Proton loss/elimination

Thus in the above case the elimination product is found to contain 82 % of (7). Unexpected alkenes may arise, however, from rearrangement of the initial carbocationic intermediate before loss of proton. El elimination reactions have been shown as involving a dissociated carbocation they may in fact often involve ion pairs, of varying degrees of intimacy depending on the nature of the solvent (cf. SN1, p. 90). 

The anion presumably plays only a minor role, if any, especially in aqueous systems. Now the formation of 3-hexene may be explained in either of two ways. The intermediate carbonium ion, written in brackets, can undergo hydride migration to form a new carbonium ion, which can then collapse by proton loss to form the 3-hexene. Such a process does not require the intermediate formation of 2-hexene. The alternate explanation involves the discrete formation of 2-hexene followed by addition and elimination of a proton to give the desired 3-hexene. There is no question but that the hydride migration occurs, and with great... 

The order in which various NMR data are acquired is largely one of user preference. Acquisition of the proton reference spectrum will invariably be undertaken first. Whether a user next seeks to establish homo- or heteronuclear shift correlations is where individual preferences come into play. Many spectro-scopists proceed from the proton reference spectrum to either a COSY or a TOCS Y spectrum next, while others may prefer to establish direct proton-carbon chemical shift correlations. This author s preference is for the latter approach. From a multiplicity-edited HSQC spectrum you obtain not only the carbon chemical shifts, which give an indication of the location of heteroatoms, the degree of unsaturation and the like, but also the number of directly attached protons, which eliminates the need for the acquisition of a DEPT spectrum [51, 52]. The statement in the prior sentence presupposes, of course, that there the sensitivity losses associated with the acquisition of multiplicity-edited HSQC data are tolerable. 

Hematite forms by a combination of aggregation-dehydration-rearrangement process for which the presence of water appears essential. Structural details about this process at 92 °C were obtained from EXAFS (Combes et al. 1989 1990) face-sharing between Fe octahedra developed before XRD showed any evidence for hematite. It is followed by internal redistribution of vacancies in the anion framework and by further dehydration. The dehydration process involves removal of a proton from an OH group and this in turn leads to elimination of a water molecule and formation of an 0X0 linkage. The local charge inbalance caused by proton loss is compensated for by migration and redistribution of Fe " within the cation sublattice. 

Certain epoxy steroids, however, yielded dienes rather than ketones, apparently by proton loss from the initially generated csrboninm ion, subsequent elimination of water from the resultant allylic alcohol, and finally double-bond migration. Among epoxides... 

The nitrosation of phenol and cresols in buffer solutions involves a diffusion-controlled C-nitrosation followed by rate-limiting proton loss. /r-Crcsol is much less reactive than the other substrates.79 Nitrosation in trifluoroacetic acid or in acetic-sulfuric acid mixtures is regioselective (e.g. 4-nitroso-m-xylene is fonned from m-xylene) and possible non-selective nitrous acid-catalysed nitration can be eliminated by purging reaction solutions with nitric oxide.80... 

Similarly, proton induced elimination of neutral molecules has also been utilized to generate 1,3-diselenolylium and 1,3-thiaselenolylium ions. As shown in Scheme 8, protonation of 2-methylthio- or 2-methylseleno-l,3-dichalcogenoles with fluoroboric acid results in loss of methanethiol or methaneselenol and in formation of the corresponding 1,3-dichalcogenolylium ions (80H(14)27l, 75TL1259). 

The driving force for this reaction is the formation of N2, an exceptionally stable molecule. The carbocations generated in this manner react like others we have seen by nucleophilic attack to give substitution, by proton loss to give elimination, and by rearrangement. Because of the many competing reaction pathways, alkanediazonium salts usually decompose to give complex mixtures of products. Therefore, the diazo-tization of primary alkylamines is not widely used for synthesis. 

As with other aromatic substitutions, the substitution step itself can be considered to involve an approximately sps hybridization at the carbon atom under attack (10). In the idealized substitution process shown in Eq. (16), 10 may constitute either an intermediate or a transition state. If proton loss ensues directly, the process is properly called a substitution. In other situations the intermediate 10 may become allied with a radical or an anion, leading thereby to a covalent adduct 11. The final substituted product 12 may then be formed either by the elimination of H—Z (first H, then Z) or by the reversal to 10, followed by proton loss. The first case is a classical example of an addition-elimination halogenation, where the adduct is an essential species in the process. In the second case, structure 10 is a common intermediate for both the substitution and the addition reactions. Being merely a diversion of 10, the addition product is not essential to the substitution. In consequence of this, the isolation of adduct 11 may not mean that addition-elimination is the principal pathway of substitution reversal to 10 may be faster than the elimination of H—Z ( 2, k3>ki). On the other hand, the mere failure to detect adduct 11 does not rule out an addition-elimination process, for dehydrohalogenation of adduct 11 may be much faster than its formation (ki>klt k2). 

Steps 1 - 3 El elimination (protonation, loss of HOCH3, deprotonation). 

Imidazole reacts very slowly with singlet oxygen to form the imida-zolidone (86)42S through an elimination reaction involving proton loss and cleavage of the oxygen-oxygen bond of the transannular peroxide (87). On the other hand, 4-phenylimidazole forms a hydantoin derivative (88) and V-benzoyl-V -methoxycarbonylurea (89). This... 

Activation of the acetylene by coordination of the triple bond to the silver cation enables a 5-endo-dig cydization via nucleophilic attack of the amine [14]. Protonation of the resulting vinyl silver complex leads to an iminium ion. Subsequent p-hydride elimination affords metallic silver and a pyrrylium ion which aromatizes by proton loss to the pyrrole. For trimethylsilyl-substituted homopropargylamines (R = SiMes), the resulting pyrrole (R = SiMe3) undergoes protodesilylation to the 1,2-disubstituted pyrrole. 

A carbonium ion which, for any reason, cannot easily rearrange in the sense leading to a carbonyl compound, will generally take the alternative route of proton loss, either with or without skeletal rearrangement, to give a hydroxy-olefin e,g. I —>2). In some situations a second elimination reaction may give a diene. Such behaviour is illustrated by reactions... 

The existence of a free carbonium ion such as VII in a strongly solvating medium is highly improbable. Only if VII could exist in association with the palladium could decomposition to vinyl acetate be expected to occur with a reasonable degree of frequency, in competition with the reaction with acetate to form ethylidene diacetate. Similar results have been reported in the Wacker acetaldehyde synthesis when D2O is used as the solvent (25). Stern (54) has reported results in which 2-deuteropropylene was used as substrate in the reaction. Based on assumed /J-acetoxyalkylpalladium intermediates, on the absence of an appreciable isotope effect in the proton-loss step, and on the product distribution observed, excellent agreement between calculated (71%) and observed (75%) deuterium retention was obtained. Several problems inherent in this study (54) have been discussed in a recent review (I). Hence, considerable additional effort must be expended before a clear-cut decision can be made between a simple / -hydrogen elimination and a palladium-assisted hydride shift in this reaction. 

Comparable studies have been performed for the formation of m-cyclo-hexyl fatty acids in Alicydobacillus acidocaldarius and the pathways are identical [100]. A recent publication concerning this later pathway has shown that the final remaining stereochemical ambiguity, the stereochemistry of proton loss at C-6 in the initial 1,4-conjugate elimination of shikimate occurs with loss of the pro-6R proton [102]. This mirrors the stereochemistry of normal shikimate metabolism in the formation of chorismate from 5-enolpyruvyl-shikimate 3-phosphate. 

Only rarely is proton loss observed from the intermediate cation derived from acylation of a terminal alkyne. In contrast, acylation of alkynylsilanes provides an excellent strategy for the synthesis of alkynyl ketones. " Here, it is apparent that elimination of the silyl substituent competes very effectively with capture of a nucleophile. Acylation of bis(trimethylsilyl)ethyne" provides a convenient route to terminal alkynic ketones, particularly since desilylation can be carried out using bases as weak as dilute aqueous borax, conditions mild enough to prevent the ready hydration of the triple bond (Scheme 25)." Silylated diynes are also excellent substrates for Friedel-Crafts acylation. ... 

The fundamental nature of the substitution-versus-elimination competition is illustrated in Figure 4.10, which is applicable to carbocations such as tertiary alkyl and secondary benzylic that have lifetimes on the order of 10 s in hydroxylic solvents (SOH). The carbocation is at a relatively high energy, with very small barriers to either solvent capture k, substitution product) or proton loss k, elimination product.) The... 

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