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-(trimethylsilyl)-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 ¹H NMR spectroscopy, varied from 2.8% (161°C) to 0.5% (100.5°C), whereas the amount of 10 present at equilibrium with 9 was distinctly less. These measurements allowed the calculation of values of Δ G° (25°C) = 4.4 kcal/mol, Δ H° = 6.9±(1.3) kcal/mol, and Δ S° = 8.5 (±3.2) cal/deg mol for the conversion of 7 to 8 , and the equilibrium concentration of 8 at 25°C was estimated to be 0.06%. The triisopropylsilyl analog 12 of 8 was prepared and at 66°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-carbomethoxy-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 .