Reaction of 2,4-Di-tert-butyl-6-[(dimethylamino)methyl]phenol with diazabicyclo[5.4.0]undec-7-ene

Abstract

There are no published data on reactions of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as carbon-centered nucleophile, which could lead to C–C bond formation. Only a few examples have been reported on reactions of DBU with electrophiles involving the C atom [1–3] and reactions of DBU anion with alkyl and acyl halides, isocyanates, oxiranes, ketones, and organophosphorus compounds [4–8]. We have found that 2,4-di-tert-butyl-6-[(dimethylamino)methyl]phenol (1) reacts with 2 equiv of DBU in DMF under reflux to give 57% of alkylation product 2 (Scheme 1). Presumably, the reaction involves generation of highly electrophilic o-quinone methide A [9–11] by thermolysis of Mannich base 1 and subsequent conjugate addition of DBU molecule. Another possible path is quaternization of the imino nitrogen atom in DBU with o-quinone methide, followed by migration of the hydroxybenzyl group to C [2]. The described transformation is likely to be related to α-alkylation of ketenimines with o-quinone methides [12]. No [4 + 2]-cycloaddition of o-quinone methide to the C=N bond was observed [13]. As by-product, we isolated less than 10% of 2,2′-methylenebis(4,6-di-tertbutylphenol) 3 which may be formed via deamination of Mannich base 1 and subsequent alkylation of 2,4-ditert-butylphenol with o-quinone methide A (Scheme 2).