Transition metal complexes decomposition pathways

The most common pathway for catalysis of autoxidations by transition metal complexes involves the decomposition of alkyl hydroperoxides. Another route that may be possible for chain initiation involves direct oxygen activation, whereby the complexation of molecular oxygen by a transition metal would lower the energy of activation for direct reaction with the substrate [reaction (9)]. For example, oxygen coordinated to a metal might be expected to possess properties similar to alkylperoxy radicals and undergo hydrogen transfer with a hydrocarbon ... 

Pure alkyl transition metal complexes are rare, owing to the -Elimination decay mechanism. An effective strategy to prepare such compounds must therefore aim at blocking possible decomposition pathways. Neopentyl is void of hydrogen in the jS-position, and neopentyl... 

By varying metal and ligands, transition metal complexes can be designed to serve as intermediates that are not too reactive or too unreactive. For a catalytic turnover to occur, each intermediate in the cycle must be reactive enough to proceed to the next stage, yet not so reactive that other pathways (e.g., decomposition or a different bonding mode) become feasible. 

All the common HDN schemes of e.g. pyridine require, after prehydrogenation a first ring-opening C-N bond breaking, followed by the decomposition of n-pentylamine to remove the nitrogen as NH,. It is also possible to envisage pathways or steps that involve extrusion of nitrogen from related molecules such as imines and amides, and therefore it is of interest to consider at this point some reactions of such substrates induced by transition metal complexes. 

The intramolecular insertion of a hydride into a coordinated olefin is a crucial step in olefin hydrogenation catalyzed by late transition metal complexes, such as those of iridium, rhodium, and ruthenium (Chapter 15), in hydroformylation reactions catalyzed by cobalt, rhodium, and platinum complexes (Chapter 16), and in many other reactions, including the initiation of some olefin polymerizations. The microscopic reverse, 3-hydride elimination, is the most common pathway for the decomposition of metal-alkyl complexes and is a mechanism for olefin isomerizations. 

Finally, the metal-perfluoroalkyl linkage also appears to be less susceptible to facile decomposition by the a- or -elimination pathways that dominate much of the chemistry of hydrocarbon alkyls and lead to metal hydrides. The absence of these reaction pathways, at least for the later transition metals, may reflect the relative strength of the C—F bond versus the M—F bond compared to C—H/M—H analogues (32). However, a-fluoride abstraction reactions can be accomplished with exogenous fluoride acceptors to give fluorinated carbene complexes (see Section III,B,1). One example of an apparent -fluorine elimination reaction is shown in Eq. (2) (33) and presumably is driven by the stronger bond to fluorine formed by early transition... 

For a long time it was thought that binary complexes of transition metals containing only o-alkyl- or aryl ligands could be thermodynamically stable only if they are coordinatively saturated (18 electron rule). A series of illuminating papers by Wilkinson 26), Lappert27) and others 28) dispelled this fallacy. It became clear that transition metal alkyls can decompose via kinetically low energy pathways such as P-hydride elimination or abstraction, but rarely via primary metal-carbon homolysis. Mechanisms and theories of decomposition have been proposed and reviewed 29). 

This is a disadvantage because cationic transition metal NHC complexes are subject to carbene decomposition pathways, unless they are stabilised as a chelating bis-carbene... 

---