Cation-anion combination reactions. XIII. Correlation of the reactions of nucleophiles with esters

Towards a General Scale of Nucleophilicity?

“…[47] If only reactions of nucleophiles with carbocations are considered, the electrophile-specific slope parameter s E becomes 1 and Equation (6) [20] Water, originally selected as the reference nucleophile of the Ritchie relationship, [7] was later reported to deviate from the constant selectivity relationship in Equation (2). [43,48] We have recently shown that this deviation is due to the slope parameter s N (H 2 O) = 0.89 [21] which differs from those of the other nucleophiles studied by Ritchie. Note, however, that other groups of reactions, for example, reactions of terminal alkenes (s N = 1) with carbocations (s E = 1) also constitute discrete sets of constant selectivity relationships which can be described by Equation (2).…”

Auf dem Weg zu einer allgemeinen Nucleophilie‐Skala?

“…[43,48] Wir haben kürzlich gezeigt, dass dieses Abweichen auf den Steigungsparameter s N (H 2 O) = 0.89 [21] zurückzuführen ist, der sich von den s N -Parametern der anderen von Ritchie untersuchten Nucleophile unterscheidet. Es ist jedoch zu beachten, dass es weitere Reaktionsserien gibt, z. B.…”

Comparison of the Electrophilic Reactivities of N‐Acylpyridinium Ions and Other Acylating Agents

“…However, the fact that 20 reacts 1.5 orders of magnitude faster with 5 than isobasic amines [morpholine (24), benzylamine (15)] is in line with the previous statement that the magnitude of the α-effect increases with the slope of the Brønsted correlation. [4] If both nucleophilic attack and subsequent expulsion of the leaving group contribute to the second-order rate constants, as in the reactions of less reactive acylating agents (for example esters), good correlations of the second-order rate constants for the reactions with the nucleophilicity parameters N according to Equation (2) cannot be expected. [4] If both nucleophilic attack and subsequent expulsion of the leaving group contribute to the second-order rate constants, as in the reactions of less reactive acylating agents (for example esters), good correlations of the second-order rate constants for the reactions with the nucleophilicity parameters N according to Equation (2) cannot be expected.…”

“…As shown in Scheme 1 (top), acylium ions can be formed in different ways before they undergo fast subsequent reactions with nucleophiles. [4] We have previously reported that the reactions of carbocations and quinone methides with different classes of σ-, π-, and n-nucleophiles follow the linear free energy relation-ship Equation (2), where electrophiles are characterized by one parameter (E) and nucleophiles are characterized by two solvent-dependent parameters (N and s N ). There has been much debate concerning whether attack of the nucleophiles at the carbonyl group displaces Xin a concerted manner or whether tetrahedral intermediates are involved.…”