Activation of C-H Bonds: Catalytic Reactions

Theoretical Study of Nickel-Catalyzed Selective Alkenylation of Pyridine: Reaction Mechanism and Crucial Roles of Lewis Acid and Ligands in Determining the Selectivity

Selective alkenylation of pyridine is challenging in synthetic organic chemistry due to the poor reactivity and regioselectivity of the aromatic ring. We theoretically investigated Ni-catalyzed selective alkenylation of pyridine with DFT. The first step is coordination of the pyridine-AlMe₃ adduct with the active species Ni⁽⁰⁾(NHC)(C₂H₂) 1 in an η²-fashion to form an intermediate Int1. After the isomerization of Int1, the oxidative addition of the C-H bond of pyridine across the nickel-acetylene moiety occurs via a transition state TS2 to form a Ni^(II)(NHC) pyridyl vinyl intermediate Int3. This oxidative addition is rate-determining. The next step is C-C bond formation between pyridyl and vinyl groups leading to the formation of vinyl-pyridine (P1). One of the points at issue in this type of functionalization is how to control the regioselectivity. With the use of Ni(NHC)/AlMe₃ catalyst, the C⁴- and C³-alkenylated products (ΔG°^⧧ = 17.4 and 21.5 kcal mol^-1, respectively) are formed preferably to the C² one (ΔG°^⧧ = 22.0 kcal mol^-1). The higher selectivity of the C⁴-alkenylation over the C³ and the C² ones is attributed to the small steric repulsion between NHC and AlMe₃ in the C⁴-alkenylation. Interestingly, with Ni(P(i-Pr)₃)/AlMe₃ catalyst, the C²-alkenylation occurs more easily than the C³ and C⁴ ones. This regioselectivity arises from the smaller steric repulsion induced by P(i-Pr)₃ than by bulky NHC. It is notable that AlMe₃ accelerates the alkenylation by inducing the strong CT from Ni to pyridine-AlMe₃. In the absence of AlMe₃, pyridine strongly coordinates with the Ni atom through the N atom, which increases Gibbs activation energy (ΔG°^⧧ = ∼27 kcal mol^-1) of the C-H bond activation. In other words, AlMe₃ plays two important roles, acceleration of the reaction and enhancement of the regioselectivity for the C⁴-alkenylation.

Ruthenium-Catalyzed C-H Bond Activation Approach to Azolyl Aminals and Hemiaminal Ethers, Mechanistic Evaluations, and Isomer Interconversion

C(sp³)-N bond-forming reactions between benzotriazole and 5,6-dimethylbenzotriazole with N-methylpyrrolidinone, tetrahydrofuran, tetrahydropyran, diethyl ether, 1,4-dioxane, and isochroman have been conducted using RuCl₃•3H₂O/t-BuOOH in 1,2-dichloroethane. In all cases, N1 and N2 alkylation products were obtained, and these are readily separated by chromatography. One of these products, 1-(isochroman-1-yl)-5,6-dimethyl-1H-benzotriazole, was examined by X-ray crystallography. It is the first such compound to be analyzed by this method, and notably, the benzotriazolyl moiety is quasi-axially disposed, consistent with the anomeric effect. This has plausible consequences, not observed previously. In contrast to other hemiaminal ether-forming reactions, which proceed via radicals, this Ru-catalyzed process is not suppressed in the presence of a radical inhibitor. Therefore, an oxoruthenium-species-mediated rapid formation of an oxocarbenium intermediate is believed to occur. In the radical-trapping experiment, previously unknown products containing both the benzotriazole and the TEMPO unit have been identified. In these products, it is likely that the benzotriazole is introduced via a Ru-catalyzed C-N bond formation, whereas C-O bond-formation with TEMPO occurs via a radical reaction. We show that reactions of THF with TEMPO are influenced by ambient light. A competitive reaction of THF and THF-d₈ with benzotriazole indicated that C-H bond cleavage occurs ca. 5 times faster than C-D cleavage. This is comparable to other metal-mediated radical reactions of THF, but lower than that observed for a reaction catalyzed by n-Bu₄N⁺I^-. Detailed mechanistic experiments and comparisons are described. The catalytic system was also evaluated for reactions of benzimidazole, imidazole, 1,2,4-triazole, and 1,2,3-triazole with THF, and successful reactions were achieved in each case. In the course of our studies, we discovered an unexpected but significant isomerization of some of the benzotriazolyl hemiaminal ethers. This is plausibly attributable to the pseudoaxial orientation of the heterocycle in the products and the stability of oxocarbenium ions, both of which can contribute to C-N bond cleavage and reformation. Predominantly, the N2-isomers rearrange to the N1-isomers even upon storage at low temperature! This previously unknown phenomenon has also been studied and described.

Overcoming naphthoquinone deactivation: rhodium-catalyzed C-5 selective C-H iodination as a gateway to functionalized derivatives

We report a Rh-catalyzed method for the C-5 selective C-H iodination of naphthoquinones and show that complementary C-2 selective processes can be achieved under related conditions. C-C bond forming derivatizations of the C-5 iodinated products provide a gateway to previously inaccessible A-ring analogues. The present study encompasses the first catalytic directed ortho-functionalizations of simple (non-bias) naphthoquinones. The strategic considerations outlined here are likely to be applicable to C-H functionalizations of other weakly coordinating and/or redox sensitive substrates.

Palladium-catalyzed direct C–H arylation of ferrocenecarboxamides with aryl halides

A simple and facile protocol for palladium-catalyzed ortho-arylation of ferrocenecarboxamides with aryl halides was developed with the assistance of the bidentate directing group.

Catalytic Transformation of Aldehydes with Nickel Complexes through η(2) Coordination and Oxidative Cyclization

Chemists no longer doubt the importance of a methodology that could activate and utilize aldehydes in organic syntheses since many products prepared from them support our daily life. Tremendous effort has been devoted to the development of these methods using main-group elements and transition metals. Thus, many organic chemists have used an activator-(aldehyde oxygen) interaction, namely, η(1) coordination, whereby a Lewis or Brønsted acid activates an aldehyde. In the field of coordination chemistry, η(2) coordination of aldehydes to transition metals by coordination of a carbon-oxygen double bond has been well-studied; this activation mode, however, is rarely found in transition-metal catalysis. In view of the distinctive reactivity of an η(2)-aldehyde complex, unprecedented reactions via this intermediate are a distinct possibility. In this Account, we summarize our recent results dealing with nickel(0)-catalyzed transformations of aldehydes via η(2)-aldehyde nickel and oxanickelacycle intermediates. The combination of electron-rich nickel(0) and strong electron-donating N-heterocyclic carbene (NHC) ligands adequately form η(2)-aldehyde complexes in which the aldehyde is highly activated by back-bonding. With Ni(0)/NHC catalysts, processes involving intramolecular hydroacylation of alkenes and homo/cross-dimerization of aldehydes (the Tishchenko reaction) have been developed, and both proceed via the simultaneous η(2) coordination of aldehydes and other π components (alkenes or aldehydes). The results of the mechanistic studies are consistent with a reaction pathway that proceeds via an oxanickelacycle intermediate generated by the oxidative cyclization with a nickel(0) complex. In addition, we have used the η(2)-aldehyde nickel complex as an effective activator for an organosilane in order to generate a silicate reactant. These reactions show 100% atom efficiency, generate no wastes, and are conducted under mild conditions.

Transition metal-catalyzed direct nucleophilic addition of C–H bonds to carbon–heteroatom double bonds

Compared with the traditional Grignard reaction, direct insertion of polar double bonds to C–H bonds via transition-metal catalysis is ideal from the viewpoint of atom-, step- and cost-economy and the avoidance of the waste emission, as well as of the complex manipulation of sensitive reagents.

1,5-Rhodium shift in rearrangement of N-arenesulfonylazetidin-3-ols into benzosultams

Benzosultams are synthesized in an enantiopure form starting from α-amino acids through a rhodium-catalyzed rearrangement reaction of N-arenesulfonylazetidin-3-ols. Mechanistically, this reaction involves C-C bond cleavage by β-carbon elimination and C-H bond cleavage by a 1,5-rhodium shift.

Purinyl N1-directed aromatic C-H oxidation in 6-arylpurines and 6-arylpurine nucleosides

Palladium-catalyzed C-H bond activation and oxidation of C6 arylpurines as well as C6 arylpurine nucleosides can be accomplished using Pd(OAc)2/PhI(OAc)2 in CH3CN. Despite the presence of four nitrogen atoms in the purine moiety as well as the polyoxygenated saccharide and a labile glycosidic bond in the nucleosides, these reactions can be effectively conducted. Notably, the generally more labile 2'-deoxyribonucleosides also undergo reaction. The reaction conditions can be tuned to yield either monoacetoxylated or diacetoxylated products predominantly. In the course of these investigations, a dimeric Pd(II)-containing cyclopalladated C6 naphthylpurine derivative has been obtained and crystallographically characterized. This compound is competent in catalyzing the oxidization with PhI(OAc)2, indicating its plausible intermediacy in the chemistry. The X-ray structure of a monoacetoxylated product from this reaction has also been obtained.

Scope and Mechanistic Study of the Coupling Reaction of α, β-Unsaturated Carbonyl Compounds with Alkenes: Uncovering Electronic Effects on Alkene Insertion vs Oxidative Coupling Pathways

The cationic ruthenium-hydride complex [(C(6)H(6))(PCy(3))(CO)RuH](+)BF(4) (-) (1) was found to be a highly effective catalyst for the intermolecular conjugate addition of simple alkenes to α,β-unsaturated carbonyl compounds to give (Z)-selective tetrasubstituted olefin products. The analogous coupling reaction of cinnamides with electron-deficient olefins led to the oxidative coupling of two olefinic C-H bonds in forming (E)-selective diene products. The intramolecular version of the coupling reaction efficiently produced indene and bicyclic fulvene derivatives. The empirical rate law for the coupling reaction of ethyl cinnamate with propene was determined as: rate = k[1](1)[propene](0)[cinnamate](-1). A negligible deuterium kinetic isotope effect (k(H)/k(D) = 1.1±0.1) was measured from both (E)-C(6)H(5)CH=C(CH(3))CONHCH(3) and (E)-C(6)H(5)CD=C(CH(3))CONHCH(3) with styrene. In contrast, a significant normal isotope effect (k(H)/k(D) = 1.7±0.1) was observed from the reaction of (E)-C(6)H(5)CH=C(CH(3))CONHCH(3) with styrene and styrene-d(10). A pronounced carbon isotope effect was measured from the coupling reaction of (E)-C(6)H(5)CH=CHCO(2)Et with propene ((13)C(recovered)/(13)C(virgin) at C(β) = 1.019(6)), while a negligible carbon isotope effect ((13)C(recovered)/(13)C(virgin) at C(β) = 0.999(4)) was obtained from the reaction of (E)-C(6)H(5)CH=C(CH(3))CONHCH(3) with styrene. Hammett plots from the correlation of para-substituted p-X-C(6)H(4)CH=CHCO(2)Et (X = OCH(3), CH(3), H, F, Cl, CO(2)Me, CF(3)) with propene and from the treatment of (E)-C(6)H(5)CH=CHCO(2)Et with a series of para-substituted styrenes p-Y-C(6)H(4)CH=CH(2) (Y = OCH(3), CH(3), H, F, Cl, CF(3)) gave the positive slopes for both cases (ρ = +1.1±0.1 and +1.5±0.1, respectively). Eyring analysis of the coupling reaction led to the thermodynamic parameters, Δ H(‡) = 20±2 kcal mol(-1) and S(‡) = -42±5 e.u. Two separate mechanistic pathways for the coupling reaction have been proposed on the basis of these kinetic and spectroscopic studies.

DENSITY FUNCTIONAL COMPUTATIONS OF Rh(I)-CATALYZED HYDROACYLATION OF ETHENE OR ETHYNE

Density functional theory has been used to study Rh(I) -catalyzed hydroacylation of ethene or ethyne. All the intermediates and the transition states were optimized completely at the B3LYP/6-311++G(d,p) level (LANL2DZ(d) for Rh , P). Calculation results confirm that Rh(I) -catalyzed hydroacylation of ethene is exothermic and the total released energy is -54 kJ/mol, and that Rh(I) -catalyzed hydroacylation of ethyne is also exothermic and the total released energy is -122 kJ/mol. In Rh(I) -catalyzed hydroacylation, ethene and ethyne have similar reactivity. Rh(I) -catalyzed oxidative addition of aldehyde is the rate-determinating step for the Rh(I) -catalyzed hydroacylation of ethene or ethyne. Hydrogen transfer reaction is prior to the C – C bond-forming reaction for Rh(I) -catalyzed hydroacylation of ethene. Thus hydrogen transfer reaction and the C – C bond-forming reaction may be co-existed for Rh(I) -catalyzed hydroacylation of ethyne. The effect of solvent in the hydroacylation of ethyne is greater than that in the hydroacylation of ethene.

Synthesis of N-(2-pyridyl)indoles via Pd(II)-catalyzed oxidative coupling

Readily available Pd(II) chloride catalysts can catalyze selective and efficient oxidative coupling between N-aryl-2-aminopyridines and internal alkynes to yield N-(2-pyridyl)indoles. This process involves the ortho C-H activation of N-aryl-2-aminopyridines, and CuCl(2) was used as an oxidant. Compared to our previously reported Rh(III)-catalyzed synthesis of this class of product, this method is advantageous with a wider scope of alkynes and cost-effective Pd(II) catalysts. Molecular oxygen can be used as a terminal oxidant.