Dissociative conjugate base mechanism

Bob s research interests and knowledge across chemistry were great. Throughout his career he retained an interest in biomimetic chemistry, specifically the study of metal ion-promoted reactions and reactions of molecules activated by metal ion coordination. His early interests in carbohydrate chemistry inspired him to study metal ion catalysis of both peptide formation and hydrolysis as well as studies in inorganic reaction mechanisms. He was particularly interested in the mechanisms of base-catalyzed hydrolysis within metal complexes and the development of the so-called dissociative conjugate-base (DCB) mechanism for base-catalyzed substitution reactions at inert d6 metal ions such as Co(III). 

Acid hydrolysis of an octahedral metal ion complex is typically a dissociative or SNl-type reaction. In the case of base hydrolysis, reactions tend to display SN2-type reaction mechanisms, although others take place by what is termed an SnI-conjugate base mechanism. The latter involves attack by an electrophile to abstract a proton... 

Deprotonation trans to the leaving group is especially effective at promoting the dissociation step. The conjugate base mechanism cannot operate if a tertiary amine with no ionisable proton is placed trans to the leaving group as expected the rate of substitution is then slower and does not depend on [OH-]. 

The mechanism becomes more dissociative for 3d ions later in the series. Substitution rates may be increased by the conjugate base mechanism. 

OH to produce the coordinated dimethylphosphate (DMP) anion and methanol in this case, however, the reaction is accompanied by considerable Rh-O bond cleavage. This reaction presumably occurs by the characteristic dissociative conjugate base, SNl(cb), mechanism (136) for metal-ligand bond rupture, which also has a first-order dependence on hydroxide ion concentration. At zero buffer concentration the rate follows the rate law ... 

Dissociative mechanisms lead to products where the stereochemistry may be the same or different than the starting complex. Table 12.9 shows that cw-[Co(en)2L(H20)] is a hydrolysis product of both ct5 -[Co(en)2LX] and trfl 5-[Co(en)2LX] in acid solution. While these aquation reactions with pure ci5-[Co(en)2LX] lead exclusively to cis products, retention of the trans ligand orientation in fra 5-[Co(en)2LX] depends on both L andX. The conjugate base mechanism is unlikely in these reactions they are carried out in acidic solution. 

These data can be accommodated by a simultaneous addition of the hydride and proton to form an alcohol that is initially hydrogen bonded to the amine and forms the alkoxide without dissociation of free alcohol or by a transition state with a structure resembling those proposed for the direct, simultaneous transfer of the hydride and proton, but leading to the alkoxide complex directly. After this transfer of the hydride to form an alkoxide, a base-assisted elimination of the alkoxide to form the amide complex is proposed to occur. This step parallels the classic conjugate-base mechanism for dissociative Hgand substitution discussed in Chapter 5. 

The early work was done on the hydroxide ion catalyzed hydrolysis of cobalt(III) amines, for which there was evidence that the reaction proceeds by a dissociative conjugate base (DCB) mechanism (S ICB in earlier terminology), as shown in Scheme 3.1. 

Partial molar volumes, and AK measurements for the base hydrolysis of a series of [CrCl(3,3-tri)(AA)] complexes suggest an associative interchange conjugate base mechanism may be more applicable to Cr(III) systems than the dissociative SNlcb. ... 

Hydrolysis and racemization of [Co(tmen)3] (tmen = 2,3-diamino-2,3-dimethylbutane) follow the rate law, rate = fc[Co(tmen)3+][OH ] in the pH range 4.8 to 13.8. For base hydrolysis, fcoH = 2.5 0.8dm mol" s at 25 °C, with = 132kJmol- , A5+= 144 J mol-, and AF+= 57.6 cm mol". A conjugate base mechanism Dcb is assigned to the hydrolysis reaction which occurs from the singly deprotonated complex via a five-coordinate intermediate, followed by rapid dissociation of the tran -Ugand and cis-trans isomerization to the final product trans-[Co(tmen)2(H20)2] . Rate constants and activation parameters have also been obtained for the racemization reaction. The racemization reaction has k = TTi dm mol" s" at 25 °C and is faster than base hydrolysis. For the faster racemization, deprotonation is followed by the inversion process without Co—N bond... 

In this mechanism, the rate-determining step involves the dissociative reaction of the conjugate base. Because of this, the mechanism is known as the SN1CB mechanism, in which the substitution is... 

The second possibility is a dissociation mechanism, involving not the reactant complex itself, but rather, its conjugate base (designated CB)... 

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