Reagents Recently

The most common method of epoxidation is the reaction of olefins with per-acids. For over twenty years, perbenzoic acid and monoperphthalic acid have been the most frequently used reagents. Recently, m-chloroperbenzoic acid has proved to be an equally efficient reagent which is commercially available (Aldrich Chemicals). The general electrophilic addition mechanism of the peracid-olefin reaction is currently believed to involve either an intra-molecularly bonded spiro species (1) or a 1,3-dipolar adduct of a carbonyl oxide, cf. (2). The electrophilic addition reaction is sensitive to steric effects. 

Due to the extensively represented oxidative behaviour of the carbenium ions as hydride abstractor or one-electron oxidant [157], attempts were made to employ the carbocations as reagents. Recently the enantioselective outcome in a hydride transfer reaction was reported [158, 159]. The abstraction of the exo hydrogen atoms from the tricarbonyliron complex 57 resulted in a yield up to 70% and enantiose-lectivity of 53% (Scheme 62) [158]. 

Another reagent, recently used for C2-elongation of a-amino aldehydes, is iso-... 

Coupling of functional groups onto the walls of mesoporous silica materials can also be achieved by surfactant-templated assembly of an organosilicon alkoxide and silicon alkoxide reagents. Recent reports that use this synthetic strategy include hybrids of methacrylate-mesoporous silica [104], phenyl-amino (thiol or allyl)-mesoporous silica [105], and organic chromo-phore-mesoporous silica [106]. 

One drawback associated with this type of chromium species is the frequoit requirement for a large excess of reagent. Recent attempts to combat this problem have involved the use of a PCC-celite mixture in benzene solution under reflux and more successfully a r-butyl hydroperoxide-pyridinium dichromate mixture (equation 33). ... 

The alkene pseudohalogen adducts (15) of Scheme 8 are also useful intermediates for aziridine synthesis. These adducts are discussed later in Sections 3.S.6.2-4. The iodine azide and bromine azide adducts may be reduced to aziridines with many reagents recent references report use of lidiium aluminum hydride - and dimethylamineborane. The iodine isocyanate aziridination continues to prove useful, as in Scheme Since the recent reviews, - the mechanism of the triphenylphosphine-based... 

Finally, using PNA as an affinity capture reagent recently was extended to probing RNA-protein complexes (RNPs) in cells (33). In this application, the PNA is functionalized with a peptide that allows uptake into cells and is complementary to an RNA component of an RNP. The PNA also bears two affinity tags, the first of which is a benzophenone-modifled phenylalanine residue that can photocross-link the PNA to a protein present in the RNP. The second tag is a biotin group, which allows the purification of the cross-linked PNA-protein. Subsequent analysis by mass spectrometry identifies both the protein and its cross-linking site. As is the case for PNA used to deliver a fluorophore to a specific site in an RNA, this method requires that the PNA not disrupt the structure being probed. 

The metallic and Ionic cleaning procedures studied by Kern were evaluated by radiotracer methods. This technique requires the use of radionuclide enriched reagents. Recently, a procedure utilizing secondary Ion mass spectrometry (SIMS) has been developed for studying very low levels of contamination on silicon surfaces (18). 

Effenberger, R 1980. Electrophilic reagents—recent developments and their preparative applications. Angew. Chem., Int. Ed. Engl. 19 151-171. 

The necessity of the cisoid conformation was questioned since 2-cyelohexenone, which has a transoid conformation, was able to form 1,4-adducts with various Grignard reagents. Recently it was found, however, that methyl- and phenyl-magnesium bromide are unable to add 1,4 to an O, /l-unsaturated substrate unless the cisoid conformation is possible 1861. Isopropyl- and i-bulylmagnesiuni chloride do, however, react 1,4, but by a homolylic mechanism. 

Surprisingly, there have been only few synthetic studies on polymer-supported asymmetric superbase reagents. Recently, Wannaporn and Ishikawa prepared a new chiral guanidine based polymer catalyst and applied it to the asymmetric Michael addition reaction of iminoacetate with methyl vinyl ketone [39] (Scheme 6.7). Although the catalyst shows only moderate levels of reactivity and enantioselectivity, the result demonstrates the possibility of expanding an exciting field of asymmetric synthesis using polymer-supported chiral superbase catalysts. 

Although the introduction of silyl moieties is usually accomplished by electrophilic silyl reagents, recently their anionic congeners began to enjoy more and more popularity, as they are readily available by metal-halogen exchange or by cleavage reactions. 

Nucleophilic Substitution of Alcohols. The use of alcohols as electrophiles instead of halides and acetate compounds is an ideal method to prevent waste salt formation (Fig. 5). However, cataljdic substitution of the hydroxyl group in alcohols is difficult due to their poor leaving ability, which requires equimolar or greater amounts of reagents. Recently, several homogeneous catalysts, such as NaAuCh, InCla, ZrCh, La, Yb, Sc, Hf triflate, BCCeFsls, BF3, and p-toluenesulfonic acid have been used for nucleophilic substitution reactions of alcohols with amides (33), 1,3-dicarbonyl compounds (34), and allylsilanes (35). However, these catalysts are often limited by low catalytic activity and selectivity, can be difficult to reuse, and require the use of halogenated solvents. 

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