Carbonyl O oxides

Carbonyl O-oxides 144, generated in situ by the ozonolysis of vinyl ethers at low temperature (—70 °C), readily undergo [3+2] cycloaddition with imines 145 added to the reaction mixture to afford the corresponding 1,2,4-dioxazolidines 15-18 in 14-97% isolated yields (Scheme 41) <1994J(P1)2449, 1995J(P1)41>. The repetition of these reactions in MeOH results in the quantitative recovery of the initial imines 145. The steric bulk of the a-substituents did not appear to diminish the imines 145 reactivity, because the isolated yields of 1,2,4-dioxazolidines... 

This reaction involving asymmetrically substituted imines and carbonyl O-oxides occurs with a high degree of stereoselectivity and only 3,4,5-trisubstituted dioxazolidines 16a-g were obtained as a mixture of as- and trans-isomers with predominance of fir-isomers. The structure and ratio of isomers were established by NMR spectroscopy <1995J (P1)41>. 

Photolysis of 5-chloro-2-hydroxybenzonitrile (70) in aqueous solution gives the triplet carbene (71), which can be detected by transient absorption spectroscopy (X,max at 368 and 385 nm). The carbene was recognized by its reactions, e.g. with O2, to produce the corresponding carbonyl O-oxide (A ax at 470 nm), and with propan-2-ol to give 2-cyanophenoxyl radical. In deoxygenated solutions the main stable products are 2,5-dihydroxybenzonitriIe and two substituted biphenyls. 

The mechanism of the ozonolysis reaction of alkenes has been investigated in the gas phase and solid state using matrix isolation spectroscopy. While alkene/ozone 7C-complexes and the primary ozonides are readily observed by IR und UV/vis spectroscopy, there is no direct spectroscopic evidence for the Criegee intermediate (carbonyl O oxide) in these reactions. However, these elusive species can be synthesized and characterized via the carbene/oxygen route. Comparison of experimental and calculated spectroscopic data allows for the prediction of the spectroscopic properties of carbonyl oxides which are not accessible by this method. 

Ozone plays a major role in the degradation of unsaturated VOCs in the troposphere, especially during night-time. The rate constants of the ozonolysis of a variety of alkenes have been reported [1]. However, in most instances the fate of the primary products of the ozonolysis is unknown, although the secondary reaction products are of crucial importance for the overall understanding of the alkene/ozone chemistry. The classical Criegee mechanism of the ozonolysis reaction involves the primary ozonide (POZ, 1,2,3-trioxolane), which cleaves to the Criegee intermediate (carbonyl O oxide) and a carbonyl compound [2, 3]. The secondary ozonide (SOZ, 1,2,4-trioxolane) is formed from these components in a [l,3]-dipolar cycloaddition reaction. 

The Criegee intermediate has been claimed to be of importance in tropospheric chemistry [4] but never been observed by direct spectroscopic methods in the ozonolysis reaction. The aims of our research were therefore (i) to provide spectroscopic (IR, UVA is) data of a variety of substituted carbonyl O oxides, (ii) to develop a theoretical model which allows the prediction of the spectra of carbonyl O oxides which are not accessible by laboratory studies, but might be of importance to tropospheric chemistry, and (iii) to elucidate the mechanism of the ozonolysis of alkenes and investigate the role of carbonyl O oxides in these reactions. 

The only route to carbonyl O oxides 1 which is suitable for matrix isolation is the oxidation of free carbenes 2 with molecular oxygen 02 [5-7]. The carbenes are generated by photolysis of the corresponding diazo compounds or diazirines. Prerequisite for this method is that the carbenes are stable under the conditions of matrix isolation (solid rare gas at 10 K). This excludes simple alkyl substituents, because these carbenes rapidly undergo [1,2]-H shifts to yield alkenes in essentially quantitative yields. During the last years we were able to study the spectroscopy and chemistry of a number of carbonyl oxides (R and R were H, Ph, HC=C, CF3, cyclic systems etc.) [8-12] by IR and UV/vis spectroscopy. 

The reaction of ozone and alkenes is sufficiently fast that it can compete with other removal processes and provide sinks for both ozone and alkenes in the troposphere. While kinetic data for a series of alkene/ozone reactions have been reported, not much is known about details of the reaction mechanisms, the role that carbonyl O oxides play, and the role that free radicals play in these processes. Our laboratory experiments provide the spectroscopic data (both infrared and UV/visible) that are important for the spectroscopic identification of Criegee intermediates in the troposphere. In addition, we were able to characterize secondary partially oxidized products (aldehydes, peroxides etc.) that are produced during the gas-phase ozonolysis. These products might lead to a net increase of ozone, if oxygen atoms are formed during their decomposition. 

Carbonyl O-oxides and dioxiranes The influence of substituents on spectroscopic properties,... 

The ozonolyses of enol ethers has been reviewed <91MI 4l6-0l>. The relative dipolarophilicity of certain species to attack by carbonyl oxides has been investigated and, in general, the order of reactivity is aldehydes > enol ethers > esters ss ketones. It is apparent that enol ethers are very reactive towards carbonyl oxides, so much so that 1,2-dioxolane formation can be a major reaction pathway (especially for formaldehyde-O-oxide) <85JOC3365>. 

AUtyl, allyl, benzyl and cycloalkyl sulfones can be converted to aldehydes or ketones in good to excellent yields by treatment with n-BuLi followed by BTSP. Carbonyl-O labeling was achieved in situ by using oxidative desulfonylation with (Me3Si)2 02 (equation 50) . 

The mechanism is similar to that of the Skraup synthesis (Problem 20.37) in that carbonyl O is protonated and the electrophilic C attacks the benzene ring in cyclization, to be followed by dehydration and oxidation. 

Silylenes 1 are highly reactive homologues of the carbenes, and we have been interested to compare the reactivity and primary products of the oxidation of these divalent species. In principal one can expect two different primary adducts of a silylene and molecular oxygen the formal "end-on" adducts silanone 0-oxide 2 or "side-on" adducts dioxasilirane 3 (Scheme 1). It was shown by numerous matrix studies [8-12], experiments in solution using time resolved spectroscopy [13-16], and a preparative scale synthesis in solution [17] that triplet as well as singlet carbenes yield carbonyl 0-oxides as the primary oxidation products, while dioxiranes are products of secondary photolysis. Ando et al. reported on the synthesis of the silanone 0-oxide 2e by the reaction of dimesitylsilylene le and O2 in solid argon [1]. This is so far the only experimental evidence for a silanone O-oxide. 

Bolm, C., Luong, T. K. K., Beckmann, O. Oxidation of carbonyl compounds asymmetric Baeyer-Villiger oxidation. Asymmetric Oxidation Reactions). 2001,147-151... 

Dehydrogenation of Carbonyl Compounds. Oxidation of thioflavanones gave thioflavones in excellent yield whereas dehydrogenation with o-chloranil under like conditions failed (eq 27). Inductive effects play an important role in such dehydrogenations. For example, the sulfone analog resisted dehydrogenation whereas the sulfoxide derivative gave the thioaurone product in low yield. 

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