Transition metal compounds reaction mechanisms

In addition, transition metal compounds have the ability to donate additional electrons or accept electrons from organic substrates and can change both their valence and their coordination number reversibly. These properties play an important role in organic synthesis, especially in catalytic processes. The ability to serve as catalysts in organic reactions is the most important property of the transition metal compounds. Reaction mechanisms involving intermediate organic structures, which are prohibitively endothermic in the absence of transition metal catalysts, are made feasible in their presence. 

The present investigation was carried out in order to extend eel to other types of transition metal compounds including organo-metallics. In addition to the search for new systems the modification of a well-known eel was used to learn more about the reaction mechanism. 

Aldehydes are oxidized by dioxygen by the chain mechanism in reactions brought about in different ways initiated, thermal, photochemical, and induced by radiation as well as in the presence of transition metal compounds [4-8]. Oxidation chains are usually very long from 200 to 50,000 units [4], Acyl radicals add dioxygen very rapidly with a rate constant of 10s—109 Lmol V1 [4], Therefore, the initiated chain oxidation of aldehyde includes the following elementary steps at high dioxygen pressures [4-7] ... 

This reaction is invariably catalyzed by transition metal compounds and its mechanism is of special interest. The first explanation for this transformation is based on the so-called pairwise mechanism , in which two olefins coordinate to a transition metal center to form a transient cyclobutane-like intermediate [2], However, this idea was later replaced... 

The first mechanism appears to be the better basis for describing most of the results referred to by Cramer (56). It will, however, be noted that the addition-elimination mechanism requires that the metal catalyst be supplied as a metal hydride. Where the catalyst has not been supplied in this form, the reaction has usually been carried out in the presence of reagents known to convert transition metal compounds to hydrides (e.g. protonic acids, alcohols or hydrogen). These substances are known as co-catalysts and, where they have been used, induction periods have been encountered which are consistent with hydride formation as required in mechanism (a), but which would not be expected for (b). 

These reactions, and in particular the ease of formation of the N(7)-0(6) chelate, may be relevant to the mechanism of action of the anticancer transition metal compounds such as m-[PtCl2(NH3)2].230... 

Thus, with compounds of type (a), the stereochemistry of substitution, insertion, and cleavage reactions can be studied (8, II), whereas with compounds of type (b), the mechanism of racemization and epimerization at the chiral metal atom can be investigated (II, 92). In the following sections, representative examples of both types of study will be given, from which it will become evident how optically active labels at metal centers in organo-transition-metal compounds contribute to the elucidation of the stereochemical course of reactions. 

A similar mechanism might operate in the activation of an azolium salt by a transition metal compound forming the metal carbene complex. However, since a basic substituent on the metal (acetate, alkoxide, hydride) usually reacts with the H -proton, the proton is removed from the reaction as the conjugate acid and reductive elimination does not occur. 

---