INSERTION OF CO2, COS AND CS INTO THE N-SI BOND OF 2 EXTREMELY ELECTROPHILIC 1,4-BIS(TRIMETHYLSILYL)-1,4-DIHYDROPYRAZINE AND FORMATION OF A NEW POLY(N,S-)HETEROCYCLIC SYSTEM BY MULTIPLE CYCLOADDITION

1,4-Bis(trimethylsilyl)-1,4-dihydropyrazine (1), a very-electron-rich N-silyl-enamine with eight conjugated pi electrons in the planar six-membered ring, exhibits different reactivity with respect to heterocumulenes X=C=Y (X,Y = O,S). Carbon dioxide is slowly inserted into one N-Si bond to give an electron-rich O-silylurethane functionality. Significant insertion into the second N-Si bond occurs only under CO2 pressure. H-1 NMR spectroscopy of the products from all insertion reactions reveals strongly solvent-dependent chemical shifts and restricted rotation around the N-C(=X) bonds. Carbonyl sulphide reacts more rapidly with 1 than does CO2 to yield a structurally characterized O-silylthiourethane derivative, only one molecule of COS could be added. Carbon disulphide reacts rapidly with 1 under partial oxidative desilylation as evident from the formation of pyrazine; in addition to some insertion, a novel polyheterocyclic system 4 is formed as the main product. The crystallographically characterized red 5r,10 c;6t,9t-diepiazano-thiocino[4,5-b]pyrazine-8-thione derivative 4 is formed by cycloaddition between CS, and two equivalents of 1; the molecule 4 contains one boat- and one chair-shaped polyhydropyrazine ring and a close to tetrahedral arrangement of the four Me(3)Si groups.