Asymmetric synthesis of benzylic and heterobenzylic amines

C2-Symmetrical, enantiopure 2,6-di[1-(1-aziridinyl)alkyl]pyridines (DIAZAPs) were prepared by a high-yielding, three-step sequence starting from 2,6-pyridinedicarbaldehyde and (S)-valinol or (S)-phenylglycinol. The new compounds were tested as ligands in palladium-catalyzed allylation of carbanions in different solvents. Almost quantitative yield and up to 99% enantiomeric excess were obtained in the reactions of the enolates derived from malonate, phenyl- and benzylmalonate dimethyl esters with 1,3-diphenyl-2-propenyl ethyl carbonate. Asymmetric synthesis of 2-(2-pyridyl)aziridines from chiral 2-pyridineimines bearing a stereogenic center at the nitrogen atom was development. The envisioned route involves the addition of chloromethyllithium to the imine derived from 2-pyridinealdehyde and (S)-valinol, protected as O-trimethylsilyl ether. The analogous reaction performed on the imine derived from (S)-valine methyl ester gave the product containing the aziridine ring as well as the α-chloro ketone group coming from the attack of chloromethyllithium to the ester function. Other stereogenic alkyl substituents at nitrogen gave less satisfactory results. Moreover, the aziridination protocol did not work on other aromatic imines, e.g. 3-pyridineimine and benzaldimine, which are not capable of bidentate chelation. The N-substituent could not be removed, but aziridine underwent ring-opening by attack of nitrogen, sulfur, and oxygen nucleophiles. Complete or prevalent regioselectivity was obtained using cerium trichloride heptahydrate as a catalyst. In some cases, the N-substituent could be removed by an oxidative protocol. The addition of organometallic (lithium, magnesium, zinc) reagents to 2-pyrroleimines derived from (S)-valinol and (S)-phenylglycinol gave the N-substituted-1-(2-pyrrolyl)alkylamines with high yields and diastereoselectivities. The (S,S)-diastereomers were useful intermediates for the preparation of enantiopure 1-[1-(2-pyrrolyl)alkyl]aziridines by routine cyclization of the β-aminoalcohol moiety and of (S)-N-benzoyl 1-[1-(2-pyrrolyl)alkyl]amines and their N-substituted derivatives by oxidative cleavage of the chiral auxiliary. 1-Allyl-2-pyrroleimines obtained from (S)-phenylglycinol and (S)-valinol underwent highly diastereoselective addition of allylmetal reagents, used in excess amounts, to give the corresponding secondary amines with concomitant allyl to 1-propenyl isomerisation of the 1-pyrrole substituent. Protection of the 2-aminoalcohol moiety as oxazolidinone, amide or Boc derivate followed by ring closing metathesis of the alkene groups gave the unsaturated bicyclic compound, whose hydrogenation afforded the indolizidine derivative as a mixture of separable diastereomers. The absolute configuration of the main diastereomer was assessed by X-ray crystallographic analysis.

Carbonates and sulfides in orogenic garnet-bearing peridotites from the Ulten Zone as witnesses for Carbon and Sulfur mobility in orogenic settings

Convergent plate boundaries are sites of sustained chemical exchanges between the Earth’s surface and deep geochemical reservoirs, playing a major role in the global cycle of carbon and sulfur. However, carbon and sulfur recycling processes continue to be hotly debated. A critical gap in the knowledge of the whole subduction factory is given by the limited accessibility to the upper mantle residing above the subducting plate, the so-called mantle wedge. This thesis investigates the carbonate and sulfide metasomatism taking place during the whole metamorphic evolution of a mantle wedge involved in the Variscan continental collision. We integrate different detailed geochemical and petrological techniques to orogenic carbonated spinel and garnet peridotites from the Ulten Zone of the Eastern Italian Alps. Our data show that the Ulten Zone peridotite experienced multiple stages of addition and removal of carbon and sulfur throughout its metamorphic evolution, as follows: (1) The Variscan lithospheric mantle was initially depleted and sulfide-poor. It subsequently inherited a sulfur and carbon component during an early metasomatic stage, when hot, H2S-CO2-bearing melts leaving a subduction-modified source percolated the overlying spinel-facies peridotite in the mantle wedge; (2) Under peak eclogite-facies P-T conditions, pervasive carbonation and sulfidation occurred. Heterogeneous melt and fluid sources variably enriched in carbon, isotopically heavy sulfur and radiogenic Sr were involved; (3) Shortly after the attainment of peak-P conditions, peridotite bodies were incorporated in a tectonic mélange with the neighboring gneisses. Here, the Ulten Zone peridotite was exposed to channelized infiltration of hybridized C-O-H fluids that promoted the formation of veinlets of carbonates locally associated with sulfide grains. (4) Upon late retrogression, infiltration of serpentinizing fluids promoted C and S remobilization at shallow crustal levels.