INDOLPhosphole and INDOLPhos Palladium−Allyl Complexes in Asymmetric Allylic Alkylations

The new INDOLPhosphole ligands 3a, b are obtained in good yield in a two-step synthetic sequence from 3-methylindole, ( S )-BINOL, and the corresponding cyanophosphole. Palladium−allyl complexes have been prepared from INDOLPhosphole ( 3b ) and INDOLPhos ( 1a ) of the type [Pd(INDOLPhos(phole))(η 3 -C 3 H 5 )]PF 6 and studied by multidimensional NMR spectroscopy and X-ray crystallography. The allyl ligand undergoes a η 3 −η 1 −η 3 isomerization in these complexes, which is selective when 1a is the ligand. A tetrameric, boxlike structure, encapsulating a PF 6 − counteranion, is formed in the solid state in the case of complex 5 ([Pd( 3b )(η 3 -C 3 H 5 )]PF 6 ). INDOLPhosphole ligands 3a, b and a small library of INDOLPhos ligands were screened in the Pd-catalyzed asymmetric allylic alkylation of mono- and disubstituted allylic acetates. The catalysts derived from these ligands were highly active, and enantioselectivities were obtained for 1,3-diphenylprop-2-enyl acetate up to 90% ee. Cinnamyl acetate was converted quantitatively with low regioselectivity (b/l = 14/86) and good enantioselectivity up to 81% ee. In the case of disubstituted substrates, the absolute configuration of the product could be rationalized by a model of selective attack on one of the enantiotopic termini in the Pd−allyl intermediate.