INDOLPhosphole and INDOLPhos palladium−allyl complexes in asymmetric allylic alkylations

Abstract

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))(η3-C3H5)]PF6 and studied by multidimensional NMR spectroscopy and X-ray crystallography. The allyl ligand undergoes a η3−η1−η3 isomerization in these complexes, which is selective when 1a is the ligand. A tetrameric, boxlike structure, encapsulating a PF6− counteranion, is formed in the solid state in the case of complex 5 ([Pd(3b)(η3-C3H5)]PF6). 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.