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.

3d Transition Metals for C-H Activation.

Alkylation reactions represent an important organic transformation to form C–C bonds. In addition to conventional approaches with alkyl halides or sulfonates as alkylating agents, the use of unactivated olefins for alkylations has become attractive from both cost and sustainability viewpoints. This Review summarizes transition-metal-catalyzed alkylations of various carbon–hydrogen bonds (addition of C–H bonds across olefins) using regular olefins or 1,3-dienes up to May 2016. According to the mode of activation, the Review is divided into two sections: alkylation via C–H activation and alkylation via olefin activation.

Transition-Metal-Catalyzed C–H Alkylation Using Alkenes

Titanium, as the second most abundant transition metal in the earth's crust, lends itself as a sustainable and inexpensive resource in catalysis. Its nontoxicity and biocompatibility are also attractive features for handling and disposal. Titanium has excelled as a catalyst for a broad range of transformations, including ethylene and α-olefin polymerizations. However, many reactions relevant to fine chemical synthesis have preferrentially employed late transition metals, and reactive, inexpensive early transition metals have been largely overlooked. In addition to promising reactivity, titanium complexes feature more robust character compared with some other highly Lewis-acidic metals such as those found in the lanthanide series. Since the advent of modulating ligand scaffolds, titanium has found use in a growing variety of reactions as a versatile homogeneous catalyst. These catalytic transformations include hydrofunctionalization reactions (adding an element-hydrogen (E-H) bond across a C-C multiple bond), as well as the ring-opening polymerization of cyclic esters, all of which are atom-economic transformations. Our investigations have focused on tight bite angle monoanionic N,O-chelating ligands, forming four-membered metallacycles. These ligand sets, including amidates, ureates, pyridonates, and sulfonamidates, have flexible binding modes offering a range of stable and reactive intermediates necessary for catalytic activity. Additionally, the simple form of these ligands leads to easily prepared proligands, along with facile tuning of steric and electronic factors. A sterically bulky titanium amidate complex has proven to be a leading catalyst for the selective formation of anti-Markovnikov addition products via intermolecular hydroamination of terminal alkynes, while sterically less demanding titanium pyridonates have opened the path to the selective formation of amine substituted cycloalkanes via the intramolecular hydroaminoalkylation of aminoalkenes over the competing hydroamination pathway. Sulfonamidates have boosted reactivity for hydrofunctionalization and polymerization reactions compared with amide ligands not bearing a sulfonyl group. N,O-Chelated titanium complexes have been used to synthesize ultrahigh molecular weight polyethylene and have been utilized in the challenging task of realizing equal incorporation of two different cyclic esters in a random ring-opening copolymerization. These discrete complexes have allowed for careful study of fundamental coordination chemistry and stoichiometric organometallic investigations. With inexpensive starting materials and modular ligands, titanium N,O-chelated complexes are well-suited to address the challenges of achieving greener chemical processes while accessing useful reaction manifolds for sustainable synthesis.

N,O-Chelating Four-Membered Metallacyclic Titanium(IV) Complexes for Atom-Economic Catalytic Reactions.

The design and synthesis of a mixed 2-pyridonate-Ta(NMe2)3Cl complex for the direct C–H alkylation adjacent to nitrogen in unprotected secondary amines are reported. The hydroaminoalkylation of sterically demanding internal alkenes gives the direct, catalytic formation of C(sp3)–C(sp3) bonds. Substrate scope investigations reveal key strategies for further catalyst development efforts in this 100% atom-economic synthesis of α-alkylated amines.

2-Pyridonate Tantalum Complexes for the Intermolecular Hydroaminoalkylation of Sterically Demanding Alkenes

TaMe3Cl2-Catalyzed Intermolecular Hydroaminoalkylation: A Simple Complex for Enhanced Reactivity and Expanded Substrate Scope

Gallium(III) iodide, which is conveniently formed in situ from gallium and iodine, is a competent catalyst for the inter- and intramolecular addition of p-toluenesulfonamides to alkenes. After each reaction, the metallic gallium can easily be recycled and used for subsequent transformations.

Recyclable Gallium as CatalystPrecursor for a Convenient and Solvent-Free Method for the IntermolecularAddition of Sulfonamides to Alkenes

Potassium hydroxide is a competent and cheap catalyst for theintermolecular addition of arylamines to styrenes. The reactionsare performed in nontoxic dimethyl sulfoxide and can be used forthe large-scale synthesis of β-phenylethylamines.

Potassium Hydroxide CatalyzedAddition of Arylamines to Styrenes

A novel dinuclear titanium complex is synthesized and successfully applied to the title reactions.

Dinuclear Titanium Complexes with Sulfamide Ligands as Precatalysts for Hydroaminoalkylation and Hydroamination Reactions.

The simple procedure can be applied to the addition of various arylamines and phenylhydrazine to arenes and is amenable to large scale synthesis.

Potassium Hydroxide Catalyzed Addition of Arylamines to Styrenes.

A new dinuclear titanium sulfamide complex was synthesized from N,N′-diphenylsulfamide and Ti(NMe2)4 and used as a precatalyst for the intermolecular hydroaminoalkylation of alkenes as well as the intramolecular hydroamination of alkenes.

Dinuclear Titanium Complexes with Sulfamide Ligands as Precatalysts for Hydroaminoalkylation and Hydroamination Reactions

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