Intermolecular Noncovalent Hydroxy-Directed Enantioselective Heck Desymmetrization of Cyclopentenol: Computationally Driven Synthesis of Highly Functionalized cis-4-Arylcyclopentenol Scaffolds

Enantioselective synthesis of β-aryl-γ-lactam derivatives via Heck-Matsuda desymmetrization of N-protected 2,5-dihydro-1H-pyrroles

We report herein an enantioselective palladium-catalyzed Heck-Matsuda reaction for the desymmetrization of N-protected 2,5-dihydro-1H-pyrroles with aryldiazonium salts, using the chiral N,N-ligand (S)-PyraBox. This strategy has allowed straightforward access to a diversity of 4-aryl-γ-lactams via Heck arylation followed by a sequential Jones oxidation. The overall method displays a broad scope and good enantioselectivity, favoring the (R) enantiomer. The applicability of the protocol is highlighted by the efficient enantioselective syntheses of the selective phosphodiesterase-4-inhibitor rolipram and the commercial drug baclofen as hydrochloride.

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.

Palladium-Catalyzed Synthesis of Linked Bis-Heterocycles─Synthesis and Investigation of Photophysical Properties

A palladium-catalyzed domino C-N coupling/Cacchi reaction is reported. Design of photoluminescent bis-heterocycles, aided by density functional theory calculations, was performed with synthetic yields up to 98%. The photophysical properties of the products accessed via this strategy were part of a comprehensive study that led to broad emission spectra and quantum yields of up to 0.59. Mechanistic experiments confirmed bromoalkynes as competent intermediates, and a density functional theory investigation suggests a pathway involving initial oxidative addition into the cis C-Br bond of the gem-dihaloolefin.

Immobilization of Trifluoromethyl-Substituted Pyridine-Oxazoline Ligand and Its Application in Asymmetric Continuous Flow Synthesis of Benzosultams

This study presents an improved synthetic route to ligand (S)-4-(tert-butyl)-2-(5-(trifluoromethyl)pyridin-2-yl)-4,5-dihydrooxazole and its application as a highly active and enantioselective catalyst in the addition of arylboronic acids to cyclic N-sulfonylketimines. Immobilization of such a ligand was achieved using a commercially available starting material and a PS-PEG TentaGel S NH2 support, resulting in a stable heterogeneous catalyst. Although the anchored catalyst exhibited a slight reduction in enantioselectivity and a 4-fold decrease in reaction rate, it displayed remarkable stability, enabling 10 consecutive reaction cycles. Furthermore, the successful transition to a continuous flow system demonstrated even higher turnover numbers compared to batch arrangements. These findings provide valuable insights into the development of efficient flow reactors for continuous synthesis of benzosultams, further advancing the field of asymmetric catalysis.

A DFT study on the formation of heterocycles via iodine(iii)-promoted ring expansion reactions

Combined computational–experimental study to elucidate the mechanism of HTIB-promoted ring expansion reaction of bicyclic heterocycles bearing an exocyclic double bond.

Direct access to tetrasubstituted cyclopentenyl scaffolds through a diastereoselective isocyanide-based multicomponent reaction

An efficient strategy combining the stereocontrol of organocatalysis with the diversity-generating character of multicomponent reactions is described to produce structurally unique, tetrasubstituted cyclopentenyl frameworks. An asymmetric Michael addition-hemiacetalization between α-cyanoketones and α,β-unsaturated aliphatic aldehydes was performed for constructing cyclic hemiacetals, which were next employed as chiral bifunctional substrates in a new diastereoselective intramolecular isocyanide-based multicomponent reaction. This approach furnished a diversity of structurally complex compounds - including peptidomimetics and natural product hybrids in high stereoselectivity (up to >99% ee and up to >99 : 1 dr) and in moderate to high yields.

A late-stage diversification via Heck-Matsuda arylation: Straightforward synthesis and cytotoxic/antiproliferative profiling of novel aryl-labdane-type derivatives

In the current study a late-stage diversification of unactivated olefins labd-8(17)-en-15-oic acid (1a) and methyl labd-8(17)-en-15-oate (1b) via Heck-Matsuda arylation is described. The reaction provided straightforward and practical access to a series of novel aryl-labdane-type derivatives (HM adducts 3a-h) in moderate to good yields in a highly regio- and stereoselective manner at room temperature under air atmosphere. The cytotoxic activity of these compounds was investigated in vitro against three different human cell lines (THP-1, K562, MCF-7). Of these, HM adduct 3h showed a selective effect in all cancer cell lines tested and was selected for extended biological investigations in a leukemia cell line (K562), which demonstrated that the cytotoxic/antiproliferative activity observed in this compound might be mediated by induction of cell cycle arrest at the sub-G1 phase and by autophagy-induced cell death. Taken together, these findings indicate that further investigation into the anticancer activity against chronic myeloid leukemia from aryl-labdane-type derivatives may be fruitful.

Enantioselective Heck-Matsuda Reactions: From Curiosity to a Game-Changing Methodology

The enantioselective palladium-catalyzed Heck arylation of olefins using arenediazonium salts is one of the last features in the evolution of a synthetic method known as the Heck-Matsuda reaction. This personal account highlights the development of the enantioselective Heck-Matsuda reaction in its initial stages, the challenges faced along the way, and the interesting findings that opened new synthetic opportunities, mainly from our laboratory, featuring the Heck-Matsuda reaction as a central player in the synthesis of bioactive and functional molecules.

Relay-Heck Cross-Coupling Between Alkenyl Halides and Unsaturated Alcohols in the Synthesis of Open-Chain Analogues of Musk Odorant Vulcanolide

Unactivated alkenyl iodides and bromides underwent an unprecedented palladium-catalyzed relay-Heck cross-coupling with a whole range of alkenols of different chain lengths linking the alkene and the alcohol, affording unsaturated aldehydes and ketones in moderate to good yields. In contrast, alkenyl triflates were not suitable partners for this reaction. This method allowed the preparation of open-chain analogues of the musk odorant Vulcanolide, several of which retained key olfactory properties of the parent molecule.

Harnessing Noncovalent Interactions in Dual-Catalytic Enantioselective Heck-Matsuda Arylation

The use of more than one catalyst in one-pot reaction conditions has become a rapidly evolving protocol in the development of asymmetric catalysis. The lack of molecular insights on the mechanism and enantioselectivity in dual-catalytic reactions motivated the present study focusing on an important catalytic asymmetric Heck-Matsuda cross-coupling. A comprehensive density functional theory (M06 and B3LYP-D3) investigation of the coupling between a spirocyclic cyclopentene and 4-fluorophenyl diazonium species under a dual-catalytic condition involving Pd₂(dba)₃ (dba = trans, trans-dibenzylideneacetone) and chiral 2,2'-binaphthyl diamine (BINAM)-derived phosphoric acids (BDPA, 2,2'-binaphthyl diamine-derived phosphoric acids) is presented. Among various mechanistic possibilities examined, the pathway with explicit inclusion of the base (in situ generated sodium bicarbonate/sodium biphosphate) is found to be energetically more preferred over the analogous base-free routes. The chiral phosphate generated by the action of sodium carbonate on BDPA is found to remain associated with the reaction site as a counterion. The initial oxidative addition of Pd(0) to the aryl diazonium bond gives rise to a Pd-aryl intermediate, which then goes through the enantiocontrolling migratory insertion to the cyclic alkene, leading to an arylated cycloalkene intermediate. Insights on how a series of noncovalent interactions, such as C-H···O, C-H···N, C-H···F, C-H···π, lp···π, O-H···π, and C-F···π, in the enantiocontrolling transition state (TS) render the migration of the Pd-aryl to the si prochiral face of the cyclic alkene more preferred over that to the re face are utilized for modulating the enantioselectivity. Aided by molecular insights on the enantiocontrolling transition states, we predicted improved enantioselectivity from 37% to 89% by changes in the N-aryl substituents of the catalyst. Subsequent experiments in our laboratory offered very good agreement with the predicted enantioselectivities.

Enantioselective Synthesis of Phthalides and Isochromanones via Heck-Matsuda Arylation of Dihydrofurans

In this communication, the enantioselective synthesis of phthalides and isochromanones is described through a new palladium-catalyzed Heck-Matsuda arylation/NaBH₄ -reduction/lactonization sequence of 2,3- and 2,5-dihydrofurans in good overall yields and excellent enantioselectivities (up to 98:2 er). This expeditious synthesis of chiral Heck lactol intermediates allowed the diversification of the strategy to obtain medicinally relevant chiral lactones, amines, and olefins. The natural product 3-butylphthalide was obtained in three steps with an overall yield of 33 % yield in 98:2 er.

Recent Advances in Phthalan and Coumaran Chemistry

Oxygen-containing heterocycles are common in biologically active compounds. In particular, phthalan and coumaran cores are found in pharmaceuticals, organic electronics, and other useful medical and technological applications. Recent research has expanded the methods available for their synthesis. This Minireview presents recent advances in the chemistry of phthalans and coumarans, with the goal of overcoming synthetic challenges and facilitating the applications of phthalans and coumarans.

Palladium-Catalyzed Ortho C-H Arylation of Aniline Carbamates with Diazonium Salts under Mild Conditions: Expedient Synthesis of Carbazole Alkaloids

Despite the significant progress, C-H arylation with aryldiazonium salts is a major challenge because of the faster rate of oxidative addition compared to the C-H insertion, leading to a deleterious homocoupling product. Recently, this limitation has been overcome by merging a photoredox catalyst with transition-metal catalysts which proceeds through a distinct single electron-transfer mechanism. However, we have observed that the photoredox catalyst is not necessary for the C-H arylation of aniline rather chemical reactivity can be controlled by tuning the electronic nature of the substrate. We report, herein, a palladium-catalyzed C-H arylation of aniline carbamates with aryldiazonium salts under external oxidant, acid, base free conditions at room temperature. Mechanistic studies suggest that the present reaction proceeds through a directed electrophilic metalation pathway which is the slowest step. However, the oxidative addition may take place through either ionic (2e^-) or radical (1e^-) pathway to generate hypervalent Pd(IV) or Pd(III) intermediate, respectively. A facile reductive elimination from the hypervalent palladium complex furnishes the C-H arylation product under mild conditions. The carbamate directing group is easily removed from the product to obtain the corresponding ortho-arylated aniline, which is a precursor for plethora of carbazole alkaloids and other biologically active molecules. The reaction is scaled-up to gram scale to furnish the desired product in comparable yields. Finally, we have applied this C-H arylation methodology for the synthesis of series of carbazole alkaloids such as clausine V, clauszoline K, O-methoxymahanine, and O-methylmurrayamine-D.

Copper-Catalyzed Enantioselective Arylative Desymmetrization of Prochiral Cyclopentenes with Diaryliodonium Salts

A copper-catalyzed enantioselective arylative desymmetrization of prochiral cyclopentenes with diaryliodonium salts was developed. In the presence of a catalytic amount of a chiral copper-bisoxazoline complex, which was generated in situ, the reaction of 4-substituted or 4,4-disubstituted cyclopent-1-enes with diaryliodonium hexafluoroarsenates afforded the chiral arylated products in good yields with excellent enantioselectivity. A cyclohexyl-containing Box ligand was essential for the high enantioselectivity. Transformation of the enantiomerically enriched adducts into other chiral building blocks is also documented.

Regioselective and Stereoselective Heck-Matsuda Arylations of Trisubstituted Allylic Alkenols and Their Silyl and Methyl Ether Derivatives To Access Two Contiguous Stereogenic Centers: Expanding the Redox-Relay Process and Application in the Total Synthesis of meso-Hexestrol

Novel palladium-catalyzed redox-relay Heck arylation reactions of trisubstituted allylic alkenols were developed employing silyl and methyl ethers. The reactions proceeded under mild conditions in moderate to high yields in an excellent anti diastereoselectivity to form α,β-disubstituted methyl ketones containing two contiguous stereocenters. The new redox-relay arylations using silyl and methyl ethers of the starting alkenols demonstrate that the presence of a free hydroxyl group is not a sine qua non condition for an effective redox-relay process as previously thought. Deuterium-labeled alkenols 2-d-10a, 2-d-10b, and 2-d-10c permitted tracking the palladium-hydride reinsertion steps in the conversion of the starting free alcohols, silyl, and methyl ethers into the corresponding methyl ketone 3-d-11a, with >98% deuterium retention. Moreover, the synthetic potential of the method was demonstrated with a straightforward synthesis of the meso-hexestrol in 4 steps, in 41% overall yield from alkenol 10a.

Enantioselective Heck-Matsuda Arylations through Chiral Anion Phase-Transfer of Aryl Diazonium Salts

A mild, asymmetric Heck-Matsuda reaction of five-, six- and seven-membered ring alkenes and aryl diazonium salts is presented. High yields and enantioselectivities were achieved using Pd0 and chiral anion co-catalysts, the latter functioning as a chiral anion phase-transfer (CAPT) reagent. For certain substrate classes, the chiral anion catalysts were modulated to minimize the formation of undesired by-products. More specifically, BINAM-derived phosphoric acid catalysts were shown to prevent alkene isomerization in cyclopentene and cycloheptene starting materials. DFT(B3LYP-D3) computations revealed that increased product selectivity resulted from a chiral anion dependent lowering of the activation barrier for the desired pathway.

Regio- and Stereoselective Synthesis of Functionalized Cyclopentene Derivatives via Mizoroki-Heck Reactions

Pd(0)-catalyzed Mizoroki-Heck alkenylations and arylations of protected aminocyclopentenes, prepared in a few steps from Vince lactam, afforded functionalized cyclopentenes in high yields and stereoselectivities. DFT calculations were performed to rationalize the high diastereoselectivities. Functionalized cyclopentene products were transformed into valuable chiral building blocks, such as cyclic γ-amino acids and carbocyclic nucleoside precursors.

I2/TBHP-mediated oxidative coupling of ketones and toluene derivatives: a facile method for the preparation of α-benzoyloxy ketones

Synthesis of α-benzoyloxy ketones from ketones and toluenes using TBHP as the surrogate of toxic heavy-metal oxidants and the oxygen source of the incorporated oxygen.

An in Situ Generated Palladium on Aluminum Oxide: Applications in Gram-Scale Matsuda-Heck Reactions

In situ generated palladium on aluminum oxide provides an active catalytic system for Matsuda-Heck reactions in gram-scale. The novel catalyst proceeded through a significantly higher catalytic activity compared to the classical Pd/C system. Based on the high catalytic activity the first α,β,β-triarylation of methyl acrylate in good yields could be provided in one-step.

Enantioselective total synthesis of the highly selective sphingosine-1-receptor VPC01091 by the Heck desymmetrization of a non-activated cyclopentene-fused spiro-pyrrolidinone

The total synthesis of VPC01091 was accomplished in a concise manner, in a good overall yield of 40% from ethyl nitroacetate and 94% ee.