Protic medium

A recent modification of this technique utilizes A,A-d2-propylamine as the solvent for the lithium reduction, thereby eliminating the inconveniences associated with the preparation and handling of liquid deuterioammonia. Under these conditions the reaction can be carried out at room temperature and less overreduction of the carbonyl group is observed. For example, the reduction of A" -3-keto steroids (159) under these conditions, followed by back exchange in protic media, leads to the corresponding 5a-di-3-ketones (160) which exhibit good isotopic purity. ... 

Cesium fluonde in dimethylformamide catalyzes the isomerization offluori-nated cyclobutenes, perfluorobipyntmdines, and their oligomers to products with expanded rings [30, 31, 32] The product distribution in cobalt tnfluonde fluorina-tion depends strongly on the temperature of the reaction [33] Fluorinated 1-dimethylamino-5,6,7,8-tetrafluoro-l,4-dihydro-l,4-ethenonaphthalene rearranges in protic media to a biphenyl derivative [34] (equation 8)... 

Here, the relative stability of the anion radical confers to the cleavage process a special character. Thus, at a mercury cathode and in organic solvents in the presence of tetraalkylammonium salts, the mechanism is expected16 to be an ECE one in protic media or in the presence of an efficient proton donor, but of EEC type in aprotic solvents. In such a case, simple electron-transfer reactions 9 and 10 have to be associated chemical reactions and other electron transfers (at the level of the first step). Those reactions are shown below in detail ... 

At this point, special mention37 should be made of the behaviour of highly conjugated ethylenic sulphones in weakly acidic media. For example, in the case when R1 =Ph (Z isomer), a fairly stable anion radical was obtained in dry DMF. However, either in aprotic (consecutive two one-electron transfer) or in protic media (ECE process, occurrence of the protonation step on anion radical), C—S bond cleavage is observed. The formation of the corresponding olefins by C—S bond cleavage may occur in high yield, and is nearly quantitative when R1 = H and R2 = Ph for an electrolysis conducted in... 

The third primary intermediate in the oxidation chemistry of a-tocopherol, and the central species in this chapter, is the orr/zo-quinone methide 3. In contrast to the other two primary intermediates 2 and 4, it can be formed by quite different ways (Fig. 6.4), which already might be taken as an indication of the importance of this intermediate in vitamin E chemistry. o-QM 3 is formed, as mentioned above, from chromanoxylium cation 4 by proton loss at C-5a, or by a further single-electron oxidation step from radical 2 with concomitant proton loss from C-5a. Its most prominent and most frequently employed formation way is the direct generation from a-tocopherol by two-electron oxidation in inert media. Although in aqueous or protic media, initial... 

Many mechanistic aspects of the hydrolysis of phosphate esters in protic media remain uncertain. In spite of predictions that racemization at phosphorus should be the final outcome if indeed the (hypothetical) metaphosphate intermediate is involved in the solvolysis of monoesters, the results of several studies on the methanolysis of appropriately O-isotopically labelled compounds are consistent with reactions proceeding with inversion of configuration, as observed for all enzymic and non-enzymic systems so far examined this has resulted in the suggestion that if metaphosphate is actually formed, then it must be in a masked form. 

Thus, these relationships can be used to define a pH scale for non-aqueous protic media, consistent with the pH scale for aqueous solutions. For standard hydrogen pressure, the potential of the hydrogen electrode depends on the pH(s) according to the relationship... 

Furthermore, it is often possible to extract from the structural analysis of solid solvates a significant information on solvation patterns and their relation to induced structural polymorphism. An interesting illustration has been provided by crystal structure determinations of solvated 2,4-dichloro-5-carboxy-benzsulfonimide (5)35). This compound contains a large number of polar functions and potential donors and acceptors of hydrogen bonds and appears to have only a few conformational degrees of freedom associated with soft modes of torsional isomerism. It co-crystallizes with a variety of solvents in different structural forms. The observed modes of crystallization and molecular conformation of the host compound were found to be primarily dependent on the nature of the solvent environment. Thus, from protic media such as water and wet acetic acid layered structures were formed which resemble intercalation type compounds. 

Cyclopropenone formation should involve the bisketene 46 and its decarbonylation to the monoketene 47 (a valence tautomer of cyclopropenone 44), since photolysis in protic media like ethanol produces diethoxy diethyl tartrate 45) (meso and d,l). This method was also successful in the case of l,2-diphenyl-3,3-dichloro-cyclo-butene dione (48) giving rise to diphenyl cyclopropenone49 (but as for 44 only a moderate yield was produced) ... 

This is further accentuated by the surprising results of the reaction between aziridine and diphenyl cyclopropenone which was elucidated by Dehmlow224. In aprotic media two molecules of aziridine react with a cyclopropenone moiety eliminating ethylene and forming enamino amide 527, whereas in protic media one molecule of aziridine reacts with the exclusive formation of the aziridide 326 ... 

Allyl pyridinium betaines 441 isoelectronic with enol betaines 427 likewise reacted with diphenyl cyclopropenone by elimination of pyridine272,213 The product formation, different in aprotic and protic media (phenol 443 in aprotic solvent, A3,5-hexadienoic esters 445 in alcohol solvent), suggested that the diene... 

For the cydative cleavage step, it turned out that aprotic conditions were definitely superior to the use of protic media. Thus, employing N,N-dimethylformamide as solvent at somewhat elevated temperatures furnished the desired compounds in high yields and excellent purities. Having established the optimized conditions, various phthalic acids and amines were employed to prepare a set of phthalimides (Scheme 7.51). However, the nature of the amine was seen to have an effect on the outcome of the reaction. Benzyl derivatives furnished somewhat lower yields, probably due to the reduced activities of these amines. Aromatic amines could not be included in the study as auto-induced ring-closure occurred during the conversion of the polymer-bound phthalic acid. 

Bromination is less stereoselective, and the reactions of cis- and trans-olefins tend to be stereoconvergent. The stereospecific formation of the mixed bromoadducts in protic media, such as methanol or acetic acid, could be interpreted in the light of the recent finding (Ruasse et al., 1991) that these solvents assist the formation of the ionic intermediate nucleophilically. If a solvent molecule is close to the cationic part of the transition state in the rate-limiting step, the intermediate can be trapped by this solvent molecule in a necessarily trans mode with respect to the first bromine, before the two components of the ion-pair diffuse away from each other (15). This would... 

The most significant difference between brominations in protic and non-protic solvents concerns the kinetic law. Whereas in protic media the reaction is first-order in bromine, in halogenated media it is second-order (Bellucci et ai, 1980). CTC ionization is electrophilically assisted by hydrogen bonding by a protic solvent to the leaving bromide and leads to a bromonium-bromide ion pair. In non-protic media, assistance to the bromination step is provided by a second bromine molecule, leading to a bromonium-tribromide ion pair. In other words, in protic media bromination is solvent-assisted (56) while in halogenated media it is bromine-catalysed (57). 

According to (57), the main driving force for the reaction in non-protic media is the formation of a tribromide ion from bromine and the developing bromide. Kinetic (Ruasse et al., 1986) and thermodynamic (Bienvenue-Goetz et al., 1980) data on equilibrium (58) are therefore relevant to the effect of non-protic solvents on bromination rates. 

If the formation of bromination intermediates is reversible, the experimental rate constants obtained by following bromine uptake are not those of the first ionization steps. It is therefore important to know whether return, shown to occur in halogenated media, can also occur in protic media, in which most of the kinetic data have been measured and structure- or solvent-reactivity relationships established. 

Generally, these equilibrium constants are very solvent dependent. In particular, the values of KEA are expected to be relatively high in protic solvents because halide anions can be stabilized through solvation in such media [143], Also, on the other hand, Kx values will be likewise affected with changes in solvent polarity [50], Low values of Kx in polar and protic media have direct implications on the degree of polymerization control because of the decreased amounts of deactivator (Mt"+1X/L), as a result of halide anion dissociation from the metal center. 

Compounds 161 and 162 were found to be unstable in basic, protic media, presumably due to the presence of the acidic hydrogen atom of the malonic ester. As a result, their synthetic utility is somewhat limited. It was, however, found possible to alkylate 162 with ethyl bromoacetate in the presence of sodium hydride,125 to give triethyl 1 - (2,3,5-tri-O-benzoyl- /3-d-ribofuranosyl) -1,1,2-ethanetricarboxylate (166). This product was obtained in 20% yield by treatment of 81... 

As exemplified above, the chemistry of carbenes in protic media raised the prospect that carbocations could be formed. Experiments were designed to identify the intervening species by means of product and/or label distributions (Section II). The results thus obtained set the stage for the application of... 

Allylic cations (180) were also generated by LFP of allenes (174) in TFE.86 Deuterium labels revealed that the cations 180 originate predominantly from vinylcarbenes (177), which are formed from 174 by way of a 1,2-H shift. Protonation at the central carbon of the photoexcited allenes87 is a minor reaction path with 174a,b,d. Vinylcarbenes are also known to arise in photolyses of cyclopropenes, 175 — 177.85bi88 However, LFP of 175 in protic media proved to be rather inefficient in generating allylic cations, presumably due to low quantum yields. 

Not only arylboronic acids but also 1-alkenylboronic acids or esters add to aldehydes to give the corresponding allylic alcohols (Equation (39)).395 Isomerization of the allylic alcohols to saturated ketones occurs in less protic media and at higher temperatures. 

In non-aqueous solution, the copper catalyzed autoxidation of catechol was interpreted in terms of a Cu(I)/Cu(II) redox cycle (34). It was assumed that the formation of a dinuclear copper(II)-catecholate intermediate is followed by an intramolecular two-electron step. The product Cu(I) is quickly reoxidized by dioxygen to Cu(II). A somewhat different model postulated the reversible formation of a substrate-catalyst-dioxy-gen ternary complex for the Mn(II) and Co(II) catalyzed autoxidations in protic media (35). 

Reductions at noble metal electrodes in acidic protic media often form adsorbed hydrogen, which is the actual reductant. For example, reduction of nitrobenzene at a Pd/C electrode in acetic acid-methanol mixtures affords aniline via adsorbed hydrogen28. This reaction is more closely related to catalytic hydrogenation of nitro groups than to the... 

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