Iminium salts, addition cyanide

Epoxynitrone (742) can be transformed to a positively charged heterodiene (743) using CF3S03SiMe3 or CF3S03Si(Bu )Me2 as electrophilic reagents (79HCA205). The diene adds to isolated double bonds to afford oxazines such as (744) via an inverse electron demand Diels-Alder pathway. Subsequent addition of cyanide to the iminium salt leads to a stable derivative (745) which can be converted through its mesylate (746) to an a-methylene y-lactone (747) on base treatment followed by acid hydrolysis (Scheme 171). 

Petrzilka, Felix, and Eschenmoser (19) have shown that the reaction of cyanide ion on the ion 219 gave mainly the addition product 40. Similarly, Riedi-ker and Graf (20) observed that the addition of cyanide on the ion 41 gave preferentially 42 which is the result of a stereoelectronically controlled reaction of the most hindered face of the iminium ion. Stereospecific addition of cyanide to enamine 43 via the iminium salt 44 to yield compound 45 has also been reported (21). 

This general synthesis of a-aminonitriles is explained by the iminium salt formation, via the formal hydride abstraction with the ruthenium catalyst, and addition of cyanide. The oxidant has the role of reoxidizing intermediate ruthenium species formed during the catalytic cycle. Slightly different catalytic steps have been proposed for the reaction using oxygen or hydrogen peroxide as oxidant [25, 26]. 

Iminium halides are intermediates in the hydrolysis of nitriles and in a variety of chemical reactions. For example, in the Ritter reaction addition of a nitrile or hydrogen cyanide to a carbonium ion occurs, leading to the intermediate formation of a nitrilium salt. Nitrilium salt intermediates have also been postulated in the Schmidt reaction and in the Beckmann rearrangement, provided the latter was performed in concentrated sulfuric acid. However, since we are predominantly concerned with imidoyl halides, these reactions are not discussed in this monograph. 

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