Electrophiles protonation

If a given vinylidene complex is not sufficiently electrophilic, protonation at Cp can promote nucleophilic addition at C by intermediate formation of an electrophilic carbyne complex [89] (Figure 2.9, Section 2.1.8). 

Mechanism. The double bond ti electrons attack the electrophile. Protonation of the double bond yields a secondary carbocation inter mediate. The bromine nucleophile attacks the carbocation to form 2-bromopropane. 

Metal-free corrin is stable only in the protonated form and reversibly deprotonates at C-8 and C-l 3 in neutral or basic media (pX mcs 8.6 in dimethyicellosolve- water, 1 l).250 Therefore, deutera-tion under basic conditions (Na0D-KCN/Bul0D-D2O (1 1), r.t., M = H2, CoHI(CN)2, Ni2+) occurs at those positions. The electrophilic proton-deuterium exchange (TFA/Bu 0D-D20 (1 1), r.t., M = H2, Coih(CN)2, Ni2+) is more facile at C-5 and C-15 than at C-10 as predicted from MO calculations.239,250... 

Electrophilic proton exchange occurs readily for sapphyrin (39) and meso deuteration is completed in trifluoroacetic acid and at room temperature overnight.274 The substitution proceeds at a moderate rate for two of the meso protons but very slowly for the other two in the dioxa analogue... 

Aryloxy derivatives 197 can be cyclized into benzofuroquinoxalines 198 in 50-70% yields on prolonged heating in PPA at 140-160°C (74JCS(P1)129). Under these conditions the more electrophilic protonated form of the substrate is subjected to an intramolecular nucleophilic attack at the carbon atom ortho to the aryloxy group. 

Because of the presence of the nucleophilic oxygen and electrophilic proton, alcohols can act both as weak acids and as weak bases when dissolved in water (Following fig.). However, the equilibrium in both cases is virtually completely weighted to the unionised form. 

In this example the electrophile is a proton and the nucleophile is a chloride anion. The mechanism is just as described in the previous section first the electrophilic proton adds to produce a carbocation intermediate, and then the chloride nucleophile bonds to the carbocation. 

Thiazolylium and benzothiazolylium salts react normally with aqueous sodium borohy-dride yielding the corresponding thiazolidine or benzothiazoline. The mechanism and the stereochemistry of the reaction have been studied with thiazolylium salts chosen as models for thiamine, using borodeuteride/hydride and deuterium/protium oxide. The pathway described in Scheme 30 was suggested it involves the addition of a nucleophilic hydride at C-2 (50 -> 51), the addition of an electrophilic proton at C-5 (51 -> 52) and the addition of a second nucleophilic hydride at C-4 (52 — 53). 

Protonation of an alcohol, for example, converts it to a strong electrophile, which can undergo attack or lose water to give a carbocation, an even stronger electrophile. Protonation of an alkene converts it to a carbocation. 

Initially, a loosely bound hydrogen bonded adduct 10 is formed, from the carboxyhc acid 8 and the carbonyl compound 6. The next step is the formation of the a-adduct 11 by reaction of the isocyanide with the nucleophilic carboxylate and the electrophilic protonated carbonyl compound. a-Adduct 11 then rearranges to give oc-acyloxy amide 9. 

As expected, 3-Li reacts readily with electrophiles protonation with cyclopentadiene or treatment with trimethylchlorosilane gave 3-H and 3-TMS, respectively (Schemes 1 and 2). Surprisingly, the reactions with 1,2-dibromoethane or diaryldichlorosilane gave rise to the formation of the cyclotrisilane 1 in moderate yield. Other silicon containing ring systems should be accessible via reaction with bis-electrophiles. 

Hydrogenolysis of the toluene-4-sulfonate of an alcohol may be carried out with a nucleophilic hydride such as lithium aluminium hydride. There are also a series of radical methods based on the reduction of alkyl halides with tri- -butyltin hydride (BUjSnH). Finally, the source of the hydrogen may be the electrophilic proton, exemplified by the decomposition of organometallic reagents such as the Grignard reagent with water. 

The initial addition of the electrophilic proton is not always followed by the addition of a nucleophile. The carbocationic intermediate may be discharged by the loss of a proton from another position, leading to double bond isomerization (Scheme 3.6). 

Bond formation at the radical anion stage (Scheme 25) usually occurs either by reaction with an electrophile (proton, cation, unsaturated electron-poor system) or with another odd-electron species (charged or neutral radical). 

The electrophilic protonated imine reacts with nucleophilic hydroxylamine. 

Acceptor ability of a group can be enhanced by interaction with a suitable electrophile (proton, Lewis acid). This is very likely in the Ni(acac)2-catalysed reaction of cyclopropyl ketones with trimethylaluminium and in the synthetically more significant addition of several cuprates to alkyl cyclopropyl ketones. The BFs-activated organometallic reagent attacks the less substituted cyclopropane carbon regioselectively. Yet steric hindrance can be a problem in this reaction (equation 40) . 

Kinetic study has shown that appreciable positive charge develops on the 1-carbon in the transition state. The electrophilic proton attacks the C(l)-C(2) bonding electrons, probably at the edge of the ring Opening of optically active truns-l-methyl-2-phenylcyclopropanol (11) gives optically active 4-phenyl-2-butanone with retention of configuration (equation 8) . 

Cyclopropenones react readily with most electrophiles. Protonation results in the formation of hydroxycyclopropenyl salts (equation 50) Alkyl- and cyclopropyl-substituted cyclopropenones are markedly more basic than phenyl-substituted cyclopropenones (or other a, j -unsaturated compounds) This is emphasized by the observation that di-n-propylcyclopropenone can be extracted from 12 N HCl whereas diphenylcyclo-... 

Example (c) uses HBr as the reagent. This is a strong acid with an electrophilic proton. The best nucleophile in the organic compound is the oxygen atom so the first step is a proton transfer and the second step uses the bromide ion, as a nucleophile for the methyl group. Direct attack at would give impossible four-valent oxygen. 

The present work gives analytical treatment to the main types of heterolytic processes with the participation of silica surface sites, the specific features for reactions of the electrophilic proton substitution in the structural silanol groups at interaction with some halo-and pseudohalosilanes, halogenides and oxohalogenides of various elements. Some possibilities of the addition reactions in the synthesis of the surface chemical compounds are considered. 

When comparing the reactivities of the trimethylsilyl-sustituted chloro- and alkoxysilanes, activities of the corresponding organosiloxanes and aminosilanes in the reactions of electrophilic proton substitution in the isolated silanol groups of the silica surface the following sequence was obtained Si-N(H,R) > Si-0(H,R) > Si-Cl > Si-O(Si) > Si-C(H,R)... 

Formerly the data were obtained [22] that thionyl chloride enters into interaction with the hydroxyl groups of silica surface by electrophilic proton substitution, and after that the forming surface compounds transforms (elimination process), and the ultimate result of such transformation corresponds to nucleophilic substitution on silicon atom. 

In this case, the carbonyl oxygen is attacked first by the electrophilic proton. Draw another canonical structure for this cationic species. 

For efBdent cationic polymerization of vinyl monomers it is necessary that the carbon-carbon double bond be the strongest nucleophile (electron donor) in the molecule. If more than one nucleophilic site exists in a monomer and the rr electron system of the double bond does not represent the most nucleophilic site, the other site(s) may complex with the electrophile (proton, cation, Lewis acid). Thus, for methyl methacrylate ... 

The terf-butyl (2-methylprop-2-yl) cation, which is the electrophile in reaction (5.2), has a vacant orbital which can accept the electron pair from the nucleophile without the need for any further movement of electrons. However, in many reactions a further electron pair movement is needed. The electrophilic proton in H30 +, HBr or H20 (reactions 5.3-5.5) can only support one covalent bond, so as the nucleophile attacks with its electron pair, the bond from the proton to the oxygen or bromine atom has to break, with the electrons from that bond forming a lone pair on the oxygen or bromine atom. 

The first step of the reaction is a relatively slow addition of the electrophilic proton to the nucleophilic alkene to form a carbocation intermediate. In the second step, the positively charged carbocation intermediate (an electrophile) reacts rapidly with the negatively charged bromide ion (a nucleophile). 

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