Hydrogen atom trapping agent

This method is often utilized for preparation of carbene compounds which do not contain heteroatoms. Trityl salts and Me3P = CH2 are utilized as hydrogen atom trapping agents ... 

In the case of carbanion and radical intermediates the solvent is less important but the products are partially determined by the resistance of the medium to proton or hydrogen atom abstraction respectively. The increased stability of these intermediates compared with carbonium ions allows the reaction mechanism to be more readily modified by the addition of trapping agents. For example, carbanions are trapped in high yields by the presence of carbon dioxide in the electrolysis medium (Wawzonek and Wearring, 1959 Wawzonek et al., 1955). 

The most important methodology for the aliphatic C-C bond formation via radical reactions is the addition of the radical to an alkene double bond, both inter -and intramolecularly (with the 5-exo-ring cyclisation mode preferred in the latter case). This reaction leads to adduct radicals that must be converted to non-radical products before polymerisations can take place. For this reason, polymerisation is avoided either by intermolecular trapping of adduct radicals or by intramolecular, homolytic bond cleavage. Hydrogen atom donors X-H, heteroatom donors X-Z or electron donors M"+ are used as trapping agents (Scheme 7.1). 

The hydrogen atom of position 3 of the derivative 18a is expected to display acidic properties. Thus when treated with sodium methoxide in methanol-di, the ester I8a underwent racemization with incorporation of deuterium at position 3. In principle, the carbanion 148, the species formally involved in the foregoing reaction, may isomerize to the enethiolate 149 by a jS-elimination process. There is good evidence (Section V,C,2,b) that such isomerizations do occur and are reversible (Section V,B,4,b). Normally, alkylating agents selectively trap species of type 149 thus compound 150 was isolated when the derivative Ifo was treated with sodium hydride and methyl iodide. However, in the presence of potassium t-butoxide and methyl iodide, the derivative 140c was converted into 151 evidence for the intermolecular nature of the reaction was provided by the observation that the same product was formed when a 1 1 mixture of the derivatives 18c and 140c was treated with the base. ... 

Iminyl radicals are particularly useful for the construction of nitrogen heterocycles, since the cyclization products are functionally disposed for further elaboration into a variety of useful systems. Zard and coworkers have pioneered the development of methodology for the generation and cyclization of iminyl radicals (Table 8) [5, 71]. Iminyl radicals cyclize one order of magnitude more rapidly than the related neutral aminyl radicals but react less rapidly than the aminyl radicals with hydrogen atom transfer trapping agents, and one would predict that iminyl radicals formed in chain reaction sequences could prove to be as versatile as carbon radicals [78]. 

---