Surprising Stability of an Al/P‐Based Frustrated Lewis‐Pair Towards Protolysis: HX Adducts (X = F, Cl) with Intramolecular H···X Hydrogen Bonds

Abstract

The Al/P‐based Frustrated Lewis Pair (FLP) Mes2P‐C(=CH‐Ph)‐Al(CMe3)2 (1) reacted with anhydrous HF and HCl as an ion pair receptor by coordination of the proton to phosphorus and the halide anion to aluminum. Short intramolecular distances and the molecular conformation indicate the presence P–H···X–Al hydrogen bonds (X = F, Cl) and an approach of the phosphorus and halogen atoms with a possible weak P···X interaction.

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

“…In most cases, the inherently high Lewis acidity of coordinatively unsaturated Al atoms does not require activation by electron‐withdrawing substituents, and simple alkyl groups are sufficient to facilitate fascinating FLP activity. These compounds have been applied for the coordination and activation of various substrates: carbonyl compounds, terminal alkynes, alkenes, boron halides, chalcogen atoms, hydrogen halides, transition‐metal atoms and so forth . They have found application in phase‐transfer catalysis with the solubilisation of alkali‐metal hydrides in toluene or the catalytic dehydrogenation of H 3 B←NHMe 2 .…”

Dehydrochlorination of Carboxylic Acid Chlorides by an Aluminum/Phosphorus‐Based Frustrated Lewis Pair and the Formation of Ketene Adducts

“…Compounds 4 to 6 showed singlets in the proton-decoupled 31 P NMR spectra in a narrow range between δ = 14.9 and 18.4. Doublets in the 1 H NMR spectra for the vinylic hydrogen atoms of the bridging alkenyl groups, P(Al)C=C(H)Ph, at δ = 7.67 with characteristic 3 J PH coupling constants of 37.8 to 39.1 Hz indicated a cis arrangement of phosphorus and hydrogen atoms across the C=C double bonds [3,9,10,12] and an unchanged configuration of these alkenyl groups. Resonances at δ = 4.45 to 4.89 with relatively small 3 J PH coupling constants of 10.7 to 13.6 Hz were observed for the vinylic hydrogen atoms of the ketene ligands.…”

“…One compound precipitated as a pale yellow solid and was identified by its characteristic NMR spectroscopic data [10] as the HCl adduct of the FLP (3). Particularly indicative were (i) the 31 P NMR resonance at δ = -6.5 and (ii) doublets in the 1 H NMR spectra for the vinylic hydrogen atom (δ = 7.93; 3 J PH = 49.7 Hz) and the P-H group with the expectedly large 1 J PH coupling constant (δ = 9.49; 1 J PH = 506 Hz).…”

“…FLP 1 coordinates HCl very effectively, [10] and we hoped to generate further ketene complexes similar to 2 by dehydrochlorination of suitable carboxylic acid chlorides, which have acidic C-H groups bonded to their molecular backbone as in acetyl chloride. HCl may be captured by one equivalent of 1, whereas a second equivalent of the FLP may trap the reactive ketene intermediates.…”

“…[2] An advantage of these aluminum compounds is the inherently high Lewis acidity of aluminum atoms, which do not need an activation by electron withdrawing fluorinated substituents. The outstanding properties of these FLPs allowed the activation or complexation of various substrates such as carbon dioxide, [3][4][5] terminal alkynes, [3,6] alkenes, [7] hydrogen, [8] carbonyl compounds, [9] isocyanates, [5] hydrogen halides, [10] or transition metal complexes. [11] Their specific acceptor and donor capability was used for the unprecedented solubilization of alkali metal hydrides in hydride transfer reactions [12] and phase-transfer catal-of the C=O groups to the Lewis acidic aluminum and the Lewis basic phosphorus atoms (4 to 6).…”