Transition state diesters

For a ctiprale addition reaction to a diester derivative sudi as 88, it miglii be expected diai die ami addition product would be favored, since a pronounced allylic a " strain in diese substrates along "modided" Felfcin-Anb lines should favor transition state 52 fsee Fig. 6.1). However, experiments produced die opposite result, widi die syn product 89 being obtained as die major diastereomer (Sdieme 6.18) [36, 37]. 

Sulfate monoesters can react by dissociative paths, and this is the favored path. Whether such reactions are concerted or involve a very short-lived sulfur trioxide intermediate has been the subject of debate. ° Benkovic and Benkovic reported evidence suggesting that the nucleophile is present (though there is little bond formation) in the transition state for the reaction of amines with p-nitrophenyl sulfate. Alkyl esters of sulfuric or sulfonic acids normally react with C-0 cleavage only when this is disfavored, as in aryl esters, does one see S-0 cleavage. Sulfate diester... 

Deng et al. (1993,1998) and Ray et al. (1993) have used V " as an analogue of in an attempt to model the transition state of the hydrolysis of phospho-diesters by ribonuclease A since is assumed to adopt the expected five coordination more readily than Examination of the vibrational spectrum of the vanadate analogue indicates that the terminal V-O bonds are only slightly weakened when bonded to the protein. A quantitative bond valence analysis effectively rules out two proposed mechanisms that involve the protonation of the terminal O atoms. 

Fig. 17.29. The Ru04 cleavage of a phenyl ring is exemplified by the case of an altylated aromatic compound using the modified Lemieux-von Rudloff conditions (Figure 17.26). The reaction involves three key intermediates the cr-ketoalde-hyde A, the a-keto acid E, and the ruthenium(VIII) acid diester G. It is unclear how the reaction of E —> G proceeds. In the last step, G undergoes a fragmentation via a cyclic transition state and forms a substituted acetic acid I, Ru03 and carbon dioxide.

Methyl-p-nitrophenyl phosphate coordinated to the two metal centers in 37 undergoes hydrolysis by a two-step addition-elimination mechanism [73]. The free phosphate hydrolyzes by a concerted mechanism. In both phosphate monoester and diester hydrolysis, the two Co(m) centers in 32 and 37 stabilize the five-coordinate phosphate species (transition state or intermediate) by bringing the phosphate and nucleophile together. This stabilization leads to a change in mechanism from dissociative to concerted for a phosphate monoester hydrolysis [96] and from concerted to stepwise for phosphate diester hydrolysis [73]. 

Fig. 14.19. The standard mechanism (transition state C) and the alternative mechanism (transition state B) of glycol cleavages with Pb(OAc)4. The trans-glycol A reacts slowly via the monoester B of Pb(IV) acid, while the isomeric di-glycol reacts fast via the cyclic diester C of lead(IV) acid.

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