Delocalisation of the nitrogen lone electron pair by an acyl and acylvinylogue group simultaneously, results in a lowering of both CN rotational barriers. MeC(O)CBrCHNMe2 and MeC(O)C(OCOMe)CHNMe2 exhibit restricted rotation around what is formally a double bond CC.
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Dąbrowski et al. (1973) studied this question.
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