Initial Nucleophilic Substitution Research Articles

Experimental and Theoretical Analysis of the Thiol-Promoted Fragmentation of 2-Halo-3-tosyl-oxanorbornadienes.

2-Halo-3-tosyl-oxanorbornadienes are able to accept two thiol molecules through an initial nucleophilic substitution, giving isolable oxabicyclic thiovinyl sulfones that, subsequently, can react with a second thiol molecule via thio-Michael addition. The resulting oxanorbornenic thioketals undergo retro-Diels-Alder (rDA) fragmentation to release a furan derivative and a ketene S,S-acetal. The substitution pattern of the oxanorbornadienic skeleton influences the rate of the rDA through electronic and steric factors examined by quantum mechanical calculations.

Diastereoselective Synthesis of Coumarin‐Based Fused Heterocycles via Intramolecular Diels‐Alder and 1,3‐Dipolar Cycloaddition Reactions

AbstractIn this work, a two‐steps strategy for the synthesis of novel coumarin‐based fused heterocycles is presented. The strategy involved i) initial nucleophilic substitution / intramolecular Diels‐Alder reaction of furfurylamine with 4‐chloro‐3‐vinylcoumarins followed by ii) base‐mediated 1,3‐dipolar cycloaddition reaction of resultant coumarin‐based bicyclic products with dibromoformaldoxime. The methodology provides a straightforward access to compounds containing the coumarin, isoquinoline, and isoxazole motifs in a single molecular entity in satisfactory yields. The reaction proceeded with remarkable diastereoselectivity that leads to the formation of five new bonds and six new stereogenic centres. Furthermore, the observed diastereoselectivity was analysed with the help of DFT calculations to substantiate the thermodynamic stability of the obtained products.

Realizing the Trifunctional Potential of Alkyl 4-Chloro-2-diazo-3-oxobutanoates: Convenient Assembly of 6,7-Dihydro-4H-[1,2,3]triazolo[5,1-c][1,4]thiazine Core

The first example of exploiting the trifunctional character of alkyl 4-chloro-2-diazo-3-oxobutanoates in the reactions with vicinal N,S-bis-nucleophiles leading to the formation of bicyclic 6,7-dihydro-4H-[1,2,3]triazolo[5,1-c][1,4]thiazines is presented. The key to the successful realization of the atom-economical synthetic strategy is the initial S N 2 event, which facilitates the subsequent domino 1,2,3-triazole formation via the Wolff reaction. Equally important is the choice of sodium acetate as the base: acetic acid formed in the course of the initial nucleophilic substitution acts as an efficient catalyst for the Wolff reaction, which suppresses the competing, unwanted fragmentation path (observed when other bases are used).

Dipyrido[1,2‐b:3′,4′‐e][1,2,4]triazine Scaffolds from Pentafluoropyridine

Annelation reaction between pentafluoropyridine and diaminodihydro substituted pyridine derivatives gave dipyrido[1,2‐b:3′,4′‐e][1,2,4]triazine systems as major products arising from the initial nucleophilic substitution of more nucleophilic N‐amino group of pyridone ring at 4‐position of the pyridine ring followed by intramolecular cyclization at 3‐position of pyridine ring. Also, 6‐amino‐2‐oxo‐1‐((perfluoropyridin‐4‐yl)amino)‐4‐aryl‐1,2‐dihydropyridine‐3,5‐dicarbonitrile derivatives were obtained as side products. The structures of all the compounds were confirmed by IR, 1H NMR, 13C NMR, and 19F NMR spectroscopy as well as elemental analysis.

Metal-Free, Brønsted Acid-Catalyzed Formal [3+2] Annulation of Quinone Monoacetals with 2-Naphthols.

An operationally simple and metal-free cross-coupling of quinone monoacetals (QMAs) with 2-naphthols catalyzed by triflic acid is reported. This formal [3+2] annulation allowed the synthesis of diverse naphtho[2,1-b]benzofuran derivatives in moderate to good yields. Preliminary mechanistic studies reveal the initial nucleophilic substitution of QMAs with 2-naphthols in preference to the mixed acetal formation and subsequent [3,3] sigmatropic rearrangement.

Diverse Reactivity of an Electrophilic Phosphasilene towards Anionic Nucleophiles: Substitution or Metal-Amino Exchange.

The reaction of MesLi (Mes=2,4,6-trimethylphenyl) with the electrophilic phosphasilene R2 (NMe2 )Si-RSi=PNMe2 (2, R=Tip=2,4,6-triisopropylphenyl) cleanly affords R2 (NMe2 )Si-RSi=PMes and thus provides the first example of a substitution reaction at an unperturbed Si=P bond. In toluene, the reaction of 2 with lithium disilenide, R2 Si=Si(R)Li (1), apparently proceeds via an initial nucleophilic substitution step as well (as suggested by DFT calculations), but affords a saturated bicyclo[1.1.0]butane analogue as the final product, which was further characterized as its Fe(CO)4 complex. In contrast, in 1,2-dimethoxyethane the reaction of 1 with 2 results in an unprecedented metal-amino exchange reaction.

ChemInform Abstract: 1,2‐Disilabicyclo[1.1.1]pentan‐4‐ones from a Disilenide and Acryloyl Chlorides.

Reactions of a disilenide, a disila analogue of vinyl lithiums, with two α,β-unsaturated carboxylic acid chlorides yield the first examples of compounds with a bicyclo[1.1.1]pentane scaffold that feature one carbon and one silicon atom at bridgehead positions; the formation of 1,2-disilabicyclo[1.1.1]pentan-4-ones is explained with initial nucleophilic substitution at the carbonyl C-atom by the nucleophilic disilenide and subsequent intramolecular [2+2]-cycloaddition of the Si=Si to the C=C double bond in a highly diastereoselective fashion.

1,2-Disilabicyclo[1.1.1]pentan-4-ones from a Disilenide and Acryloyl Chlorides

Reactions of a disilenide, a disila analogue of vinyl lithiums, with two α,β-unsaturated carboxylic acid chlorides yield the first examples of compounds with a bicyclo[1.1.1]pentane scaffold that feature one carbon and one silicon atom at bridgehead positions; the formation of 1,2-disilabicyclo[1.1.1]pentan-4-ones is explained with initial nucleophilic substitution at the carbonyl C-atom by the nucleophilic disilenide and subsequent intramolecular [2+2]-cycloaddition of the Si=Si to the C=C double bond in a highly diastereoselective fashion.

ChemInform Abstract: Reaction of 1,2-Dioxetanes with Heteroatom Nucleophiles: Adduct Formation by Nucleophilic Attack at the Peroxide Bond.

The reactions of 3,3-disubstituted 1,2-dioxetanes 1 with numerous heteroatom nucleophiles, e.g., R 2 NH, R 3 N, RSH R 2 S, CN - , SCN - , Br - , Cl - , OH - , and O 2 .- , were investigated. Initial nucleophilic substitution at the sterically less hindered site of the dioxetane peroxide bond leads to addition, deoxygenation, and fragementation products. The observed S N 2 chemistry was substantiated with the dioxetane 1c because bromide ion displacement by the proximate alkoxide ion site afforded epoxide products

Imidazopyridine and pyrimidinopyridine systems from perfluorinated pyridine derivatives

Annelation of pentafluoropyridine via an intramolecular nucleophilic aromatic substitution process with benzamidine gave an imidazopyridine system in high yield in a two step process whilst alkyl amidines gave 4-aminotetrafluoropyridine by a competing elimination reaction. 4-Phenylsulfonyl tetrafluoropyridine reacts with amidines to give the corresponding imidazo[4,5- b]pyridine systems. In contrast, 4-cyanotetrafluoropyridine gave a [6,6]-fused pyrimidinopyridine system arising from initial nucleophilic substitution at the C-3 position of the pyridine ring followed by intramolecular cyclization onto the pendant cyano group. The systems prepared by this annelation methodology further demonstrate the utility of perfluorinated heterocyclic substrates for the synthesis of heterocyclic scaffolds that possess multiple functionality and have potential applications in the drug discovery arena.

An Improved Understanding of the Reaction of Bis(bromomethyl)quinoxaline 1-N-Oxides with Amines Using Substituent Effects

The reaction of bis(bromomethyl)quinoxaline N-oxides with amines is interesting from a reaction mechanism perspective and due to the reported biological activity of compounds in this general class. The complex mechanism of this reaction (particularly in the case of primary amines) is complicated further when C6 or C7 substituted mono-N-oxides are considered. In this study, the synthesis and subsequent characterization of a series of 2,3-bis(bromomethyl)quinoxaline 1-N-oxides is reported. Experimental and computational evidence is used to show that the observed product ratios from the reaction with diethylamine reflect the influence of both the C6/C7 substituent and the N-oxide functional group on the initial nucleophilic substitution reaction.

The Loss of Carbon Dioxide from Activated Perbenzoate Anions in the Gas Phase: Unimolecular Rearrangement via Epoxidation of the Benzene Ring

The unimolecular reactivities of a range of perbenzoate anions (X-C6H5CO3-), including the perbenzoate anion itself (X = H), nitroperbenzoates (X = para-, meta-, ortho-NO2), and methoxyperbenzoates (X = para-, meta-OCH3) were investigated in the gas phase by electrospray ionization tandem mass spectrometry. The collision-induced dissociation mass spectra of these compounds reveal product ions consistent with a major loss of carbon dioxide requiring unimolecular rearrangement of the perbenzoate anion prior to fragmentation. Isotopic labeling of the perbenzoate anion supports rearrangement via an initial nucleophilic aromatic substitution at the ortho carbon of the benzene ring, while data from substituted perbenzoates indicate that nucleophilic attack at the ipso carbon can be induced in the presence of electron-withdrawing moieties at the ortho and para positions. Electronic structure calculations carried out at the B3LYP/6-311++G(d,p) level of theory reveal two competing reaction pathways for decarboxylation of perbenzoate anions via initial nucleophilic substitution at the ortho and ipso positions, respectively. Somewhat surprisingly, however, the computational data indicate that the reaction proceeds in both instances via epoxidation of the benzene ring with decarboxylation resulting--at least initially--in the formation of oxepin or benzene oxide anions rather than the energetically favored phenoxide anion. As such, this novel rearrangement of perbenzoate anions provides an intriguing new pathway for epoxidation of the usually inert benzene ring.

Catalytic Mechanism of Class B2 Metallo-β-lactamase

The initial nucleophilic substitution step of biapenem hydrolysis catalyzed by a subclass B2 metallo-beta-lactamase (CphA from Aeromonas hydrophila) is investigated using hybrid quantum mechanical/molecular mechanical methods and density functional theory. We focused on a recently proposed catalytic mechanism that involves a non-metal-binding water nucleophile in the active site of the monozinc CphA. Both theoretical models identified a single transition state featuring nearly concomitant nucleophilic addition and elimination steps, and the activation free energy from the potential of mean force calculations was estimated to be approximately 14 kcal/mol. The theoretical results also identified the general base for activating the water nucleophile to be the metal-binding Asp-120 rather than His-118, as suggested earlier. The protonation of Asp-120 leads to cleavage of the O(delta2)-Zn coordination bond, whereas the negatively charged nitrogen leaving group resulting from the ring opening replaces Asp-120 as the fourth ligand of the sole zinc ion. The electrophilic catalysis by the metal ion provides sufficient stabilization for the leaving group to avoid a tetrahedral intermediate. The theoretical studies provided detailed insights into the catalytic strategy of this unique metallo-beta-lactamase.

Solid-phase synthesis of quinoxalines on SynPhase™ Lanterns

A convenient and straightforward solid-phase synthesis of quinoxalines is described. A polymer-bound o-phenylenediamine was reacted with α-bromoketones in DMF at 60°C to give quinoxalines in good purity and yield after TFA cleavage. The quinoxalines were presumably formed via an initial nucleophilic substitution, followed by subsequent cyclization–oxidation. A small library of ten quinoxalines was prepared on SynPhase™ Lanterns using this simple one-pot procedure.

Selective transformation of α,α-dibromomethyl ketones into α-monosulfenylated ketones

When α,α-dibromomethyl ketones are treated with sodium thiolates only the α-monosulfenylated ketones are formed. Evidence is put forward that the reaction mechanism proceeds by an initial nucleophilic substitution of one bromo atom and reduction by single electron transfer (SET) - hydrogen atom abstraction of the second bromo atom.

Reaction of 1,2-dioxetanes with heteroatom nucleophiles: adduct formation by nucleophilic attack at the peroxide bond

The reactions of 3,3-disubstituted 1,2-dioxetanes 1 with numerous heteroatom nucleophiles, e.g., R 2 NH, R 3 N, RSH R 2 S, CN - , SCN - , Br - , Cl - , OH - , and O 2 .- , were investigated. Initial nucleophilic substitution at the sterically less hindered site of the dioxetane peroxide bond leads to addition, deoxygenation, and fragementation products. The observed S N 2 chemistry was substantiated with the dioxetane 1c because bromide ion displacement by the proximate alkoxide ion site afforded epoxide products

Nucleophilic substitution at bivalent sulphur. Part IV. A mechanistic study of the base-catalysed hydrolysis of derivatives of 2-nitro-4-trifluoromethylbenzenesulphenic acid using 19F nuclear magnetic resonance spectroscopy

The various intermediates involved in the base-catalysed hydrolysis of bis-2-nitro-4-trifluoromethylphenyl disulphide were detected using the 19F n.m.r. chemical shift of the singlet from the 4-trifluoromethyl group. The reaction involves an initial nucleophilic substitution at sulphur to give the sulphenate anion and a thiolate ion. A second intermediate, presumably the thiolsulphinate, in which the aryl groups have different environments is formed by condensation of the sulphenate anion. The second intermediate gives the corresponding sulphinate and thence sulphonate ions. Surprisingly 2-amino-4-trifluoromethylbenzenesulphonate ion was also detected and isolated. A mechanism for its formation is proposed. The corresponding ethyl arenesulphenate gave similar results but with a higher percentage of the second intermediate. Hydrolysis of the sulphenyl chloride and bromide was much faster at all steps of the reaction and gave disulphide and sulphinate anion as the initial products. The sulphenyl thiocyanate reacted in a similar manner to the sulphenyl halides but gave a substantially higher yield of thiolate ion which is attributed to the simultaneous reaction of hydroxide ion at the thiocyanate sulphur. The aryl thiocyanate did not undergo substitution at sulphur and gave only the thiolate ion.

Reaction of α-Halosulfoxides with Amines

Abstract When optically active bromomethyl p-toly sulfoxide was treated with sodium methylmercaptide or sodium ethoxide, optically active α-substituted methyl p-toly sulfoxides were obtained in good yields. By contrast, reaction with secondary amines produced achiral p-toluenesulfenamides, methylenediamines, and amine hydrobromides in nearly quantitative yields. The reaction with an amine is presumed to proceed through the initial nucleophilic substitution at α-carbon of the sulfoxide to form aminomethyl p-toly sulfoxide, followed by rearrangement to the corresponding sulfenate, and subsequent attack of the amine at the sulfur atom of the sulfenate to form the final product. Meanwhile, a facile interconversion of alkoxymethyl p-toly sulfoxide and corresponding sulfenate was also observed.