López Carlos Silva, Faza Olalla Nieto, Lera Angel R de
Departamento de Quimica Organica, Universidade de Vigo, Lagoas Marcosende, 36310 Vigo, Galicia, Spain.
J Org Chem. 2008 Jan 18;73(2):467-73. doi: 10.1021/jo701962t. Epub 2007 Dec 15.
Changing the dienophile moiety of an intramolecular Diels-Alder (IMDA) cycloaddition from an allyl ether to an allenyl ether can dramatically change the regioselectivity. We hereby show by density functional theory computations that such unprecedented divergence is produced by an underlying change in the mechanism of the reaction. The allyl ether yields a fused tetrahydrofuran through a classical Diels-Alder reaction, whereas the allenyl ether yields a (methylidene)tetrahydropyran through a stepwise process. The latter reaction involves an extreme asynchronism in the bond-forming events with a diradicaloid intermediate that is stabilized by conjugation and synergistic (captodative) effects. Comparison with intermolecular model D-A reactions, which are concerted processes with various degrees of asynchrony, helps explain the change in regioselectivity for the IMDA reaction of allyl systems and the shift in mechanism for the IMDA reaction of the allenyl derivatives studied.
将分子内狄尔斯-阿尔德（IMDA）环加成反应中的亲双烯体部分从烯丙基醚变为联烯基醚，可显著改变区域选择性。我们在此通过密度泛函理论计算表明，这种前所未有的差异是由反应机理的潜在变化产生的。烯丙基醚通过经典的狄尔斯-阿尔德反应生成稠合四氢呋喃，而联烯基醚则通过逐步过程生成（亚甲基）四氢吡喃。后一反应在成键过程中涉及极端的不同步性，生成一个通过共轭和协同（给体-受体）效应得以稳定的双自由基中间体。与分子间模型D-A反应（具有不同程度不同步性的协同过程）进行比较，有助于解释烯丙基体系IMDA反应区域选择性的变化以及所研究的联烯基衍生物IMDA反应机理的转变。
