Electrophilic Additions to Highly Reactive Enol Ethers. The Particular Role of Resonance in Determining the Kinetic Selectivity Evidenced by Extensive Comparison of Olefin Bromination and Hydration

Abstract

AbstractThe kinetic selectivity of aliphatic enol ethers, EtOCR = CHR' (R and R′ = H or Me), towards electrophiles, I2, Br2 and H3O+, is expressed by the kinetic effect of a methyl substituent in the α position with respect to the ethoxy group, kα‐Me/kH. As expected from the reactivity–selectivity principle, RSP, these selectivities are small, 16, 18 and 330, respectively, as compared to those observed for less reactive olefins. However, a more general comparison of the selectivities of various XCH = CH2 olefins (X = Br, Me, Ph, OAc, OEt) reveals anomalies in their behavior with respect to the RSP: (i) enol ether iodination and bromination exhibit the same selectivity although their rates differ by 4 powers of ten, (ii) enol acetate and enol ether show similar selectivities in bromination but the rate of acetate is 3 × 105 times smaller than that of ether and (iii) in hydration the selectivities of these two olefins are similar to that of styrene although rates range over 7 powers of ten from styrene to enol ether. In contrast with what was previously observed for homogeneous series of R‐substituted styrenes (Ph(R)C = CH2), there is no reactivity–selectivity relationship for electrophilic additions to XCH = CH2 olefins. There is a parallelism, however, between the selectivities and the transition‐state position estimated by the Brønsted exponents for hydration and by the Winstein–Grunwald coefficients for solvent effects on halogenations. These results are discussed in terms of different resonance effects on transition states and on reactivities which could arise from differences in the relative contributions of thermodynamic and intrinsic kinetic (Hammond effects) factors on the selectivities.