Energy barriers to the Diels–Alder cycloadditions and cycloreversions of cation-radicals in the gas phase
作者:František Tureček、Zdeněk Havlas
DOI:10.1039/p29860001011
日期:——
mechanisms are operative in reactions which occur at their thermochemical thresholds. The reverse process, the Diels–Alder reaction of a cation-radical with a molecule, has a negligible activation energy for symmetrical and slightly unsymmetrical systems, [(Z)-1,2-dihydroxyethene]+˙+ cyclopentadiene, [cyclopentadiene]+˙+ vinyl alcohol, [(E)-1-hydroxybuta-1,3-diene]+˙+ cyclopentadiene, and [(Z)-1-hydroxybuta-1
取代的双环[2.2.1]庚-2-烯阳离子自由基的逆Diels-Alder反应机理可能与碎片的能量有关。协调一致的同步或异步机制可在发生在其热化学阈值的反应中起作用。逆过程,即阳离子自由基与分子的Diels–Alder反应,对于对称和稍微不对称的体系[(Z)-1,2-二羟基乙烯] + ˙+环戊二烯,[环戊二烯] +的活化能可忽略不计˙+乙烯醇,[(E)-1-羟基丁-1,3-二烯] + ˙+环戊二烯和[(Z)-1-羟基丁-1,3-二烯] +˙+环戊二烯。高度不对称的2-乙烯基双环[2.2.1]庚-5-烯-2-醇阳离子自由基的逆Diels–Alder分解分两步进行,并克服了热化学反应之上的58–67 kJ mol –1的障碍临界点。讨论了键离解能对反应机理的影响。