Stoichiometric reactions reactive intermediates

Chloroformates are reactive intermediates that combine acid chloride and ester functions. They undergo many reactions similar to those of acid chlorides however, the rates are usually slower (4—8). Those containing smaller organic (hydrocarbon) substituents react faster than those containing large organic (hydrocarbon) substituents (3). Reactions of chloroformates and other acid chlorides proceed faster with better yields when alkaU hydroxides or tertiary amines are present to react with the HCl as it forms. These bases act as stoichiometric acid acceptors rather than as tme catalysts. 

In the last decade, a new aspect of nickel-catalyzed reactions has been disclosed, where nickel serves to selectively activate dienes as either an al-lyl anion species or a homoallyl anion species (Scheme 1). These anionic species are very important reactive intermediates for the construction of desired molecules. Traditionally they have been prepared in a stoichiometric manner from the corresponding halides and typical metals, e.g., Li, Mg. In this context, the catalytic generation method of allyl anions and homoallyl anions disclosed here might greatly contribute to synthetic organic chemistry and organotransition metal chemistry. 

Today, iridium compounds find so many varied applications in contemporary homogeneous catalysis it is difficult to recall that, until the late 1970s, rhodium was one of only two metals considered likely to serve as useful catalysts, at that time typically for hydrogenation or hydroformylation. Indeed, catalyst/solvent combinations such as [IrCl(PPh3)3]/MeOH, which were modeled directly on what was previously successful for rhodium, failed for iridium. Although iridium was still considered potentially to be useful, this was only for the demonstration of stoichiometric reactions related to proposed catalytic cycles. Iridium tends to form stronger metal-ligand bonds (e.g., Cp(CO)Rh-CO, 46 kcal mol-1 Cp(CO)Ir-CO, 57 kcal mol ), and consequently compounds which act as reactive intermediates for rhodium can sometimes be isolated in the case of iridium. 

The search for low-molecular weight (phenoxyl)copper(II) complexes as functional models for GO, which would mimick this reactivity, had a promising start in 1996 when Tolman and co-workers (202) reported that electrochemical one-electron oxidation of Cull(,L,lil 2)(bcnzylalcoholatc) (Fig. 27) resulted in the formation of benzaldehyde (46%) and some other decomposition products of the ligand H L,Bu2 in <5% yield and probably a Cu(I) species of unknown composition. These authors suggest that a (phenoxyl)copper(II) intermediate Cull(,L,l l 2 )(bcn-zylalcoholate)]+ is formed and that the reaction sequence, as in Fig. 8, leads to the observed products. Although this represents a stoichiometric reaction, it demonstrated for the first time that GO chemistry can be successfully modeled. 

The importance of these radicals in catalytic processes may be evaluated by studying their behavior in stoichiometric reactions and by extrapolating this information to catalytic conditions. In following the stoichiometric reactions, magnesium oxide has been an excellent model surface since the three types of oxygen ions may be selectively formed and are stable at temperatures where most hydrocarbons of interest will react. Magnesium oxide, on the other hand, is basic and reactive itself therefore intermediates may react differently on this surface than on silica, for example. 

Acceptor-substituted carbene complexes are highly reactive intermediates, capable of transforming organic compounds in many different ways. Typical reactions include insertion into o-bonds, cyclopropanation, and ylide formation. Generally, acceptor-substituted carbene complexes are not isolated and used in stoichiometric amounts, but generated in situ from a carbene precursor and transition metal derivative. Usually only catalytic quantities of a transition metal complex are required for complete conversion of a carbene precursor via an intermediate carbene complex into the final product. 

A modification of an earlier procedure for debromination of v/c-dibromides in the presence of catalytic amounts of diorganotellurides has allowed the synthesis of terminal alkenes and cis- and frani-l,2-disubstituted alkenes from appropriate precursors the relative substrate reactivities suggest that, as for the stoichiometric reaction, the catalytic reaction involves intermediate bromonium ion formation. The Te(IV) dibromides formed in the debrominative elimination are reduced back to the catalysts by either sodium ascorbate or the thiol glutathione. 

This section will describe the various applications of HP IR spectroscopy to determine reaction mechanisms of transition metal catalysed reactions. It will begin by looking at truly in situ studies, carried out under catalytic conditions, and then consider investigations of stoichiometric reaction steps and characterisation of reactive intermediates. 

The polycondensation of dibromomethane with bis(benzenethiol) instead of bisphenol also shows stoichiometrically imbalanced polymerization behavior (Scheme 63) [253]. When 1.5 equivalents of dibromomethane was reacted with dithiol in NMP at 75 °C for 4 h, polysulfide with the maximum inherent viscosity (//jn, = 0.50 dbg ) was obtained. On the basis of the model reaction, a reactive intermediate, in which a bromine in dibromomethane is substituted with the mercapto group of the monomer, is estimated to be 61 times more reactive than dibromomethane. 

In general, the sensitivity of the overall rate to the forward rate constant for step i is always proportional to the reversibility of the preceding step that produces the reaction intermediate in step i however, the proportionality constant depends on the stoichiometric coefficients for the formation and consumption of the reactive intermediates. For example, if the reactive intermediate is consumed once in step i and produced twice in the preceding step j, then [Pg.184]

The double silylation of unsaturated organic compounds catalyzed by group 10 metals is a convenient synthetic route to disilacyclic compounds. Nickel and platinum complexes, in particular, are excellent catalysts for the transformation of disilanes. Cyclic bis(silyl)metal complexes2,3 have been implicated as key intermediates in the metal-catalyzed double silylation of alkynes, alkenes, and aldehydes however, the intermediates have not been isolated due to their instability. We now describe (i) the isolation of the reactive intermediates cyclic bis(silyl)metal compounds (1) with bulky o-carborane unit 4 (ii) the generation of a new class of heterocyclic compounds (4-5) by the stoichiometric reaction of the intermediates with a variety of substrates such as an alkyne, dione, and nitrile 4 and (iii) the facile double silylation of alkenes and alkynes (10,12-14) catalyzed by the intermediate under mild conditions.5... 

In this chapter, a reaction is considered acid or base catalyzed if its rate is proportional to the concentration of acid or base, respectively. According to Ostwald s definition of catalysis, however, it is required that the acid or base be not consumed in the reaction [12]. A true catalyst combines with the substrate to form a reactive intermediate, and the catalyst is regenerated in one of the final steps of the mechanism. In Bell s definition, a catalyst appears in the rate expression to a power higher than that to which it appears in the stoichiometric equation [1]. On the other hand, it merely depends on the acidity or basicity of the products (and also on the pH of the solution) whether or not the catalyst will be regenerated at the end. Therefore, it is not essential for the classification of reaction type and mechanism whether the acid or base is a true catalyst according to the more restricted definition, or a reactant which is consumed [12]. In both cases, the formation of an intermediate from substrate (S) and acid or base opens a low free energy pathway for the reaction. 

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