Carbon organometallic reactions

Organometallic Reactions (Carbon-Carbon Bond-forming Reactions)... 

The first examples of the use of palladium as a catalyst for carbon-carbon coupling reactions were reported almost thirty years ago [14], and over recent decades a massive effort has been devoted to the extension of the scope of palladium-catalyzed reactions. Organic and organometallic chemists have received extensive input from palladium-coordination chemistry in the task of understanding the mechanisms behind these efficient synthetic procedures [14]. 

Copper has long played a dominant role in stoichiometric organometallic reactions in synthesis. Organocuprate mediated conjugate addition reactions are a cornerstone of carbon-carbon bond-forming reactions. Its preeminence has not been overlooked in the search for asymmetric versions of the reaction (134-136). However, the requirement for stoichiometric amounts of the metal has dampened efforts to introduce chirality into this process. 

Carbonvlation of Benzyl Halides. Several organometallic reactions involving anionic species in an aqueous-organic two-phase reaction system have been effectively promoted by phase transfer catalysts(34). These include reactions of cobalt and iron complexes. A favorite model reaction is the carbonylation of benzyl halides using the cobalt tetracarbonyl anion catalyst. Numerous examples have appeared in the literature(35) on the preparation of phenylacetic acid using aqueous sodium hydroxide as the base and trialkylammonium salts (Equation 1). These reactions occur at low pressures of carbon monoxide and mild reaction temperatures. Early work on the carbonylation of alkyl halides required the use of sodium amalgam to generate the cobalt tetracarbonyl anion from the cobalt dimer(36). 

After the discovery of the first terminal vinylidene-metal complex in 1972, it was established that the stoichiometric activation of terminal alkynes by a variety of suitable metal complexes led to 1,2-hydrogen transfer and the formation of metal-vinylidene species, which is now a classical organometallic reaction. A metal-vinylidene intermediate was proposed for the first time in 1986 to explain a catalytic anti-Markovnikov addition to terminal alkynes. Since then, possible metal-vinylidene intermediate formation has been researched to achieve catalytic regiose-lective formation of carbon-heteroatom and carbon-carbon bonds involving the alkyne terminal carbon. 

Combinatorial chemistry and solid-phase synthesis have evolved in the last decade to become one of the most important techniques to save time for drug discovery. To reach its full potential, the solid-phase synthesis has to incorporate many versatile organometallic reactions developed over recent several decades. The first example of the Nicholas reaction on solid phase was reported by Kann and his co-workers in 2002, which involves the reaction of polymer-bound cobalt complexes 51 with various carbon-centered nucleophiles in the presence of a Lewis acid to... 

Reactions other than Lewis acid-base associations/dissociations are frequently observed wit donor molecules, leading notably to solvolysis, oxygen or sulfur abstraction, insertion reaction and carbon-carbon coupling reactions. The tendency to form metal-element multiple bonds i remarkable in this respect the activation of dinitrogen by tantalum or niobium is unique. Th formation and chemistry of constrained reactive metallacycles open another promisin fast-developing area, on the frontier with organometallic chemistry. 

SCHORIG1N (or Shorygin) REACTION. Organometallic reactions of the Grignard type, employing sodium in place of magnesium the reaction of alkyl sodium compounds with carbon dioxide to give monobasic acids is sometimes known as the Wanklyn reaction. 

When a nucleophile containing a heteroatom reacts at a carboxyl carbon SN, reactions occur that convert carboxylic acid derivatives into other carboxylic acid derivatives, or they convert carbonic acid derivatives into other carbonic acid derivatives. When an organometallic compound is used as the nucleophile, SN reactions at the carboxyl carbon make it possible to synthesize aldehydes (from derivatives of formic acid), ketones (from derivatives of higher carboxylic acids), or—starting from carbonic acid derivatives—carboxylic acid derivatives. Similarly, when using a hydride transfer agent as the nucleophile, SN reactions at a carboxyl carbon allow the conversion of carboxylic acid derivatives into aldehydes. 

The nature of the electrode plays a significant role in the direction and often the products of electrochemical processes, particularly reduction. Metals that can form relatively stable organometallics with the substrate under study often intervene directly to produce a product like that of direct organometallic reaction. The electro reduction of alkylmercury halides was studied on Pt, Hg and carbon electrodes. Whereas at Pt and carbon electrodes two-electron reduction was observed, at mercury-coated electrodes multistep reduction occurred and RHgHgX was observed213. 

Redistribution of electron density in CT complexes results in a modification of the chemical properties of coordinated arenes, and this effect is widely used in organometallic catalysis [2]. To demonstrate the relationship between charge transfer in arene complexes and their reactivity, we focus our attention on carbon-hydrogen bond activation, nucleophilic/ electrophilic umpolung, and the donor/acceptor properties of arenes in a wide variety of organometallic reactions. 

For these complexes, the rate of carbonylation increases rapidly with increasing tr-alkyl character. Thus, (5-cis-butadiene)ZrCp2 (5a) is carbony-lated 2.5 times faster than the zirconocene complex of 1,2,5,6-tetramethyl-3,4-bis(methylene)tricyclo[3.1.0.0 ] hexane (51) (ambient temperature and 1 bar CO pressure) taken as a standard. Introduction of methyl groups at the internal carbon centers C2/C3 of the diene chain increases the carbonylation rate by a factor of 5 with phenyl groups at these positions increasing the rate by a factor of 250 (Table VIII). The organometallic reaction products have yet to be isolated or completely identified. Ultimately, zirconium enolate complexes (20) are probably formed in these... 

The secondary reduction of the terminal radical by Sml2 generates samarium alkyl species which are suitable for classical organometallic reactions, e.g. protonation, acylation, reactions with carbon dioxide, disulfides, diselenides, or the Eschenmoser salt. A broad variety of products is available (hydroxy-substituted alkanes, esters, carboxylic acids, thioethers, selenoethers, tertiary amines) by use of the double-redox four-step (reduction-radical reaction-reduction-anion reaction) route (Scheme 20) [73]. 

The unique antagonistic features of the (butadiene)zirconocene isomers 3a/5a have been used as a probe for the elucidation of organometallic reaction mechanisms. In some cases it was possible to distinguish between mechanistic alternatives by simply allowing the isomeric substrates 3 and 5 to compete for a reagent. An example is as follows. Transition metal-induced C—C coupling between a conjugated diene and an olefin can occur by two basically different types of reaction sequence. Either a new C— C bond can be formed by olefin insertion into a metal-carbon bond of a ( T-allyl)M-type intermediate (24) (95), or, alternatively, the alkene may... 

The C—I bond is very unstable and more reactive than C—Br, C—Cl and C—F bonds. Iodine is the most expensive of the common halogens and is much less frequently used in synthesis than bromine, chlorine or fluorine. Organometallic reactions proceed with iodinated aliphatic or aromatic compounds more easily than with the other halogens. Noble metal catalysis with palladium complexes is most effective with iodinated compounds. A useful synthetic procedure is the facile reduction of iodinated derivatives under mild conditions. Replacement of iodine by hydrogen at an sp carbon is an exothermic reaction with A// = -25 kJ mol . ... 

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