| A potentially general solution to anti-Markovnikov olefin hydrofunctionalization was explored. Each of the three steps of the proposed mechanistic pathway was successfully mediated by rhodium tetraphenylporphyrin hydride, (TPP)Rh-H. Step A, olefin insertion into the [Rh]-H bond, produced remarkably stable sigma-alkyl adducts and proceeded readily in the presence of alcohols, aldehydes, carboxylic acids, anilines, carbamates, nitroalkanes and sulfonamides. The anti-Markovnikov regioselectivity (>97%) of the overall hydrofunctionalization reaction was dictated by this step since formation of the less-substituted sigma-alkyl rhodium species is kinetically favored. Step B, intramolecular displacement of [RhI] - from the sigma-alkyl adduct, was accelerated in the presence of base and successfully generated 3-, 5-, and 6-membered heterocycles, including epoxides, tetrahydrofurans, pyrrolidines, lactones, lactams and piperidines. This carbon(sp3)-heteroatom, bond-forming reductive elimination was found to proceed via an SN2 mechanism. Finally, step C required the facile protonation of [RhI]- to regenerate the starting [Rh]-H complex.; A rare example of anti-Markovnikov olefin cyclohydroamination catalyzed by rhodium tetraarylporphyrin hydrides was observed and investigated. An NMR study of this reaction, using 2-isopropenyl-N-(trifluoromethanesulfonyl)aniline as the olefin substrate and (TPP)Rh-H as the catalyst, provided valuable evidence in support of the proposed three-step mechanism. Since nucleophilic cyclization was determined to be the rate-limiting process in C6D6, the application of a polar co-solvent and a more electron-withdrawing porphyrin ligand successfully amplified the reaction velocity. Indeed, when the olefin was treated with 2 mol % (F12TTP)Rh-H (F12TTP = 5,10,15,20-tetrakis( p-trifluoromethylphenyl)porphyrin) and 2 mol % K2CO 3/18-crown-6 in 9:1 alpha,alpha,alpha-trifluorotoluene/sulfolane at 110°C, fifty catalytic turnovers within several hours proceeded with >97% anti-Markovnikov regioselectivity to provide a 97% yield of 3-methyl-N- (trifluoromethanesulfonyl)indoline.; The cyclohydroamination reaction of N-(3-butenyl)trifluoromethanesulfonamide, catalyzed by rhodium tetraarylporphyrin hydrides, was also pursued. The transformation was initially studied with 7 mol % (TPP)Rh-H and 14 mol % K2CO 3/18-crown-6 in [D4]-1,2-dichlorobenzene. The formation of ∼10% of N-(trifluoromethanesulfonyl)pyrrolidine was indeed observed by 1H NMR spectroscopy after 21 hours at 150°C. However, it was discovered that (TPP)Rh(CH2)4NHSO2CF 3, a necessary organometallic intermediate in the catalytic cycle, underwent an oxidative rearrangement that ultimately impeded the desired cyclohydroamination reaction. Despite this difficulty, the early evidence suggests that the catalytic hydroamination of aliphatic alpha-olefins with rhodium tetraarylporphyrin hydrides will be feasible. |
