Ab initio computational studies of conformationally restricted cope rearrangements. First examples of fully concerted allenyl cope rearrangements
Ab initio computational studies of conformationally restricted cope rearrangements. First examples of fully concerted allenyl cope rearrangements
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构象限制性上位重排的从头算计算研究。
DOI：
10.1021/jo000571
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发表时间：
2000
期刊：
影响因子：
--
通讯作者：
Spong
中科院分区：
文献类型：
--
作者：
Duncan;Spong
在（8，8）CASPT 2/6- 31 G//（8，8）CASSCF/6- 31 G水平上，对顺式-5-丙二烯基双环[2.1.0]己-2-烯（14）和顺式-6-丙二烯基双环[2.1.1]己-2-烯（15）的构象限制性联烯科普重排进行了势能面计算.两者都是通过协调一致的途径进行的。在（6，6）CASPT 2/6- 31 G//（6，6）CASSCF/6- 31 G水平上计算了顺式-5-乙烯基双环[2.1]的科普重排反应。0]己-2-烯（18）、顺式-6-乙烯基双环[2.1.1]己-2-烯（19）和顺式-7-乙烯基降冰片烯（20），发现它们分别涉及二烯丙基双自由基中间体26、30和36。以前的研究表明，1，2，6-庚三烯（1）到3-亚甲基-1，5-己二烯（2）的联烯基科普重排涉及一个单一的过渡结构，该结构要么生成单烯丙基环己烷-1，4-二基衍生物3，要么绕过3直接生成2。(4)最近，还发现顺式-7-丙二烯基环己烯（7）的构象限制的丙二烯基科普重排涉及三环单烯丙基环己烷-1，4-二基中间体11。(7)14和15的重排似乎代表了首次报道的完全协同的丙二烯基科普重排的例子。在这些情况下的一致性归因于两个平行的因素：（1）可能的三环双自由基中间体16和17的相对不稳定性，相比于双自由基中间体3和11分别在1和7的重排中形成;以及（2）存在在过渡结构21和23中形成分别通向产物22和24的sp-sp（2）σ键的机会。相比之下，只有较弱的sp（2）-sp（2）σ键可以在未观察到的协同过渡结构中形成，从而导致在18和19的非协同重排中形成的产物28和32。
Results of (8,8)CASPT2/6-31G//(8,8)CASSCF/6-31G level calculations on the potential surface for the conformationally restricted allenyl Cope rearrangements of syn-5-propadienylbicylco[2.1.0]pent-2-ene (14) and syn-6-propadienylbicyclo[2.1.1]hex-2-ene (15) are reported. Both are found to proceed through concerted pathways. Also included are the results of (6,6)CASPT2/6-31G//(6,6)CASSCF/6-31G level calculations on the Cope rearrangements of syn-5-ethenylbicyclo[2.1. 0]pent-2-ene (18), syn-6-ethenylbicyclo[2.1.1]hex-2-ene (19), and syn-7-vinylnorborene (20), which are found to involve diallylic diradical intermediates 26, 30, and 36, respectively. Previous studies have shown that the allenyl Cope rearrangement of 1,2, 6-heptatriene (1) to 3-methylene-1,5-hexadiene (2) involves a single transition structure that either proceeds to the monoallylic cyclohexane-1,4-diyl derivative 3 or bypasses 3 to form 2 directly. (4) More recently, the conformationally restricted allenyl Cope rearrangement of syn-7-allenylnorbornene (7) has also been found to involve tricyclic monoallylic cyclohexane-1,4-diyl intermediate 11. (7) The rearrangements of 14 and 15 appear to represent the first reported examples of fully concerted allenyl Cope rearrangements. Concertedness in these cases is ascribed to two parallel factors: (1) the relative instability of possible tricyclic diradical intermediates 16 and 17, compared to diradical intermediates 3 and 11 formed in the rearrangements of 1 and 7, respectively; and (2) the opportunity that exists to form sp-sp(2) sigma bonds in transition structures 21 and 23 that lead, respectively, to products 22 and 24. By contrast, only weaker sp(2)-sp(2) sigma bonds could form in unobserved concerted transition structures leading to products 28 and 32, formed in the nonconcerted rearrangements of 18 and 19.