A broadly applicable NHC-Cu-catalyzed approach for efficient, site-, and enantioselective coupling of readily accessible (pinacolato)alkenylboron compounds to allylic phosphates and applications to natural product synthesis

Enantioselective Allylboration of Acetylene: A Versatile Tool for the Stereodivergent Synthesis of Natural Products

Efficient catalytic methods that allow the use of simple and abundant chemical feedstocks for the preparation of synthetically versatile compounds are central to modern synthetic chemistry. Acetylene is a basic feedstock with a remarkable production over one million tons per year, although it is underutilized in the stereoselective synthesis of fine chemicals. Here we report a facile catalytic multicomponent reaction that allows for the enantio- and diastereoselective allylboration of acetylene gas. This process is catalyzed by a chiral copper catalyst, operates without specialized equipment or pressurization, and provides chiral skipped dienes bearing stereodefined and orthogonally functionalized olefins with excellent levels of chemo-, regio-, enantio- and diastereoselectivity. The combined stereochemical features and orthogonal functionalization make the products privileged structural scaffolds to access the complete set of stereoisomers of the chiral skipped diene core through simple enantio- and diastereodivergent pathways. The utility of the method is demonstrated with the enantioselective synthesis of three bioactive natural skipped diene products, namely (+)-Nyasol, (+)-Hinokiresinol and Phorbasin C, and other related synthetically relevant chiral molecules.

Chelation enables selectivity control in enantioconvergent Suzuki-Miyaura cross-couplings on acyclic allylic systems

Asymmetric Suzuki-Miyaura cross-couplings with aryl boronic acids and allylic electrophiles are a powerful method to convert racemic mixtures into enantioenriched products. Currently, enantioconvergent allylic arylations are limited to substrates that are symmetrical about the allylic unit, and the absence of strategies to control regio-, E/Z- and enantioselectivity in acyclic allylic systems is a major restriction. Here, using a system capable of either conjugate addition or allylic arylation, we have discovered the structural features and experimental conditions that allow an acyclic system to undergo chemo- and regioselective, enantioconvergent allylic Suzuki-Miyaura-type arylation. A wide variety of boronic acid coupling partners can be used, and both alkyl and aromatic substituents are tolerated on the allylic unit so that a wide variety of structures can be obtained. Preliminary mechanistic studies reveal that the chelating ability of the ester group is crucial to obtaining high regio- and enantioselectivity. Using this method, we were able to synthesize the natural products (S)-curcumene and (S)-4,7-dimethyl-1-tetralone and the clinically used antidepressant sertraline (Zoloft).

Organophosphates as Versatile Substrates in Organic Synthesis

This review summarizes the applications of organophosphates in organic synthesis. After a brief introduction, it discusses cross-coupling reactions, including both transition-metal-catalyzed and transition-metal-free substitution reactions. Subsequently, oxidation and reduction reactions are described. In addition, this review highlights the applications of organophosphates in the synthesis of natural compounds, demonstrating their versatility and importance in modern synthetic chemistry.

Cobalt catalyzed practical hydroboration of terminal alkynes with time-dependent stereoselectivity

Stereodefined vinylboron compounds are important organic synthons. The synthesis of E-1-vinylboron compounds typically involves the addition of a B-H bond to terminal alkynes. The selective generation of the thermodynamically unfavorable Z-isomers remains challenging, necessitating improved methods. Here, such a proficient and cost-effective catalytic system is introduced, comprising a cobalt salt and a readily accessible air-stable CNC pincer ligand. This system enables the transformation of terminal alkynes, even in the presence of bulky substituents, with excellent Z-selectivity. High turnover numbers (>1,600) and turnover frequencies (>132,000 h-1) are achieved at room temperature, and the reaction can be scaled up to 30 mmol smoothly. Kinetic studies reveal a formal second-order dependence on cobalt concentration. Mechanistic investigations indicate that the alkynes exhibit a higher affinity for the catalyst than the alkene products, resulting in exceptional Z-selective performance. Furthermore, a rare time-dependent stereoselectivity is observed, allowing for quantitative conversion of Z-vinylboronate esters to the E-isomers.

Nickel-Catalyzed Ligand-Controlled Regioselective Allylic Alkenylation of Allylic Alcohols with Easily Accessible Alkenyl Boronates: Synthesis of 1,4-Dienes

A nickel-catalyzed direct reaction of allylic alcohols with easily accessible alkenyl boronates has been developed, which provides valuable 1,4-dienes with high regio- and stereoselectivity in good to excellent yields, wide substrate scope, and functional group compatibility. The catalytic system simply consists of Ni(cod)2 as the catalyst and a ligand, without a need for a base and alcohol activator in most cases. The proper choice of ancillary ligands is highly important for this reaction. Depending on the substitution pattern of allylic alcohols and/or alkenyl boronates, different ligands were used for improving the reaction efficiency.

Catalytic Alkyne Allylboration: A Quest for Selectivity

Catalytic methodologies that enable the synthesis of complex organic molecules from simple and readily available starting materials represent a goal in modern synthetic chemistry. In particular, multicomponent carboboration reactions that provide stereoselective access to densely functionalized building blocks are particularly valuable to achieve molecular diversity. This Perspective covers the developments in the area of catalytic allylboration of alkynes and highlights the key features that have allowed for the control of the regio-, diastereo-, and enantioselectivity in these transformations.

Palladium-Catalyzed Multicomponent Assembly of (Z)-Alkenylborons via Carbopalladation/Boronation/Retro-Diels-Alder Cascade Reaction

A palladium-catalyzed multicomponent cascade reaction of aryl iodides, oxanorbornadiene, and diborns to access (Z)-alkenylborons is reported. This transformation proceeds through the sequential carbopalladation/boronation/retro-Diels-Alder domino reaction. The oxanorbornadiene used in this reaction serves as an acetylene surrogate, which is generated via a retro-Diels-Alder reaction. Such a stereoselective and scalable approach has a wide range of functional group tolerance and good substrate universality.

Copper-Catalyzed Enantioselective Borylative Allyl-Allyl Coupling of Allenes and Allylic gem-Dichlorides

A catalytic asymmetric reaction between allenes, bis(pinacolato)diboron, and allylic gem-dichlorides is reported. The method involves the coupling of a catalytically generated allyl copper species with the allylic gem-dichloride and provides chiral internal 1,5-dienes featuring (Z)-configured alkenyl boronate and alkenyl chloride units with high levels of chemo-, regio-, enantio-, and diastereoselectivity. The synthetic utility of the products is demonstrated with the synthesis of a range of optically active compounds. DFT calculations reveal key noncovalent substrate-ligand interactions that account for the enantioselectivity outcome and the diastereoselective formation of the (Z)-alkenyl chloride.

Enantioselective Synthesis of Skipped Dienes via Iridium-Catalyzed Allylic Alkylation of Phosphonates

An enantioselective synthesis of skipped dienes has been developed based on an iridium-catalyzed allylic alkylation of phosphonates and Horner-Wadsworth-Emmons olefination. This two-step protocol uses easily accessible substrates and delivers C2-substituted skipped dienes bearing a C3 stereogenic center, generally with outstanding enantioselectivities (up to 99.5:0.5 er). This is the first catalytic enantioselective allylic alkylation of phosphonates, and the overall process represents a formal enantioselective α-C(sp²)-H allylic alkylation of α,β-unsaturated carbonyls and acrylonitrile.

Catalytic Cross-Metathesis Reactions That Afford E- and Z-Trisubstituted Alkenyl Bromides: Scope, Applications, and Mechanistic Insights

Stereochemically defined trisubstituted alkenes with a bromide and a methyl group at a terminus can be readily and stereoretentively derivatized through catalytic cross-coupling, affording unsaturated fragments found in many bioactive natural products. A direct method for generating such entities would be by stereocontrolled catalytic cross-metathesis (CM). Such methods are scarce however. Here, we present a stereoretentive strategy for CM between tri-, Z- or E-di, or monosubstituted olefins and Z- or E-2-bromo-2-butene, affording an assortment of E- or Z-trisubstituted alkenyl bromides. The majority of the transformations were catalyzed by two Mo monoaryloxide pyrrolide (MAP) complexes, one purchasable and the other accessible by well-established protocols. Substrates, such as feedstock trisubstituted olefins, can be purchased; the alkenyl bromide reagents are commercially available or can be prepared in two steps in a multigram scale. The catalytic process can be used to generate products that contain polar moieties, such as an amine or an alcohol, or sterically hindered alkenes that are α- or β-branched. The utility of the approach is highlighted by a brief and stereocontrolled synthesis of an unsaturated fragment of phomactin A and a concise total synthesis of ambrein. An unexpected outcome of these investigations was the discovery of a new role for the presence of a small-molecule alkene in an olefin metathesis reaction. DFT studies indicate that this additive swiftly reacts with a short-lived Mo alkylidene and probably helps circumvent the formation of catalytically inactive square pyramidal metallacyclobutanes, enhancing the efficiency of a transformation.

Density Functional Theory Study on the H2-Acceptorless Dehydrogenative Boration of Alkenes Catalyzed by a Zirconium Complex

For the synthesis of vinyl boronate esters, the direct catalytic H₂-acceptorless dehydrogenative boration of alkenes is one of the promising strategies. In this paper, the density functional theory method was employed to investigate the reaction mechanism of dehydrogenative boration and transfer boration of alkenes catalyzed by a zirconium complex (Cp₂ZrH₂). There are two possible pathways for this reaction: the alkene insertion followed by the dehydrogenative boration (path A) and the alkene insertion after the dehydrogenative boration (path B). The calculated results showed that path A is more favorable than path B, and that the rate-determining step is the C-B coupling step with an energy barrier of 18.7 kcal/mol. The reaction modes of the C-B coupling assisted dehydrogenative boration and the alkene insertion were also discussed. These analyses reveal a novel hydrogen release behavior in dehydrogenative boration and the alkene insertion modes and sequences were proposed to be of importance in the chemoselectivity of this reaction. In addition, the X ligand effect (X = H, Cl) on the catalytic activity of the zirconium complex was explored, indicating that the H ligand could enhance the catalytic activity of the complex for styrene dehydrogenative boration.

Divergent Reactivity of α,α-Disubstituted Alkenyl Hydrazones: Bench Stable Cyclopropylcarbinyl Equivalents

Herein we report the divergent reactivity of 2,2-dialkyl-3-(E)-alkenyl N-tosylhydrazones using Pd-catalyzed cross-coupling conditions, which enable the Z-selective synthesis of 3-aryl-1,4-dienes and gem-dialkyl vinylcyclopropanes. We found that the dialkylbiaryl phosphine ligand SPhos was the optimal ligand for this transformation producing skipped dienes in up to 83% isolated yield. The ratio of skipped diene to vinylcyclopropane is dependent on both the structure of the α,α-disubstituted hydrazones and the aryl halide partner. Using sterically encumbered aryl bromides provided the trans-cyclopropane products selectively in up to 69% yield. The reaction is stereospecific and stereoselective and occurs alongside a competing 1,2-alkenyl group migration pathway.

Skipped dienes in natural product synthesis

Covering: 2000-2020The 1,4-diene motif, also known as a skipped diene, is widespread across various classes of natural products including alkaloids, fatty acids, terpenoids, and polyketides as part of either the finalized structure or a biosynthetic intermediate. The prevalence of this nonconjugated diene system in nature has resulted in numerous encounters in the total synthesis literature. However, skipped dienes have not been extensively reviewed, which could be attributed to overshadowing by the more recognized 1,3-diene system. In this review, we aim to highlight the relevance of skipped dienes in natural products through the lens of total synthesis. Subjects that will be covered include nomenclature, structural properties, prevalence in natural products, synthetic strategies and the future direction of the field.

Catalytic Asymmetric Allylic Substitution with Copper(I) Homoenolates Generated from Cyclopropanols

By using copper(I) homoenolates as nucleophiles, which are generated through the ring-opening of 1-substituted cyclopropane-1-ols, a catalytic asymmetric allylic substitution with allyl phosphates is achieved in high to excellent yields with high enantioselectivity. Both 1-substituted cyclopropane-1-ols and allylic phosphates enjoy broad substrate scopes. Remarkably, various functional groups, such as ether, ester, tosylate, imide, alcohol, nitro, and carbamate are well tolerated. Moreover, the present method is nicely extended to the asymmetric construction of quaternary carbon centers. Some control experiments argue against a radical-based reaction mechanism and a catalytic cycle based on a two-electron process is proposed. Finally, the synthetic utilities of the product are showcased by means of the transformations of the terminal olefin group and the ketone group.

Rhodium-Catalyzed Regio- and Enantioselective Allylic Amination of Racemic 1,2-Disubstituted Allylic Phosphates

Alkynylphosphines are rarely used as ligands in asymmetric metal catalysis. We synthesized a series of chiral bis(oxazoline)alkynylphosphine ligands and used them in Rh-catalyzed highly regio- and enantioselective allylic amination reactions of 1,2-disubstituted allylic phosphates. Chiral 1,2-disubstituted allylic amines were synthesized in up to 95% yield with >20:1 branched/linear (b/l) ratio and 99% ee from racemic 1,2-disubstituted allylic precursors. The sterically smaller linear alkynyl group on the P atom in the bis(oxazoline)alkynylphosphine ligands was the key to fit the new requirements of the introduction of bulky 2-R' groups.

Sulfonate N-Heterocyclic Carbene-Copper Complexes: Uniquely Effective Catalysts for Enantioselective Synthesis of C-C, C-B, C-H, and C-Si Bonds

A copper-based complex that contains a sulfonate N-heterocyclic carbene ligand was first reported 15 years ago. Since then, these organometallic entities have proven to be uniquely effective in catalyzing an assortment of enantioselective transformations, including allylic substitutions, conjugate additions, proto-boryl additions to alkenes, boryl and silyl substitutions, hydride-allyl additions to alkenyl boronates, and additions of boron-containing allyl moieties to N-H ketimines. In this review article, we detail the shortcomings in the state-of-the-art that fueled the development of this air stable ligand class, members of which can be prepared on multigram scale. For each reaction type, when relevant, the prior art at the time of the advance involving sulfonate NHC-Cu catalysts and/or subsequent key developments are briefly analyzed, and the relevance of the advance to efficient and enantioselective total or formal synthesis of biologically active molecules is underscored. Mechanistic analysis of the structural attributes of sulfonate NHC-Cu catalysts that are responsible for their ability to facilitate transformations with high efficiency as well as regio- and enantioselectivity are detailed. This review contains several formerly undisclosed methodological advances and mechanistic analyses, the latter of which constitute a revision of previously reported proposals.

NHC-Ni catalyzed enantioselective synthesis of 1,4-dienes by cross-hydroalkenylation of cyclic 1,3-dienes and heterosubstituted terminal olefins

Enantioenriched 1,4-dienes are versatile building blocks in asymmetric synthesis, therefore their efficient synthesis directly from chemical feedstock is highly sought after. Here, we show an enantioselective cross-hydroalkenylation of cyclic 1,3-diene and hetero-substituted terminal olefin by using a chiral [NHC-Ni(allyl)]BArF catalyst. Using a structurally flexible chiral C2 NHC-Ni design is key to access a broad scope of chiral 1,4-diene 3 or 3' with high enantioselectivity. This study also offers insights on how to regulate chiral C2 NHC-Ni(II) 1,3-allylic shift on cyclic diene 1 and to build sterically more hindered endocyclic chiral allylic structures on demand.

Heliannuol: Distribution, Biological Activities, 1H and 13C-NMR Spectral Data

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Heliannuols are an important class of compounds isolated from Helianthus annuus, a common sunflower, with allelopathic properties. Interestingly, two of these fourteen compounds have been isolated from a gorgonian, Pseudopterogorgia rigida. This review compiles data on the isolation and biological activity of heliannuols described in the literature and briefly reports the papers describing their syntheses. This review could stimulate research into other natural species and syntheses. Further, it describes the 1H and 13C-NMR spectral data described in the literature for the 14 heliannuols already isolated and correlates them with their respective structures. Therefore, we believe that these searchable data enable a rapid identification in routine analysis of the crude extracts of sunflowers or gorgonians.

A Ba/Pd Catalytic System Enables Dehydrative Cross-Coupling and Excellent E-Selective Wittig Reactions

A Ba/Pd cooperative catalysis system was developed to enable the dehydrative cross-coupling of allylic alcohols with P-ylides to occur directly and promote a subsequent Wittig reaction in one pot. A variety of multisubstituted 1,4-dienes were isolated in good to excellent yields with broad P-ylides (stabilized by both ester and ketone carbonyl groups) and aldehyde (aliphatic and aromatic) substrates with excellent E selectivity.

Copper-Hydride-Catalyzed Enantioselective Processes with Allenyl Boronates. Mechanistic Nuances, Scope, and Utility in Target-Oriented Synthesis

Synthesis of complex bioactive molecules is substantially facilitated by transformations that efficiently and stereoselectively generate polyfunctional compounds. Designing such processes is hardly straightforward, however, especially when the desired route runs counter to the inherently favored reactivity profiles. Furthermore, in addition to being efficient and stereoselective, it is crucial that the products generated can be easily and stereodivergently modified. Here, we introduce a catalytic process that delivers versatile and otherwise difficult-to-access organoboron entities by combining an allenylboronate, a hydride, and an allylic phosphate. Two unique selectivity problems had to be solved: avoiding rapid side reaction of a Cu-H complex with an allylic phosphate, while promoting its addition to an allenylboronate as opposed to the commonly utilized boron-copper exchange. The utility of the approach is demonstrated by applications to concise preparation of the linear fragment of pumiliotoxin B (myotonic, cardiotonic) and enantioselective synthesis and structure confirmation of netamine C, a member of a family of anti-tumor and anti-malarial natural products. Completion of the latter routes required the following noteworthy developments: (1) a two-step all-catalytic sequence for conversion of a terminal alkene to a monosubstituted alkyne; (2) a catalytic S_N2'- and enantioselective allylic substitution method involving a mild alkylzinc halide reagent; and (3) a diastereoselective [3+2]-cycloaddition to assemble the polycyclic structure of a guanidyl polycyclic natural product.