Subject lead, reaction with ozone

Nitric oxide is quite rapidly oxidized to N02 by reaction with ozone. Nitrogen dioxide, in turn, is subject to photolysis whereby NO is regenerated. This leads to the following reaction sequence... 

Hydrocarbon CXXXIV was prepared from abieta-6,8-diene (CXXXV) by the following route. Reaction with maleic anhydride gave the adduct CXXXVI, whose dimethyl ester was ozonized to give CXXXVII in 35% yield. Oxidation of the latter with H2O2-BF3 gave the keto diester CXXXVIII in 60% yield. The diacid of CXXXVIII was subjected to oxidative bis decarboxylation with lead tetraacetate in pyridine to afford the keto olefin CXXXIX. Hydrogenation of CXXXIX over palladium/... 

Thus, when studying atmospheric chemistry, it is necessary always to take into account the vertical and horizontal movements in the atmosphere, as well as the conditions controlling those chemical reactions that do not spontaneously lead to photochemical equilibrium. These conditions are applicable not only to ozone in the lower stratosphere, but also to atomic oxygen in the upper mesosphere above 75 km. In fact, equation (4) shows that, with increasing height, the formation of O3 becomes less and less important because of the decrease in the concentration of 02 and N2. Above 60 km the concentration of atomic oxygen exceeds that of ozone, but it is still in photochemical equilibrium up to 70 km. However, at the mesospause (85 km), it is subject to atmospheric movements, and its local concentration depends more on transport than on the rate of production. 

The exact mechanism arises in the process of inverse pre-dissociation, as discussed in detail by Herzberg (1966). During an atom-molecule collision, the reactants interact with one another subject to the relevant potential energy surface. The lifetime of this excited intermediate is on the order of molecular vibrational periods, or 10 s. The lifetime is a complex function of the chemical reaction dynamics, which in turn depends on the number of available states. In this specific instance, there is a state dependence for the isotopically substimted species. Ozone of pure has a Cav symmetry and has half the rotational complement of the asymmetric isotopomers. As a result, it was suggested that the extended lifetime for the asymmetric species leads to a greater probability of stabilization. While these assumptions are valid for a gas phase molecular reaction, they do not sufficiently account for the totality of the experimental ozone isotopic observations. Reviews by Weston (1999) and Thiemens (1999) have detailed the physical-chemical reasons. 

In sequence A, the final step is acetal deprotection using acetic acid and water, which will produce the desired product, aldehyde 2. Under these conditions no undesired side reactions are expected to occur. In sequence B, ozonolysis of the terminal alkene will certainly produce an aldehyde, however the molecule also contains two additional alkenes within the 8-membered ring. Since ozone is a non-selective oxidant, if this compound were subjected to O3/DMS, all three alkene groups would react Therefore, the major product of the reaction will not be aldehyde 2, but a compound with three different carbonyl groups (as well as two other fi agments). In sequence C, hydroboration/oxidation will convert the terminal alkene to a primary alcohol, which will then be transformed into the desired product via oxidation with PCC. However, just like we saw in sequence B, the two other tc bonds will react under the hydroboration/oxidation conditions the product isolated will not be aldehyde 2. In conclusion, after a thorough analysis, oidy sequence A will lead to the desired product... 

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