Stereo- and Site-Selective Crotylation of Alcohol Proelectrophiles via Ruthenium-Catalyzed Hydrogen Auto-Transfer Mediated by Methylallene and Butadiene

Hydrogen Source Tuned Regiodivergent Asymmetric Hydroalkylations of 2-Substituted 1,3-Dienes with Aldehydes by Cobalt-Catalysis

Catalytic methods allowing for the reliable prediction and control of diverse regioselectivity along with the control of enantioselectivity to access different regio- and enantiomers by switching the least reaction parameters are one of the most attractive ways in organic synthesis, which provide access to diverse enantioenriched architectures from identical starting materials. Herein, a Co-catalyzed regiodivergent and enantioselective reductive hydroalkylation of 1,3-dienes with aldehydes has been achieved, furnishing different enantioenriched homoallylic alcohol architectures in good levels of enantioselectivity. The reaction features the switch of regioselectivity tuned by the selection of proton source. The use of an acid as proton source provided asymmetric 1,2-hydroalkylation products under reductive conditions, yet asymmetric 4,3-hydroalkylation products were obtained with silane as hydride source. This catalytic protocol allows for the access of homoallylic alcohols with two continuous saturated carbon centers in good levels of regio-, diastereo-, and enantioselectivity.

β-Hydroxy Esters as Malonic Semialdehyde Proelectrophiles in Enantioselective Butadiene-Mediated Crotylation: Total Synthesis of Octalactins A and B

Tractable and commercially available esters (and amides) of β-hydroxypropionic acid serve as malonic semialdehyde proelectrophiles in enantioselective ruthenium-catalyzed hydrogen autotransfer crotylations mediated by butadiene. Through iterative asymmetric butadiene-mediated crotylations of ethyl 3-hydroxypropanoate, total syntheses of the polyketide natural products octalactin A and B were achieved in fewer steps than previously possible.

Asymmetric Synthesis of Trisubstituted Vicinal Diols through Copper(I)-Catalyzed Diastereoselective and Enantioselective Allylation of Ketones with Siloxypropadienes

Chiral trisubstituted vicinal diols are a type of important organic compounds, serving as both common structure units in bioactive natural products and chiral auxiliaries in asymmetric synthesis. Herein, by using siloxypropadienes as the precursors of allyl copper(I) species, a copper(I)-catalyzed diastereoselective and enantioselective reductive allylation of ketones was achieved, providing both syn-diols and anti-diols in good to excellent enantioselectivity. DFT calculations show that cis-γ-siloxy-allyl copper species are generated favorably with either 1-TBSO-propadiene or 1-TIPSO-propadiene. Moreover, the steric difference of TBS group and TIPS group distinguishes the face selectivity of acetophenone, leading to syn-selectivity for 1-TBSO-propadiene and anti-selectivity for 1-TIPSO-propadiene. Easy transformations of the products were performed, demonstrating the synthetic utility of the present method. Moreover, one chiral diol prepared in the above transformations was used as a suitable organocatalyst for the catalytic asymmetric reductive self-coupling of aldimines generated in situ with B2(neo)2.

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.

Leveraging the Stereochemical Complexity of Octahedral Diastereomeric-at-Metal Catalysts to Unlock Regio-, Diastereo-, and Enantioselectivity in Alcohol-Mediated C-C Couplings via Hydrogen Transfer

Experimental and computational studies illuminating the factors that guide metal-centered stereogenicity and, therefrom, selectivity in transfer hydrogenative carbonyl additions of alcohol proelectrophiles catalyzed by chiral-at-metal-and-ligand octahedral d6 metal ions, iridium(III) and ruthenium(II), are described. To augment or invert regio-, diastereo-, and enantioselectivity, predominantly one from among as many as 15 diastereomeric-at-metal complexes is required. For iridium(III) catalysts, cyclometalation assists in defining the metal stereocenter, and for ruthenium(II) catalysts, iodide counterions play a key role. Whereas classical strategies to promote selectivity in metal catalysis aim for high-symmetry transition states, well-defined low-symmetry transition states can unlock selectivities that are otherwise difficult to achieve or inaccessible.

Cu-Catalyzed Asymmetric Dicarboxylation of 1,3-Dienes with CO2

Dicarboxylic acids and derivatives are important building blocks in organic synthesis, biochemistry, and the polymer industry. Although catalytic dicarboxylation with CO2 represents a straightforward and sustainable route to dicarboxylic acids, it is still highly challenging and limited to generation of achiral or racemic dicarboxylic acids. To date, catalytic asymmetric dicarboxylation with CO2 to give chiral dicarboxylic acids has not been reported. Herein, we report the first asymmetric dicarboxylation of 1,3-dienes with CO2 via Cu catalysis. This strategy provides an efficient and environmentally benign route to chiral dicarboxylic acids with high regio-, chemo-, and enantioselectivities. The copper self-relay catalysis, that is, Cu-catalyzed boracarboxylation of 1,3-dienes to give carboxylated allyl boronic ester intermediates and subsequent carboxylation of C-B bonds to give dicarboxylates, is key to the success of this dicarboxylation. Moreover, this protocol exhibits broad substrate scope, good functional group tolerance, easy product derivatizations, and facile synthesis of chiral liquid crystalline polyester and drug-like scaffolds.

Radical Crotylation of Aldehydes with 1,3-Butadiene by Photoredox Cobalt and Titanium Dual Catalysis

Metal-hydride hydrogen atom transfer (MHAT) has been recognized as a powerful method for alkene functionalization; however, photochemical MAT-mediated chemoselective functionalization of dienes remains undeveloped. In this study, we report a radical strategy (1e-) through MHAT using photoredox cobalt and titanium dual catalysis for aldehyde crotylation with butadiene, achieving excellent regio- and diastereoselectivity.

Synthesis of the C28-C41 Side Chain of (Proposed) Neaumycin B: A Contiguous Stereohextet

The C28-C41 side-chain of the proposed structure of neaumycin B, which encompasses a contiguous stereohextet, is prepared in 8 steps (longest linear sequence). Convergency is maximized via Williams' Felkin-Anh-selective triorganozincate-mediated vinylation of an α,β-stereogenic aldehyde. The relative stereochemical assignment of the C33-C35 stereotriad was accomplished via the 13C NMR analysis of the related acetonide. Relative stereochemistry of C33 carbinol and C36-C37 glycidol was determined by comparative NOE analysis of the related diastereomeric furans.

Chiral-at-Ruthenium-SEGPHOS Catalysts Display Diastereomer-Dependent Regioselectivity: Enantioselective Isoprene-Mediated Carbonyl tert-Prenylation via Halide Counterion Effects

The first correlation between metal-centered stereogenicity and regioselectivity in a catalytic process is described. Alternate pseudo-diastereomeric chiral-at-ruthenium complexes of the type RuX(CO)[η3-prenyl][(S)-SEGPHOS] form in a halide-dependent manner and display divergent regioselectivity in catalytic C-C couplings of isoprene to alcohol proelectrophiles via hydrogen autotransfer. Whereas the chloride-bound ruthenium-SEGPHOS complex prefers a trans-relationship between the halide and carbonyl ligands and delivers products of carbonyl sec-prenylation, the iodide-bound ruthenium-SEGPHOS complex prefers a cis-relationship between the halide and carbonyl ligands and delivers products of carbonyl tert-prenylation. The chloride- and iodide-bound ruthenium-SEGPHOS complexes were characterized in solution and solid phase by 31P NMR and X-ray diffraction. Density functional theory calculations of the iodide-bound catalyst implicate a Curtin-Hammett-type scenario in which the transition states for aldehyde coordination from an equilibrating mixture of sec- and tert-prenylruthenium complexes are rate- and product-determining. Thus, control of metal-centered diastereoselectivity has unlocked the first catalytically enantioselective isoprene-mediated carbonyl tert-prenylations.

Dual Ruthenium-Catalyzed Alkene Isomerization-Hydrogen Auto-Transfer Unlocks Skipped Dienes as Pronucleophiles for Enantioselective Alcohol C-H Allylation

The first use of 1,4-pentadiene and 1,5-hexadiene as allylmetal pronucleophiles in regio-, anti-diastereo-, and enantioselective carbonyl addition from alcohol proelectrophiles is described. As corroborated by deuterium labeling experiments, primary alcohol dehydrogenation delivers a ruthenium hydride that affects alkene isomerization to furnish a conjugated diene, followed by transfer hydrogenative carbonyl addition. Hydrometalation appears to be assisted by the formation of a fluxional olefin-chelated homoallylic alkylruthenium complex II, which exists in equilibrium with its pentacoordinate η1 form to enable β-hydride elimination. This effect confers remarkable chemoselectivity: while 1,4-pentadiene and 1,5-hexadiene are competent pronucleophiles, higher 1,n-dienes are not, and the olefinic functional groups of the products remain intact under conditions in which the 1,4- and 1,5-dienes isomerize. A survey of halide counterions reveals iodide-bound ruthenium-JOSIPHOS catalysts are uniquely effective in these processes. This method was used to prepare a previously reported C1-C7 substructure of (-)-pironetin in 4 vs 12 steps.

Asymmetric Ruthenium-Catalyzed Carbonyl Allylations by Gaseous Allene via Hydrogen Auto-Transfer: 1° vs 2° Alcohol Dehydrogenation for Streamlined Polyketide Construction

Iodide-bound ruthenium-JOSIPHOS complexes catalyze the redox-neutral C-C coupling of primary alcohols 2a-2r with the gaseous allene (propadiene) 1a to form enantiomerically enriched homoallylic alcohols 3a-3r with complete atom-efficiency. Using formic acid as reductant, aldehydes dehydro-2a and dehydro-2c participate in reductive C-C coupling with allene to deliver adducts 3a and 3c with comparable levels of asymmetric induction. Deuterium labeling studies corroborate a mechanism in which alcohol dehydrogenation triggers allene hydroruthenation to form transient allylruthenium-aldehyde pairs that participate in carbonyl addition. Notably, due to a kinetic preference for primary alcohol dehydrogenation, chemoselective C-C coupling of 1°,2°-1,3-diols occurs in the absence of protecting groups. As illustrated by the synthesis of C7-C15 of spirastrellolide B and F (7 vs 17 steps), C3-C10 of cryptocarya diacetate (3 vs 7 or 9 steps), and a fragment common to C8'-C14' of mycolactone F (1 vs 4 steps) and C22-C28 marinomycin A (1 vs 9 steps), this capability streamlines type I polyketide construction.

Ruthenium-Catalyzed Butadiene-Mediated Crotylation and Oxazaborolidine-Catalyzed Vinylogous Mukaiyama Aldol Reaction for The Synthesis of C1-C19 and C23-C35 of Neaumycin B

Neaumycin B is a femtomolar inhibitor of U87 human glioblastoma. Using a newly developed anti-diastereoselective ruthenium-catalyzed butadiene-mediated crotylation of primary alcohol proelectrophiles via hydrogen auto-transfer, as well as a novel variant of the catalytic asymmetric vinylogous Mukaiyama aldol (VMA) reaction applicable to linear aliphatic aldehydes and terminally methylated dienyl ketene acetals, preparation of the key C1-C19 and C23-C35 substructures of neaumycin B is achieved in 12 and 7 steps (LLS), respectively.

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.