The α-effect in the S_NAr reaction of 1-(4-nitrophenoxy)-2,4-dinitrobenzene with anionic nucleophiles: effects of solvation and polarizability on the α-effect

Abstract

A kinetic study on S N Ar reactions of 1-(4-nitrophenoxy)-2,4-dinitrobenzene (1a) with various anionic nucleophiles in 80 mol% water -20 mol% DMSO at 25.0°C is reported. The Brønsted-type plot for the reaction of 1a with a series of substituted phenoxides and HOO − results in an excellent linear correlation with ␤ nuc = 1.17. However, OH − exhibits dramatic negative deviation from the Brønsted-type plot, while N 3 − , C 6 H 5 S − , and butane-2,3-dione monoximate (Ox − ) deviate positively from linearity. HOO − is 680-fold more reactive than OH − but does not exhibit the ␣-effect. In contrast, Ox − is 166-fold more reactive than isobasic 4-Cl − C 6 H 4 O − and exhibits the ␣-effect. Differential solvation effects have been suggested to be responsible for the ␣-effect in this study, i.e., Ox − exhibits the ␣-effect, since it is 5.7 kcal/mol less strongly solvated than 4-Cl − C 6 H 4 O − in the reaction medium, while HOO − does not show the ␣-effect due to a strong requirement for partial desolvation before nucleophilic attack. The highly enhanced reactivity of polarizable N 3 − and C 6 H 5 S − and extremely decreased reactivity of nonpolarizable OH − are in accord with the hard-soft acid and base principle.Résumé : Nous présentons une étude cinétique des réactions S N Ar entre le 1-(4-nitrophénoxy)-2,4-dinitrobenzène (1a) et divers nucléophiles anioniques dans un mélange constitué d'eau à 80 % molaire et de DMSO à 20 % molaire à une température de 25,0°C. La courbe de type Brønsted pour les réactions du composé 1a avec une série de phénoxydes substitués et avec l'ion HOO − donne lieu à une excellente corrélation linéaire dont le coefficient ␤ nuc = 1,17. Cependant, les ions OH − sont associés à une déviation négative importante de la courbe de type Brønsted, tandis que les ions N 3 − , C 6 H 5 S − et butane-2,3-dionemonoximate (Ox − ) dévient de la linéarité de façon positive. L'ion HOO − est 680 fois plus réactif que l'ion OH − , mais ne montre pas d'effet ␣. À l'inverse, l'ion Ox − est 166 fois plus réactif que l'ion isobasique 4-ClC 6 H 4 O − et présente l'effet ␣. Nous avons proposé des effets de solvatation variables d'un composé à l'autre pour expliquer l'effet ␣ observé dans cette étude, c.-à -d. que l'ion Ox − présente l'effet ␣ parce qu'il est moins fortement solvaté dans le milieu réactionnel que l'ion 4-Cl-C 6 H 4 O − (différence de 5,7 kcal/mol), tandis que l'ion HOO − ne montre pas d'effet ␣ en raison de fortes contraintes à la désolvatation partielle préalable à l'attaque nucléophile. La réactivité fortement accrue des ions polarisables N 3 − et C 6 H 5 S − et la réduction extrême de la réactivité de l'ion non polarisable OH − sont des phénomènes en accord avec le principe de dureté chimique des acides et des bases. [Traduit par la Rédaction]

Kinetic Study on Nucleophilic Substitution Reactions of Aryl Diphenylphosphinates with Butane‐2,3‐dione Monoximate and Aryloxide Anions: Reaction Mechanism and Origin of the α‐Effect

“…In contrast, the α‐effect found in the corresponding reactions of S ‐4‐nitrophenyl thioacetate ( 1b ) increases with increasing the mol % DMSO up to near 50 mol % DMSO and then remains nearly constant beyond that point 9a . A similar result has been observed for the corresponding reactions of 4‐nitrophenyl benzoate ( 2a ) 9b and O ‐4‐nitrophenyl thionobenzoate ( 2b ), 9c indicating that solvent effect is indeed an important factor that affects the magnitude of the α‐effect …”

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

“…Regarding the mechanism of this transformation, the authors propose that the desired neutral hydrazonyl radical was generated through formal homolysis of the N–H bond via sequential Brønsted base (p K a = 15.74 in H 2 O) 353 mediated deprotonation of the allylic hydrazone (e.g., for acetophenone hydrazone, p K a = 4.7 in MeOH) 354 and single-electron oxidation of the resulting anion by the excited state of the Ru(II) photocatalyst ( E 1/2 *Ru(II)/Ru(I) = +0.77 V vs SCE in MeCN). 64 The resulting neutral, N -centered hydrazonyl radical undergoes a 5- exo- trig cyclization onto a pendant alkene.…”

Medium effect (water versus MeCN) on reactivity and reaction pathways for the SNAr reaction of 1-aryloxy-2,4-dinitrobenzenes with cyclic secondary amines

“…anionic versus neutral and; the effect of solvent. [1][2][3][4] Our interests have included studies of S N Ar reactions of highly reactive, α-nucleophiles, 33,34 effects of alkali metal ions, 35 reactions of primary and secondary cyclic amines [24][25][26] with Sanger's reagent (1-fluoro-2,4dinitrobenzene), a series of 1-halo-2,4-dinitrobenzenes and with 1-(substituted phenoxy)-2,4dinitrobenzenes as electron deficient substrates, usually in acetonitrile (MeCN) solvent often with comparison to the results in water, as a standard reaction medium. Nucleophilic reaction of amines in the S N Ar process is traditionally described, 36 as shown in Scheme 1, as a partition between two pathways after formation of the initial zwitterionic MC, i.e., MC ± ; expulsion of the leaving group, a substituted phenoxide (ArO -) in current work, gives the protonated product, PH + , that equilibrates rapidly to give the observed 2,4dinitroaniline product, P (Scheme 1) in the basic medium.…”