Transition structures hydrolysis

Intramolecular additions generally follow the same trends of stereoselectivity as observed in the bimolecular reactions. Eor example, allylic boronates ( )- and (Z)-118 provide the respective trans- and cis-fused products of intramolecular aUylation. As shown with allylboronate ( )-118, a Yb(OTf)3-catalyzed hydrolysis of the acetal triggers the intramolecular aUylboration and leads to isolation of the trans-fused product 119 in agreement with the usual cyclic transition structure (Eq. 96). 

The transition structures for the hydrolysis reactions of methyl, /-butyl, and ada-mantyl chlorides in the gas phase and in water were calculated using the B3LYP/6-31-G(d) level of theory and the PCM solvation model.82 In the gas phase, backside attack is strongly favoured for the methyl chloride reaction and slightly favoured for the t -butyl chloride reaction. Frontside attack is favoured for the adamantyl chloride... 

The preferential formation of Z-homoallylic alcohols and the significant 1,5-stereoinduction have been rationalized in terms of a directed, stereoselective transmetallation to form oxastannacyclobutane intermediate x followed by reaction via a closed, six-membered ring transition structure to form trichlorostannyl ether xi, which after hydrolysis affords the observed homoallylic alcohol (Scheme 10-56). While neither the intermediacy of x nor the origin of its selective formation have been established, similar four-membered ring structures have been reported [95j. 

Figure 6 Hydrolysis of aryl benzoates by alkali has a transition structure located at an edge of the reaction surface...

The alkaline hydrolysis of methyl esters of substituted benzoic acids has a Hammett p of 2.23. What would be the Bronsted 3 for the reaction when measured against the of the corresponding benzoic acid Comment on the magnitude of the effective charge at the reaction centre in the transition structure compared with that of the carboxylate anion product in the reference dissociation reaction ... 

Reference to Chapter 3 (Scheme 3) shows that the alkaline hydrolysis of aryl carbamate esters has a large Hammett p coefficient (2.54) whereas that for the alkaline hydrolysis of aryl acetate esters is only small (0.67) (Scheme 2). The small p value for alkaline hydrolysis of the acetate case is consistent with little bond fission in the transition structure and the large p for the carbamate case (see Chapter 3 for mechanism) indicates substantial bond fission and a gross difference in mechanism between the two reactions. The simple Hammett relationship only correlates the alkaline hydrolysis rates of the carbamate when meta substituents are employed. Para substituents, which withdraw negative charge by resonance (4-NO2, 4-CN, etcf require values to obtain a good correlation. This is interpreted to mean that the transition structure has sufficient aryl oxide ion character for resonance to occur. In... 

On the other hand, phosphorane intermediates are not expected to be involved in the hydrolysis of phosphate monoesters, so the effective observed catalysis by the carboxyl group of salicyl phosphate 3.21 [51] (Scheme 2.26) is presumed to be concerted vith nucleophilic attack. (The hydrolysis reaction involves the less abundant tautomer 3.22 of the dianion 3.21, and the acceleration is >10 -fold relative to the expected rate for the pH-independent hydrolysis of the phosphate monoester dianion of a phenol of pK 8.52.) However, this system differs from the methoxy-methyl acetals discussed above, in that there is a clear distinction between neutral nucleophiles, which react through an extended transition structure similar to 3.16 in Scheme 2.23, and anions, which do not react at a significant rate, presumably because of electrostatic repulsion. This distinction is well-established for the dianions of phosphate monoesters with good leaving groups (p-nitrophenyl [52] and... 

Experiments described by Corey constitute a noteworthy example of double asymmetric induction where neither participant in the reaction is chiral [95] As illustrated in Figure 4.18 two different catalysts are necessary to achieve the best results. Control experiments indicated that the nucleophile is probably free cyanide, introduced by hydrolysis of the trimethylsilylcyanide by adventitious water, and continuously regenerated by silylation of the alkoxide product. Note that the 82.5% enantioselectivity in the presence of the magnesium complex shown in Figure 4.18a is improved to 97% upon addition of the bisoxazoline illustrated Figure 4.18b as a cocatalyst. Note also that the bisoxazoline 4.18b alone affords almost no enantioselectivity, and that the enantioselectivity is much less when the enantiomer of the bisoxazoline (Figure 4.18b) when used as the cocatalyst. Thus 4.18a and 4.18b constitute a matched pair of co-catalysts and 4.18a and ent-A. %h are a mismatched pair (see Chapter 1 for definitions). The proposed transition structure... 

An asymmetric synthesis of amino alcohols by asymmetric addition of Grignard reagents to chiral a-bromoglycine esters provides a convenient synthesis of a-amino esters (Scheme 4.8, [99]). Hydrolysis of the product ester produces racemized amino acids, but reduction affords amino alcohols that can be subsequently oxidized to the amino acids with no loss of enantiomeric purity. Note that in the proposed transition structure, the phenyl effectively shields the Re face (toward the viewer) of the imine, which is chelated to the carbonyl by magnesium halide formed in the dehydrohalogenation. 

Dialkyl H-phosphonates are hydrolytically unstable compounds. The hydrolysis of H-phos-phonate diesters and phosphate esters is believed to proceed via pentacoordinated intermediates and transition structures that are formed by nucleophilic attack of the tetracoordinated phosphorus atom [14-16]. These intermediates undergo further pseudorotation [17] and elimination of alcohol to form the final products of the nucleophilic substitution. 

Possible transition structures for acid-catalyzed hydrolysis of an acetal. (Adapted from reference 151.)... 

The proposed mechanisms for acetal hydrolysis can be tested with the tools used to study other organic reactions. An A1 mechanism proceeding through transition structure II would lead to a carbocation centered on the departing R group, so racemization of a chiral center would be expected. On... 

Intermediate values of pH (3-8) represent conditions in which a spectrum of transitional structures might be expected. The dissolution and condensation rates smoothly increase with pH, whereas the hydrolysis rate goes through a minimum at approximately neutral pH. Increased H2O Si ratios, r, increase the dissolution and hydrolysis rates at any particular pH value. 

Transition metal hydrolysis, 22-25 Transition metal oxide sols, 30-42. See also listing by composition anion effect, 39-42 polyanions, 30-33 polycations, 28, 29 structure, 31-39 Transition metal See listing by element Transition state... 

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