Secondary amines and imines were used to prepare unsymmetrically substituted, non-aromatic N-heterocyclic carbenes via imidazolidine-2-thiones (rac-1) as intermediates. The presence of only one sterically demanding substituent on one of the N atoms in rac-2 is sufficient to prevent dimerization to the enetetramine.
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Hahn et al. (2003) studied this question.
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