Technical note: Improved synthetic routes to cis- and trans-(2-methyloxirane-2,3-diyl)dimethanol (cis- and trans-β-isoprene epoxydiol)

Abstract

Abstract. We report improved synthetic routes to the isomeric isoprene-derived β-epoxydiols (β-IEPOX) in high yield (57 %–69 %) from inexpensive, readily available starting compounds. The syntheses do not require the protection/deprotection steps or time-consuming purification of intermediates and can readily be scaled up to yield the target IEPOX isomers in gram quantities. Emissions of isoprene (2-methyl-1,3-butadiene, C5H8), primarily from deciduous vegetation, constitute the largest source of nonmethane atmospheric hydrocarbons. In the gas phase under low-nitric-oxide (NO) conditions, addition of the atmospheric hydroxyl radical (OH) followed by rapid addition of O2 yields isoprene-derived hydroxyperoxyl radicals. The major sink (>90 %) for the peroxyl radicals is a sequential reaction with the hydroperoxyl radical (HO2), OH, and O2, which is then followed by the elimination of OH to yield a ∼2:1 mixture of cis- and trans-(2-methyloxirane-2,3-diyl)dimethanol (cis- and trans-β-IEPOX). The IEPOX isomers account for about 80 % of closed-shell hydroxyperoxyl products and are rapidly taken up into acidic aerosols to form secondary organic aerosol (SOA). IEPOX-derived SOA makes a significant mass contribution to fine particulate matter (PM2.5), which is known to be a major factor in climate forcing as well as adversely affecting respiratory and cardiovascular systems of exposed populations. Prediction of ambient PM2.5 composition and distribution, both in regional- and global-scale atmospheric chemistry models, crucially depends on the accuracy of identification and quantitation of uptake product formation. Accessibility of authentic cis- and trans-β-IEPOX in high purity and in large quantity for laboratory studies underpins progress in developing models as well as identification and quantitation of PM2.5 components.