Selective C−H Activation of Haloalkanes using a Rhodiumtrispyrazolylborate Complex

C-Cl Oxidative Addition and C-C Reductive Elimination Reactions in the Context of the Rhodium-Promoted Direct Arylation

A cycle of stoichiometric elemental reactions defining the direct arylation promoted by a redox-pair Rh(I)-Rh(III) is reported. Starting from the rhodium(I)-aryl complex RhPh{κ3-P,O,P-[xant(PiPr2)2]} (xant(PiPr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene), the reactions include C-Cl oxidative addition of organic chlorides, halide abstraction from the resulting six-coordinate rhodium(III) derivatives, C-C reductive coupling between the initial aryl ligand and the added organic group, oxidative addition of a C-H bond of a new arene, and deprotonation of the generated hydride-rhodium(III)-aryl species to form a new rhodium(I)-aryl derivative. In this context, the kinetics of the oxidative additions of 2-chloropyridine, chlorobenzene, benzyl chloride, and dichloromethane to RhPh{κ3-P,O,P-[xant(PiPr2)2]} and the C-C reductive eliminations of biphenyl and benzylbenzene from [RhPh2{κ3-P,O,P-[xant(PiPr2)2]}]BF4 and [RhPh(CH2Ph){κ3-P,O,P-[xant(PiPr2)2]}]BF4, respectively, have been studied. The oxidative additions generally involve the cis addition of the C-Cl bond of the organic chloride to the rhodium(I) complex, being kinetically controlled by the C-Cl bond dissociation energy; the weakest C-Cl bond is faster added. The C-C reductive elimination is kinetically governed by the dissociation energy of the formed bond. The C(sp3)-C(sp2) coupling to give benzylbenzene is faster than the C(sp2)-C(sp2) bond formation to afford biphenyl. In spite of that a most demanding orientation requirement is needed for the C(sp3)-C(sp2) coupling than for the C(sp2)-C(sp2) bond formation, the energetic effort for the pregeneration of the C(sp3)-C(sp2) bond is lower. As a result, the weakest C-C bond is formed faster.

Photolysis of Tp'Rh(CNneopentyl)(PhNCNneopentyl) in the presence of ketones and esters: kinetic and thermodynamic selectivity for activation of different aliphatic C-H bonds

The active fragment [Tp'Rh(CNneopentyl)], generated from the precursor Tp'Rh(CNneopentyl)(PhN[double bond, length as m-dash]CNneopentyl), underwent oxidative addition of substituted ketones and esters resulting in Tp'Rh(CNneopentyl)(R)(H) complexes (Tp' = tris-(3,5-dimethylpyrazolyl)borate). These C-H activated complexes underwent reductive elimination at varying temperatures (24-70 °C) in C6D6 or C6D12. Using previously established kinetic techniques, the relative Rh-C bond strengths were calculated. Analysis of the relative Rh-C bond strengths vs. C-H bond strengths shows a linear correlation with slope RM-C/C-H = 1.22 (12). In general, α-substituents increase the relative Rh-C bond strengths compared to the C-H bond that is broken.

The Essential Role of Bond Energetics in C-H Activation/Functionalization

The most fundamental concepts in chemistry are structure, energetics, reactivity and their inter-relationships, which are indispensable for promoting chemistry into a rational science. In this regard, bond energy, the intrinsic determinant directly related to structure and reactivity, should be most essential in serving as a quantitative basis for the design and understanding of organic transformations. Although C-H activation/functionalization have drawn tremendous research attention and flourished during the past decades, understanding the governing rules of bond energetics in these processes is still fragmentary and seems applicable only to limited cases, such as metal-oxo-mediated hydrogen atom abstraction. Despite the complexity of C-H activation/functionalization and the difficulties in measuring bond energies both for the substrates and intermediates, this is definitely a very important issue that should be more generally contemplated. To this end, this review is rooted in the energetic aspects of C-H activation/functionalization, which were previously rarely discussed in detail. Starting with a concise but necessary introduction of various classical methods for measuring heterolytic and homolytic energies for C-H bonds, the present review provides examples that applied the concept and values of C-H bond energy in rationalizing the observations associated with reactivity and/or selectivity in C-H activation/functionalization.

Gold(I) Carbenoids: On-Demand Access to Gold(I) Carbenes in Solution

Chloromethylgold(I) complexes of phosphine, phosphite, and N-heterocyclic carbene ligands are easily synthesized by reaction of trimethylsilyldiazomethane with the corresponding gold chloride precursors. Activation of these gold(I) carbenoids with a variety of chloride scavengers promotes reactivity typical of metallocarbenes in solution, namely homocoupling to ethylene, olefin cyclopropanation, and Buchner ring expansion of benzene.

Synthesis of Mono-, Di-, and Trinuclear Rhodium Diphosphine Complexes Containing Light-Harvesting Fluorene Backbones

Syntheses and structural determination of rhodium complexes ligated by diphosphines containing light-harvesting fluorene backbones are presented. In these rhodium complexes, the rhodium center is surrounded by a light-absorbing diphosphine unit. The presence of Rh-Rh bonding interactions is suggested by density functional theory studies.

Tuning the Reactivity of Radical through a Triplet Diradical Cu(II) Intermediate in Radical Oxidative Cross-Coupling

Highly selective radical/radical cross-coupling is paid more attention in bond formations. However, due to their intrinsic active properties, radical species are apt to achieve homo-coupling instead of cross-coupling, which makes the selective cross-coupling as a great challenge and almost untouched. Herein a notable strategy to accomplish direct radical/radical oxidative cross-coupling has been demonstrated, that is metal tuning a transient radical to a persistent radical intermediate followed by coupling with another transient radical. Here, a transient nitrogen-centered radical is tuned to a persistent radical complex by copper catalyst, followed by coupling with a transient allylic carbon-centered radical. Firstly, nitrogen-centered radical generated from N-methoxybenzamide stabilized by copper catalyst was successfully observed by EPR. Then DFT calculations revealed that a triplet diradical Cu(II) complex formed from the chelation N-methoxybenzamide nitrogen-centered radical to Cu(II) is a persistent radical species. Moreover, conceivable nitrogen-centered radical Cu(II) complex was observed by high-resolution electrospray ionization mass spectrometry (ESI-MS). Ultimately, various allylic amides derivatives were obtained in good yields by adopting this strategy, which might inspire a novel and promising landscape in radical chemistry.

Intermolecular hydroacylation: high activity rhodium catalysts containing small-bite-angle diphosphine ligands

Readily prepared and bench-stable rhodium complexes containing methylene bridged diphosphine ligands, viz. [Rh(C(6)H(5)F)(R(2)PCH(2)PR'(2))][BAr(F)(4)] (R, R' = (t)Bu or Cy; Ar(F) = C(6)H(3)-3,5-(CF(3))(2)), are shown to be practical and very efficient precatalysts for the intermolecular hydroacylation of a wide variety of unactivated alkenes and alkynes with β-S-substituted aldehydes. Intermediate acyl hydride complexes [Rh((t)Bu(2)PCH(2)P(t)Bu(2))H{κ(2)(S,C)-SMe(C(6)H(4)CO)}(L)](+) (L = acetone, MeCN, [NCCH(2)BF(3)](-)) and the decarbonylation product [Rh((t)Bu(2)PCH(2)P(t)Bu(2))(CO)(SMePh)](+) have been characterized in solution and by X-ray crystallography from stoichiometric reactions employing 2-(methylthio)benzaldehdye. Analogous complexes with the phosphine 2-(diphenylphosphino)benzaldehyde are also reported. Studies indicate that through judicious choice of solvent and catalyst/substrate concentration, both decarbonylation and productive hydroacylation can be tuned to such an extent that very low catalyst loadings (0.1 mol %) and turnover frequencies of greater than 300 h(-1) can be achieved. The mechanism of catalysis has been further probed by KIE and deuterium labeling experiments. Combined with the stoichiometric studies, a mechanism is proposed in which both oxidative addition of the aldehyde to give an acyl hydride and insertion of the hydride into the alkene are reversible, with the latter occurring to give both linear and branched alkyl intermediates, although reductive elimination for the linear isomer is suggested to have a considerably lower barrier.

The role of solvent in organometallic chemistry — Oxidative addition with dichloromethane or chloroform

Dimethylplatinum(II) complexes [PtMe2(NN)], with NN = diimine ligand, can react with dichloromethane or chloroform solvent to give the corresponding organoplatinum(IV) complexes [PtClMe2(CH2Cl)(NN)] or [PtClMe2(CHCl2)(NN)], respectively. The products can exist in isomeric forms, corresponding to products of cis or trans oxidative addition. The structures of three dichloromethane adducts and one chloroform adduct are reported.

Net oxidative addition of C(sp3)-F bonds to iridium via initial C-H bond activation

Carbon-fluorine bonds are the strongest known single bonds to carbon and as a consequence can prove very hard to cleave. Alhough vinyl and aryl C-F bonds can undergo oxidative addition to transition metal complexes, this reaction has appeared inoperable with aliphatic substrates. We report the addition of C(sp(3))-F bonds (including alkyl-F) to an iridium center via the initial, reversible cleavage of a C-H bond. These results suggest a distinct strategy for the development of catalysts and promoters to make and break C-F bonds, which are of strong interest in the context of both pharmaceutical and environmental chemistry.

Autocatalysis for C-H bond activation by ruthenium(II) complexes in catalytic arylation of functional arenes

Kinetic data for the C-H bond activation of 2-phenylpyridine by Ru(II)(carboxylate)(2)(p-cymene) I (acetate) and I' (pivalate) are available for the first time. They reveal an irreversible autocatalytic process catalyzed by the coproduct HOAc or HOPiv (acetonitrile, 27 °C). The overall reaction is indeed accelerated by the carboxylic acid coproduct and water. It is retarded by a base, in agreement with an autocatalytic process induced by HOAc or HOPiv that favors the dissociation of one carboxylate ligand from I and I' and consequently the ensuing complexation of 2-phenylpyridine (2-PhPy). The C-H bond activation initially delivers Ru(O(2)CR)(o-C(6)H(4)-Py)(p-cymene) A or A', containing one carboxylate ligand (OAc or OPiv, respectively). The overall reaction is accelerated by added acetates. Consequently, C-H bond activation (faster for acetate I than for pivalate I') proceeds via an intermolecular deprotonation of the C-H bond of the ligated 2-PhPy by the acetate or pivalate anion released from I or I', respectively. The 18e complexes A and A' easily dissociate, by displacement of the carboxylate by the solvent (also favored by the carboxylic acid), to give the same cationic complex B(+) {[Ru(o-C(6)H(4)-Py)(p-cymene)(MeCN)](+)}. Complex B(+) is reactive toward oxidative addition of phenyl iodide, leading to the diphenylated 2-pyridylbenzene.