Lee Hyejung, Feakins Sarah J, Lu Zhiyao, Schimmelmann Arndt, Sessions Alex L, Tierney Jessica E, Williams Travis J
Department of Earth Sciences, University of Southern California, Los Angeles, CA, 90089, USA.
Department of Chemistry, University of Southern California, Los Angeles, CA, 90089, USA.
Rapid Commun Mass Spectrom. 2017 Oct 15;31(19):1633-1640. doi: 10.1002/rcm.7947.
Methylation protocols commonly call for acidic, hot conditions that are known to promote organic H/ H exchange in aromatic and aliphatic C-H bonds. Here we tested two such commonly used methods and compared a third that avoids these acidic conditions, to quantify isotope effects with each method and to directly determine acidic-exchange rates relevant to experimental conditions.
We compared acidic and non-acidic methylation approaches catalyzed by hydrochloric acid, acetyl chloride and EDCI (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide)/DMAP (4-dimethylaminopyridine), respectively. These were applied to two analytes: phthalic acid (an aromatic) and octacosanoic acid (an aliphatic). We analyzed yield by gas chromatography/flame ionization (GC/FID) and hydrogen and carbon isotopic compositions by isotope ratio mass spectrometry (GC/IRMS). We quantified the H/ H exchange rate on dimethyl phthalate under acidic conditions with proton nuclear magnetic resonance ( H-NMR) measurements.
The δ H and δ C values and yield were equivalent among the three methods for methyl octacosanoate. The two acidic methods resulted in comparable yield and isotopic composition of dimethyl phthalate; however, the non-acidic method resulted in lower δ H and δ C values perhaps due to low yields. Concerns over acid-catalyzed H/ H exchange are unwarranted as the effect was trivial over a 12-h reaction time.
We find product isolation yield and evaporation to be the main concerns in the accurate determination of isotopic composition. H/ H exchange reactions are too slow to cause measurable isotope fractionation over the typical duration and reaction conditions used in methylation. Thus, we are able to recommend continued use of acidic catalysts in such methylation reactions for both aliphatic and aromatic compounds.
甲基化方案通常需要酸性、高温条件，已知这些条件会促进芳香族和脂肪族碳氢键中的有机氢/氘交换。在此，我们测试了两种常用方法，并比较了第三种避免这些酸性条件的方法，以量化每种方法的同位素效应，并直接确定与实验条件相关的酸交换速率。
我们比较了分别由盐酸、乙酰氯和EDCI（1-乙基-3-（3-二甲基氨基丙基）碳二亚胺）/DMAP（4-二甲基氨基吡啶）催化的酸性和非酸性甲基化方法。这些方法应用于两种分析物：邻苯二甲酸（一种芳香族化合物）和二十八烷酸（一种脂肪族化合物）。我们通过气相色谱/火焰离子化（GC/FID）分析产率，并通过同位素比质谱法（GC/IRMS）分析氢和碳的同位素组成。我们用质子核磁共振（¹H-NMR）测量定量了酸性条件下邻苯二甲酸二甲酯上的氢/氘交换速率。
三种方法制备的二十八烷酸甲酯的δ¹H和δ¹³C值以及产率相当。两种酸性方法得到的邻苯二甲酸二甲酯产率和同位素组成相当；然而，非酸性方法得到的δ¹H和δ¹³C值较低，可能是由于产率较低。由于在12小时的反应时间内酸催化的氢/氘交换效应微不足道，因此对其无需担忧。
我们发现产物分离产率和蒸发是准确测定同位素组成的主要问题。在甲基化反应的典型持续时间和反应条件下，氢/氘交换反应太慢，不会导致可测量的同位素分馏。因此，我们能够推荐在这类甲基化反应中继续使用酸性催化剂来处理脂肪族和芳香族化合物。
