Liao Wei, Yu Zhi-Xiang
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking University , Beijing 100871, China.
J Org Chem. 2014 Dec 19;79(24):11949-60. doi: 10.1021/jo5017844. Epub 2014 Oct 16.
Diels-Alder reaction between electronically neutral dienes and dienophiles is usually sluggish under thermal conditions and has to be catalyzed by transition metal catalysts. We report here our DFT study of the mechanism and stereochemistry of the Rh-catalyzed Diels-Alder reaction between electronically neutral dienes and dienophiles (alkenes and alkynes), finding that this reaction includes a reaction sequence of oxidative cyclization between diene and alkene/alkyne and a reductive elimination step. The alkyne's oxidative cyclization is much faster than alkene's due to the additional coordination of alkyne to the Rh center in the oxidative cyclization transition state. For both intermolecular and intramolecular reactions, the reductive elimination step in the catalytic cycle is rate-determining. The different reactivity of ene-diene and yne-diene substrates can be rationalized by the model that reductive elimination to form a C(sp(2))-C(sp(3)) bond is easier than that for the formation of a C(sp(3))-C(sp(3)) bond, due to the additional coordination of the double bond to the Rh center in the transition state in the former. We also uncovered the reasons for the high para-selectivity of the intermolecular Diels-Alder reaction of dienes and alkynes. In addition, DFT calculations aiming to understand the high diastereoselectivity of an intramolecular [4 + 2] reaction of ene-dienes with substituents adjacent to the diene and ene moieties of the substrates found that the substituents in the substrates favor staying away from the Rh center in the oxidative cyclization transition states. This preference leads to the generation of the final [4 + 2] products with the substituents and the bridgehead hydrogen atoms in a cis-configuration.
电子中性双烯体和亲双烯体之间的狄尔斯-阿尔德反应在热条件下通常较为缓慢，必须由过渡金属催化剂催化。我们在此报告了对电子中性双烯体和亲双烯体（烯烃和炔烃）之间铑催化的狄尔斯-阿尔德反应的机理和立体化学的密度泛函理论（DFT）研究，发现该反应包括双烯体与烯烃/炔烃之间的氧化环化反应序列以及还原消除步骤。由于在氧化环化过渡态中炔烃与铑中心的额外配位，炔烃的氧化环化比烯烃的氧化环化快得多。对于分子间和分子内反应，催化循环中的还原消除步骤是速率决定步骤。烯-双烯体和炔-双烯体底物的不同反应性可以通过以下模型来解释：由于前者过渡态中双键与铑中心的额外配位，形成C(sp(2))-C(sp(3))键的还原消除比形成C(sp(3))-C(sp(3))键的还原消除更容易。我们还揭示了双烯体和炔烃分子间狄尔斯-阿尔德反应高对位选择性的原因。此外，旨在理解烯-双烯体与底物双烯体和烯部分相邻的取代基的分子内[4 + 2]反应的高非对映选择性的DFT计算发现，底物中的取代基在氧化环化过渡态中倾向于远离铑中心。这种偏好导致最终[4 + 2]产物中取代基和桥头氢原子呈顺式构型。
