Nicholas Reaction Research Articles

Substituent effects in the Nicholas epimerization of glycosides

AbstractThe Nicholas reaction is a proven and versatile tool in glycoside chemistry. Its application is less clear‐cut, if the intermediate Nicholas cation has two reactive sides that enable different reactions. Density functional theory (DFT) calculations are used to rationalize the observed difference in product yields for the Nicholas epimerization and substitution obtained from experiments with very similar glycosides. Substituents, which can increase the rigidity of the backbone, should be avoided in epimerization reactions as they can slow down reorientation of the intramolecular ion pair so that the substitution reaction dominates.

Nicholas Reactions of Alkynyl- and Alkenyltrifluoroborates

The Lewis acid mediated Nicholas reaction of potassium alkynyltrifluoroborates and propargyl acetate-hexacarbonyldicobalt complexes affords 1,4-diyne dicobalt hexacarbonyl complexes in good yields. The analogous Nicholas reactions of potassium alkenyltrifluoro­borates give 1,3-enyne dicobalt hexacarbonyl complexes in most cases, although the initial site of reaction can vary. Potassium vinyltrifluoroborate itself affords alkynylcyclopropane complexes.

2-Aminobenzenesulfonamide-Containing Cyclononyne as Adjustable Click Reagent for Strain-Promoted Azide-Alkyne Cycloaddition.

The synthesis of 2-aminobenzenesulfonamide-containing cyclononyne (ABSACN), starting from 2-nitrobenzenesulfonamide and but-2-yne-1,4-diol via Mitsunobu and Nicholas reactions, is described for the development of an adjustable alkyne reagent in click reactions. In a strain-promoted azide-alkyne cycloaddition (SPAAC) reaction, the reactivity of the alkyne is controlled by introducing various N-functionalities. The structure-reactivity relationship is found to be influenced by a transannular hydrogen bond between amino and sulfonyl groups.

ChemInform Abstract: Remote Functionalization in Nicholas Reactions of Vinylogous γ‐Carbonyl Cations.

AbstractNicholas reactions of enynoate‐Co2(CO)6 complexes are demonstrated to give ε‐functionalized carbonyl compounds with high selectivity.

Alkyne Ligation Handles: Propargylation of Hydroxyl, Sulfhydryl, Amino, and Carboxyl Groups via the Nicholas Reaction.

The Nicholas reaction has been applied to the installation of alkyne ligation handles. Acid-promoted propargylation of hydroxyl, sulfhydryl, amino, and carboxyl groups using dicobalt hexacarbonyl-stabilized propargylium ions is reported. This method is useful for introduction of propargyl groups into base-sensitive molecules, thereby expanding the toolbox of methods for the incorporation of alkynes for bio-orthogonal reactions. High-value molecules are used as the limiting reagent, and various propargylium ion precursors are compared.

Enantioconvergent Nucleophilic Substitution Reaction of Racemic Alkyne-Dicobalt Complex (Nicholas Reaction) Catalyzed by Chiral Brønsted Acid.

Catalytic enantioselective syntheses enable a practical approach to enantioenriched molecules. While most of these syntheses have been accomplished by reaction at the prochiral sp(2)-hybridized carbon atom, little attention has been paid to enantioselective nucleophilic substitution at the sp(3)-hybridized carbon atom. In particular, substitution at the chiral sp(3)-hybridized carbon atom of racemic electrophiles has been rarely exploited. To establish an unprecedented enantioselective substitution reaction of racemic electrophiles, enantioconvergent Nicholas reaction of an alkyne-dicobalt complex derived from racemic propargylic alcohol was developed using a chiral phosphoric acid catalyst. In the present enantioconvergent process, both enantiomers of the racemic alcohol were transformed efficiently to a variety of thioethers with high enantioselectivity. The key to achieving success is dynamic kinetic asymmetric transformation (DYKAT) of enantiomeric cationic intermediates generated via dehydroxylation of the starting racemic alcohol under the influence of the chiral phosphoric acid catalyst. The present fascinating DYKAT involves the efficient racemization of these enantiomeric intermediates and effective resolution of these enantiomers through utilization of the chiral conjugate base of the phosphoric acid.

Silver(I)-Catalyzed Addition of Phenols to Alkyne Cobalt Cluster Stabilized Carbocations.

A smooth catalytic method to use phenols as the nucleophilic partner in the Nicholas reaction has been developed. The method uses either Ag(I) or Au(I) catalysts with AgClO4 or AgBF4 as the most efficient catalysts tested. Neither additional additives nor cocatalysts were required and the formation of the corresponding phenol adducts occurred in excellent yields. The process has the single limitation of the inability of less nucleophilic phenols (4-nitrophenol) to generate the corresponding adducts. Additionally, the reaction is highly diastereoselective. DFT calculations allow a catalytic cycle to be proposed that involves trimetallic intermediates; the rate-determining step of the reaction is hydroxy-group elimination in a cobalt-silver trimetallic intermediate.

Surface-Guided Formation of an Organocobalt Complex.

Organocobalt complexes represent a versatile tool in organic synthesis as they are important intermediates in Pauson-Khand, Friedel-Crafts, and Nicholas reactions. Herein, a single-molecule-level investigation addressing the formation of an organocobalt complex at a solid-vacuum interface is reported. Deposition of 4,4'-(ethyne-1,2-diyl)dibenzonitrile and Co atoms on the Ag(111) surface followed by annealing resulted in genuine complexes in which single Co atoms laterally coordinated to two carbonitrile groups undergo organometallic bonding with the internal alkyne moiety of adjacent molecules. Alternative complexation scenarios involving fragmentation of the precursor were ruled out by complementary X-ray photoelectron spectroscopy. According to density functional theory analysis, the complexation with the alkyne moiety follows the Dewar-Chatt-Duncanson model for a two-electron-donor ligand where an alkyne-to-Co donation occurs together with a strong metal-to-alkyne back-donation.

Surface‐Guided Formation of an Organocobalt Complex

AbstractOrganocobalt complexes represent a versatile tool in organic synthesis as they are important intermediates in Pauson–Khand, Friedel–Crafts, and Nicholas reactions. Herein, a single‐molecule‐level investigation addressing the formation of an organocobalt complex at a solid–vacuum interface is reported. Deposition of 4,4′‐(ethyne‐1,2‐diyl)dibenzonitrile and Co atoms on the Ag(111) surface followed by annealing resulted in genuine complexes in which single Co atoms laterally coordinated to two carbonitrile groups undergo organometallic bonding with the internal alkyne moiety of adjacent molecules. Alternative complexation scenarios involving fragmentation of the precursor were ruled out by complementary X‐ray photoelectron spectroscopy. According to density functional theory analysis, the complexation with the alkyne moiety follows the Dewar–Chatt–Duncanson model for a two‐electron‐donor ligand where an alkyne‐to‐Co donation occurs together with a strong metal‐to‐alkyne back‐donation.

Remote Functionalization in Nicholas Reactions of Vinylogous γ-Carbonyl Cations

The Nicholas reactions of Co₂(CO)₆ complexes of allyl/propargyl alcohol–ynoates and ynone ethylene acetals give selective to exclusive reaction with nucleophiles at the site ε to the carbonyl or carbonyl equivalent. For nucleophiles giving modest selectivity in the above cases, the analogous Nicholas reactions may be accomplished with propargyl acetate–Co₂(CO)₆ complexes of conjugated enynoates, to give complex ε selectivity.

Cobalt-Catalysed Bond Formation Reactions; Part 2

This review summarises the cobalt-catalysed reactions of the years 2008 through 2014. Covered are addition reactions, such as the hydrogenation of double bonds, the addition of C–H bonds to double and triple bonds, C–H activation reactions, cobalt-catalysed cross-coupling reactions and selected additions of H–B and H–Si bonds to unsaturated starting materials. The cobalt-catalysed cycloadditions range from cyclopropanations, [2+2], Pauson–Khand reactions, [2+2+2], [4+2], [6+2] cycloadditions as well as benzannulations. Also covered are Nicholas reactions, cobalt-catalysed Jacobsen’s kinetic resolution of epoxides as well as selected miscellaneous cobalt-catalysed processes mainly for carbon–carbon bond formations. 1 Introduction 2 Addition Reactions 3 Homo- and Cross-Coupling Reactions 4 Jacobsen Kinetic Resolution of Epoxides 5 Nicholas Reactions 6 Cycloaddition Reactions 7 Miscellaneous 8 Conclusion

ChemInform Abstract: Domino Nicholas and Pauson—Khand Process Induced by Nitroarene Reduction.

AbstractThe enyne substrate (I) undergoes an unprecedented transformation into product (III) in which a nitroarene reduction step initiates an endocyclic Nicholas reaction followed by an intramolecular Pauson—Khand cyclization.

ChemInform Abstract: Synthesis of Tenuifolin Through Intramolecular Nicholas Reaction.

The synthesis of the Cinnamomum homosesquiterpenoid tenuifolin has been accomplished by way of an intramolecular Nicholas reaction of the [Co 2 (CO) 6 ] complex of an alkyne-substituted biaryl for construction of the seven-membered ring. The cyclization features the reaction of a nonactivated arene ring with the propargyldicobalt cation to give the dibenzocycloheptyne-Co 2 (CO) 6 .

Vinylogous Nicholas reactions in the synthesis of bi- and tricyclic cycloheptynedicobalt complexes.

The Lewis acid mediated intramolecular Nicholas reactions of allylic acetate enyne-Co2(CO)6 complexes afford cycloheptenyne-Co2(CO)6 complexes in three manifestations. Electron rich aryl substituted alkyne complexes give tricyclic 6,7,x-benzocycloheptenyne complexes, with x = 5, 6, or 7. Allylsilane substituted complexes afford exo methylene bicyclic x,7-cycloheptenyne complexes (x = 6,7). The allyl acetate function may also be replaced by a benzylic acetate, to afford dibenzocycloheptyne-Co2(CO)6 complexes. Following reductive complexation, the methodology may be applied to the synthesis of the icetexane diterpene carbon framework.

The Reversible Nicholas Reaction in the Synthesis of Highly Symmetric Natural‐Product‐Based Macrocycles

AbstractTwo different approaches to highly symmetric macrocycles by reversible Nicholas reactions have been developed. The first sequence uses a bis[Co2(CO)6] complex derived from a double propargylic alcohol supporting two natural product moieties. The reactions with BF3·OEt2 and different benzene dimethanols yielded either (1:1) or (2:2) adducts, depending essentially on the nature of the tether joining both Co clusters. Alternatively, the Nicholas reactions of Co‐complexed propargyl alcohol templates with one steroid and one monoterpene fragment as well as one aromatic terminus containing an alcohol moiety yielded the corresponding macrocycles derived from self‐dimerization. Both routes to macrocycles are complementary and efficiently produce sophisticated natural‐product‐containing structures.

Heteroatom-embedded medium-sized cycloalkynes: concise synthesis, structural analysis, and reactions.

A variety of medium-sized cycloalkynes were efficiently synthesized by the double Nicholas reaction of cobalt complex and bis(hetero)substituted acyclic compound. The alkyne moiety within the ring has a unique bent structure and high reactivity toward cycloaddition reactions. Furthermore, preparation of multifunctionalized alkynes was achieved by embedding the cycloalkyne within a peptide chain.

Heteroatom‐embedded Medium‐Sized Cycloalkynes: Concise Synthesis, Structural Analysis, and Reactions

AbstractA variety of medium‐sized cycloalkynes were efficiently synthesized by the double Nicholas reaction of cobalt complex and bis(hetero)substituted acyclic compound. The alkyne moiety within the ring has a unique bent structure and high reactivity toward cycloaddition reactions. Furthermore, preparation of multifunctionalized alkynes was achieved by embedding the cycloalkyne within a peptide chain.

ChemInform Abstract: A New Route to Dithia‐ and Thiaoxacyclooctynes via Nicholas Reaction.

ChemInform Abstract: A New Route to Dithia‐ and Thiaoxacyclooctynes via Nicholas Reaction.

Application of the Nicholas reaction to the synthesis of dicobalt hexacarbonyl complexed diyne ethers

Addition of propargylic or homopropargylic ynols to dicobalt hexacarbonyl propargylium cations gave the expected ether products of the Nicholas reaction, with the exception of 4-phenyl-3-butyn-1-ol which gave a dihydrofuran due to propargylium cation promoted 5-endo ring-closure. Use of chiral dicobalt hexacarbonyl complexed propargylic ynols as both propargylium cation precursor and nucleophile gave racemic doubly complexed disymmetric diyne ethers, each as a single diastereoisomer. Addition of dicobalt hexacarbonyl complexed 1-phenyl-1-butyne-3-ene to the propargylium cation derived from dicobalt hexacarbonyl complexed 1-phenyl-1-butyn-3-ol, followed by the addition of a nucleophile (NuH), gave double complexed 1,7-diphenyl-3-Nu-5-methylhepta-1,6-diynes (d.r. > 10:1).

Synthesis of Tenuifolin through Intramolecular Nicholas Reaction

The synthesis of the Cinnamomum homosesquiterpenoid tenuifolin has been accomplished by way of an intramolecular Nicholas reaction of the [Co 2 (CO) 6 ] complex of an alkyne-substituted biaryl for construction of the seven-membered ring. The cyclization features the reaction of a nonactivated arene ring with the propargyldicobalt cation to give the dibenzocycloheptyne-Co 2 (CO) 6 .