Bicarbonyl

Aldehydes are very reactive substances. They react with OH by the abstraction of H (almost from the chain) to produce finally bicarbonyls, e. g. dialdehydes and ketoaldehydes (the carbonyl group denoted by C=0). Nevertheless, more important is the photolysis of aldehydes and ketones that initiates radical chains. We have seen that the simplest aldehyde HCHO is photolyzed (reaction 5.46) and gains H (which turns into HO2) and the formyl radical HCO (a carbon radical), which reacts with O2 to HO2 and CO. Therefore, HCHO is an efficient radical source. In analogy, higher aldehydes are photolyzed and produce H atoms and acyl radicals RCO, for example acetyl CH3CO, which do not decompose like the formyl radical HCO and add O2, thereby giving acyl peroxo radicals R—C(0)02i... 

Chapter 5.3.3, HCHO provides a net source of radicals. The formation of formic acid is likely to be negligible in the gas phase. But methanol, formaldehyde and formic acid (all produced and/or emitted in huge quantities from biogenic sources) will be scavenged and provide more efficient oxidation, finally with an accumulation of formic acid, but partly until its mineralization to CO2 and, most interestingly, a pathway in the formation of highly reactive bicarbonyls such as glyoxal and oxalic acid. 

Finally, the CH2OH radicals react with O2 to give HCHO and HO2 (5.327). Thus, C2 is broken down into Ci species. Fig. 5.34 shows schematically the C2 gas phase chemistry. It is obvious that there is no ethanol formation and acetic acid decomposition, whereas acetaldehyde provides many pathways back to Ci chemistry. Glycolaldehyde is a highly water-soluble product from several C2 species (ethene, acetaldehyde and ethanol) other bicarbonyls, however, are likely to be produced preferably in solution. The aqueous phase produces other C2 speeies but also deeomposes them (Fig. 5.35). By contrast, in aqueous solution from Ci, C2 species can be given as shown by the formation of glyoxal from the formyl radicals (5.351) the latter is... 

Nevertheless, direct evidence that the triplet state of glyoxal is not populated at any stage of Si decay [i.e., that L(t) = 0] would be of highest interest. This may be done by showing that the T-T absorption is totally absent after the excitation of the Sj state in collision-free conditions. Such an experiment, attempted by one of us, failed because of the weakness of the visible T-T absorption of aliphatic bicarbonyls. 

Two waves arise with the indicated half-wave potentials. The wave height for the process P2 + Zb -> P3 occurring at the more negative potential will obviously depend on the rate at which electroactive P2 is generated from Pi, which is itself assumed to be electroinactive. Examples where this type of behavior arises are the reductions of a-substituted ketones, aromatic bicarbonyl compounds such as p-diacetylbenzene, terephthaldialdehyde and a,/8-unsaturated ketones. Restricting ourselves to one type of example discussed by Zuman, " the following is the scheme for reduction of an a-substituted... 

An interesting catalytic system, which operates through a not yet clear mechanism, was described by Alper and co-workers. It consists of the use of PdCl2, CUCI2, and HCl and O2 to obtain mainly the oxidative bicarbonylation products with terminal alkynes and the additive monocarbonylation products with internal alkynes (Scheme 23). 

It has been shown that a variety of nucleophilic species able to attack Pd-coordinated carbon monoxide provide an efficient way to initiate insertion steps, which can be terminated by H-addition, H-elimination, or attack by nucleophilic species. At the end of the catalytic cycle palladium is found in the same oxidation state as at the beginning of the process or in the reduced state and in this case a reoxidation is needed to reestablish the initial oxidation state. Several olefinic and acetylenic substrates can undeigo these processes. Car-bonylation and bicarbonylation products can be prepared under mild conditions. 

In the reaction, two carbonyl groups and the entire VCP moiety were incorporated into the products, rendering an unprecedented bicarbonylative cycloaddition mode of 1-yne-VCP. As indicated by its name, the reaction can be viewed as a formal cascade reaction consisting of a [5+1] cycloaddition (between the VCP and CO) and a subsequent intramolecular Pauson-Khand [(2+2)+l] cycloaddition... 

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