Early transition states, methane

Examine the structures of the two transition states (chlorine atom+methane and chlorine+methyI radical). For each, characterize the transition state as early (close to the geometry of the reactants) or as late (close to the geometry of the products) In Ught of the thermodynamics of the individual steps, are your results anticipated by the Hammond Postulate Explain. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

In another recent study of CO reduction using organometallic compounds of the early transition elements, Caulton et al. report that CH4 is evolved when toluene solutions of (C5H5)2Ti(CO)2 are heated at 150°C under H2 or H2 -l- CO atmospheres (55). When deuterium is used, the observed product is CD4, although some deuteration of the cyclopentadienyl rings is noted. Caulton does not report the percent yield of methane, but does state that the reaction is not catalytic. One of the most novel aspects of this investigation is the isolation of a blue Tig cluster that is inert to further reaction... 

In this reaction, a C—H bond in methane is made, and a C—H bond of benzene is cleaved, but the oxidation state of the scandium center remains +3. This class of reaction, which is not hmited to early transition metals, is called sigma-bond metathesis. In this mechanism, the metal is first postulated to coordinate the bond to be activated in an rf- fashion, followed by formation of a four-centered transition state that leads to an exchange of ligands at the metal (Figure 14.8). 

The activation of C-H bonds by early transition metals or /-block elements in their highest possible formal oxidation states, for some of which the first direct observations of methane activation were verified, " is particularly interesting from a mechanistic point of view since their r/ - (or r/ /"-) electron configurations preclude any of the... 

The ALMO-EDA scheme was used to analyze a wide range of methane CH bond cleavage transition states for metal-ligand complexes that have been reported experimentally to promote or catalyze CH activation [57]. This included late transition metals such as Rh(lll), Rh(l), lr(ni), Pt(n), Pd(ll), Ru(II), and Au(III) and early transition metals such as Sc(lll) and W(ll). Ligands attached to these metal centers range from phosphine, N-heterocycle, and halide to 0-donors. Three specific examples of transition states analyzed and their ALMO-EDA interaction energy components are shown in Scheme 10. 

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