Synthesis and Reactivity of Captodative Diradical Oligomers Incorporating the 3,5,5-Trimethyl-2-oxomorpholin-3-yl (TM-3) Unit

Article abstract of DOI:10.1021/jo00248a025

The synthesis and reactivity of macrocyclic oligomers of diradicals incorporating the 3,5,5-trimethyl -2-oxomorpholin-3-yl radical stabilizing unit are described, oligomers (8 and 9) of dl-6,6'-bi(3,5,5-trimethyl-2-oxomorpholin-3-yl) (7) and dl-1,3-propanediyl-6,6'-bis(3,5,5-trimethyl-2-oxomorpholin-3-yl) (10), respectively.The diradicals differ by zero and trimethylene bridges connecting oxomorpholinyl radicals at the 6-position.The oligomers of 7 are predominantly mixtures of stereoisomeric trimers and the oligomers of 10 are predominantly mixtures of stereoisomeric pentamers.The oligomers were formed by photoreduction in 2-propanol of the corresponding bisoxazinones, dl-6,6'-bi(5,6-dihydro-3,5,5-trimethyl-2-oxo-1,4-oxazinyl) (11) and dl-1,3-propanediyl-6,6'-bis (5,6-dihydro-3,5,5-trimethyl-2-oxo-1,4-oxazinyl) (12), prepared from diamino diols and pyruvoyl chloride.Warm degassed solutions of the oligomers showed doublet EPR signals characteristic of the 3,5,5-trimethyl-2-oxomorpholin-3-yl unit indicating that the macrocycles undergo ring opening via bond homolysis.The oligomers are oxidatively cleaved by molecular oxygen, N-methylisatin (26), diphenylpicrylhydrazyl (DPPH), and daunomycin (6) to give back bisoxazinones 11 and 12, in analogy with the oxidative cleavage of the dimers of 3,5,5-trimethyl -2-oxomorpholin-3-yl (1, TM-3).The kinetics of oxidative deoligomerization with DPPH as the oxidizing agent are first order in oligomer.The activation parameters for deoligomerization of 9 are consistent with those for cleavage of a model, monoradical dimer, bi(3,5,5,6-tetramethyl-2-oxomorpholin-3-yl) (30, 31), suggesting that all weak bonds cleave at about the same rate and that the kinetics areactually first order in weak bonds.The activation parameters for deoligomerization of 8, especially in benzene, were anomalous, suggesting a more complicated mechanism.A reversible first bond cleavage followed by a slow second bond cleavage is proposed.Metal ion binding ability of these highly functionalized macrocycles was also explored and found to be low except for the small alkali and alkaline-earth cations, Li⁺, Mg²⁺, Ca²⁺.