A Frustrated Lewis Pair Based on a Cationic Aluminum Complex and Triphenylphosphine

Abstract

The highly Lewis acidic, cationic aluminum species [DIPP-nacnacAlMe] + [B(C 6 F 5 ) 4 ] − (1, DIPP-nacnac = [HC{C(Me)N(2,6-i Pr 2 C 6 H 3 )} 2 ] − ) has been shown to undergo reactions with a wide variety of small molecules, in both the presence and absence of an external weak phosphine base, PPh 3 . Cycloaddition reactions of unsaturated C−C bonds across the aluminum diketiminate framework are reported, and the first structural confirmation of this type of cycloaddition product is presented. Addition of PPh 3 to 1 produces the cationic aluminum phosphine complex [DIPP-nacnacAl(Me)PPh 3 ] + [B(C 6 F 5 ) 4 ] − , which undergoes fluxional dissociation/ coordination of the phosphine in solution. This weak Al−P interaction can be utilized in frustrated Lewis pair type reactions to activate alkenes, alkynes, CO 2 , propylene oxide, and the C−Cl bonds of CH 2 Cl 2 . The CO 2 adduct [DIPP-nacnacAl(Me)OC-(PPh 3 )O] + [B(C 6 F 5 ) 4 ] − undergoes further stoichiometric reduction with Et 3 SiH to produce an aluminum formate species.

On the non-innocence of “Nacnacs”: ligand-based reactivity in β-diketiminate supported coordination compounds

“…Similar cycloaddition reactions of unsaturated C-C bonds across the aluminium BDI framework in Al(BDI*)R + (R = i Bu, Et) cations were previously reported by Jordan (Scheme 27). 130 Yet it is unclear if metal-BDI cooperation is playing a role in these reactions. For example, the bicyclic ethylene adduct shown on Scheme 27 is stable and does not release ethylene under vacuum, but undergoes rapid and complete ethylene exchange at the bridging site with ethylene-d 4 or 2-butyne, presumably by initial ethylene extrusion.…”

Electronically Unsaturated Three‐Coordinate Aluminum Hydride and Organoaluminum Cations

“…A number of four‐, five‐ and six‐coordinate cationic aluminum species are known with their applications ranging from polymerization of cyclic ethers (propylene oxide), esters (ϵ‐capralactone), isobutene, and β‐hydrogen‐transfer reactions to olefins or ketones . It is not surprising that two and three‐coordinate cationic aluminum species, due to the coordinative unsaturation at the metal, are very rare, and the well‐characterized ones are shown in Figure …”

Frustrated Lewis Pair Chemistry with Magnesium Lewis Acids

“…The anilido imine complexes (L H MgBr) 2 ( 1 ) and (L Me MgBr) 2 ( 2 ) were screened as Lewis acids in FLP reactivity. For the Lewis base we chose the very simple air‐stable Ph 3 P which, in combination with Lewis‐acidic cationic Al complexes, displayed FLP reactivity . Addition of Ph 3 P to solutions of 1 and 2 in C 6 D 6 did not change the 1 H NMR spectra, and the 31 P NMR spectrum shows a signal for free Ph 3 P. Various electrophiles were added to a solution of (L H MgBr) 2 or (L Me MgBr) 2 and Ph 3 P in C 6 D 6 at room temperature, and samples were investigated by 1 H and 31 P NMR spectroscopy (Scheme ).…”

“…Although the majority of systems is limited to P/B or N/B pairs, the use of Al‐, C‐, or Si‐based Lewis acids emerged. Especially FLPs with Al Lewis acids have been able to add unique metal‐based reactivities to the growing repertoire: for example, the complexation of alkali metal hydrides, stoichiometric and catalytic dehydrocoupling of amine–boranes, activation of alkenes,, alkynes,, carbodiimides, isocyanates, hydrogen, CO 2 , and other carbonyl groups .…”

“…In contrast to the complex in Scheme , we aim to focus on intermolecular P/Mg FLP reactivity. Our goal is to synthesize Lewis‐acidic complexes of the type L–Mg–X (in which L is a nonreactive spectator ligand and X is a heavier halide) and test its FLP reactivity with Ph 3 P, a simple air‐stable phosphine that recently was found to be active as an FLP in combination with a cationic Al complex …”