Mn-Catalyzed Aromatic C–H Alkenylation with Terminal Alkynes

Mn(i)-Catalyzed nucleophilic addition/ring expansion via C–H activation and C–C cleavage

The Mn(i)-Catalyzed C–H alkenylation/carbonyl addition/retro-Aldol cascade was realized leading to the convenient synthesis of seven- or eight-membered carbocycles.

Manganese(ii)-catalysed dehydrogenative annulation involving C–C bond formation: highly regioselective synthesis of quinolines

An inexpensive nontoxic manganese(ii)-catalysed dehydrogenative annulation was developed for C–C bond formation.

Synthesis of benzoazepine derivatives via Rh(iii)-catalyzed inert C(sp2)–H functionalization and [4 + 3] annulation

A novel synthesis of benzoazepine derivatives via Rh(iii)-catalyzed inert C(sp²)–H bond functionalization and [4 + 3] annulation is presented.

Group 9 [Cp*MIII] complex-catalyzed C–H olefination of arenes in water at room temperature: a study on the catalytic activity

Herein, we report our discovery of group 9 [Cp*M^III] complexes catalyzing the olefination of arenes in water at ambient temperature.

ReI-Catalyzed highly regio- and stereoselective C–H addition to terminal and internal alkynes

Re(i)-Catalyzed ortho alkenylation of arylpyridines and N-pyrimidyl indoles with alkynes provides excellent regio- and stereoselectivity with lower catalytic loadings.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Group 4 Metallocene Difluoride/Palladium Bimetallic Catalysts for the Reductive Cross-Coupling of Alkynes with Aryl Iodides and Bromides

A novel protocol has been developed for the selective synthesis of ( E)-alkenes via the reductive cross-coupling of alkynes and aryl halides using a bimetallic catalyst system composed of a group 4 metallocene difluoride (Cp₂[M]F₂; [M] = Hf or Zr; Cp = cyclopentadienide) and palladium dichloride. This reaction proceeds via a coupling between an aryl halide and an in situ generated alkenyl metallocene intermediate derived from the group 4 metallocene difluoride, a hydrosilane, and an alkyne. For a catalytic reductive coupling, the addition of sodium fluoride (NaF) to the reaction system is required. Moreover, in the presence of NaF, a ligand exchange was observed by NMR spectroscopy in hafnocene diiodide (Cp₂HfI₂) to afford hafnocene difluoride (Cp₂HfF₂).

Recent Advances on Mechanistic Studies on C–H Activation Catalyzed by Base Metals

During the last ten years, base metals have become very attractive to the organometallic and catalytic community on activation of C-H bonds for their catalytic functionalization. In contrast to the statement that base metals differ on their mode of action most of the manuscripts mistakenly rely on well-studied mechanisms for precious metals while proposing plausible mechanisms. Consequently, few literature examples are found where a thorough mechanistic investigation have been conducted with strong support either by theoretical calculations or experimentation. Therefore, we consider of highly scientific interest reviewing the last advances on mechanistic studies on Fe, Co and Mn on C-H functionalization in order to get a deep insight on how these systems could be handle to either enhance their catalytic activity or to study their own systems in a similar systematic fashion. Thus, in this review we try to cover the most insightful articles for mechanistic studies on C-H activation catalyzed by Fe, Co and Mn based on kinetic and competition experiments, stoichiometric reactions, isolation of intermediates and theoretical calculations.

Selective Hydroarylation of 1,3-Diynes Using a Dimeric Manganese Catalyst: Modular Synthesis of Z-Enynes

The transition-metal-catalyzed selective hydroarylation of unsymmetrical alkynes represents the state-of-art in organic chemistry, and still mainly relies on the use of precious late-transition-metal catalysts. Reported herein is an unprecedented Mn^I -catalyzed hydroarylation of unsymmetrical 1,3-diyne alcohols with commercially available arylboronic acids with predictive selectivity. This method addresses the challenges in regio-, stereo-, and chemoselectivity. It offers a general, convenient and practical strategy for the modular synthesis of multisubstituted Z-configurated conjugated enynes. This protocol is distinguished by its operational simplicity, complete selectivity, excellent functional-group compatibility, and gram-scale potential. A dimeric Mn^I species, Mn₂ (CO)₈ Br₂ , was proven to be a much more efficient catalyst precursor than Mn(CO)₅ Br.

Manganese-Catalyzed Hydrosilylation Reactions

Over the past few decades, manganese-catalyzed hydrosilylation of C=O or C=C/C≡C unsaturated bonds have undergone enormous developments. In this focus review, the hydrosilylation reactions of alkenes, alkynes, and carbonyl-containing substrates catalyzed by manganese complexes are summarized. Moreover, the mechanisms of the manganese-catalyzed hydrosilylation are briefly discussed.

Continuous Visible-Light Photoflow Approach for a Manganese-Catalyzed (Het)Arene C-H Arylation

Manganese photocatalysts enabled versatile room-temperature C-H arylation reactions by means of continuous visible-light photoflow, thus allowing for efficient C-H arylations in 30 minutes with ample scope. The robustness of the manganese-catalyzed photoflow strategy was shown by visible light-induced gram-scale synthesis, clearly outperforming the batch performance.

Palladium-Catalyzed Regioselective Olefination of O-Acetyl Cyanohydrins

A practical palladium-catalyzed ortho-olefination of O-acetyl cyanohydrins assisted by synergetic directing groups has been developed. Thus, a range of olefinated O-acetyl cyanohydrins were synthesized in moderate to good yields. The reaction occurs efficiently with high regioselectivity and with a satisfactory tolerance of functional groups.

Inert C-H Bond Transformations Enabled by Organometallic Manganese Catalysis

Traditional organic synthesis relies heavily on the transformations of various preinstalled functional groups, such as cross-coupling reactions using organohalides and organometallic reagents. The strategy of C-H activation enables the direct formation of C-C/C-X (X = heteroatom) bonds from inert C-H bonds, which can enhance the atom- and step-economy of organic synthesis. To date, precious metals have overwhelmingly dominated the C-H activation field; however, the rarity and high cost of these metals necessitate the development of more sustainable catalysts. In this regard, catalysts based on manganese are highly desirable owing to the abundant reserve of manganese in the earth's crust and its economic benefits, low toxicity, and potentially unique reactivity. Although the first stoichiometric manganese-mediated C-H activation reaction was reported as early as 1970, manganese-catalyzed C-H activation reactions are largely underdeveloped. How to construct an efficient catalytic cycle for manganese in C-H activation reactions remains as a key issue to be addressed. In this Account, we summarize our recent advances in the manganese-catalyzed transformations of inert C-H bonds. To overcome the challenges associated with building manganese-based catalytic cycles, we developed two novel strategies, namely, synergy between manganese catalysts and bases and between manganese catalysts (with or w/o bases) and acids. By implementing the former strategy, we developed cooperative manganese/base catalytic systems that facilitate a new mode of C-H bond activation by manganese via a redox-neutral base-assisted deprotonation mechanism. As such, the requirement for the tedious preparation of MnR(CO)₅ complexes (R = Me, Bn, Ph) in stoichiometric reactions was eliminated, and a series of manganese-catalyzed C-H activation reactions of arenes with various reaction partners having C≡C and C═C bonds were achieved. Through the latter strategy of synergy between manganese catalysts (with or w/o bases) and acids, we disclosed a "dual activation" mode for performing manganese-catalyzed C-H bond transformations, that is, merging C-H activation by manganese catalysts and C-X multiple bond activation by Lewis acids. Consequently, the scope of C-H substrates could be expanded to include challenging ketones and olefinic C-H compounds. Additionally, the range of reaction partners could be significantly broadened to include those bearing more polarized C═O, C═N, and C≡N bonds such as aldehydes, imines, and nitriles. Remarkably, the innate reactivity of different C-H bonds in ketones could be reversed by manganese catalysis, and the reactions could even be carried out at room temperature. Our findings provide guiding information for the future development of manganese-catalyzed C-H activation reactions and beyond. Related important contributions from other groups are mentioned, and the remaining challenges and future perspective in this emerging area are also presented.

Central-metal effect on intramolecular vibrational energy transfer of M(CO)5Br (M = Mn, Re) probed by two-dimensional infrared spectroscopy

Vibrational energy transfer in transition metal complexes with flexible structures in condensed phases is of central importance to catalytical chemistry processes. In this work, two molecules with different metal atoms, M(CO)₅Br (where M = Mn, Re), were used as model systems, and their axial and radial carbonyl stretching modes as infrared probes. The central-metal effect on intramolecular vibrational energy redistribution (IVR) in M(CO)₅Br was investigated in polar and nonpolar solvents. The linear infrared (IR) peak splitting between carbonyl vibrations increases as the metal atom changes from Mn to Re. The waiting-time dependent two-dimensional infrared diagonal- and off-diagonal peak amplitudes reveal a faster IVR process in Re(CO)₅Br than in Mn(CO)₅Br. With the aid of density functional theory (DFT) calculations, the central-metal effect on IVR time linearly correlates with the vibrational coupling strength between the two involved modes. In addition, the polar solvent is found to accelerate the IVR process by affecting the anharmonic vibrational potentials of a solute vibration mode.

Manganese-catalyzed direct C2-allylation of indoles

Development of manganese-catalyzed synthesis of functionalized indoles is reported. This method involves direct C–H activation and allylation of indoles with broad substrate tolerance, leading to a series of C2-allylated indole derivatives under mild reaction conditions.

Recent advances in manganese-catalysed C–H activation: scope and mechanism

Manganese catalysed C–H activation has emerged as a promising green alternative to transition metal mediated processes.

Isolation of Cp*CoIII -Alkenyl Intermediate in Efficient Cobalt-Catalyzed C-H Alkenylation with Alkynes

A general and efficient procedure for C-H alkenylation of arenes with a broad substrate scope catalyzed by Cp*Co^III was demonstrated with alkynes. A highly selective mono-alkenylation and sequential bis-C-H bond functionalization was displayed to exemplify the versatility of the cobalt catalyst. Isolation of cationic Cp*Co^III -alkenyl intermediate was achieved under identical catalytic conditions to further establish the proposed pathway.

Aromatic C-H addition of ketones to imines enabled by manganese catalysis

Selectivity control of varied C–H bonds in a complex molecule is a long-standing goal and still a great challenge in C–H activation field. Most often, such selectivity is achieved by the innate reactivity of different C–H bonds. In this context, the classic Mannich reaction of acetophenone derivatives and imines is ascribed to the more reactive C(sp³)–H bonds α to the carbonyl, with the much less reactive aromatic C(sp²)–H bonds remaining intact. Herein we report an aromatic C(sp²)–H addition of ketones to imines enabled by manganese catalysis, which totally reverses the innate reactivity of C–H bonds α to the carbonyl and those on the aromatic ring. Diverse products of ortho-C–H aminoalkylated ketones, cyclized exo-olefinic isoindolines, and three-component methylated isoindolines can be successfully accessed under mild reaction conditions, which significantly expands the synthetic utilities of ketones as simple bulk chemicals.

The Mannich reaction proceeds by functionalization of C(sp³)-H bonds alpha to carbonyls. Here, the authors suppress the Mannich reactivity of acetophenones in presence of a manganese catalyst and obtain valuable o-aminoalkylacetophenones and isoindolines via C(sp²)-H functionalization with imines.