Diastereo- and enantioselective catalytic carbometallative aldol cycloreduction: tandem conjugate addition-aldol cyclization

A catalytic diastereo- and enantioselective method for tandem conjugate addition-aldol cyclization is described. This methodology enables the formation of five- and six-membered ring products from aromatic and aliphatic mono-enone mono-ketone precursors. Notably, in a single manipulation, three contiguous stereogenic centers are created with high levels of relative and absolute stereocontrol.     

Tandem conjugate reduction-aldol cyclization using Stryker's reagent

[see reaction]. Conjugate reduction by Stryker's reagent to form copper enolates, followed by intramolecular aldol cyclization, successfully generated five- and six-membered carbocycles in one pot efficiently. This tandem reaction is generally diastereoselective and provides good yields of the beta-hydroxyketones without any dehydration at low temperatures.     

Chemoselective conjugate addition of dimethylzinc-mediated ether and acetal radicals to alkylidenemalonates and asymmetric reactions

Cyclic and acyclic ether or acetal radicals were generated directly from ethers or acetals by the action of dimethylzinc-air, and their subsequent conjugate addition reaction with alkylidenemalonates afforded the corresponding conjugate adducts in reasonably high yields. The reaction with benzylidenemalonates bearing formyl and imino groups gave chemoselectively the conjugate addition products. The asymmetric reaction of bis(8-phenylmenthyl) benzylidenemalonate proceeded diastereoselectively to provide the adduct with 93:7 dr.     

Stereoselective synthesis of 2-methylenepyrrolizidines by tandem cyclization of N-propargylaminyl radicals

Tandem cyclization of N-propargylaminyl radicals, generated by N-chlorination of (E)-alk-4-enylamines 2a-d and 2f followed by treatment with tributyltin radical, afforded 2-methylenepyrrolizidines 3a-d and 3f in a highly stereoselective manner. A similar radical cyclization of (Z)-N-propargyl-1-methyl-5-phenylpent-4-enylamine (2e) gave pyrrolizidine 3b having the same stereochemistry as that obtained from the E isomer 2b.     

Tandem dual C(sp3)-H/C(sp2)-H functionalization: a radical cyclization of 2-isocyanobiphenyl with ether to 6-alkylated phenanthridine

<正>1 General experimental details 1H NMR and 13 C NMR spectra were measured on 400 MHz spectrometer, using CDCl3 as the solvent with tetramethylsilane(TMS) as the internal standard at room temperature. Chemical shifts(δ) are given in ppm relative to TMS, the coupling constants J are given in Hz. HRMS were recorded on a TOF LC/MS equipped with electrospray ionization(ESI) probe operating in positive or negative ion mode.     

Tandem cross-metathesis/hydrogenation/cyclization reactions by using compatible catalysts

[reaction: see text] A one-pot tandem cross-metathesis/hydrogenation/cyclization procedure was achieved at room temperature, under 1 atm of hydrogen, in the presence of a ruthenium catalyst and PtO(2) showing the compatibility of the two catalysts. This tandem reaction allows the synthesis of substituted lactones and lactols from acrylic acid and acrolein, respectively, in the presence of unsaturated alcohols.     

Stereoselective cyclization reactions of IBX-generated alkoxyamidyl radicals

In this paper, a method for the generation of alkoxyamidyl radicals is presented. These N-centered radicals can efficiently be formed starting from the corresponding acylated alkoxyamines using IBX as an oxidant. Stereoselective 5-exo and 6-exo reactions with these N-heteroatom-centered radicals leading to isoxazolidines and [1,2]oxazinanes are discussed. The N-O bond in the heterocycles can readily be cleaved with SmI(2) to provide N-acylated 1,3-amino alcohols.     

Pushing radical cyclization from regioselective to regiospecific: cyclization of amidyl radicals controlled by vinylic halogen substitution

Efficient and regiospecific 6-exo, 7-endo, 7-exo, and even 8-endo amidyl radical cyclizations can be accomplished by the direct reactions of unsaturated N-H amides if they bear a vinylic halogen (Cl, Br, I) substituent. This remarkable halogen-substitution effect could be rationalized in terms of lone pair-lone pair electron repulsion between the N radical and the vinylic halogen atom, in addition to the well-known steric and radical-stabilizing effect. [reaction: see text]     

Negishi-type coupling of bromoarenes with dimethylzinc

Treatment of bromoarenes with dimethylzinc in the presence of a palladium catalyst provides a high-yielding route to methylarenes. The process accommodates a wide range of aromatic substituents and, in the majority of cases, is free of side reactions.     

Probing the diastereoselectivity in the cyclization of cationic aminyl radicals

The diastereoselective intramolecular cyclization of cationic aminyl radicals into the corresponding pyrrolidines has been investigated. The selectivities obtained are good to excellent.     

Alkenylthioimidoyl radicals: competition between beta-scission and cyclization to dihydrothiophen-2-ylidene-amines

[reaction: see text] But-3-enylthioimidoyl radicals were shown by EPR spectroscopy and end product analysis to ring-close predominantly in the 5-exo mode with a rate constant of 2.4 x 10(4) s(-)(1) at 300 K to afford substituted dihydrothiophenylmethyl radicals. This ring closure was in competition with dissociation to but-3-enyl radicals and an isothiocyanate. The dissociation predominated at temperatures above ca. 300 K.     

Direct ESR detection and spin trapping of radicals generated by reaction of oxygen radicals with sulfonated poly(ether ether ketone) (SPEEK) membranes

The stability of membranes under the strong oxidizing conditions in fuel cells is one of the major challenges in the development of fuel cells based on proton exchange membranes (PEMs). This study is centered on the determination of the susceptibility to degradation of SPEEK membranes exposed to OH radicals, using both direct ESR and spin trapping with 5,5-dimethyl-1-pyrroline-1-oxide (DMPO). In order to achieve a complete picture on SPEEK degradation, two types of experiments were performed: 1. UV irradiation at 77 K of SPEEK membranes swollen by aqueous solutions of H₂O₂; 2. UV irradiation of SPEEK membranes swollen by aqueous solutions of H₂O₂ in the presence of DMPO as a spin trap. UV irradiation without oxygen of SPEEK at 77 K in acid or basic form in the presence of H₂O₂/H₂O produced phenoxyl radicals as the predominant radicals detected by direct ESR or spin trapping methods. At pH 4, the oxygen radicals produced phenyl radicals as the predominant species detected by spin trapping methods. The hydroperoxyl radical, as DMPO/OOH adduct, was detected only when the DMPO/OH adduct was absent. The appearance of phenyl and phenoxyl radicals provides the evidence that OH radicals react with the aromatic ring of SPEEK or leading to the scission of its ether bridge.     

Memory of chirality in the transannular cyclization of cyclodecenyl radicals

The transannular cyclization of an enantioenriched cyclodecenyl radical proceeds in a 5-exo fashion to produce scalemic bicyclo[5.3.0]decanes. This cyclization is notable because it demonstrates chirality transfer through conformational memory of an unstabilized secondary radical. [reaction: see text]     

Kinetics of the reaction of OH radicals with t-amyl methyl ether revisited

The kinetics of the reaction of OH radicals with t-amyl methyl ether (TAME) have been reinvestigated using both absolute (flash photolysis resonance fluorescence) and relative rate techniques. Relative rate experiments were conducted at 295 K in 99 kPa (740 torr) of synthetic air using ethyl t-butyl ether, cyclohexane, and di-isopropyl ether as reference compounds. Absolute rate experiments were performed over the temperature range 240–400 K at a total pressure of 4.7 kPa (35 torr) of argon. Rate constant determinations from both techniques are in good agreement and can be represented by k₁=(6.32 ± 0.72) × 10^?12 exp[(?40 ± 70)/T] cm³ molecule^?1 s^?1. Quoted errors represent 2σ from the least squares analysis and do not include any estimate of systematic errors. We show that results from the previous kinetic study of reaction (1) are in error due to the presence of a reactive impurity. Results are discussed in terms of the atmospheric chemistry of TAME. © 1993 John Wiley & Sons, Inc.     

Tandem conjugate addition-elimination reaction promoted by chiral pyrrolidinyl sulfonamide (CPS)

Chiral pyrrolidinyl sulfonamides have been found to promote the conjugate addition-elimination reaction between activated allylic bromides and 1,3-dicarbonyl compounds with high enantioselectivities and the highly functionalised products can be used to generate a variety of interesting enantiomerically pure compounds via simple transformations.     

Oxidation of dimethyl ether: Absolute rate constants for the self reaction of CH3OCH2 radicals,the reaction of CH3OCH2 radicals with O2, and the thermal decomposition of CH3OCH2 radicals

The rate constant for the reaction of CH₃OCH₂ radicals with O₂ (reaction (1)) and the self reaction of CH₃OCH₂ radicals (reaction (5)) were measured using pulse radiolysis coupled with time resolved UV absorption spectroscopy. k₁ was studied at 296K over the pressure range 0.025–1 bar and in the temperature range 296–473K at 18 bar total pressure. Reaction (1) is known to proceed through the following mechanism: CH₃OCH₂ + O₂ ↔ CH₃OCH₂O₂^# → CH₂OCH₂O₂H^# → 2HCHO + OH (k_prod) CH₃OCH₂ + O₂ ↔ CH₃OCH₂O₂^# + M → CH₃OCH₂O₂ + M (k_RO2) k = k_RO2 + k_prod, where k_RO2 is the rate constant for peroxy radical production and k_prod is the rate constant for formaldehyde production. The k₁ values obtained at 296K together with the available literature values for k₁ determined at low pressures were fitted using a modified Lindemann mechanism and the following parameters were obtained: k_RO2,0 = (9.4 ± 4.2) × 10⁻³⁰ cm⁶ molecule⁻² s⁻¹, k_RO2,∞ = (1.14 ± 0.04) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, and k_prod,0 = (6.0 ± 0.5) × 10⁻¹² cm³ molecule⁻¹ s⁻¹, where k_RO2,0 and k_RO2,∞ are the overall termolecular and bimolecular rate constants for formation of CH₃OCH₂O₂ radicals and k_prod,0 represents the bimolecular rate constant for the reaction of CH₃OCH₂ radicals with O₂ to yield formaldehyde in the limit of low pressure. k_RO2,∞ = (1.07 ± 0.08) × 10⁻¹¹ exp(−(46 ± 27)/T) cm³ molecule⁻¹ s⁻¹ was determined at 18 bar total pressure over the temperature range 296–473K. At 1 bar total pressure and 296K, k₅ = (4.1 ± 0.5) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ and at 18 bar total pressure over the temperature range 296–523K, k₅ = (4.7 ± 0.6) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. As a part of this study the decay rate of CH₃OCH₂ radicals was used to study the thermal decomposition of CH₃OCH₂ radicals in the temperature range 573–666K at 18 bar total pressure. The observed decay rates of CH₃OCH₂ radicals were consistent with the literature value of k₂ = 1.6 × 10¹³exp(−12800/T)s⁻¹. The results are discussed in the context of dimethyl ether as an alternative diesel fuel. © 1997 John Wiley & Sons, Inc.