title: Coexistence of Lewis acid and base functions: a generalized view of the frustrated Lewis pair concept with novel implications for reactivity.
abstract: Sterically congested Lewis pairs cannot form Lewis adducts; instead they establish encounter complexes of "frustrated" Lewis pairs (FLPs). These encounter complexes have recently been recognized to be capable of activating, i.e., splitting, homopolar and polar single and double bonds, which rendered a new reactivity principle. With the help of qualitative orbital considerations this chapter reviews and explains the reactivity of FLPs toward homopolar Z-Z or Z-Z' single bonded molecules, such as H-H and C-H single bonds, assuming in the encounter complexes the action of strongly polarizing Coulombic fields originating from the FLP constituents. This reactivity principle has been extended in its view to the activating potential for homopolar Z-Z or Z-Z' single bonds of strongly polarized [X-Y ↔ X(+)-Y|(-)] σ and [X=Y ↔ X(+)-Y|(-)] π bonded molecules (X,Y = atoms or molecular fragments; electronegativity of X < electronegativity of Y). A striking analogy in the reaction behavior of FLPs and strongly polarized σ and π bonded molecules could be revealed based on the analyses of selected examples of "metal-free" (main group element reactions) or metal-based (containing transition metals) σ bond metathesis reactions and σ bond additions of H2 and alkanes to polarized main group element and metal to ligand π bonds. Related to the described polar reaction types are Z,Z' double atom or group transfers between highly polarized double bonds of XY and X'Y' molecules combining a Z,Z' elimination with an addition process. Multiple consecutive Z,Z' double atom or group transfers are denoted as double H transfer cascade reactions. Analyzed by examples are concerted or stepwise double H transfers and double H transfer cascades with Z,Z'=H,H from the "metal-free" and metal-based realms: the Meerwein- Pondorf-Verley reduction, H,H exchanges between amine boranes and between amine boranes and unsaturated organic compounds, and the crucial H,H transfer steps of Noyori's bifunctional and Shvo type transfer hydrogenation catalyses.
authors.affiliation: Anorganisch-chemisches Institut, Universität Zürich, Winterthurer Strasse 190, 8057, Zürich, Switzerland, hberke@aci.uzh.ch.
authors.forename: Heinz
authors.initials: H
authors.lastname: Berke
authors.forename: Yanfeng
authors.initials: Y
authors.lastname: Jiang
authors.forename: Xianghua
authors.initials: X
authors.lastname: Yang
authors.forename: Chunfang
authors.initials: C
authors.lastname: Jiang
authors.forename: Subrata
authors.initials: S
authors.lastname: Chakraborty
authors.forename: Anne
authors.initials: A
authors.lastname: Landwehr