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.

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The alkenylation with alkyl acrylates of ferrocenyl alkyl ketones is performed with the [RuCl 2 (pcymene)] 2 /AgSbF 6 catalytic system and leads, via ferrocene C−H bond activation, to moderate yields of the 1,2-disubstituted ferrocene derivatives in the presence of Cu-(OAc) 2 ·H 2 O under aerobic conditions at 80−110°C. The alkenylation of ferrocenyl phenyl ketone, in contrast, takes place at room temperature to afford quantitative yields of the phenyl monoalkenylated product, demonstrating the strong influence of the ferrocenyl group on arene C−H bond functionalization. Small amounts of 2-alkenylferrocenyl 2′-phenyl ketones can also be obtained. F unctional ferrocene derivatives constitute a family of chemicals offering useful properties in several areas. They are the basis of practical ligands, 1 including planar chirality ligands for efficient enantioselective catalysis. 2 They have found applications in material science as metallocene-containing polymers 3 and as materials for optical applications. 4 Most of the synthetic methods functionalizing ferrocene involve either a Friedel−Crafts type electrophilic substitution or a stoichiometric C−H bond metalation followed by coupling with an electrophile. 5 It thus remains a challenge to develop new, general catalytic methods to derivatize the stable ferrocene derivatives.The direct catalytic sp 2 C−H bond activation/functionalization of arenes and heterocycles has considerably improved synthetic methods via selective C−C bond cross-couplings. 6 In contrast, simple ferrocene derivatives have shown resistance to their catalytic C−H bond functionalization, likely due to the electron richness of this metallocene and its inertness to C−H bond deprotonation via a concerted metalation−deprotonation process. 7 However, several examples of stoichiometric C−H bond activation of ferrocene derivatives leading to cyclometalated complexes have been reported. 8 Only a few reports have already demonstrated the possibility of catalytic ferrocene C−H bond functionalization leading to arylated and alkenylated products. Ferrocene itself was first alkenylated by Moritani and Fujiwara 9 with Pd(OAc) 2 catalyst in dioxane/AcOH solvent in modest yields. A cross-coupling reaction between two C−H bonds of ferrocenyloxazoline and of an arene has been performed using a stoichiometric amount of Pd(OAc) 2 , but in the additional presence of Cu(OAc) 2 the cross-coupling became catalytic and led to a good yield of the diarylated ferrocene. 10 Recently the direct alkenylation of ferrocene with acrylates has been observed with Pd(OAc) 2 catalyst in acetic acid, leading only to moderate yields in monoalkenylated ferrocenes. 11 In contrast, in the presence of a directing dimethylaminomethyl group, ferrocene C−H bonds were recently functionalized with Pd(II) catalyst on reaction with diarylacetylenes to give highly arylated naphthalenes, 12 and effective borylation of ferrocene C−H bonds has been performed with iridium catalyst. 13 Recently it was shown that stable and inexpensive ruthenium(II) cat...