36. Silver Phosphanes Partnered with Carborane Monoanions: Synthesis, Structures and Use as Highly Active Lewis Acid Catalysts in a Hetero-Diels-Alder Reaction
Chemistry-A European Journal 2002, 8, 2088-2098
DOI: 10.1002/1521-3765(20020503)8:9
Patmore, N. J.; Hague, C.; Cotgreave, J. H.; Mahon, M. F.; Frost, C. G.; Weller, A. S.
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.C-arene contact in the solid state. In solution. the carborane remains closely connected with the {Ag(PPh3)}+ fragment, as evidenced by B11 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 ate 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 Lex is 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 4000h-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 Bronsted 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.