Ni-catalyzed stereoselective arylation of inert C-O bonds at low temperatures

Investigations of new potential photo-acid generators: crystal structures of 2-[(E)-2-phenyl-ethen-yl]phenol (ortho-rhom-bic polymorph) and (2E)-3-(2-bromo-phen-yl)-2-phenyl-prop-2-enoic acid

The title compounds, C14H12O, (I), and C15H11BrO2, (II), were prepared and characterized as part of our studies of potential new photo-acid generators. In (I), which crystallizes in the ortho-rhom-bic space group Pca21, compared to P21/n for the previously known monoclinic polymorph [Cornella & Martin (2013 ▸). Org. Lett. 15, 6298-6301], the dihedral angle between the aromatic rings is 4.35 (6)° and the OH group is disordered over two sites in a 0.795 (3):0.205 (3) ratio. In the crystal of (I), mol-ecules are linked by O-H⋯π inter-actions involving both the major and minor -OH disorder components, generating [001] chains as part of the herringbone packing motif. The asymmetric unit of (II) contains two mol-ecules with similar conformations (weighted r.m.s. overlay fit = 0.183 Å). In the crystal of (II), both mol-ecules form carboxyl-ate inversion dimers linked by pairs of O-H⋯O hydrogen bonds, generating R 2 (2)(8) loops in each case. The dimers are linked by pairs of C-H⋯O hydrogen bonds to form [010] chains.

[Stereoselective Synthesis of Multi-substituted Fluoro-β-lactams and Their Conversion to Fluorinated β-Amino Acid Core]

Multi-substituted β-lactam compounds have not only attracted considerable interest as core structures of pharmaceutical compounds such as antibiotics but also have been used as building blocks for the construction of β-amino acids. Electrophilic β-lactams can be used to enhance essential biological activities. Furthermore, the ring-opening reactions of electrophilic β-lactams can be used to provide facile access to β-amino acids. The introduction of an electronegative fluorine atom to a β-lactam ring to give the corresponding fluoro-β-lactam can be used as an effective strategy for the preparation of electrophilic β-lactams. In this review, we provide a summary of our recent research towards the direct functionalization of fluoro-β-lactams. This review has been divided into four topics, including: 1) the alkylation and hydroxyalkylation of α-bromo-α-fluoro-β-lactams (1); 2) the nickel-catalyzed cross coupling reaction of 1; 3) the asymmetric synthesis of fluoro-β-lactams using chiral ligands; and 4) the utilization of fluoro-β-lactams as highly electrophilic building blocks.

Cross-Couplings Using Aryl Ethers via C-O Bond Activation Enabled by Nickel Catalysts

Arene synthesis has been revolutionized by the invention of catalytic cross-coupling reactions, wherein aryl halides can be coupled with organometallic and organic nucleophiles. Although the replacement of aryl halides with phenol derivatives would lead to more economical and ecological methods, success has been primarily limited to activated phenol derivatives such as triflates. Aryl ethers arguably represent one of the most ideal substrates in terms of availability, cost, safety, and atom efficiency. However, the robust nature of the C(aryl)-O bonds of aryl ethers renders it extremely difficult to use them in catalytic reactions among the phenol derivatives. In 1979, Wenkert reported a seminal work on the nickel-catalyzed cross-coupling of aryl ethers with Grignard reagents. However, it was not until 2004 that the unique ability of a low-valent nickel species to activate otherwise unreactive C(aryl)-O bonds was appreciated with Dankwardt's identification of the Ni(0)/PCy3 system, which significantly expanded the efficiency of the Wenkert reaction. Application of the nickel catalyst to cross-couplings with other nucleophiles was first accomplished in 2008 by our group using organoboron reagents. Later on, several other nucleophiles, including organozinc reagents, amines, hydrosilane, and hydrogen were shown to be coupled with aryl ethers under nickel catalysis. Despite these advances, progress in this field is relatively slow because of the low reactivity of benzene derivatives (e.g., anisole) compared with polyaromatic substrates (e.g., methoxynaphthalene), particularly when less reactive and synthetically useful nucleophiles are used. The "naphthalene problem" has been overcome by the use of N-heterocyclic carbene (NHC) ligands bearing bulky N-alkyl substituents, which enables a wide range of aryl ethers to be coupled with organoboron nucleophiles. Moreover, the use of N-alkyl-substituted NHC ligands allows the use of alkynylmagnesium reagents, thereby realizing the first Sonogashira-type reaction of anisoles. From a mechanistic perspective, nickel-catalyzed cross-couplings of aryl ethers are at a nascent stage, in particular regarding the mode of activation of C(aryl)-O bonds. Oxidative addition is one plausible pathway, although such a process has not been fully verified experimentally. Nickel-catalyzed reductive cleavage of aryl ethers in the absence of an external reducing agent provides strong support for this oxidative addition process. Several other mechanisms have also been proposed. For example, Martin demonstrated a new possibility of the involvement of a Ni(I) species, which could mediate the cleavage of the C(aryl)-O bond via a redox-neutral pathway. The tolerance of aryl ethers under commonly used synthetic conditions enables alkoxy groups to serve as a platform for late-stage elaboration of complex molecules without any tedious protecting group manipulations. Aryl ethers are therefore not mere economical alternatives to aryl halides but also enable nonclassical synthetic strategies.

Access to enantioenriched 2,3- and 2,5-dihydrofurans with a fully substituted C2 stereocenter by Pd-catalyzed asymmetric intermolecular Heck reaction

A palladium catalyzed intermolecular asymmetric Heck reaction provides access to valuable 2,3- and 2,5-dihydrofurans with a fully substituted C2 stereocenter with high levels of regio- and enantiocontrol.

A palladium-catalyzed intermolecular asymmetric Heck reaction with dihydrofurans with a trisubstituted double bond is reported. The use of two different chiral ligands provides access to valuable 2,3- and 2,5-dihydrofurans with a fully substituted C2 stereocenter with high levels of regio- and enantiocontrol.

Ipso-Borylation of Aryl Ethers via Ni-Catalyzed C-OMe Cleavage

A Ni-catalyzed ipso-borylation of aryl ethers via C(sp(2))-OMe and C(sp(3))-OMe cleavage is described. The transformation is characterized by its wide substrate scope under mild conditions and an exquisite divergence in site selectivity that can be easily switched by selecting the appropriate boron reagent.

Nickel catalyzed dealkoxylative Csp2–Csp3 cross coupling reactions – stereospecific synthesis of allylsilanes from enol ethers

The application of cyclic and acyclic enol ethers as electrophiles in cross coupling reactions offers new possibilities for the preparation of functional compounds.

1,3-Dicyclohexylimidazol-2-ylidene as a superior ligand for the nickel-catalyzed cross-couplings of aryl and benzyl methyl ethers with organoboron reagents

A new catalytic system has been developed involving the use of Ni(cod)2 in conjunction with 1,3-dicyclohexylimidazol-2-ylidene for the cross-coupling of aryl and benzyl methyl ethers with organoboron reagents. This method not only allows for the use of readily available methyl ethers as halide surrogates but also provides a functional group tolerant method for the late-stage derivatization of complex molecules.

Stereospecific cross-coupling reactions of aryl-substituted tetrahydrofurans, tetrahydropyrans, and lactones

The stereospecific ring-opening of O-heterocycles to provide acyclic alcohols and carboxylic acids with controlled formation of a new C-C bond is reported. These reactions provide new methods for synthesis of acyclic polyketide analogs with complex stereochemical arrays. Stereoselective synthesis of the cyclic template is utilized to control relative configuration; subsequent stereospecific nickel-catalyzed ring-opening affords the acyclic product. Aryl-substituted tetrahydrofurans and tetrahydropyrans undergo nickel-catalyzed Kumada-type coupling with a range of Grignard reagents to furnish acyclic alcohols with high diastereoselectivity. Enantioenriched lactones undergo Negishi-type cross-coupling with dimethylzinc to afford enantioenriched carboxylic acids. Application in a two-step enantioselective synthesis of an anti-dyslipidemia agent is demonstrated.

Metal-catalyzed activation of ethers via C–O bond cleavage: a new strategy for molecular diversity

This review highlights the prospective impact of simple ethers as aryl halide surrogates in the cross-coupling arena via C–O bond-activation.