Asymmetric Iridium-Catalyzed C-C Coupling of Chiral Diols via Site-Selective Redox-Triggered Carbonyl Addition

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

Iodide-bound ruthenium-JOSIPHOS complexes catalyze the redox-neutral C-C coupling of primary alcohols with methylallene (1,2-butadiene) or 1,3-butadiene to form products of anti-crotylation with good to excellent levels of diastereo- and enantioselectivity. Distinct from other methods, direct crotylation of primary alcohols in the presence of unprotected secondary alcohols is possible, enabling generation of spirastrellolide B (C9-C15) and leucascandrolide A (C9-C15) substructures in significantly fewer steps than previously possible.

Chiral Amines via Enantioselective π-Allyliridium-C,O-Benzoate-Catalyzed Allylic Alkylation: Student Training via Industrial-Academic Collaboration

ConspectusCyclometalated π-allyliridium-C,O-benzoate complexes discovered in the Krische laboratory display unique amphiphilic properties, catalyzing both nucleophilic carbonyl allylation and electrophilic allylation of diverse amines as well as nitronates. Given the importance of chiral amines in FDA-approved small-molecule drugs, a collaboration with medicinal chemists at Genentech that included on-site graduate student internships was undertaken to explore and expand the scope of π-allyliridium-C,O-benzoate-catalyzed allylic amination and related processes. As described in this Account, our collective experimental studies have unlocked asymmetric allylic aminations of exceptionally broad utility and scope. Specifically, using racemic branched alkyl-substituted allylic acetate proelectrophiles, primary and secondary aliphatic or aromatic amines, including indoles, engage in highly regio- and enantioselective allylic amination. Additionally, unactivated nitronates were found to be competent nucleophilic partners for regio- and enantioselective allylic alkylation, enabling entry to β-stereogenic α-quaternary primary amines. Notably, these π-allyliridium-C,O-benzoate-catalyzed allylic substitutions, which display complete branched regioselectivity in reactions of alkyl-substituted allyl electrophiles, complement the scope of corresponding iridium phosphoramidite-catalyzed allylic aminations, which require aryl-substituted allyl electrophiles to promote high levels of branched regioselectivity. Computational, kinetic, ESI-CID-MS, and isotopic labeling studies were undertaken to understand the mechanism of these processes, including the origins of regio- and enantioselectivity. Isotopic labeling studies suggest that C-N bond formation occurs through outer-sphere addition to the π-allyl. DFT calculations corroborate C-N bond formation via outer-sphere addition and suggest that early transition states and distinct trans effects of diastereomeric chiral-at-iridium π-allyl complexes render the reaction less sensitive to steric effects, accounting for complete levels of branched regioselectivity in reactions of hindered amine and nitronate nucleophiles. Reaction progress kinetic analysis (RPKA) reveals a zero-order dependence on allyl acetate, a first-order dependence on the catalyst, and a fractional-order dependence on the amine. As corroborated by ESI-CID-MS analysis, the 0.4 kinetic order dependence on the amine may reflect the intervention of cesium-bridged amine dimers, which dissociate to form monomeric cesium amide nucleophiles. Hence, the requirement of cesium carbonate (vs lower alkali metal carbonates) in these processes may reside in cesium's capacity for Lewis acid-enhanced Brønsted acidification of the amine pronucleophile. Beyond the development of catalytic processes for the synthesis of novel chiral amines, the present research was conducted by graduate students who benefited from career development experiences associated with training in both academic and industrial laboratories.

Stereo- and Site-Selective Conversion of Primary Alcohols to Allylic Alcohols via Ruthenium-Catalyzed Hydrogen Auto-Transfer Mediated by 2-Butyne

The first enantioselective ruthenium-catalyzed carbonyl vinylations via hydrogen autotransfer are described. Using a ruthenium-JOSIPHOS catalyst, primary alcohols 2a-2m and 2-butyne 1a are converted to chiral allylic alcohols 3a-3m with excellent levels of absolute stereocontrol. Notably, 1°,2°-1,3-diols participate in site-selective C-C coupling, enabling asymmetric carbonyl vinylation beyond premetalated reagents, exogenous reductants, or hydroxyl protecting groups. Using 2-propanol as a reductant, aldehydes dehydro-2a, 2l participate in highly enantioselective 2-butyne-mediated vinylation under otherwise identical reaction conditions. Regio-, stereo-, and site-selective vinylations mediated by 2-pentyne 1b to form adducts 3n, 3o, and epi-3o also are described. The tiglyl alcohol motif obtained upon butyne-mediated vinylation, which is itself found in diverse secondary metabolites, may be converted to commonly encountered polyketide stereodiads, -triads, and -tetrads, as demonstrated by the formation of adducts 4a-4d. The collective mechanistic studies, including deuterium labeling experiments, corroborate a catalytic cycle involving alcohol dehydrogenation to form a transient aldehyde and a ruthenium hydride, which engages in alkyne hydrometalation to form a nucleophilic vinylruthenium species that enacts carbonyl addition. A stereochemical model for carbonyl addition invoking formyl CH···I[Ru] and CH···O≡C[Ru] hydrogen bonds is proposed based on prior calculations and crystallographic data.

Bi(cyclopentyl)diol-Derived Boronates in Highly Enantioselective Chiral Phosphoric Acid-Catalyzed Allylation, Propargylation, and Crotylation of Aldehydes

In this study, we disclose the catalytic addition of bi(cyclopentyl)diol-derived boronates to aldehydes promoted by chiral phosphoric acids, allowing for the formation of enantioenriched homoallylic, propargylic, and crotylic alcohols (up to >99% enantiomeric excess (ee), diastereomeric ratio (dr) >20:1). These boronate substrates provided superior enantioselectivities, allowing for the reactions to proceed with low catalyst loading (0.5-5 mol %) and reduced reaction time (15 min at room temperature for aldehyde allylboration). A wide substrate scope was exhibited, and the novel boronates provided high enantiocontrol. Reactions with substituted allylboronates and aldehydes yielded vicinal stereogenic alcohols bearing β-tertiary or quaternary carbon centers. High enantio- and diastereoselectivities were found due to the closed six-membered chair-like transition state, with backbone modifications of the boronate and its interactions with the chiral phosphoric acid being the most likely contributing factor.

Feedstock Reagents in Metal-Catalyzed Carbonyl Reductive Coupling: Minimizing Preactivation for Efficiency in Target-Oriented Synthesis

Use of abundant feedstock pronucleophiles in catalytic carbonyl reductive coupling enhances efficiency in target-oriented synthesis. For such reactions, equally inexpensive reductants are desired or, ideally, corresponding hydrogen autotransfer processes may be enacted wherein alcohols serve dually as reductant and carbonyl proelectrophile. As described in this Minireview, these concepts allow reactions that traditionally require preformed organometallic reagents to be conducted catalytically in a byproduct-free manner from inexpensive π-unsaturated precursors.

Total Synthesis of Clavosolide A via Asymmetric Alcohol-Mediated Carbonyl Allylation: Beyond Protecting Groups or Chiral Auxiliaries in Polyketide Construction

The 20-membered marine macrodiolide clavosolide A is prepared in 7 steps (LLS) in the absence of protecting groups or chiral auxiliaries via enantioselective alcohol-mediated carbonyl addition. In 9 prior total syntheses, 11-34 steps (LLS) were required.

Enantioselective iridium-catalyzed carbonyl isoprenylation via alcohol-mediated hydrogen transfer

Highly enantioselective iridium catalyzed carbonyl (2-vinyl)allylation or "isoprenylation" is achieved via hydrogen auto-transfer or 2-propanol-mediated reductive coupling from primary alcohol or aldehyde reactants, respectively. Using this method, asymmetric total syntheses of the terpenoid natural products (+)-ipsenol and (+)-ipsdienol were achieved.

Catalytic Enantioselective Allylations of Acetylenic Aldehydes via 2-Propanol-Mediated Reductive Coupling

Cyclometalated π-allyliridium C,O-benzoates modified by ( S)-SEGPHOS or ( S)-Cl,OMe-BIPHEP catalyze enantioselective 2-propanol-mediated reductive couplings of diverse nonmetallic allyl pronucleophiles with the acetylenic aldehyde TIPSC≡CCHO. Absolute stereochemistries of the resulting secondary homoallylic-propargylic alcohols were assigned using Rychnovsky's competing enantioselective conversion method.

Ruthenium(0)-Catalyzed Cycloaddition of 1,2-Diols, Ketols, or Diones via Alcohol-Mediated Hydrogen Transfer

Merging the characteristics of transfer hydrogenation and carbonyl addition, a broad new class of ruthenium(0)-catalyzed cycloadditions has been developed. As discussed in this Minireview, fused or bridged bicyclic ring systems are accessible in a redox-independent manner in C-C bond-forming hydrogen transfer reactions of diols, α-ketols, or 1,2-diones with diverse unsaturated reactants.

Enantioselective Iridium-Catalyzed Phthalide Formation through Internal Redox Allylation of Phthalaldehydes

An inside job: Enantioselective phthalide synthesis was achieved through internal redox allylation of o-phthalaldehydes. Oxidative esterification is balanced by reductive carbonyl addition to achieve an overall redox-neutral process. This method enabled formal syntheses of ent-spirolaxine methyl ether and CJ-12,954.

Ir-Catalyzed reactions in natural product synthesis

This review highlights the recent applications of Ir-catalyzed reactions in the total synthesis of natural products.

Synthesis of the C(1)-C(13) Fragment of Leiodermatolide via Hydrogen-Mediated C-C Bond Formation

The C(1)-C(13) fragment of the antimitotic marine macrolide leiodermatolide is prepared in seven steps via hydrogenative and transfer-hydrogenative reductive C-C couplings. A hydrogen-mediated reductive coupling of acetylene with a Roche-type aldehyde is used to construct C(7)-C(13). A 2-propanol-mediated reductive coupling of allyl acetate with (E)-2-methylbut-2-enal at a low loading of iridium (1 mol %) is used to construct C(1)-C(6), which is converted to an allylsilane using Oestereich's copper-catalyzed allylic substitution of Si-Zn reagents. The union of the C(1)-C(6) and C(7)-C(13) fragments is achieved via stereoselective Sakurai allylation.

Reductive C-C Coupling via Hydrogenation and Transfer Hydrogenation: Departure from Stoichiometric Metals in Carbonyl Addition

Metal catalyzed reductive couplings of π-unsaturated reagents with carbonyl compounds via hydrogenation or transfer hydrogenation has emerged as an alternative to the use of stoichiometric organometallic reagents in carbonyl addition.

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.

Alkynes as Electrophilic or Nucleophilic Allylmetal Precursors in Transition-Metal Catalysis

Diverse late transition metal catalysts convert terminal or internal alkynes into transient allylmetal species that display electrophilic or nucleophilic properties. Whereas classical methods for the generation of allylmetal species often form stoichiometric by-products, the recent use of alkynes as allylmetal precursors enables completely atom-efficient catalytic processes to be carried out, including enantioselective transformations.

Asymmetric Allylation of Glycidols Mediated by Allyl Acetate via Iridium-Catalyzed Hydrogen Transfer

Glycidols prepared via Sharpless asymmetric epoxidation participate in asymmetric redox-neutral carbonyl allylation with good levels of catalyst-directed diastereoselectivity. Equally stereoselective allylations may be performed from the aldehyde oxidation level using 2-propanol as the terminal reductant. An epoxide ring-opening reaction using AlMe3-n-BuLi is used to prepare the propionate-based stereotetrad spanning C17-C23 of dictyostatin, illustrating how this method may be applied to polyketide construction.

Total Synthesis of Swinholide A: An Exposition in Hydrogen-Mediated C-C Bond Formation

Diverse hydrogen-mediated C-C couplings enable construction of the actin-binding marine polyketide swinholide A in only 15 steps (longest linear sequence), roughly half the steps required in two prior total syntheses. The redox-economy, chemo- and stereoselectivity embodied by this new class of C-C couplings are shown to evoke a step-change in efficiency.

Total Synthesis of (+)-SCH 351448: Efficiency via Chemoselectivity and Redox-Economy Powered by Metal Catalysis

The polyketide natural product (+)-SCH 351448, a macrodiolide ionophore bearing 14 stereogenic centers, is prepared in 14 steps (LLS). In eight prior syntheses, 22-32 steps were required. Multiple chemoselective and redox-economic functional group interconversions collectively contribute to a step-change in efficiency.

Enantioselective Alcohol C-H Functionalization for Polyketide Construction: Unlocking Redox-Economy and Site-Selectivity for Ideal Chemical Synthesis

The development and application of stereoselective and site-selective catalytic methods that directly convert lower alcohols to higher alcohols are described. These processes merge the characteristics of transfer hydrogenation and carbonyl addition, exploiting alcohols and π-unsaturated reactants as redox pairs, which upon hydrogen transfer generate transient carbonyl-organometal pairs en route to products of C-C coupling. Unlike classical carbonyl additions, stoichiometric organometallic reagents and discrete alcohol-to-carbonyl redox reactions are not required. Additionally, due to a kinetic preference for primary alcohol dehydrogenation, the site-selective modification of glycols and higher polyols is possible, streamlining or eliminating use of protecting groups. The total syntheses of several iconic type I polyketide natural products were undertaken using these methods. In each case, the target compounds were prepared in significantly fewer steps than previously achieved.