Ruthenium-catalyzed reductive coupling of 1,3-enynes and aldehydes by transfer hydrogenation: anti-diastereoselective carbonyl propargylation

Catalyst-Free Propargylboration of Ketones with Allenyl-Bpins: Highly Stereoselective Synthesis of tert-Homopropargyl Alcohols Bearing Vicinal Stereocenters

A practical and efficient propargylboration of ketones is presented using general allenylboronic acid pinacol esters (allenyl-Bpins) without a catalyst. This reaction is triggered by in-situ activation of stable allenyl-Bpins through the sequential addition of 1.25 equiv. of n BuLi and the prerequisite 2.0 equiv. of TFAA. Under the optimized reaction conditions, the versatile trisubstituted allenyl-Bpins react with various ketones smoothly to afford a wide range of tert-homopropargyl alcohols bearing vicinal stereocenters in high yields with good to excellent diastereoselectivities. Furthermore, propargylboration of ketones with chiral trisubstituted allenyl-Bpins allows for the asymmetric synthesis of chiral tert-homopropargyl alcohols with a full chirality transfer.

Development of regiodivergent asymmetric reductive coupling reactions of allenamides to access heteroatom-rich organic compounds

Organic compounds of biological importance often contain multiple stereogenic C-heteroatom functional groups (e.g. amines, alcohols, and ethers). As a result, synthetic methods to access such compounds in a reliable and stereoselective fashion are important. In this feature article, we present a strategy to enable the introduction of multiple C-heteroatom functional groups in a regiodivergent cross-coupling approach through the use of reductive coupling chemistry employing allenamides. Such processes allow for opportunities to access different heteroatom substitution patterns from the same starting materials.

Copper(I)-catalyzed asymmetric 1,3-dipolar cycloaddition of 1,3-enynes and azomethine ylides

Herein, we report a copper(I)-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides and 1,3-enynes, which provides a series of chiral poly-substituted pyrrolidines in high regio-, diastereo-, and enantioselectivities. Both 4-aryl-1,3-enynes and 4-silyl-1,3-enynes serve as suitable dipolarophiles while 4-alkyl-1,3-enynes are inert. Moreover, the method is successfully applied in the construction of both tetrasubstituted stereogenic carbon centers and chiral spiro pyrrolidines. The DFT calculations are also conducted, which imply a concerted mechanism rather than a stepwise mechanism. Finally, various transformations started from the pyrrolidine bearing a triethylsilylethynyl group and centered on the alkyne group are achieved, which compensates for the inertness of 4-alkyl-1,3-enynes in the present reaction.

Recent Advances in the Synthesis of Propargyl Derivatives, and Their Application as Synthetic Intermediates and Building Blocks

The propargyl group is a highly versatile moiety whose introduction into small-molecule building blocks opens up new synthetic pathways for further elaboration. The last decade has witnessed remarkable progress in both the synthesis of propargylation agents and their application in the synthesis and functionalization of more elaborate/complex building blocks and intermediates. The goal of this review is to highlight these exciting advances and to underscore their impact.

Historical perspective on ruthenium-catalyzed hydrogen transfer and survey of enantioselective hydrogen auto-transfer processes for the conversion of lower alcohols to higher alcohols

Ruthenium-catalyzed hydrogen auto-transfer reactions for the direct enantioselective conversion of lower alcohols to higher alcohols are surveyed. These processes enable completely atom-efficient carbonyl addition from alcohol proelectrophiles in the absence of premetalated reagents or metallic reductants. Applications in target-oriented synthesis are highlighted, and a brief historical perspective on ruthenium-catalyzed hydrogen transfer processes is given.

Structure-guided insights into non-catalytic (α-hydroxy)alkylation of olefins with alcohols

New experimental data on the mutual reactivity of alcohols and olefins were obtained at 350 °C and a surrogate for olefins was suggested in return.

Allylic alcohol synthesis by Ni-catalyzed direct and selective coupling of alkynes and methanol

Methanol is an abundant and renewable chemical raw material, but its use as a C1 source in C-C bond coupling reactions still constitutes a big challenge, and the known methods are limited to the use of expensive and noble metal catalysts such as Ru, Rh and Ir. We herein report nickel-catalyzed direct coupling of alkynes and methanol, providing direct access to valuable allylic alcohols in good yields and excellent chemo- and regioselectivity. The approach features a broad substrate scope and high atom-, step- and redox-economy. Moreover, this method was successfully extended to the synthesis of [5,6]-bicyclic hemiacetals through a cascade cyclization reaction of alkynones and methanol.

Preparation of Chiral Allenes through Pd-Catalyzed Intermolecular Hydroamination of Conjugated Enynes: Enantioselective Synthesis Enabled by Catalyst Design

In this study, we establish that conjugated enynes undergo selective 1,4-hydroamination under Pd catalysis to deliver chiral allenes with pendant allylic amines. Several primary and secondary aliphatic and aryl-substituted amines couple with a wide range of mono- and disubstituted enynes in a nonenantioselective reaction where DPEphos serves as the ligand for Pd. Benzophenone imine acts as an ammonia surrogate to afford primary amines in a two-step/one-pot process. Examination of chiral catalysts revealed a high degree of reversibility in the C-N bond formation that negatively impacted enantioselectivity. Consequently, an electron-poor ferrocenyl-PHOX ligand was developed to enable efficient and enantioselective enyne hydroamination.

Catalytic Enantioselective Carbonyl Propargylation Beyond Preformed Carbanions: Reductive Coupling and Hydrogen Auto-Transfer

Chiral metal complexes catalyze enantioselective carbonyl propargylation via reductive coupling or as hydrogen auto-transfer processes, in which reactant alcohols serve dually as reductant and carbonyl proelectrophile. Unlike classical propargylation protocols, which rely on allenylmetal reagents or metallic reductants (e.g. NHK reactions), reductive protocols for carbonyl propargylation can occur in the absence of stoichiometric metals, precluding generation of metallic byproducts. Propargylations of this type exploit both enyne and propargyl halide pronucleophiles.

Copper-Catalyzed Regioselective and Stereoselective Coupling of Grignard Reagents with Pent-1-en-4-yn-3-yl Benzoates: A Shortcut to ( Z) -1,5-Disubstituted Pent-3-en-1-ynes from Accessible Starting Materials

Copper-catalyzed coupling of Grignard reagents with pent-1-en-4-yn-3-yl benzoates occurs regioselectively at the terminal alkenyl carbon rather than the alkynyl site, leading to the stereoselective formation of unexpected ( Z) -1,5-disubstituted pent-3-en-1-ynes without generation of the initially expected alkenyl allene products. By using easily accessible starting materials, this reaction can provide direct access to thermodynamically unfavorable Z-configured enynes, which widely exist in many bioactive natural products, such as the anti-inflammatory components in henna.

Intermolecular Metal-Catalyzed Reductive Coupling of Dienes, Allenes, and Enynes with Carbonyl Compounds and Imines

Metal-catalyzed reductive coupling has emerged as an alternative to the use of stoichiometric organometallic reagents in an increasingly diverse range of carbonyl and imine additions. In this review, the use of diene, allene, and enyne pronucleophiles in intermolecular carbonyl and imine reductive couplings are surveyed, along with related hydrogen autotransfer processes.

A Simple Procedure for the Synthesis of β-Hydroxyallenamides via Homoallenylation of Aldehydes

A simple one-pot synthesis of β-hydroxyallenamides is reported. This procedure entails chemo- and regioselective hydroboration of 3-en-1-ynyl-sulfonylamides with Cy₂BH followed by homoallenylation of aldehydes to yield β-hydroxyallenamides (up to 94% yield and >20:1 dr). Controlled synthesis of up to three continuous stereochemical elements was realized. Density functional theory (DFT) calculations suggest a concerted Zimmerman-Traxler chair-like transition state. Initial results suggest that enantio- and diastereoselective synthesis of β-hydroxyallenamides with optically active hydroboration reagents is viable.

Organocatalytic reductive coupling of aldehydes with 1,1-diarylethylenes using an in situ generated pyridine-boryl radical

A pyridine-boryl radical promoted reductive coupling reaction of aldehydes with 1,1-diarylethylenes has been established.

A pyridine-boryl radical promoted reductive coupling reaction of aldehydes with 1,1-diarylethylenes has been established via a combination of computational and experimental studies. Density functional theory calculations and control experiments suggest that the ketyl radical from the addition of the pyridine-boryl radical to aldehydes is the key intermediate for this C–C bond formation reaction. This metal-free reductive coupling reaction features a broad substrate scope and good functional compatibility.

Synthesis of 2-aminopyridines via ruthenium-catalyzed [2+2+2] cycloaddition of 1,6- and 1,7-diynes with cyanamides: scope and limitations

A direct route to access 2-aminopyridines using Ru-catalyzed-[2+2+2] cycloaddition of 1,n-diynes with cyanamides is described in excellent yields and regioselectivities.

Catalytic Enantioselective Carbonyl Allylation and Propargylation via Alcohol-Mediated Hydrogen Transfer: Merging the Chemistry of Grignard and Sabatier

Merging the characteristics of transfer hydrogenation and carbonyl addition, we have developed a new class of catalytic enantioselective C-C bond formations. In these processes, hydrogen transfer between alcohols and π-unsaturated reactants generates carbonyl-organometal pairs that combine to deliver products of addition. On the basis of this mechanistic paradigm, lower alcohols are converted directly to higher alcohols in the absence of premetalated reagents or discrete alcohol-to-carbonyl redox reactions. In certain cases, due to a pronounced kinetic preference for primary versus secondary alcohol dehydrogenation, diols and higher polyols are found to engage in catalytic stereo- and site-selective C-C bond formation-a capability that further enhances efficiency by enabling skeletal construction events without extraneous manipulations devoted to the installation and removal of protecting groups. While this Account focuses on redox-neutral couplings of alcohols, corresponding aldehyde reductive couplings mediated by 2-propanol were developed in parallel for most of the catalytic transformations reported herein. Mechanistically, two distinct classes of alcohol C-H functionalizations have emerged, which are distinguished by the mode of pronucleophile activation, specifically, processes wherein alcohol oxidation is balanced by (a) π-bond hydrometalation or (b) C-X bond reductive cleavage. Each pathway offers access to allylmetal or allenylmetal intermediates and, therefrom, enantiomerically enriched homoallylic or homopropargylic alcohol products, respectively. In the broadest terms, carbonyl addition mediated by premetalated reagents has played a central role in synthetic organic chemistry for well over a century, but the requisite organometallic reagents pose issues of safety, require multistep syntheses, and generate stoichiometric quantities of metallic byproducts. The concepts and catalytic processes described in this Account, conceived and developed wholly within the author's laboratory, signal a departure from the use of stoichiometric organometallic reagents in carbonyl addition. Rather, they reimagine carbonyl addition as a hydrogen autotransfer process or cross-coupling in which alcohol reactants, by virtue of their native reducing ability, drive the generation of transient organometallic nucleophiles and, in doing so, serve dually as carbonyl proelectrophiles. The catalytic allylative and propargylative transformations developed to date display capabilities far beyond their classical counterparts, and their application to the total synthesis of type-I polyketide natural products have evoked a step-change in efficiency. More importantly, the present data suggest that diverse transformations traditionally reliant on premetalated reagents may now be conducted catalytically without stoichiometric metals. This Account provides the reader and potential practitioner with a catalog of enantioselective alcohol-mediated carbonyl additions-a user's guide, 10-year retrospective, and foundation for future work in this emerging area of catalytic C-C bond formation.

Regioselective Hydrohydroxyalkylation of Styrene with Primary Alcohols or Aldehydes via Ruthenium-Catalyzed C-C Bond Forming Transfer Hydrogenation

Transfer hydrogenative coupling of styrene with primary alcohols using the precatalyst HClRu(CO)(PCy3 )2 modified by AgOTf or HBF4 delivers branched or linear adducts from benzylic or aliphatic alcohols, respectively. Related 2-propanol mediated reductive couplings also are described.

Metal-catalyzed reductive coupling of olefin-derived nucleophiles: Reinventing carbonyl addition

α-Olefins are the most abundant petrochemical feedstock beyond alkanes, yet their use in commodity chemical manufacture is largely focused on polymerization and hydroformylation. The development of byproduct-free catalytic C-C bond-forming reactions that convert olefins to value-added products remains an important objective. Here, we review catalytic intermolecular reductive couplings of unactivated and activated olefin-derived nucleophiles with carbonyl partners. These processes represent an alternative to the longstanding use of stoichiometric organometallic reagents in carbonyl addition.

C-Propargylation Overrides O-Propargylation in Reactions of Propargyl Chloride with Primary Alcohols: Rhodium-Catalyzed Transfer Hydrogenation

The canonical SN 2 behavior displayed by alcohols and activated alkyl halides in basic media (O-alkylation) is superseded by a pathway leading to carbinol C-alkylation under the conditions of rhodium-catalyzed transfer hydrogenation. Racemic and asymmetric propargylations are described.

Ruthenium-Catalyzed Transfer Hydrogenation for C-C Bond Formation: Hydrohydroxyalkylation and Hydroaminoalkylation via Reactant Redox Pairs

Merging the chemistry of transfer hydrogenation and carbonyl or imine addition, a broad new family of redox-neutral or reductive hydrohydroxyalkylations and hydroaminomethylations have been developed. In these processes, hydrogen redistribution between alcohols and π-unsaturated reactants is accompanied by C-C bond formation, enabling direct conversion of lower alcohols to higher alcohols. Similarly, hydrogen redistribution between amines to π-unsaturated reactants results in direct conversion of lower amines to higher amines. Alternatively, equivalent products of hydrohydroxyalkylation and hydroaminomethylation may be generated through the reaction of carbonyl compounds or imines with π-unsaturated reactants under the conditions of 2-propanol-mediated reductive coupling. Finally, using vicinally dioxygenated reactants, that is, diol, ketols, or diones, successive transfer hydrogenative coupling occurs to generate 2 C-C bonds, resulting in products of formal [4+2] cycloaddition.

Ruthenium-BINAP Catalyzed Alcohol C-H tert-Prenylation via 1,3-Enyne Transfer Hydrogenation: Beyond Stoichiometric Carbanions in Enantioselective Carbonyl Propargylation

The chiral ruthenium complex formed in situ from (TFA)2Ru(CO)(PPh3)2 and (R)-BINAP is found to catalyze the enantioselective C-C coupling of diverse primary alcohols with the 1,3-enyne, TMSC≡CC(Me)═CH2, to form secondary homopropargyl alcohols bearing gem-dimethyl groups. All reagents for this byproduct-free coupling are inexpensive and commercially available, making this protocol a practical alternative to stoichiometric carbanions in enantioselective carbonyl reverse prenylation.