High catalysis

C—H borylation of indoles has been reported by a number of groups. Chirik and coworkers utilized pincer-ligated cobalt complexes with N-methyhndole and recorded the C-2 borylated indole (133) as the major product (2014JAC4133). The catalysts employed demonstrated high catalysis turnover and low catalytic loading and also demonstrated efficacy with other electron-rich heteroarenes (furan, thiophene, benzofuran) as well as electron-deficient pyridines. More recendy, platinum-NHC complexes have been used in the selective C—H borylation of indoles (2015JAC12211). The authors reported higher isolated yields with this... 

Adsorption is of technical importance in processes such as the purification of materials, drying of gases, control of factory effluents, production of high vacua, etc. Adsorption phenomena are the basis of heterogeneous catalysis and colloidal and emulsification behaviour. 

Recently, in situ studies of catalytic surface chemical reactions at high pressures have been undertaken [46, 47]. These studies employed sum frequency generation (SFG) and STM in order to probe the surfaces as the reactions are occurring under conditions similar to those employed for industrial catalysis (SFG is a laser-based teclmique that is described in section A 1.7.5.5 and section BT22). These studies have shown that the highly stable adsorbate sites that are probed under vacuum conditions are not necessarily tlie same sites that are active in high-pressure catalysis. Instead, less stable sites that are only occupied at high pressures are often responsible for catalysis. Because the active... 

In tire preceding example, tire stmcture of tire catalyst combined witli reactants and products was detennined and tire data were used to infer a cycle. Stmctures of tire highly reactive intennediates in catalysis are generally elusive and infonnation about tliem based only on inference. In prospect, tire most incisive infonnation about tire workings of a catalyst can be obtained by observations of tire catalyst in action. The following example illustrates this... 

Ti, B, Ni, Cr, Fe, Co, Mn) has been described, as was the synthesis of nonsiliceous materials such as oxides of W, Fe, Pb, Mo, and Sb [18]. Although these materials do not represent tme zeolites, they are highly interesting materials which are commonly covered in the zeolite literature with great potential for shape-selective catalysis of bulky molecules. 

The use of indium in acpieous solution has been reported by Li and co-workers as a new tool in org nometallic chemistry. Recently Loh reported catalysis of the Mukaiyama-aldol reaction by indium trichloride in aqueous solution". Fie attributed the beneficial effect of water to a eg tion phenomena in connection with the high internal pressure of this solvenf This woric has been severely criticised by... 

Copper is clearly the most selective metal-ion catalyst. Interestingly, proton catalysis also leads to high selectivities. This is a strong indication that selectivity in this catalysed Diels-Alder reaction does not result from steric interactions. 

Inspired by the many hydrolytically-active metallo enzymes encountered in nature, extensive studies have been performed on so-called metallo micelles. These investigations usually focus on mixed micelles of a common surfactant together with a special chelating surfactant that exhibits a high affinity for transition-metal ions. These aggregates can have remarkable catalytic effects on the hydrolysis of activated carboxylic acid esters, phosphate esters and amides. In these reactions the exact role of the metal ion is not clear and may vary from one system to another. However, there are strong indications that the major function of the metal ion is the coordination of hydroxide anion in the Stem region of the micelle where it is in the proximity of the micelle-bound substrate. The first report of catalysis of a hydrolysis reaction by me tall omi cell es stems from 1978. In the years that... 

Interestingly, at very low concentrations of micellised Qi(DS)2, the rate of the reaction of 5.1a with 5.2 was observed to be zero-order in 5.1 a and only depending on the concentration of Cu(DS)2 and 5.2. This is akin to the turn-over and saturation kinetics exhibited by enzymes. The acceleration relative to the reaction in organic media in the absence of catalyst, also approaches enzyme-like magnitudes compared to the process in acetonitrile (Chapter 2), Cu(DS)2 micelles accelerate the Diels-Alder reaction between 5.1a and 5.2 by a factor of 1.8710 . This extremely high catalytic efficiency shows how a combination of a beneficial aqueous solvent effect, Lewis-acid catalysis and micellar catalysis can lead to tremendous accelerations. 

Chapter 5 also demonstrates that a combination of Lewis-acid catalysis and micellar catalysis can lead to accelerations of enzyme-like magnitudes. Most likely, these accelerations are a consequence of an efficient interaction between the Lewis-acid catalyst and the dienophile, both of which have a high affinity for the Stem region of the micelle. Hence, hydrophobic interactions and Lewis-acid catalysis act cooperatively. Unfortunately, the strength of the hydrophobic interaction, as offered by the Cu(DS)2 micellar system, was not sufficient for extension of Lewis-acid catalysis to monodentate dienophiles. 

Chapter 5 describes a study of the effect of micelles on the Diels-Alder reaction of 1 with 2. Literature studies on micellar catalysis of Diels-Alder reactions invariably failed to reveal significant accelerations. These results are unexpected, since most Diels-Alder reactants have a high affinity for... 

Chloroanisole and p-nitrophenol, the nitrations of which are susceptible to positive catalysis by nitrous acid, but from which the products are not prone to the oxidation which leads to autocatalysis, were the subjects of a more detailed investigation. With high concentrations of nitric acid and low concentrations of nitrous acid in acetic acid, jp-chloroanisole underwent nitration according to a zeroth-order rate law. The rate was repressed by the addition of a small concentration of nitrous acid according to the usual law rate = AQ(n-a[HN02]atoioh) -The nitration of p-nitrophenol under comparable conditions did not accord to a simple kinetic law, but nitrous acid was shown to anticatalyse the reaction. 

The catalysis was very strong, for in the absence of nitrous acid nitration was very slow. The rate of the catalysed reaction increased steeply with the concentration of nitric acid, but not as steeply as the zeroth-order rate of nitration, for at high acidities the general nitronium ion mechanism of nitration intervened. 

The thioboration of terminal alkynes with 9-(alkylthio)-9-borabicyclo[3.3.1]-nonanes (9-RS-9-BBN) proceeds regio- and stereoselectively by catalysis of Pd(Ph,P)4 to produce the 9-[(Z)-2-(alkylthio)-l-alkeny)]-9-BBN derivative 667 in high yields. The protonation of the product 667 with MeOH affords the Markownikov adduct 668 of thiol to 1-alkyne. One-pot synthesis of alkenyl sulfide derivatives 669 via the Pd-catalyzed thioboration-cross-coupling sequence is also possible. Another preparative method for alkenyl sulfides is the Pd-catalyzed cross-coupling of 9-alkyl-9-BBN with l-bromo-l-phe-nylthioethene or 2-bromo-l-phenylthio-l-alkene[534]. 

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