Silver phosphanes partnered with carborane monoanions: Synthesis, structures and use as highly active Lewis acid catalysts in a hetero-Diels-Alder reaction

Nathan J. Patmore, Catherine Hague, Jamie H. Cotgreave, Mary F. Mahon, Christopher G. Frost, Andrew S. Weller

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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 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 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.

Original languageEnglish
Pages (from-to)2088-2098
Number of pages11
JournalChemistry - A European Journal
Issue number9
Early online date24 Apr 2002
Publication statusPublished - 3 May 2002
Externally publishedYes


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