| The objectives of this research focus on the investigation of carbon-hydrogen bond activation (CHA) and carbon-carbon bond activation (CCA) of alkanes by rhodium porphyrin complexes as well as the mechanistic understanding.;Base-promoted CHA of unstrained alkanes with 5,10,15,20-tetratolylporphyrinatorhodium complexes, Rh(ttp)X (X = Cl, H, Rh(ttp)), has been achieved. Rh(ttp)Cl, reacted with n-pentane, n-hexane, n-heptane, c-pentane and c-hexane in the presence of potassium carbonate at 120 °C in 6 to 24 h to give rhodium porphyrin alkyls, Rh(ttp)R, in 29--76% yields. Mechanistic investigations suggested that Rh 2(ttp)2 and Rh(ttp)H are key intermediates for the parallel CHA step. The roles of base are (i) to facilitate the formation of Rh(ttp)Y (Y- = OH-, KCO3 -), (ii) to enhance the CHA rate with alkane and generate Rh(ttp)H by a Rh(ttp)Y species which is more reactive than Rh(ttp)Cl, and (iii) to provide a parallel CHA pathway by Rh2(ttp)2.;K2CO3-promoted CHA of the ring-strained cycloheptane with Rh(ttp)Cl at 120 °C in 6 h gave the CHA product Rh(ttp)( c-heptyl) and together with, unexpectedly, the CCA product Rh(ttp)Bn, in 30% and 24% yields, respectively. Mechanistic studies revealed that Rh(ttp)( c-heptyl) undergoes beta-hydride elimination in neutral condition or beta-proton elimination in basic condition followed by reprotonation to give rhodium(III) porphyrin hydride, Rh(ttp)H, and c-heptene. Successive base-promoted CHA of c-heptene with Rh(ttp)H, followed by beta-proton elimination, generates cycloheptatriene. The CHA of cycloheptatriene with Rh(ttp)H formed Rh(ttp)(c-heptatrienyl), which underwent rearrangement with carbon-carbon cleavage at 120 °C in 16 d to yield Rh(ttp)Bn in 96% yield.;c-Octane reacted with Rh(ttp)Cl at 120 °C in 7.5 h in the presence of K2CO3 to yield Rh(ttp)( n-octyl) and Rh(ttp)H in 33% and 58% yields, respectively. Mechanistic investigations indicate that the CCA product is generated from the Rh II(ttp)-catalyzed 1,2-addition of c-octane with Rh(ttp)H. Reaction of c-octane and Rh(ttp)H/Rh2(ttp) 2 (10:1) selectively yielded Rh(ttp)(n-octyl) in 73% at 120 °C in 15 h. The catalyst RhII(ttp) radical cleaves the C-C bond of c-octane to form to a Rh(ttp)-alkyl radical, which then abstracts a hydrogen atom from Rh(ttp)H to generate the Rh(ttp)( n-octyl), and subsequently leading to regeneration of the Rh II(ttp) radical. (Abstract shortened by UMI.). |
