Adventitious oxygen

Quenching by adventitious oxygen occurs to a small extent only, as most of the oxygen is consumed in the chain-propagation reaction. 

As a part of a program directed toward the synthesis of the potent topisomerase I inhibitors, the lamellarins (e.g., 153 and 154), Porco has reported the silver triflate-catalyzed tandem cycloisomerization-azomethine ylide cycloaddition of 155 (Scheme 2.42).75 The postulated mechanism of this intriguing and highly efficient process is shown in Scheme 2.43. Silver-catalyzed addition of the imine nitrogen to the alkyne results, on subsequent deprotonation, in the formation of an azomethine ylide 160. This ylide participates in [3+2] cycloaddition with the alkyne component leading to formation of a dehydropyrrole 161. Finally, oxidation by adventitious oxygen leads to formation of the product 162. 

The reaction of 0-acyl thiohydroxamates with tris(phenylthio)phosphorus, initiated by adventitious oxygen at room temperature, leads in the first instance to alkylbis(phenylthio)phosphines (Scheme 2) by a chain mechanism. Combination of the latter with the disulfide by product affords a phosphotus(V) species which on woik-up gives a]kylbis(phenylduo)phosphonates (Scheme 2 and equation 38) widh moderate to good yields. lliis reaction sequence provides a convenient meduxl for die overall transformation of a carboxylic acid into a readily hydrolyzable ester of the analogous phosphonic acid. 

A related oxidative rearrangement of cephem dioxides has been reported in which an alkene is oxidized stereospecifically with rearrangement to the allylic alcohol in good yield by simple exposure to a palladium/caibon catalyst, as depicted in equation (12). Adventitious oxygen preadsotbed on the catalyst seems the likely oxida The reaction fails on the parent ccphem or its monoxide, or on the free acid of the dioxide. This reaction would seem to hold some promise for furdier utility in the cephem field and odier related systems. 

The results reported here provide the reason for adopting the rather elaborate procedure described above for the preparation of most of the reaction systems used in this investigation. It was considered essential to eliminate the complex effects associated with the presence of adventitious oxygen, and the evidence presented here suggests that this objective can be attained by using the procedure described. 

In the synthesis of octacarbonyldicobalt(O), an important by-product is the hydride CoH(CO)4, which is promptly decomposed to Co2(CO)s by adventitious oxygen or spontaneously (equation 18) see Hydride Complexes of the Transition Metals). 

Quite similarly, superoxide may react with some activated aromatic halides to give, through substitution, the corresponding phenols [262,263]. A consequence of the results and reactions described earlier is that for electrolyses conducted in aprotic solutions, adventitious oxygen may quite often cause large discrepancies, not only in coulometric... 

The hydrosilylation of water-soluble propiolic acid was also tested under the same conditions. The reaction proceeded efficiently, giving the (Z)-alkene in optimal yield. It is worth mentioning that this hydrosilylation has recently been reported to be efficient also in neat conditions, where the initiation was linked to the presence of adventitious oxygen.24,26... 

The catalyst efficiency in these reactions varies from a TON of 20 to 91. It is possible that the catalyst is deactivated by the presence of adventitious oxygen and water, something which is more difficult to avoid at low catalyst loadings and on smaller scales. Examination of the P-NMR spectrum of the catalyst indicates that the phosphine monoxide and dioxide are formed in the presence of nickel prior to addition of the substrate [23]. Rigorous exclusion of oxygen and water is a necessity for every reaction in this series. 

Platinum and palladium dominate the enantioselective catalysis scene as far as alkaloid modifiers are concerned. Of the other platinum group metals, Ir follows Pt in cinchona-modified pyruvate ester hydrogenation,132 the fact that Rh can hydrogenolyse alkaloids adsorbed at its surface133 makes it unlikely to function successfully and Ru tends to be too easily oxidised by adventitious oxygen in these reactions. 

Consequently, the Rovis group identified a productive dual-catalysis mode that enables the asymmetric a-acylation of tertiary amines with aldehydes facilitated by the powerful combination of chiral NHC catalysis and photoredox catalysis. m-DNB (dinitrobenzene) is likely to induce an oxidative quenching cycle of [Ru(bpy)3] under these conditions, with adventitious oxygen likely being the terminal oxidant (Scheme 7.14). 

Stratton, P. F, Chang, E.,Takenaka, L, Onishi, H.,Tsujimoto, Y., The Effect of Adventitious Oxygen on Nitrogen Inerted IR Reflow Soldering with Low Residue Pastes, Soldering and Surface Mount Technology, No. 13,February 1993, pp. 12-15. 

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