Carbenes catalysis, substitution

Chiang PC, Bode JW (2011) N-mesityl substituted chiral triazoliiun salts opening a new world of N-heterocyclic carbene catalysis. TCI MAIL 149 2-17... 

Incorporation of the new catalysis concept and N-heterocyclic carbene catalysis into multicatalytic systems emerged as a new direction to build up a complex scaffold such as those often found in natural products. Liu et al. developed an enantioselective Diels-Alder reaction via trienamine catalysis of indole-2,3-quinodimethane and activated alkenes. More recently, they combined this Diels-Alder process with N-heterocyclic carbene catalysis for the rapid generation of chiral cyclopentanone-fused tetrahydrocarbazoles with diverse substitution [30]. Mechanistically, with the... 

Rhodium(II) acetate was found to be much more superior to copper catalysts in catalyzing reactions between thiophenes and diazoesters or diazoketones 246 K The outcome of the reaction depends on the particular diazo compound 246> With /-butyl diazoacetate, high-yield cydopropanation takes place, yielding 6-eco-substituted thiabicyclohexene 262. Dimethyl or diethyl diazomalonate, upon Rh2(OAc)4-catalysis at room temperature, furnish stable thiophenium bis(alkoxycarbonyl)methanides 263, but exclusively the corresponding carbene dimer upon heating. In contrast, only 2-thienylmalonate (36 %) and carbene dimer were obtained upon heating the reactants for 8 days in the presence of Cul P(OEt)3. The Rh(II)-promoted ylide formation... 

As mentioned in Sections 3.1.6 and 4.1.3, cyclopropenes can also be suitable starting materials for the generation of carbene complexes. Cyclopropenone di-methylacetal [678] and 3-alkyl- or 3-aryl-disubstituted cyclopropenes [679] have been shown to react, upon catalysis by Ni(COD)2, with acceptor-substituted olefins to yield the products of formal, non-concerted vinylcarbene [2-1-1] cycloaddition (Table 3.6). It has been proposed that nucleophilic nickel carbene complexes are formed as intermediates. Similarly, bicyclo[1.1.0]butane also reacts with Ni(COD)2 to yield a nucleophilic homoallylcarbene nickel complex [680]. This intermediate is capable of cyclopropanating electron-poor alkenes (Table 3.6). 

Abstract Allenylidene complexes have gained considerable significance in the context of transition-metal carbene chemistry due to their potential applications in organic synthesis. The aim of this chapter is to draw together a general presentation of the most efficient synthetic routes, the main structural features and reactivity patterns, as well as current applications in homogeneous catalysis, of aU-carbon-substituted allenylidenes and related cumulenylidene complexes containing an odd number of carbon atoms. 

Keywords Absolute configuration, Amines, Amino acids, Carbenes, Cascade reactions, 2-chloro-2-cyclopropylideneacetates. Combinatorial libraries. Cycloadditions, Cyclobutenes, Cyclopropanes, Diels-Alder reactions. Heterocycles, Michael additions. Nitrones, Nucleophilic substitutions, Peptidomimetics, Palladium catalysis. Polycycles, Solid phase synthesis, Spiro compounds. Thiols... 

Arasabenzene, with chromium, 5, 339 Arcyriacyanin A, via Heck couplings, 11, 320 Arduengo-type carbenes with titanium(IV), 4, 366 with vanadium, 5, 10 (Arene(chromium carbonyls analytical applications, 5, 261 benzyl cation stabilization, 5, 245 biomedical applications, 5, 260 chiral, as asymmetric catalysis ligands, 5, 241 chromatographic separation, 5, 239 cine and tele nucleophilic substitutions, 5, 236 kinetic and mechanistic studies, 5, 257 liquid crystalline behaviour, 5, 262 lithiations and electrophile reactions, 5, 236 as main polymer chain unit, 5, 251 mass spectroscopic studies, 5, 256 miscellaneous compounds, 5, 258 NMR studies, 5, 255 palladium coupling, 5, 239 polymer-bound complexes, 5, 250 spectroscopic studies, 5, 256 X-ray data analysis, 5, 257... 

Oxazoles of various substitution patterns are well known heterocycles for which a number of methods of synthesis have been reported.129 Acyl carbenes or functionally equivalent species have been found to undergo cyclization with nitriles to give oxazoles in high yield via nitrile ylide intermediates.130,131 This reaction can be induced to occur under thermal, photolytic, or catalytic conditions.129,132,133 Huisgen and coworkers were the first to study this process in some detail.132 Thermolysis (or copper catalysis) of a mixture of ethyl diazoacetate and benzonitrile resulted in the formation of oxazole 254. The isolation of this product is... 

Bipyridocarbene 9a was first synthesized by Weiss et al. and is a very electron-rich NHC (Fig. 4) [76,77]. This can be seen from the very strong high-held shift of its carbene signal in the 13C NMR spectrum at 196 ppm [78]. However, the lability of this compound hinders its application in catalysis. Kunz et al. recognized that ferf-butyl substitution results in the formation of more stable NHC 9b, which has very recently allowed the first X-ray structural analysis of these types of carbenes [27]. 

Because these insertion reactions create new bonds at completely unfunctionalized centres, they can be very useful in synthesis. This next carbene is created between two carbonyl groups from a diazocompound with rhodium catalysis and selectively inserts into a C-H bond five atoms away to form a substituted cyclopentanonc. 

Apart form a great number of chiral NHC carbenes that have been used as ligands in enantioselective transition-metal catalysis (Gade and Bellemin-Laponnanz 2007), some less usual heterazolium salts have been tested in organocatalytic transformations. A planar-chiral thia-zolium salt (Pesch et al. 2004) and a rotaxane-derived precatalyst were reported (Tachibana et al. 2004), as well as catalytically active peptides containing an unnatural thiazolium-substituted alanine amino acid (Fig. 3 Mennen et al. 2005a,b). 

Analogous to epoxides, aziridines can be prepared by the methylenation of imines. In this case, ethyl diazoacetate is the most common source of carbenes. For example, the imine derived from p-chlorobenzaldehyde 148 is converted to the c/j-aziridinyl ester 149 upon treatment with ethyl diazoacetate in the presence of lithium perchlorate <03TL5275>. These conditions have also been applied to a reaction medium of the ionic liquid l-n-butyl-3-methylimidazolium hexafluorophosphate (bmimPFe) with excellent results <03TL2409>. An interesting enantioselective twist to this protocol has been reported, in which a diazoacetate derived from (TJ)-pantolactone 150 is used. This system was applied to the aziridination of trifluoromethyl-substituted aldimines, which were prepared in situ from the corresponding aminals under the catalysis of boron trifluoride etherate <03TL4011>. 

Keeping our target of homogeneous catalysis in mind, we adopted a broad definition of organometallic complexes and included compounds without metal-carbon bonds (e. g., metal-phosphine and metal-nitrosyl complexes) as far as they retain the structural and reactivity features of typical organometallic compounds. Recent developments, e. g., the substitution of phosphine by carbene ligands (cf Section 3.1.1.1), support the validity of this view. 

The reaction of either isomeric esters 4 with bromoform under phase-transfer catalysis conditions did not yield a tetrabromospiropentane derivative, the product of further transformations of the carbene adducts. The corresponding tetrachlorospiropentanes were formed if j8-substituted aerylates were allowed to react with chloroform under phase-transfer catalysis conditions (see Section 1.2.1.4.3.1.6.1.). 

Transient phosphoryl-substituted carbenes, generated by photolysis or by copper catalysis, react selectively with one double bond of a 1,3-diene affording mixtures of trans- and c -isomers which, in some cases, can be separated by column chromatography on silica gel or by distillation (Table3). ... 

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