RESUMO
Catalysis with remote-stereocontrol provides special challenges in design and comprehension. One famous example is the dienamine catalysis, for which high ee values are reported despite insufficient shielding of the second double bond. Especially for dienamines with variable Z/E-ratios of the second double bond, no correlations to the ee values are found. Therefore, the structures, thermodynamics, and kinetics of dienamine intermediates in SN-type reactions are investigated. The NMR studies show that the preferred dienamine conformation provides an effective shielding if large electrophiles are used. Calculations at SCS-MP2/CBS-level of theory and experimental data of the dienamine formation show kinetic preference for the Z-isomer of the second double bond and a slow isomerization toward the thermodynamically preferred E-isomer. Modulations of the rate-determining step, by variation of the concentration of the electrophile, allow the conversion of dienamines to be observed. With electrophiles, a faster reaction of Z- than of E-isomers is observed experimentally. Calculations corroborate these results by correlating ee values of three catalysts with the kinetics of the electrophilic attack and reveal the significance of CH-π and stacking interactions in the transition states. Thus, for the first time a comprehensive understanding of the remote stereocontrol in γ-functionalization reactions of dienamines and an explanation to the "Z/E-dilemma" are presented. The combination of bulky catalyst subsystems and large electrophiles provides a shielding of one face and causes different reactivities of E/Z-dienamines in nucleophilic attacks from the other face. Kinetic preferences for the formation of Z-dienamines and their unfavorable thermodynamics support high ee values.
RESUMO
During the last decade, phosphoramidites have been established as a so-called privileged class of ligands in various transition metal catalyses. However, the interactions responsible for their favorable properties have hitherto remained elusive. To address this issue, the formation trends, structural features, and interligand interaction patterns of several trans- and cis-[PdLL'Cl2] complexes have been investigated by NMR spectroscopy. The energetic contribution of their interligand interactions has been measured experimentally using the supramolecular balance for transition-metal complexes. The resulting energetics combined with an analysis of the electrostatic potential surfaces reveal that in phosphoramidites not only the aryl groups but the complete (CH)CH3 Ph moieties of the amine side chains form extended quasi-planar CH-π and π-π interaction surfaces. Application of the supramolecular balance has shown that modulations in these extended interaction surfaces cause energetic differences that are relevant to enantioselective catalysis. In addition, the energetics of these interligand interactions are quite independent of the actual structures of the complexes. This is shown by similar formation and aggregation trends of complexes with the same ligand but different structures. The extended quasi-planar electrostatic interaction surface of the (CH)CH3 Ph moiety explains the known pattern of successful ligand modulation and the substrate specificity of phosphoramidites. Thus, we propose modulations in these extended CH-π and π-π interaction areas as a refined stereoselection mode for these ligands. Based on the example of phosphoramidites, this study reveals three general features potentially applicable to various ligands in asymmetric catalysis. First, specific combinations of alkyl and aryl moieties can be used to create extended anisotropic interaction areas. Second, modulations in these interaction surfaces cause energetic differences that are relevant to catalytic applications. Third, bulky substituents with matching complementary interaction surfaces should not only be considered in terms of steric hindrance but also in terms of attractive and repulsive interactions, a feature that may often be underestimated in asymmetric catalysis.
