Gas-phase rearrangements of deprotonated ketoximes, ketoxime ethers, and aldoximes. A negative-ion beckmann rearrangement

Abstract

Evidence is presented which indicates that the pronounced loss of water from deprotonated ketoximes involves specific proton transfer followed by a negative-ion Beckmann rearrangement.For example, Me,C=NO---+ -CH,(Me)C=NOH 0 [(CH,=C=NMe)-OH] --+ CHpC=NCH2-+ H20. Deprotonated aldoximes, e.g. MeCH=NO-, fragment in this way, but also undergo the competing process MeCH=NO---+ MeC=NOH --[(MeCN) -OH] --+ -CH,CN + H20. Other rearrangements occur when proton transfer to oxygen does not occur; e.g. PhO-+ PhCN.We have recently reported a number of simple 'rules' for fragmentations of even-electron negative ions including enolates, C-, N -, and 0species.'T2 Most fragmentations involve loss of a neutral molecule, and many such reactions are initiated from the charged centre through ion complexes [e.g. equation (l), R' = H, alkyl or aryl]. When such reactions are either unfavourable or not possible, one of two events generally occurs, viz. ( i ) proton transfer to the original charged centre produces a new anion which may fragment [e.g. carboxylate species, equations ( 2 ) and (3)3Jt or (ii) some type of internal (skeletal) rearrangement occurs (e.g. sigmatropic 5 * 6 and Smiles [equation (4)] rearrangements). Deprotonated oximes (R')(R2CH2)C=NO-(R' and R2 = H, alkyl or aryl), are somewhat akin to carboxylate species [see equations ( 2) and (3)], since it is unlikely that fragmentation can be directly effected through 0-. Either proton transfer to oxygen, or some internal rearrangement would be expected to precede fragmentation. Proton transfer [see equation (5)] could be facile since the acidities at the two centres should differ only by some 30 kJ mol-'. For example, the gas phase AH&i, values for Me2C=NOHs and (CH3),C=NOMe9 are 1 532 and 1 561 kJ mol-' respectively.This paper reports the basic fragmentations of deprotonated ketoximes, ketoxime ethers and aldoximes, and provides evidence in favour of a number of rearrangement reactions including the negative-ion Beckmann rearrangement. Results and DiscussionCollision-induced Dissociations of Deprotonated Alkyl Ketoximes.-Alkyl ketoxime spectra are listed in Table 1 or recorded in Figures 1-3. Deprotonation was effected by NH,: under these conditions, R2C=NOD systems yield M -D+ and M -H + ions in the approximate ratio 4-5: 1. This is the expected result since although the O H position is the more acidic, O H and -CHC=N-differ in acidity by only some 30 kJ mol-'. Labelling experiments are crucial for this study, and exchange reactions must be carried out with care because of the similarities in acidity at the two described positions. Full details are provided in the Experimental section.The oxime of acetone is prototypical in this series; its decompositions are shown in Figures 1 and 2. Major fragmentations shown in Figure 1 are the loss of H', the losses of H 2 0 and CH, and the formation of HO-. Less abundant * Numbers listed in the table refer to relative abundances of peaks with reference to base peak (100%) of that spectrum.a Loss of MeOH yields -CH,CN. approximate rat...