Alkylation

About: Alkylation is a research topic. Over the lifetime, 29915 publications have been published within this topic receiving 464944 citations. The topic is also known as: alkylation reaction.

Importance of planar chirality in chiral catalysts with three chiral elements: the role of planar chirality in 2'-substituted 1,1'-P,N-ferrocene ligands on the enantioselectivity in Pd-catalyzed allylic substitution.

Abstract: A series of novel planar chiral 2‘-substituted 1,1‘-P,N-ferrocene ligands 9−11, 14, and 16 were prepared with diastereopurity >99:1 and found to be effective in asymmetric allylic alkylation and amination reactions. Ligand 14 furnished the highest enantiomeric excess, 98.5% and 96.5% ee in alkylation and amination reactions, respectively. The role of planar chirality in asymmetric reactions has been examined, and decisive effects on enantioselectivity as well as the control of absolute configuration in palladium-catalyzed allylic alkylation and amination reactions were observed. To clarify why and how the planar chirality governed the stereochemical outcome, X-ray crystallographic structures of η3-diphenylallyl Pd complexes, 1H NMR, 31P NMR spectra of palladium dichloride complexes, and η3-diphenylallyl Pd complexes of three 1,1‘-P,N-ferrocene ligands were analyzed with the aid of COSY and 2D NOESY experiments. All results led to the conclusion that planar chirality influences the stereochemical outcome b...

On the mode of activation of the catalytically essential sulfhydryl group of papain.

Abstract: The reaction of the thiol group of papain with chloroacetamide, iodoacetamide, d- and l-2-bromopropionamides was studied. In the acidic pH-range the reaction rate is higher than expected for an ordinary SH-group and shows a double-sigmoid pH-rate profile. In addition to the already described pKa of 8.5, we found a pKa of 4.0. This indicates that in the pH-range where the enzyme is catalytically active the thiol group of Cys-25 interacts with some amino acid side chain, presumably with the imidazole group of the neighboring His-159. In the light of the X-ray diffraction studies (Drenth et al., 1971), it is probable that the anomalous PKa of His-159 is due to its interaction with Trp-177. The comparison of the pH-rate profiles of d- and l-2-bromopropionamide reactions indicates that there is no sifgnificant alteration in the geometry of the active site around pH 4, whereas at slightly alkaline pH structural changes can be observed. 2H2O has no effect on the rate constants of the alkylation reaction. This rules out the possibility of general base catalysis by the imidazole group of His-159. The experimental data indicate that the dissociated thiol group forms an ion pair with the protonated imidazole group. On the basis of the formation of thiolate-imidazolium ion pair, the mechanism of action of papain is reconsidered. It is concluded that the acylation and deacylation steps are not symmetrical processes, and they are one-encounter type reactions. The interaction between His-159 and Trp-177 may be important in the catalysis.