16282-50-1Relevant academic research and scientific papers
An Asymmetric SN2 Dynamic Kinetic Resolution
Rezayee, Nomaan M.,Enem?rke, Valdemar J.,Linde, Sif T.,Lamhauge, Johannes N.,Reyes-Rodríguez, Gabriel J.,J?rgensen, Karl Anker,Lu, Chenxi,Houk
, p. 7509 - 7520 (2021/05/26)
The SN2 reaction exhibits the classic Walden inversion, indicative of the stereospecific backside attack of the nucleophile on the stereogenic center. Observation of the inversion of the stereocenter provides evidence for an SN2-type displacement. However, this maxim is contingent on substitution proceeding on a discrete stereocenter. Here we report an SN2 reaction that leads to enantioenrichment of product despite starting from a racemic mixture of starting material. The enantioconvergent reaction proceeds through a dynamic Walden cycle, involving an equilibrating mixture of enantiomers, initiated by a chiral aminocatalyst and terminated by a stereoselective SN2 reaction at a tertiary carbon to provide a quaternary carbon stereocenter. A combination of computational, kinetic, and empirical studies elucidates the multifaceted role of the chiral organocatalyst to provide a model example of the Curtin-Hammett principle. These examples challenge the notion of enantioenriched products exclusively arising from predefined stereocenters when operating through an SN2 mechanism. Based on these principles, examples are included to highlight the generality of the mechanism. We anticipate the asymmetric SN2 dynamic kinetic resolution to be used for a variety of future reactions.
Copper-catalyzed vinylogous aerobic oxidation of unsaturated compounds with air
Zhang, Hai-Jun,Schuppe, Alexander W.,Pan, Shi-Tao,Chen, Jin-Xiang,Wang, Bo-Ran,Newhouse, Timothy R.,Yin, Liang
, p. 5300 - 5310 (2018/04/24)
A mild and operationally simple copper-catalyzed vinylogous aerobic oxidation of β,γ- and α,β-unsaturated esters is described. This method features good yields, broad substrate scope, excellent chemo- and regioselectivity, and good functional group tolerance. This method is additionally capable of oxidizing β,γ- and α,β-unsaturated aldehydes, ketones, amides, nitriles, and sulfones. Furthermore, the present catalytic system is suitable for bisvinylogous and trisvinylogous oxidation. Tetramethylguanidine (TMG) was found to be crucial in its role as a base, but we also speculate that it serves as a ligand to copper(II) triflate to produce the active copper(II) catalyst. Mechanistic experiments conducted suggest a plausible reaction pathway via an allylcopper(II) species. Finally, the breadth of scope and power of this methodology are demonstrated through its application to complex natural product substrates.
Reactivity and selectivity in the oxidation of styrene derivatives. V. studies on the oxidation of α-substituted styrenes
Suprun
, p. 52 - 58 (2007/10/03)
The liquid phase oxidation of α-phenyl-1a, α-trimethylsilyloxy 1b, α-cyclopropyl-1c, α-trifluoromethyl-1d styrene, and styrene 1e with oxygen in chlorobenzene and cumene solution in the temperature range 55-125°C was investigated. The product yields were determined gaschromatographically. The epoxide selectivity increases up to 90°C with increasing temperature. The epoxides of 1a and 1c rearrange at higher temperatures, therefore their yield decreases. The relative chain propagation constants (kpC=C) were determined by competitive oxidations of cumene. WILEY-VCH Verlag GmbH, 1999.
Cyclopropylketenes: preparation and nucleophilic additions
Allen, Annette D.,Baigrie, Lynn M.,Gong, Leyi,Tidwell, Thomas T.
, p. 138 - 145 (2007/10/02)
Phenylcyclopropylketene (4), tert-butylcyclopropylketene (5), and dicyclopropylketene (6) were formed by dehydrochlorination of the corresponding acyl chlorides by Et3N in THF, and are the first cyclopropylketenes to be isolated and purified.Reaction of 4 with n-BuLi and capture of the intermediate enolates with Me3SiCl gave the stereoisomeric silyl enol ethers c-PrCPh=C(OSiMe3)-n-Bu with a 79:21 preference for formation of the Z isomer resulting from nucleophilic attack syn to cyclopropyl, whereas the corresponding reaction of t-BuLi gave 9:91 preference for attackanti to cyclopropyl.Some isopropyl-, cyclopentyl-, and cyclohexylketenes gave comparable results.Analyses of the relative sizes of the ketene substituents in the ground state by steric parameters, and of the product stabilities by molecular mechanics, both fail to predict the observed similarities in the results with different secondary alkyl groups.The hydration reactivities of 4 and 6 show that, in neutral H2O/CH3CN, c-PrCPh=C=O is more reactive than i-PrCPh=C=O, a result ascribed as mainly due to the smaller size of cyclopropyl. c-Pr2C=C=O has the same reactivity in neutral water as Et2C=C=O, but is 22 times less reactive with acid, a result attributed to the inability of the β-cyclopropyl groups to directly stabilize the cationic transition state for protonation. Key words: cyclopropylketenes, ketenes, nucleophilic addition, hydration kinetics.
Ring and C-O Bond Fragmentation as Tools for Fingerprinting the Extent of Homolysis during Base-Catalyzed Carbon-Carbon Bond Cleavages of the Haller-Bauer, Cram, and Gilday Types
Paquette, Leo A.,Maynard, George D.
, p. 5054 - 5063 (2007/10/02)
The mechanisms of the base-catalyzed cleavage of non-enolizable ketones (Haller-Bauer reaction), fragmentation of the alkali-metal salts of diphenylcarbinols (Cram cleavage), and decarboxylative elimination of methyllithium-carboxylic acid adducts (Gilday process) are probed by attaching a small ring or a carbon-oxygen bond proximal to the ultimate seat of reaction.Particular attention is given to whether product formation in the first case is accompanied by fission of the cyclopropane or cyclobutane subunit.The product distributions constitute a serviceable diagnostic of the relative extent to which carbanion and radical pathways operate concurrently.This distinction is also possible in the oxa analogues since the homolysis/heterolysis dichtomy is matched by retention of an intact C-O bond and the extent of its cleavage, respectively.A key feature of the Haller-Bauer process is its ability to deliver debenzoylated products having intact cyclopropane or cyclobutane rings because of its strong predilection for the generation of carbanions during C-C bond fragmentation.Counterion influences are minimal.The Cram cleavages show a very different product distribution profile.The results can be plausibly fitted to the involvement of free radicals, although the distinction between direct C-C bond hemolysis or heterolysis followed by rapid black-electron transfer cannot be made at this time.Because the Gilday reaction leads directly to styrenes and these products suffer destruction under the reaction conditions, this transformation lacks synthetic value in this particular context.
