Modular pyridine-type P,N-ligands derived from monoterpenes: application in asymmetric Heck addition

Novel (diphenylphosphinophenyl)pyridine ligands (+)-8, (+)-15, (−)-21, and (−)-26 were synthesized from (−)-β-pinene, (+)-3-carene, (+)-2-carene, and (−)-α-pinene, respectively, via Kröhnke annulation as the key step, and shown to effect ≤88% ee in Heck addition (27→28). Ligands (+)-15 and (−)-21 are quasi-enantiomeric; ligands 8 and 26 can be prepared in both enantiomeric forms from (+)- and (−)-enantiomers of α- and β-pinene, respectively.