Copper hydride, catalysis

The successes described above notwithstanding, synthetic chemistry in the 1990s was in large measure characterized by catalysis , which encouraged development of organocopper processes that were in line with the times. The cost associated with the metal was far from the driving force that was more (and continues to be) a question of transition metal waste. In other words, proper disposal of copper salt by-products is costly, and so precludes industrial applications based on stoichiometric copper hydrides. 

From a chemical point of view, the 1,2-reduction of the enone involves nucleophilic attack of a hydride to the carbonyl moiety. In theory, nucleophiles other than hydride (e.g. carbon nucleophiles) could also be added to the carbonyl under copper (I) catalysis. This was achieved by the groups of Harutyunyan and Minnaard using Grignard reagents [10]. 

In 1992 Murahashi, Hosokawa, and co-workers described the anti-Markovnikov oxidative addition of amides and carbamates to electron-deficient olefins by applying a palladium and copper cooperative catalysis under oxygen atmosphere [41]. The proposed mechanism involved a ff-bonded palladium(II) intermediate resulting from the addition of the nucleophile to the olefin, and subsequent ) -palladium hydride elimination to yield the functionalized alkene. Interestingly, both lactams and cyclic carbamates gave predominantly the corresponding E-enamide derivatives. Acyclic amides, conversely, afforded ElZ mixtures of products. The addition of a catalytic amount (5 mol%) of hexamethylphosphoric triamide (HMPA) was found notably beneficial for the reaction of 5-membered lactams and reduced the reaction time of such particular oxidative amidations (Scheme 2). 

Cobalt B12 Enzymes Coenzymes Copper Hemocyanin/Tyrosinase Models Heterogeneous Catalysis by Metals Hydride Complexes of the Transition Metals Hydrocyanation by Homogeneous Catalysis Hydrogen Inorganic Chemistry Mechanisms of Reaction of Organometalhc Complexes Nickel OrganometaUic Chemistry Ohgomerization Polymerization by... 

Multicomponent metallic hydrogenation catalysts, based on intermetallic compounds (IMC) of rare-earth elements with nickel, copper, cobalt, and other bimetallic systems. Most studies were devoted to two structural systems LnMs and LnMs, where Ln = La, Sm, Gd, Ce, Pr, and Nd and M = Ni (see Klabunovskii, Konenko s group 183,251,252 Compaiison of LnNis catalysts with Ni catalysts supported on oxides of Ln, show higher activities of the IMC s and their hydrides in hydrogenation of propene (100°C, 1 bar), where LaNis proved to be the most active catalyst... 

Copper has a long history in chemistry. Nevertheless, Al-heterocycUc carbene (NHC)-copper systems have been known and used only these last 20 years since Arduengo et al. reported the first NHC-copper system in 1993 [1]. Since then, the NHC-copper chemistry has undergone continuous expansion with the synthesis of new complexes (well-defined systems, hydrides, hydroxides, and cationic species) and the development of various applications (catalysis, transme-talation reagents, antitumor reagents, etc.). NHC-copper systems have become an example of a best-seller in organometallic chemistry. 

The synthesis of indoles and benzofiirans by the palladium-catalysed cyclization of o-alkynyl anilines and phenols can also form part of tandem processes through interception of the -intermediate (Scheme 6.41). Inclusion of an alkene results in a Heck process, giving 3-vinylated indoles 6.129." As the 3-hydride elimination step causes reduction of the palladium(II) to palladium(O), an oxidizing agent, such as copper(n) must be included to maintain catalysis. The intermediate may also be intercepted by alkoxycarbonylation, giving an ester 6.130." Again an oxidant is needed. 

Other metals can catalyze Heck-type reactions, although none thus far match the versatility of palladium. Copper salts have been shown to mediate the arylation of olefins, however this reaction most probably differs from the Heck mechanistically. Likewise, complexes of platinum(II), cobalt(I), rhodium(I) and iridium(I) have all been employed in analogous arylation chemistry, although often with disappointing results. Perhaps the most useful alternative is the application of nickel catalysis. Unfortunately, due to the persistence of the nickel(II) hydride complex in the catalytic cycle, the employment of a stoichiometric reductant, such as zinc dust is necessary, however the nickel-catalyzed Heck reaction does offer one distinct advantage. Unlike its palladium counterpart, it is possible to use aliphatic halides. For example, cyclohexyl bromide (108) was coupled to styrene to yield product 110. 

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