Direct evidence for anchimeric assistance in alcohol elimination from gas-phase MH+ ions of 1,4-dialkoxycyclohexanes under chemical ionisation. Experiment and theory
作者:Ronit Shvily、Thomas Müller、Yitzhak Apeloig、Asher Mandelbaum
DOI:10.1039/a605638e
日期:——
trans-1,4-Dialkoxycyclohexanes afford very abundant
[MH - ROH]+ ions upon chemical ionisation
(CI), in contrast to the cis-isomers, suggesting anchimeric
assistance in the alcohol elimination from the MH+ ions of
the trans-diethers. Collision induced dissociation (CID)
measurements of the [MH - ROH]+ ions,
obtained from various suitably deuterium labelled stereoisomeric
1-ethoxy-4-methoxycyclohexanes, indicate formation of symmetrical
bicyclic ethyl and methyl oxonium ions by an anchimerically assisted
alcohol elimination from the trans-diethers. On the other hand
these measurements suggest that the cis-isomers afford isomeric
monocyclic O-protonated 4-alkoxycyclohexene cations, in which
the hydrogens at positions 2 and 3 (as well as those at positions 5 and
6, and 1 and 4) are not equivalent. The two results, namely the
symmetrical bicyclic structure and the high abundance of the
[MH - ROH]+ ions in the CI mass spectra
of the trans-diethers, in contrast to the non-symmetrical
monocyclic structure and low abundance of these ions in the
cis-isomers, are suggested to be direct evidence for anchimeric
assistance in a gas-phase ion dissociation process. Ab initio
calculations at the MP3/6-31G*//6-31G* level support the anchimerically
assisted elimination mechanism observed in
trans-1-ethoxy-4-methoxycyclohexane, but also show that the
energy difference between the anchimerically assisted and non-assisted
elimination mechanisms is small (ca. 2–3 kcal
mol-1) (1 cal = 4.184 J).
trans-1,4-二烷氧基环己烷在化学电离(CI)过程中产生非常丰富的[MH - ROH]+离子,这与顺式异构体形成鲜明对比,表明在转移二醚的MH+离子中醇消除过程中的邻助效应。通过碰撞诱导解离(CID)测量,从各种适当氘标记的立体异构的1-乙氧基-4-甲氧基环己烷获得的[MH - ROH]+离子,表明形成了对称的二环乙基和甲基氧鎓离子,这是通过顺式二醚中的邻助助醇消除形成的。另一方面,这些测量表明,顺式异构体产生异构的单环O-质子化的4-烷氧基环己烯阳离子,其中2和3位(以及5和6位,1和4位)的氢原子不等效。两种结果,即顺式二醚的CI质谱中对称的二环结构和[MH - ROH]+离子的高丰度,与顺式异构体中的非对称单环结构和这些离子的低丰度形成对比,被认为是气相离子解离过程中邻助效应的直接证据。MP3/6-31G*//6-31G*级别的从头计算支持在trans-1-乙氧基-4-甲氧基环己烷中观察到的邻助消除机制,但也显示邻助和非邻助消除机制之间的能量差异很小(约2-3 kcal mol-1)(1 cal = 4.184 J)。