SILYL STABILIZATION OF UNSYMMETRICAL BISKETENES - 3-(TRIMETHYLSILYL) AND 3-(TRIISOPROPYLSILYL)-2-SUBSTITUTED-1,3-BUTADIENE-1,4-DIONES

The bisketenes 2-phenyl and 2-methyl-3-(trimethyl-silyl)-1,3-butadiene-1,4-dione (8 and 10) are calculated on the basis of additivity of substituent effects to be less stable than the 3-phenyl and 3-methyl-4-(trimethylsilyl)cyclobut-3-ene-1,2-diones (7 and 9) by 1.9 and 2.6 kcal/mol, respectively. In agreement with this prediction, 8 and 10 are formed by photolysis of 7 and 9, respectively, and undergo thermal reversion to their precursors at similar rates. The concentration of 8 in thermal equilibrium with 7 in CDCl3, as measured by H-1 NMR spectroscopy, varied from 2.8% (161-degrees-C) to 0.5% (100.5-degrees-C), whereas the amount of 10 present at equilibrium with 9 was distinctly less. These measurements allowed the calculation of values of DELTAG-degrees (25-degrees-C) = 4.4 kcal/mol, DELTAH-degrees = 6.9 +/- (1.3) kcal/mol, and DELTAS-degrees = 8.5 (+/- 3.2) kcal/deg mol for the conversion of 7 to 8, and the equilibrium concentration of 8 at 25-degrees-C was estimated to be 0.06%. The triisopropylsilyl analog 12 of 8 was prepared and at 66-degrees-C was 2.6 times more reactive in ring closure to the corresponding cyclobutenone compared to 8. Reactions of 8 and 10 with MeOH in CDCl3 give the isolable monoketenes 3-phenyl and 3-methyl-2-(trimethylsilyl)-3-carbo-methoxy-1-ene-1-one (20,21). Reaction of 20 with excess MeOH or H2O gave the diastereomeric dimethyl 2-(trimethylsilyl)-3-phenylsuccinates (22) or ester-acids 24, respectively. Reaction of 8 with excess N-methylaniline gave the diamide 25.