Cyclic Phosphonium Bis(fluoroaryl)boranes – Trends in Lewis Acidities and Application in Diels–Alder Catalysis

The rigid cyclic Lewis acids [(C 6 F 5 )B(CH 2 )(C 6 F 4 )P( t Bu) 2 ] + ([ 1 ] + ), [(C 6 F 5 )B(CH 2 )(C 6 F 3 H)P( t Bu) 2 ] + ([ 2 ] + ), and [(C 6 H 5 )B(CH 2 )(C 6 F 4 )P( t Bu) 2 ] + ([ 3 ] + ), possess Gutmann acceptor numbers of AN = 87.3, 85.7, and 85.7, respectively, which are among the highest for organoboranes. Starting from ( 1 )OTf, adducts [( 1 )Do][OTf] (Do = OPEt 3 , pyridine, H 2 O) have been prepared and fully characterized. In all three cases, X‐ray crystallography revealed significantly shorter B–O/N bond lengths than those in the corresponding adducts (C 6 F 5 ) 3 B · Do. Both the free Lewis acid [ 1 ][Al(O( t Bu F )) 4 ] and its triflate adduct ( 1 )OTf have successfully been employed as catalysts for the [4+2] cycloaddition reaction between 2,5‐dimethyl‐1,4‐benzoquinone and cyclopentadiene. Moreover, we have shown that chiral phosphonium boranes are accessible by replacement of one t Bu group in [ 1 ] + by a Me substituent {cf. [ 4 ] + = [(C 6 F 5 )B(CH 2 )(C 6 F 4 )P(Me)( t Bu)] + }.