Gas-phase polar [4++2] cycloaddition of cationic 2-azabutadienes with enol ethers

Abstract

Polar [4++2] cycloaddition of the prototype cationic 2-azabutadiene with ethyl vinyl ether was recently reported, and its potential application for the structural analysis of enol ethers suggested [R. Augusti, F.C. Gozzo, L.A.B. Moraes, R. Sparrapan, M.N. Eberlin, J. Org. Chem. 63 (1998) 4889]. We now report that the gaseous N-methyl 2-azabutadienyl cation (MAB+) reacts similarly, readily and generally with alkyl, silyl, and thio-enol ethers. The intact cycloadducts are formed abundantly, and collision-induced dissociation of acyclic enol ether cycloadducts occurs competitively by RDA and by the characteristic loss of either a ROH, (CH3)3SiOH, or RSH neutral molecule. Cycloadducts of acyclic enol ethers that bear no double bond substituents also form upon RO(S)H loss a characteristic fragment of m/z 96. For the cycloadducts of enol ethers bearing double-bound substituents (R1), the RO(S)H loss fragment displays a mass shift that corresponds to the mass of R1. Endocyclic enol ethers also react readily by MAB+ cycloaddition, but unless affected by ring substituents, the dissociation of their cycloadducts is dominated by RDA. Detailed structural information of the reactant enol ethers is therefore provided, and positional isomers are easily distinguished. Gas-phase polar [4++2] cycloaddition with cationic 2-azabutadienes, as demonstrated here for MAB+, is therefore class-selective and structurally diagnostic for enol ethers and their analogs.