Hydride Elimination Dependent Reactions

The sodium hydride eliminated in Reaction 5-73 may also participate in hydrogen abstraction from XXI. The order of stability of polystyryl carbanions depends on the alkali metal in the order K > Na > Li. 

Intramolecular insertion of alkenes into ir-allylpalladium complexes has been shown to proceed regio-selectively to yield five-membered rings.2621263 TTiis transformation is equivalent to a palladium ene reaction and is completed by a (3-hydride elimination. The C==C bond of ketenes is also capable of undergoing insertion into ir-allylpalladium intermediates.264 The final course of this reaction is dependent on the leaving group in the initial allyl-X precursor. For X = OAc, dienes are observed for X = Me0C02, a methoxycarbonyl adduct is obtained (equation 303).264... 

The most important feature of organocobalt cyclizations is that a variety of functionalized products can be obtained, depending on the nature of the substrate and the reaction conditions. The most common transformation has been formation of an alkene by cobalt hydride elimination. Alkenes are often formed in situ during the photolysis, and with activated alkene acceptors the formation of these products by cobalt hydride elimination is very facile. Scheme 31 provides a representative example from the work of Baldwin and Li.143 The alkene that is formed by cobalt hydride elimination maintains the correct oxidation state in the product (54) for formation of the pyrimidone ring of acromelic acid. Under acidic conditions, protonation of the cyclic organocobalt compound may compete 144 however, if protonated products are desired, the cyclization can probably be conducted by the reductive method with only catalytic quantities of cobalt (see Section 4.2.2.2.2). 

The reaction proceeds well with unhindered secondary amines as both nucleophiles and bases. The yield of allylic amine formed depends upon how easily palladium hydride elimination occurs from the intermediate. In cases such as the phenylation of 2,4-pentadienoic acid, elimination is very facile and no allylic amines are formed with secondary amine nucleophiles, while phenylation of isoprene in the presence of piperidine gives 29% phenylated diene and 69% phenylated allylic amine (equation 30).84 Arylation occurs at the least-substituted and least-hindered terminal diene carbon and the amine attacks the least-hindered terminal ir-allyl carbon. If one of the terminal ir-allyl carbons is substituted with two methyl groups, however, then amine substitution takes place at this carbon. The reasons for this unexpected result are not clear but perhaps the intermediate reacts in a a- rather than a ir-form and the tertiary center is more accessible to the nucleophile. Primary amines have been used in this reaction also, but yields are only low to moderate.85 A cyclic version occurs with o-iodoaniline and isoprene.85... 

Various mechanisms for the aerobic oxidation of alcohols catalysed by (NHC)Pd (carboxylate)2(H20) complexes [NHC = l,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] were investigated using DFT combined with a solvent model. Of these, reductive j3-hydride elimination, in which the -hydrogen of a palladium-bound alkoxide is transferred directly to the free oxygen of the bound carboxylate, provided the lowest-energy route and explained the published kinetic isotope effect, activation enthalpy, reaction orders, and dependence of rate on carboxylate pKa.26S... 

In general, the termination reactions of these polymerizations are not well understood but, depending upon the metal and the monomer, reductive coupling of the metal carbene fragments to give alkene and reduced metal complexes is one possibility. Another termination reaction appears to be initiated by -Hydride Elimination from the carbene complex. These mechanisms have been observed in well-defined catalyst systems, and are possible in the ill-defined systems also. The fact that most catalysts are sensitive to oxygen and moisture (or other proton sources) means that termination of the polymer chain by added or adventitious sources of water is a common problem, especially for the ill-defined catalysts. 

The effectiveness of an oxidant is dependent upon the oxidation-reduction potential of the intermediate a-adduct and the oxidant (Scheme 10) (76RCR454). For the sake of simplicity, pyridine is used to represent a TT-deficient nitrogen aromatic heterocycle. When the a-adduct is formed readily (k, k i) and hydride elimination is facile (kj is large), the product is formed easily. In cases where the adduct formation is facile (k, k.J but hydride elimination is very slow (kj is small), the reaction proceeds to the stage of a-adduct formation and stops there. When [Pg.10]

Iridium-catalyzed intramolecular l,n-enyne metathesis has been studied as a unique tool for the synthesis of various types of cyclic compounds. Reactions of this type depend on both the structure of substrates and the nature of catalyst systems used (411). Recently, the cycloisomerization of various 1,6-enynes have been shown to be catalyzed by [Ir(cod)Cl]2/dppf (494). These reactions are highly stereoselective, and generate the (Z)-isomer preferentially over the ( )-isomer (Scheme 63). The proposed mechanism (Scheme 64) involves oxidative cyclization of the enyne at Ir(I) to give the trivalent iridacyclopentene. The intermediate undergoes (3-hydride elimination to give the irida-1,3-diene, which experiences steric repulsion between the metal fragment and the cis substituent on the... 

The Mizoroki-Heck reaction is a subtle and complex reaction which involves a great variety of intermediate palladium complexes. The four main steps proposed by Heck (oxidative addition, alkene insertion, )3-hydride elimination and reductive elimination) have been confirmed. However, they involved a considerable number of different Pd(0) and Pd(Il) intermediates whose structure and reactivity depend on the experimental conditions, namely the catalytic precursor (Pd(0) complexes, Pd(OAc)2, palladacycles), the Ugand (mono- or bis-phosphines, carbenes, bulky monophosphines), the additives (hahdes, acetates), the aryl derivatives (ArX, ArOTf), the alkenes (electron-rich versus electron-deficient ones), which may also be ligands for Pd(0) complexes, and at least the base, which can play a... 

The insertion reactions of ethene into the nickel-carbon bond of four-membered oxanickelacycles 25 leads to the expected derivatives 50, which depending on the workup conditions, yield protonolytic or P-hy-dride elimination products (Scheme 16)7 On the other hand, azanickelacycle 51, formed in the nickel(0)-promoted reaction of ethene and phenylisocyanate, reacts with a second molecule of ethene to yield an insertion product 52, which afford a secondary carboxamide by p-hydride elimination (Eq. 21). The process is carried out catalytically with moderate efficiency. ... 

---