Ion-neutral complex formation and hydrogen-shifts in the alkene loss from ionized alkyl phenyl thioethers in the gas phase
作者:Margot W. Van Amsterdam、Steen Ingemann、Nico M. M. Nibbering
DOI:10.1002/jms.1190300109
日期:1995.1
The mechanistic aspects of alkene loss from ionized ethyl-and n-propyl phenyl thioethers have been studied by use of deuterium labelling and tandem mass spectrometry. Loss of ethene from the molecular ion of ethyl phenyl thioether proceeds predominantly by a specific H-shift from the β-position of the ethyl group to the remaining part of the ion. In contrast, deuterium labelling reveals that elimination of propene from the molecular ions of the n-propyl phenyl and n-propyl 2,6-dimethylphenyl thioethers involves three competing reactions: (i) a 1,2-hydride shift assisted cleavage of the bond between the propyl group and the sulfur atom yielding an ion–neutral complex of a thiophenyoxy radical and a secondary propyl carbenium ion, which reacts further by proton transfer prior to dissociation; (ii) a partially reversible 1,5-H shift from the β-position to the 2- or 6-position of the ring; and (iii) a 1,3-H shift from the β-position to the sulfur atom. The preference for transfer of a hydrogen atom from the β-position is particularly pronounce for the reactions of the metastable molecular ions of the n-propyl phenyl and n-propyl 2,6-dimethylphenyl thioethers. This indicates that the critical energies of the processes involving H-shifts are lower than the critical energy for propene loss with intermediate formation of an ion–neutral complex. In addition to ethene elimination, ionized ethyl phenyl thioether expels a HS+· radical. This reaction is preceded by an interchange between the hydrogen atoms at the α-methylene group and the hydrogen atoms at the 2- and 6-positions of the ring as observed also for ionized methyl phenyl thioether.
我们利用氘标记和串联质谱法研究了从电离乙基和正丙基苯基硫醚中损失烯烃的机理。乙基苯基硫醚分子离子中乙烯的损失主要是通过特定的 H 移位从乙基的 β 位转移到离子的其余部分。相反,氘标记显示,正丙基苯基和正丙基 2,6-二甲基苯基硫醚分子离子中丙烯的消除涉及三个竞争反应:(i) 1,2-氢转移辅助丙基和硫原子之间的键的裂解,产生一个噻吩氧自由基和一个仲丙基硒离子的离子中性复合物,该复合物在解离之前通过质子转移进一步反应;(ii) 部分可逆的 1,5-H 转移,从 β 位转移到环的 2 位或 6 位;以及 (iii) 1,3-H 转移,从 β 位转移到硫原子。在正丙基苯基和正丙基 2,6-二甲基苯基硫醚的逸散分子离子的反应中,从 β 位转移一个氢原子的偏好尤其明显。这表明,涉及氢转移的过程的临界能量低于丙烯损失的临界能量,中间形成离子中性复合物。除了乙烯消除外,电离的乙基苯基硫醚还排出一个 HS+- 自由基。在发生这一反应之前,α-亚甲基上的氢原子与环上 2-和 6-位置上的氢原子发生了互换,这一点在电离的甲基苯基硫醚中也可以观察到。