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Synthesis of chiral supramolecular bisphosphinite palladacycles through hydrogen transfer-promoted self-assembly process

Abstract : P-Chiral secondary phosphine oxides react with Pd 2 (dba) 3 in an acidic medium to provide chiral supramolecular bisphosphinite palladacycles through a H-transfer-based self-assembly process prior to SPO-promoted oxidative addition of an acid to a Pd(0) centre. The one-pot methodology allows variations of the X-type ligand as desired. Eight complexes have been characterised by X-ray diffraction. Supramolecular chemistry has witnessed huge growth due to a host of applications in a variety of areas including self-assembly phenomena. 1 A broad spectrum of research in coordination has been conducted since supramolecular interactions are nowadays used to self-assemble ligands to tune the coordination spheres of metal catalysts. For instance, there is scope for the synthesis of supramolecular bidentate ligands through non-covalent interactions such as hydrogen bonding, coordinate bonding, and ionic bonding between two monodentate ligands. 2 In this respect, secondary phosphine oxides (SPOs) have become an appealing preligand class owing to their potential for the construction of self-assembled bidentate ligands via hydrogen interaction. In concrete terms, the pentavalent tetracoordinated phosphorous form P(V)(s 4 l 5) is usually in favourable tautomeric equilibrium with the trivalent tricoordinated phosphorous form, namely, phosphinous acid (PA) P(III)(s 3 l 3). This equilibrium is shifted towards the trivalent phosphorous tautomer in the presence of a late transition metal via coordination through the unshared electron pair of the phosphorous atom (Scheme 1a). 3 Although seldom exhibited, two cis-coordinated PAs arising from enantio-pure SPOs may lead to a chiral six-membered hydrogen-bonded monoanionic diphosphorous chelate structure, 4 resulting from the deprotonation of one of the PAs by either an added base 5 or the anionic X-type ligand dissociated from the metal (Scheme 1b). 6 As bidentate ligands are of crucial importance to steer the selectivity outcome of a transition-catalysed reaction, it is thus not surprising that such PA-based supramolecular chelate structures are extensively used in homogenous catalysis 7 with avant la lettre examples disclosed by Van Leeuwen as the backdrop. 8 Achieving variations of the chelate structure seems rather straightforward as evidenced by the extensive available metal(II) complexes displaying relevant and subtle alterations of bite and cone angles by changing R 1 and R 2. In contrast, accessing complexes in which the anionic X-type ligand is different from chloride or acetate may be a challenging task since it heavily depends on the used starting metal(II) source (Scheme 1b). To date, this challenge necessitates either the presynthesis of a non-commercial metal(II) starting source or subsequent additional synthetic steps from appropriately chosen home-prepared PA-based metallacycles such as the silver salt-promoted substitution of the chloride ligand of [Pd{(1) 2 H}] 2 (m-Cl) 2 when more original pallada-cycles are desired. 3 Given the influence of this ligand type on the course of Pd-catalysed reactions, 9 developing a one-pot method that allows direct access to any supramolecular bisphosphinite palladacycles appears highly desirable but is a huge challenge. Scheme 1 Coordination of PAs to soft transition-metal centre.
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Alexandre Vasseur, Romain Membrat, Davide Palpacelli, Michel Giorgi, Didier Nuel, et al.. Synthesis of chiral supramolecular bisphosphinite palladacycles through hydrogen transfer-promoted self-assembly process. Chemical Communications, Royal Society of Chemistry, 2018, 54 (72), pp.10132 - 10135. ⟨10.1039/c8cc06283h⟩. ⟨hal-01870774⟩



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