Air-Stable Secondary Phosphine Oxide or Chloride (Pre)Ligands for Cross-Couplings of Unactivated Alkyl Chlorides

Metal-catalysed C-C bond formation at cyclopropanes

Cyclopropanes are important substructures in natural products and pharmaceuticals. Although traditional methods for their incorporation rely on cyclopropanation of an existing scaffold, the advent of transition-metal catalysis has enabled installation of functionalized cyclopropanes using cross-coupling reactions. The unique bonding and structural properties of cyclopropane render it more easily functionalized in transition-metal-catalysed cross-couplings than other C(sp³) substrates. The cyclopropane coupling partner can participate in polar cross-coupling reactions either as a nucleophile (organometallic reagents) or as an electrophile (cyclopropyl halides). More recently, single-electron transformations featuring cyclopropyl radicals have emerged. This Review will provide an overview of transition-metal-catalysed C-C bond formation reactions at cyclopropane, covering both traditional and current strategies, and the benefits and limitations of each.

Visible-light-induced cross-coupling of aryl iodides with hydrazones via an EDA-complex

A visible-light-induced, transition-metal and photosensitizer-free cross-coupling of aryl iodides with hydrazones was developed. In this strategy, hydrazones were used as alternatives to organometallic reagents, in the absence of a transition metal or an external photosensitizer, making this cross-coupling mild and green. The protocol was compatible with a variety of functionalities, including methyl, methoxy, trifluoromethyl, halogen, and heteroaromatic rings. Mechanistic investigations showed that the association of the hydrazone anion with aryl halides formed an electron donor–acceptor complex, which when excited with visible light generated an aryl radical via single-electron transfer.

Visible-light-induced catalyst-free cross-coupling of aryl iodides with hydrazones via single-electron-transfer was reported. The mechanistic investigations showed that the association of hydrazone anion with aryl iodides formed an EDA complex.

A general alkyl-alkyl cross-coupling enabled by redox-active esters and alkylzinc reagents

Alkyl carboxylic acids are ubiquitous in all facets of chemical science, from natural products to polymers, and represent an ideal starting material with which to forge new connections. This study demonstrates how the same activating principles used for decades to make simple C-N (amide) bonds from carboxylic acids with loss of water can be used to make C-C bonds through coupling with dialkylzinc reagents and loss of carbon dioxide. This disconnection strategy benefits from the use of a simple, inexpensive nickel catalyst and exhibits a remarkably broad scope across a range of substrates (>70 examples).

An unusually stable chlorophosphite: What makes BIFOP-Cl so robust against hydrolysis?

Two chlorophosphites, the biphenyl-based BIFOP-Cl and the diphenyl ether-based O-BIFOP-Cl, exhibit striking differences regarding their reaction with water. While BIFOP-Cl is nearly completely unreactive, its oxo-derivative O-BIFOP-Cl reacts instantly with water, yielding a tricyclic hydrocarbon unit after rearrangement. The analysis of the crystal structure of O-BIFOP-Cl and BIFOP-Cl revealed that the large steric demand of encapsulating fenchane units renders the phosphorus atom nearly inaccessible by nucleophilic reagents, but only for BIFOP-Cl. In addition to the steric effect, a hypervalent P(III)-O interaction as well as an electronic conjugation effect causes the high reactivity of O-BIFOP-Cl. A DFT study of the hydrolysis in BIFOP-Cl verifies a higher repulsive interaction to water and a decreased leaving tendency of the chloride nucleofuge, which is caused by the fenchane units. This high stability of BIFOP-Cl against nucleophiles supports its application as a chiral ligand, for example, in Pd catalysts.

A diphenyl ether derived bidentate secondary phosphine oxide as a preligand for nickel-catalyzed C–S cross-coupling reactions

A bidentate secondary phosphine oxide is a more effective preligand than Ph₂P(O)H in nickel-catalyzed C–S cross-coupling reactions.

Developing five-membered heterocycle substituted phosphinous acids as ligands for palladium-catalyzed Suzuki–Miyaura and Catellani reactions

Several secondary phosphine oxides (SPOs) were prepared. Organic products with unexpected conformations (9 and 10) were obtained besides 8 from Heck-type Catellani reactions with the assistance of the selected SPO ligand 5f.

Cross-coupling of alkyl halides with aryl or alkyl Grignards catalyzed by dinuclear Ni(II) complexes containing functionalized tripodal amine-pyrazolyl ligands

Structurally distinctive dinuclear Ni(II) complexes with furan or thiophene tethered amine-pyrazolyl tripodal hybrid ligands have been synthesized and crystallographically characterized. All complexes are catalytically active towards cross-coupling of aryl/alkyl Grignard reagents with β-H containing alkyl halides at room temperature in the presence of N,N,N',N'-tetramethylethylenediamine (TMEDA). The catalytic efficacy of the complexes is dependent on the tether substituent at the central amine. Two species, Ni(II) TMEDA and Mg(II) TMEDA complexes, have been isolated from the catalytic reaction mixtures under different conditions. Some ligand-stabilized Ni(II) and Mg(II) bimetallic species have also been identified in the ESI-MS spectra.

Palladium(II)-catalyzed Heck reaction of aryl halides and arylboronic acids with olefins under mild conditions

A series of general and selective Pd(II)-catalyzed Heck reactions were investigated under mild reaction conditions. The first protocol has been developed employing an imidazole-based secondary phosphine oxide (SPO) ligated palladium complex (6) as a precatalyst. The catalytic coupling of aryl halides and olefins led to the formation of the corresponding coupled products in excellent yields. A variety of substrates, both electron-rich and electron-poor olefins, were converted smoothly to the targeted products in high yields. Compared with the existing approaches employing SPO-Pd complexes in a Heck reaction, the current strategy features mild reaction conditions and broad substrate scope. Furthermore, we described the coupling of arylboronic acids with olefins, which were catalyzed by Pd(OAc)2 and employed N-bromosuccinimide as an additive under ambient conditions. The resulted biaryls have been obtained in moderate to good yields.