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.

Metal‐Free σ‐Bond Metathesis in 1,3,2‐Diazaphospholene‐Catalyzed Hydroboration of Carbonyl Compounds

“…Recently, we reported that 1,3,2‐diazaphospholene 1 (Scheme ) effectively catalyzes transfer hydrogenation of a NN bond with ammonia–borane 9. The rate‐determining step involves a concerted double‐hydrogen transfer where the electrophilic P atom accepts a hydride ion (H − ) via a six‐membered‐ring transition state (Scheme ) 10. We reasoned that, similar to the cases of highly Lewis acidic boranes,7, 8 the strong electron‐accepting ability of the P atom of 1 may allow for interaction with a σ‐bond of other substrates, presumably leading to the formation of a four‐membered‐ring transition state followed by σ‐bond metathesis, if the pathway is favored thermodynamically.…”

Cooperative Catalysis at Metal–Sulfur Bonds

“…Among the various strategies that are known for MLC, the bond activation can occur across the M–L bond of a Lewis-acidic metal center and an adjacent Lewis-basic ligand. This structural motif can be considered as a transition metal frustrated Lewis pair (FLP) . Complexes with polar metal–nitrogen bonds are the most prominent representatives and have evolved as well-established catalysts in hydrogenation chemistry .…”

Metal‐Free σ‐Bond Metathesis in 1,3,2‐Diazaphospholene‐Catalyzed Hydroboration of Carbonyl Compounds

“…[9] The rate-determining step involves a concerted double-hydrogen transfer where the electrophilic P atom accepts a hydride ion (H À ) via a sixmembered-ring transition state (Scheme 1 f). [10] We reasoned that, similar to the cases of highly Lewis acidic boranes, [7,8] the strong electron-accepting ability of the P atom of 1 may allow for interaction with a s-bond of other substrates, presumably leading to the formation of a four-membered-ring transition state followed by s-bond metathesis, if the pathway is favored thermodynamically. To examine our hypothesis, we employed a hydroboration reaction [11] so as to let an interaction between boron and oxygen atoms in the transition state be involved, expecting the subsequent formation of the strong BÀO bond which could be a driving force to induce the desired metathesis.…”