Reaction of Frustrated Lewis Pairs with Ketones and Esters

The frustrated Lewis pair (FLP) Mes2PCH2CH2B(C6F5)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 tBu3P/B(C6F5)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(C6F5)3 reacts analogously with acetylenic ester by trans-addition to the carboncarbon triple bond. An excess of the intermolecular FLP tBu3P/B(C6F5)3, which contains a more nucleophilic phosphane, reacts differently with acetylenic ester examples, namely by O?C(alkyl) bond cleavage to give the {R-CO2[B(C6F5)3]2-}[alkyl-PtBu3+] 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.