Tuning N-heterocyclic carbenes in T-shaped Pt(II) complexes for intermolecular C-H bond activation of arenes

Small change matters: T-shaped Pt(II) complexes with less flexible substituents, than, for example, isopropyl or tert-butyl groups, on N-heterocyclic carbene (NHC) ligands allow for C-H bond activation reactions of aromatic compounds (see scheme; BAr(f)(4)(-) =tetrakis[(3,5-trifluoromethyl)phenyl]borate; F yellow, Pt red). NHC substituents that are not highly branched prevent agostic interactions and reduce the barriers to achieve the C-H bond cleavage.     

Triangles and Squares for a Unique Molecular Crystal Structure: Unsupported Two‐Coordinate Lithium Cations and CC Agostic Interactions in Cyclopropyllithium Derivatives

Understanding and controlling the aggregation state is germane to alkyllithium chemistry. Lewis base‐free alkyllithium compounds normally form tetrahedral tetramers or octahedral hexamers in the solid state with the lithium cations being three‐coordinate. We report that the unsupported cyclopropyl derivative 1‐(trimethylsilyl)cyclopropyllithium [{μ‐c‐C(SiMe₃)C₂H₄}Li]₄ ( 1 ), synthesized by the reduction of 1‐(phenylthio)‐1‐(trimethylsilyl)cyclopropane, crystallizes as a tetramer in the space group I‐4 with the two‐coordinate lithium atoms forming a square. CC agostic interactions complete the coordination sphere around each lithium. The aggregate is preserved in hydrocarbon solution. Furthermore, CC agostic interactions compete intra‐ and intermolecularly with Lewis base donation as in the unsaturated dimer of 1‐(phenylthio)cyclopropyllithium [Li(thf)₂{μ‐c‐C(SPh)C₂H₄}₂Li (thf)] ( 3 ) which is also fully characterized.     

基于铁催化的C-H键活化构建C-C键的研究进展

过渡金属催化的C-H键活化及在此基础上的C-C键形成的反应因其高原子经济性和高效的合成效率而备受人们的关注.铁元素具有含量丰富、廉价、易得、环境友好等优点,在催化反应中得到了越来越广泛的应用.近几年来,人们关于Fe催化的C-H键活化构建C-C键反应的研究也取得了一定的进展.本文对铁催化的C-H键活化构建C-C键的最新研究进展作了综述,并且按照铁催化剂的不同价态进行了分类归纳,也对催化机理进行了阐述与总结.     

μ3‐η2:η2:η2‐Coordination of Primary Silane on a Triruthenium Plane

A μ₃‐η²:η²:η²‐silane complex, [(Cp*Ru)₃(μ₃‐η²:η²:η²‐H₃SitBu)(μ‐H)₃] ( 2 a ; Cp*=η⁵‐C₅Me₅), was synthesized from the reaction of [{Cp*Ru(μ‐H)}₃(μ₃‐H)₂] ( 1 ) with tBuSiH₃. Complex 2 a is the first example of a silane ligand adopting a μ₃‐η²:η²:η² coordination mode. This unprecedented coordination mode was established by NMR and IR spectroscopy as well as X‐ray diffraction analysis and supported by a density functional study. Variable‐temperature NMR analysis implied that 2 a equilibrates with a tautomeric μ₃‐silyl complex ( 3 a ). Although 3 a was not isolated, the corresponding μ₃‐silyl complex, [(Cp*Ru)₃(μ₃‐η²:η²‐H₂SiPh)(H)(μ‐H)₃] ( 3 b ), was obtained from the reaction of 1 with PhSiH₃. Treatment of 2 a with PhSiH₃ resulted in a silane exchange reaction, leading to the formation of 3 b accompanied by the elimination of tBuSiH₃. This result indicates that the μ₃‐silane complex can be regarded as an “arrested” intermediate for the oxidative addition/reductive elimination of a primary silane to a trinuclear site.     

基于C-H键活化的铜催化二苄胺氧化酰胺化反应

基于铜催化的C-H键活化构建了一类高效的二苄胺氧化酰基化生成N-苄基苯甲酰胺的反应. 该反应使用CuBr作为催化剂, 二乙酸碘苯作为氧化剂, 在温和条件下生成两种酰胺, 总产率最高可达92.0%. CuBr催化剂价廉易得, 催化活性高. 同时这也是首次使用二乙酸碘苯作为氧化剂, 通过二苄胺的直接氧化酰基化反应合成N-苄基苯甲酰胺.     

Directing group assists in transition metal-catalyzed site-selective C-H bond activation/transformations

In the review paper, we attempt to point out the importance of the directing group in the transition metal-catalyzed C-H bond activation/transformations, especially in the site-selectivity and reactivity. The employment of the directing groups in the type of reaction can enable a chemist to straightforwardly approach the desired molecular skeletons. Thus, understanding the role of the directing groups in the C-H bond activation/transformations would benefit chemists to design better auxiliary groups and develop more efficient and practical synthetic methodologies. We believe that the synthetic strategy based on the utility of the directing group to assist in the transition metal-catalyzed C-H bond activation/transformations will keep playing as a mainstream role in the research field.     

Hydrocarbon oxidation by beta-halogenated dioxoruthenium(VI) porphyrin complexes: effect of reduction potential (RuVI/V) and C-H bond-dissociation energy on rate constants

beta-Halogenated dioxoruthenium(VI) porphyrin complexes [Ru(VI)(F(28)-tpp)O(2)] [F(28)-tpp=2,3,7,8,12,13, 17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato(2-)] and [Ru(VI)(beta-Br(8)-tmp)O(2)] [beta-Br(8)-tmp=2,3,7,8,12,13,17,18-octabromo-5,10,15,20- tetrakis(2,4,6-trimethylphenyl)porphyrinato(2-)] were prepared from reactions of [Ru(II)(por)(CO)] [por=porphyrinato(2-)] with m-chloroperoxybenzoic acid in CH(2)Cl(2). Reactions of [Ru(VI)(por)O(2)] with excess PPh(3) in CH(2)Cl(2) gave [Ru(II)(F(20)-tpp)(PPh(3))(2)] [F(20)-tpp=5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato(2-)] and [Ru(II)(F(28)-tpp)(PPh(3))(2)]. The structures of [Ru(II)(por)(CO)(H(2)O)] and [Ru(II)(por)(PPh(3))(2)] (por=F(20)-tpp, F(28)-tpp) were determined by X-ray crystallography, revealing the effect of beta-fluorination of the porphyrin ligand on the coordination of axial ligands to ruthenium atom. The X-ray crystal structure of [Ru(VI)(F(20)-tpp)O(2)] shows a Ru=O bond length of 1.718(3) A. Electrochemical reduction of [Ru(VI)(por)O(2)] (Ru(VI) to Ru(V)) is irreversible or quasi-reversible, with the E(p,c)(Ru(VI/V)) spanning -0.31 to -1.15 V versus Cp(2)Fe(+/0). Kinetic studies were performed for the reactions of various [Ru(VI)(por)O(2)], including [Ru(VI)(F(28)-tpp)O(2)] and [Ru(VI)(beta-Br(8)-tmp)O(2)], with para-substituted styrenes p-X-C(6)H(4)CH=CH(2) (X=H, F, Cl, Me, MeO), cis- and trans-beta-methylstyrene, cyclohexene, norbornene, ethylbenzene, cumene, 9,10-dihydroanthracene, xanthene, and fluorene. The second-order rate constants (k(2)) obtained for the hydrocarbon oxidations by [Ru(VI)(F(28)-tpp)O(2)] are up to 28-fold larger than by [Ru(VI)(F(20)-tpp)O(2)]. Dual-parameter Hammett correlation implies that the styrene oxidation by [Ru(VI)(F(28)-tpp)O(2)] should involve rate-limiting generation of a benzylic radical intermediate, and the spin delocalization effect is more important than the polar effect. The k(2) values for the oxidation of styrene and ethylbenzene by [Ru(VI)(por)O(2)] increase with E(p,c)(Ru(VI/V)), and there is a linear correlation between log k(2) and E(p,c)(Ru(VI/V)). The small slope (approximately 2 V(-1)) of the log k(2) versus E(p,c)(Ru(VI/V)) plot suggests that the extent of charge transfer is small in the rate-determining step of the hydrocarbon oxidations. The rate constants correlate well with the C-H bond dissociation energies, in favor of a hydrogen-atom abstraction mechanism.     

Complexation and C-H bond activation of 1,5-cyclooctadiene on triosmium carbonyl clusters

Reaction of Os₃(CO)₁₀(NCMe)₂ and 1,5-cyclooctadiene (C₈H₁₂) affords the diene complex Os₃(CO)₁₀(η⁴-C₈H₁₂) (1) with the two alkene moieties coordinated to an equatorial and an axial positions of one osmium atom. Thermolysis of 1 in refluxing n-hexane results in a vinylic C-H bond activation to form (μ-H)Os₃(CO)₉(μ,η⁴-C₈H₁₁) (2) in good isolated yield. The crystal structures of 1 and 2 have been established by an X-ray diffraction study.     

Rhodium-catalyzed alkylation of terephthaldialdimine with terminal alkenes via C-H bond activation

Terephthaldialdimine 1a reacted with alkenes to give 2,6-dialkylated products selectively in moderate to high isolated yields. In the case of terephthaldialdimine 1b having a substituent at site 2 in the phenyl ring, the alkylation takes place selectively at site 6 in the phenyl ring. In this alkylation, a meta-substituent affected the reactivity significantly because of the steric hindrance.     

Rhodium-catalyzed alkylation of aromatic azines with alkenes via C-H bond activation

The aromatic azines reacted with terminal alkenes under a rhodium catalyst [RhCl(coe)₂]₂ and Cy₃P to give the alkylated products with good to high isolated yields. The azines bearing H and o-CH₃, p-CH₃ and p-CH₃O groups have high reactivities, but m-CH₃O, p-Cl, p-F exhibit low reactivities.