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Silver Phosphanes Partnered with Carborane Monoanions: Synthesis, Structures and Use as Highly Active Lewis Acid Catalysts in a Hetero‐Diels–Alder Reaction
Author(s) -
Patmore Nathan J.,
Hague Catherine,
Cotgreave Jamie H.,
Mahon Mary F.,
Frost Christopher G.,
Weller Andrew S.
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20020503)8:9<2088::aid-chem2088>3.0.co;2-l
Subject(s) - lewis acids and bases , carborane , chemistry , catalysis , monomer , solvent , medicinal chemistry , stereochemistry , polymer chemistry , organic chemistry , polymer
Abstract Four Lewis acidic silver phosphane complexes partnered with [1‐ closo ‐CB 11 H 12 ] − and [1‐ closo ‐CB 11 H 6 Br 6 ] − have been synthesised and studied by solution NMR and solid‐state X‐ray diffraction techniques. In the complex [Ag(PPh 3 )(CB 11 H 12 )] ( 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(PPh 3 )} + fragment, as evidenced by 11 B chemical shifts. Complex 2 [Ag(PPh 3 ) 2 (CB 11 H 12 )] 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(PPh 3 ) 2 ][CB 11 H 12 ] and dimeric [Ag(PPh 3 ) 2 (CB 11 H 12 )] 2 . With the more weakly coordinating anion [CB 11 H 6 Br 6 ] − and one phosphane, complex 3 [Ag(PPh 3 )(CB 11 H 6 Br 6 )] is isolated. Complex 4 , [Ag(PPh 3 ) 2 (CB 11 H 6 Br 6 )], 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 [BF 4 ] − , [OTf] − and [ClO 4 ] − has also been studied. Significant decreases in reaction rate and final product yield are observed on changing the anion from [CB 11 H 6 Br 6 ] − , thus demonstrating the utility of weakly coordinating carborane anions in organic synthesis.