ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

The selective transformation of ubiquitous but inert C–H bonds to other functional groups has far-reaching practical implications, ranging from more efficient strategies for fine chemical synthesis to the replacement of current petrochemical feedstocks by less expensive and more readily available alkanes. The past twenty years have seen many examples of C–H bond activation at transition-metal centres, often under remarkably mild conditions and with high selectivity. Although profitable practical applications have not yet been developed, our understanding of how these organometallic reactions occur, and what their inherent advantages and limitations for practical alkane conversion are, has progressed considerably. In fact, the recent development of promising catalytic systems highlights the potential of organometallic chemistry for useful C–H bond activation strategies that will ultimately allow us to exploit Earth's alkane resources more efficiently and cleanly.

Understanding and exploiting C–H bond activation

In 2010, Richard Heck, Ei-ichi Negishi, and Akira Suzuki joined the prestigious circle of Nobel Laureate chemists for their roles in discovering and developing highly practical methodologies for C-C bond construction. From their original contributions in the early 1970s the landscape of the strategies and methods of organic synthesis irreversibly changed for the modern chemist, both in academia and in industry. In this Review, we attempt to trace the historical origin of these powerful reactions, and outline the developments from the seminal discoveries leading to their eminent position as appreciated and applied today.

Palladium-Catalyzed Cross-Coupling: A Historical Contextual Perspective to the 2010 Nobel Prize

One {open_quotes}Holy Grail{close_quotes} of C-H activation research, therefore, is not simply to find new C-H activation reactions but to obtain an understanding of them that will allow the development of reagents capable of selective transformations of C-H bonds into more reactive functionalized molecules. Although a few interesting organic and main group reagents have been discovered that show intriguing potential for meeting this goal, much of the activity in this field has been in transition metal chemistry. The present Account therefore reflects this emphasis. 109 refs., 3 figs.

Selective Intermolecular Carbon-Hydrogen Bond Activation by Synthetic Metal Complexes in Homogeneous Solution

ConspectusIn recent years, carbon–hydrogen bond functionalization has evolved from an organometallic curiosity to a tool used in mainstream applications in the synthesis of complex natural products and drugs. The use of C–H bonds as a transformable functional group is advantageous because these bonds are the most abundant functionality in organic molecules. One-step conversion of these bonds to the desired functionality shortens synthetic pathways, saving reagents, solvents, and labor. Less chemical waste is generated as well, showing that this chemistry is environmentally beneficial. This Account describes the development and use of bidentate, monoanionic auxiliaries for transition-metal-catalyzed C–H bond functionalization reactions. The chemistry was initially developed to overcome the limitations with palladium-catalyzed C–H bond functionalization assisted by monodentate directing groups. By the use of electron-rich bidentate directing groups, functionalization of unactivated sp3 C–H bonds under palla...

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Bidentate, monoanionic auxiliary-directed functionalization of carbon-hydrogen bonds.

Oxidative addition of the carbon-hydrogen bonds of neopentane and cyclohexane to a photochemically generated iridium(I) complex

Addition oxydante du methane a un complexe carbonyle de pentamethylcyclopentadienyl-iridium

Activation of methane by iridium complexes

There is now strong evidence that {eta}{sup 2}-benzene intermediates are involved in benzene activation to form complexes such as ({eta}-C{sub 5}Me{sub 5})(Me{sub 3}P)Rh(H)(C{sub 6}H{sub 5}), a possibility envisaged earlier. It was thus natural to assume that an {eta}{sup 2} intermediate would be involved in the activation of olefinic C-H bonds by metal complexes. This notion was upset by the importance observation that ({eta}-C{sub 5}Me{sub 5})(Me{sub 3}P)Ir({eta}-C{sub 2}H{sub 4}) was more stable than its hydridovinyl isomer ({eta}-C{sub 5}Me{sub 5})(Me{sub 3}P)Ir(H)(CH=CH{sub 2}) and thus could not be an intermediate in formation of the latter. Other examples of this stability order, L{sub n}M({eta}{sup 2}-C{sub 2}H{sub 4}) > L{sub n}M(H)(CH=CH{sub 2}), have been reported. In experiments with an iridium complex of a new tris(pyrazolyl)borate ligand, we have encountered a particularly clear case in which this stability order is reversed. The new ligand is the tris(3-(trifluoromethyl)-5-methylpyrazol-1-yl)borato anion, designated HBPf{sub 3}{sup {minus}}. {sup 1}H NMR spectra is presented for this compound.

A hydridovinyliridium complex more stable than its .eta.2-ethylene isomer

Protonation of [M(HBPz*3)(Co)2](Pz*= 3,5-dimethylpyrazol-1-yl) occurs at the nitrogen for M = Rh forming [Rh{η2-HBPz*2(Pz*H)}(CO)2][BF4], of which the X-ray crystal structure has been determined; for M = Ir a cationic IrIII hydride is formed which is attacked by strong nucleophiles at a carbonyl carbon.

James K. Hoyano

Papers

Oxidative addition of the carbon-hydrogen bonds of neopentane and cyclohexane to a photochemically generated iridium(I) complex

Activation of methane by iridium complexes

A hydridovinyliridium complex more stable than its .eta.2-ethylene isomer

Oxidative addition of the carbon-hydrogen bonds of neopentane and cyclohexane to a photochemically generated iridium(I) complex

Nitrogen versus metal protonation in pyrazolyborate complexes of rhodium(I) and iridium(I). Unexpected reactions of a cationic iridium(III) hydride