Stereochemical analysis of deuterated alkyl chains by charge-remote fragmentations of protonated parent ions

Abstract : MH+ ions from sec-alkyl m-dimethylamino ethers have been studied by MS/MS using chemical ionization (CI) and electrospray ionization (ESI). Collisionally activated dissociation (CAD) of MH+ from the vicinally perdeuterated 3-hexyl ether, CH3CD2CHOArCD2CH2CH3, shows that alkene expulsion is >96% regioselective, yielding m/z 139 and m/z 124 fragment ions. The fact that alkene elimination is not 100% regioselective (but gives about 3% m/z 138) is attributed to a small amount of ion-neutral complex formation. CAD of MH+ ions from vicinally monodeuterated ethers (the 3-hexyl-4-d1 ether (1) and the sec-butyl-3-d1 ether (2)) demonstrates that the erythro and threo diastereomers give significantly different m/z 139:m/z 138 fragment ion intensity ratios. These intensity ratios are sufficiently reproducible that they can be used to quantitate the proportions of erythro and threo in mixtures. DFT calculations show substantial differences between the 4-center transition states for charge-remote elimination vs. alkene elimination from a neutral precursor. © 2003 Published by Elsevier Science B.V.