CATALYSIS WITH PLATINUM-GROUP ALKYLAMIDO COMPLEXES, THE ACTIVE PALLADIUM AMIDE IN CATALYTIC ARYL HALIDE AMINATIONS AS DEDUCED FROM KINETIC DATA AND INDEPENDENT GENERATION

Mechanistic studies of the palladium-catalyzed coupling between aryl b romides and tin amides were conducted as-a means to evaluate the pathw ay of this reaction as well as the general potential of low valent ami do complexes to be reactive intermediates in catalysis. The specific s ystems involved reactions between Bu(3)SnNMe(2) and aryl halides catal yzed by {Pd[P(o-Tol)(3)](2)} (1), {Pd[P(o-Tol)(3)](p-MeC(6)H(4))(Br)}( 2) (2a), and {Pd[P(o-Tol)(3)](NHMe(2))(p-MeC(6)H(4))(Br)} (3a). A comb ination of kinetic studies and independent synthesis of reaction inter mediates indicated that the three-coordinate platinum-group amido comp lex {Pd[P(o-Tol)(3)](Ar)(NMe(2))} was an intermediate in these reactio ns. Thus, these aryl halide aminations are rare examples of catalysis with a platinum-group amido complex. Kinetic data were obtained by H-1 NMR spectroscopy, and the rate behavior was determined to be zero ord er in added phosphine, zero order in aryl halide, and first order in t in amide under conditions of equal or greater concentrations of aryl b romide compared to tin amide. Reactions catalyzed by 3a were first ord er in the palladium complex. Reaction rates were inhibited by added ti n bromide, but not by the arylamine product. The inhibition by tin bro mide showed that reversible transmetalation between an aryl halide com plex and the tin reagent was occurring. Subsequent to reversible trans metalation, a rate-determining reductive elimination of arylamine occu rred. Under conditions with a 10-fold excess of tin amide and high pho sphine concentrations, the rate-determining-step became oxidative addi tion of aryl bromide, and reactions became first order, rather than ze ro order, in aryl bromide. The amido intermediate deduced from kinetic studies appeared to be generated by reacting {Pd[P(o-Tol)(3)](p-BuC(6 )H(4))(Br)}(2) (2b) with lithium arylamides or by deprotonating {Pd[P( o-Tol)(3)](NHEt(2))(p-BuC(6)H(4))(Br)} (3b) with MN(SiMe(3))(2) (M = K , Li). Both reactions gave yields of arylamine that were comparable to those of catalytic reactions. Competition and relative rate studies r evealed an equilibrium between aryl halide complexes 2a-c and a till a mide adduct of it. In competition studies involving an in situ selecti vity for reaction of Bu(3)SnNMe(2) or Bu(3)SnNEt(2) a with p-t-BuC(6)H (4)Br catalyzed by 1, the ratio of N,N-dimethylaniline to N,N-diethyla niline was 2.9. However, kinetic measurements of individual reactions showed that Bu(3)SnNMe(2) reacted only 1.4 times faster than Bu(3)SnNE t(2), consistent with a reversible equilibrium involving tin amide bin ding to the catalyst, similar to that resulting from substrate binding preequilibria in enzyme-systems.