Oxygen Participation

Both the hydroxyl group and the alkoxyl group have low driving forces for participation, and it is not surprising that, in sugar chemistry, only a 

1-thio-a-n-galactofuranoside is, apparently, an intermediate in this reaction. An intermediate such as (13), similar to (11), in which the mercuric salt assists the displacement by the C-4 hydroxyl group, could explain the products of these reactions. 

Barker and coworkers noted the rapid solvotysis of 4-O-benzyl-l-O-p-tolylsulfonyl-1,4-pentanediol in ethanol to give tetrahydro-2-methylfuran, benzyl ethyl ether, and p-toluenesulfonic acid, a reaction where the rate and products clearly indicate anchimerio assistance from the benzyloxy group by way of a five-membered, cychc, oxonium-ion intermediate. They extended their study to three pentitol derivatives, (14), (18), and (20)  

In a reaction that resembles the work of Gray and coworkers described above, Hughes and Speakman noted a 1— 4 migration of a methoxyl group in the reaction of the dimethyl acetal (22) with tetrabutylam-monium benzoate in iV-methylpyrrolidinone. The immediate product was the 4-0-methyl-ii-lyxose hemiacetal benzoate (24), which could be converted into the known 2-0-methyl-L-arabinitol (4-0-methyl-L-lyxitol) 

Lemieux and Fraser-Reid noted that the brominolysis of methyl 

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Phenolic oxygen participates in facile oxypalladation. The intramolecular reaction of 2-hydroxychalcone (105) produces the flavone 106[127]. The ben-zofuran 107 is formed from 2-allyIphenol by exo cyclization with Pd(OAc)2, but benzopyran 108 is obtained by endo cyclization with PdChf S], Normal cyclization takes place to form the furan 109 from 2-(l-phenylethenyl)phe-nol[129]. Benzofuran formation by this method has been utilized in the synthesis of aklavinione (110)[130]. 

Wada, N. (1989). The active oxygen participating in the primary process of firefly D-(—)-luciferin chemiexcitation in dimethylsulfoxide. Toyo Daigaku Kogakubu Kenkyu Hokoku 25 27-34. 

The effect of alkali presence on the adsorption of oxygen on metal surfaces has been extensively studied in the literature, as alkali promoters are used in catalytic reactions of technological interest where oxygen participates either directly as a reactant (e.g. ethylene epoxidation on silver) or as an intermediate (e.g. NO+CO reaction in automotive exhaust catalytic converters). A large number of model studies has addressed the oxygen interaction with alkali modified single crystal surfaces of Ag, Cu, Pt, Pd, Ni, Ru, Fe, Mo, W and Au.6... 

These experiments clearly showed that it is a-oxygen participation that provides FeZSM-5 zeolites with such a remarkable catalytic performance in the reaction of benzene to phenol oxidation. Equations (1-3) written above are the main stages of the reaction mechanism. 

Data from Pasto and Serve, 1965. Carbonyl oxygen participation is uncertain for n = 7 e Data from Illuminati et al., 1974a... 

When allenyl aldehydes are allowed to react with DMDO, the aldehyde moiety is not oxidized to the acid except for monosubstituted allenes [21]. In all other cases, the carbonyl oxygen participates as a nucleophile in the opening of the intermediate epoxide. From 2,2,5-trimethy]-3,4-hexadienal 67, for example, five different products can be synthesized selectively under different reaction conditions (Scheme 17.22). When p-toluenesulfonic acid (TsOH) is present or DMDO is formed in situ, then the initially formed allene (mono)oxide reacts with the aldehyde moiety to give 68 or 69. In the presence of excess DMDO and the absence of acid, three other products (70-72) can be formed via the spirodioxide intermediate. These reactions, however, seem to be less general compared with similar reactions of allenyl acids and allenyl alcohols. y-Allenylaldehydes 73 can be cyclized to five-membered hemiacetals 74 via the spirodioxide intermediate. 

An oxazine has been obtained320 by oxygen participation between... 

Halofluorinations can also be carried out with transannular oxygen participation in the case of unsaturated bicyclic epoxides. Thus, 9-oxabicyclo[6.1,0]non-4-ene (7) is attacked stereo-specifically by A -halosuccinimide and triethylamine tris(hydrogen fluoride) (or 70% HF/py-ridine), to give endo,e fo-2-fluoro-6-halo-9-oxabicyclo[3.3.1]nonane 8 (main product) and eHr/o,eWo-2-fluoro-5-halo-9-oxabicyclo[4.2.1]nonane 9.194... 

Halofluorinations with Transannular Oxygen Participation by THethylamine Tris(hydrogen Fluoride) General Procedure 194... 

A successful correlation is obtained for the combustion reaction. For the first period tansition metal oxides, it is found that the combustion activity increases with decreasing heat of reduction (per atom of O) of the oxide to the next lower oxidation state (40). This correlation implies that lattice oxygen participates in the combustion reaction. Similar successful correlations have been observed for the combustion of CO, H2, CH4, and propene (47). 

This scheme includes a-oxygen loading (7.14), its interaction with benzene at room temperature (7.15) and product extraction from the catalyst surface (7.16). A nearly theoretical yield of phenol was obtained, with no other products detected. These results proved clearly the a-oxygen participation, which was confirmed additionally by isotopic experiments using 18Oa [119, 133]. 

A wide variety of these superelectrophilic carboxonium ions have been studied. It has long been recognized that carboxonium ions are highly stabilized by strong oxygen participation and therefore are much less reactive than alkyl cations. For example, trivalent carbocations are efficient hydride abstractors from tertiary isoalkanes (eq 41). 

A phenolic oxygen participates in facile oxypalladation. 2-Allylphenol (55) undergoes clean cyclization to 2H-l-benzopyran (56) in DMSO under air with a catalytic amount of Pd(OAc)2 without a reoxidant. 2-Methylbenzofuran (57) is obtained when PdCl2 is used [50]. But different chemoselectivity with the Pd(II) salts was also reported [51]. Catalytic asymmetric cyclization of the tetrasubstituted 2-allylic phenol 58 using the binaphthyl-type chiral ligand 60, called (.S, .S )-ip-borax, afforded the furan 59 with 96% ee. Use of Pd(CF3 002)2 as a catalyst is essential in the presence of benzoquinone [52]. Formation of the benzofuran 62 from 61 has been utilized in the synthesis of aklavinione [53]. The intramolecular reaction of 2-hydroxychalcone (63) produces the flavone 64 [54]. 

G(2d,p) levels of theory.11 The results indicated that the mechanism is a two-step process. The first step, which is rate determining, occurs via a six-membered cyclic transition state in which the carbonyl oxygen participates, followed by a rapid decomposition via a four-membered cyclic transition state of the alkoxy acid so formed. 

Atomic oxygen participates mainly as a p2- or (i3-bridgc ligand [26]. The structures 48 [W304F9] 3H20 and 49 [0s60(C0)19] [26] are illustrations of these kinds of ligands ... 

Thermodynamic calculation results are shown in Table 4.1. For reaction (5), the main parameters are the following free energy variation 5165 kJ, equilibrium constant at 600 °C 3.4 10-3 and the reagent conversion to reaction products is negligibly low. Much less favorable is the equilibrium state in the reaction (6). Therefore, both reactions are not practically executed. Reaction (6) described in the monograph by Zeldovich el al. [39] and in the article by Anbar [40] runs at a temperature above 1273 K with nitric oxide formation by the mechanism, which includes elementary stages with atomic oxygen participation. However, atomic... 

Atomic oxygen formation from H202 at conjugated dehydrogenation of hydrocarbons on the reactor walls is of low probability, but to justify the possibility of oxygen participation in dehydrogenation, nitrous oxide decomposition [3] ... 

When an oxidizable organic species R is present in the solution, the physisorbed active oxygen causes predominantly its complete mineralization (reaction (8)), and the chemisorbed active oxygen participates in the formation of partially oxidized products RO (reaction (9)) ... 

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