On the mechanism of the ethyl elimination from the molecular ion of 6‐methoxy‐1‐hexene

Abstract

It is shown by 13C and D labelling that the ethyl radical elimination from the molecular ion of Bmethoxy-1-hexene is a very complex process involving at least two different channels. The major channel (80%) is induced by an initial 1,lhydrogen shift in the molecular ion from C(5) to C(l) leading via a series of steps to methoxycyclohexane, which then undergoes a ring contraction to 2-methyl-l-methoxycyclopentane, being the key intermediate for the ethyl loss. The same key intermediate is formed in the other, minor channel (20%) by ring closure directly following an initial 1,bhydrogen shift in the molecular ion of bmethoxy-1-hexene from C(6) to C(1). Collision-induced dissociation experiments on the [Methyl] + ion from 6-methoxy-1-hexene have further estab-lished that it has the unique structure of oxygen methyl cationized 2-methylpropen-2-al. This ion is also generated by ethyl loss from the molecular ion of 2-methyl-1-methoxycyclopentane itself, as shown by collision-induced dissociation experiments, thus confirming the key role of the intermediate mentioned.

Investigation of the mechanism of alkyl radical elimination from ionised pentenyl methyl and hexenyl methyl ethers by analysis of the collision-induced dissociation mass spectra of C4H7O+ and C5H9O+ ions

“…This spectrum indicates that the structure of the product ion is predominantly 28. A similar conclusion was reached in an earlier combined CID and labelling study of the mechanism of C2Hselimination from 12+' (4). In view of this detailed investigation, which showed that 12+'expels C,H,.…”

Four decades of joy in mass spectrometry

“…Another interesting observation for methoxycyclohexane was the sudden increase of a non‐stereospecific 1,3‐elimination of methanol between 10 −10 and 10 −9 sec at the expense of the 1,4‐elimination for which on the basis of energetic arguments the key intermediates of ionized 1‐methoxyhexene‐1 and 6‐methoxy‐hexene‐1 have been proposed, respectively (Molenaar‐Langeveld & Nibbering, 1986). The latter ionized molecule was shown by 13 C and deuterium labeling to rearrange for the major part to methoxycyclohexane that after ring contraction to ionized 2‐methyl‐1‐methoxycyclopentane lost an ethyl radical (Molenaar‐Langeveld, Fokkens, & Nibbering, 1988). In an earlier study also the formation of 3‐methoxyhexene‐1 from ionized methoxycyclohexane had been found at ion lifetimes of 10 −10 sec that is responsible for the elimination of a propyl radical, being quite different from the textbook propyl loss found at <10 −10 sec (Molenaar‐Langeveld & Nibbering, 1983), which proceeds via successive cleavage of the C(1)C(2) bond, 1,5‐H shift from C(6) to C(2), and rupture of the C(4)C(5) bond (Budzikiewicz, Djerassi, & Williams, 1967).…”

The mechanism of alkyl radical elimination from ionised γγ-disubstituted alkenyl methyl ethers

“…Reference spectra of ions of known structure are also given in these Tables. The structures of these ions (13)(14)(15)(16)(17)(18), which were generated by ionisation and alkyl radical loss from secondary and tertiary alkenyl methyl ethers (8)(9)(10)(11)(12) , as shown by their similar CID spectra, which are distinct from that of the isomeric ion, 18, Table 3.…”

“…4 In contrast, a thorough investigation of the reactions of ionised 6-methoxyhex-1-ene revealed that this C 7 H 14 O +• • species deserved scrutiny in its own right: ethyl radical loss was found to occur by at least two mechanisms, one of which involved rearrangement to species accessible to ionised methoxcyclohexane. 11 A subsequent survey of the 70 and 12 eV electron impact (EI) ionisation mass spectra of numerous alkenyl ethers C m H 2m -1 OCH 3 (m = 3-6) indicated that several mechanisms operate for alkyl radical elimination from these C n H 2n O +• • species. 12 Extension of this work to encompass determination of the structure of the fragment ions produced by alkyl radical loss by analysis of collision-induced dissociation (CID) 13 mass spectra established that hydrogen transfers and occasional skeletal isomerisations allow groups attached to either carbon atom of the double bond to be lost.…”