Multicatalytic Approach to One-Pot Stereoselective Synthesis of Secondary Benzylic Alcohols

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.

Variable structure diversification by multicatalysis: the case of alcohols

Given that alcohol moieties are present in a great diversity of valuable fine chemicals from nature and synthesis, methods enabling their structure diversification are highly sought after. Catalysis proved to enable the development of new transformations that are beyond the inherent reactivity of alcohols. However, modifying the structure of alcohols at certain unbiased positions remains a major challenge or requires tedious multistep procedures. Recently, increased attention has been given to multicatalyis, which combines multiple reactions and catalysts within one system, creating room for discovering previously inaccessible reactivities or increasing the overall efficiency of multistep transformations. This feature article focuses on demonstrating various aspects of devising such multicatalytic systems that modify the structure of alcohol-containing compounds. Special attention is given to highlighting the challenges and advantages of multicatalysis, and in a broader context discussing how the field of catalysis may progress toward more complex systems.

Ruthenium-Catalyzed Reductive Coupling of Epoxides with Primary Alcohols via Hydrogen Transfer Catalysis

Herein, we report the ruthenium-catalyzed synthesis of β-alkylated secondary alcohols via the regioselective ring-opening of epoxides with feedstock primary alcohols. The reaction utilized alcohol as the carbon source and the terminal reductant. Kinetic and labeling experiments elucidate the hydrogen transfer catalysis that operates via tandem Markovnikov selective transfer hydrogenation of terminal epoxides and hydrogen transfer-mediated cross-coupling of the resulting alcohol with primary alcohol substrates. A broad scope (40 examples including drugs/natural product derivatives) and excellent regioselectivity for a variety of substrates were shown.

Employing Visible-Light Photoredox Catalysis in Multicomponent-Multicatalyst Reactions: One-Pot Synthesis of Spiroquinazolin-2-(thi)ones

The multicomponent-multicatalyst reaction ((MC)²R) and visible-light catalysis have emerged as green and powerful strategies for achieving ideal syntheses. Here, we report the first example of a visible-light-induced approach toward spiroquinazolin(thi)ones. This (MC)²R features an eco-friendly energy source and solvent, metal-free catalysts, step- and atom-economy, a relay catalysis strategy, air as green oxidant, mild conditions, and easily accessible starting materials.

Conversion of Primary Alcohols and Butadiene to Branched Ketones via Merged Transfer Hydrogenative Carbonyl Addition-Redox Isomerization Catalyzed by Rhodium

The first examples of rhodium-catalyzed carbonyl addition via hydrogen autotransfer are described, as illustrated in tandem butadiene-mediated carbonyl addition-redox isomerizations that directly convert primary alcohols to isobutyl ketones. Related reductive coupling-redox isomerizations of aldehyde reactants mediated by sodium formate also are reported. A double-labeling crossover experiment reveals that the rhodium alkoxide obtained upon carbonyl addition enacts redox isomerization without dissociation of rhodium at any intervening stage.

Diastereo-, Enantio-, and Z-Selective α,δ-Difunctionalization of Electron-Deficient Dienes Initiated by Rh-Catalyzed Conjugate Addition

Metal-catalyzed enantioselective conjugate additions are highly reliable methods for stereoselective synthesis; however, multicomponent reactions that are initiated by conjugate arylation of acyclic π-systems are rare. These reactions generally proceed with poor diastereoselectivity while requiring basic, moisture sensitive organometallic nucleophiles. Here, we show that Rh-catalysts supported by a tetrafluorobenzobarrelene ligand (Ph-tfb) enable the enantio-, diastereo-, and Z-selective α,δ-difunctionalization of electron-deficient 1,3-dienes with organoboronic acid nucleophiles and aldehyde electrophiles to generate Z-homoallylic alcohols with three stereocenters. The reaction accommodates diene substrates activated by ester, amide, ketone, or aromatic groups and can be used to couple aryl, alkenyl, or alkyl aldehydes. Diastereoselective functionalization of the Z-olefin unit in the addition products allows for the generation of compounds with five stereocenters in high dr and ee. Mechanistic studies suggest aldehyde allylrhodation is the rate-determining step, and unlike reactions of analogous Rh-enolates, the Rh-allyl species generated by δ-arylation undergoes aldehyde trapping rather than protonolysis, even when water is present as a cosolvent. These findings should have broader implications in the use of privileged metal-catalyzed conjugate addition reactions as entry points toward the preparation of acyclic molecules containing nonadjacent stereocenters.

Enantio- and Diastereodivergent Sequential Catalysis Featuring Two Transition-Metal-Catalyzed Asymmetric Reactions

This study demonstrates the feasibility and inherent benefits of combining two distinct asymmetric transition-metal-catalyzed reactions in one pot. The reported transformation features a Pd-catalyzed asymmetric allylic alkylation and a Rh-catalyzed enantioselective 1,4-conjugate addition, effectively converting simple allyl enol carbonate precursors into enantioenriched cyclic ketones with two remote stereocenters. Despite the anticipated challenges associated with controlling stereoselectivity in such a complex system, the products are obtained in enantiomeric excesses ranging up to >99 % ee, exceeding those obtained from either of the individual asymmetric reactions. In addition, since the stereoselectivity of both steps is under catalyst control, this one-pot reaction is enantio- and diastereodivergent, enabling facile access to all stereoisomers from the same set of starting materials.

Enantioselective α-Arylation of Primary Alcohols under Sequential One-Pot Catalysis

Secondary benzylic alcohols and diarylmethanols are common structural motifs of biologically active and medicinally relevant compounds. Here we report their enantioselective synthesis by α-arylation of primary aliphatic and benzylic alcohols under sequential catalysis integrating a Ru-catalyzed hydrogen transfer oxidation and a Ru-catalyzed nucleophilic addition. The method can be applied to various alcohols and aryl nucleophiles tolerating a range of functional groups, including secondary alcohols, ketones, alkenes, esters, NH amides, tertiary amines, aryl halides, and heterocycles.