Marcello Crucianelli

Ligand-controlled regioselectivity in the hydrothiolation of alkynes by rhodium N-heterocyclic carbene catalysts.

Rh-N-heterocyclic carbene compounds [Rh(μ-Cl)(IPr)(η(2)-olefin)](2) and RhCl(IPr)(py)(η(2)-olefin) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-carbene, py = pyridine, olefin = cyclooctene or ethylene) are highly active catalysts for alkyne hydrothiolation under mild conditions. A regioselectivity switch from linear to 1-substituted vinyl sulfides was observed when mononuclear RhCl(IPr)(py)(η(2)-olefin) catalysts were used instead of dinuclear precursors. A complex interplay between electronic and steric effects exerted by IPr, pyridine, and hydride ligands accounts for the observed regioselectivity. Both IPr and pyridine ligands stabilize formation of square-pyramidal thiolate-hydride active species in which the encumbered and powerful electron-donor IPr ligand directs coordination of pyridine trans to it, consequently blocking access of the incoming alkyne in this position. Simultaneously, the higher trans director hydride ligand paves the way to a cis thiolate-alkyne disposition, favoring formation of 2,2-disubstituted metal-alkenyl species and subsequently the Markovnikov vinyl sulfides via alkenyl-hydride reductive elimination. DFT calculations support a plausible reaction pathway where migratory insertion of the alkyne into the rhodium-thiolate bond is the rate-determining step.

Methyltrioxorhenium Catalysis in Nonconventional Solvents: A Great Catalyst in a Safe Reaction Medium

The requirement that chemical processes are sustainable, reflected in waste reduction and the use of safe reagents and reaction conditions, is becoming even more stringent as a result of pressure by society and governments to preserve the environment and protect human health. Catalysis offers numerous benefits related to green chemistry, including lowered energetic reaction requirements; catalytic, rather than stoichiometric, amounts of materials; increased selectivity; lowered consumption of processing and separation agents; and, in many cases, the use of less-toxic compounds. Our research group has for a long time been studying methyltrioxorhenium in the oxyfunctionalization of different substrates, by using H(2)O(2) or its urea-hydrogen peroxide complex as the primary oxidant. In this Review paper we aim to provide a full literature account on the catalytic activity and selectivity of methyltrioxorhenium in the oxyfunctionalization reaction, either in nonconventional solvents or under solvent-free conditions, with a particular emphasis on the use of ionic liquids as green reaction media.

Exploring the frontiers of synthetic eumelanin polymers by high-resolution matrix-assisted laser/desorption ionization mass spectrometry.

New trends in material science and nanotechnologies have spurred growing interest in eumelanins black insoluble biopolymers derived by tyrosinase-catalysed oxidation of tyrosine via 5,6-dihydroxyindole (DHI) and its 2-carboxylic acid (DHICA). Efficient antioxidant and photoprotective actions, associated with peculiar optoelectronic properties, are recognised as prominent functions of eumelanin macromolecules within the human and mammalian pigmentary system, making them unique candidates for the realisation of innovative bio-inspired functional soft materials, with structure-based physical-chemical properties. An unprecedented breakthrough into the mechanism of synthetic eumelanin buildup has derived from a detailed investigation of the oxidative polymerization of DHI and its N-methyl derivative (NMDHI) by linear and reflectron matrix-assisted laser/desorption ionization mass spectrometry. Regular collections of oligomers of increasing masses, spanning the entire m/z ranges up to 5000 Da (>30-mer) and 8000 Da (> 50-mer) for the two building blocks, respectively, were disclosed. It is the first time that the in vitro polymerisation of dihydroxyindoles to form synthetic eumelanins is explored up to its high mass limits, giving at the same time information on the polymerisation mode, whether it follows a stepwise pattern (being this the conclusion in our case) or a staking sequencing of small-sized entities. It also highlighted the influence of the N-methyl substituent on the polymerization process; this opens the way to the production of N-functionalized, synthetic eumelanin-inspired soft materials, for possible future technological applications.

Unusual non-oxidative pummerer rearrangement of γ-trifluoro-β-aminosulfoxides

Abstract Trifluoroacetic anhydride promoted Pummerer rearrangement of γ-trifluoro-β-amino sulfoxides 1 follows an unusual pathway, in which a migration of the p -tolylthio group to the nitrogen atom provides the corresponding α-sulfenamidotrifluoroacetates 5 . The usual removal of the proton in α to the sulfinyl moiety does not take place, as shown by maintenance of deuterium during the rearrangement. This procedure is exploited for the stereoselective synthesis of 2D- and 2H- ( R )-2-amino-3,3,3-trifluoropropan-1-ol 10 .

Enhanced photovoltaic performance with co-sensitization of quantum dots and an organic dye in dye-sensitized solar cells

CdSe quantum dots of two different sizes exhibiting a maximum emission at 495 nm (CdSe495) and 545 nm (CdSe545) were combined with di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4-dicarboxylato)ruthenium(II) (N719) or 2-cyano-3-{5-[7-(4-diphenylamino-phenyl)benzo[1,2,5]thiadiazol-4-yl]-thiophen-2-yl}-acrylic acid (TBTCA) resulting in four novel hybrid organic–inorganic sensitizers, which were used in the fabrication of dye-sensitized solar cells. The results showed that with N719, both the CdSe dots decreased the power conversion efficiencies when compared to a standard device consisting only of N719 as the sensitizer. With the organic dye TBTCA, CdSe545 showed no significant effect, while CdSe495 interacted favorably, leading to a 25% increase in power conversion efficiency compared to a device sensitized solely by TBTCA. Studies on excited-state lifetimes of N719 in the presence of CdSe did not distinguish between energy and/or charge transfer mechanisms. On the other hand, time correlated single photon counting experiments on the photoelectrodes suggest that the advantages due to the CdSe495–TBTCA combination could be ascribed to FRET from quantum dots to the organic dye and to a further contribution, as suggested by IPCE spectra, consisting of electron transfer via cascade from the LUMO level of TBTCA to CdSe495 to TiO2, which produces a higher flux of electrons in the external circuit.

Enantioseparation of amino acid derivatives by capillary zone electrophoresis using vancomycin as chiral selector

The separation of racemic derivatized amino acids (N‐acetyl) into their enantiomers was achieved using capillary zone electrophoresis employing vancomycin as a chiral selector. Due to the strong absorption properties of the chiral selector at the low wavelengths used, the partial‐filling countercurrent method was adopted in order to improve method sensitivity. In the separation system studied, the chiral selector filled only a part of the capillary and, due to the appropriate selection of the pH, was moving in the opposite direction of the analytes keeping the detector free from absorbing compounds. The effect of several experimental parameters on the enantioresolution of analytes was studied, e.g., vancomycin concentration (0–5 mM), pH of the background electrolyte (pH 4–7), capillary temperature (15–35°C), and the presence of an organic modifier in the run buffer (methanol or ethanol or n‐propanol). N‐Acetyl glutamic acid, serine, cystine, tyrosine, and proline were all baseline‐resolved into their enantiomers and the enantioresolution factor (Rs) was increased by raising the vancomycin concentration. pH 4 allowed the baseline resolution of the five studied analytes in the presence of 2.5 mM of chiral selector and an increase in pH caused a decrease of Rs.

Stereoselective synthesis of optically pure γ-fluoro-β-enaminosulfoxides and reduction to γ-fluoro-β-aminosulfoxides

Abstract The aza-Wittig reaction of phospha- λ 5 -azenes with optically pure γ-fluorosubstituted β-ketosulfoxides leads to the corresponding β-enaminosulfoxides, which were easily isolated in diastereomerically pure form. The stereoselective reduction of the enamine 3cb afforded the optically pure β-sulphinyl-amine 5cb in high overall yield.

Stereoselective synthesis of the antibacterial 3-fluoro-d-alanine

Abstract The wide spectrum antibacterial 3-fluoro- d -alanine (S)- 9 has been stereoselectively synthesized via “chiral sulfoxide chemistry”. Key steps are the azidation of the α-fluoro α′ -sulfinyl alcohol (2S,R s )- 1 under Mitsunobu conditions and the one-pot transformation of the N-Cbz α-sulfinyl amine (2R,R s )- 5 into the N-Cbz aminoalcohol (S)- 7 , through a “non-oxidative Pummerer reaction”.

Synthesis of fluorinated chirons: Stereoselective oxirane formation by reaction of diazomethane on 1-fluoro-3-arylsulfinyl-2-propanone and ring opening by selected nucleophiles

Abstract (R)-1-fluoro-3-[(4-methylphenyl)sulfinyl]-2-propanone (1) reacts with diazomethane affording (S)-2-(fluoromethyl)-2-[(R)-(4-methylphenyl)sulfinyl]methyl oxirane (2) as the main product. The influence of reaction conditions (solvent, temperature) on the chemo-and stereoselectivity has been studied. Several elaborations of 2, including reactions on the chiral auxiliary and opening of the oxirane ring by carbon, nitrogen, oxygen, phosphorus and halogen nucleophiles, are described. Full structural elucidation of the products is provided.

Selective catalytic oxidation of olefins by novel oxovanadium(IV) complexes having different donor ligands covalently anchored on SBA-15: a comparative study

Using a post-synthetic grafting method, mesoporous SBA-15 was functionalized first with aminopropyl silane reagents such as (3-aminopropyl)trimethoxysilane (APTMS) or bis[3-(trimethoxysilyl)propyl]amine (BTMSPA) and then with chlorinated ligands such as 4-(2-chloroacetyl)-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one (H-AP) or 5-chloromethyl-8-quinolinol hydrochloride (H-HQ). Vanadyl cations [(VO)2+] were then immobilized over functionalized silica samples in order to prepare new oxovanadium(IV) based “quasi-homogeneous” catalysts, namely Ia (Ia′), Ib (Ib′), and IIa. Alternatively, a preformed oxovanadium(IV) complex such as [VO(AP)2(H2O)] has been immobilized over a previously functionalized SBA-15-NH2 silica support, affording catalyst Ic. Elemental analysis (C, H, N), N2 adsorption–desorption isotherms, FT-IR, 29Si and 13C CP-MAS NMR, XPS, ICP-AES, SEM-EDX and TG-DTG techniques have been used for the full characterization of these materials. Their catalytic properties in the H2O2 promoted oxidation of conjugated olefins like styrene, α-methyl and β-methylstyrene were investigated, in terms of activity, selectivity and recyclability, and compared with that shown by more simple systems prepared by the direct grafting of the vanadyl cation onto SBA-15 (catalyst A) or SBA-15-NH2 (catalyst B). Generally, high activity, selectivity and recyclability (over the first three runs) were shown by all anchored catalysts, with the only exception of A and B systems. A full account of the obtained results, along with insights into the effects due to the different strategies employed for the functionalization of SBA-15 on the properties of final anchored catalysts, are reported.