Reactions of an Aluminium/Phosphorus Frustrated Lewis Pair (FLP) with α,β‐Unsaturated Carbonyl Compounds: FLPs as Efficient Two‐Electron Reductants with the Formation of Enolates, a cis‐Enediolate, and an Allene

다국어 초록 (Multilingual Abstract)

The Al/P‐based frustrated Lewis pair (FLP) Mes2P−C(AltBu)2=C(H)Ph (1; Mes=mesityl) reacted as an efficient two‐electron reductant with benzil to afford a cis‐enediolate that was coordinated to the FLP through P−O and Al−O bonds and the formation of a seven‐membered heterocycle (2). The phosphorus atom is oxidised from +III to +V. Similar heterocycles (3 a to 3 f) were formed if 1 was treated with various enones (acrolein, acrylate, acrylamide). The resulting enolates are bound to the FLP through P−C and Al−O bonds. Cyclopropenone gave an adduct (4) with the C=O bond coordinated by P and Al. Ynones gave a fascinating variety of different structures. 1,3‐Diphenylprop‐2‐yn‐1‐one afforded a remarkable allene‐type moiety with two cumulated C=C bonds (5); 3‐hexyn‐2‐one yielded a ligand with two conjugated C=C bonds by C−H bond activation at the carbonyl methyl group (7); and 4‐(trimethylsilyl)‐3‐butyn‐2‐one reacted by C−H bond cleavage, formation of an enolate group with a terminal C=C bond, and shift of the proton to the P atom (8). The C≡C bond was not affected. Allene compound 5 rearranged at elevated temperature and in daylight through the formation of a tricyclic compound by C−H bond activation and C−C bond formation. DFT calculations on this unusual rearrangement suggest insertion of the central allene C atom into the C−H bond of a methyl group and the intermediate formation of a C3 ring.
Polarising reactants: An Al/P‐based frustrated Lewis pair (FLP) reacted as an effective two‐electron reductant towards α,β‐unsaturated carbonyl compounds. Coordination of the O atoms to Al polarised the substrates and facilitated the attack of P at the β position, with formation of seven‐membered heterocycles (see scheme). An allenic ligand showed C−H bond activation and rearrangement to a tricyclic compound probably via a three‐membered cyclic transition state.