Reinvestigating Catalytic Alcohol Dehydrogenation with an Iridium Dihydroxybipyridine Catalyst

Transition-Metal Free C-C Bond Cross-Coupling of Aryl Ethers with Diarylmethanes

We describe a general and efficient transition-metal free C-C bond cross-coupling of (hetero)aryl ethers and diarylmethanes via C(sp²)-O bond cleavage. The coupling reactions mediated by KHMDS proceeded well with high efficiency, broad substrate scope, and good functional group tolerance. The robustness and practicability of this protocol also have been demonstrated by easy gram-scale preparation and diversified product derivatization.

Ruthenium Complexes with Protic Ligands: Influence of the Position of OH Groups and π Expansion on Luminescence and Photocytotoxicity

Protic ruthenium complexes using the dihydroxybipyridine (dhbp) ligand combined with a spectator ligand (N,N = bpy, phen, dop, Bphen) have been studied for their potential activity vs. cancer cells and their photophysical luminescent properties. These complexes vary in the extent of π expansion and the use of proximal (6,6'-dhbp) or distal (4,4'-dhbp) hydroxy groups. Eight complexes are studied herein as the acidic (OH bearing) form, [(N,N)₂Ru(n,n'-dhbp)]Cl₂, or as the doubly deprotonated (O^- bearing) form. Thus, the presence of these two protonation states gives 16 complexes that have been isolated and studied. Complex 7_A, [(dop)₂Ru(4,4'-dhbp)]Cl₂, has been recently synthesized and characterized spectroscopically and by X-ray crystallography. The deprotonated forms of three complexes are also reported herein for the first time. The other complexes studied have been synthesized previously. Three complexes are light-activated and exhibit photocytotoxicity. The log(D_o/w) values of the complexes are used herein to correlate photocytotoxicity with improved cellular uptake. For Ru complexes 1-4 bearing the 6,6'-dhbp ligand, photoluminescence studies (all in deaerated acetonitrile) have revealed that steric strain leads to photodissociation which tends to reduce photoluminescent lifetimes and quantum yields in both protonation states. For Ru complexes 5-8 bearing the 4,4'-dhbp ligand, the deprotonated Ru complexes (5_B-8_B) have low photoluminescent lifetimes and quantum yields due to quenching that is proposed to involve the ³LLCT excited state and charge transfer from the [O₂-bpy]^2- ligand to the N,N spectator ligand. The protonated OH bearing 4,4'-dhbp Ru complexes (5_A-8_A) have long luminescence lifetimes which increase with increasing π expansion on the N,N spectator ligand. The Bphen complex, 8_A, has the longest lifetime of the series at 3.45 μs and a photoluminescence quantum yield of 18.7%. This Ru complex also exhibits the best photocytotoxicity of the series. A long luminescence lifetime is correlated with greater singlet oxygen quantum yields because the triplet excited state is presumably long-lived enough to interact with ³O₂ to yield ¹O₂.

Activation of H-H, HO-H, C(sp2)-H, C(sp3)-H, and RO-H Bonds by Transition-Metal Frustrated Lewis Pairs Based on M/N (M = Rh, Ir) Couples

Reaction of the dimers [(Cp*MCl)₂(μ-Cl)₂] (Cp* = η⁵-C₅Me₅) with Ph₂PCH₂CH₂NC(NH(p-Tolyl))₂ (H₂L) in the presence of NaSbF₆ affords the chlorido complexes [Cp*MCl(κ²N,P-H₂L)][SbF₆] (M = Rh, 1; Ir, 2). Upon treatment with aqueous NaOH, solutions of 1 and 2 yield the corresponding complexes [Cp*M(κ³N,N′,P-HL)][SbF₆] (M = Rh, 3; Ir, 4) in which the ligand HL presents a fac κ³N,N′,P coordination mode. Treatment of THF solutions of complexes 3 and 4 with hydrogen gas, at room temperature, results in the formation of the metal hydrido-complexes [Cp*MH(κ²N,P-H₂L)][SbF₆] (M = Rh, 5; Ir, 6) in which the N(p-Tolyl) group has been protonated. Complexes 3 and 4 react with deuterated water in a reversible fashion resulting in the gradual deuteration of the Cp* group. Heating at 383 K THF/H₂O solutions of the complexes 3 and 4 affords the orthometalated complexes [Cp*M(κ³C,N,P-H₂L_-H)][SbF₆] [M = Rh, 7; Ir, 8, H₂L_-H = Ph₂PCH₂CH₂NC(NH(p-Tolyl))(NH(4-C₆H₃Me))], respectively. At 333 K, complexes 3 and 4 react in THF with methanol, primary alcohols, or 2-propanol giving the metal-hydrido complexes 5 and 6, respectively. The reaction involves the acceptorless dehydrogenation of the alcohols at a relatively low temperature, without the assistance of an external base. The new complexes have been characterized by the usual analytical and spectroscopic methods including the X-ray diffraction determination of the crystal structures of complexes 1–5, 7, and 8. Notably, the chlorido complexes 1 and 2 crystallize both as enantiopure conglomerates and as racemates. Reaction mechanisms are proposed based on stoichiometric reactions, nuclear magnetic resonance studies, and X-ray crystallography as well as density functional theory calculations.

In solution, masked transition-metal frustrated Lewis pairs (TMFLPs) give rise to the corresponding TMFLP species which activate dihydrogen, water, and alcohols following FLP reaction pathways. When D₂O or alcohols with deuterated OH groups were employed, H/D exchange at the Cp* ligand (involving C(sp³)−H activation) was observed. C(sp²)−H bond activation involving orthometalation of the p-Tolyl ring was also observed.

Tuning the selectivity in iridium-catalyzed acceptorless dehydrogenative coupling of primary alcohols

An acceptorless dehydrogenative coupling of primary alcohols to carboxylic acids/carboxylates, esters, and Guerbet alcohols (via both homo- and cross-β-alkylation of the alcohols) in the presence of an N-heterocyclic carbene iridium(I) catalyst was developed under aerobic conditions. The product selectivity can be easily tuned among the products with a single catalyst through simple modification of the reaction conditions, such as the catalyst and base amounts, the choice of base, and the reaction temperature.

Light-responsive and Protic Ruthenium Compounds Bearing Bathophenanthroline and Dihydroxybipyridine Ligands Achieve Nanomolar Toxicity towards Breast Cancer Cells

We report new ruthenium complexes bearing the lipophilic bathophenanthroline (BPhen) ligand and dihydroxybipyridine (dhbp) ligands which differ in the placement of the OH groups ([(BPhen)₂ Ru(n,n'-dhbp)]Cl₂ with n = 6 and 4 in 1_A and 2_A , respectively). Full characterization data are reported for 1_A and 2_A and single crystal X-ray diffraction for 1_A . Both 1_A and 2_A are diprotic acids. We have studied 1_A , 1_B , 2_A , and 2_B (B = deprotonated forms) by UV-vis spectroscopy and 1 photodissociates, but 2 is light stable. Luminescence studies reveal that the basic forms have lower energy ³ MLCT states relative to the acidic forms. Complexes 1_A and 2_A produce singlet oxygen with quantum yields of 0.05 and 0.68, respectively, in acetonitrile. Complexes 1 and 2 are both photocytotoxic toward breast cancer cells, with complex 2 showing EC₅₀ light values as low as 0.50 μM with PI values as high as >200 vs. MCF7. Computational studies were used to predict the energies of the ³ MLCT and ³ MC states. An inaccessible ³ MC state for 2_B suggests a rationale for why photodissociation does not occur with the 4,4'-dhbp ligand. Low dark toxicity combined with an accessible ³ MLCT state for ¹ O₂ generation explains the excellent photocytotoxicity of 2.

Direct Oxidation of Primary Alcohols to Carboxylic Acids

Oxidation of primary alcohols to carboxylic acids is a fundamental transformation in organic chemistry, yet despite its simplicity, extensive use, and relationship to pH, it remains a subject of active research for synthetic organic chemists. Since 2013, a great number of new methods have emerged that utilize transition-metal compounds as catalysts for acceptorless dehydrogenation of alcohols to carboxylates. The interest in this reaction is explained by its atom economy, which is in accord with the principles of sustainability and green chemistry. Therefore, the methods for the direct synthesis of carboxylic acids from alcohols is ripe for a modern survey, which we provide in this review.

1 Introduction

2 Thermodynamics of Primary Alcohol Oxidation

3 Oxometalate Oxidation

4 Transfer Dehydrogenation

5 Acceptorless Dehydrogenation

6 Electrochemical Methods

7 Outlook