Ketones base effect

It was suggested in the 1950s that the reduction of aliphatic ketones by dissolving metals proceeded by two sequential one-electron additions to provide a dianion (equation 1). This mechanism was based on the observation that benzophenone affords a dianion on reaction with excess Na in liquid NH3, and it was inferred that aliphatic ketones would behave similarly. A number of workers presented mechanistic rationalizations for the stereochemical course of the dissolving metal reductions of cyclic aliphatic ketones based on this dianion concept. However, in a 1972 review, it was noted that the reduction potentials of alkali metals were not sufficient to effect the addition of two electrons to an aliphatic carbonyl group, and an alternative mechanism was suggested which with some modification is now generally accepted. ... 

Methyl, ethyl, isopropyl, and r-butyl magnesium bromides were compared in enolization reactions [Eq. (11)]. The relative rates were Et>/-Pr> Me>f-Bu, which reflects a combination of steric and electronic effects. The bulkier Grignard reagents react slower, whereas the more substituted Grignard reagents are stronger bases. With more-hindered ketones, steric effects predominate over electronic. The structures of magnesium enoiates were confirmed by reaction with benzoyl chloride to form enol-benzoates [14,15]. 

Poly(ether ether ketone)-based membranes (PEEK) have also been studied in low-temperature hydrogen pumping as more cost effective alternatives to perfluorosulfonic... 

Conjugate addition of O-alkylhydroxylamines to a,P-unsaturated ketones was effectively catalyzed by an isolable complex of scandium with a chiral phosphate ligand (Sc[(R)-BNP]3) [124]. The corresponding P-amino ketones were generated in almost quantitative yields with high enantioselectivities (Scheme 12.48). These products could be quantitatively converted into the 2-acyl aziridines upon treatment with a base catalyst without losing their enantiopurities. 

Abstract With the advent of new synthetic methodologies, carbon-carbon bond (C-C) activation strategies have uncovered not only new fundamental reactivity but also the potential for use as a highly efficient synthetic protocol. This chapter specifically discusses the use of four-membered ketone-based starting materials for C-C activation initiated transformations using a variety of transition metals. The two major modes of activation, oxidative addition and p-C elimination, are presented as each pathway shows different mechanistic details and the ability to effect several types of reactions. Applications to the synthesis of complex molecules are presented and perspectives on future applications are considered. 

Several materials have been studied on the goal of producing cost-effective PFMs [55-62]. Some of these are PBI-based membranes, polysterene membranes, sulfonated polyimide, cross-Unked poly(vinyl alcohol), and phosphobenzene, sulfonated poly(aryl ether ketone) —based manbranes. Sulfonation of aromatic thermoplastics such as polyether sulfone, polybenzimidazole, polyimides, and poly(ether ether ketone) makes them proton conductive suitable for fuel cell... 

Well, that s about as rounded an education on Leuckart reactions as Strike can give. Strike feels that after reading all of those similar, repetitious steps, one can start to get a good feel for where a product is at any given moment. Stuff like what happens to MDA when it s mixed with acid or base, or what happens to ketones (P2P) under the same circumstances. One can see now that it is possible to not only isolate safrole and P2Ps chemically but that the same can be true for the final MDA or meth freebase oil. Repeated washings with acid or base and solvent can effectively clean up a compound to an almost presentable state without the use of vacuum distillation, it can happen, one only needs have confidence in the chemistry. 

The ketone is added to a large excess of a strong base at low temperature, usually LDA in THF at -78 °C. The more acidic and less sterically hindered proton is removed in a kineti-cally controlled reaction. The equilibrium with a thermodynamically more stable enolate (generally the one which is more stabilized by substituents) is only reached very slowly (H.O. House, 1977), and the kinetic enolates may be trapped and isolated as silyl enol ethers (J.K. Rasmussen, 1977 H.O. House, 1969). If, on the other hand, a weak acid is added to the solution, e.g. an excess of the non-ionized ketone or a non-nucleophilic alcohol such as cert-butanol, then the tautomeric enolate is preferentially formed (stabilized mostly by hyperconjugation effects). The rate of approach to equilibrium is particularly slow with lithium as the counterion and much faster with potassium or sodium. 

Electron-deficient alkenes add stereospecifically to 4-hydroxy-THISs with formation of endo-cycloadducts. Only with methylvinyl-ketone considerable amounts of the exo isomer are produced (Scheme 8) (16). The adducts (6) may extrude hydrogen sulfide on heating with methoxide producing 2-pyridones. The base is unnecessary with fumaronitrile adducts. The alternative elimination of isocyanate Or sulfur may be controlled using 7 as the dipolarenOphile. The cycloaddition produces two products, 8a (R = H, R = COOMe) and 8b (R = COOMe, R =H) (Scheme 9) (17). Pyrolysis of 8b leads to extrusion of furan and isocyanate to give a thiophene. The alternative S-elimi-nation can be effected by oxidation of the adduct and subsequent pyrolysis. 

Chemical Properties. A combination of excellent chemical and mechanical properties at elevated temperatures result in high performance service in the chemical processing industry. Teflon PEA resins have been exposed to a variety of organic and inorganic compounds commonly encountered in chemical service (26). They are not attacked by inorganic acids, bases, halogens, metal salt solutions, organic acids, and anhydrides. Aromatic and ahphatic hydrocarbons, alcohols, aldehydes, ketones, ethers, amines, esters, chlorinated compounds, and other polymer solvents have Httle effect. However, like other perfluorinated polymers,they react with alkah metals and elemental fluorine. 

The exploration of the chemistry of azirines has led to the discovery of several pyrrole syntheses. From a mechanistic viewpoint the simplest is based upon their ability to behave as a-amino ketone equivalents in reactions analogous to the Knorr pyrrole synthesis cf. Section 3.03.3.2.2), as illustrated in Schemes 91a and 91b for reactions with carbanions. Parallel reactions with enamines or a-keto phosphorus ylides can be effected with electron-deficient 2//-azirines (Scheme 91c). Conversely, electron-rich azirines react with electron deficient alkynes (Scheme 91d). 

There are at least two mechanisms available for aziridine cis-trans isomerism. The first is base-catalyzed and proceeds via an intermediate carbanion (235). The second mechanism can be either thermally or photochemically initiated and proceeds by way of an intermediate azomethine ylide. The absence of a catalytic effect and interception of the 1,3-dipole intermediate provide support for this route. A variety of aziridinyl ketones have been found to undergo equilibration when subjected to base-catalyzed conditions (65JA1050). In most of these cases the cis isomer is more stable than the trans. Base-catalyzed isotope exchange has also been observed in at least one molecule which lacks a stabilizing carbonyl group (72TL3591). 

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