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
A. Ehlend et al., 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, Journal of organometallic chemistry, 501(1-2), 1995, pp. 283-292
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.