Alkyl C–O bond cleavage in aryl alkyl ethers was achieved with Rh(ttp)Cl (1a; ttp = 5,10,15,20-tetrakis(p-tolyl)porphyrinato dianion) together with competitive alkyl C–H bond activation in alkaline media. In contrast, selective alkyl C–O bond cleavage occurred with the iridium–porphyrin Ir(ttp)(CO)Cl (1b)/KOH. Mechanistic investigations indicate the coexistence of MI(ttp)− and M2II(ttp)2 (M = Rh, Ir) under basic conditions. With a weaker Rh(ttp)–Rh(ttp) bond, RhII(ttp)· metalloradical exists in an appreciable amount to cleave the alkyl C–H bond, competing with the alkyl C–O bond cleavage via RhI(ttp)−. In contrast, the more nucleophilic IrI(ttp)− cleaves the alkyl C–O bond exclusively.

ConspectusThe 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.

Rhodium porphyrin catalyzed hydrogenation of the aliphatic carbon–carbon σ-bond of [2.2]paracyclophane with water has been examined with a variety of tetraarylporphyrins and axial ligands. Mechanistic investigations show that RhIII(ttp)H, which can be derived from the reaction of [RhII(ttp)]2 with water without a sacrificial reductant, plays an important role in promoting bimetallic reductive elimination to give the hydrogenation product.

The mild and selective aliphatic C(CO)–C(α) bond activation (CCA) of ketones was successfully achieved at room temperature using rhodium(II) porphyrins in the presence of H2O. RhII(tmp) (tmp = tetrakismesitylporphyrinate dianion) disproportionates in H2O to generate the highly reactive intermediate RhIII(tmp)(OH) for cleaving the C–C bond of ketone, giving up to 90% of RhIII(tmp)(COR) and the corresponding oxidized carbonyl product in up to 76% yield within 10 min. Substrate scopes cover aliphatic as well as aromatic ketones. Both isopropyl and cyclic ketones worked well.

•C–C sigma bond hydrogenolyis with hydrogen by transition metal complexes.•Metalloporphyrins catalyzed hydrogenolysis of C–C sigma bond with water.•Bimetalloradical C–C bond activation in transition state.Catalytic hydrogenolysis of carbon–carbon sigma bond in organic substrates represents an interesting, yet less common reduction methodology in chemical transformations. In the past few decades, there are impressive works aiming at mechanistic understanding via strategic reaction design. Efforts to expand synthetic usefulness have been observed. This digest summarizes the recent achievements in transition metal catalyzed reductive C–C sigma bond cleavage to give C–H bonds by hydrogenolysis.Figure optionsDownload full-size imageDownload high-quality image (101 K)Download as PowerPoint slide

C60 at 1 mol % loading catalyzed the direct C–H arylation of benzene with aryl iodides in air to yield biaryls. The in situ generation of electron-rich C60(OH)2−, from C60 and OH−, reduced the aryl iodides to aryl radicals for arylation of benzene. The large surface area and spherical shape of C60 facilitated this process.

CoII(por) (por = porphyrinato dianion) reacted selectively with isopropyl ketones at the carbon (CO)–carbon (α) bond at room temperature to give high yields of CoIII(por) acyls and the corresponding oxidized carbonyl compounds in up to 89% yields. CoIII(por)OH is proposed to be the C–C bond activation (CCA) intermediate. The stoichiometric reaction is further developed into the photocatalytic CCA using both UV and visible light sources (λ 405 nm). Under ambient conditions, the photocatalytic C–C oxidation of 2,6-dimethylcyclohexanone gives 2-heptanone in up to 24 turnovers in the presence of isopropyl alcohol as the H atom donor and H2O as the oxidant. Various isopropyl ketones successfully undergo photocatalysis.

Cobalt porphyrins were found to catalyze the transfer hydrogenation of the carbon–carbon σ bond of [2.2]paracyclophane (PCP) with the solvent DMF serving as the hydrogenating agent. Successful trapping experiments with benzene solvent and the kinetic isotope effect (4.9) suggested the presence of benzyl radical intermediates in undergoing hydrogen atom transfer from DMF as the rate-limiting step. The rate law was established by initial rate measurements to be rate = kobs[CoII(ttp)][PCP].

Base-promoted vinyl carbon–bromine bond cleavage of styryl bromide by group 9 metalloporphyrin complexes was achieved to give the metal(III) porphyrin styryls M(ttp)(styryl) (ttp = 5,10,15,20-tetra-p-tolylporphyrinato dianion). Mechanistic studies suggest that [MII(ttp)]2 (M = Rh, Ir) cleaves the vinyl C–Br bond via an addition–elimination mechanism. The much less reactive CoII(ttp) undergoes a radical recombination with styryl radical which is generated by the hydroxide reduction of styryl bromide to give a radical anion with subsequent C–Br cleavage.

Base-promoted aerobic carbon–hydrogen bond activation (CHA) of thiophene was achieved with high regioselectivity at the 2-position by group 9 metalloporphyrins in moderate to high yields. Mechanistic investigations suggest a homolytic aromatic substitution through MII(ttp) (M = Rh, Ir) metalloradical addition, followed by β-elimination of a metal hydride pathway.

Iridium porphyrins were found to be good catalysts for the carbon–carbon σ-bond hydrogenation of [2.2]paracyclophane using water in neutral conditions. Mechanistic investigations reveal the promoting effects of iridium porphyrin hydride, IrIII(ttp)H, for efficient hydrogenation in the catalysis. The bimolecular reductive elimination from IrIII(ttp)H and carbon–carbon bond activation intermediates speeds up the hydrogenation process.Keywords: bimolecular reductive elimination; C−C bond hydrogenation; iridium hydride; iridium porphyrins; water

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 [IrI(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.

Cobalt porphyrin catalyzed the dehydrodehalogenation of aryl bromides using KOH in THF or 2-propanol.

The selective aliphatic carbon–carbon activation of cyclo-octane (c-octane) was achieved via the RhII(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.

Alkylation of rhodium porphyrins was achieved in moderate to high yields in the presence of air and water. With this facile alkylation method, various alkyl RhIII(por) species, including those with tertiary alkyl, were synthesized. Mechanistic investigations suggest a parallel SN2 via [RhI(ttp)]− with halogen atom transfer pathway via [RhII(ttp)]•.

Aryl–bromine bonds are successfully cleaved by cobalt(II) porphyrins in basic media to give Co(por)Ar (por = porphyrin) in good yields. Mechanistic studies suggested that the aryl–bromine bond is cleaved through a halogen atom transfer mechanism, which is different from the aryl–halogen bond cleavage mechanism with other group 9 metalloporphyrins.

Transition-metal free direct C–H arylation of benzene with aryl halides was achieved by meso-aryl-substituted [14]triphyrins(2.1.1) catalysts in an air atmosphere. Various aryl halides underwent successful direct C–H arylation of benzene to give moderate to high yields of biaryls. A radical mechanism is proposed for this triphyrin catalyzed C–H arylation reaction.

•Selective C(α)–C(β) bond activation of aliphatic ethers.•Rhodium porphyrin hydroxides as intermediates.•Ether oxidation by water to esters.The selective aliphatic C(α)–C(β) bond activation (CCA) of ethers by rhodium(III) porphyrin halides in the presence of KOH was achieved to give Rh–C(β) alkyls up to 88% yield. The addition of H2O and a phase transfer agent Ph4PBr improved the homogeneity of the reaction mixture and significantly brought down the reaction temperature to 60 °C. At this mild temperature, the C(α) co-product was oxidized to the corresponding esters in up to 89% yield. KOH promotes the bond activation by transferring the hydroxyl group to rhodium porphyrin to generate the key intermediate RhIII(ttp)OH (ttp = 5,10,15,20-tetratolylporphyrinate dianion).Selective C(α)–C(β) bond activation of aliphatic ethers.

Consecutive aromatic C–F bond and C–H bond activations of aryl fluorides were achieved by iridium porphyrins to initially give aryl and finally fluoroaryl iridium porphyrins. The C–F bond activation product is generated first, which is the precursor for the C–H bond activation. Both experimental and theoretical results support that the C–F bond is cleaved by iridium porphyrin anion through nucleophilic aromatic substitution, and the C–H bond cleavage is through homolytic aromatic substitution by iridium porphyrin radical followed by hydrogen atom abstraction. Moreover, the meta-fluorophenyl iridium porphyrin is the most thermodynamic stable regioisomers.

K2CO3-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 RhII2(ttp)2. The metalloradical RhII(ttp), formed via dissociation of RhII2(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. K2CO3 promoted the dehydrogenation of Rh(ttp)H to give RhII2(ttp)2 and H2. Both Rh(ttp)H and RhII2(ttp)2 activate the cycloheptatriene to give Rh(ttp)(cycloheptatrienyl), which further undergoes a RhII(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.

Base-promoted competitive Ar–F and Ar–X (X = Cl, Br) bond cleavage with iridium porphyrin complexes was investigated. Mechanistic studies suggested that Ir(ttp)− (ttp = 5,10,15,20-tetra-p-tolylporphyrinato dianion) cleaves the Ar–F bond via nucleophilic aromatic substitution and Ir2(ttp)2 cleaves the Ar–X (X = Cl, Br) bond via metalloradical ipso substitution. Therefore, a stronger base, polar solvent, lower temperature, and iridium anion precursor favor Ar–F bond cleavage, while a weaker base, nonpolar solvent, higher temperature, and Ir2(ttp)2 precursor favor Ar–X (X = Cl, Br) bond cleavage.

Rhodium(III) porphyrin β-hydroxyethyl, RhIII(ttp)CH2CH2OH (ttp = 5,10,15,20-tetratolylporphyrinato dianion), was found to serve as a precursor of the highly reactive RhIII(ttp)OH for the C(CO)–C(α) bond activation (CCA) of ketones under mild and aerobic conditions of 25–50 °C.

Photocatalytic carbon–carbon σ-bond oxidation of unstrained ketones by water using rhodium(III) porphyrin catalyst was accomplished. The catalysis yielded the corresponding one-carbon-less carbonyl compound and H2 with up to 30 turnovers in both aliphatic and cyclic ketones with α substituents. No carbon loss was observed in aromatic ketone. Mechanistic studies suggest that (Ph3P)RhIII(ttp)OH (ttp = tetratolylporphyrinato dianion) is the key intermediate in the carbon–carbon σ-bond anaerobic oxidation.

The catalytic carbon–carbon σ-bond activation and hydrogenation of [2.2]paracyclophane with water in a neutral reaction medium is demonstrated. The hydrogen from water is transferred to the hydrocarbon to furnish hydrogen enrichment in good yields.

Direct C–H arylation of unactivated heteroaromatics with aryl halides catalyzed by cobalt porphyrin is reported. The reaction is proposed to go through a homolytic aromatic substitution reaction. The aryl radical is electrophilic and a SOMO–HOMO interaction is predominant in the aryl radical addition process.

A general procedure for free porphyrin catalyzed direct C–H arylation of benzene was demonstrated. This air tolerant, transition-metal-free process provides a promising system for cheap and efficient synthesis of biaryls in a user-friendly approach.

Base-promoted aryl carbon–iodine and carbon–bromine bond (Ar–X, X = I, Br) cleavage by rhodium porphyrin complexes was achieved to give the rhodium(III) porphyrin aryls Rh(ttp)Ar (ttp = tetra-p-tolylporphyrinato dianion). Mechanistic studies showed that RhII2(ttp)2 is the intermediate for Ar–X (X = I, Br) cleavage. The Ar–X cleavage process goes through a rhodium(II) porphyrin radical mediated ipso-substitution mechanism.

Rhodium(III) porphyrins were found to undergo selective C(CO)–C(α) bond activation (CCA) of ketones promoted by water at temperatures as low as 50 °C. The acyl group of the ketone was transferred to the rhodium center, and the alkyl fragment was oxidized to a carbonyl moiety accordingly. The hydroxyl group of water is transferred to the rhodium porphyrin through hydrolysis of the kinetic α-carbon–hydrogen bond activation (α-CHA) product to give RhIII(ttp)OH (ttp = 5,10,15,20-tetratolylporphyrinato dianion), which subsequently cleaves the C(CO)–C(α) bond of ketone.

Rh(ttp)(C7H7) rearranged to give Rh(ttp)(CH2Ph) quantitatively at 120 °C in 12 d (ttp = 5,10,15,20-tetratolylporphyrinato dianion). This process is 1010 faster than for the organic analogue. Mechanistic investigation suggests that a RhII(ttp)-catalyzed pathway is operating.

A general procedure for cobalt-catalyzed direct C–H arylation of unactivated arenes has been discovered. This method employs aryl halides as the direct coupling partners with arenes without using any Grignard-type reagents. This catalysis opens a new methodology for the preparation of symmetrical as well as unsymmetrical biaryls in a user-friendly approach.

Highly reactive rhodium(III) porphyrin hydroxides were formed from the ligand substitution of rhodium porphyrin halides in benzene and were rapidly reduced to rhodium(II) porphyrins and hydrogen peroxide. Thus hydroxide acted as the reducing agent. Oxidative addition of rhodium(II) porphyrin with hydrogen peroxide proceeded rapidly at room temperature to give back rhodium(III) porphyrin hydroxides. Rhodium(II) porphyrins and H2O2 therefore were thermally reversible with rhodium porphyrin hydroxides.

Chemoselective carbonyl carbon and α-carbon bond activation (CCA) of ketones (RCOR′) was successfully achieved with various iridium(III) tetrakis-4-tolylporphyrinato complexes Ir(ttp)X (X = (BF4)(CO), Cl(CO), and Me) to give the corresponding Ir(ttp)COR (R = Ar, Me, or Et) and Ir(ttp)R′ (R′ = Me or Et) complexes. Ir(ttp)(BF4)(CO) exhibited the highest reactivity toward CCA, as it possesses a higher Lewis acidity in catalyzing the aldol condensation of ketones to give water, which hydrolyzes the kinetic products, C−H bond activation (CHA) complexes, into the proposed Ir(ttp)OH for a subsequent CCA process. The CCA step is nonregioselective in giving both Ir(ttp)R′ and Ir(ttp)COR. However, Ir(ttp)R′ was kinetically less stable toward hydrolysis to give Ir(ttp)OH. Thus, only Ir(ttp)COR was observed as the sole CCA product.

K2CO3 was found to promote selective aryl carbon−bromine bond (Ar−Br) cleavage by a high-valent iridium(III) porphyrin carbonyl chloride (IrIII(ttp)(CO)Cl, ttp = 5,10,15,20-tetra-p-tolylporphyrinato dianion) in benzene solvent at elevated temperature to give iridium(III) porphyrin aryls (IrIII(ttp)Ar) in high yields. Ir(ttp)(CO)Cl is reduced in alkaline media to give an [Ir(ttp)]2 intermediate. [Ir(ttp)]2 then cleaves the Ar−Br bond via a radical-type addition−elimination reaction (radical ipso -substitution) to yield Ir(ttp)Ar and a bromine radical.

Selective aliphatic carbon(α)–carbon(β) bond activation of ethers by (5,10,15,20-tetramesitylporphyrinato)rhodium(II) (Rh(tmp) (1)) was achieved at room temperature to yield corresponding rhodium porphyrin alkyls and the functionalized esters. Rh(tmp)OH was the proposed intermediate responsible for cleaving the C(α)–C(β) bond. The reaction is general for both straight- and branch-chain ethers.

Base-promoted selective aryl carbon–bromine and carbon–iodine bond (Ar–X, X = Br, I) cleavage by iridium(III) porphyrin carbonyl chloride (IrIII(ttp)(CO)Cl) was achieved in the presence of base (K2CO3, NaOH) to give iridium(III) porphyrin aryls (IrIII(ttp)Ar). Mechanistic studies revealed that the base undergoes ligand substitution with Ir(ttp)(CO)Cl to yield an iridium(III) hydroxo species (IrIII(ttp)OH). The hydroxo ligand most likely reduces the Ir(III) center to give iridium(II) porphyrin dimer ([IrII(ttp)]2) and H2O2. In a competitive pathway, [IrII(ttp)]2 disproportionates in the presence of base and residual water to give an iridium(III) hydride (IrIII(ttp)H) and Ir(ttp)OH. In a productive process, [Ir(ttp)]2 undergoes IrII(ttp) metalloradical-mediated ipso substitution of Ar–X via an addition–elimination pathway to form Ir(ttp)Ar and Ir(ttp)X. Ir(ttp)X is recycled by reacting with base to regenerate [Ir(ttp)]2 for subsequent Ar–X cleavage.

Base-promoted aryl carbon–chlorine bond (Ar–Cl) cleavage by iridium(III) porphyrin carbonyl chloride (IrIII(ttp)(CO)Cl; ttp = 5,10,15,20-tetrakis(p-tolyl)porphyrinato dianion) was achieved in the presence of K2CO3 to give iridium(III) porphyrin aryls (IrIII(ttp)Ar). Mechanistic studies revealed that K2CO3 promotes the reduction of Ir(ttp)(CO)Cl to give the iridium(II) porphyrin dimer intermediate [IrII(ttp)]2. [Ir(ttp)]2 is the source of IrII(ttp) metalloradical, which cleaves Ar–Cl to give Ir(ttp)Ar and a chlorine radical (Cl•) via radical ipso substitution in an addition–elimination pathway. Cl• reacts with [Ir(ttp)]2 to yield Ir(ttp)Cl for subsequent base-promoted reduction and Ir(ttp) for radical chain propagation. Additionally, the base-promoted Ar–Cl cleavage of chlorobenzene (PhCl) by Ir(ttp)(CO)Cl gives both Ir(ttp)Ph and 1,4-bis-iridium(III)-porphyrin benzene, IrIII(ttp)(p-C6H4)IrIII(ttp). The reactive Cl• can simultaneously react with PhCl via homolytic aromatic substitution to give 1,4-dichlorobenzene, which further undergoes double Ar–Cl cleavage to form Ir(ttp)(p-C6H4)Ir(ttp).

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 RhII(ttp)-catalyzed and was very sensitive to porphyrin sterics.

Reaction of hydrido[5,10,15,20-tetrakis(p-tolyl)porphyrinato]iridium(III) (Ir(ttp)H) (1) with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) (2) at room temperature gave a 90% yield of the unsupported iridium(II) porphyrin dimer, IrII2(ttp)2 (3). Kinetic measurements revealed that the oxidation followed overall second-order kinetics: rate = k[Ir(ttp)H][TEMPO], k(25 °C) = 6.65 × 10−4 M−1. The entropy of activation (ΔS‡ = −25.3 ± 2.5 cal mol−1 K−1) and the kinetic isotope effect of 7.2 supported a bimolecular associative mechanism in the rate-determining hydrogen atom transfer from Ir(ttp)H to TEMPO.

Toluenes underwent selective benzylic carbon−hydrogen bond activation (BnCHA) with rhodium(III) porphyrin methyl. The ortho-, meta-, and para-substituted toluenes yielded the corresponding rhodium porphyrin benzyls in high yields in solvent-free conditions as well as in benzene solvent. Mechanistically, Rh(ttp)Me likely undergoes a σ-bond metathesis pathway. The small value of the kinetic isotope effect (2.7) indicates a bent transition state. The negative slope (−1.1) of the linear free energy relationship Hammett plot supports that the benzylic carbon builds up a positive charge in the transition state.

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+.

Selective carbonyl carbon (C(═O)) and α-carbon (C(methyl)) bond activation of acetophenones was discovered by the high-valent, iridium(III) 5,10,15,20-tetrakis-4-tolylporphyrinato carbonyl chloride (Ir(ttp)Cl(CO)), which also acted as a Lewis acid in catalyzing the aldol condensation of acetophenones together with release of the coproduct water. Preliminary mechanistic studies suggest that both aliphatic and aromatic carbon−hydrogen bond activation products are kinetic products, which can be converted by reaction with water to iridium porphyrin hydride (Ir(ttp)H) via iridium porphyrin hydroxide (Ir(ttp)OH). Both Ir(ttp)OH and Ir(ttp)H were the possible intermediates to cleave the C(═O)−C(methyl) bond of acetophenones and to generate iridium porphyrin acyl complexes as the thermodynamic products.

Rh(tmp) underwent Ph3P-enhanced aliphatic carbon−carbon bond activation with various nitroxides. (Ph3P)Rh(tmp), rapidly formed from Rh(tmp) and Ph3P, enhanced the rate, selectivity, and yield in comparison to Rh(tmp). From kinetic studies, the rate of reaction showed a first-order dependence on both Rh(tmp) and TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl) and saturation kinetics on Ph3P. The rate enhancement of (Ph3P)Rh(tmp) over Rh(tmp) was estimated to be about 11 at 70 °C.

Selective carbon(CO)−carbon(α) bond activation of ketones was achieved by rhodium(III) 5,10,15,20-tetrakis-4-toylporphyrinato methyl (Rh(ttp)Me (1)) to yield the corresponding rhodium porphyrin acyls at temperatures as low as 50 °C. More hindered isopropyl ketones were much more reactive than ethyl or methyl ketones. Rh(ttp)OH (3a) was proposed to be the intermediate to cleave the C(CO)−C(α) bond.

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)CH3 (2a), up to 87%. Rh(ttp)H (1d) is suggested to be the key intermediate for the carbon−oxygen bond cleavage.

Base-promoted, selective aliphatic carbon(α)−carbon(β) bond activation (CCA) of ethers by (5,10,15,20-tetramesitylporphyrinato)rhodium(III) iodide was achieved.

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.

Iridium(III) porphyrin silyls were synthesized in moderate to high yields conveniently from the reactions of iridium(III) porphyrin carbonyl chloride and methyl with silanes, via silicon−hydrogen bond activation (SiHA) in solvent-free conditions and nonpolar solvents. Base was found to promote the SiHA reactions. Specifically, K3PO4 accelerated the SiHA with iridium porphyrin carbonyl chloride, while KOAc promoted the SiHA by iridium porphyrin methyl. Mechanistic experiments suggested that iridium(III) porphyrin carbonyl chloride initially formed iridium porphyin cation, which then reacted with silanes likely via heterolysis to give iridium porphyrin hydride. Iridium porphyrin hydride further reacted with silanes to yield iridium porphyrin silyls. On the other hand, iridium(III) porphyrin methyl and silyls underwent either oxidative addition or σ-bond metathesis to form the products. In the presence of base, a pentacoordinated silicon hydride species likely formed and reacted with iridium porphyrin methyl to form iridium porphyrin anion, which could further react with silanes to yield iridium porphyrin hydride after protonation. Ir(ttp)H finally reacted with excess silanes to give iridium porphyrin silyl complexes.

K2CO3 and NaOPh promoted the rate of benzylic carbon−hydrogen bond activation (BnCHA) of toluenes with iridium(III) porphyrin carbonyl chloride (Ir(ttp)Cl(CO)) to give iridium porphyrin benzyls in high yields. Mechanistic studies suggested that K2CO3 initially converted Ir(ttp)Cl(CO) to Ir(ttp)X (X = OH−, KCO3−), which reacted very fast with toluenes to yield Ir(ttp)H. Ir(ttp)H then reduced the carbonyl ligand in unreacted Ir(ttp)Cl(CO) to yield Ir(ttp)Me. Ir(ttp)H also dimerized dehydrogenatively to give [Ir(ttp)]2, especially promoted in the presence of base, which further reacted with toluenes to yield iridium benzyls. Weaker base of NaOPh converted Ir(ttp)Cl(CO) to Ir(ttp)OPh, which selectively promoted BnCHA to yield iridium benzyls.

The first examples of intermolecular activation of aliphatic carbon–carbon bonds by transition metal complexes are demonstrated in the reaction of nitroxides with a rhodium(II) porphyrin radical.

Palladium catalyzed phosphination of substituted aryl bromides using triarylphosphines as the phosphinating agents has been developed; this method tolerates ketone, aldehyde, ester, nitrile, ether and chloride functional groups.

This work reports the facile synthesis and photophysical properties of two novel near infrared (NIR) absorbing conjugates based on orthogonally arranged rhodium(III) tetrakis-4-tolylporphyrin (RhIII(ttp)) and BF2-chelated tetraarylazadipyrromethene (aza-BODIPY) linked by a covalent Rh–C(aryl) bond. These conjugates exhibit intense absorption and moderate fluorescence bands around 700 nm, which do not correspond with those of their aza-BODIPY precursors, due to the strong ground-state interaction between the aza-BODIPY and metal porphyrin moieties. DFT calculations predict that the energies of the 4dxy, 4dxz and 4dyz orbitals of the central metal and the highest occupied MO of the aza-BODIPY moiety lie close to one another. Minor changes in the linkage position on the aza-BODIPY moiety can alter the relative energies of these MOs and hence have a significant impact on the optical and photophysical properties.

Rh(ttp)(C7H7) rearranged to give Rh(ttp)(CH2Ph) quantitatively at 120 °C in 12 d (ttp = 5,10,15,20-tetratolylporphyrinato dianion). This process is 1010 faster than for the organic analogue. Mechanistic investigation suggests that a RhII(ttp)-catalyzed pathway is operating.

Aryl–bromine bonds are successfully cleaved by cobalt(II) porphyrins in basic media to give Co(por)Ar (por = porphyrin) in good yields. Mechanistic studies suggested that the aryl–bromine bond is cleaved through a halogen atom transfer mechanism, which is different from the aryl–halogen bond cleavage mechanism with other group 9 metalloporphyrins.

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 [IrI(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.

The mild and selective aliphatic C(CO)–C(α) bond activation (CCA) of ketones was successfully achieved at room temperature using rhodium(II) porphyrins in the presence of H2O. RhII(tmp) (tmp = tetrakismesitylporphyrinate dianion) disproportionates in H2O to generate the highly reactive intermediate RhIII(tmp)(OH) for cleaving the C–C bond of ketone, giving up to 90% of RhIII(tmp)(COR) and the corresponding oxidized carbonyl product in up to 76% yield within 10 min. Substrate scopes cover aliphatic as well as aromatic ketones. Both isopropyl and cyclic ketones worked well.

Rhodium porphyrin catalyzed hydrogenation of the aliphatic carbon–carbon σ-bond of [2.2]paracyclophane with water has been examined with a variety of tetraarylporphyrins and axial ligands. Mechanistic investigations show that RhIII(ttp)H, which can be derived from the reaction of [RhII(ttp)]2 with water without a sacrificial reductant, plays an important role in promoting bimetallic reductive elimination to give the hydrogenation product.