IQUIR   05412
INSTITUTO DE QUIMICA ROSARIO
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Studies toward the development of catalytic Diels-Alder reactions of vinylboronates
Autor/es:
SAROTTI, A.M.; PISANO, P.L.; PELLEGRINET, S.C.
Lugar:
Honolulu, Hawaii, EEUU.
Reunión:
Congreso; The 2010 International Chemical Congress of Pacific Basin Societies (Pacifichem 2010); 2010
Resumen:
Unsaturated boronates are attractive dienophiles but they exhibit poor reactivity.1 Recent reports on the use of chiral diols as organocatalysts in addition reactions of different boronates to carbonyl compounds2 encouraged us to apply this strategy to Diels-Alder reactions. We have studied the effect of the addition of different compounds with a hydroxy group to the reaction of dibutyl vinylboronate (1) with cyclopentadiene (2). The cycloadducts (3) were oxidized in situ to 5-norbornen-2-ol (4) under standard conditions. The background reaction (room temperature, 6 days) afforded the alcohols with a 10% yield in a 47:53 endo/exo ratio. We first investigated the use of catalytic amounts of various chiral biphenols (5-7). Unfortunately, the yields were not significantly improved. We then tested three different hydroxyacids (mandelic acid (8a), tartaric acid (9) and salicylic acid (10)). We were glad to note that mandelic acid gave increased yields in reduced reaction times (up to 50% in 8 h, endo/exo 65:35). Similar yields were obtained when using either optically pure or racemic mandelic acid. However, low enantioselectivity was observed. O-acetylmandelic acid (8b) and methyl mandelate (8c) were unsuccessful, which suggested that both functionalities in the hydroxyacid were necessary. Tartaric acid (9) and salicylic acid (10) were not effective either. To optimize the results, we studied the use of several additives and microwave irradiation. Best yields were obtained with isobutanol under very mild microwave irradiation (up to 97% in 1 h at 70 ºC, endo/exo 63:37). We are currently doing more experiments to rationalize the results and to extend the scope of the reaction.