Acetal Substitution Reactions: Stereoelectronic Effects, Conformational Analysis, Reactivity vs Selectivity, and Neighboring-Group Participation

Abstract

Acetal substitution reactions can proceed by a number of mechanisms, but oxocarbenium ion intermediates are involved in many of these reactions. Our research has focused on understanding the conformational preferences, structures, and reactions of these intermediates. This account summarizes our observations that electrostatic effects play a significant role in defining the preferred conformations, and that torsional effects determine how those intermediates react. Neighboring-group effects are not as straightforward as they might seem, considering that oxocarbenium ion intermediates are in equilibrium with structures that involve stabilization by a nearby substituent.1 Introduction2 Unexpected Stereoselectivities3 Determining Conformational Preferences of Oxocarbenium Ions4 Structures of Carbocations by NMR Spectroscopy and X-ray Crystallography5 Stereoelectronic Models for Reactions Involving Other Oxocarbenium Ions6 Stereoselectivity and Reactivity: When They Correlate, When They Do Not7 Neighboring–Group Participation Is Not as Simple as It Seems8 What Is True for Carbocations Is True for Carbonyl Compounds9 Stereoelectronic and Torsional Effects in Reactions of Enolates10 Summary of Expected Selectivities for Reactions of Cyclic Acetals11 Conclusion