Palladacycles with a Metal-Bonded sp3-Hybridized Carbon as Intermediates in the Synthesis of 2,2,3,4-Tetrasubstituted 2H-1-Benzopyrans and 1,2-Dihydroquinolines. Effects of Auxiliary Ligands and Substitution at a Palladium-Bonded Tertiary Carbon

Abstract

Prompted by the failure to prepare stable oxapalladacycles L2Pd-1-C6H4-2-OC(R1)COOEt with PPh3 ligands (L = PPh3) and substituents R1 other than hydrogen, systematic studies on the effects of steric and electronic properties of auxiliary ligands L and substituents R1 (alkyl, aryl) on the feasibility of formation, stability, and reactivity of the oxapalladacycles were performed. Using N,N,N‘,N‘-tetramethyl-1,2-ethylenediamine bidentate ligand (L−L = TMEDA), stable palladacycles featuring substituents R1 (Me, Et, i-Pr, and Ph) were prepared, and the presence of palladium-bonded tertiary sp3-hybridized carbons was confirmed by X-ray crystallographic analyses on palladacycles with i-Pr and Ph substituents R1. Relying on 31P NMR analyses of crude reaction mixtures, ligand displacement reactions of palladacycles (TMEDA)Pd-1-C6H4-2-OC(R1)COOEt with monodentate phosphines (PPh3 and PPh2Me) were studied, as well as base-mediated ring-closure reactions of arylpalladium(II) iodo complexes (PPh3)2Pd(-1-C6H4-2-OCHR1COOEt)I and (PPh2Me)2Pd(-1-C6H4-2-OCHR1COOEt)I. In both series, higher conversions to corresponding palladacycles L2Pd-1-C6H4-2-OC(R1)COOEt were achieved with PPh2Me ligands, and both the transformations were negatively affected by an increase in the steric bulk of substituents R1. Although the palladacycles with monodentate phosphines could not be isolated as stable entities, IR spectroscopic analyses supported the presence of a Csp3−Pd bond. On the basis of these studies, a one-pot conversion of stable arylpalladium(II) iodo complexes with PPh2Me ligands into 2,2,3,4-tetrasubstituted 2H-1-benzopyrans and 1,2-dihydroquinolines via the insertion of unsymmetrical alkynes into in situ formed palladacycles was developed, providing the heterocycles in good yields (44−80%) as single regioisomers.