Metalloradical-Catalyzed Selective 1,2-Rh-H Insertion into the Aliphatic Carbon–Carbon Bond of Cyclooctane

Abstract

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-...

Rhodium Porphyrin Catalyzed Regioselective Transfer Hydrogenolysis of C–C σ-Bonds in Cyclopropanes with iPrOH

“…On the basis of the above results and established mechanisms for rhodium porphyrin metalloradical mediated C–C bond activation of cyclopropanes and cyclooctane, Scheme depicts a plausible mechanism. Rh­(ttp)­Me undergoes alcoholysis with i PrOH to give the reactive Rh­(ttp)­H as well as methane and acetone rapidly, analogous to the reported hydrolysis process with water .…”

“…Rh­(ttp)H then undergoes thermal reductive dimerization and equilibrates with H 2 and Rh II 2 (ttp) 2 upon heating . Rh II (ttp)·, generated from the homolysis of Rh II 2 (ttp) 2 , cleaves the C–C bond of cyclopropylbenzene through a homolytic radical substitution to form a more stable secondary alkyl radical intermediate A . , Intermediate A can undergo reversible fast ring closure to regenerate cyclopropylbenzene or abstract a hydrogen atom from the weak (ttp)­Rh– H (∼60 kcal/mol) or CH 3 C– H (OH)­CH 3 (∼91 kcal/mol) bond to form the stable Rh­(ttp)­CH 2 CH 2 CH 2 Ph complex (intermediate B ). At high temperature, intermediate B undergoes facile β-hydride elimination to give allylbenzene and Rh­(ttp)­H .…”

Selective Aliphatic Carbon–Carbon Bond Activation by Rhodium Porphyrin Complexes

“…Independent reactions of 9 or 10 with cyclooctane gave only low yields of Rh III (ttp)­( n -octyl) ( 7 ) in 15 h, which suggests that they were also not the active species in cleaving the C–C bond of cyclooctane (Table , entries 1 and 2). Nonetheless, prolonged heating of Rh III (ttp)H ( 9 ) with cyclooctane for a period of 12 d produced the CCA product Rh III (ttp)­( n -octyl) ( 7 ) in 94% yield (Table , entry 3) …”

“…The calculated activation barrier of Rh II (ttp)-metalloradical-mediated CCA (Δ G ⧧ = 36.8 kcal mol –1 ) is reasonably consistent with the experimental conditions . We did not consider any electron transfer pathway in this case because both substrate and solvent medium are nonpolar in nature.…”

“…Nonetheless, prolonged heating of Rh III (ttp)H ( 9) with cyclooctane for a period of 12 d produced the CCA product Rh III (ttp)(n-octyl) (7) in 94% yield (Table 5, entry 3). 32 The formation of Rh III (ttp)(n-octyl) ( 7) is a formal 1,2addition of Rh III (ttp)H ( 9) across the C−C bond of cyclooctane. We reason that the Rh II (ttp) metalloradical that dissociates from Rh II 2 (ttp) 2 (10), which is formed from the dehydrogenative dimerization of Rh III (ttp)H ( 9), catalyzes such 1,2-addition reaction in analogy to the Rh II (oep)-catalyzed 1,2addition of Rh III (oep)H to styrene to produce Rh III (oep)-CH 2 CH 2 Ph, as reported by Halpern.…”