of (S,S)-transdiaminocyclohexane and (norbornadiene)Mo(CO)(4). The results of these studies, along with x-ray crystallography and solution NMR structural analysis of the pi-allyl intermediate, conclusively established the reaction proceeded by a retention-retention pathway. This mechanism contrasts with that defined for Pd-catalyzed allylic alkylations, which proceed by an inversion-inversion pathway. A proposed rationale for the retention pathway for nucleophilic substitution involves CO-coordination to form a tri-CO intermediate, followed by complexation with the anion of dimethyl malonate to produce a seven-coordinate intermediate, which reductively eliminates to afford product with retention of configuration.

Conclusive evidence for a retention-retention pathway for the molybdenum-catalyzed asymmetric alkylation.

Enantiomerically enriched, deuterated branched carbonates (Z)-(S)-PhCH(O2COMe)-CH=CHD (1a), (Z)-(R)-PhCH(O2COMe)CH=CHD (2a), and linear carbonate (E)-(S)-PhCH=CHCHD(O2COMe) (12) were employed as probes in the Mo-catalyzed asymmetric allylic alkylation with sodium dimethyl malonate, catalyzed by ligand complex 10 derived from the mixed benzamide/picolinamide of (S,S)-trans-diaminocyclohexane and (norbornadiene)Mo(CO)4. X-ray crystallography, along with solution NMR structural analysis of the pi-allyl intermediate and competition experiments, conclusively established the reaction proceeded via a retention-retention pathway. This mechanism contrasts with that defined for Pd-catalyzed allylic alkylations, which proceed via an inversion-inversion pathway.