INSERTION OF CO2, COS AND CS INTO THE N-S I 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-m embered ring, exhibits different reactivity with respect to heterocumu lenes 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. Signif icant insertion into the second N-Si bond occurs only under CO2 pressu re. H-1 NMR spectroscopy of the products from all insertion reactions reveals strongly solvent-dependent chemical shifts and restricted rota tion around the N-C(=X) bonds. Carbonyl sulphide reacts more rapidly w ith 1 than does CO2 to yield a structurally characterized O-silylthiou rethane derivative, only one molecule of COS could be added. Carbon di sulphide 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 ;6t,9t-diepiazano-thioci no[4,5-b]pyrazine-8-thione derivative 4 is formed by cycloaddition bet ween CS, and two equivalents of 1; the molecule 4 contains one boat- a nd one chair-shaped polyhydropyrazine ring and a close to tetrahedral arrangement of the four Me(3)Si groups.