Multistep reactions, radical structures

The major carbon centered reaction intermediates in multistep reactions are carboca-tions (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [ ]+, [ ]-, [ ], and [ CI I21-... 

The product of the reduction of [Co(bpy)3] by Cr, upon aerial oxidation, is a red dimeric species, postulated to have the structure [(H20)4Cr(/x-OH)2Cr(OH2)2]. This product and the stoichiometry of the reaction suggests a two-electron process, with the bpy ligand serving as a temporary bridging radical. An investigation of the Cr(II) reduction of [Co(pd)3] (pd = pentane-2,4-dione) in water/acetone mixtures reveals outer-sphere, and mono- and di-bridged ([H ] dependent) pathways.The effect of the cosolvent on the activation parameters is observed at an acetone mole fraction of 0.06, at which point its solvation of the activated complex becomes important. The reduction of [Co(en)2(dppd)] (dppd = l,3-diphenylpropane-l,3-dione) by Cr occurs by a multistep mechanism in which the first step is the formation of the [Co(en)2(dppd )] radical, which catalyzes the inner-sphere Co(III)/Cr(II) electron transfer process. " A molecular orbital study indicates that the [Co(en)2(dppd)] reduction likely involves attack of Cr " at the methine carbon of dppd, in contrast to the attack on an oxygen in the [Co(en)(pd)2] reduction. 

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