Chak Po Lau

Palladium-Catalyzed Amination of Aryl Mesylates†

Palladium-catalyzed C(sp2) N bond-forming reactions have evolved into a highly versatile and synthetically attractive transformation in targeting pharmaceutically useful intermediates. Since the discovery of the first catalytic amination method, efforts have been made toward increasing the reaction efficacy. Notable ligands, such as tBu3P, [4] Beller and co-workers( PAP, Buchwald and co-workers( biaryl phosphines, Hartwig and co-workers( Q-Phos, and Verkade and co-workers( amino phosphine (Figure 1) provide excellent catalytic activity in the cross-coupling of aryl halides (especially aryl chlorides). X-Phos, in particular, is effective in the handling of aryl tosylate/benzenesulfonate substrates in coupling reactions.

Suzuki−Miyaura Coupling of Aryl Tosylates Catalyzed by an Array of Indolyl Phosphine−Palladium Catalysts

A family of indolyl phosphine ligands was applied to Suzuki-Miyaura cross-coupling of aryl tosylates. Catalyst loading can be reduced to 0.2 mol % for coupling of nonactivated aryl tosylate. A challenging example for room temperature coupling is realized. The scope of this highly active Pd/L2 system can be extended to other boron nucleophiles, including trifluoroborate salts and boronate esters. The ligand structural comparisons toward the reactivity in tosylate couplings are also described.

A Mild and Efficient Palladium-Catalyzed Cyanation of Aryl Chlorides with K4[Fe(CN)6]

An efficient palladium-catalyzed cyanation of aryl chlorides is established. In the presence of a highly effective Pd/CM-phos catalyst, cyanation of aryl chlorides proceeds at 70 °C in general, which is the mildest reaction temperature achieved so far for this process. Common functional groups such as keto, aldehyde, ester, nitrile and -NH(2), and heterocyclic coupling partners including N-H indoles are well tolerated. Moreover, a sterically hindered nonactivated ortho,ortho-disubstituted electrophile is shown to be a feasible coupling partner in cyanation.

Palladium−Indolylphosphine-Catalyzed Hiyama Cross-Coupling of Aryl Mesylates

Aryl mesylates are found to be applicable as electrophiles in organosilicon-mediated coupling reactions. The catalyst system comprising 2 mol % of Pd(OAc)(2) and CM-phos supporting ligand is highly effective in catalyzing Hiyama cross-coupling of various aryl and heteroaryl mesylates. Interesting acid additive effects show that the presence of 0.25-0.50 equiv of acetic acid efficiently suppresses the mesylate decomposition and generally promotes the coupling product yields.

A New Family of Tunable Indolylphosphine Ligands by One-Pot Assembly and Their Applications in Suzuki−Miyaura Coupling of Aryl Chlorides

This study describes a new class of indolylphosphine ligands, which can be easily accessed by a simple one-pot assembly from commercially available indoles, acid chlorides, and chlorophosphines. A combination of these three starting materials provides a high diversification of the ligand structure. The application of this ligand array in palladium-catalyzed Suzuki-Miyaura coupling reaction of aryl chlorides with arylboronic acids is described. A catalyst loading down to 0.01 mol % of Pd can be achieved.

Palladium-Catalyzed Direct and Regioselective C−H Bond Functionalization/Oxidative Acetoxylation of Indoles†

The first general examples of palladium-catalyzed direct and selective oxidative C3-acetoxylation of indoles are presented. The mild reaction conditions (70 °C and with weak base, KOAc) in this indole C-H-acetoxylation are notable.

A decade advancement of transition metal-catalyzed borylation of aryl halides and sulfonates

This review describes the recent advancement of transition metal-catalyzed aromatic carbon-boron bond construction processes. The efficacy of palladium, nickel and copper catalysis are comparatively illustrated. Particular focus is placed on the application of ligands, for instance tailor-made phosphines and carbenes that can effectively enable the borylation of challenging and sterically demanding substrates. Selected applications of this methodology for the synthesis of pharmaceutically useful and materially interesting molecules are mostly documented. This review includes literatures up to late 2012.

The effect of weak Brönsted acids on the electrocatalytic reduction of carbon dioxide by a rhenium tricarbonyl bipyridyl complex

Abstract The effect of four weak Bronsted acids, 2,2,2-trifluoroethanol, phenol, methanol and water on the electrocatalytic reduction of CO 2 by [Re(bpy)(CO) 3 (py)] 2+ (bpy=2,2′-bipyridine, py=pyridine) in acetonitrile was investigated. The addition of weak Bronsted acids enhances the rate of the catalytic process and improves the lifetime of the rhenium catalyst. Under the experimental conditions employed in this study, CO was produced as the only product with faradaic efficiency close to 100%. Kinetic analysis indicated that the reaction order in acid is 2. The Bronsted acids probably stabilise the rhenium–carbon dioxide intermediate through protonation and facilitate the cleavage of one of the C–O bonds to yield CO. The efficiency of the acid increases with its acidity. Thus, trifluoroethanol and phenol are more effective in enhancing the rate of catalysis than methanol and water. Even though the acid strength of water and methanol are very similar, water is much less effective in enhancing the rate of CO 2 reduction. This can be attributed to the relatively strong ligating property of water, which would compete with CO 2 for the binding site on rhenium.

Remarkably Effective Phosphanes Simply with a PPh2 Moiety: Application to Pd‐Catalysed Cross‐Coupling Reactions for Tetra‐ortho‐substituted Biaryl Syntheses

ortho-Substituted biaryl compounds are important structural motifs present in a number of natural products from various origins and have a wide range of biological properties. The biaryl substructures in vancomycin, a glycopeptide antibiotic from Streptomyces orientalis, steganacin, a cytotoxic tubulin-binding dibenzocyclootadiene lignan from Steganotaenia araliacea, and michellamine B, an anti-HIV naphthylisoquinoline alkaloid from Ancistrocladus abbreviatus, have aroused the interest of many synthetic chemists within the field of sterically congested biaryl synthesis. However, the ability to prepare extremely hindered asymmetric biaryl compounds by using Suzuki–Miyaura coupling reactions has proven to be an extremely difficult task. Recent superb findings by the Buchwald group have demonstrated that PCy2-substituted phenanthrene-based phosphanes and 2-dicyclohexylphosphino-2’,6’-dimethoxybiphenyl (S-Phos) are effective ligands for the synthesis of tetra-ortho-substituted biaryl compounds, mainly from aryl bromides (Figure 1). To date, in the exploration of phosphane ligands, only the ruthenocenylphosphane R-Phos developed by Hoshi/Hagiwara and, very recently, the diazaphospholidine chloride disclosed by Ackermann allowed the successful synthesis of tetra-ortho-substituted biaryls from unactivated aryl chlorides (Figure 1). Besides phosphanes, carbene IBiox12·OTf with a tuneable ring size, “flexible-steric-bulk” PEPPSI-IPent and phenanthryl-based H2-ICP·HCl, reported by Glorius, [10] Organ and Ma/ Andrus, respectively, are also appropriate for sterically congested couplings (Figure 1). The lore in the field of demanding cross-coupling reactions reveals that successful phosphane ligands often contain dialkylphosphane groups, such as PCy2 or PtBu2 (Figure 1). In contrast to ArPCy2 and ArPtBu2 ligands, the corresponding ArPPh2 compounds have received less attention due to the accepted rationale indicating that they are less electronrich and less bulky than the corresponding ArPCy2 and ArPtBu2 compounds (Figure 2) [13] and, thus, provide a less favourable outcomes for the oxidative addition and reductive elimination steps in unactivated aryl chloride couplings. However, triarylphosphanes are attractive compounds as they are reasonably air stable and can be readily prepared from the relatively inexpensive Ph2PCl. [17] Herein, we report an unexpected finding that advances coupling technology by showing that the previously omitted triarylphosphane family can effectively deal with difficult coupling reactions. We disclose their ability to perform the most difficult biaryl cou[a] C. M. So, W. K. Chow, P. Y. Choy, Prof. Dr. C. P. Lau, Prof. Dr. F. Y. Kwong State Key Laboratory of Chiroscience and Department of Applied Biology and Chemical Technology The Hong Kong Polytechnic University Hung Hom, Kowloon, Hong Kong (Hong Kong) Fax: (+852) 2364-9932 E-mail : bcfyk@inet.polyu.edu.hk Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201000723.

An efficient palladium–benzimidazolyl phosphine complex for the Suzuki–Miyaura coupling of aryl mesylates: facile ligand synthesis and metal complex characterization

A new class of easily accessible hemilabile benzimidazolyl phosphine ligands has been developed. The ligand skeleton is prepared from commercially available and inexpensive o-phenylenediamine and 2-bromobenzoic acid. With catalyst loading down to 0.5 mol% palladium, excellent catalytic activity towards the Suzuki-Miyaura coupling of aryl mesylates is still observed. This represents the lowest catalyst loading achieved so far for this reaction in general. X-Ray crystallography shows that new ligand L2 is coordinated with Pd in a κ(2)-P,N fashion.