Intermolecular Dehydrative Coupling Reaction of Arylketones with Cyclic Alkenes Catalyzed by a Well-Defined Cationic Ruthenium-Hydride Complex: A Novel Ketone Olefination Method via Vinyl C-H Bond Activation

Transition-Metal-Catalyzed C-H Bond Activation for the Formation of C-C Bonds in Complex Molecules

Site-predictable and chemoselective C-H bond functionalization reactions offer synthetically powerful strategies for the step-economic diversification of both feedstock and fine chemicals. Many transition-metal-catalyzed methods have emerged for the selective activation and functionalization of C-H bonds. However, challenges of regio- and chemoselectivity have emerged with application to highly complex molecules bearing significant functional group density and diversity. As molecular complexity increases within molecular structures the risks of catalyst intolerance and limited applicability grow with the number of functional groups and potentially Lewis basic heteroatoms. Given the abundance of C-H bonds within highly complex and already diversified molecules such as pharmaceuticals, natural products, and materials, design and selection of reaction conditions and tolerant catalysts has proved critical for successful direct functionalization. As such, innovations within transition-metal-catalyzed C-H bond functionalization for the direct formation of carbon-carbon bonds have been discovered and developed to overcome these challenges and limitations. This review highlights progress made for the direct metal-catalyzed C-C bond forming reactions including alkylation, methylation, arylation, and olefination of C-H bonds within complex targets.

Recent Advances in C–H Bond Functionalization with Ruthenium-Based Catalysts

The past decades have witnessed rapid development in organic synthesis via catalysis, particularly the reactions through C–H bond functionalization. Transition metals such as Pd, Rh and Ru constitute a crucial catalyst in these C–H bond functionalization reactions. This process is highly attractive not only because it saves reaction time and reduces waste,but also, more importantly, it allows the reaction to be performed in a highly region specific manner. Indeed, several organic compounds could be readily accessed via C–H bond functionalization with transition metals. In the recent past, tremendous progress has been made on C–H bond functionalization via ruthenium catalysis, including less expensive but more stable ruthenium(II) catalysts. The ruthenium-catalysed C–H bond functionalization, viz. arylation, alkenylation, annulation, oxygenation, and halogenation involving C–C, C–O, C–N, and C–X bond forming reactions, has been described and presented in numerous reviews. This review discusses the recent development of C–H bond functionalization with various ruthenium-based catalysts. The first section of the review presents arylation reactions covering arylation directed by N–Heteroaryl groups, oxidative arylation, dehydrative arylation and arylation involving decarboxylative and sp3-C–H bond functionalization. Subsequently, the ruthenium-catalysed alkenylation, alkylation, allylation including oxidative alkenylation and meta-selective C–H bond alkylation has been presented. Finally, the oxidative annulation of various arenes with alkynes involving C–H/O–H or C–H/N–H bond cleavage reactions has been discussed.

Intermolecular Dehydrative Coupling and Intramolecular Cyclization of Ruthenium Vinylidene Complexes Formed from Aromatic Propynes Containing Carbonyl Functionalities

A remarkable intermolecular dehydrative coupling reaction with the formation of a C-C bond was found for the vinylidene complex 2 a, yielding the dinuclear bisvinylidene complex 4 a. Complex 2 a containing 1-hydroxyindan moiety was first formed from the reaction of o-propynyl benzaldehyde 1 a with [Ru]-Cl ([Ru]=Cp(PPh₃ )₂ Ru) by a cyclization process. For analogous aldehyde 1 b containing an additional 1,3-dioxolane group on the aryl ring, similar intermolecular coupling yields the dinuclear bisvinylidene complex 4 b. However, the fluoro group on the aryl ring in aldehyde 1 c inhibits the coupling reaction, giving only the vinylidene complex 2 c. For the reactions of [Ru]-Cl in MeOH with compounds 1 f, 1 g and 1 h, each with a ketone functionality, cyclization gives vinylidene complexes 2 f, 2 g and 2 h, respectively. However, no similar intermolecular coupling was observed, instead, the intramolecular dehydration yields 8 f, 8 g and 8 h, respectively. In CDCl₃ , catalytic cyclization is observed for the o-propynylphenyl ketone 1 h with [Ru]-Cl at 50 °C giving the isochromene products 14 h. Furthermore, treatment of the o-propynylaryl α,β-unsaturated ketones 1 k-m and 1 n with [Ru]-Cl in MeOH affords the corresponding vinylidene complexes 10 k-m and 11 n each with 1-benzosuberone moiety in the presence of NH₄ PF₆ . These intramolecular cyclization products were formed by the addition of Cβ onto the terminal carbon of the alkene moiety. All these reaction products were characterized by spectroscopic methods. In addition, structures of complexes 4 a, and 10 l were confirmed by single crystal X-ray diffraction analysis.

Recent advances in the ruthenium(ii)-catalyzed chelation-assisted C–H olefination of substituted aromatics, alkenes and heteroaromatics with alkenes via the deprotonation pathway

The transition-metal-catalyzed chelation-assisted alkenylation at the inert C–H bond of aromatics with alkenes is one of the efficient methods to synthesize substituted vinylarenes in a highly regio- and stereoselective manner.

Mechanistic insight into ruthenium catalysed meta-sulfonation of 2-phenylpyridine

The mechanism of the ruthenium catalysed meta-sulfonation has been studied and a novel cycloruthenated phenylpyridine complex tosylated at the para position to the metal has been structurally characterised.

Dehydrative C-H alkylation and alkenylation of phenols with alcohols: expedient synthesis for substituted phenols and benzofurans

A well-defined cationic Ru-H complex catalyzes the dehydrative C-H alkylation reaction of phenols with alcohols to form ortho-substituted phenol products. Benzofuran derivatives are efficiently synthesized from the dehydrative C-H alkenylation and annulation reaction of phenols with 1,2-diols. The catalytic C-H coupling method employs cheaply available phenols and alcohols, exhibits a broad substrate scope, tolerates carbonyl and amine functional groups, and liberates water as the only byproduct.

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.

Chelate-Assisted Oxidative Coupling Reaction of Arylamides and Unactivated Alkenes: Mechanistic Evidence for Vinyl C-H Bond Activation Promoted by an Electrophilic Ruthenium-Hydride Catalyst

The cationic ruthenium-hydride complex [(η(6)-C(6)H(6))(PCy(3))(CO)RuH](+)BF(4) (-) was found to be a highly regioselective catalyst for the oxidative C-H coupling reaction of aryl-substituted amides and unactivated alkenes to give ortho-alkenylamide products. The kinetic and spectroscopic analyses support a mechanism involving a rapid vinyl C-H activation followed by a rate-limiting C-C bond formation steps.

Chain- and Regioselective Ethylene and Styrene Dimerization Reactions Catalyzed by a Well-Defined Cationic Ruthenium-Hydride Complex: New Insights on the Styrene Dimerization Mechanism

The cationic ruthenium-hydride complex [(eta(6)-C(6)H(6))(PCy(3))(CO)RuH](+)BF(4) (-) was found to be a highly regioselective catalyst for the ethylene dimerization reaction to give 2-butene products (TOF = 1910 h(-1), >95% selectivity for 2-butenes). The dimerization of styrene exclusively produced the head-to-tail dimer (E)-PhCH(CH(3))CH=CHPh at an initial turnover rate of 2300 h(-1). A rapid and extensive H/D exchange between the vinyl hydrogens of styrene-d(8) and 4-methoxystyrene was observed within 10 min without forming the dimer products at room temperature. The inverse deuterium isotope effect of k(H)/k(D) = 0.77+/-0.10 was measured from the first order plots on the dimerization reaction of styrene and styrene-d(8) in chlorobenzene at 70 degrees C. The pronounced carbon isotope effect on both vinyl carbons of styrene as measured by using Singleton's method ((13)C(recovered)/(13)C(virgin) at C(1) = 1.096 and C(2) = 1.042) indicates that the C-C bond formation is the rate-limiting step for the dimerization reaction. The Eyring plot of the dimerization of styrene in the temperature range of 50-90 degrees C led to DeltaH(double dagger) = 3.3(6) kcal/mol and DeltaS(double dagger) = -35.5(7) e.u. An electrophilic addition mechanism has been proposed for the dimerization of styrene.