Reactions of Aryl Phenylacetates with Secondary Amines in MeCN. Structure−Reactivity Relationship in the Ketene-Forming Eliminations and Concurrent E2 and E1cb Mechanisms

Abstract

Elimination reactions of aryl esters of arylacetic acids 1 and 2 promoted by R2NH in MeCN have been investigated kinetically. The reactions are second-order and exhibit β = 0.44−0.84, |βlg| = 0.41−0.50, and ρH = 2.0−3.6. Brønsted β and |βlg| decrease with the electron-withdrawing ability of the β-aryl substituent. Hammett ρH values remain nearly the same, but the |βlg| value increases as the base strength becomes weaker. Both ρH and β decrease with the change of the leaving group from 4-nitrophenoxide to 2,4-dinitrophenoxide. The results are consistent with an E2 mechanism and a reaction coordinate with a large horizontal component corresponding to proton transfer. When the base−solvent system is changed from R2NH−MeCN to R2NH/R2NH2 +−70 mol % MeCN(aq), the Brønsted β, ρH, and |βlg| decrease. Finally, the ketene-forming elimination reactions from p-nitrophenyl p-nitrophenylacetate promoted by R2NH/R2NH2 + buffers in 70 mol % MeCN(aq) have been shown to proceed by concurrent E2 and E1cb mechanisms.

Elimination Reactions of (E)-2,4,6-Trinitrobenzaldehyde O-Aryloximes Promoted by R3N/R3NH+in 70 mol% MeCN(aq). Effect of β-Aryl Group the Nitrile-Forming Transition-State

“…15,16 Therefore, the measured values of |β lg | = 0.34-0.46 for R 3 N-promoted eliminations from 1 can most reasonably be interpreted with a limited extent of N α -OAr bond cleavages in the transition state. The combined results reveal that the nitrile-forming elimination from 1d proceeds by the E2 mechanism with significant C β -H bond cleavage and limited N α -OAr bond cleavage.…”

Ketene-Forming Elimination Reactions from Aryl Thienylacetates Promoted by R2NH in MeCN. Effects of Base-Solvent and β-Aryl Group

“…The product of reaction between 1a and i-Bu 2 NH in MeCN was identified by TLC and GC as before. 14 The products were N,N-di(isobutyl)thenylamide and 4-nitrophenoxide. The yield of N,N-di(isobutyl)thenyl-amide determined by GC was 91%.…”

“…Reactions of 1 with R 2 NH in MeCN were followed by monitoring the increase in the absorbance of the aryloxides at 411-448 nm with a UV-vis spectrophotometer as described. [14][15][16] Product Studies. The products were also identified by periodically monitoring the UV absorption of the reactions mixtures under the reaction condition.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14] Whereas the former is the major reaction pathway in protic solvents with hydroxide or alkoxide as the base, the latter becomes predominant with amine bases in aprotic solvents. In protic solvents, the elimination reaction mechanism changes from E2 to E1cb as the good leaving group is made poorer and as the electron withdrawing ability of the β-aryl substituent increases.…”

“…In contrast, the reactions of aryl phenylacetates [PhCH 2 C(O)OC 6 H 3 -2-X-4-NO 2 (2)] proceeded via an E2 mechanism under the same condition. 14,16 The remarkable difference between the two reactions was attributed to the smaller aromatic resonance energy of thiophene than benzene. 17 Because the negative charge density at the β-carbon could be delocalized by the thienyl group, the E1cb intermediate seemed to be stabilized and the E1cb mechanism competed.…”