Proton acid acceleration

Several reports deal with the action of heterocycle-chromate agents such as quinolin-ium dichromate on five-membered heteroaldehydes340 and quinolinium bromochromate on benzaldehydes,341,342 all in acetic acid solution. The latter studies show a second-order dependence on proton concentration, acceleration by electron-withdrawing para-substituents, and a substantial kinetic isotope effect for the deuterated aldehyde. 

The difference is in the mechanism of reaction. In the case of carbaryl, the reaction proceeds by an elimination process in which the proton acidity on the nitrogen atom determines the reactivity. On the other hand, the chlorpropham reaction proceeds in a manner analogous to the hydrolysis of carboxylic acid esters. Much like carboxylic acid esters, electron-withdrawing substituents in carbamates accelerate the reaction by an amount that depends on whether the substituents are on N or O. Conversely, electron-donating substituents (methyl in the case of chlorpropham, above) slow the rate of hydrolysis. 

An NMR kinetic study of a phosphine-catalysed aza-Baylis-Hillman reaction of but-3-enone with arylidene-tosylamides showed rate-limiting proton transfer in the absence of added protic species, but no autocatalysis.175 Brpnsted acids accelerate the elimination step. Study of the effects of BINOL-phosphinoyl catalysts sheds light not only on the potential for enantioselection with such bifunctional catalysis, but also on their scope for catalysing racemization. 

Indium-mediated reaction of enamines with allyl bromides gives homoallylamines. The addition of one equivalent of acetic acid accelerates the reaction. An analogous reaction of methyl bromoacetate in place of allylic bromides is also possible. The iminium salts formed by protonation of the enamines are considered to be the intermediates (Scheme 70).271 272... 

For benzoic acid acceleration in Yb(OTf)3-catalyzed allylation of aldehydes in acetonitrile, (a) As-pinall, H.C. Greeves, N. Mclver, E. G. Tetrahedron Lett. 1998, 39, 9283. For acetic acid acceleration in Yb(fod)3-catalyzed ene reaction of aldehydes with alkyl vinyl ethers, ene reaction of aldehydes with alkyl vinyl ethers, (b) Deaton, M. V. Ciufolini, M.A. Tetrahedron Lett. 1993, 34, 2409. Yamamoto et al. reported Brpnsted acid-assisted chiral Lewis acids and Lewis acid-assisted Brpnsted acids which were used for catalytic asymmetric Diels-Alder reactions and protonations and stoichiometric asymmetric aza Diels-Alder reactions, aldol-type reactions of imines, and an aldol reaction, (c) Ishihara, K. Yamamoto, H. J. Am. Chem. Soc. 1994, 116, 1561. (d) Ishihara, K. Kurihara, H. Yamamoto, H. J. Am. Chem. Soc. 1996, 118, 3049. (e) Ishihara, K. Nakamura, S. Kaneeda, M. Yamamoto, H. J. Am. Chem. Soc. 1996, 118, 12854. (f) Ishihara, K. Miyata, M. Hattori, K. Tada, T. Yamamoto, H. J. Am. Chem. Sc c. 1994, 116, 10520. (g) Yamamoto, H. J. Am. Chem. Soc 1994, 116, 10520. (h) ishihara, K. Kurihara, H. Matsumoto, M. Yamamoto Ishihara, K. Kurihara, H. Matsumoto, M. Yamamoto, H. J. Am. Chem. Soc 1998, 120, 6920. 

Friedel-Crafts acylation usually involves the reaction of an acyl halide, a Lewis acid catalyst, and the aromatic reactant. Several species may function as the active electrophile, depending on the reactivity of the aromatic compound. For activated aromatics, the active electrophile can be a discrete positively charged acylium ion or a complex formed between the acyl halide and the Lewis acid catalyst. For benzene and less reactive aromatics, it is believed that the active electrophile is a protonated acylium ion or an acyiium ion complexed by a Lewis acid. Reactions using acylium salts are slow with toluene or benzene as the reactant and do not proceed with chlorobenzene. The addition of triflic acid accelerates the reactions with benzene and toluene and permits reaction with chlorobenzene. These results suggest that a protonation step must be involved. 

It has been found that the presence of Lewis acids or protons can accelerate carbonyl insertion reactions, providing another possibility of modifying catalysts. Mixtures of transition metal carbonyls and Lewis acids could in future be of interest as catalysts for CO hydrogenation, for example, in Fischer-Tropsch reactions (Eq. 2-82) [Til]. 

Palladium (0) catalysis promised to be an efficient tool to accelerate aza-Claisen rearrangements. Indeed, the reaction temperatures could be kept between 50 and 100 °C using a Pd(0) catalyst and a strong proton acid. The major drawback reported was that the palladium-assisted aza-Claisen reaction delivered 1,3 as well as 3,3 rearrangement products [21a,b]. The least hindered allyl terminus always formed the new C-C bond restricting the process to symmetric allyl systems. 

Boron trifluoride and other Lewis acid protonic acids have also been found to function as effective accelerators for epoxy-anhydride systems. Their effectiveness has been attributed to the ease of proton formation according to reaction (74), which then facilitates anhydride rind-opening according to reaction (75) in Scheme 25. ... 

It is obvious that the reaction is accelerated markedly by water. However, for the first time, the Diels-Alder reaction is not fastest in water, but in 2,2,2-trifiuoroethanol (TFE). This might well be a result of the high Bronsted acidity of this solvent. Indirect evidence comes from the pH-dependence of the rate of reaction in water (Figure 2.1). Protonation of the pyridyl nitrogen obviously accelerates the reaction. 

An alternative view of these addition reactions is that the rate-determining step is halide-assisted proton transfer, followed by capture of the carbocation, with or without rearrangement Bromide ion accelerates addition of HBr to 1-, 2-, and 4-octene in 20% trifluoroacetic acid in CH2CI2. In the same system, 3,3-dimethyl-1-butene shows substantial rearrangement Even 1- and 2-octene show some evidence of rearrangement, as detected by hydride shifts. These results can all be accoimted for by a halide-assisted protonation. The key intermediate in this mechanism is an ion sandwich. An estimation of the fate of the 2-octyl cation under these conditions has been made ... 

The acid-catalyzed hydrolysis of imidazolides can also be accelerated by protonation of N-3, which increases the leaving-group ability of the ring. Accumulation of additional... 

The exchange of active hydrogens in —OH, —SH, —NH2, —NH— and —COOH is usually a very fast reaction. The rate of the reaction depends on the acidity of these protons and it can be strongly accelerated by the presence of traces of acid catalysts. 

The 0X0 group, will tend to activate nucleophihc substitution when its oxygen atom is protonated (e.g., 201 when Z is H) in acid-catalyzed reactions 223b,298 qj. hydrogen bonding to the solvent. Acceleration... 

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