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

Abstract

The Al/P-based frustrated Lewis pair (FLP) Mes P-C(AltBu) =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 C ring.

Al/P‐ and Ga/P‐Based Frustrated Lewis Pairs and Electronically Unsaturated Substrates: Ring Cleavage and Ring Closure, C−C and C−N Bond Formation

“…We assumed that FLPs 1 and 2 , which have two mesityl groups attached to P, are not able to show similar reactions, because in that case the migration of a bulky mesityl group by cleavage of a P−C bond is required. The previously reported reaction of the t Bu 2 Al/P FLP 1 with cyclopropenone at room temperature resulted selectively in the formation of the cyclic addition product 13 , in which the CO group is bonded to the FLP backbone through Al−O and P−C bonds with an intact C 3 ring (Scheme ) . We did not detect an intermediate and did not recognize the slow secondary reaction discussed below.…”

“…At 200 K the 31 P NMR resonance is shifted to 25.6 ppm ( 3 J HP =38.9 Hz) and may indicate that the equilibrium is on the side of the heterocyclic compound 15 . A chemical shift of δ ( 31 P)=21.7 ppm and relatively broad resonances in the NMR spectra at room temperature may confirm a similar equilibrium for the di( tert ‐butyl) compound 13 in solution.…”

“…In contrast, 15 shows a localized bonding situation in the C 3 ring with C−C distances of 150 (2x) and 131 pm. The presence of two different molecules in a single crystal reflects the comparable energy of both isomers (see below) and gives a nice experimental insight into the mechanism of such carbonyl‐activation processes– which proceed through the approach of the nucleophilic O atom to the Lewis acidic Al atom. Coordination of the O atom results in an increased polarization of the C−O bond and an enhanced electrophilicity of the carbonyl C atom, which facilitates attack of the P atom by P−C bond formation.…”

Cooperative activation of azides by an Al/N-based active Lewis pair – unexpected insertion of nitrogen atoms into C–Si bonds and formation of AlCN3 heterocycles

“…Usually the O or N atoms of substrates (carbonyl compounds, imines) approach the Lewis acidic Al atoms of the FLPs or ALPs as the initiating step of the respective transformation. By the interaction with the metal atom the polarity of the C=O or C=N bonds and the electrophilicity of the C atoms increase and facilitate the nucleophilic attack of the Lewis basic ALP or FLP centers at this position by ring closure [7,8] or by initiating another secondary reaction such as an insertion into a M-C bond [12,13,16]. Azides with a homonuclear N 3 group react with Al/P-based FLPs by coordination of the terminal N atom to the Lewis acidic and basic atoms (see Introduction).…”

“…These highly promising materials are considered as attractive alternatives or complements for transition metal catalysts. The majority of FLPs is based on systems with B and P atoms as Lewis acids and Lewis bases [1][2][3][4], but it has recently been shown that Al/P based systems are also highly efficient in various transformations [5][6][7][8][9][10]. Al atoms possess an inherently high Lewis acidity, which makes an activation by electron-withdrawing substituents, as required in B based systems, unnecessary.…”