Heterogeneous reactions of epoxides in acidic media.

Epoxides have recently been identified as important intermediates in the gas phase oxidation of hydrocarbons, and their hydrolysis products have been observed in ambient aerosols. To evaluate the role of epoxides in the formation of secondary organic aerosols (SOA), the kinetics and mechanism of heterogeneous reactions of two model epoxides, isoprene oxide and α-pinene oxide, with sulfuric acid, ammonium bisulfate, and ammonium sulfate have been investigated using complementary experimental techniques. Kinetic experiments using a fast flow reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS) show a fast irreversible loss of the epoxides with the uptake coefficients (γ) of (1.7 ± 0.1) × 10(-2) and (4.6 ± 0.3) × 10(-2) for isoprene oxide and α-pinene oxide, respectively, for 90 wt % H(2)SO(4) and at room temperature. Experiments using attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) reveal that diols are the major products in ammonium bisulfate and dilute H(2)SO(4) (<25 wt %) solutions for both epoxides. In concentrated H(2)SO(4) (>65 wt %), acetals are formed from isoprene oxide, whereas organosulfates are produced from α-pinene oxide. The reaction of the epoxides with ammonium sulfate is slow and no products are observed. The epoxide reactions using bulk samples and Nuclear Magnetic Resonance (NMR) spectroscopy reveal the presence of diols as the major products for isoprene oxide, accompanied by aldehyde formation. For α-pinene oxide, organosulfate formation is observed with a yield increasing with the acidity. Large yields of organosulfates in all NMR experiments with α-pinene oxide are attributed to the kinetic isotope effect (KIE) from the use of deuterated sulfuric acid and water. Our results suggest that acid-catalyzed hydrolysis of epoxides results in the formation of a wide range of products, and some of the products have low volatility and contribute to SOA growth under ambient conditions prevailing in the urban atmosphere.

Synthesis of substituted acetylenic epoxides followed by indium-catalyzed rearrangement to 2,3,5-trisubstituted furans.

Syntheses of various aromatic and aliphatic 2,3,5-trisubstituted furans from acetylenic epoxides are described. These epoxides are directly prepared by nucleophilic ring closure of propargylic alkoxides generated by lithium acetylide addition to α-haloketones.

Palladium-catalyzed alkynylation of secondary α-bromo carbonyl compounds via Stille coupling.

A catalytic intermolecular cross-coupling reaction that couples secondary α-bromo carbonyl compounds with alkynylstannanes to form secondary alkynyl carbonyl compounds via palladium catalysis employing the XPhos ligand is described.

Chromium-catalyzed homoaldol equivalent reaction employing a nucleophilic propenyl acetate.

A scalable, highly regioselective chromium-catalyzed homoaldol equivalent reaction employing 3-bromopropenyl acetate as a masked homoenolate nucleophile in additions to aromatic, aliphatic, and alpha,beta-unsaturated aldehydes under mild Cr/Mn redox conditions in good to excellent yields is reported. The resulting vinyl acetate-containing adducts are easily hydrolyzed with mild base to provide formal homoaldol adducts, or transformed to other more functionalized products by stereoselective transformations including epoxidation and cyclopropanation.

Isomer-selective study of the OH-initiated oxidation of isoprene in the presence of O(2) and NO: 2. the major OH addition channel.

We report the first isomeric-selective study of the dominant isomeric pathway in the OH-initiated oxidation of isoprene in the presence of O2 and NO using the laser photolysis-laser induced fluorescence (LP-LIF) technique. The photolysis of monodeuterated/nondeuterated 2-iodo-2-methylbut-3-en-1-ol results exclusively in the dominant OH-isoprene addition product, providing important insight into the oxidation mechanism. On the basis of kinetic analysis of OH cycling experiments, we have determined the rate constant for O2 addition to the hydroxyalkyl radical to be 1.0(-0.5)+1.7 x 10(-12) cm3 s(-1), and we find a value of 8.1-2.3+3.4 x 10(-12) cm3 s(-1) for the overall reaction rate constant of the resulting hydroxyperoxy radical with NO. We also report the first clear experimental evidence of the (E) form of the delta-hydroxyalkoxy channel through isotopic labeling experiments and quantify its branching ratio to be (10 +/- 3)%. This puts a rigorous upper limit on the branching of the (E)-delta-hydroxyalkoxy radical channel. Since our measured isomeric-selective rate constants for the dominant outer channel in OH-initiated isoprene chemistry are similar to the overall rate constants derived from nonisomeric kinetics, we predict that the remaining outer addition channel will have similar reactivity.

OH radical initiated oxidation of 1,3-butadiene: isomeric selective study of the dominant addition channel.

We report the first isomeric selective kinetic study of the dominant isomeric pathway in the OH initiated oxidation of 1,3-butadiene in the presence of O(2) and NO using the laser photolysis-laser induced fluorescence (LP-LIF) technique. The photodissociation of the precursor 2-iodo-but-3-en-1-ol results exclusively in the dominant OH-butadiene addition product, permitting important insight into the OH initiated oxidation mechanism. On the basis of analysis of the time dependent OH/OD signals, we have determined a rate constant for O2 addition to the hydroxyalkyl radical of 7.0(-3.0)(+7.0) x 10(-13) cm(3) s(-1), and we find a value of 1.5(-0.6)(+1.0) x 10(-11) cm(3) s(-1) for the overall reaction rate constant of the hydroxy peroxy radical with NO. We also report the first clear experimental evidence of the (E) form of the delta-hydroxyalkoxy channel through isotopic labeling experiments and provide an upper bound of 13 +/- 5% to its branching ratio. This species provides a mechanistic pathway for the formation of 4-hydroxy-2-butenal, which has been identified as a first generation end product. This isomeric selective kinetic study, together with a previous study on the minor channel of the 1,3-butadiene oxidation, yields a comprehensive picture of butadiene oxidation under high NOx conditions relevant to most regions in the continental US.

Acceleration of the Morita-Baylis-Hillman reaction by a simple mixed catalyst system.

By using a catalytic amount of 4-dimethylaminopyridine (DMAP) as a nucleophile in the presence of an equal amount of tetramethylethylenediamine (TMEDA) and MgI(2), Morita-Baylis-Hillman adducts can be obtained in good to excellent yields from various aromatic and aliphatic aldehydes and cyclic enones/enoates at room temperature after convenient reaction times.

Conversion of nitrosobenzenes to isoxazolidines: an efficient cascade process utilizing reactive nitrone intermediates.

Reactive nitrones can be generated directly in situ by an unusual reaction of nitrosobenzene with styrene.

A highly active and selective palladium pincer catalyst for the formation of α-aryl ketones via cross-coupling.

Several air and moisture stable Pd(II) pincer complexes were synthesized via oxidative addition of Pd(0) to novel PheBox pincer ligand precursors. Low loadings (1 mol%) of the Pd complex [t-BuPhebox-Me(2)]PdBr are capable of efficiently promoting the selective α-monoarylation of a variety of ketones with numerous aryl bromides in only 1 h at 70 °C with 82-99% yields.

MgI2-accelerated enantioselective Morita-Baylis-Hillman reactions of cyclopentenone utilizing a chiral DMAP catalyst.

Fu's planar chiral DMAP catalyst efficiently promotes the asymmetric Morita-Baylis-Hillman reactions of cyclopentenone with a variety of aldehydes in the presence of MgI(2) as a cocatalyst.

Efficient, direct alpha-methylenation of carbonyls mediated by diisopropylammonium trifluoroacetate.

A very efficient method for the direct alpha-methylenation of carbonyl compounds, with yields up to 99%, utilizing paraformaldehyde, diisopropylammonium trifluoroacetate, and catalytic acid or base, is presented.

Synthesis of the antifungal macrolide antibiotic (+)-roxaticin.

The total synthesis of the antifungal macrolide antibiotic roxaticin has been accomplished. The synthesis relies principally on aldol and directed reduction steps to construct the extended 1,3-polyol array present in the natural product. Three principal nonpolyene containing fragments were assembled and then coupled using Julia olefination and methyl ketone aldol addition reactions. A series of functionalization reactions incorporated the sensitive polyene and provided the protected roxaticin seco-acid, which was lactonized in good yield. Acidic deprotection completed this convergent synthesis of roxaticin.

1,5-asymmetric induction in boron-mediated beta-alkoxy methyl ketone aldol addition reactions.

This article presents studies that illustrate beta-alkoxy methyl ketone-derived boron enolates undergo diastereoselective aldol addition to afford the 1,5-anti diol relationship. The stereochemical outcome of this reaction is documented to be general for a variety of beta-alkoxy methyl ketone analogues and aldehyde partners. The double stereodifferentiating reactions of these enolates with chiral beta-alkoxy aldehydes have also been investigated in conjunction with the possibility of controlling the absolute stereochemistry of the aldol process. With the proper selection of reaction conditions, the proximal alkoxy substituent on either the aldehyde (1,3-induction) or the enolate fragment (1,5-induction) can be employed to control facial selectivity of the aldol addition. Selection of a boron enolate ensures dominant 1,5-anti induction from the beta-alkoxy methyl ketone-derived enolate partner while negating any influence of the beta-alkoxy aldehyde substituent. Conversely, if stereochemical control from the beta-alkoxy aldehyde is desired, a Lewis acid-catalyzed enolsilane addition ensures dominant 1,3-induction from the aldehyde beta-oxygen substituent.