Base-Promoted Carbon−Hydrogen Bond Activation of Alkanes with Rhodium(III) Porphyrin Complexes

Synthesis and applications of rhodium porphyrin complexes

A review on rhodium porphyrin chemistry, ranging from synthesis and properties to reactivity and application.

Selective Aliphatic Carbon-Carbon Bond Activation by Rhodium Porphyrin Complexes

The carbon-carbon bond activation of organic molecules with transition metal complexes is an attractive transformation. These reactions form transition metal-carbon bonded intermediates, which contribute to fundamental understanding in organometallic chemistry. Alternatively, the metal-carbon bond in these intermediates can be further functionalized to construct new carbon-(hetero)atom bonds. This methodology promotes the concept that the carbon-carbon bond acts as a functional group, although carbon-carbon bonds are kinetically inert. In the past few decades, numerous efforts have been made to overcome the chemo-, regio- and, more recently, stereoselectivity obstacles. The synthetic usefulness of the selective carbon-carbon bond activation has been significantly expanded and is becoming increasingly practical: this technique covers a wide range of substrate scopes and transition metals. In the past 16 years, our laboratory has shown that rhodium porphyrin complexes effectively mediate the intermolecular stoichiometric and catalytic activation of both strained and nonstrained aliphatic carbon-carbon bonds. Rhodium(II) porphyrin metalloradicals readily activate the aliphatic carbon(sp³)-carbon(sp³) bond in TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl) and its derivatives, nitriles, nonenolizable ketones, esters, and amides to produce rhodium(III) porphyrin alkyls. Recently, the cleavage of carbon-carbon σ-bonds in unfunctionalized and noncoordinating hydrocarbons with rhodium(II) porphyrin metalloradicals has been developed. The absence of carbon-hydrogen bond activation in these systems makes the reaction unique. Furthermore, rhodium(III) porphyrin hydroxide complexes can be generated in situ to selectively activate the carbon(α)-carbon(β) bond in ethers and the carbon(CO)-carbon(α) bond in ketones under mild conditions. The addition of PPh₃ promotes the reaction rate and yield of the carbon-carbon bond activation product. Thus, both rhodium(II) porphyrin metalloradical and rhodium(III) porphyrin hydroxide are very reactive to activate the aliphatic carbon-carbon bonds. Recently, we successfully demonstrated the rhodium porphyrin catalyzed reduction or oxidation of aliphatic carbon-carbon bonds using water as the reductant or oxidant, respectively, in the absence of sacrificial reagents and neutral conditions. This Account presents our contribution in this domain. First, we describe the chemistry of equilibria among the reactive rhodium porphyrin complexes in oxidation states from Rh(I) to Rh(III). Then, we present the serendipitous discovery of the carbon-carbon bond activation reaction and subsequent developments in our laboratory. These aliphatic carbon-carbon bond activation reactions can generally be divided into two categories according to the reaction type: (i) homolytic radical substitution of a carbon(sp³)-carbon(sp³) bond with a rhodium(II) porphyrin metalloradical and (ii) σ-bond metathesis of a carbon-carbon bond with a rhodium(III) porphyrin hydroxide. Finally, representative examples of catalytic carbon-carbon bond hydrogenation and oxidation through strategic design are covered. The progress in this area broadens the chemistry of rhodium porphyrin complexes, and these transformations are expected to find applications in organic synthesis.

Light induced catalytic intramolecular hydrofunctionalization of allylphenols mediated by porphyrin rhodium(iii) complexes

Catalytic intramolecular hydrofunctionalization of allylphenols to heterocyclic compounds mediated by rhodium(iii) porphyrin complexes was described. The Wacker-type intermediate β-heterocyclic alkyl rhodium complex was independently synthesized and crystallized.

User-friendly aerobic reductive alkylation of iridium(III) porphyrin chloride with potassium hydroxide: scope and mechanism

Alkylation of iridium 5,10,15,20-tetrakistolylporphyrinato carbonyl chloride, Ir(ttp)Cl(CO) (1), with 1°, 2° alkyl halides was achieved to give (ttp)Ir-alkyls in good yields under air and water compatible conditions by utilizing KOH as the cheap reducing agent. The reaction rate followed the order: RCl < RBr < RI (R = alkyl), and suggests an SN2 pathway by [Ir(I)(ttp)](-). Ir(ttp)-adamantyl was obtained under N2 when 1-bromoadamantane was utilized, which could only undergo bromine atom transfer pathway. Mechanistic investigations reveal a substrate dependent pathway of SN2 or halogen atom transfer.

Iridium porphyrins in CD3OD: reduction of Ir(III), CD3-OD bond cleavage, Ir-D acid dissociation and alkene reactions

Methanol solutions of iridium(III) tetra(p-sulfonatophenyl)porphyrin [(TSPP)Ir(III)] form an equilibrium distribution of methanol and methoxide complexes ([(TSPP)Ir(III)(CD3OD)(2-n)(OCD3)n]((3+n)-)). Reaction of [(TSPP)Ir(III) with dihydrogen (D2) in methanol produces an iridium hydride [(TSPP)Ir(III)-D(CD3OD)](4-) in equilibrium with an iridium(I) complex ([(TSPP)Ir(I)(CD3OD)](5-)). The acid dissociation constant of the iridium hydride (Ir-D) in methanol at 298 K is 3.5 × 10(-12). The iridium(I) complex ([(TSPP)Ir(I)(CD3OD)](5-)) catalyzes reaction of [(TSPP)Ir(III)-D(CD3OD)](4-) with CD3-OD to produce an iridium methyl complex [(TSPP)Ir(III)-CD3(CD3OD)](4-) and D2O. Reactions of the iridium hydride with ethene and propene produce iridium alkyl complexes, but the Ir-D complex fails to give observable addition with acetaldehyde and carbon monoxide in methanol. Reaction of the iridium hydride with propene forms both the isopropyl and propyl complexes with free energy changes (ΔG° 298 K) of -1.3 and -0.4 kcal mol(-1) respectively. Equilibrium thermodynamics and reactivity studies are used in discussing relative Ir-D, Ir-OCD3 and Ir-CD2- bond energetics in methanol.