Protonic catalysts

When no protonic catalyst is added it is the carboxylic acid which acts as a catalyst in 1896, Goldschmidt already showed that in this case the reaction order with respect to acid is greater than unity and Hinshelwood2511 suggested the following relation for the esterification rate ... 

Studies at High Temperatures (above 100 °C) in the Presence of Protonic Catalysts... 

To ensure that proton transfer takes place from the protonated catalyst 64-H and not from the acidic reagent itself, apolar solvents favoring contact rather than solvent separated ion pairs as well as a slow addition of the acidic substrate RX-H are required. In addition, it was sometimes found beneficial to lower the basicity of the catalyst, thus rendering the protonated species [catalyst-H" ] more acidic for the stereo-determining protonation of the enolate. This was accomplished by formally replacing NR2 by Me (see 64e, Fig. 36). 

The oxazaborolidines B and C derived from proline are also effective catalysts. The protonated forms of these catalysts, generated using triflic acid or triflimide, are very active catalysts,95 and the triflimide version is more stable above 0° C. Another protonated catalyst D is derived from 2-cyclopentenylacetic acid. 

Chen, L.-W. and Chen, J.-W., Kinetics of diethylene glycol formation from bishydroxyethyl terephthalate with proton catalyst in the preparation of poly(ethylene terephthalate),. /. Appl. Polym. Sci, 75, 1221-1228 (2000). 

Enolization and ketonization kinetics and equilibrium constants have been reported for phenylacetylpyridines (85a), and their enol tautomers (85b), together with estimates of the stability of a third type of tautomer, the zwitterion (85c). The latter provides a nitrogen protonation route for the keto-enol tautomerization. The two alternative acid-catalysed routes for enolization, i.e. O- versus Af-protonation, are assessed in terms of pK differences, and of equilibrium proton-activating factors which measure the C-H acidifying effects of the binding of a proton catalyst at oxygen or at nitrogen. 

The metal halide thus functions in similar manner to the proton and may be considered to be an acidic catalyst (cf. Luder and Zuffanti, 19). The catalyst-olefin complex differs in one significant respect from the product formed by the addition of the proton (or the corresponding acid) to the olefin the halide catalyst is a neutral but electronically deficient molecule and combines with the pi electrons of the double bond to form a coordinate bond between the carbon atom and the aluminum or boron. On the other hand, the addition of the positive proton to the double bond results in the formation of a true (covalent) link between carbon and hydrogen. In other words, the complex, while it contains an electron-deficient (hence, positive) carbon atom, is in itself electronically neutral the product of the addition of a proton to the alkene contains a similar carbon atom but is itself electrically positive. It has been suggested (Whitmore and Meunier, 20) that this difference is related to the fact that metal halide catalysts tend to yield much higher polymers than do the acid (proton) catalysts. 

Two distinct reaction pathways can be envisioned for the C—C bond formation step of this catalytic process (see Scheme 3.7). According to the mechanism proposed by Takemoto et al. [30], the nitroolefin interacts with the thiourea moiety of complex 3 (Scheme 3.7, route A), forming a ternary complex, wherein both substrates are activated, and C—C bond formation can occur to produce the nitronate form of the adduct Alternatively, the facile interconversion between 3 and 3" may allow an interaction of the nitroolefin with the cationic ammonium group of the protonated catalyst (Scheme 3.7, route B). In both cases, ternary complexes result... 

Xenon difluoride reacts with alcohols to form unstable alkoxyxenon fluoride intermediates. Alkoxyxenon fluorides react as positive oxygen electrophiles when boron trifluoride-diethyl ether complex is used as a catalyst. However, these alkoxyxenon fluorides react as apparent fluorine electrophiles with proton catalysts (hydrogen fluoride generated in situ).49... 

The above results are consistent with a steric specific syn 1,2-addition-elimination of metal hydride intermediate which is formed fast in a pre-equilibrium [MH] [MD] and adds to the olefinic substrate to form the metal alkyl intermediate (equation 261). The /1-hydride elimination of the most stable rotamer (equation 262) is the RDS in the rearrangement, leading to a metal hydride-product complex, which starts a new cycle faster than uncoordinated metal hydride. The protonated catalyst, 434, produces a precursor... 

Alkoxyxenon fluorides (ROXeF), generated from XeF2 on reaction with alcohols, react with indene as positive oxygen electrophiles when BF3.Et20 is used as catalyst. By contrast, with proton catalysts they react as apparent fluorine electrophiles87. 

Mechanism IV Adsorption-Desorption Reaction in the Presence of a Proton Catalyst. The mechanism in which H+ acts as a catalyst is... 

A theoretical study of the acetylation of Bu OII by acetic anhydride catalysed by 4-(dimethylamino)pyridine has confirmed the generally accepted pathway in which an initially formed acetylpyridinium/acetate ion pair suffers attack by the alcohol in a rate-determining step to form the ester together with deactivitated (protonated) catalyst. Regeneration of the latter requires an auxiliary base such as triethylamine. Deprotonation of the alcohol in the rate-determining step is effected by the acetate counterion.26... 

L. A. Heredy, et al. employed BFo as a catalyst. BF3 forms a stable adduct with pTienol which yields the proton catalyst due to the dissociation equilibrium ... 

In a full paper we described detailed studies of our various approaches to chiral induction in the amino acid products of transaminations. In that paper we also pointed out the odd fact that the Tabushi laboratory had reported the use of large concentrations of buffer in their reaction, which would have been expected to interfere with the selective proton transfer by an internal catalytic group if the buffer itself could start playing the role of protonating catalyst. 

Metal-free bisoxazolines catalyzed Diels-Alder reaction of A-substituted maleim-ides 137 with anthrone 136 derivatives was achieved by Gbbel, et al. Scheme 3.45 [61]. With the photoelectron spectra and other studies, the Br0nsted-base catalysis is assumed to be involved with formation of an ion pair between the protonated catalyst 138 and the anthrone enolate, acting as diene. 

Sulfuric acid provides a proton catalyst as shown in the following equihbrium H-rOSOjH H + "OSO3H. 

IV) The adsorption-desorption reaction in the presence of proton catalyst. 

Anode Proton Catalyst Exchange Layer Membrane... 

Interestingly, an unsymmetrical bifunctional protonated catalyst was developed by Johnston and Singh to promote the asymmetric aza-Henry reaction of a-alkyl a-nitroesters with iV-Boc imines, providing the corresponding chiral a-substituted syra-a,p-diamino acid derivatives of phenyl alanine in high yields, excellent enantioselectivities of up to 99% ee and moderate to high diastereoselectivities of up to 90% de, as shown in Scheme 3.27. ... 

Scheme 3.27 Aza-Henry reactions catalysed by bifunctional protonated catalyst.

Thiols, the sulfur analogs of alcohols (see Section 9-10), react with aldehydes and ketones by a mechanism identical with the one described for alcohols. Instead of a proton catalyst. 

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