Kin Shing Chan

Carbon-carbon bond activation of 2,2,6,6-tetramethyl-piperidine-1-oxyl by a Rh-II metalloradical: A combined experimental and theoretical study

Competitive major carbon-carbon bond activation (CCA) and minor carbon-hydrogen bond activation (CHA) channels are identified in the reaction between rhodium(II) meso-tetramesitylporphyrin [Rh(II)(tmp)] (1) and 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO) (2). The CCA and CHA pathways lead to formation of [Rh(III)(tmp)Me] (3) and [Rh(III)(tmp)H] (5), respectively. In the presence of excess TEMPO, [Rh(II)(tmp)] is regenerated from [Rh(III)(tmp)H] with formation of 2,2,6,6-tetramethyl-piperidine-1-ol (TEMPOH) (4) via a subsequent hydrogen atom abstraction pathway. The yield of the CCA product [Rh(III)(tmp)Me] increased with higher temperature at the cost of the CHA product TEMPOH in the temperature range 50-80 degrees C. Both the CCA and CHA pathways follow second-order kinetics. The mechanism of the TEMPO carbon-carbon bond activation was studied by means of kinetic investigations and DFT calculations. Broken symmetry, unrestricted b3-lyp calculations along the open-shell singlet surface reveal a low-energy transition state (TS1) for direct TEMPO methyl radical abstraction by the Rh(II) radical (SH2 type mechanism). An alternative ionic pathway, with a somewhat higher barrier, was identified along the closed-shell singlet surface. This ionic pathway proceeds in two sequential steps: Electron transfer from TEMPO to [Rh(II)(por)] producing the [TEMPO]+ [RhI(por)]- cation-anion pair, followed by net CH3+ transfer from TEMPO+ to Rh(I) with formation of [Rh(III)(por)Me] and (DMPO-like) 2,2,6-trimethyl-2,3,4,5-tetrahydro-1-pyridiniumolate. The transition state for this process (TS2) is best described as an SN2-like nucleophilic substitution involving attack of the d(z)2 orbital of [Rh(I)(por)]- at one of the C(Me)-C(ring) sigma* orbitals of [TEMPO]+. Although the calculated barrier of the open-shell radical pathway is somewhat lower than the barrier for the ionic pathway, R-DFT and U-DFT are not likely comparatively accurate enough to reliably distinguish between these possible pathways. Both the radical (SH2) and the ionic (SN2) pathway have barriers which are low enough to explain the experimental kinetic data.

Synthesis of β-aryl substituted porphyrins by palladium-catalysed cross-coupling reactions

β-Bromoporphyrins undergo Suzuki cross-coupling reactions with aryl boronic acids to give β-arylporpyrins in high yields and X-ray analysis shows that H2TPP(Ph)42a(TPP = tetraphenylporphyrin) is centrosymmetric and possesses a hydrogen-bonded inner core.

Metalloradical-Catalyzed Aliphatic Carbon−Carbon Activation of Cyclooctane

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.

Catalytic Carbon–Carbon σ-Bond Hydrogenation with Water Catalyzed by Rhodium Porphyrins

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.

Synthesis of β-octasubstituted sterically bulky porphyrins by Suzuki cross coupling

β-Octasubstituted tetramesitylporphyrins have been prepared in good yields by Suzuki cross coupling of β-octabromotetramesitylporphyrin with aryl- and alkyl-boronic acids. A single-crystal X-ray analysis of -octamethyltetramesitylporphyrin 3a shows a saddled non-planar structure.

Synthesis of a β-linked porphyrin dimer and some homo-, and hetero-bimetallic complexes

The porphyrin boronate H2L2 undergoes Suzuki cross-coupling reaction with β-monobromoporphyrin H2L1 to give the unsymmetrically substituted porphyrin dimer H4L3 in high yield; homo-, and heter-bimetallic complexes of H4L3 are prepared.

Synthesis of novel dinickel(II) and nickel(II)–copper(II) bimetallic complexes derived from an acyclic dinucleating Schiff base–pyridine ligand

Novel phenolic bridged dinickel(II) and nickel(II)–copper(II) complexes are synthesized by stepwise metallation of a dinucleating compartmental Schiff base–pyridine ligand.

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.

Synthesis of an oxorhenium(V) corrolate from porphyrin with detrifluoromethylation and ring contraction

Metallation of highly electron deficient 5,10,15,20-tetrakis(trifluoromethyl)porphyrin 1 with [Re2(CO)10] in refluxing PhCN resulted in a novel synthesis of oxorhenium(V) 5,10,15-tris(trifluoromethyl)corrolate 2 characterized by single crystal X-ray crystallography.

Dramatic temperature effect in asymmetric catalysis in the enantioselective addition of diethylzinc to aldehydes

The enantioselectivity of the addition of diethylzinc to aryl aldehydes catalysed by (S)-2-(3-methyl-2-pyridyl)-3,5-di-tert-butylphenol have been found to depend heavily on temperature with the inversion temperatures affected by the para-substituents of aryl aldehydes.