Air-stable catalysts

The ionic liquid butylmethylimidazolium tetrachloroferrate (bmim-FeCU) has been found to be a very effective and completely air-stable catalyst for the biphasic Grignard cross-coupling with primary and secondary alkyl halides bearing /3-hydrogens.93 The ionic liquid catalyst has been successfully recycled four times. 

Low hydrogen pressures Mild reaction temperatures Air-stable catalyst precursor... 

High ee s have been observed in analogous reactions with titanium catalysts derived from tartaric acid,56-81-90 and bisoxazolines.17-70-96-98-99 The development of a robust air-stable catalyst that... 

Reaction of alkenes with carbonyl compounds or carbonyl derivatives in the presence of Lewis acids, the ene reaction, enables the stereoselective preparation of highly functionalized compounds. Copper Lewis acids activate both aldehydes and imines in ene reactions. Evans has reported that Cu(II) Lewis acids catalyze glyoxylates in reactions with alkenes (Sch. 56) [103]. The homoallylic alcohols 257 and 259 are produced in high yield and enantioselectivity. The bis aquo complex 260 is a readily prepared and air-stable catalyst and gave high chemical yield and excellent selectivity in the ene reactions. Another point of note is that catalysts 260 and 261 furnish enantiomeric products even though they differ from each other only by the substituent at the 4-posi-tion of the oxazoline. 

The complex trans-RhX(CO)L2 (ref. 1) is an important type of rhodium complex, which is used as an air stable catalyst precursor in organic syntheses. A number of syntheses for these rhodium complexes have been summarized. Among recent methods of synthesis, the most useful, using RhCl(CO)L2, Ag2C03, NH4F, and MX (alkali metal salt), was reported by Vaska et al." ... 

The first problem may be drcumvented using air-stable catalysts such as Pd(OAc)2 [44] or Pd(dppf)Cl2. This mefhod has been shown to provide consistently high coupling yields with a variety of substrates [8, 9, 44]. The phosphine-free catalysts... 

In 2005, Fan and coworkers developed a highly effective and air stable catalyst system Ir/P Phos/l2 for the asymmetric hydrogenation of quinoline derivatives (Scheme 10.4) [6]. They found that THF was the best solvent that gave the highest enantioselectivity (92% ee). The reactions were carried out at room temperature, and a series of quinoline derivatives were examined with full conversions and excellent enantioselectivity. More important, the catalyst could be effectively immobilized in DMPEG with retained reactivity and enantioselectivity in eight catalytic runs. 

Anhydrous lithium salts are soluble in organic solutions which can be considered to be air-stable catalysts. The most common is lithium perchlorate which is generally used in solution in diethyl ether (LPDE) [9], nitromethane [15], and dichloro-methane [16]. Its catalytic role comes from the properties of the lithium cation [17] which gives rise to specific solute-Liinteractions modulated by complexation to appropriate solvents and counterions [18]. A recent proposal denies lithium catalysis and emphasizes electrostatic stabilization of the transition state by LPDE... 

Utilizing this principle, Heck reactions were performed [57]. The silica gel was derivatized with a CgHiy-trimethoxysilane to obtain the desired reversed phase properties, followed by treatment with palladium acetate and triph-enylphosphine in cyclohexane. After removal of the solvent, an air-stable catalyst supported on reversed-phase silica gel (RPSG) was formed. This supported catalyst was employed with variable success in Heck C-C coupling reactions, as depicted in Scheme 16. 

A highly efficient Pd(OA)2/guanidine aqueous system for the room temperature Suzuki cross coupling reaction was developed. The new water-soluble and air-stable catalyst from Pd(OA)2 and 2-butyl-1,1,3,3-tetramethlyguanidine was synthesized and characterized by X-ray crystallography [86]. The catalyst catalyses reaction of arylboronic acids... 

Buchwald reported a more cost-effective, high-yielding method for aryl thioether formation, including substrates for which the Palomo conditions were ineffective (2). Notably, this process employs a relatively low loading of an air-stable catalyst (Cul) and K2CO3 as the base. The simplicity of the reported conditions, the high degree of chemoselectivity and the broad substrate scope render this process an attractive choice for C-S bond formation [35]. 

An enantioselective 1,3-dipolar cycloaddition of nitrones 187 with ethyl vinyl ether 194 catalyzed by Brpnsted acid catalyst 195 was reported by Yamamoto and co-workers. Scheme 3.63 [80]. Only 5 mol% of this air-stable catalyst was used, and the reactions were completed within 1 h. The endo-selectivity of this cycloaddition is different to the previously reported cjto-selectivity of the aluminum-catalyzed reaction (Lewis acid catalysis). 

The use of air-stable catalyst, a combination of [Cp lrCl2]2 (222) with (223), for selective transfer hydrogenation of quinoxalines in H2O with HC02Na (H2 source) produced corresponding tetrahydroquinoxalines in good to excellent yields. The aqueous phase reduction was highly pH dependent, with acidic pH leading to better results. The HOAc/NaOAc buffer solution was used to maintain the pH of the reaction mixture at which optimum rate was obtained. ... 

Kumbhar et al. found that calcined and rehydrated MgAl-COs-LDH is a highly active, reusable and air stable catalyst for the cyanoethylation of primary and secondary alcohols with very high conversion and selectivity at 50°C (574). Chou-dary et al. reported that MgAl-tetra-butanoate-LDH can be directly used as catalyst in the cyanoethylation of alcohols and thiols with very high conversion and selectivity at room temperature (575). 

Phenols 26 substitute chloride in 2-chloro-4,6-dimethoxy-l,3,5-tri-azine 27 to give a series of corresponding 2-aryloxy-4,6-dimethoxy-l, 3,5-triazines 28 in high yields. This quite trivial reaction deserves special attention because the aryl—O bond in 28 can be selectively cleaved with inexpensive, air-stable catalyst NiCl2(dppf) in the Suzuki-Miyaura coupling of 28 with arylboronic acids.The reaction proceeds more easily in comparison with other aryl ethers or esters yielding different diaryls 29 in 50—90% yields (13JOC5078). 

While the formation of C—O bond in Fenton and Gif chemistry is well documented, the formation of C—N bonds through the activation of unactivated sp C—H bonds has been challenging. The primary nitrogen source in most reported procedures involve the use of nitrenes or their derivatives. " Much of this chemistry anploys the expensive late transition metals. It would be beneficial to employ inexpensive first row transition metals as catalysts to generate C—bonds. An early example that demonstrates this is an efficient, inexpensive, and air-stable catalyst/an oxidant (FeCl2 and A-bromosuccinimide or NBS) system that promotes amidation of ben-zylic sp C—H bonds in ethyl acetate under mild conditions. ... 

An air-stable catalyst, prepared from dimer 1 and M-hetero-cyclic carbene (iVA -bis(2,6-diisopropylphenyl)4,5-dihydro-imidazol)-2-ylidene), afforded a-arylated ketones from aryl chlorides, bromides, or triflates (eq 87). The reaction conditions are compatible with aryl or alkyl methyl ketones. 

Heck Reactions. The Heck reaction is a Pd-catalyzed olefi-nation usually performed between an aryl halide or triflate and an acrylate ester. While phosphines are traditionally used as ancillary ligands, new Pd(dba)2-mediated reactions have been performed with a variety of other ligand types. These include chelating Wheterocyclic carbene/phosphine ligands, benzimidazoles, and quinolinyl oxazolines. Air stable catalysts have been prepared from Pd(dba)2 and sterically hindered thiourea ligands (eq 24). An effective immobilized catalyst has been prepared from Pd(dba)2 and a dendritic phosphine-containing polymer. ... 

The ruthenium-catalyzed approach to this chemistry is particularly attractive due to the mild conditions, high selectivity, and the use of an air-stable catalyst [136]. A rhodium-catalyzed process using an easily handled catalyst precursor has also been reported [141]... 

In the year 2005, Nobel Prize in Chemistry was awarded to Yves Chauvin, Robert H. Grubbs, and Riehard R. Schrock for the development of the metathesis method in organie synthesis [41-43], It is a general view that, this reaction revolutionized different fields of synthetic chemistry. Chauvin proposed the carbene mechanism to explain this reaction [44] and Schrock prepared the first well-defined highly active metathesis catalysts [45], while, Grubbs developed ruthenium based active and air stable catalysts, which are tolerant to various functional groups in the presence of olefins [46]. 

To date, several different catalysts, both organocatalysts and metal-based catalysts, are available for the asymmetric Michael-type addition reactions. Indeed, a high level of achievement has been reached in terms of enanatioselectiv-ity and product yield. However, specihc windows for particular substrates, especially in natural product motif synthesis, are stdl in great demand. Thus, the exploration of more gen-eraL as well as more operationally simple (e.g., moisture stable and air stable), catalysts is attainable. Through the further in-depth structural investigation of catalyst-substrate interaction in Michael addition, a more sophisticated, yet more efficient, catalyst can be developed, and thus, the Turn Over Number (TON) can be expected to be increased. These future developments certainly will be fmitfiil to pharmaceutically and industrially related processes. 

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