Theoretical Investigation of the Diels-Alder Reactions of Pinacol Alkenylboronates with cyclopentadiene

GRIMBLAT, NICOLÁS; VALLEJOS, MARGARITA M.; PELLEGRINET, SILVINA C.

Santiago de Chile

Congreso; 10th Congress of the World Association of Theoretical and Computational Chemists; 2014

Boron activated dienophiles are synthetically attractive due to their versatility. Although alkenylboronates are more stable than alkenylboranes, they are less reactive and harsh conditions are needed to carry out their Diels-Alder (DA) reactions. [1] Recently, we reported the microwave-assisted [4+2] cycloadditions of vinylboronates in excellent yields and short reaction times. We have now studied the DA reactions of pinacol alkenylboronates with cyclopentadiene under thermal conditions, noting that yields vary significantly from system to system and exo-stereoselectivities are moderate to excellent. To gain a deeper insight into the mechanism of the reactions under study we performed a theoretical study. In particular, we intended to examine whether the starting material/products distribution was determined by thermodynamic or kinetic control (Figure 1). Therefore, we optimized the geometries of the reactants, the transition structures and the products in toluene to compute the activation and reaction energies at 170 °C. In addition, we analyzed the geometries and the properties of the dienophiles and the transition structures with the aim of rationalizing the reactivity and selectivity trends. We have found that the thermal DA reactions of alkenylboronates with aryl groups in the 2-position give low yields because they are highly reversible. For the α-phenyl vinylboronic acid pinacol ester, a non-conventional hydrogen bond seems to explain the high reactivity observed