Arkadi Vigalok

Metal‐Mediated Formation of Carbon–Halogen Bonds

Organic halides represent basic starting materials for numerous metal-catalyzed organic transformations. Generally, the carbon-halogen is broken in the first step, that is, an oxidative addition reaction, of the catalytic cycle. On the other hand, very little is known about the reverse reaction, carbon-halogen reductive elimination from a transition-metal center. In this Concept article, we describe the examples of C(sp(3))-halide and C(sp(2))-halide reductive-elimination reactions which demonstrate that this type of reactivity can be quite common in organometallic chemistry. Although the thermodynamic driving force for the formation of carbon-halogen bonds is relatively small, the kinetic barrier for these reactions can also be low. Thus, C-halide reductive elimination can compete favorably with the more established organic transformations, such as C-C reductive elimination.

Reagent-Dependent Formation of C−C and C−F Bonds in Pt Complexes: An Unexpected Twist in the Electrophilic Fluorination Chemistry

Cyclometalated platinum(II) complexes (C-P)Pt(Aryl)py undergo oxidative addition reaction upon treatment with electrophilic fluorination reagents, XeF(2) or N-fluoro-2,4,6-trimethylpyridinium tetrafluoroborate, giving eventually products of C-C coupling. However, when aryl = mesityl, the two reagents give completely different products: the N-F salt still favors the C-C coupling reaction while XeF(2) gives the unprecedented benzylic fluorination of one of the ortho-methyl groups of the mesityl ligand.

Electrophilic halogenation-reductive elimination chemistry of organopalladium and -platinum complexes.

CONSPECTUS: Transition metal-catalyzed organic transformations often reveal competing reaction pathways. Determining the factors that control the selectivity of such reactions is of extreme importance for the design of reliable synthetic protocols. Herein, we present the account of our studies over the past decade aimed at understanding the selectivity of reductive elimination chemistry of organotransition metal complexes under electrophilic halogenation conditions. Much of our effort has focused on finding the conditions for selective formation of carbon (aryl)-halogen bonds in the presence of competing C-C reductive elimination alternatives. In most cases, the latter was the thermodynamically preferred pathway; however, we found that the reactions could be diverted toward the formation of aryl-iodine and aryl-bromine bonds under kinetic conditions. Of particular importance was to maintain the complex geometry that prohibits C-C elimination while allowing for the elimination of carbon-halogen bonds. This was achieved by employing sterically rigid diphosphine ligands which prevented isomerization within a series of Pt(IV) complexes. It was also important to understand that the neutral M(IV) products often observed or isolated in the oxidative addition reactions are not necessarily the intermediates in the reductive elimination chemistry as it generally takes place from unsaturated species formed en route to relatively stable M(IV) complexes. While aryl-halide reductive elimination for heavier halogens can be competitive with aryl-aryl coupling in diaryl M(IV) complexes, the latter reaction always prevails over aryl-fluoride bond formation. Even when one of the aryl groups is a part of a rigid cyclometalated ligand C-C coupling is still the dominant reaction pathway. However, when one of the aryl groups is replaced with a phenolate donor aryl-F bond formation becomes preferred over C-O bond elimination. During our studies, other interesting reactions have been discovered. For example, the fluorination of the C(sp(3))-H bond can be very selective and compete favorably with C-C coupling. Also, in electron-poor complexes, metal oxidation can have higher energy than oxidation of the coordinated iodo ligand resulting in I-F elimination instead of the formation of aryl-I bond. Overall, electrophilic fluorination can lead to often very selective elimination reactions giving new C-C, C-I, C-F, or I-F bonds, with this selectivity dependent on the metal center, supporting ligands, complex geometry, and electrophilic fluorine source. Together with the many reports on the halogenation of organometallic compounds that appeared in recent years, our results contribute to understanding the requirements for selective transformations under electrophilic conditions and design of new synthetic methods for making organohalogen compounds.

Evidence for metal-ligand cooperation in a Pd-PNF pincer-catalyzed cross-coupling.

The first Pd-pincer complex bearing a halogen (fluorine) arm has been prepared via the base-assisted dearomatization of a phosphine-quinoline (P~N) ligand. This dearomatization is reversible and has been used to facilitate catalytic Sonogashira-type cross-coupling that, contrary to the typical mechanistic approach, is based on a metal-ligand cooperation mode.

Aryl-bromide reductive elimination from an isolated Pt(IV) complex

A Pt(IV) complex bearing two aryl and two bromo ligands, which undergoes selective elimination of a bromoarene molecule has been prepared and fully-characterized. The mechanistic studies of this reaction are presented.

Late Transition Metal-Mediated Formation of Carbon-Halogen Bonds

This chapter reviews the synthesis of nonactivated aromatic halides assisted by late transition metal complexes. Although some of these reactions have been known for over half-a-century, there was a tremendous progress in the stoichiometric and catalytic applications of late transition metals in halogenation of aromatic compounds. Both nucleophilic and electrophilic halogenation pathways have been explored, resulting in practical methods for making aryl-halides. The chapter provides a metal-by-metal discussion of the above reactions, including the possible mechanisms of the metal-promoted formation of C-Halogen bonds.

A unique dioxo alkene hydride metal complex: [RhH(O2){CH2C(CH2CH2PBut2)2}]

A unique, unexpectedly stable rhodium complex containing hydride, alkene and dioxygen ligands in cis-positions to each other is synthesized and fully characterized spectroscopically and crystallographically.

Solvent-stabilized alkylrhodium(III) hydride complexes: a special mode of reversible C-H bond elimination involving an agostic intermediate

Reaction of the complex [Rh(coe)2(solv)n]BF4 (coe=cyclooctene) with the phosphane 1-di-tert-butylphosphinomethyl-2,4,6-trimethylbenzene (1) results in selective C-H bond activation, yielding the spectroscopically characterized solvento complexes [(solv)nRhH(CH2C6H2(CH3)2[CH2P(tBu)2]]]BF4 (solv = acetone, 2a; THF, 2b; methanol, 2c). The stability of these complexes is solvent dependent, alcohols providing significant stabilization. Although cis-alkylrhodium hydride complexes containing labile ligands are generally unstable, 2a-c are stable at room temperature. Complex [ (acetone)(ketol)RhH[CH2C6H2(CH3)2[CH2P(t-Bu)2]]]BF4 (2d, ketol 4-hydroxy-4-methyl-2-pentanone, the product of acetone aldol condensation), crystallized from a solution of 2a in acetone and was structurally characterized. Unusual solvent- and temperature-dependent selectivity in reversible C-H bond elimination of these complexes, most probably controlled by a special mode of strong agostic interactions, is observed by spin saturation transfer experiments.

Straightforward radical organic chemistry in neat conditions and “on water”

Radicals generated during aldehyde oxidation to carboxylic acids can be efficiently trapped under environmentally friendly conditions, either in neat conditions or “on water”.

The Inside of Metal Calixarene Chemistry

Calixarene molecules are often associated with their ability to form inclusion complexes with organic or inorganic guests. These complexes are usually formed due to the guest molecule interactions with the calixarene aromatic cavity and/or substituents attached to it. The present mini-review discusses a different type of calixarene inclusion complexes, where the inclusion event occurs due to the coordination of an organic or inorganic ligand to a metal centre attached to the lower rim of calix[4]arene. Structural and chemical properties of such complexes of ‘uninvited’ guests are discussed.