Heteroatom-substituted secondary phosphine oxides for Suzuki-Miyaura cross-coupling reactions

• Di-substituted diimines and heteroatom-substituted unsaturated HASPO pre-ligands were prepared.
• Four crystal structures were presented.
• The effectiveness of HAPA ligands, the tautomer of HASPOs, in Suzuki reaction is revealed by computational results.
• Pre-ligand of HASPO 6a/ligand of HAPA 7a was found to give satisfactory catalytic performances in cross-coupling reactions.

Several di-substituted diimines (3a–3f) and heteroatom-substituted unsaturated secondary phosphine oxides (HASPO, 6a–6f) were prepared and characterized. Compounds 6a–6f are regarded as pre-ligands because of their ability of tautomerization to heteroatom-substituted phosphinous acid (HAPA, 7a–7f). An unexpected 3e-coordinated palladium dibromide 8e was observed from the reaction of compound 6e with PdBr2. Molecular structures of pre-ligands 6a, 6c, and 6e, as well as palladium complexes 8e were determined by single crystal X-ray diffraction methods. When pre-ligand 6a was applied to palladium-catalyzed Suzuki-Miyaura cross-coupling reactions, satisfactory yields were obtained. Density functional theory were employed to examine the electronic properties of HASPO 6a–6f pre-ligands, their corresponding 1,3-di-N-substituted tautomers 7a–7f, and the saturated counterpart 7as of 7a. Compound 7a is the most effective and genuine ligand in Suzuki-Miyaura reaction that is confirmed by its higher-lying lone-pair (LP) molecular orbital (HOMO-1). The LP orbital of 7c–7f is lower-lying HOMO-5. For each 7c–7f, two conformational rotamers with minute energy difference were located. Hirshfeld charge and population analysis of 7c–7f were also calculated in order to comprehend the electronic properties for these two rotamers for each HAPAs. Besides, the steric effect of HAPAs was estimated in terms of the Percent Buried Volume (%Vbur). This model has shown that 7a has similar steric property to that of PCy3, which is an effective ligand in Suzuki-coupling reactions.

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