Synthesis and Characterization of Novel Quinone Methides: Reference Electrophiles for the Construction of Nucleophilicity Scales

Abstract

Keywords: Electrophilicity / Nucleophilicity / Kinetics / Linear free-energy relationships / Quinone methides Novel synthetic routes to the aryl-substituted quinone methides 3a-f have been developed, and a previously reported Mannich approach has been used for the syntheses of the acceptor substituted quinone methides 2e-g. The secondorder rate constants for the reactions of 3c-f and 2e-g with the carbanions 9a-h were determined photometrically in DMSO. With Equation (1), log k 2 = s (N + E), and the known nucleophilicity parameters N and s for the carbanions 9a-h,

Reactions of N1-Methyl-4-nitro-2,1,3-benzoselenadiazolium Tetrafluoroborate with Aliphatic and Cyclic Amines in Acetonitrile: Kinetic and Structure-Reactivity Correlations

“…As can be seen, the electrophilic character of benzoselenadiazolium cation 1 of −14.72 can be compared with values of −14.36 for 2,6‐di‐tert.butyl‐4‐(4‐nitrobenzylidene)cyclohexa‐2,5‐dienone 5 , −14.70 for 4‐methoxy‐nitrostyrene 6 , and −14.91 for 1,2‐diaza‐1,3‐diene in DMSO 7 .…”

The generation and reactions of quinone methides

“…3 Consequently, the rate constants k Nu for addition of nucleophiles to a di-CF 3 substituted quinone methide (Scheme 3A) are correlated by the N + scale for nucleophile addition to strongly resonance stabilized carbocations, 4,5 and quinone methides have been used by Mayr, alongside other carbocations, as reference electrophiles in the construction of nucleophilicity scales. 6 The efficient and synthetically useful 2 + 4 addition of electron-rich alkenes to 1,2 quinone methides can be similarly understood as the stepwise electrophilic addition of a resonance stabilized benzylic carbocation to nucleophilic alkene, followed by capture of the oxocarbenium ion intermediate by phenoxide ion (Scheme 3B). 7 …”

Nucleophilic reactivities of tertiary alkylamines

“…Because the nucleophilic activities of amines are known to correlate only poorly with their Brønsted basicities (p K aH ),33–35 we now set out to include the tertiary amines 1(a–d) in our comprehensive nucleophilicity scales36–40 by studying the rates of their reactions with benzhydrylium ions 36 …”

“…Numerous kinetic investigations have shown that the rate constants for the reactions of n ‐, π ‐, and σ ‐nucleophiles with carbocations can be described by Eqn (1),36–40 where nucleophiles are characterized by two parameters ( N, s ) and electrophiles are characterized by one parameter ( E ). By employing benzhydrylium ions ( 2 , Table 1)36 and structurally related quinone methides40 as reference electrophiles, it became possible to compare reactivities of a large number of nucleophiles in a single scale 36–40. With this methodology, we have previously quantified the nucleophilicities of numerous n ‐, π ‐, and σ ‐nucleophiles,41 including primary and secondary amines,42 pyridines,43 amidines,44 cinchona alkaloids,45 as well as the tertiary alkylamines 1e,f 46.…”