SN2反应的机理的解释问题

btm

刚开始学的时候做了许多无用功，然后当时将卤代烷亲核取代直接构建C-C键【1】判定为SN2，最近发现好像应该加成到金属中心【2】才是对的，但是直接构建C-C生成构型翻转的能垒也不是很高，所以想加到文章里，故想请教大家如图【1】这种反应形式合理吗？如果合理请给赐个名字

wjc404

图1看上去不太容易发生，亲核位点的空间位阻较大。你是做过量子化学计算吗？
以下是wikipedia上Corey-House合成的部分内容，比较可能的机理是富电子的铜原子去进攻亲电试剂，文中还提到了碘代物的反应路径比较复杂可能有自由基中间体。

In the case of alkyl bromides and tosylates, inversion of configuration is observed when an configurationally pure alkyl electrophile is used. The reaction is believed to proceed via an SN2-like mechanism to give a copper(III) species, which undergoes reductive elimination to give the coupling product. When alkyl iodides are used, scrambling of configuration is observed, and cyclization products are observed to form for alkyl iodides with an olefin tether, both of which are indicative of the involvement of radicals.

It is important to note that for this reaction to work successfully, the alkyl (pseudo)halide coupling partner should be methyl, benzylic, allylic, 1° alkyl, or 2° cycloalkyl. In most cases, 3° and acyclic 2° electrophiles give unsatisfactory results. (However, see below for recent modifications that allow 2° electrophiles to be used successfully.) On the other hand, sterically hindered organocopper reagents, including 3° and other branched alkyl reagents, are generally tolerated. However, aryl bromides, iodides and sulfonates, which do not ordinarily undergo nucleophilic substitution in the absence of a transition metal, can be used successfully as coupling partners.

btm

wjc404 发表于 2023-6-2 00:45
图1看上去不太容易发生，亲核位点的空间位阻较大。你是做过量子化学计算吗？
以下是wikipedia上Corey-Hous ...

算是吧，其实我的课题Cu所连接的配体是一个烯基，卤代烷是一个酰胺，都整体是类平面位阻还好。因为保密嘛，所以就用别人算过的来提问。我算的图一反应形式能垒22左右，最佳的反应路径（自由基路径）也才19，所以我很晕。我的工作也讨论了自由基反应形式。因为投过一次人家说机理探究的不全，所以这次整的很多，但这个不甚理解。