RESUMO
A dinickel catalyst promotes reductive cyclization reactions of 1,1-dichloroalkenes containing pendant olefins. The reactions can be conducted with a Zn reductant or electrocatalytically using a carbon working electrode. Mechanistic studies are consistent with the intermediacy of a Ni2(vinylidene) species, which adds to the alkene and generates a metallacyclic intermediate. ß-Hydride elimination followed by C-H reductive elimination forms the cyclization product. The proposed dinickel metallacycle is structurally characterized and its stoichiometric conversion to product is demonstrated. Spin polarized, unrestricted DFT calculations are used to further examine the cyclization mechanism. These computational models reveal that both nickel centers function cooperatively to mediate the key oxidative addition, migratory insertion, ß-hydride elimination, and reductive elimination steps.
RESUMO
Attempts to perform the OsO4-catalyzed enantioselective base-free aminohydroxylation of ß,ß-disubstituted enoates are described. Low yields and racemic products were obtained in the presence of standard chiral ligands, suggesting the occurrence of a "Second Cycle" process due to slow hydrolysis of the amino alcohol product from the Os metal center. Support for this hypothesis was provided by the slightly improved enantioselectivity (60:40 er) obtained with an amino alcohol ligand. Based on density functional theory calculations, it is proposed that the lack of significant enantioselectivity is due to a low-energy (3 + 2) oxo/imido cycloaddition transition state without the chiral ligand in the Second Cycle that outcompetes protonolysis in the First Cycle.
RESUMO
Arylmetals are highly valuable carbon nucleophiles that are readily and inexpensively prepared from aryl halides or arenes and widely used on both laboratory and industrial scales to react directly with a wide range of electrophiles. Although C-C bond formation has been a staple of organic synthesis, the direct transfer of primary amino (-NH2) and hydroxyl (-OH) groups to arylmetals in a scalable and environmentally friendly fashion remains a formidable synthetic challenge because of the absence of suitable heteroatom-transfer reagents. Here, we demonstrate the use of bench-stable N-H and N-alkyl oxaziridines derived from readily available terpenoid scaffolds as efficient multifunctional reagents for the direct primary amination and hydroxylation of structurally diverse aryl- and heteroarylmetals. This practical and scalable method provides one-step synthetic access to primary anilines and phenols at low temperature and avoids the use of transition-metal catalysts, ligands and additives, nitrogen-protecting groups, excess reagents and harsh workup conditions.
RESUMO
PURPOSE: To determine if a system of astigmatic keratomy at the time of cataract surgery is safe and effective. SETTING: Academic Ophthalmology Center. METHODS: One hundred and ten consecutive patients with greater than 0.50 diopters (d) of cylinder with the rule (WTR) and 0.75 d against the rule (ATR) were enrolled. Those with an axis between 16 and 74 degrees and 106 and 164 degrees were defined as oblique axis. Peripheral clear corneal relaxing incisions 600 mum deep were used with a nomogram of 60 degrees length per 1.0 d of attempted correction. Keratometry was taken from IOL Master biometry and refractive results at 4-12 weeks were compared. RESULTS: 60.5% of ATR, 62% of WTR and 23% of oblique attempted correction was achieved. Splitting the patients to under and over 70 years of age had no impact on the results. Of the 14 overcorrected, 7 had oblique cylinder (p = 0.009) and 10 had less than 1 d of preoperative cylinder (p = 0.00015). CONCLUSIONS: Using the IOL Master topography with this nomogram was quite effective in reducing preoperative cylinder. Oblique axis results were unpredictable, however, and much more likely to lead to an overcorrection.
