Substitution reactions ligand field effect

It will not have escaped the reader s attention that the kinetically inert complexes are those of (chromium(iii)) or low-spin d (cobalt(iii), rhodium(iii) or iridium(iii)). Attempts to rationalize this have been made in terms of ligand-field effects, as we now discuss. Note, however, that remarkably little is known about the nature of the transition state for most substitution reactions. Fortunately, the outcome of the approach we summarize is unchanged whether the mechanism is associative or dissociative. 

One strong indication of the importance of ligand field effects is the fact that substitution reactions in octahedral complexes of Pt4+, Rh3+, and Ru3+ occur without rearrangement. The following are examples of reactions of this type ... 

In this spirit, an attempt will be made to account for the magnitude of pressure effects on ligand substitution reaction rates. Attention will necessarily be confined to a few simple model systems two recent reviews (1, 2) of pressure effects on reactions of transition metal complexes in solution may be consulted for more comprehensive surveys of the field. 

Thanks to the fundamental studies of Tsuji, Trost, and others, palladium-catalyzed allylic substitution has become a versatile, widely used process in organic synthesis [40]. The search for efficient enantioselective catalysts for this class of reactions is an important goal of current research in this field [41]. It has been shown that chiral phosphine ligands can induce substantial enantiomeric excesses in Pd-catalyzed reactions of racemic or achiral allylic substrates with nucleophiles [42]. Recently, promising results have also been obtained with chiral bidentate nitrogen ligands [43]. We have found that palladium complexes of neutral aza-semicorrin or methylene-bis(oxazoline) ligands are effective catalysts for the enantioselective allylic alkylation of l,3-diphenyl-2-propenyl acetate or related substrates with dimethyl malonate (Schemes 18 [25,30] and 19 [44]). 

Finally, we will only mention the ultrasonic absorption relaxation method which is, in fact, several different experimental techniques that permit the determination of relaxation times in the range 10 to 5 X 10 sec. Several of the dissociation field effect results in Table II have been confirmed by this method, and much new data on ligand substitution of alkali metal complexes have also been acquired by ultrasonic techniques (Eigen, 1963). Eigen and Tamm (1962) have thoroughly reviewed the role these techniques have played in revealing the mechanism of fast consecutive substitution reactions in metal complexes. 

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