Boosting the Nucleophilicity of the Diphenylphosphide Anion with Crown Ether Supported Heavy Alkali Metals to Facilitate Highly Efficient Catalytic Alkene Isomerisation

Caesium and rubidium have long experienced an interest drought in organoelement chemistry in comparison to the vast ocean of applications accomplished mainly by lithium and to a lesser extent, by sodium and potassium in this field. Here, we report a breakthrough study in catalytic alkene isomerisation using crown ether-supported alkali metal phosphides in which the activity increases sequentially and significantly as Group One is descended with Cs(18-crown-6)PPh₂ performing best even at 1 mol% loadings with high turnover frequencies (TOFs) and good functional group tolerance. Elevating its profile further, Cs(18-crown-6)PPh₂ is also successful in a stepwise double catalysis on combining alkene isomerisation with its newly established hydrophosphination (HP) capacity to access challenging to make Markovnikov products. This remote functionalisation approach was also applied to alkynes with terminal alkynes preferentially producing allenes over internal alkynes in one-pot reactions and then under HP, generating highly functionalised vinyl phosphines. Kinetic studies and DFT calculations have also been performed to shed light on mechanistic aspects of the Cs-mediated isomerisation of model alkene allylbenzene.