Catalytic C−O Bond Cleavage of Allylic Alcohols Using Diphosphinidenecyclobutene-Coordinated Palladium Complexes. A Mechanistic Study

The mechanism of C-O bond cleavage of allylic alcohols promoted by the hydridopalladium complexes PdH(OTf)(DPCB-Y) (2), bearing 1,2-diaryl-3,4-bis[(2,4,6-tri-tert-butylphenyl)-phosphinidene]cyclobutene ligands (DPCB-Y), has been investigated (aryl = 4-(trifluoromethyl)phenyl (DPCB-CF 3 ), phenyl (DPCB), 4-methoxyphenyl (DPCB-OMe), 4-octyloxyphenyl (DPCB-OOct)). This reaction forms the (π-allyl)palladium complexes [Pd(π-allyl)(DPCB-Y)]-OTf (1), which are key intermediates for the catalytic allylation of aniline with allylic alcohols. The platinum analogue of 2 is obtained as the hydrido-bridged dimer [Pt 2 (p-H) 2 (DPCB) 2 ]-(OTf) 2 (4) by the treatment of PtMe(OTf)(DPCB) (5) with HSiMe 2 Ph in the presence of a small amount of water. Complex 4 cleaves the C-O bond of allylic alcohols at 50 °C, yielding the π-allyl complexes [Pt(π-allyl)(DPCB)]OTf (7). Although complex 2, similarly prepared by the reaction of PdMe(OTf)(DPCB) (5) with HSiMe 2 Ph and water, is too unstable to be identified, its formation is confirmed by trapping experiments using dienes to give the corresponding π-allyl complexes. Complex 2, thus generated, instantly reacts with allylic alcohols at room temperature to afford the π-allyl complex 1 in high yield. The intermediacy of 2 in the catalytic allylation is further examined by kinetic experiments on actual catalytic systems, leading to mechanistic details of C-O bond cleavage promoted by 2.