Boosting conjugate addition to nitroolefins using lithium tetraorganozincates : synthetic strategies and structural insights

Dell'Aera, Marzia and Perna, Filippo Maria and Vitale, Paola and Altomare, Angela and Palmieri, Alessandro and Maddock, Lewis C. H. and Bole, Leonie J. and Kennedy, Alan R. and Hevia, Eva and Capriati, Vito (2020) Boosting conjugate addition to nitroolefins using lithium tetraorganozincates : synthetic strategies and structural insights. Chemistry - A European Journal, 26 (40). pp. 8742-8748. ISSN 0947-6539

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    Abstract

    We report the first transition metal catalyst- and ligand-free conjugate addition of lithium tetraorganozincates (R 4ZnLi 2) to nitroolefins. Displaying enhanced nucleophilicity combined with unique chemoselectivity and functional group tolerance, homoleptic aliphatic and aromatic R 4ZnLi 2 provide access to valuable nitroalkanes in up to 98 % yield under mild conditions (0 °C) and short reaction time (30 min). This is particularly remarkable when employing β-nitroacrylates and β-nitroenones, where despite the presence of other electrophilic groups, selective 1,4 addition to the C=C is preferred. Structural and spectroscopic studies confirmed the formation of tetraorganozincate species in solution, the nature of which has been a long debated issue, and allowed to unveil the key role played by donor additives on the aggregation and structure of these reagents. Thus, while chelating N,N,N',N'-tetramethylethylenediamine (TMEDA) and (R,R)-N,N,N',N'-tetramethyl-1,2-diaminocyclohexane (TMCDA) favour the formation of contacted-ion pair zincates, macrocyclic Lewis donor 12-crown-4 triggers an immediate disproportionation process of Et 4ZnLi 2 into equimolar amounts of solvent-separated Et 3ZnLi and EtLi.