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

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Abstract
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.