Synthesis and asymmetric catalytic application of chiral imidazolium–phosphines derived from (1R,2R)-trans-diaminocyclohexane

Reaction between a chiral imidazole–amine precursor derived from (1R,2R)-trans-diaminocyclohexane and P1Cl (where P1 = PPh2, P(1,3,5-Me3C6H3)2, P(2,2′-O,O′-(1,1′-biphenyl), P((R)-(2,2′-O,O′-(1,1′-binaphthyl))) and P((S)-(2,2′-O,O′-(1,1′-binaphthyl)))) followed by RX (where R = nPr, iPr, CHPh2, X = Br; R = iPr, X = I), respectively, gives a selection of chiral imidazolium–phosphine compounds. Deprotonation of the imidazolium salt gives the corresponding NHC–P ligands that can be used in metal-mediated asymmetric catalytic applications. Catalytic reactions show that NHC–P ligands give a significantly greater rate of reaction for a palladium catalysed allylic substitution reaction in comparison to analogous di-NHC or NHC–imine ligands and that NHC–P hybrids are also effective for iridium catalysed transfer hydrogenation.

Graphical abstractReaction between the imidazole amine 4 and chlorophosphines gives imidazole–phosphines that serve as precursors to chiral N-heterocyclic carbene (NHC)-phosphine derivatives. The NHC–P compounds have been applied as ancillary ligands to two transition metal catalysed asymmetric reactions.Figure optionsDownload full-size imageDownload as PowerPoint slide