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P–H Functionalized Al/P-Based Frustrated Lewis Pairs in Dipolar Activation and Hydrophosphination: Reactions with CO2 and SO2

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posted on 2019-07-10, 19:14 authored by Niklas Aders, Lukas Keweloh, Damian Pleschka, Alexander Hepp, Marcus Layh, Friedhelm Rogel, Werner Uhl
The P–H functionalized FLPs R­(H)­PC­(AlBis2)C­(H)-CMe3 (1a, R = Bis; 1b, R = Mes; Bis = CH­(SiMe3)2) combine the typical FLP properties based on Lewis acidic Al and basic P atoms with the reactivity of a P–H bond. They allow the coordination of substrates followed by hydrophosphination with the activated P–H group. Reactions of 1a with R′–NCS (R′ = Ph, CMe3) or of both FLPs with Et–NCCPh2 afforded five-membered AlCPCS/N heterocycles (4 and 5) via coordination of CS or CN bonds to the FLP backbone. Isomerization by a 1,3-H shift from P to N or C (6 and 7) was only achieved in the presence of an auxiliary base (DABCO; 1–5 mol %). (Z)-1a coordinated CO2 to yield the adduct 8, which features a five-membered AlCPCO heterocycle with an exocyclic CO bond. Addition of bases such as DABCO and DBU afforded by deprotonation of the P atom the compounds [Bis-PC­(AlBis2)C­(H)–CMe3(CO2)][HB]+ (9a, B = DABCO; 9b, B = DBU) that displayed hydrogen bonding between the ammonium ions and the exocyclic O atom of the FLP adducts in solution and the solid state. The stronger base n-BuLi afforded the dimeric Li compound 10, in which the Li cation was coordinated in a chelating manner to the oxygen atoms of one FLP adduct and additionally to the exocyclic O atom of the second adduct, resulting in a four-membered Li2O2 heterocycle. The related reaction of (Z)-1a with DABCO­(SO2)2 led in contrast to the elimination of BisH and formation of a SO2-bridged dimer that features a central (AlOSO)2 heterocycle with Al–O and S–O single bonds. The resulting unusual structural motifs may be derived from those of dialkylcarbamic or dialkylamidosulfinic acids with the N atoms replaced by P atoms.

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