Inorganometallics (Transition Metal-Metalloid Complexes) and Catalysis

While the formation and breaking of transition metal (TM)-carbon bonds plays a pivotal role in the catalysis of organic compounds, the reactivity of inorganometallic species, that is, those involving the transition metal (TM)-metalloid (E) bond, is of key importance in most conversions of metalloid derivatives catalyzed by TM complexes. This Review presents the background of inorganometallic catalysis and its development over the last 15 years. The results of mechanistic studies presented in the Review are related to the occurrence of TM-E and TM-H compounds as reactive intermediates in the catalytic transformations of selected metalloids (E = B, Si, Ge, Sn, As, Sb, or Te). The Review illustrates the significance of inorganometallics in catalysis of the following processes: addition of metalloid-hydrogen and metalloid-metalloid bonds to unsaturated compounds; activation and functionalization of C-H bonds and C-X bonds with hydrometalloids and bismetalloids; activation and functionalization of C-H bonds with vinylmetalloids, metalloid halides, and sulfonates; and dehydrocoupling of hydrometalloids. This first Review on inorganometallic catalysis sums up the developments in the catalytic methods for the synthesis of organometalloid compounds and their applications in advanced organic synthesis as a part of tandem reactions.

Ruthenium-Catalyzed Reductive Cleavage of Unstrained Aryl-Aryl Bonds: Reaction Development and Mechanistic Study

Cleavage of carbon-carbon bonds has been found in some important industrial processes, for example, petroleum cracking, and has inspired development of numerous synthetic methods. However, nonpolar unstrained C(aryl)-C(aryl) bonds remain one of the toughest bonds to be activated. As a detailed study of a fundamental reaction mode, here a full story is described about our development of a Ru-catalyzed reductive cleavage of unstrained C(aryl)-C(aryl) bonds. A wide range of biaryl compounds that contain directing groups (DGs) at 2,2' positions can serve as effective substrates. Various heterocycles, such as pyridine, quinoline, pyrimidine, and pyrazole, can be employed as DGs. Besides hydrogen gas, other reagents, such as Hantzsch ester, silanes, and alcohols, can be employed as terminal reductants. The reaction is pH neutral and free of oxidants; thus a number of functional groups are tolerated. Notably, a one-pot C-C activation/C-C coupling has been realized. Computational and experimental mechanistic studies indicate that the reaction involves a ruthenium(II) monohydride-mediated C(aryl)-C(aryl) activation and the resting state of the catalyst is a η4-coordinated ruthenium(II) dichloride complex, which could inspire development of other transformations based on this reaction mode.

Silver-catalyzed intermolecular amination of fluoroarenes

A novel highly selective Ag-catalyzed intermolecular amination of fluoroarenes has been developed.

Addition of Carbon-Fluorine Bonds to a Mg(I)-Mg(I) Bond: An Equivalent of Grignard Formation in Solution

Addition of the carbon–fluorine bond of a series of perfluorinated and polyfluorinated arenes across the Mg–Mg bond of a simple coordination complex proceeds rapidly in solution. The reaction results in the formation of a new carbon–magnesium bond and a new fluorine–magnesium bond and is analogous to Grignard formation in homogeneous solution.

Synthesis of a rhodium(i) germyl complex: a useful tool for C–H and C–F bond activation reactions

The rhodium(i) germyl complex [Rh(GePh₃)(PEt₃)₃] is a useful tool for C–F and C–H bond activation reactions. For instance, treatment with hexafluoropropene results in the formation of two isomeric C–F activation products [Rh{(E)-CFCF(CF₃)}(PEt₃)₃] and [Rh{(Z)-CFCF(CF₃)}(PEt₃)₃] in a 3 : 1 ratio.

Oxidative addition of carbon-fluorine and carbon-oxygen bonds to Al(I)

Addition of fluoroarenes, fluoroalkanes or benzofuran to [{(2,6-ⁱPr₂C₆H₃NCMe)₂CH}Al] results in facile oxidative addition of either a C–F or C–O bond to the Al(i) centre.

Addition of fluoroarenes, fluoroalkanes or benzofuran to [{(2,6-ⁱPr₂C₆H₃NCMe)₂CH}Al] results in facile oxidative addition of either a C–F or C–O bond to the Al(i) centre.

Synthesis and structure of rhodium(i) silyl carbonyl complexes: photochemical C-F and C-H bond activation of fluorinated aromatic compounds

The rhodium(i) silyl carbonyl complexes [Rh{Si(OEt)3}(CO)(dippp)] () and [Rh{Si(OEt)3}(CO)(dippe)] () (dippp = 1,3-bis(diisopropylphosphino)propane, dippe = 1,2-bis-(diisopropylphosphino)ethane) were synthesized on treatment of the methyl compounds [Rh(CH3)(CO)(dippp)] () or [Rh(CH3)(CO)(dippe)] () with HSi(OEt)3 at low temperature. The methyl complexes and were prepared starting from the binuclear complexes [{Rh(μ-Cl)(dippp)}2] () and [{Rh(μ-Cl)(dippe)}2] (), respectively. The silyl complexes and as well as the precursors [{Rh(μ-I)(dippp)}2] (), [Rh(X)(CO)(dippp)] (: X = CH3, : X = I) and [Rh(X)(CO)(dippe)] (: X = CH3, : X = Cl) were characterized by NMR and IR spectroscopy and the structures in the solid state were determined by X-ray crystallography. The silyl complex converts into the carbonyl-bridged complex [{Rh(μ-CO)(dippp)}2] () above temperatures of -30 °C by loss of the silyl ligand, whereas is more thermally stable and a reaction to the binuclear complex [{Rh(μ-CO)(dippe)}2] () was observed at 50 °C. The silyl complex reacted under irradiation with hexafluorobenzene and pentafluoropyridine to give the C-F activation products [Rh(C6F5)(CO)(dippe)] () and [Rh(2-C5F4N)(CO)(dippe)] (), respectively. As additional products the silyl dicarbonyl complex [Rh{Si(OEt)3}(CO)2(dippe)] () and the cationic complex [Rh2(μ-H)(μ-CO)2(dippe)2](+)[SiF5](-) () were identified. Compound was synthesized independently by treatment of with gaseous CO. In a similar manner, the dippp analogue [Rh{Si(OEt)3}(CO)2(dippp)] () was also prepared starting from . Photochemical reaction of with pentafluorobenzene and 2,3,5,6-tetrafluoropyridine resulted selectively in C-H bond activation to afford and [Rh(4-C5F4N)(CO)(dippe)] (), respectively.

Catalytic defluorinative [3 + 2] cycloaddition of trifluoromethylalkenes with alkynes via reduction of nickel(ii) fluoride species

A catalytic synthesis of 2-fluoro-1,3-cyclopentadienes was achieved via [3 + 2] cycloaddition of 2-trifluoromethyl-1-alkenes and alkynes using a nickel catalyst and a diboron-based ternary additive system.

Activator-free palladium-catalyzed silylation of aryl chlorides with silylsilatranes

The palladium-catalyzed silylation of aryl chlorides with silylsilatranes proceeds under activator-free conditions; hence, wide functional group compatibility is displayed and boryl and siloxy groups are able to survive. Experimental and computational studies revealed that smooth transmetalation from the silylsilatrane to the arylpalladium chloride is facilitated by strong interaction between the Lewis acidic silicon and the chloride.

Nickel-catalyzed Suzuki-Miyaura reaction of aryl fluorides

Two protocols for the nickel-catalyzed cross-coupling of aryl fluorides with aryl boronic esters have been developed. The first employs metal fluoride cocatalysts, such as ZrF(4) and TiF(4), which enable Suzuki-Miyaura reactions of aryl fluorides bearing electron-withdrawing (ketones, esters, and CF(3)), aryl and alkenyl groups as well as those comprising fused aromatic rings, such as fluoronaphthalenes and fluoroquinolines. The second protocol employs aryl fluorides bearing ortho-directing groups, which facilitate the difficult C-F bond activation process via cyclometalation. N-heterocycles, such as pyridines, quinolines, pyrazoles, and oxazolines, can successfully promote cross-coupling with an array of organoboronic esters. A study into the substituent effects with respect to both coupling components has provided fundamental insights into the mechanism of the nickel-catalyzed cross-coupling of aryl fluorides.

C-F activation at rhodium boryl complexes: formation of 2-fluoroalkyl-1,3,2-dioxaborolanes by catalytic functionalization of hexafluoropropene

Fluorinated building blocks by C-F bond cleavage: Catalytic C-F activation reactions that give novel dioxaborolanes have been developed (see scheme). The reactions proceed at room temperature, and catalytic intermediates are presumably rhodium hydride and boryl species.

High ortho preference in Ni-catalyzed cross-coupling of halophenols with alkyl grignard reagents

High preference of substitution at the position ortho to the hydroxy group was observed for Ni-catalyzed cross-coupling reactions of dihalophenols with alkyl Grignard reagents. Reactions of 2,4-dihalophenols, with various combinations of F, Cl, and Br, were shown to afford ortho-cross-coupled products selectively. In addition, high ortho preference was also observed in competitive cross-coupling reactions between 2-halophenols and other halophenols.