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Aryl-aryl bond formation one century after the discovery of the Ullmann reaction.

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

Homogeneous catalysis by transition metal oxygen anion clusters

Phthalocyanine Metal Complexes in Catalysis

The recent surge of interest in metal-catalysed C-H bond functionalisation reactions reflects the importance of such reactions in biomimetic studies and organic synthesis. This critical review focuses on metalloporphyrin-catalysed saturated C-H bond functionalisation reported since the year 2000, including C-O, C-N and C-C bond formation via hydroxylation, amination and carbenoid insertion, respectively, together with a brief description of previous achievements in this area. Among the metalloporphyrin-catalysed reactions highlighted herein are the hydroxylation of steroids, cycloalkanes and benzylic hydrocarbons; intermolecular amination of steroids, cycloalkanes and benzylic or allylic hydrocarbons; intramolecular amination of sulfamate esters and organic azides; intermolecular carbenoid insertion into benzylic, allylic or alkane C-H bonds; and intramolecular carbenoid C-H insertion of tosylhydrazones. These metalloporphyrin-catalysed saturated C-H bond functionalisation reactions feature high regio-, diastereo- or enantioselectivity and/or high product turnover numbers. Mechanistic studies suggest the involvement of metal-oxo, -imido (or nitrene), and -carbene porphyrin complexes in the reactions. The reactivity of such metal-ligand multiple bonded species towards saturated C-H bonds, including mechanistic studies through both experimental and theoretical means, is also discussed (244 references).

Selective functionalisation of saturated C–H bonds with metalloporphyrin catalysts

Irontetrakis(pentafluorophenyl)β-octabromoporphyrinato complexes have been synthesized for the first time and shown to have unprecedented catalytic activity for the reaction of molecular oxygen with isobutane to givetert-butyl alcohol. This is the first report of the use of a perhaloporphyrin complex for mild, selective air-oxidation of an alkane and extends the trend of increased activity with halogen substitution established previously. Replacing the eight β-(pyrrolic) hydrogens in Fe(TPPF20) complexes with bromines gives catalysts having twice the room temperature air-oxidation activity of the Fe(TPPF20) complexes. Room temperature reaction of isobutane with oxygen catalyzed by Fe(TPPF20β6-Br8)Cl produces 190 moles product per mole catalyst per hour with over 90% selectivity to the alcohol. The catalyst activity is unchanged after 74 hours.

Selective low temperature hydroxylation of isobutane by molecular oxygen catalyzed by an iron perhaloporphyrin complex

Die Gleichgewichtskonstanten fur die Komplexe (Ia)-(Id) mit 1-Methyl- und 1,2-Dimethylimidazol sowie einer Reihe anderer basischer Liganden (PBu3, P(O-Et)3, Ph-CH2-CN, primare Amine, Sulfide, Thiole, Pyridine) wurden gemessen und relative Bindungsordnungen bestimmt.

Axial ligation constants of iron(II) and cobalt(II) "capped" porphyrins

Halogenation of the porphyrin ring of porphyrinatoiron(III) complexes greatly increases their catalytic activity for the selective mild reaction of alkanes with molecular oxygen. The greater the halogen content the greater is the catalytic activity of the complex. Selective reaction of isobutane and propane with molecular oxygen is catalyzed bytetrakis(pentafluorophenyl)porphyrinatoiron(III) azido, hydroxo, or halo complexes under mild conditions of temperature and pressure. No added co-reductant is required in these systems. Catalytic oxidation of isobutane in the presence oftetrakis (pentafluorophenyl)porphyrinatoiron(III) hydroxide at room temperature gavetert-butyl alcohol in 95% selectivity. The catalyst activity was virtually unchanged after over 140 hours at room temperature and over 12,000 turnovers (moles of alcohol produced/mole of metal complex used) of isobutane to the alcohol were observed.

Halogen substituent effects on the catalytic activity of iron porphyrin complexes for selective air-oxidation of alkanes in the liquid phase

Much work has been done in an effort to understand the nature of iron oxo complexes and their roles in the selective catalytic oxidation of alkanes. Iron oxo (ferryl) species (Fe=O) have been proposed to be the active intermediates responsible for both the enzymatic and biomimetic oxidations of alkanes to alcohols, while it is generally accepted that iron(III) μ-oxo species [Fe(III)-O-Fe(III)] are not catalytically active. We have synthesized a number of iron complexes having μ-oxo bridges in several molecular environments including porphyrinato, polyoxometallate, and silicometallate structures and examined the catalytic activity of these compounds for alkane oxidation in both liquid and vapor phase. The activity and selectivity of these catalysts depend upon the molecular environment of the μ-oxo species used as the catalyst precursor. In some instances in situ conversion of μ-oxo to ferryl oxo species may be the key to catalysts capable of direct hydroxylation of alkanes with air or oxygen.

Paul E. Ellis

Papers

Selective low temperature hydroxylation of isobutane by molecular oxygen catalyzed by an iron perhaloporphyrin complex

Axial ligation constants of iron(II) and cobalt(II) "capped" porphyrins

Halogen substituent effects on the catalytic activity of iron porphyrin complexes for selective air-oxidation of alkanes in the liquid phase

Active Iron Oxo Centers for the Selective Catalytic Oxidation of Alkanes

Oxygenation of iron(II) and cobalt(II) "capped" porphyrins