The basicity of enones. Substituent effects and the correlation of protonation with HA

Abstract

1365 (1971). (17) Expected to be closely analogous to the p-toluenesulfonate leaving group, which departs about 5 X 103 more easily than bromide: R. C. Bingham and P. v. R. Schleyer, J. Am. Chem. Soc., 93, 3189 (1971). (18) In this case LiEtaBH evidently acts as a carbonium Ion trap in much the same way as does BH4-: . M.

Stereoselective Synthesis of 2,6-Cis-Substituted Tetrahydropyrans: Brønsted Acid-Catalyzed Intramolecular Oxa-Conjugate Cyclization of α,β-Unsaturated Ester Surrogates

“…In line with the theoretical study by Houk and Strozier, it has been proposed that acid-catalyzed hydration of aliphatic α,β-unsaturated ketones I , such as 3-buten-2-one derivatives, involves protonation of the carbonyl oxygen followed by addition of H 2 O to the β-carbon atom (Scheme ). Because of the contribution of the canonical structures II and III , the transition structure of the reaction would have an allylic cationic character . On the basis of these previous studies, we propose that Brønsted acid-catalyzed IOCC of ( E )-α,β-unsaturated ester surrogates would involve an allylic cationic transition structure and proceed via an S N 1-type mechanism (Scheme B).…”

Acidic Ruthenium PNNP Complexes of Non-enolized 1,3-Dicarbonyl Compounds as Catalysts for Asymmetric Michael Addition

“…As the Michael addition smoothly occurs to give 8 upon addition of triflic acid to the palladium enolato complex, it was suggested that the role of the acid is to activate the enone toward nucleophilic attack by the enolato complex . However, it seemed unlikely to us that such an explanation would hold for the ruthenium system, as, on the basis of the p K a value between 2 and 5 estimated for 2a , we anticipated that the α-hydrogen of the ruthenium-bound β-keto ester would not be acidic enough to protonate methyl vinyl ketone to such an extent to trigger the 1,4-addition. An alternative mechanism based on the activation of the enone toward nucleophilic attack by coordination to the metal, as suggested for the iron(III)-catalyzed Michael addition, seemed also improbable, as 3a is a coordinatively saturated complex.…”

The stereodivergent formation of 2,6-cis and 2,6-trans-tetrahydropyrans: experimental and computational investigation of the mechanism of a thioester oxy-Michael cyclization

“… 8 Although there is no generally accepted mechanism for the acid-mediated cyclization, it has been proposed that the formation of the 2,6- cis -THP is favored due to greater stereoelectronic stabilization of the transition state from both the FMO coefficients of the allylic cation and orbital overlap with the oxygen lone pair, compared to the transition state leading to the 2,6- trans -THP. 9 …”