Intermolecular reactions active catalysts

Section B gives some examples of metal-catalyzed cyclopropanations. In Entries 7 and 8, Cu(I) salts are used as catalysts for intermolecular cyclopropanation by ethyl diazoacetate. The exo approach to norbornene is anticipated on steric grounds. In both cases, the Cu(I) salts were used at a rather high ratio to the reactants. Entry 9 illustrates use of Rh2(02CCH3)4 as the catalyst at a much lower ratio. Entry 10 involves ethyl diazopyruvate, with copper acetylacetonate as the catalyst. The stereoselectivity of this reaction was not determined. Entry 11 shows that Pd(02CCH3) is also an active catalyst for cyclopropanation by diazomethane. 

Concept Most of the synthetic asymmetric catalysts show limited activity in terms of either enantioselec-tivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activate only one side of the substrate in an intermolecular reaction, whereas the latter can not... 

The development of catalytic asymmetric reactions is one of the major areas of research in the field of organic chemistry. So far, a number of chiral catalysts have been reported, and some of them have exhibited a much higher catalytic efficiency than enzymes, which are natural catalysts.111 Most of the synthetic asymmetric catalysts, however, show limited activity in terms of either enantioselectivity or chemical yields. The major difference between synthetic asymmetric catalysts and enzymes is that the former activate only one side of the substrate in an intermolecular reaction, whereas the latter can not only activate both sides of the substrate but can also control the orientation of the substrate. If this kind of synergistic cooperation can be realized in synthetic asymmetric catalysis, the concept will open up a new field in asymmetric synthesis, and a wide range of applications may well ensure. In this review we would like to discuss two types of asymmetric two-center catalysis promoted by complexes showing Lewis acidity and Bronsted basicity and/or Lewis acidity and Lewis basicity.121... 

A development of the last two decades is the use of Wacker activation for intramolecular attack of nucleophiles to alkenes in the synthesis of organic molecules [9], In most examples, the nucleophilic attack is intramolecular, as the rates of intermolecular reactions are very low. The reaction has been applied in a large variety of organic syntheses and is usually referred to as Wacker (type) activation of alkene (or alkynes). If oxygen is the nucleophile, it is called oxypalladation [10], Figure 15.4 shows an example. During these reactions the palladium catalyst is often also a good isomerisation catalyst, which leads to the formation of several isomers. 

The use of mediators to improve reactivity or selectivity in nitrone cycloaddition chemistry begins with the nitrone generation step. As is well known, the N-alkyla-tion of oximes provides one of the most direct and convenient synthetic routes to N-alkylated nitrones from readily available aldehydes and ketones. Electrophilic mediators have been employed to activate alkenes for N-alkylation, both in intramolecular and intermolecular reactions. They include activation of the internal alkene function by the action of (a) strong nonmetallic electrophiles such as phenyl-selenenyl sulfate (159), and (b) metallic catalysts such as Ag(I) (160) and Pd(II) ions... 

This chapter will focus on preparative applications of iron catalysts in the synthesis of cyclic ring systems by means of intra- and intermolecular cycloadditions, Alder-ene reactions and ring expansion reactions. Previous reviews concerning iron-catalyzed chemistry have either focused their attention on different aspects involving iron-containing compounds as active catalysts [1] or have concentrated on certain reactions [2] and on the synthesis of specific substance classes [3]. A more recent general review concerning all aspects of modern applications in iron-catalyzed reactions has been summarized by Bolm et al. [4]. 

Although arylation or alkenylation of active methylene compounds can be carried out using a Cu catalyst, the reaction is sluggish. However, the arylation of malononitrile (390) or cyanoacetate proceeds smoothly in the presence of a base and Pd catalysts [189], Tetracyanoquinodimethane (392) is prepared by the coupling of / -diiodoben-zene with malononitrile (390) to give 391, followed by oxidation [190], Presence of the cyano group seems to be essential for intermolecular reactions. However, the intramolecular arylation of malonates, / -keto esters and /i-diketones proceeds smoothly [191]. The bromoxazole 393 reacts with phenylsulphonylacetonitrile (394)... 

The additive-modified catalyst mixture developed for intramolecular olefin coupling is sufficiently active to catalyze the analogous intermolecular reaction. A variety of five-membered N-heterocydes can be catalytically alkylated at the C-2 position (Table 2) [8], Additional functional groups on the heterocyde[15] (Table 2, Entry 1) and on the olefin (Table 3) are well tolerated. Products corresponding to linear addition are usually obtained exclusively, even when the olefin is rapidly isomerized under the reaction conditions. 

This chapter exemplifies how the development of highly active catalysts and creative reaction design have revitalized the interest in Stetter reactions as a valuable tool for efficient C-C-bond-forming reactions. The next challenges to be met in this field are the reduction of the catalyst loading and the catalytic asymmetric intermolecular Stetter reactions. 

During our investigation of the reactivity of the 10 % Pd/C catalyst, we found that le reacts with itself forming dimeric compounds (Scheme 32). This problem was solved by slow addition of the silane to a mixture of the alcohol and catalyst in the second alcoholysis step. We proposed that the silyl ether is activated by the catalyst and intramolecular dimerization occurs on the surface of the catalyst (Figure 8). This reaction is faster than the usual intermolecular reaction of the incoming alcohol with the silane, resulting in the dimeric products. 

Following their works on immobilized heterobimetallic nanoparticle catalysts, Chung s group has synthesized Ru/Co nanoparticles immobilized in charcoal and shown the ability of this system to catalyze a PKR-type reaction in the presence of pyridylmethyl formiate as a CO source. They used these conditions with intra- and intermolecular reactions and showed that the catalyst can be reused without loss of catalytic activity (Scheme 40) [146]. 

Thus, surface effects and adsorption equilibria can dramatically influence the relative reactivity of photoelectrochemical transformations. Not only does the surface effectively control the movement of reagents from the electrolyte to the photo-activated surface and the desorption of products (avoiding overreaction or complete mineralization), but it also influences the stability and accessibility of photogenerated intermediates toward secondary intermolecular reactions [87]. Because the efficiency of diffusion and mass transfer to and from the photocatalyst surface depends on the solvent and catalyst pretreatment, quantitative predictions of photocatalytic reactions have proved to be difficult, although the qualitative principles governing each step of these events can be easily recognized. 

As nitrobenzene is the result of the oxidation of NSB, this indicated that the latter did not react with phenylhydroxylamine to form AZY. One explanation might be that the products were strongly adsorbed on the surface of V-containing catalysts, thus preventing intermolecular reactions. However, this did not exclude the possibility of a mechanism involving two TBHP molecules over V active sites, transforming directly PH into NB. 

Additional catalytic investigation of p-diketiminate scandium complexes by Piers and coworkers showed that well-characterized complexes 121 and 122 with the bulky ligand L27 were highly active catalysts for intramolecular hydroamination to form nitrogen heterocycles. The catalytic reaction was monitored by determining starting material and product with NMR. Both the neutral complex 121 and the CIP complex 122 are effective catalysts (10 mol%) for the intramolecular hydroamination of 5-phenyl-4-pentyl-l-amine (R = H, R = Ph, n = 1 in Scheme 42). However, they are not active catalysts for the potential application to the intermolecular hydroamination of 1-hexyne with alkylamines [82],... 

In general, however, the activity of the triazolium salts in this asymmetric Stetter protocol is quite low, i.e. the total turnover numbers obtained ranged from 0.5 to 8. The development of more active catalysts which are also suitable for the intermolecular Stetter reaction is desirable. 

The microwave-assisted asymmetric Heck reaction has also been used with success for intermolecular bond formation (Scheme 2.28). Using 2,3-dihydrofuran and phenyl triflate as model substrates, the most active catalyst screened was a combination of Pd2dba3 and a phosphine-thiazole supporting ligand. Heating a THF/DIPEA... 

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