The (pi-allyl)palladium complex bearing an sp2-hybridized phosphorus ligand (DPCB-OMe: 1,2-bis(4-methoxyphenyl)-3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutene) efficiently catalyzes direct conversion of allylic alcohols in the absence of activating agents of alcohols such as Lewis acids. N-Allylation of aniline proceeds at room temperature to afford monoallylated anilines in 90-97% yields. C-Allylation of active methylene compounds is also successful at 50 degrees C using a catalytic amount of pyridine as a base, giving monoallylation products in 85-95% yields. The catalytic mechanism involving hydrido- and (pi-allyl)palladium intermediates has been proposed on the basis of stoichiometric examinations using model compounds of presumed intermediates.

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-CF3), 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 [Pt2(μ-H)2(DPCB)2](OTf)2 (4) by the treatment of PtMe(OTf)(DPCB) (5) with HSiMe2Ph 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 HSiMe2Ph 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.

Dimethylplatinum(II) complexes coordinated with 1,2-diaryl-3,4-bis[(2,4,6-tri-tert-butylphenyl)phosphinidene]cyclobutenes (DPCB-Y; aryl = 4-methoxyphenyl, phenyl, 4-trifluoromethylphenyl, 3,5-bis(trifluoromethyl)phenyl] are prepared by ligand displacement of Pt2Me4(μ-SMe2)2 with corresponding DPCB-Y, which is a novel class of ligands having sp2-hybridized phosphorus as coordination atoms. Comparison of the NMR data with those of diimine- and diphosphine-coordinated analogues indicates intermediate magnitude of trans-influence of DPCB-Y ligands. This observation is supported by X-ray structural analysis. On the other hand, structural deviations of the coordinated DPCB-Y from free DPCB-Y suggest the occurrence of strong π-back-donation from the platinum to the ligands. Reflecting the strong π-back-donation, the ethylene protons in [PtMe(η2-C2H4)(DPCB-Y)]OTf appear at δ 5.22−5.02 in the 1H NMR spectra, the chemical shifts of which are significantly lower than those so far reported for platinum(II) complexes (ca. δ 4.0) and comparable to that of free ethylene (δ 5.30). Ethylene polymerization using methylpalladium catalysts bearing DPCB-Y ligands is also reported.

Methylplatinum triflate coordinated with 1,2-diphenyl-3,4-bis[(2,4,6-tri-t-butylphenyl)phosphinidene]cyclobutene serves as a highly selective catalyst for dehydrogenative silylation of ketones with HSiMe2Ph in the presence of pyridine as a co-catalyst, giving silyl enol ethers in high yields.