Meurer Eduardo C, Eberlin Marcos N
Institute of Chemistry, State University of Campinas, UNICAMP, 13083-970, Brazil.
J Mass Spectrom. 2006 Apr;41(4):470-6. doi: 10.1002/jms.1005.
We have already shown that the in-vacuum gas-phase Meerwein reaction of (thio)acylium ions is general in nature and useful for class-selective screening of cyclic (thio)epoxides. Herein we report that this gas-phase reaction can also be performed efficiently at atmospheric pressure under both electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) conditions. This alternative expands the range of molecules that can be reacted by gas-phase Meerwein reaction. Phenyl epoxide, thiirane, 3-methoxy-2,2-dimethyloxirane, propylene oxide, 2,2'-bioxirane, trans-1,3-diphenyl-2,3-epoxypropan-1-one, epichloridrine and propylene oxide are shown to react efficiently in both ESI and APCI conditions. Tetramethylurea (TMU) and (thio)TMU were both used as dopants, being co-injected with either toluene, acetonitrile or methanol solutions of the (thio)epoxides, with similar results. In both ESI and APCI, (thio)TMU is protonated preferentially, and these labile species dissociate promptly to yield (CH3)2N-C+=O and (CH3)2NCS+, which are the least acidic and most reactive (thio)acylium ions so far tested in the gas-phase Meerwein reaction. Under the low-energy ESI conditions set to favor both the formation of the (thio)acylium ion and ion/molecule reactions, (CH3)2NCO(S)+ react competitively with (thio)TMU to form acylated (thio)TMU and with the (thio)epoxide to form the characteristic Meerwein products. Enhanced selectivity in structural characterization or for the screening of (thio)epoxides is achieved by performing on-line collision-induced dissociation of Meerwein products, particularly for the more structurally complex (thio)epoxides.
我们已经表明，（硫）酰基离子的真空气相 Meerwein 反应本质上具有普遍性，可用于环状（硫）环氧化物的类别选择性筛选。在此我们报告，这种气相反应在电喷雾电离（ESI）和大气压化学电离（APCI）条件下的大气压下也能高效进行。这种替代方法扩展了可通过气相 Meerwein 反应进行反应的分子范围。苯基环氧化物、硫杂环丙烷、3-甲氧基-2,2-二甲基环氧乙烷、环氧丙烷、2,2'-双环氧乙烷、反式-1,3-二苯基-2,3-环氧丙烷-1-酮、环氧氯丙烷和环氧丙烷在 ESI 和 APCI 条件下均能高效反应。四甲基脲（TMU）和（硫）TMU 均用作掺杂剂，与（硫）环氧化物的甲苯、乙腈或甲醇溶液共注入，结果相似。在 ESI 和 APCI 中，（硫）TMU 优先被质子化，这些不稳定的物种迅速解离生成（CH3）2N-C+=O 和（CH3）2NCS+，它们是迄今为止在气相 Meerwein 反应中测试的酸性最低且反应性最高的（硫）酰基离子。在设定为有利于（硫）酰基离子形成和离子/分子反应的低能量 ESI 条件下，（CH3）2NCO(S)+ 与（硫）TMU 竞争性反应形成酰化（硫）TMU，并与（硫）环氧化物反应形成特征性的 Meerwein 产物。通过对 Meerwein 产物进行在线碰撞诱导解离，特别是对于结构更复杂的（硫）环氧化物，可提高结构表征或（硫）环氧化物筛选的选择性。
