Double-displacement Mechanisms - Second Step

Both mechanisms account for the observed inversion of stereochemistry. The second mechanism proceeds by an adjacent displacement occurring with retention, followed by an in-line displacement with inversion. This doubledisplacement mechanism requires a pseudorotation at phosphorus, the intermediacy of multiple TBP intermediates, and the intermediacy of a covalent phosphoryl enzyme intermediate. An adenylated enzyme has been isolated in poor yield from a phosphodiesterase capable of hydrolysing cAMP (Landt and Butler, 1978), and the phosphodiesterase from snake venom has been shown to operate via a double-displacement mechanism (Burgers et al., 1979a). Eckstein points out that the choice between the two proposed mechanisms is dependent (i) on the conformational preference of a six-membered ring in a TBP species, and (ii) on proof for a two-step mechanism for cAMP phosphodiesterase. 

The reactions in this chapter follow the same general mechanism. First, the nucleophile bonds to the carbonyl carbon, displacing the pi electrons of the CO double bond onto the oxygen and forming the tetrahedral intermediate. In the second step the unshared electrons on the oxygen reform the pi bond as the leaving group leaves ... 

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