Intermolecular Insertion of Ethylene and Octene into a Palladium-Amide Bond. Spectroscopic Evidence for an Ethylene Amido Intermediate Patrick S. Hanley, Dean Markovic ´, and John F. Hartwig* Department of Chemistry, UniVersity of Illinois, 600 South Mathews AVenue, Urbana, Illinois 61801 Received March 15, 2010; E-mail: jhartwig@illinois.edu Additions to olefins comprise some of the most utilized catalytic reactions. 1 Many of these reactions occur by transfer of a hydride or an alkyl group from a transition-metal complex to a coordinated alkene in a reaction commonly termed migratory insertion. 2 Migratory insertion of an olefin into a transition metal-amide bond is much less established but has been proposed to occur during several classes of palladium-catalyzed reactions, including carboaminations, 3 oxidative aminations, 4 diaminations, 5 aminoacetoxylations, 6 chloroaminations, 7 and hetero-Heck type transformations. 8 Stereochemical evidence for syn-aminopalladation by migratory insertion has been gained during studies of some of these catalytic reactions, 3-6,9 but intermolecular insertion of an unactivated alkene into an isolated palladium-amido complex has not been reported. 10,11 Here, we describe a series of palladium diarylamido complexes that react with unactivated alkenes, including the simplest alkene, ethylene, to form enamine products from initial intermolecular insertion. The stereochemistry of the enamine products, along with kinetic data, supports a migratory insertion pathway, and we have obtained spectroscopic evidence at -100 °C for an ethylene amido intermediate that undergoes migratory insertion at -40 °C. The preparation of the palladium-amido complexes in this study (2a-e) is depicted in Figure 1. To generate complexes that are isoelectronic with the rhodium amido complexes reported previously by our group to insert alkenes, 11 an anionic ancillary ligand is necessary. To encourage monomeric structures and to discourage C-N reductive elimination from unsaturated arylpalladium amido com- plexes, 12 a cyclometalated complex generated from a hindered benzylic phosphine was studied. The stable THF-ligated Pd-amido complexes 2a-d were formed from the reaction of [(P-C)PdCl] 2 (1) with KNAr 2 in THF. Complex 2e was synthesized by proton transfer between 2a and HN((3,5-CF 3 ) 2 C 6 H 3 ) 2 . The bound THF in 2a-e was evidenced by broad resonances in the 1 H NMR spectrum between δ 3.56-3.32 and 1.35-0.94 ppm that integrated to 1 equiv of THF in C 6 D 6 . Apparently, the bulky tert-butyl groups and the weak basicity of the diarylamido nitrogen inhibit formation of an N-bridged amido dimer. An ORTEP diagram of 2a is shown in Figure 1. Complex 2a possesses a square planar geometry, and the substituents on the diarylamide lie on either side of the square plane. The Pd-N distance is within error of that of a related three-coordinate diarylamide 12 and the average for palladium-amido complexes (2.083 Å) in the CCD. The Pd-C bond is only ∼0.03 Å longer than that in the previous arylpalladium diarylamido complex. The P-Pd-C angle (82.4°) is typical for a five-membered palladacycle. Amido complexes 2a-e reacted with alkenes to form enamine products, and the yields and rate constants for these reactions are shown in Table 1. The reaction of 2a with ethylene for 2 h at -10 °C formed the N-vinyldiarylamine product in 89% yield. The reaction of 2a with neat 1-octene at 80 °C for 30 min generated a mixture of three isomeric enamines in 74% yield. Reactions with 1-octene were conducted in both polar and nonpolar solvents, including diethyl ether, DMF, benzene, and toluene; the highest yields were obtained from reactions in toluene or benzene. The data in Table 1 show that the complexes containing the more electron-donating amido groups occurred faster than those containing the less electron-donating amido groups. Complex 2b containing the most electron-donating di-p-ansiylamido ligand reacted approximately 2 times faster than complex 2c containing di-p-tolylamido groups and 10 times faster than complexes 2a and 2d containing the less electron- donating diphenyl and p-fluorophenylamido ligands. Likewise, com- plexes 2a and 2d reacted much faster than complex 2e containing the bis-trifluoromethyl-substituted diarylamido ligand. Figure 1. Preparation of THF ligated amido complexes, and an ORTEP drawing of 2a with 35% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond angles (degrees) and lengths (Å): P-Pd-C, 82.4(2); P-Pd-N, 176.5(2); C-Pd-N, 94.2(2); N-Pd-O, 85.8(2); P-Pd-O, 97.59(9); C-Pd-O, 179.4(2); Pd-C, 2.001(5); Pd-P, 2.249(1); Pd-N, 2.082(2); Pd-O, 2.264(3). Table 1. Reactions of Ethylene and 1-Octene with Amides 2a-2e a entry complex For R ) H (yield) for R ) H k obs × 10 3 (s -1 ) For R ) C 6 H 13 (Yield) b 1 2a 89% 0.91 (neat) 74% 2 2a - - (25 equiv) 69% 3 2a - - (10 equiv) 48% 4 2b 94% 9.6 97% 5 2c 63% 4.3 64% 6 2d 60% 0.79 52% 7 c 2e 98% 0.053 ND d a Conditions for reactions with 1-octene: benzene, 80 °C for 30 min. Conditions for reactions with ethylene: toluene, -10 °C for 2 h, 20 equiv of ethylene. b Combined yield for all enamine isomers. c Reaction at 85 °C. d This reaction did not form detectable amounts of the enamine product. Published on Web 04/21/2010 10.1021/ja102172m  2010 American Chemical Society 6302 9 J. AM. CHEM. SOC. 2010, 132, 6302–6303