Proton-catalyzed addition

However, only the initial formation of an imine from an aldehyde and ammonia or an amine and the subsequent proton-catalyzed addition of cyanide (path ) is in accord with the chemical... 

The chemistry of 1,3-dioxins containing four double bonds is poorly developed. A few examples of nucleophilic additions have been demonstrated. The proton-catalyzed addition of alcohol or carboxylic acid nucleophiles to anhydro derivatives of acetylsalicylic acid - new prodrugs of aspirin - was reported to give the C-2 O-substituted 1,3-dioxanes in 41-60% yield (Equation 19) <2001TL5231, 2003CRC265>. 

Al3+-exchanged synthetic hectorite is a good catalyst for these conversions, and the 13C NMR spectrum obtained in the interlamellar, proton-catalyzed addition of water to 2-methylpropene is indistinguishable (Fig. 79) from that of f-butanol. Doubtless studies of this kind, where natural-abundance, 3C NMR signals are used to probe the chemical identity and motional freedom of reactant and product species situated in the interlamellar spaces of clays or pillared clays (see below), will become increasingly popular. Using l3C NMR linewidths and spin-lattice relaxation studies, Matsumoto et al. (466) have succeeded in discriminating between the internal and external surfaces of pillared montmorillonites. 

Proton-Catalyzed Addition of Alcohols to Glycals Glycals as Glycosyl Donors... 

The proton-catalyzed addition of alcohols to glycals is shown not to be trans diaxial addition. Bollitt et al. [83] reported that triphenylphosphine hydrobromide (TPP-HBr) was the catalyst of choice for mild and high-yield protonation and subsequent glycosidation of glycals (Fig. 5.44). 

The formation of alcohols by acid-catalyzed addition of water to alkenes is a fundamental organic reaction. At the most rudimentary mechanistic level, it can be viewed as involving a carbocation intermediate. The alkene is protonated, and the carbocation is then captured by water. 

A mercurinium ion has both similarities and differences as compared with the intermediates that have been described for other electrophilic additions. The proton that initiates acid-catalyzed addition processes is a hard acid and has no imshared electrons. It can form either a carbocation or a hydrogen-bridged cation. Either species is electron-deficient and highly reactive. 

The most common synthetic application of mercury-catalyzed addition to alkynes is the conversion of alkynes to ketones. This reaction is carried out under aqueous acidic conditions, where the addition intermediate undergoes protonation to regenerate Hg. ... 

General acid catalysis is a catalysis by a Br(4nsted acid (other than the lyonium ion) acting by donating a proton. The addition of thiols to the carbonyl group is general acid catalyzed. ... 

Thus, ynaminoketones with 1,2-diaminobenzene form benzodiazepines with retention of the dialkylamino group. The reaction occurs as a nucleophilic addition in the absence of catalysts. With Q, /3-acetylenic ketones 1,2-diaminobenzene reacts in the same manner, but under proton-catalyzed conditions (72LA24). At the same time, ynamines and enynamines furnish with 1,2-diaminobenzene substituted benzimidazoles as aresultof double attack at the acetylene bond(83ZOR926 84ZOR1648). 

The Ru(m)-catalyzed oxidation of glycerol by an acidified solution of bromate (BrCfi ) at 45 °C consumes the required amount of 2 moles of bromate to obtain pure glyceric acid. Traces of Hg(OAc)2 were used as scavenger for potentially formed bromide, thereby eliminating the formation of bromine (formed by reaction of bromide and bromate) as an alternative oxidant [96]. The reaction is first order in Ru(m) (0.58 ms-1 at 45 °C) and zero order in substrate and protons. The addition of RuC163 to protonated bromate is assumed to be rate limiting. Similar catalytic chemistry is obtained with Rh(m)Cl3 [97]. 

Formation of ring-substituted arylamine thio ethers occurs also by proton-catalyzed thermal rearrangement of the corresponding sulfenamides73,82,83. This alternative pathway may not completely be excluded in thio ether formation from nitrosoarenes, but it seems unlikely since these thio ethers were produced at neutral pH and low temperatures68. The discovery of the fc -conjugate additionally favors the pathway of nucleophilic ring addition of thiolate to the sulfenamide cation. 

In fact, a mechanism for this reaction can be drawn that does not involve Pd at all, but let s assume that Pd is required for it to proceed. Cl- must be nucleophilic. It can add to Cl of the alkyne if the alkyne is activated by coordination to Pd(II). (Compare Hg-catalyzed addition of water to alkynes.) Addition of Cl- to an alkyne-Pd(II) complex gives a o-bound Pd(II) complex. Coordination and insertion of acrolein into the C2-Pd bond gives a new a-bound Pd(II) complex. In the Heck reaction, this complex would undergo P-hydride elimination, but in this case the Pd enolate simply is protonated to give the enol of the saturated aldehyde. 

In proton-catalyzed hydrolysis (specific acid catalyzed hydrolysis), protonation of the carbonyl O-atom leads to polarization of the carbonyl group, facilitating addition of the nucleophile, i. e., a H20 molecule (Fig. 3.1, Pathway a). The acid-catalyzed hydrolysis of esters is reversible because the neutral alcohol or phenol released is nucleophilic, whereas hydrolysis of amides is irreversible because the amine released is protonated in the acidic medium and, hence, has considerably reduced nucleophilicity. 

Studies on the use of proton-exchanged zeolites have also been conducted.12-14 Since tert-butyl alcohol and tert-butyl methyl ether (MTBE) are important gasoline octane enhancers, their synthesis by the addition of water and methanol to isobutylene attracted special attention. Among other processes, clay-catalyzed additions have been studied.15... 

The competition between Michael addition of a,(3-unsaturated ketones and Diels-Alder reactions involving furan and 2-methylfuran is affected by the catalyst used. Methyl vinyl ketone gives the alkylation product with furan and 2-methylfuran in the presence of silica gel (88TL175). Bis(alkylated) products have also been obtained in reactions of 2-methylene-1,3-dicarbonyl compounds (90H(31)1699). An intramolecular proton catalyzed alkylation reaction of an a,(3-unsaturated ketone provided a straightforward synthesis of norpinguisone (90TL4343) and in the example shown in Equation (4) the cyclization reaction involved an a,(3-y,8-dienone (94TL4887). 

In the discussion of the general base catalyzed addition step above (p. 120) the objection was raised that it was difficult to believe that general base catalysis would be necessary for the addition of water to so reactive a species as a protonated ester. An answer to this objection is implicit in the discussion above of the mechanism of hydrolysis of orthoesters. It appears that the protonated orthoester, which would be the initial product of the simple addition of a molecule of water to a protonated ester, is too reactive a species to exist in aqueous solution, and that carbon-oxygen bond-cleavage is concerted with the transfer of the proton to the orthoester. The formation of a protortated orthoester by the addition of a molecule of water to the conjugate acid of an ester will be even less likely, and it seems entirely reasonable, therefore, that the formation of the neutral orthoester, by a general base catalyzed process, should be the favoured mechanism. 

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