Search results for "Leo"
showing 10 items of 8768 documents
Electrochemical meso-functionalization of magnesium(II) porphine
2010
International audience; Regioselective meso-functionalization of the totally unsubstituted magnesium(II) porphine was performed by controlled potential electrolysis with pyridine and triphenylphosphine as nucleophiles leading to the original pyridinium and phosphonium substituted derivatives, respectively. The crystallographic structure of the latter is described as the only meso-phosphonium porphyrin reported to date.
Preparative-Scale Synthesis of Vedejs Chiral DMAP Catalysts
2018
A scalable synthesis of chiral Vedejs-type DMAP catalysts is reported. The key step of the synthesis is amination of the enantiomerically pure 4-chloropyridine derivative using well-defined ZnCl2(amine)2 complexes. A series of Zn(II)–amine complexes have been synthesized to explore the scope of the ZnCl2-mediated amination of 4-halopyridines. Mechanistic studies support a Zn(II)-facilitated nucleophilic aromatic substitution as a plausible mechanism for the chlorine-to-amine exchange.
N-Cobaltocenium Amide as Reactive Nucleophilic Reagent for Donor–Acceptor Bimetallocenes
2017
Deprotonation of the aminocobaltocenium ion [Cc-NH2]+ ([H-1]+) generates the nucleophilic imine CcNH (1). Reaction of 1 with acid chlorides R–COCl (R = Ph, Fc, and Cc+) yields the reference amide [Ph-CO-NH-Cc]+ (2+) and the amide-linked hetero- and homobimetallocenes [Fc-CO-NH-Cc]+ (3+) and [Cc-CO-NH-Cc]2+ (42+), respectively. Cation–anion interactions of charged amides 2+–42+ in the solid state and in solution are probed by single crystal X-ray diffraction and NMR and IR spectroscopy. Intramolecular metal–metal interactions in donor–acceptor heterobimetallocene 3+ and in mixed-valent homobimetallocene 4+ (prepared electrochemically) are discussed within the Marcus–Hush framework aided by s…
Total Synthesis of (-)-Oxycodone via Anodic Aryl-Aryl Coupling.
2019
A fully regio- and diastereoselective electrochemical 4a–2′-coupling of a 3′,4′,5′-trioxygenated laudanosine derivative enables the synthesis of the corresponding morphinandienone. This key intermediate is further transformed into (−)-oxycodone through conjugate nucleophilic substitution for E-ring closure and [4 + 2] cycloaddition with photogenerated singlet oxygen to accomplish diastereoselective hydroxylation at C-14. The anodic transformation provides high yields and can be performed under constant current conditions both in a simple undivided cell or in continuous flow.
Further synthetic and structural investigations of new pre-organized picket porphyrins
2005
The straightforward synthesis of three new picket porphyrins with a restrained conformation is described. These porphyrins have an unusual behavior due to the conjugated but still flexible nature of their pickets. The crystal structure of their common precusor is also reported and confirms the presumed geometry of this type of picket. Indeed, the latter one is formally obtained by the conjugation of two aromatic rings through an amide bond. Although the specific shape of the picket is expected to overcrowd the center of the porphyrin, it is shown that different types of nucleophilic reagents can add easily on these pickets.
Influence of the solvent and R groups on the structure of (carboranyl)R2PI2 compounds in solution. Crystal structure of the first iodophosphonium sal…
2008
The influence of the electron-donor or electron-acceptor capacity of the R groups (R = (i)Pr, Ph, Et) and the solvent on the molecular geometry in solution of adducts of carboranylphosphanes [(carboranyl)(i)Pr2P, (carboranyl)Ph2P and (carboranyl)Et2P] with I2 in 1 : 1 ratios, has been studied in detail by 31P{1H} and 11B{1H} NMR spectroscopies. The more electron-accepting Ph groups make the (carboranyl)Ph2P less nucleophilic, thus stabilizing the I2 encapsulated neutral biscarboranylphosphane-diiodine adducts in solution, such as (carboranyl)Ph2PI-IPPh2(carboranyl), generating P---I-I---P motifs. Even in a polar solvent, such as EtOH, the arrangement is preserved. The expected basicity of t…
The [Pd(bipy)]2+ “merry-go-round”: Insights into the lability of the Pd–N bond
2009
Abstract Two tripods (1 and 2) featuring pyrimidinyl pendant arms have been synthesized from 5-(1H-pyrazol-3-yl)-pyrimidine (5) and 1,3,5-tribromomethylbenzene derivatives. Reaction with three equivalents of [Pd(bipy)](NO3)2 to form a macrotricycle closed by palladium coordination unexpectedly afforded the mononuclear species [Pd(1)(bipy)]2+ and [Pd(2)(bipy)]2+. These complexes show fluxional behavior on the 1H NMR timescale, the [Pd(bipy)]2+ fragment hopping between the pyrimidinyl coordinating moieties. The ΔGc‡’s estimated by the coalescence method are temperature independent, which means that ΔSc‡ = 0. This indicates that the “merry-go-round” process of [Pd(bipy)]2+ occurs intramolecula…
Cytosine Nucleobase Ligand: A Suitable Choice for Modulating Magnetic Anisotropy in Tetrahedrally Coordinated Mononuclear CoII Compounds
2017
A family of tetrahedral mononuclear CoII complexes with the cytosine nucleobase ligand is used as the playground for an in-depth study of the effects that the nature of the ligand, as well as their noninnocent distortions on the Co(II) environment, may have on the slow magnetic relaxation effects. Hence, those compounds with greater distortion from the ideal tetrahedral geometry showed a larger-magnitude axial magnetic anisotropy (D) together with a high rhombicity factor (E/D), and thus, slow magnetic relaxation effects also appear. In turn, the more symmetric compound possesses a much smaller value of the D parameter and, consequently, lacks single-ion magnet behavior.
Cross-Metathesis/Intramolecular (Hetero-)Michael Addition: A Convenient Sequence for the Generation of Carbo- and Heterocycles
2017
The high stability and functional group compatibility of ruthenium carbene complexes confer them a great ability to catalyze domino processes. For this reason, the combination of metathesis reactions with additional transformations in a domino fashion has been exploited extensively, with the result of expanding the utility of ruthenium carbene complexes beyond that of just olefin metathesis. Among those domino processes, it is worth mentioning the sequence of cross-metathesis/intramolecular Michael addition, which allows for the generation of a wide variety of carbo- and heterocycles in a very simple manner, taking advantage of the benefits of domino reactions. Carbon-, oxygen- and nitrogen…
Annually resolved δ13Cshell chronologies of long-lived bivalve mollusks (Arctica islandica) reveal oceanic carbon dynamics in the temperate North Atl…
2011
Abstract The ability of the ocean to absorb carbon dioxide is likely to be adversely affected by recent climate change. However, relatively little is known about the spatiotemporal variability in the oceanic carbon cycle due to the lack of long-term, high-resolution dissolved inorganic carbon isotope ( δ 13 C DIC ) data, especially for the temperate North Atlantic, which is the major oceanic sink for anthropogenic CO 2 . Here, we report shell carbon isotope values ( δ 13 C shell ), a potential proxy for δ 13 C DIC , of old-grown specimens of the long-lived bivalve mollusk, Arctica islandica . This paper presents the first absolutely dated, annually resolved δ 13 C shell record from surface …