Benzyl esters steric effects

A similar steric effect was observed in the reaction of benzyl carboxylate (44). When 44a-d were treated with Bu OK under solvent-free conditions at around 100 °C for 30 min, the corresponding condensation products 45a (75%), 45b (66%), 45c (64%), and 45d (84%) were obtained in the yields indicated [9] (Scheme 6). When the same reactions of 44a-d and Bu OH were carried out in toluene under reflux for 16 h, no condensation product was obtained and 44a-d were recovered unchanged. In solution reactions, exchange of the alkoxy group occurs among the substrate, reagent, and solvent. Therefore, the alkoxy groups of the ester, metal alkoxide, and alcohol used as a solvent should be identical. 

Enthalpies of formation have been measured for BF3 adducts of a number of carbonyl compounds PhCOX. The electron-donating strength of the oxygen atom is directly related to the inductive effect of the group X, although the establishment of a basicity scale for the carbonyl function must also take into account steric effects.214 The following order was found for the basicities benzaldehyde = acetophenone > hindered ketones > benzoic esters a-chloroacetophenone > benzyl chloride. 

Davies and co-workers also applied a ring-closing metathesis strategy in their preparation of constrained p-amino acid derivatives. Their work employed a chiral auxiliary, S -(l-phenyl-ethyl)-carbamic acid benzyl ester, to achieve the desired P-amino acid derivatives in a stereoselective fashion. Treatment of diene precursors 46 and 48 with 4 mol % of 3 in refluxing dichloromethane provided the corresponding carbocyclic P-amino acid derivatives 47 and 49 in 85 and 35% yield, respectively, with greater than 95% diastereoselectivity in both cases. The diminished yield in the case of 49 is presumably due to sterics effects. 

By adding one equivalent of alcohol to CDI at room temperature with or without base it is possible to isolate the imidazole-iV-carboxylate, which then reacts with a second mole of ROH to yield the carbonate. As in the case of alcoholysis of imidazolides, the reaction can be accelerated so effectively with catalytic amounts of NaOC2H5 or ImNa that it takes place in most cases exothermically, even at room temperature. However, tert-butyl alcohol, even when in excess, affords with CDI and base catalysis at room temperature only the imidazole-N-tert-butylcarboxylate, obviously for steric reasons. At higher temperature the carbonic ester is formed. Mixed carbonates such as ethyl benzyl carbonate or ethyl terf-butyl carbonate can be prepared with two different alcohols added sequentially.C9],[229]... 

Cleavage of esters and lactones (8,447-448).1 SN2-Cleavage of esters and lactones with NaSeC6H5in HMPT-THF is considered to be the most effective known method. The order of reactivity of related reagents is NaSeC6H5-18-crown-6-THF > LiSe(, H5 HMPT THF > LiSeC6H5 THF. Methyl esters are cleaved more readily than ethyl esters benzyl, isoamyl, and isopropyl esters are cleaved in high yield, but more slowly. Methyl esters are cleaved easily, irrespective of steric hindrance. Amides are completely inert. 

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