Mechanistic Investigation into the Regio-Controllable Hydroallylations of Alkynes with Allylborons under Pd-Based Synergetic Catalyses

Density functional theory calculations were employed to investigate the Pd-catalyzed regio-selective hydroallylations of alkynes with allylborons: cooperation of Cu(OAc)₂ and dppe resulting in 1,4-dienes while combination of AdCO₂H and PCy₃ leading to 1,5-dienes. A unified rationalization mechanism called "Lewis-acid-base-interaction promoted deprotonation/3,3-rearrangement" was proposed. Compared with the commonly reported metathesis pathway to only afford the metal-allyl intermediate, in the newly established mechanism, an additional Brønsted acid (as an initiator of the Pd⁰ oxidative addition) is generated by the interaction of the allylboron (Lewis acid) B atom with the ⁿBuOH (Lewis base) O atom, and subsequent 3,3-rearrangement ensures the thermodynamic feasibility of the reaction. In addition, it was found that excess Cu(OAc)₂ plays two potential roles in the oxidative addition/alkyne insertion: (i) the participation of one AcO^- of Cu(OAc)₂ ensures a large orbital overlap between the migrating H and Pd atoms, facilitating the formal AcO-H cleavage and (ii) the extra (OAc)₂Cu···O(carboxyl) σ-coordination indirectly contributes to the (Me)C≡C(Ph) insertion into the Pd-H bond. Further analysis showed that the origin of the regioselectivity is closely related to the employed phosphorus ligand. These revealed results, which have been overlooked in the previous documents, would aid the development of new related catalytic reactions.

Cobalt-catalyzed branched selective hydroallylation of terminal alkynes

Here, we reported a cobalt-hydride-catalyzed Markovnikov-type hydroallylation of terminal alkynes with allylic electrophile to access valuable and branched skipped dienes (1,4-dienes) with good regioselectivity. This operationally simple protocol exhibits excellent functional group tolerance and exceptional substrate scope. The reactions could be carried out in gram-scale with TON (turn over number) up to 1160, and the products could be easily derivatized. The preliminary mechanism of electrophilic allylation of α-selective cobalt alkenyl intermediate was proposed based on deuterium labeling experiment and kinetic studies.

Selectively generating “skipped” dienes, where two carbon–carbon double bonds are separated by a saturated carbon center, is an interesting problem in organic chemistry, with few reliable, catalytic methods currently available. Here, the authors report branched selective hydroallylation of terminal alkynes with allylic bromides to form skipped dienes, via cobalt catalysis.

Silaborative Assembly of Allenamides and Alkynes: Highly Regio- and Stereocontrolled Access to Bi- or Trimetallic Skipped Dienes

A highly stereo- and regiocontrolled multicomponent approach to skipped 1,4-dienes decorated with one boryl and two silyl functionalities is described. This Pd-catalyzed atom-economical union of allenamides, alkynes, and Me₂ PhSiBpin (or Et₃ SiBpin) proceeds without the use of phosphine ligands, instead relying on chelation through the internal amide group of the allenamide sulfonyl. A variety of alkynes, including those derived from complex bioactive molecules, can be efficiently coupled with allenamides and Me₂ PhSiBpin in good yields and with excellent selectivity. The synthetic potential was demonstrated through multiple valuable chemoselective transformations, establishing new disconnections for functionalized dienes. Density functional theory calculations revealed that the reaction first proceeded through borylation of the allenamide, followed by silylation of the alkyne and then reductive elimination, which convergently assemble the skipped 1,4-diene.

Regiodivergent hydroallylation of 1,3-diynes controlled by nickel and palladium catalysts

A highly efficient hydroallylation reaction of 1,3-diynes with allylborons was developed, with the regioselectivity governed primarily by the appropriate choice of the metal.

Nickel-catalyzed hydroalkylation and hydroalkenylation of 1,3-dienes with hydrazones

Transition-metal-catalyzed hydrofunctionalization of 1,3-dienes is a useful and atom-economical method for constructing allylic compounds. Although substantial progress on hydroalkylation of dienes with stabilized carbon nucleophiles has been made, hydroalkylation of dienes with unstabilized carbon nucleophiles has remained a challenge. In this article, we report a protocol for nickel-catalyzed hydroalkylation of dienes with hydrazones, which serve as equivalents of alkyl carbon nucleophiles. In addition, we developed a protocol for hydroalkenylation of dienes with α,β-unsaturated hydrazones, providing a new method for the synthesis of 1,4-dienes. These hydroalkylation and hydroalkenylation reactions feature mild conditions and a wide substrate scope, and the utility of the reaction products is demonstrated by the preparation of an activator of soluble guanylate cyclase.

A regioselectivity switch in Pd-catalyzed hydroallylation of alkynes

By exploiting the reactivity of a vinyl-Pd species, we control the regioselectivity in hydroallylation of alkynes under Pd-hydride catalysis. A monophosphine ligand and carboxylic acid combination promotes 1,5-dienes through a pathway involving isomerization of alkynes to allenes. In contrast, a bisphosphine ligand and copper cocatalyst favor 1,4-dienes via a mechanism that involves transmetalation. Our study highlights how to access different isomers by diverting a common organometallic intermediate.

Nanostructured cochleates: a multi-layered platform for cellular transportation of therapeutics

Among lipid-based nanocarriers, multi-layered cochleates emerge as a novel delivery system because of prevention of oxidation of hydrophobic and hydrophilic drugs, enhancement in permeability, and reduction in dose of drugs. It also improves oral bioavailability and increases the safety of a drug by targeting at a specific site with less side effects. Nanostructured cochleates are used as a carrier for the delivery of water-insoluble or hydrophobic drugs of anticancer, antiviral and anti-inflammatory action. This review article focuses on different methods for preparation of cochleates, mechanism of formation of cochleates, mechanism of action like cochleate undergoes macrophagic endocytosis and release the drug into the systemic circulation by acting on membrane proteins, phospholipids, and receptors. Advanced methods such as calcium-substituted and β-cyclodextrin-based cochleates, novel techniques include microfluidic and modified trapping method. Cochleates showed enhancement in oral bioavailability of amphotericin B, delivery of factor VII, oral mucosal vaccine adjuvant-delivery system, and delivery of volatile oil. In near future, cochleate will be one of the interesting delivery systems to overcome the stability and encapsulation efficiency issues associated with liposomes. The current limiting factors for commercial preparation of cochleates involve high cost of manufacturing, lack of standardization, and specialized equipments.

Regio- and Stereoselective Allylindation of Alkynes Using InBr₃ and Allylic Silanes: Synthesis, Characterization, and Application of 1,4-Dienylindiums toward Skipped Dienes

Regioselective anti-allylindation of alkynes was achieved using InBr₃ and allylic silanes. Various types of alkynes and allylic silanes were applicable to the present allylindation. This sequential process used the generated 1,4-dienylindiums to establish novel synthetic methods for skipped dienes. The 1,4-dienylindiums were characterized by spectral analysis and treated with I₂ to stereoselectively give 1-iodo-1,4-dienes. The Pd-catalyzed cross coupling of 1,4-dienylindium with iodobenzene successfully proceeded in a one-pot manner to afford the corresponding 1-aryl-1,4-diene.

Weinreb Amide Directed Versatile C-H Bond Functionalization under (η5 -Pentamethylcyclopentadienyl)cobalt(III) Catalysis

The (η⁵ -pentamethylcyclopentadienyl)cobalt(III) (Cp*Co^III )-catalyzed C-H bond functionalization of aromatic, heteroaromatic, and α,β-unsaturated Weinreb amides was explored. C-H allylation reactions with the use of allyl carbonate and a perfluoroalkene, oxidative alkenylation reactions with the use of ethyl acrylate, iodination reactions with the use of N-iodosuccinimide, and amidation reactions with the use of dioxazolones were catalyzed by Cp*Co(CO)I₂ in the presence of a cationic Ag salt and AgOAc to afford various synthetically useful building blocks. Mechanistic studies of the C-H allylation disclosed that the C-H activation step was rate determining and virtually irreversible.

Ligand-Controlled Regiodivergent and Enantioselective Copper-Catalyzed Hydroallylation of Alkynes

A ligand-controlled regiodivergent and enantioselective copper-catalyzed intermolecular hydroallylation of alkynes with allylic phosphates and hydrosilanes has been achieved for the first time. The chiral bidentate sulfonate-containing N-heterocyclic carbene ligated CuCl complex leads to enantioenriched S_N 2'-type products, whereas the use of the IMesCuCl catalyst affords S_N 2-type products. Thus a range of chiral branched and achiral linear 1,4-dienes could be facilely synthesized from readily available alkynes in a regiodivergent manner.

Monomer design strategies to create natural product-based polymer materials

In an effort towards enhancing function and sustainability, natural products have become of interest in the field of polymer chemistry.

Modular, Catalytic Enantioselective Construction of Quaternary Carbon Stereocenters by Sequential Cross-Coupling Reactions

The catalytic Suzuki-Miyaura cross-coupling with chiral γ,γ-disubstituted allylboronates in the presence of RuPhos ligand occurs with high regioselectivity and enantiospecificity, furnishing nonracemic compounds with quaternary centers. Mechanistic experiments suggest that the reaction occurs by transmetalation with allyl migration, followed by rapid reductive elimination.

γ-Selective Allylation of (E)-Alkenylzinc Iodides Prepared by Reductive Coupling of Arylacetylenes with Alkyl Iodides

The first examples of Cu-catalyzed γ-selective allylic alkenylation using organozinc reagents are reported. (E)-Alkenylzinc iodides were prepared by Fe-catalyzed reductive coupling of terminal arylalkynes with alkyl iodides. In the presence of a copper catalyst, these reagents reacted with allylic bromides derived from Morita-Baylis-Hillman alcohols to give 1,4-dienes in high yields. The reactions are highly γ-selective (generally γ/α > 49:1) and tolerate a wide range of functional groups such as ester, cyano, keto, and nitro.

A novel catalytic asymmetric route towards skipped dienes with a methyl-substituted central stereogenic carbon

A highly efficient method for the enantioselective synthesis of 1,4-dienes (skipped dienes) with a methyl-substituted central stereogenic carbon using copper-catalysed asymmetric allylic alkylation of diene bromides was developed. Excellent regio- and enantioselectivity (up to 97 : 3 SN2'/SN2 ratio and 99% ee) were achieved with broad substrate scope.

Chemoselective reductive cross-coupling of 1,5-diene-3-ols with alkynes: a facile entry to stereodefined skipped trienes

A convergent synthesis of highly substituted and stereodefined skipped polyenes is described from the reductive cross-coupling of substituted 1,5-diene-3-ols with alkynes. The control of site selectivity in functionalization of the substituted diene is a central feature of this complex fragment union reaction.

Convergent and stereospecific synthesis of complex skipped polyenes and polyunsaturated fatty acids

Skipped polyenes (i.e. 1,4-dienes and higher homologues) are stereodefined components of a vast array of biologically important natural products, including polyunsaturated fatty acids. While widespread in nature, these architectures are generally considered to represent significant barriers to efficient chemical synthesis. While partial reduction of skipped poly-ynes provides a pathway to a subset of such structures, general chemical methods for the preparation of skipped polyenes that contain varied stereochemistries and substitution patterns are lacking. Here, we describe a metal-promoted reductive cross-coupling reaction between vinylcyclopropanes and alkynes (or vinylsilanes) that provides stereoselective access to a diverse array of skipped polyenes through a process that establishes one C–C bond, generates up to three stereodefined alkenes, and can be used to introduce stereogenic centers at the central positions of the skipped polyene motif. We also demonstrate the significance of the present bond construction by preparing substituted and stereodefined polyunsaturated synthetic fatty acids.

Natural halogenated fatty acids: their analogues and derivatives

A comprehensive survey has been made of all fatty acids containing halogen atoms covalently bonded to carbon and which are deemed as naturally occurring. Generally thought to be minor components produced by many different organisms, these interesting compounds now number more than 300. Recent research, especially in the marine area, indicates this number will increase in the future. Sources of halogenated fatty acids include microorganisms, algae, marine invertebrates, and higher plants and some animals. Their possible biological significance has also been discussed