Silver Phosphanes Partnered with Carborane Monoanions: Synthesis, Structures and Use as Highly Active Lewis Acid Catalysts in a Hetero-Diels–Alder Reaction

Four Lewis acidic silver phosphane complexes partnered with [1-closo-CB11H12]− and [1-closo-CB11H6Br6]− have been synthesised and studied by solution NMR and solid-state X-ray diffraction techniques. In the complex [Ag(PPh3)(CB11H12)] (1), the silver is coordinated with the carborane by two stronger 3c–2e B−H−Ag bonds, one weaker B−H−Ag interaction and a very weak Ag⋅⋅⋅Carene contact in the solid state. In solution, the carborane remains closely connected with the {Ag(PPh3)}+ fragment, as evidenced by 11B chemical shifts. Complex 2 [Ag(PPh3)2(CB11H12)]2 adopts a dimeric motif in the solid state, each carborane bridging two Ag centres. In solution at low temperature, two distinct complexes are observed that are suggested to be monomeric [Ag(PPh3)2][CB11H12] and dimeric [Ag(PPh3)2(CB11H12)]2. With the more weakly coordinating anion [CB11H6Br6]− and one phosphane, complex 3 [Ag(PPh3)(CB11H6Br6)] is isolated. Complex 4, [Ag(PPh3)2(CB11H6Br6)], has been characterised spectroscopically. All of the complexes have been assessed as Lewis acids in the hetero-Diels–Alder reaction of N-benzylideneaniline with Danishefsky's diene. Exceptionally low catalyst loadings for this Lewis acid catalysed reaction are required (0.1 mol %) coupled with turnover frequencies of 4000 h−1 (quantitative conversion to product after 15 minutes using 3 at room temperature). Moreover, the reaction does not occur in rigorously dry solvent as addition of a substoichiometric amount of water (50 mol %) is necessary for turnover of the catalyst. It is suggested that a Lewis assisted Brønsted acid is formed between the water and the silver. The effect of changing the counterion to [BF4]−, [OTf]− and [ClO4]− has also been studied. Significant decreases in reaction rate and final product yield are observed on changing the anion from [CB11H6Br6]−, thus demonstrating the utility of weakly coordinating carborane anions in organic synthesis.