This work investigated theoretically, at the M06/6-311++G(d,p) level of theory, the Diels–Alder reactions of furan with substituted alkynes to afford oxanorbornadienes and the subsequent [4 + 2] and [2 + 2 + 2] cycloaddition reactions leading to the formation of heterocyclic furans, deltacyclenes, and bicyclic oxanorbornadienes, which have high utility in the chemical and pharmaceutical industries. The results show that in the initial Diels–Alder reactions to afford oxanorbornadiene intermediates, reactions of alkynes bearing electron-withdrawing substituents have lower activation barriers compared with those with electron-donating substituents. In the [4 + 2] reactions of the oxanorbornadiene intermediates to form bicyclic oxanorbornadienes, oxanorbornadienes with electron-donating groups favor addition across the unsubstituted olefinic bond, whereas those with electron-withdrawing substituents favor addition across the substituted double bond, but in both cases, the exo stereoisomer is favored. The regioselectivity of the reactions are strictly dictated by the nature of substituent on oxanorbornadiene. The retrocleavage step, regardless of the addition sequence and nature of substituents, is the rate-determining step. For the reactions of substituted oxanorbornadienes with furan along the [4 + 2] cycloaddition and the [2 + 2 + 2] cycloaddition (dimerization) of substituted oxanorbornadienes, we observed a decrease in activation barriers when electron-withdrawing substituents are involved. The global electrophilicity indices (ω) and maximum electronic charge transfer (ΔN_max) computed for various oxanorbornadiene derivatives considered in this work is consistent with energetic trends and experimental outcomes. Perturbation molecular orbital theory was employed to rationalize the results.

中文翻译：

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呋喃与取代炔烃反应形成氧降冰片二烯和随后的 [4 + 2] 和 [2 + 2 + 2] 加成反应的密度泛函理论研究

这项工作从理论上研究了在 M06/6-311++G(d,p) 理论水平上，呋喃与取代炔烃的 Diels-Alder 反应生成氧降冰片二烯以及随后的 [4 + 2] 和 [2 + 2 + 2] 环加成反应导致形成杂环呋喃、δ环烯和双环氧杂降冰片二烯，它们在化学和制药工业中具有很高的实用性。结果表明，在提供氧降冰片二烯中间体的初始 Diels-Alder 反应中，与带有给电子取代基的炔烃相比，带有吸电子取代基的炔烃的反应具有更低的活化势垒。在氧降冰片二烯中间体形成双环氧降冰片二烯的 [4 + 2] 反应中，具有给电子基团的氧降冰片二烯有利于在未取代的烯键上加成，而具有吸电子取代基的那些有利于跨越取代双键的加成，但在这两种情况下，外向立体异构体都是有利的。反应的区域选择性严格由氧降冰片二烯上的取代基的性质决定。无论取代基的添加顺序和性质如何，逆向裂解步骤都是决定速率的步骤。对于取代氧降冰片二烯与呋喃的 [4 + 2] 环加成反应和取代氧降冰片二烯的 [2 + 2 + 2] 环加成（二聚化）反应，当涉及吸电子取代基时，我们观察到活化势垒降低。全球亲电性指数（反应的区域选择性严格由氧降冰片二烯上的取代基的性质决定。无论取代基的添加顺序和性质如何，逆向裂解步骤都是决定速率的步骤。对于取代氧降冰片二烯与呋喃的 [4 + 2] 环加成反应和取代氧降冰片二烯的 [2 + 2 + 2] 环加成（二聚化）反应，当涉及吸电子取代基时，我们观察到活化势垒降低。全球亲电性指数（反应的区域选择性严格由氧降冰片二烯上的取代基的性质决定。无论取代基的添加顺序和性质如何，逆向裂解步骤都是决定速率的步骤。对于取代氧降冰片二烯与呋喃的 [4 + 2] 环加成反应和取代氧降冰片二烯的 [2 + 2 + 2] 环加成（二聚化）反应，当涉及吸电子取代基时，我们观察到活化势垒降低。全球亲电性指数（对于取代氧降冰片二烯与呋喃的 [4 + 2] 环加成反应和取代氧降冰片二烯的 [2 + 2 + 2] 环加成（二聚化）反应，当涉及吸电子取代基时，我们观察到活化势垒降低。全球亲电性指数（对于取代氧降冰片二烯与呋喃的 [4 + 2] 环加成反应和取代氧降冰片二烯的 [2 + 2 + 2] 环加成（二聚化）反应，当涉及吸电子取代基时，我们观察到活化势垒降低。全球亲电性指数（ω ) 和在这项工作中考虑的各种氧降冰片二烯衍生物计算的最大电子电荷转移 (Δ N _max ) 与能量趋势和实验结果一致。微扰分子轨道理论被用来使结果合理化。