ACCESSIBILITY OF ZWITTERIONIC COMPOUNDS FROM PRIMARY AMINES AND 2,5-DIHYDROXY-1,4- BENZOQUINONE
Following the discovery of an unprecedented transamination reaction between primary alkylamines and a quinonoid molecule of the type C6H2(NHCH2R)2(=O)2 (I), obtained from commercially available diaminoresorcinol.2HCl, we have extended this method to the use of primary arylamines and found that, in contrast, secondary amines led to a different outcome. Whereas functionalized molecules of type I, which are best described as 6π +6π zwitterions, were obtained with aniline or 4-methoxyaniline, no transamination was observed with tBuNH2 in ethanol. However, a reaction which afforded salt 2-methylpropan-2-aminium 4-(methylamino)-3,6 dioxocyclohexa-1,4-dien-1- olate (2b) took place in water and resulted from hydrolysis of the imine group and deprotonation of 5-hydroxy-2-(methylamino)-4- (methylimino)-cyclohexa-2,5-dienone (1a). Under similar conditions, secondary amines led to comparable results. The cations associated with the anionic quinonoid are readily exchanged in the presence of a primary amine. Whereas for the transamination reaction, basic amines react under mild conditions, slightly harsher conditions are needed for less basic amines such as piperidine, diisopropylamine, or diethylamine. Transamination reactions were also performed with 5-hydroxy-2-(methylamino)-4-(methylimino)-cyclohexa-2,5-dienone (1a), which is more soluble in organic solvents than 2-amino-5-hydroxy-4-iminocyclohexa-2,5-dienone (compound I) This led to the first examples of quinonoidal zwitterions functionalized with different alkyl groups on the nitrogen atoms. A number of compounds were characterized by X-ray diffraction, which allowed a better understanding of their electronic situation, and in many cases, the presence of multiple hydrogen-bond donors and acceptors results in crystal packings dominated by these interactions.