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Refluxing a mixture of (dppm)palladium(II) dichloride (where dppm is bis(diphenylphosphino)-methane) and various alkynes in the 1,2-dichloroethane/ 1,4-dioxane mixed solvents affords a variety of Pd-bound alkenyl phosphorus ylides in modest to good yields. Detailed spectroscopic data for these Pd-bound alkenyl phosphorus ylides along with the X-ray single-crystal structure of 1c are reported. The mechanism and implications for such an unprecedented apparent insertion of alkynes into the Pd-phosphine bond are also discussed.Phosphines are among the most important ancillary ligands for transition-metal catalysts. In particular, palladium phosphine complexes have been widely used to catalyze many organic transformations that utilize unsaturated hydrocarbon substrates. 1 For example, additions of HX to unsaturated hydrocarbons (e.g., hydrogenation 2 and hydrosilation 3 reactions) and carboncarbon bond forming reactions 4 (e.g., Heck and Stille reactions) are readily catalyzed by palladium phosphine complexes. Under these often forcing conditions, orthometalation and carbon-phosphorus bond cleavage of arylphosphines represent common catalyst deactivation pathways. 5 Herein we wish to report an unprecedented apparent insertion of alkynes into palladium-phosphine bonds to result in the first examples of alkenyl phosphorus ylides of palladium. Such a phosphorus-carbon bond forming process will lead to thermodynamically stable, catalytically inactive alkenyl phosphorus ylides and thus presents a potential catalyst deactivation pathway for palladium-catalyzed organic transformations with unsaturated hydrocarbon substrates.Refluxing a mixture of (dppm)palladium(II) dichloride 6 (where dppm is bis(diphenylphosphino)methane) and excess phenylacetylene in the 1,2-dichloroethane/ 1,4-dioxane mixed solvents results in the Pd-bound alkenyl phosphorus ylide 1a in 49% yield after purification by column chromatography (eq 1). The 1 H NMR spectrum of 1a exhibits a characteristic doublet of doublet for the alkenyl proton at δ 5.88; the alkenyl proton is coupled to two inequivalent phosphorus nuclei with 2 J H-P ) 47.7 Hz and 4 J H-P ) 10.3 Hz. The methylene protons of the Pd-bound alkenyl phosphorus ylide ligand also appear as a doublet of doublet at δ 3.69 due to the coupling to two inequivalent phosphorus nuclei. Consistent with the formation of the phosphorus ylide, the 31 P{ 1 H} NMR spectrum of 1a shows two downfield doublets at δ 50.1 and 47.1. The characteristic CtC stretch at ∼2106 cm -1 for phenylacetylene has also disappeared in the IR spectrum of 1a.Other alkynes with either electron-donating or electron-withdrawing substituents can be used in place of phenylacetylene in this insertion reaction to result in a variety of Pd-bound alkenyl phosphorus ylides. Compounds 1b-d were synthesized by refluxing a mixture of (dppm)palladium dichloride and n-undecyne, (p-(dimethylamino)phenyl)acetylene, 7 and (p-nitrophenyl)-acetylene 7 in the 1,2-dichloroethane/1,4-dioxane mixed solvents, respectively. These Pd-bound alkenyl phosphorus ...

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