Base-promoted carbon-hydrogen bond activation of alkanes was achieved in the reactions of alkanes with rhodium(III) porphyrin chlorides (Rh(por)Cl) at 120 °C to give rhodium porphyrin alkyls in moderate yields. This carbon-hydrogen activation (CHA) of alkane provided a facile synthesis of Rh(por)R. Mechanistic investigation of CHA suggested that Rh(por)H and [Rh(por)] 2 were key intermediates for the CHA step.

The aliphatic carbon-carbon activation of c-octane was achieved via the addition of Rh(ttp)H to give Rh(ttp)(n-octyl) in good yield under mild reaction conditions. The aliphatic carbon-carbon activation was Rh(II)(ttp)-catalyzed and was very sensitive to porphyrin sterics.

Rh(ttp)Cl (1a) (ttp = 5,10,15,20-tetrakistolylporphyrinato dianion) was found to react with methanol at a high temperature of 150 °C in the presence of inorganic bases to give a high yield of Rh(ttp)CH 3 (2a), up to 87%. Rh(ttp)H ( 1d) is suggested to be the key intermediate for the carbonoxygen bond cleavage.

Ir(ttp)Cl(CO) (1a; ttp = 5,10,15,20-tetrakis(p-tolyl)porphyrinato dianion) was found to cleave the C−O bond of CH3OH at 200 °C to give Ir(ttp)CH3 (3a). Addition of KOH promoted the reaction rate and gave a higher yield of Ir(ttp)CH3 in 70% yield in 1 day. Mechanistic studies suggest that, in the absence of KOH, Ir(ttp)Cl(CO) reacts with CH3OH initially to give Ir(ttp)OCH3, which then undergoes β-hydride elimination to produce Ir(ttp)H (4a). Ir(ttp)H further reacts slowly to cleave the C−O bond of CH3OH, likely via σ-bond metathesis, to give Ir(ttp)CH3. In the presence of KOH, Ir(ttp)Cl(CO) initially reacts with KOH more rapidly to give Ir(ttp)OH, which then cleaves the O−H bond of CH3OH by metathesis to give Ir(ttp)OCH3. Ir(ttp)OCH3 further isomerizes via β-hydride elimination/reinsertion to give Ir(ttp)CH2OH and concurrently undergoes base-assisted β-proton elimination to give Ir(ttp)−K+ (5a). Ir(ttp)CH2OH subsequently condenses with CH3OH to form Ir(ttp)CH2OCH3 (2). Finally, Ir(ttp)−K+ cleaves the C−O bond in CH3OH, most probably via nucleophilic substitution, to give Ir(ttp)CH3. Ir(ttp)CH2OCH3 also serves as the precursor of Ir(ttp)−K+ as it undergoes nucleophilic substitution by KOH to give Ir(ttp)−K+.

Rh(ttp)(C(7)H(7)) rearranged to give Rh(ttp)(CH(2)Ph) quantitatively at 120 °C in 12 d (ttp = 5,10,15,20-tetratolylporphyrinato dianion). This process is 10(10) faster than for the organic analogue. Mechanistic investigation suggests that a Rh(II)(ttp)-catalyzed pathway is operating.

The selective aliphatic carbon−carbon activation of cyclo-octane (c-octane) was achieved via the Rh II (ttp)-catalyzed 1,2-addition of Rh(ttp)H to give Rh(ttp)(n-octyl) (ttp = tetratolylporphyrinato dianion) in good yield under mild reaction conditions. This mechanism is further supported by DFT calculations. The reaction worked only with the sterically accessible Rh(ttp) porphyrin complex but not with the bulky Rh(tmp) system (tmp = tetrakismesitylporphyrinato dianion), thus showing the highly steric sensitivity of carbon−carbon bond activation by transition metal complexes. ■ INTRODUCTIONAlkane functionalization in a homogeneous medium is an important and challenging process which involves either carbon−hydrogen bond activation (CHA) 1 or carbon−carbon bond activation (CCA) 2 with organic, inorganic, and organometallics reagents followed by functionalization. Although aliphatic C−C bonds are weaker than aliphatic C−H bonds, CCA of alkanes by the attack of a transition metal complex is much less reported due to steric hindrance of the carbon atoms which are shielded by peripheral C−H bonds as well as for statistical reasons associated with C−C bonds being typically less abundant than C−H bonds in organic compounds. 3 Cyclo-octane (c-octane) is a relatively unstrained cycloalkane with a strain energy of 9.6 kcal/mol 4 and therefore serves as a commonly studied substrate in alkane functionalization, mostly involving CHA. Some examples of CHA of c-octane are the iridium(I) pincer dihydride-catalyzed dehydrogenation to coctene, 5a the FeCl 3 -catalyzed aerobic oxidation to c-octanol and c-octanone 5b as well as the MnO 2 -catalyzed bromination to coctyl bromide. 5c Reports of CCA of c-octane are rare. CCA of c-octane in a heterogeneous medium requires a very high reaction temperature of 530°C and consequently results in both CHA and CCA. 3a Oxidative CCA of c-octane catalyzed by N-hydroxyphthalides/Co(II)/Mn(II) at 100°C in 14 h gives ω-dicarboxylic acids in 2% yield only together with the major products being coctanol and c-octanone. 3b We have recently discovered the base-promoted CHA of cyclic alkanes with Rh(III) porphyrins. 6 In contrast to cpentane 6 and c-hexane, 6 c-heptane 7 undergoes both CHA and CCA to give Rh(III) porphyrin c-heptyl and benzyl. 7 Both CHA and CCA have been proposed to involve Rh(II) porphyrin as a reagent or as a catalyst. These Rh(II) porphyrins are unique metalloradicals and exhibit rich chemistry in bond activations including CHA 8,9 and CCA. 10,11 The bimetalloradical CHA of methane and toluene, based on the second order dependence of the Rh II (tmp) (tmp = tetrakismesitylporphyrinato dianion) concentration in the rate laws, has been reported by Wayland et al. 8 The CCA mechanism has been shown to be dependent on Rh II (tmp) concentration, with a first order rate dependence in both the reaction with nitroxide, 10 and in the reaction with 2-methyl substituted nitrile 11 and second order dependence in the reaction with cyclophane. 12 We have previously communicated the CCA of c-...

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K 2 CO 3 -promoted carbon−hydrogen and carbon−carbon bond activations of cycloheptane are achieved with rhodium(III) tetrakis(4-tolyl)porphyrin chloride (Rh-(ttp)Cl) at 120 °C to give Rh(ttp) cycloheptyl and benzyl complexes. On the basis of mechanistic studies, Rh(ttp)Cl first reacts by ligand substitution to give Rh(ttp)OH, which then undergoes reductive elimination to give Rh II 2 (ttp) 2 . The metalloradical Rh II (ttp), formed via dissociation of Rh II 2 (ttp) 2 , activates the CH bond of cycloheptane to form Rh(ttp)(cycloheptyl) and Rh(ttp)H. Rh(ttp)(cycloheptyl) slowly yields Rh(ttp)(cycloheptatrieneyl) by successive β-hydride elimination to olefins and Rh(ttp)H. K 2 CO 3 promoted the dehydrogenation of Rh(ttp)H to give Rh II 2 (ttp) 2 and H 2 . Both Rh(ttp)H and Rh II 2 (ttp) 2 activate the cycloheptatriene to give Rh(ttp)(cycloheptatrienyl), which further undergoes a Rh II (ttp)-catalyzed skeletal rearrangement to form Rh(ttp)Bn with rate enhancement much faster than that of the analogous organic isomerization of cycloheptatriene to toluene.