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 (CHOO), methyl vinyl ketone oxide (MVK-OO), and methacrolein oxide (MACR-OO). The nature of the substituents (R,R' = H, CH, CH═CH) 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 O. 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 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) 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 (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.

Many metal-oxo (M=0) and -imido (M=NR) complexes are known, but in most the M=X linkages are notoriously inert. The absence from the literature of monomeric group IV metallocenes of this class ((t75-C5R5)2M=0 and (7js-C5R5)2M=NR', M = Ti, Zr, Hf) suggests that if these species could be generated, they might exhibit more extensive chemistry than do oxo and imido complexes that are presently known. We now wish to report the

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This paper outlines the generation of the transient monomeric imido complexes CpzZr==NR (la-e), their successful trapping with unsaturated organic molecules and dative ligands, and their use in activating C-H bonds. The generation of Cp,Zr=NR has been accomplished by thermolysis of the alkyl amides CpzZr(NHR)(R') (2a-d) and bis(amides) CpzZr(NHR)2 (3a-c) via a-abstraction of alkane and amine, respectively. The imido intermediate la, generated from alkyl amide 2a (R = 4-tert-butylphenyl, R' = Me) in THF, resulted in the formation of the bridging imido dimer 4a in 70% yield. In contrast, the generation of intermediates lb-d ( l b R = tert-Bu; IC R = 2,6-dimethylphenyl; Id R = Si(CMe3)Mez) under similar conditions furnished the monomeric imido T H F complexes 5b-d in 71, 69, and 72% yields. Imido dimer 4a and monomer 5b were characterized crystallographically. The T H F ligands of 5b-d are weakly bound, as demonstrated by the exchange of bound and free T H F in 5c on the NMR time scale and by facile substitution of Ph3PO for T H F in 5b, giving the triphenylphosphine oxide adduct Cp2Zr(NCMe3)(OPPh3) (6b). Thermolysis of the alkyl amides 2b and 2d a t 85-95 "C in benzene resulted in the addition of the C-H bond of benzene across the Zr-N linkage to provide the phenyl amides 7b and 7d. Similarly, the T H F adduct 5b reacted with benzene under the same conditions to give the C-H activation product 7b. The imido intermediates Cp,Zr=NR undergo cycloaddition reactions with a variety of unsaturated organic compounds. In addition to formation of azametallacyclobutenes (e.g., 8b, 8c, 9c, lob) from alkynes, certain olefins react with Cp2-Zr=NR to give azametallacyclobutanes. Thus, upon reaction of norbornene or ethylene with imido complex 5b an equilibrium mixture of 5b, free olefin, free THF, and the metallacycles 12b and 13b, respectively, were observed. Addition of a large excess of norbornene shifted the equilibrium almost entirely to the azametallacyclobutane 12b and facilitated its isolation. The structure of 12b was determined after crystallization from hexamethyldisiloxane. Complex 5b also reacted with benzophenone and tert-butyl isocyanate to provide the tert-butylimine of benzophenone and di-tert-butylcarbodiimide in 89 and 65 % yields, respectively. Monomer 5b reacted with tert-butyl isocyanide to provide the couplingltrapping product CpzZr-(N(CMe3)C=NCMes)(CNCMe3) (16), while reaction of 5b with isobutene oxide gave the epoxideopened product Cp2Zr(NHCMe3)(0CHzC(Me)=CH2) (17). Finally, heating the alkyl amide CpzZr(NHCHPh2) (CH2CH2CMe3) (2e) (which contains hydrogens both a and / 3 to nitrogen) with diphenylacetylene yielded the azametallacyclobutene product formed via a-abstraction in 64% yield.

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.

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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.

The asymmetric addition of alkyl groups to aldehydes catalyzed by BINOLate-titanium complexes has become the testing grounds to evaluate the potential of new BINOL-based ligands. We have investigated the mechanism of this reaction and report our findings here. Model systems for the open form of the catalyst, (BINOLate)[Ti(O-i-Pr)(3)](2), based on mono-oxygen-alkylated BINOL ligands have been examined. Comparison of the reactivity and enantioselectivity of the mono-alkyl BINOL derivatives with those of BINOL indicate that the open form of the catalyst, (BINOLate)[Ti(O-i-Pr)(3)](2), is not active in the asymmetric addition reaction. Several BINOLate-titanium complexes have been synthesized and characterized by X-ray crystallography. These include the dinuclear (BINOLate)Ti(O-i-Pr)(2).Ti(O-i-Pr)(4), which contains a bridging naphtholate and isopropoxy group, trinuclear (BINOLate)Ti(O-i-Pr)(2).[Ti(O-i-Pr)(4)](2), and trimeric [(BINOL)Ti(O-i-Pr)(2)](3). The solid-state and solution structures reported here indicate that (BINOLate)Ti(O-i-Pr)(2) prefers to bind to titanium tetraisopropoxide rather than to itself, explaining why no nonlinear effects are observed in the catalytic reaction. Additionally, experimental evidence suggests that the BINOLate-titanium species responsible for the catalytic and stoichiometric asymmetric addition reactions are different, indicating that the proposed intermediate, (BINOLate)Ti(R)(aldehyde)(O-i-Pr), is not involved in either of these processes. Reactions were examined using different sources of the alkyl group [ZnMe(2) or MeTi(O-i-Pr)(3)]. Under similar conditions, it was found that the product ee's were the same, independent of whether ZnMe(2) or Me-Ti(O-i-Pr)(3) was used as the source of the alkyl groups. This indicates that the role of the dialkylzinc is not to add the alkyl group to the carbonyl but rather to transfer the alkyl group to titanium. On the basis of these results, we hypothesize that the intermediate in the asymmetric addition involves (BINOLate)Ti(O-i-Pr)(2)(aldehyde).MeTi(O-i-Pr)(3).