In Situ Ring‐Closing Strategy for Direct Synthesis of N‐Heterocyclic Carbene Nickel Complexes and Their Application in Coupling of Allylic Alcohols with Aryl Boronic Acids

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Migratory allylic arylation of 1,n-enols enabled by nickel catalysis

Transition-metal-catalyzed allylic substitution reactions (Tsuji-Trost reactions) proceeding via a π-allyl metal intermediate have been demonstrated as a powerful tool in synthetic chemistry. Herein, we disclose an unprecedented π-allyl metal species migration, walking on the carbon chain involving 1,4-hydride shift as confirmed by deuterium labeling experiments. This migratory allylic arylation can be realized under dual catalysis of nickel and lanthanide triflate, a Lewis acid. Olefin migration has been observed to preferentially occur with the substrate of 1,n-enols (n ≥ 3). The robust nature of the allylic substitution strategy is reflected by a broad scope of substrates with the control of regio- and stereoselectivity. DFT studies suggest that π-allyl metal species migration consists of the sequential β-H elimination and migratory insertion, with diene not being allowed to release from the metal center before producing a new π-allyl nickel species.

Regioselective Synthetic Approach to Higher Alkenes from Lower Alkenes with Sulfoxides in the Fe3+/H2O2 System via Direct Alkylation or Arylation of the Csp2-H Bond on the C═C Bond of Alkenes

The regioselective synthetic approach to higher alkenes from lower alkenes by using sulfoxides as alkyl or aryl reagents in the Fe³⁺/H₂O₂ system has been developed. This reaction realized direct alkylation or arylation of alkenes. In this reaction, sulfoxides afforded one Csp³ or Csp² atom to the C═C bond of alkenes; one new Csp²-Csp³ bond or Csp²-Csp² bond was formed. Nearly 40 products including di-, tri-, and tetra-substituted products were regioselectively synthesized. Both aliphatic and aromatic alkenes could participate in this reaction. Moreover, not only dimethyl sulfoxide but also three other sulfoxides can be applied to this reaction, including diethyl, dibenzyl, and diphenyl sulfoxide. The mechanism studies showed that this reaction may experience a coupling process via radical addition-elimination and the Fe³⁺/H₂O₂ system made the sulfoxides offered one alkyl or aryl radical to the C═C bond of alkenes.

Nickel-Catalyzed Arylative Substitution of Homoallylic Alcohols

Direct coupling of unactivated alcohols remains a challenge in synthetic chemistry. Current approaches to cross-coupling of alcohol-derived electrophiles often involve activated alcohols such as tosylates or carbonates. We report the direct arylative substitution of homoallylic alcohols catalyzed by a nickel-bisphosphine complex as a facile method to generate allylic arenes. These reactions proceed via formation of an allylic alcohol intermediate. Subsequent allylic substitution with arylboroxine nucleophiles enables the formation of a variety of allylic arenes. The presence of p-methoxyphenylboronic acid is crucial to activate the allylic alcohol to achieve high product yields.

Arylative substitutions of homoallylic alcohols with arylboron nucleophiles demonstrate the utility of unactivated alcohols as coupling partners in transition metal-catalyzed cross-coupling chemistry.

Nickel-catalyzed cross-electrophile coupling of aryl chlorides with allylic alcohols

Nickel-catalyzed cross-electrophile coupling of aryl chlorides with allylic alcohols proceeds readily under mild conditions in the presence of zinc powder and MgCl₂ to produce allylarenes in 25-92% yields. The reaction shows high regioselectivity and E/Z-selectivity, giving linear allylation products with an E configurated double bond when 1- or 3-arylallyl alcohols were used as the substrates. Functional groups including F, CF₃, COOEt, NMe₂, OMe, SiMe₃, OH and vinyl groups as well as nitrogen-containing heterocycles were tolerated.

Organocatalytic Strategy for the Fixation of CO2 via Carboxylation of Terminal Alkynes

An organocatalytic strategy for the direct carboxylation of terminal alkynes with CO₂ has been developed. The combined use of a bifunctional organocatalyst and Cs₂CO₃ resulted in a robust catalytic system for the preparation of a range of propiolic acid derivatives in high yields with broad substrate scope using CO₂ at atmospheric pressure under mild temperatures (60 °C). This work has demonstrated that this organocatalytic method offers a competitive alternative to metal catalysis for the carboxylation of terminal alkynes and CO₂. In addition, this protocol was suitable for the three-component carboxylation of terminal alkynes, alkyl halides, and CO₂.