Proton-catalyzed reactions

Micellar catalysis to enhance or diminish the rate of chemical reactions is well known [97]. Of somewhat greater interest is the influence of micelles on competing reactions, e.g., proton-catalyzed reactions. An example related to the effect of alkanesulfonates is the epoxidation of simple aliphatic olefins. The reaction of olefins and hydrogen peroxide catalyzed by strongly acidic Mo(VI)... 

S. R. Blaszlowski and R. A. van Santen, Quantum chemical studies of zeolite proton catalyzed reactions. Top. Catal. 4, 145-156 (1997). 

Hydrogen bond-promoted asymmetric aldol reactions and related processes represent an emerging facet of asymmetric proton-catalyzed reactions, with the first examples appearing in 2005. Nonetheless, given their importance, these reactions have been the subject of investigation in several laboratories, and numerous advances have already been recorded. The substrate scope of such reactions already encompasses the use of enamines, silyl ketene acetals and vinylogous silyl ketene acetals as nucleophiles, and nitrosobenzene and aldehydes as electrophiles. 

A. F. Bochkov, V. I. Betaneli, and N. K. Kochetkov, Sugar orthoesters XV. Dependence on the conditions and mechanisms of the proton-catalyzed reactions of substituted a-D-glucopyranose 1,2-(alkyl orthoacetate)s in media of low polarity, Bioorg. Khim.(EngL), 2 (1976) 671-682. 

Although methanol undergoes addition to the C-C double bond of an (alk-l-enyl)cyclopropane in a proton-catalyzed reaction, " generally methanol addition is carried out in a two-step reaction involving methoxymercuration followed by sodium borohydride reduction. The pure product from the first step can be isolated, but this isolation is usually not performed ... 

Mechanistic studies of the diacetoxylation of alkenes using (diacetoxyiodo)benzene have demonstrated a protio-catalytic nature of this reaction [255]. Systematic studies into the catalytic activity in the presence of proton-trapping and metal-complexing additives indicate that strong acids act as catalysts in the reaction. When trifluoromethanesulfonic acid is used as catalyst, the selectivity and reaction rate of the conversion is similar or superior to most efficient metal-based catalysts, such as Pd(II) and Cu(II) metal cations. Based on a kinetic study as well as in situ mass spectrometry, a mechanistic cycle for the proton-catalyzed reaction was proposed in this work [255]. 

As an example of a proton-catalyzed reaction, consider the hypothetical dissociation of a p2"Oxo site at the surface of silica [Si02(s)]. The first step is equilibrium protonation of the neutral siloxane (the p2 0 site) to form a hydroxyl bridge (the P2-OH site) ... 

The TEOS- fisier system exhibits a pH-dependent onset time for the hydrolysis thermal peak. Donatti and VoUet [46] have found k = A o/[H" = (4.6 0.4) min [H" ] at 39°C in the range of log [HC1] between 0.8 and 2.0. The dependence of the hydrolysis rate constant on [H was found to be in agreement with the proton catalyzed reaction. 

For non-diffusion-limited reactions carried out in low Al/Si ratio systems, the overall rate for a proton-catalyzed reaction increases linearly with the proton concentration, as illustrated schematically in Fig. 4.4 . The rate, when normalized against the framework proton concentration, however, is a constant. When the lattice Al/Si exceeds 10%, the proton-zeolite interaction energy increases. This increase in the proton-zeolite interaction decreases the intrinsic Brpnsted acidity of the zeolite. At this concentration, the tetrahedra containing A1 start to share a silicon tetrahedron. This increases the effective negative... 

Transition states in proton-catalyzed reactions that occur in zeolites and other solid acids proceed through activated intermediates close to the carbenium or carbonium ions found in superacid solutions. A carbenium ion is a positively charged ion, that can be formed by the protonation of an alkene. The positive charge is localized on an sp -hybridized C atom. A carbonimn ion is a protonated saturated alkane, that forms non-classical valencies such as a protonated a C-C bond or a five-coordinated C atom. In zeolites, the positively charged intermediates are compensated for by the negatively charged zeolite framework... 

Pillared clays with pillars based on A1 and also different elements (e.g., Cr, Fe) or with mixed pillars have been extensively investigated in numerous catalytic reactions of hydrocarbons, principally in proton-catalyzed reactions. Catalytic aspects have been reviewed in several articles (3,91,92). Comparison of the catalytic performances of acid solids with potential catalytic applications preferably with a reference catalyst is a straightforward way to obtain a preliminary information on their efficiency. With this approach, the activity of Al-pillared materials prepared with different smectites and pillaring methods have been compared, using the hydroisomerization of linear paraffins over Pt-impregnated pillared samples. In these bifiinctional catalysts, the metal function is necessary to dehydrogenate the paraffin and rehydrogenate the branched olefins, and the protons aid the formation of carbenium ions, which isomerize via protonated cyclopropane... 

Anhyd copper sulfate is also an excellent catalyst for the formation of acetonides from glycols and acetone (eq 2). Either 2,2-dimethoxypropane or 2-methoxypropene can sometimes improve the efficiency. The regiochemistry of the copper-catalyzed process is different from that found in the proton-catalyzed reaction. 

Hydrogen fluoride (HF) fails to convert alcohols to the corresponding fluorides. Sulfur tetrafluoride or its derivative, diethylaminosulfur trifluoride (DAST (CH3CH2)2NSp3), is required for the conversion of alcohols to the corresponding fluorides. As with analogous proton-catalyzed reactions and as shown in Scheme 8.29, alkyl fluoride formation results from mixtures of SnI and Sn2 processes. 

According to several authors in the 1990s [5,210], the active site for silica-alumina and protonic zeolites is the same (i.e., it is constituted by the bridging hydroxy groups bonded to a silicon and an aluminum atom) in contrast to the fact that the catalytic activity of protonic zeolites in proton-catalyzed reactions is by far higher than that of silica-aluminas. 

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