Reaction of frustrated lewis pairs with ketones and esters

The frustrated Lewis pair (FLP) Mes ₂PCH ₂CH ₂B(C ₆F ₅) ₂ (1) reacts with an enolizable conjugated ynone by 1,4-addition involving enolate tautomerization to give an eight-membered zwitterionic heterocycle. The conjugated endione PhCO-CH=CH-COPh reacts with the intermolecular FLP tBu ₃P/B(C ₆F ₅) ₃ by a simple 1,4-addition to an enone subunit. The same substrate undergoes a more complex reaction with the FLP 1 that involves internal acetal formation to give a heterobicyclic zwitterionic product. FLP 1 reacts with dimethyl maleate by selective overall addition to the C=C double bond to give a six-membered heterocycle. It adds analogously to the triple bond of an acetylenic ester to give a similarly structured six-membered heterocycle. The intermolecular FLP P(o-tolyl) ₃/B(C ₆F ₅) ₃ reacts analogously with acetylenic ester by trans-addition to the carbon-carbon triple bond. An excess of the intermolecular FLP tBu ₃P/B(C ₆F ₅) ₃, which contains a more nucleophilic phosphane, reacts differently with acetylenic ester examples, namely by O-C(alkyl) bond cleavage to give the {R-CO ₂[B(C ₆F ₅) ₃] ₂ ^-}[alkyl-PtBu ₃ ⁺] salts. Simple aryl or alkyl esters react analogously by using the borane-stabilized carboxylates as good leaving groups. All essential products were characterized by X-ray diffraction.