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Knorr, Rudolf; Schmidt, Barbara; Freudenreich, Johannes; Roman, Therese von (2018): β-Shielded Michael systems whose H-C(sp(2)) deprotonation and 1,2-addition reactions compete for organolithiums. In: Tetrahedron, Vol. 74, No. 52: pp. 7466-7471
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Steric shielding of an alpha,beta-unsaturated nitrile R2C = CH-C N by tert-alkyl beta-substituents (R) prevented 1,4-additions of organolithiums and showed a remarkably solvent-dependent selectivity of tert-butyllithium (tBuLi): 1,2-addition at C N and alpha-deprotonation occurred almost equally fast in THF, while C N addition was exclusive in Et2O. The resultant N-lithioimine was cleanly alpha-deprotonated (albeit with unusual slowness) in situ by an excess of tBuLi. The imine was hydrolyzed to produce an enone R2C = CH-C(R') = O with bulky substituents R and R', which admitted C=O addition but did not undergo alpha-deprotonation by mesityllithium, n-butyllithium (nBuLi), and even the very strong base tBuLi. 2,4,6-Tri-tert-butylphenyllithium ("Mes*Li") did not add or deprotonate the enone. With N-lithio-2,2,6,6-tetramethylpiperidine (LTMP) as the catalyst, however, Mes*Li achieved alpha-deprotonation simultaneously with the known Mes* radical rearrangement and a subsequent carbonyl addition. With KOH or KOtBu as a base in [D-6]DMSO (but not in DOCH3), the enone accomplished a rapid alpha-H/alpha-D exchange reaction at rt.