We report a BF3-mediated direct alkynylation of pyridines at C(2) by using a variety of alkynyllithium reagents (oxidative cross-coupling). Moreover, we have developed a novel transition-metal-free cross-coupling method between alkylmagnesium reagents and 4-substituted pyridines, such as isonicotinonitrile and 4-chloropyridine, by employing BF3⋅OEt2 as a promoter. The combination of these methods enabled us to efficiently prepare a range of di-, tri-, and tetrasubstituted pyridines.
Oxidative or non-oxidative—That is the question! Pyridines bearing a substituent at position 4 readily undergo a BF3-mediated oxidative coupling at position 2 with a wide range of alkynyllithium compounds. In contrast, 4-cyano- or 4-chloropyridines undergo a novel BF3-mediated cross-coupling at position 4 with alkylmagnesium reagents. The combination of the two transition-metal-free procedures allows the preparation of a broad range of pyridines.
Lithiation/borylation methodology has been developed for the synthesis of acyclic quaternary-tertiary motifs with full control of relative and absolute stereochemistry, thus leading to all four possible isomers of a stereodiad. A novel intramolecular Zweifel-type olefination enabled acyclic stereocontrol to be transformed into cyclic stereocontrol. These key steps have been applied to the shortest enantioselective synthesis of (−)-filiformin to date (9 steps) with full stereocontrol.
True to (fili)form: Lithiation/borylation methodology has been developed for the synthesis of acyclic quaternary-tertiary motifs with full control of relative and absolute stereochemistry, thus leading to all four possible isomers of a stereodiad. A novel intramolecular Zweifel-type olefination enabled acyclic stereocontrol to be transformed into cyclic stereocontrol. These key steps were applied to the enantioselective synthesis of (−)-filiformin.
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Paris-based designer Maud Vantours is a master at constructing incredible 3D sculptures and mesmerizing patterns out of paper. After carefully cutting out assorted shapes, she meticulously builds up layer upon layer, superimposing the papers to create intricate and colorful works of art. Her paper works range from geometric patterns to dizzying spirals to delicately crafted flowers. With amazing texture, elaborate detail, and vibrant colors, each piece is a joy to look at.
Although Vantours graduated from the Parisian school Duperré with a specialization in textiles and design, her favorite medium is paper because the works can "transcend a simple material and transform it into a work of art." She says, "I love to create original graphics with multicolored and dreamlike landscapes."
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Density functional calculations yield energy barriers for H abstraction by oxygen radical sites in Li-doped MgO that are much smaller (12±6 kJ mol−1) than the barriers inferred from different experimental studies (80–160 kJ mol−1). This raises further doubts that the Li+O.− site is the active site as postulated by Lunsford. From temperature-programmed oxidative coupling reactions of methane (OCM), we conclude that the same sites are responsible for the activation of CH4 on both Li-doped MgO and pure MgO catalysts. For a MgO catalyst prepared by sol–gel synthesis, the activity proved to be very different in the initial phase of the OCM reaction and in the steady state. This was accompanied by substantial morphological changes and restructuring of the terminations as transmission electron microscopy revealed. Further calculations on cluster models showed that CH4 binds heterolytically on Mg2+O2− sites at steps and corners, and that the homolytic release of methyl radicals into the gas phase will happen only in the presence of O2.
The energy barriers for methane activation by hydrogen abstraction at oxygen radical sites are unrealistically low. CH bonds can also heterolytically add onto a regular MgO ion pair at morphological defects such as steps and corners. Release of methyl radicals into the gas phase will happen only in the presence of O2.
But I promise, I didn’t drag you here today to sell you on a refined carb-free life as I myself have little interest in living one. What I’d hoped to share was the neat thing that many less stubborn than myself have known of eons: when you tip the food scales away from lackluster bread-fill, a wonderful thing happens: vegetables, beans and protein come back into prominence, and it was just the cooking recharge that I needed. To wit, since the beginning of the year we’ve talked about eggs baked in a nest of spinach and mushrooms (biscuits on the side), a seasonal mayo-light riff on devilled eggs, my new favorite three-bean chili (a small amount of brown rice underneath), chicken fajitas loaded with vegetables, beans, slaw, pico, and guacamole (all perched on one or two small corn tortillas) and a kale-quinoa salad I’m so addicted to, if I don’t have it for lunch at least three days a week, I feel twitchy.