[59733-54-9] · C12H24O2Si2 · 2,3-Bis(trimethylsilyloxy)-1,3-cyclohexadiene · (MW 256.54)
(useful for Diels-Alder reactions giving highly functionalized [2.2.2] bicyclic systems)
Physical Data: bp 102-103 °C/12 mmHg.
Solubility: sol C6H6, Et2O.
Handling, Storage, and Precautions: silyl enol ethers are susceptible to hydrolysis under basic and especially acidic conditions or in the presence of fluoride ion and other silyl specific nucleophiles (e.g. DMSO, DMF). Precautions should therefore be adopted to protect such enol ethers from moisture.
Silyl enol ethers are significantly more electron-rich than normal alkenes and this property dominates their reactivity.4 They react with a wide variety of electrophiles, usually in the presence of a Lewis acid. It was also recognized, particularly by Danishefsky,5 that these functional groups can serve as convenient surrogates for ketones in hetero-Diels-Alder reactions (eq 1).
1,2-Diketones, somewhat more difficult materials to work with, can also be utilized in related reactions. The diketones, by virtue of dipole repulsion, exist largely as the keto-enol form. Attempts to directly silylate cyclohexane-1,2-dione using LDA as the base were reported to be complicated by competing self-condensation.2 Sequential silylation, however, led to the bis(silyloxy)diene.2 Alternatively, with the bulky silicon base Sodium Hexamethyldisilazide, self-condensation was less problematic (eq 2).3 The bis(silyloxy)diene can also be directly prepared under less basic conditions from the dione using Me3SiOTf/NEt3 (eq 2).
Reetz has shown that 2,3-bis(trimethylsilyloxy)-1,3-cyclohexadiene, an electron-rich diene, reacts readily in a Diels-Alder fashion with a variety of activated dienophiles (eq 3). In these reactions, endo addition was exclusively observed.3
Related bis-2,3-silyloxy-1,3-dienes also undergo Diels-Alder reactions with various dienophiles, including heterodienophiles such as diazo compounds and imines (eqs 4 and 5).6,7
The products of these Diels-Alder reactions, bis(1,2-silyloxy)-1-alkenes, can undergo further synthetic elaboration. Although such reactions have not been described for the cyclohexane-1,2-dione derivatives, structurally related species may be desilylated to regenerate the 1,2-dione (eq 6).3 The high electron density in the bis(1,2-silyloxy)-1-alkenes makes them suitable educts for inverse electron demand Diels-Alder reactions (eqs 7 and 8).8,9 Finally, the silyl enol ether can undergo normal electrophilic addition to imines, leading to a-hydroxy carbonyl compounds (eq 9).10
Michael A. Brook
McMaster University, Hamilton, ON, Canada