Photodissociation dynamics of methyl vinyl ketone oxide: A four-carbon unsaturated Criegee intermediate from isoprene ozonolysis.

The electronic spectrum of methyl vinyl ketone oxide (MVK-oxide), a four-carbon Criegee intermediate derived from isoprene ozonolysis, is examined on its second π* ← π transition, involving primarily the vinyl group, at UV wavelengths (λ) below 300 nm. A broad and unstructured spectrum is obtained by a UV-induced ground state depletion method with photoionization detection on the parent mass (m/z 86). Electronic excitation of MVK-oxide results in dissociation to O (1D) products that are characterized using velocity map imaging. Electronic excitation of MVK-oxide on the first π* ← π transition associated primarily with the carbonyl oxide group at λ > 300 nm results in a prompt dissociation and yields broad total kinetic energy release (TKER) and anisotropic angular distributions for the O (1D) + methyl vinyl ketone products. By contrast, electronic excitation at λ ≤ 300 nm results in bimodal TKER and angular distributions, indicating two distinct dissociation pathways to O (1D) products. One pathway is analogous to that at λ > 300 nm, while the second pathway results in very low TKER and isotropic angular distributions indicative of internal conversion to the ground electronic state and statistical unimolecular dissociation.

Functionalized Hydroperoxide Formation from the Reaction of Methacrolein-Oxide, an Isoprene-Derived Criegee Intermediate, with Formic Acid: Experiment and Theory.

Methacrolein oxide (MACR-oxide) is a four-carbon, resonance-stabilized Criegee intermediate produced from isoprene ozonolysis, yet its reactivity is not well understood. This study identifies the functionalized hydroperoxide species, 1-hydroperoxy-2-methylallyl formate (HPMAF), generated from the reaction of MACR-oxide with formic acid using multiplexed photoionization mass spectrometry (MPIMS, 298 K = 25 °C, 10 torr = 13.3 hPa). Electronic structure calculations indicate the reaction proceeds via an energetically favorable 1,4-addition mechanism. The formation of HPMAF is observed by the rapid appearance of a fragment ion at m/z 99, consistent with the proposed mechanism and characteristic loss of HO2 upon photoionization of functional hydroperoxides. The identification of HPMAF is confirmed by comparison of the appearance energy of the fragment ion with theoretical predictions of its photoionization threshold. The results are compared to analogous studies on the reaction of formic acid with methyl vinyl ketone oxide (MVK-oxide), the other four-carbon Criegee intermediate in isoprene ozonolysis.

Formic acid catalyzed isomerization and adduct formation of an isoprene-derived Criegee intermediate: experiment and theory.

Isoprene is the most abundant non-methane hydrocarbon emitted into the Earth's atmosphere. Ozonolysis is an important atmospheric sink for isoprene, which generates reactive carbonyl oxide species (R1R2C[double bond, length as m-dash]O+O-) known as Criegee intermediates. This study focuses on characterizing the catalyzed isomerization and adduct formation pathways for the reaction between formic acid and methyl vinyl ketone oxide (MVK-oxide), a four-carbon unsaturated Criegee intermediate generated from isoprene ozonolysis. syn-MVK-oxide undergoes intramolecular 1,4 H-atom transfer to form a substituted vinyl hydroperoxide intermediate, 2-hydroperoxybuta-1,3-diene (HPBD), which subsequently decomposes to hydroxyl and vinoxylic radical products. Here, we report direct observation of HPBD generated by formic acid catalyzed isomerization of MVK-oxide under thermal conditions (298 K, 10 torr) using multiplexed photoionization mass spectrometry. The acid catalyzed isomerization of MVK-oxide proceeds by a double hydrogen-bonded interaction followed by a concerted H-atom transfer via submerged barriers to produce HPBD and regenerate formic acid. The analogous isomerization pathway catalyzed with deuterated formic acid (D2-formic acid) enables migration of a D atom to yield partially deuterated HPBD (DPBD), which is identified by its distinct mass (m/z 87) and photoionization threshold. In addition, bimolecular reaction of MVK-oxide with D2-formic acid forms a functionalized hydroperoxide adduct, which is the dominant product channel, and is compared to a previous bimolecular reaction study with normal formic acid. Complementary high-level theoretical calculations are performed to further investigate the reaction pathways and kinetics.

Direct kinetic measurements and theoretical predictions of an isoprene-derived Criegee intermediate.

Isoprene has the highest emission into Earth's atmosphere of any nonmethane hydrocarbon. Atmospheric processing of alkenes, including isoprene, via ozonolysis leads to the formation of zwitterionic reactive intermediates, known as Criegee intermediates (CIs). Direct studies have revealed that reactions involving simple CIs can significantly impact the tropospheric oxidizing capacity, enhance particulate formation, and degrade local air quality. Methyl vinyl ketone oxide (MVK-oxide) is a four-carbon, asymmetric, resonance-stabilized CI, produced with 21 to 23% yield from isoprene ozonolysis, yet its reactivity has not been directly studied. We present direct kinetic measurements of MVK-oxide reactions with key atmospheric species using absorption spectroscopy. Direct UV-Vis absorption spectra from two independent flow cell experiments overlap with the molecular beam UV-Vis-depletion spectra reported recently [M. F. Vansco, B. Marchetti, M. I. Lester, J. Chem. Phys. 149, 44309 (2018)] but suggest different conformer distributions under jet-cooled and thermal conditions. Comparison of the experimental lifetime herein with theory indicates only the syn-conformers are observed; anti-conformers are calculated to be removed much more rapidly via unimolecular decay. We observe experimentally and predict theoretically fast reaction of syn-MVK-oxide with SO2 and formic acid, similar to smaller alkyl-substituted CIs, and by contrast, slow removal in the presence of water. We determine products through complementary multiplexed photoionization mass spectrometry, observing SO3 and identifying organic hydroperoxide formation from reaction with SO2 and formic acid, respectively. The tropospheric implications of these reactions are evaluated using a global chemistry and transport model.

Experimental Evidence of Dioxole Unimolecular Decay Pathway for Isoprene-Derived Criegee Intermediates.

Ozonolysis of isoprene, one of the most abundant volatile organic compounds emitted into the Earth's atmosphere, generates two four-carbon unsaturated Criegee intermediates, methyl vinyl ketone oxide (MVK-oxide) and methacrolein oxide (MACR-oxide). The extended conjugation between the vinyl substituent and carbonyl oxide groups of these Criegee intermediates facilitates rapid electrocyclic ring closures that form five-membered cyclic peroxides, known as dioxoles. This study reports the first experimental evidence of this novel decay pathway, which is predicted to be the dominant atmospheric sink for specific conformational forms of MVK-oxide (anti) and MACR-oxide (syn) with the vinyl substituent adjacent to the terminal O atom. The resulting dioxoles are predicted to undergo rapid unimolecular decay to oxygenated hydrocarbon radical products, including acetyl, vinoxy, formyl, and 2-methylvinoxy radicals. In the presence of O2, these radicals rapidly react to form peroxy radicals (ROO), which quickly decay via carbon-centered radical intermediates (QOOH) to stable carbonyl products that were identified in this work. The carbonyl products were detected under thermal conditions (298 K, 10 Torr He) using multiplexed photoionization mass spectrometry (MPIMS). The main products (and associated relative abundances) originating from unimolecular decay of anti-MVK-oxide and subsequent reaction with O2 are formaldehyde (88 ± 5%), ketene (9 ± 1%), and glyoxal (3 ± 1%). Those identified from the unimolecular decay of syn-MACR-oxide and subsequent reaction with O2 are acetaldehyde (37 ± 7%), vinyl alcohol (9 ± 1%), methylketene (2 ± 1%), and acrolein (52 ± 5%). In addition to the stable carbonyl products, the secondary peroxy chemistry also generates OH or HO2 radical coproducts.

Synthesis, Electronic Spectroscopy, and Photochemistry of Methacrolein Oxide: A Four-Carbon Unsaturated Criegee Intermediate from Isoprene Ozonolysis.

Ozonolysis of isoprene, one of the most abundant volatile organic compounds in the earth's atmosphere, generates the four-carbon unsaturated methacrolein oxide (MACR-oxide) Criegee intermediate. The first laboratory synthesis and direct detection of MACR-oxide is achieved through reaction of photolytically generated, resonance-stabilized iodoalkene radicals with oxygen. MACR-oxide is characterized on its first π* ← π electronic transition using a ground-state depletion method. MACR-oxide exhibits a broad UV-visible spectrum peaked at 380 nm with weak oscillatory structure at long wavelengths ascribed to vibrational resonances. Complementary theory predicts two strong π* ← π transitions arising from extended conjugation across MACR-oxide with overlapping contributions from its four conformers. Electronic promotion to the 11ππ* state agrees well with experiment, and results in nonadiabatic coupling and prompt release of O 1D products observed as anisotropic velocity-map images. This UV-visible detection scheme will enable study of its unimolecular and bimolecular reactions under thermal conditions of relevance to the atmosphere.

Reactions of 2-Aryl-1,3-Dithianes and [1.1.1]Propellane.

Bicyclo[1.1.1]pentanes (BCPs) have sparked the interest of medicinal chemists due to their recent discovery as bioisosteres of aromatic rings. To study the biological activity of this relatively new class of bioisosteres, reliable methods to incorporate BCPs into target molecules are in high demand, as reflected by a flurry of methods for BCP synthesis in recent years. In this work, we disclose a general method for the synthesis of BCP-containing dithianes which, upon deprotection, provide access to BCP analogues of medicinally abundant diarylketones. A broad scope of 2-aryl-1,3-dithianes, including several heterocyclic derivatives, react with [1.1.1]propellane to afford 26 new derivatives in good to excellent yields. Further transformation of the dithiane portion into a variety of functional groups demonstrates the robustness of the products. A computational study indicates that the reaction of 2-aryl-1,3-dithianes and [1.1.1]propellane proceeds via a two-electron pathway.

Four-Carbon Criegee Intermediate from Isoprene Ozonolysis: Methyl Vinyl Ketone Oxide Synthesis, Infrared Spectrum, and OH Production.

The reaction of ozone with isoprene, one of the most abundant volatile organic compounds in the atmosphere, produces three distinct carbonyl oxide species (RR'COO) known as Criegee intermediates: formaldehyde oxide (CH2OO), methyl vinyl ketone oxide (MVK-OO), and methacrolein oxide (MACR-OO). The nature of the substituents (R,R' = H, CH3, CH═CH2) and conformations of the Criegee intermediates control their subsequent chemistry in the atmosphere. In particular, unimolecular decay of MVK-OO is predicted to be the major source of hydroxyl radicals (OH) in isoprene ozonolysis. This study reports the initial laboratory synthesis and direct detection of MVK-OO through reaction of a photolytically generated, resonance-stabilized monoiodoalkene radical with O2. MVK-OO is characterized utilizing infrared (IR) action spectroscopy, in which IR activation of MVK-OO with two quanta of CH stretch at ca. 6000 cm-1 is coupled with ultraviolet detection of the resultant OH products. MVK-OO is identified by comparison of the experimentally observed IR spectral features with theoretically predicted IR absorption spectra. For syn-MVK-OO, the rate of appearance of OH products agrees with the unimolecular decay rate predicted using statistical theory with tunneling. This validates the hydrogen atom transfer mechanism and computed transition-state barrier (18.0 kcal mol-1) leading to OH products. Theoretical calculations reveal an additional roaming pathway between the separating radical fragments, which results in other products. Master equation modeling yields a thermal unimolecular decay rate for syn-MVK-OO of 33 s-1 (298 K, 1 atm). For anti-MVK-OO, theoretical exploration of several unimolecular decay pathways predicts that isomerization to dioxole is the most likely initial step to products.

Positional Selectivity in C-H Functionalizations of 2-Benzylfurans with Bimetallic Catalysts.

Metal-catalyzed carbon-carbon bond-forming reactions are a mainstay in the synthesis of pharmaceutical agents. A long-standing problem plaguing the field of transition metal catalyzed C-H functionalization chemistry is control of selectivity among inequivalent C-H bonds in organic reactants. Herein we advance an approach to direct site selectivity in the arylation of 2-benzylfurans founded on the idea that modulation of cooperativity in bimetallic catalysts can enable navigation of selectivity. The bimetallic catalysts introduced herein exert a high degree of control, leading to divergent site-selective arylation reactions of both sp(2) and sp(3) C-H bonds of 2-benzylfurans. It is proposed that the selectivity is governed by cation-π interactions, which can be modulated by choice of base and accompanying additives [MN(SiMe3)2, M = K or Li·12-crown-4].

NiXantphos: a deprotonatable ligand for room-temperature palladium-catalyzed cross-couplings of aryl chlorides.

Although the past 15 years have witnessed the development of sterically bulky and electron-rich alkylphosphine ligands for palladium-catalyzed cross-couplings with aryl chlorides, examples of palladium catalysts based on either triarylphosphine or bidentate phosphine ligands for efficient room temperature cross-coupling reactions with unactivated aryl chlorides are rare. Herein we report a palladium catalyst based on NiXantphos, a deprotonatable chelating aryldiphosphine ligand, to oxidatively add unactivated aryl chlorides at room temperature. Surprisingly, comparison of an extensive array of ligands revealed that under the basic reaction conditions the resultant heterobimetallic Pd-NiXantphos catalyst system outperformed all the other mono- and bidentate ligands in a deprotonative cross-coupling process (DCCP) with aryl chlorides. The DCCP with aryl chlorides affords a variety of triarylmethane products, a class of compounds with various applications and interesting biological activity. Additionally, the DCCP exhibits remarkable chemoselectivity in the presence of aryl chloride substrates bearing heteroaryl groups and sensitive functional groups that are known to undergo 1,2-addition, aldol reaction, and O-, N-, enolate-α-, and C(sp(2))-H arylations. The advantages and importance of the Pd-NiXantphos catalyst system outlined herein make it a valuable contribution for applications in Pd-catalyzed arylation reactions with aryl chlorides.