Alder Roger W, Butts Craig P, Sessions Richard B
School of Chemistry , University of Bristol , Bristol , BS8 1TS , UK . Email:
School of Biochemistry , University of Bristol , Bristol , BS8 1TD , UK.
Chem Sci. 2017 Sep 1;8(9):6389-6399. doi: 10.1039/c7sc01759f. Epub 2017 Jul 17.
Overall inversion in fused cyclohexane oligomers , , and (all based on -decalin ) occurs by a rolling process involving no more than two adjacent rings in twist-boat conformations at any time. These inverting rings move along the oligomer in processes that are precisely choreographed by the adjacent chairs. Actual inversion mechanisms can be stepwise [CC → TC → TT → C'T → C'C'], as for -decalin, but it is shown that a concerted alternative [CC → TC → C'T → C'C'] is enforced in . The all-,,-isomers of perhydrohelicenes are based on the diamond lattice and have remarkably low strain energies. Helix inversion in is compared with that in helicenes . For both, the intermediates and transition states have shapes broadly like kinked old-style telephone cables. In both cases barriers increase with the length of the system to eventually reach a plateau value of 120 kJ mol for , much lower than that for (320-350 kJ mol). While rolling inversion only requires two adjacent rings in twist-boat conformations at any instant, inversion in propellane requires all three rings be converted to twist-boats, and the symmetric hydrocarbon requires all four rings to be converted to twist-boats. As a consequence, probably has the highest barrier of any non-oligomeric -decalin derived structure (87.3 kJ mol at B3LYP/6-31G*).
稠合环己烷低聚物、和（均基于 - 十氢化萘）的整体翻转通过一种滚动过程发生，在任何时刻扭转船式构象中涉及不超过两个相邻环。这些翻转环沿着低聚物移动，其过程由相邻的椅式构象精确编排。实际的翻转机制可以是逐步的[CC → TC → TT → C'T → C'C']，就像 - 十氢化萘那样，但结果表明在中存在一种协同的替代机制[CC → TC → C'T → C'C']。全 - 、、 - 过氢螺旋烯的异构体基于金刚石晶格，并且具有非常低的应变能。将中的螺旋翻转与螺旋烯中的进行比较。对于两者而言，中间体和过渡态的形状大致类似于扭结的老式电话电缆。在这两种情况下，能垒都随着体系长度的增加而升高，最终对于达到120 kJ/mol的平稳值，远低于（320 - 350 kJ/mol）。虽然滚动翻转在任何时刻仅需要两个处于扭转船式构象的相邻环，但联环丙烷中的翻转需要所有三个环都转变为扭转船式，而对称烃需要所有四个环都转变为扭转船式。因此，可能具有任何非低聚 - 十氢化萘衍生结构中最高的能垒（在B3LYP/6 - 31G*水平下为87.3 kJ/mol）。
