Olefin insertion in Pd-acyl complexes modified with 1,4-Cs-symmetrical diphosphine ligands

The insertion of ethene and propene was investigated in palladium(II) acyl complexes of the type [PdC(O)CH3(P∧P′)(CH3CN)](OTf) modified with the Cs-symmetric diphosphines 2–4 and the parent ligand 1, described by C2v-symmetry and taken as a reference. Ethene insertion was investigated for acyl complexes containing the ligands 2 and 3. Two insertion products formed in a ratio of approximately 1:1 for both systems, irrespective of the electronic properties of the ligands. Propene as an α-olefin can insert according to a 1,2- or 2,1-insertion mode into a palladium acyl bond, arising regioselectivity issues. Moreover, due to the Cs-symmetry of the ligands, two stereoisomers can result upon insertion, as the alkyl group of the formed five-membered metallacycle can be cis or trans to each non-equivalent moiety. Propene insertion was indeed neither stereo- nor regioselective in the cases of 3 and 4, in which the products arising from both 1,2- and 2,1-insertion were observed. 2 displayed total control of stereo- and regioselectivity, with the formation of one primary insertion product. Similar regioselectivity was observed for the reference ligand 1. The regioisomeric distribution was different from equimolar for propene insertion, where the ratio of the products might be controlled by a combination of steric and electronic factors.