Ion–dipole complexes in the unimolecular reactions of isolated organic ions. Fragmentation of protonated and cationated formate and acetate esters

Abstract

The unimolecular fragmentation reactions occurring on the metastable ion time scale have been determined for protonated alkyl formates and acetates and for the adducts of CD3+ and C2H5+ with these esters. The protonated esters, R1CO2RH+, fragment to form R+ + R1CO2H or, by internal hydrogen transfer, R1CO2H2+ + (R – H) with the relative yields depending on the identity of R1 and R. Using deuterium labelling it has been shown that the added proton interchanges with hydrogens of the R group prior to fragmentation. The methyl-d3-cationated esters, R1CO2R•CD3+, mostly fragment to give R1CO2CD3, H+ by elimination of (R – H), although R+ formation is observed in a few cases. The ethyl-cationated esters fragment primarily to give R1CO2C2H5•H+ by elimination of (R–H); specifically no elimination of C2H4 is observed. All of the results can be rationalized in terms of a step-wise mechanism of fragmentation involving initial formation of an ion–dipole complex of the alkyl ion R+ with a carbonyl compound. This complex may either fragment to form R+ or may proceed by way of an ion–dipole complex of a protonated carbonyl and an olefin to the protonated species plus (R – H). The results indicate that primary alkyl ions are not formed but rather that rearrangement to the more stable secondary or tertiary structure occurs. There is some evidence that s-butyl cations are not formed, but that these too rearrange to the t-butyl structure.