Generation of Heteroatom-Substituted Carbene Complexes of Iridium by Double C-H Activation of Ether and Amine Substrates This work was supported by the "Fonds zur Förderung der wissenschaftlichen Forschung" with a Schrödinger Stipendium for C.S. (J1756-CHE), the Spanish Ministry of Education (DGES, Project PB97-0733), and the Junta de Andalucía

Reversible alkoxycarbene formation by C-H activation of ethers via discrete, isolable intermediates

A cationic iridium complex bearing a pendant ether ligand is shown to be in equilibrium with the corresponding dihydridoalkoxycarbene resulting from two consecutive C-H activations. Control of the conditions allows for the crystallographic characterization of all three complexes involved in the interconversion of ether and alkoxycarbene functionalities via sequential oxidative addition and α elimination.

Steric and electronic factor comparisons in hydrotris(3-phenylpyrazolyl)borate nickel(II) aryloxides

Hydrotris(pyrazolyl)borate (Tp) ligands, also known as scorpionates, are potent tridentate donors that effectively bind metal ions in a face-capping array. Hydrotris(3-phenylpyrazolyl)borate enforces a tetrahedral environment on Ni^II to model metalloenzymes. The syntheses and structural characterizations of a number of [hydrotris(3-phenylpyrazolyl)borato]nickel(II) aryloxides were performed to provide insight into the environment of the model active site; these compounds are chlorido[hydrotris(3-phenylpyrazolyl-κN²)borato](3-phenyl-1H-pyrazole-κN²)nickel(II) chloroform monosolvate, [Ni(C₂₇H₂₂BN₆)Cl(C₉H₈N₂)]·CHCl₃, (2), [hydrotris(3-phenylpyrazolyl-κN²)borato](phenolato-κO)nickel(II), [Ni(C₂₇H₂₂BN₆)(C₆H₅O)], (3), (2,6-dimethylphenolato-κO)[hydrotris(3-phenylpyrazolyl-κN²)borato]nickel(II) [Ni(C₂₇H₂₂BN₆)(C₈H₉O)], (4), (4-tert-butylphenolato-κO)[hydrotris(3-phenylpyrazolyl-κN²)borato]nickel(II), [Ni(C₂₇H₂₂BN₆)(C₁₀H₁₃O)], (5), and [hydrotris(3-phenylpyrazolyl-κN²)borato](phenolato-κO)(tetrahydrofuran-κO)nickel(II) tetrahydrofuran monosolvate, [Ni(C₂₇H₂₂BN₆)(C₆H₅O)(C₄H₈O)]·C₄H₈O, (6). Alkyl groups, e.g. tert-butyl in (5) and methyl in (4), electronically activate the aryloxide group to intramolecular π-π stacking but can be frustrated by steric encumbrance at the interacting ring faces. The flexibility at the nickel coordination site, afforded by the uncrowded B atom of the Tp^Ph ligand, allows tetrahydrofuran coordination in the phenolate compound.

Iridium(i) PNP complexes in the sp3C–H bond activation of methyl propanoate and related esters

The utilisation of the PNP iridium pincer complex [Ir(PNP)(COE)][BF₄] [PNP = 2,6-bis{(di-tert-butylphosphino)methyl}pyridine; COE = cyclooctene] in the sp³C–H activation of methyl propanoate and other related esters was explored.

Reversible double C-H bond activation of linear and cyclic ethers to form iridium carbenes

The double C-H bond activation of a series of linear and cyclic ethers by the iridium complex [Tp(tol')Ir(C(6)H(5))(N(2))] (2⋅N(2)), which features a cyclometalated hydrotris(3-p-tolylpyrazol-1-yl)borate ligand (Tp(tol')) coordinated in a κ(4)-N,N',N'',C manner, has been studied. Two methyl ethers, namely, Me(2)O and MeOtBu, along with diethyl ether and the cyclic ethers tetrahydrofuran, tetrahydropyran (THP), and 1,4-dioxane have been investigated with formation in every case of the corresponding hydride carbene complexes 3-8, which are stabilized by κ(4)-coordination of the ancillary Tp(tol') ligand. Five of the compounds have been structurally authenticated by X-ray crystallography. A remarkable feature of these rearrangements is the reversibility of the double C-H bond activation of Me(2)O, MeOtBu, Et(2)O, and THP. This has permitted catalytic deuterium incorporation into the methyl groups of the two methyl ethers, although in a rather inefficient manner (for synthetic purposes). Although possible in all cases, C-C coupling by migratory insertion of the carbene into the Ir-C σ bond of the metalated linkage has only been observed for complex 8 that contains a cyclic carbene that results from α,α-C-H activation of 1,4-dioxane. Computational studies on the formation of iridium carbenes are also reported, which show a role for metalated Tp ligands in the double C-H activation and account for the reversibility of the reaction in terms of the relative stability of the reagents and the products of the reaction.

Palladium-catalyzed cycloaddition of alkynyl aryl ethers with internal alkynes via selective ortho C-H activation

Alkynyl aryl ethers react with internal alkynes through selective ortho C-H activation by a palladium(0) catalyst to give substituted 2-methylidene-2H-chromenes. The alkynoxy group acts as a directing group to promote ortho C-H functionalization. Deuterium-labeling experiments indicated that the arylpalladium hydride complex is a key intermediate via oxidative addition. Various functional groups tolerate the present transformation to give the corresponding products.

C-H bond activation reactions of ethers that generate iridium carbenes

Two important objectives in organometallic chemistry are to understand C-H bond activation reactions mediated by transition metal compounds and then to develop efficient ways of functionalizing the resulting products. A particularly ambitious goal is the generation of metal carbenes from simple organic molecules; the synthetic chemist can then take advantage of the almost unlimited reactivity of this metal-organic functionality. This goal remains very difficult indeed with saturated hydrocarbons, but it is considerably more facile for molecules that possess a heteroatom (such as ethers), because coordination of the heteroatom to the metal renders the ensuing C-H activation an intramolecular reaction. In this Account, we focus on the activation reaction of different types of unstrained ethers, both aliphatic and hemiaromatic, by (mostly) iridium compounds. We emphasize our recent results with the Tp(Me2)Ir(C(6)H(5))(2)(N(2)) (1.N(2)) complex (where Tp(Me2) denotes hydrotris(3,5-dimethylpyrazolyl)borate). Most of the reactivity observed with this system, and with related electronically unsaturated iridium species, starts with a C-H activation reaction, which is then followed by reversible alpha-hydrogen elimination. An alpha-C-H bond is, in every instance, broken first; when there is a choice, cleavage of the stronger terminal C(sp(3))-H bonds is always preferred over the weaker internal C(sp(3))-H (methylene) bonds of the ether. Nevertheless, competitive reactions of the unsaturated [Tp(Me2)Ir(C(6)H(5))(2)] iridium intermediate with ethers that contain C(sp(3))-H and C(sp(2))-H bonds are also discussed. We present theoretical evidence for a sigma-complex-assisted metathesis mechanism (sigma-CAM), although for other systems oxidative addition and reductive elimination events can be effective reaction pathways. We also show that additional unusual chemical transformations may occur, depending on the nature of the ether, and can result in C-O and C-C bond-breaking and bond-forming reactions, leading to the formation of more elaborate molecules. Although the possibility of extending these results to saturated hydrocarbons appears to be limited for this iridium system, the findings described in this Account are of fundamental importance for various facets of C-H bond activation chemistry, and with suitable modifications of the ancillary ligands, they could be even broader in scope. We further discuss experimental and theoretical studies on unusual alkene-to-alkylidene equilibria for some of the products obtained in the reactions of iridium complex 1.N(2) with alkyl aryl ethers. The rearrangement involves reversible alpha- and beta-hydrogen eliminations, with a rate-determining metal inversion step (supported by theoretical calculations); the alkylidene is always favored thermodynamically over the alkene. This startling result contrasts with the energetically unfavorable isomerization of free ethene to ethylidene (by about 80 kcal mol(-1)), showing that the tautomerism equilibrium can be directed toward one product or the other by a judicious choice of the transition metal complex.

Late metal carbene complexes generated by multiple C-H activations: examining the continuum of M=C bond reactivity

Unactivated C(sp(3))-H bonds are ubiquitous in organic chemicals and hydrocarbon feedstocks. However, these resources remain largely untapped, and the development of efficient homogeneous methods for hydrocarbon functionalization by C-H activation is an attractive and unresolved challenge for synthetic chemists. Transition-metal catalysis offers an attractive possible means for achieving selective, catalytic C-H functionalization given the thermodynamically favorable nature of many desirable partial oxidation schemes and the propensity of transition-metal complexes to cleave C-H bonds. Selective C-H activation, typically by a single cleavage event to produce M-C(sp(3)) products, is possible through myriad reported transition-metal species. In contrast, several recent reports have shown that late transition metals may react with certain substrates to perform multiple C-H activations, generating M=C(sp(2)) complexes for further elaboration. In light of the rich reactivity of metal-bound carbenes, such a route could open a new manifold of reactivity for catalytic C-H functionalization, and we have targeted this strategy in our studies. In this Account, we highlight several early examples of late transition-metal complexes that have been shown to generate metal-bound carbenes by multiple C-H activations and briefly examine factors leading to the selective generation of metal carbenes through this route. Using these reports as a backdrop, we focus on the double C-H activation of ethers and amines at iridium complexes supported by Ozerov's amidophosphine PNP ligand (PNP = [N(2-P(i)Pr(2)-4-Me-C(6)H(3))(2)](-)), allowing isolation of unusual square-planar iridium(I) carbenes. These species exhibit reactivity that is distinct from the archetypal Fischer and Schrock designations. We present experimental and theoretical studies showing that, like the classical square-planar iridium(I) organometallics, these complexes are best described as nucleophilic at iridium. We discuss the classification of this reactivity in the context of a scheme originally delineated by Roper. These "Roper-type" carbenes perform a number of multiple-bond metatheses leading to atom and group transfer from electrophilic heterocumulene (e.g., CO(2), CS(2), PhNCS) and diazo (e.g., N(2)O, AdN(3)) reagents. In one instance, we have extended this methodology to a process for catalytic C-H functionalization by a double C-H activation-group transfer process. Although the scope of these reactions is currently limited, these new pathways may find broader utility as the reactivity of late-metal carbenes continues to be explored. Examination of alternative transition metals and supporting ligand sets will certainly be important. Nonetheless, our findings show that carbene generation by double C-H activation is a viable strategy for C-H functionalization, leading to products not accessible through traditional C(sp(3))-H activation pathways.

Unusual fragmentation of CH2Cl2by an Ir(iii) centre bonded to a doubly metalated TpMsLigand (TpMs= hydrotris(3-mesitylpyrazol-1-yl)borate)

The Ir(III) compound Tp(Ms'')Ir(N2), that contains a pentadentate, doubly metalated 3-mesityl substituted tris(pyrazolyl)borate ligand, induces the cleavage of C-H and C-Cl bonds of CH2Cl2 to yield a highly electrophilic chlorocarbene Ir=C(H)Cl complex.

[(tBu2PCH2SiMe2)2N]RuCH3: The Origin of Extremely Facile, Double H−C(sp3) Activation Generating a “Hydrido-Carbene” Complex

The four-coordinate compound [(tBu2PCH2SiMe2)2N]RuCH3 undergoes rapid double H-C(sp3) activation at -78 degrees C to generate a "hydrido-carbene" complex. DFT calculations suggest that the origin of the low barrier to methane elimination is an alpha-agostic interaction in the low-lying singlet state of the highly unsaturated (PNP)RuMe. The hydrido-carbene complex can be viewed as a "masked" resting state of the four-coordinate cyclometalated alkyl complex, [(tBu2PCH2SiMe2)N(Me2SiCH2P(tBu)(C(CH3)2CH2)]Ru, where hydride migration from metal to carbon occurs before any subsequent reactivity.

Double C−H Activation during Functionalization of Phenyl(methyl)ketene on Iridium(I) Using Alkynes. Synthesis of 1,4-Dien-3-ones

Under the influence of an Ir(I) metal fragment, the methyl group of phenyl(methyl)ketene undergoes two C-H activations in reacting with internal alkynes, giving metallacycles 3 in 86-94% yield. Treatment of 3 with CO liberates 1,4-dien-3-ones 5 in 81-93% yield, along with CO complex 4. A possible mechanism for the very selective double C-H activation-alkyne coupling is discussed.

Isolation and crystal structure of a water-soluble iridium hydride: a robust and highly active catalyst for acid-catalyzed transfer hydrogenations of carbonyl compounds in acidic media

This paper reports the isolation and structural determination of a water-soluble hydride complex [Cp*Ir(III)(bpy)H](+) (1, Cp* = eta(5)-C(5)Me(5), bpy = 2,2'-bipyridine) that serves as a robust and highly active catalyst for acid-catalyzed transfer hydrogenations of carbonyl compounds at pH 2.0-3.0 at 70 degrees C. The catalyst 1 was synthesized from the reaction of a precatalyst [Cp*Ir(III)(bpy)(OH(2))](2+) (2) with hydrogen donors HCOOX (X = H or Na) in H(2)O under controlled conditions (2.0 < pH < 6.0, 25 degrees C) which avoid protonation of the hydrido ligand of 1 below pH ca. 1.0 and deprotonation of the aqua ligand of 2 above pH ca. 6.0 (pK(a) value of 2 = 6.6). X-ray analysis shows that complex 1 adopts a distorted octahedral geometry with the Ir atom coordinated by one eta(5)-Cp*, one bidentate bpy, and one terminal hydrido ligand that occupies a bond position. The isolation of 1 allowed us to investigate the robust ability of 1 in acidic media and reducing ability of 1 in the reaction with carbonyl compounds under both stoichiometric and catalytic conditions. The rate of the acid-catalyzed transfer hydrogenation is drastically dependent on pH of the solution, reaction temperature, and concentration of HCOOH. The effect of pH on the rate of the transfer hydrogenation is rationalized by the pH-dependent formation of 1 and activation process of the carbonyl compounds by protons. High turnover frequencies of the acid-catalyzed transfer hydrogenations at pH 2.0-3.0 are ascribed not only to nucleophilicity of 1 toward the carbonyl groups activated by protons but also to a protonic character of the hydrido ligand of 1 that inhibits the protonation of the hydrido ligand.