3794-96-5Relevant academic research and scientific papers
Solvolysis of 2-chloro-2(3,4-disubstituted) phenylpropanes: Validity of Hammett-Brown σ+ constants in assessing additive effects of substituents
Taha, Ahmed A.
, p. 514 - 523 (2012/07/28)
The objective of this study is to test the suitability of the extended Hammett-Brown equation, log (kXX/kHH) = ρ+∑σ+, in depicting satisfactorily additive effects of electronegative atom-bearing substituents, which are known to possess diverse and multicomponent influences on the side chain reactions of polysubstituted benzenes. The equation has been used to correlate, for the first time, the additive effect of substituents in the specific rates of solvolysis of 2-chloro-2-phenylpropanes (3b-3f) having 3-F,4-Me, 3-Br,4-Me, 3-I,4-Me, 3-Me,4-Me, or 3-MeO,4-Me substituents. The rates were determined titrimetrically at 288, 298, and 308 K using 90% aqueous acetone as solvent. Measured additive effects of these substituents on the solvolysis rate and activation parameters of the parent cumyl chloride (2-chloro-2-phenylpropane) are found to be well correlated using the equation given above. Plots of log (kXX/k HH) of 3b-3f together with mainly di-, but also tri- and mono-substituted cumyl chlorides from previous studies against ∑σ+ give a linear correlation coefficient of 0.990 as a measure of the validity of the equation to depict such systems. The halogen substituents' extent of conformity with additivity reflected in their relative (kobsd/kcalcd) rate ratios is found to correlate with the steric size of substituents. Plots of rate ratios against Taft's steric factor of each halogen give a linear correlation coefficient of 0.994 for the 3-halo substituents. The 3,4-dimethyl substituents' relative rate ratio of 1.03 shows excellent additivity, whereas the 3-methoxy-4-methyl ratio of 1.43 shows the methoxy group to be far less deactivating than predicted. Similar trends were found for the free energy of activation (δG? - δG 0?) differences, which correlated linearly with a coefficient of 0.983 with Taft's steric factor of halogen atoms.
Lipase-mediated resolution of substituted 2-aryl-propanols: Application to the enantioselective synthesis of phenolic sesquiterpenes
Serra, Stefano
experimental part, p. 619 - 628 (2011/07/08)
A comprehensive study of the lipase-mediated resolution of substituted 2-aryl-propanols is reported. The latter alcohols were submitted to the irreversible acetylation catalyzed either by PPL, CRL, or lipase PS. The enantioselectivity of these transformations was dependent on the type of lipase used. The type of substituents and particularly their position on the aromatic ring strongly affected the selectivity of the reaction. The experiments described prove that PPL is the more versatile lipase catalyzing the acetylation with an enantiomeric ratio (E) value that ranges from 1 up to 144, depending on the substrate used. Conversely, the same transformations were catalyzed by CRL and lipase PS with an enantiomeric ratio value, which is always less than 5. The remarkable behavior of PPL was exploited in the large scale resolution of some substituted 2-aryl-propanols whose enantiomeric forms are relevant building blocks in the enantioselective synthesis of phenolic sesquiterpenes. By these means, the synthesis of (S)-turmeronol B and the formal syntheses of (R)-curcumene, (R)-curcuphenol, (R)-xanthorrhizol, and (R)-curcuhydroquinone were accomplished.
