Kinetics of Proton Transfer from Benzo[b]-2,3-dihydrofuran-2-one and Benzo[b]-2,3-dihydrothiophene-2-one. Effect of Anion Aromaticity on Intrinsic Barriers
Abstract:Rates of the reversible deprotonation of benzo[b]-2,3-dihydrofuran-2-one (6H-O) and benzo[b]-2,3-dihydrothiophene-2-one (6H-S) by OH-, primary aliphatic amines, secondary alicyclic amines, and carboxylate ions have been determined in water at 25 degrees C. As noted earlier by Kresge and Meng, 6H-S (pKa = 8.82) is considerably more acidic than 6H-O (pKa = 11.68), which mainly reflects the greater aromatic stabilization of the conjugate base of 6H-S (thiophene derivative) compared to that of 6H-O (furan derivati… Show more
“…According to the PNS, these results imply that the development of aromaticity is ahead of proton transfer at the transition state. The study of a third reaction, eq 5, yielded Δ(O) < Δ(S) . However, a detailed analysis 19,20 revealed that the observed reactivity order is an artifact stemming from other factors that mask the Δ-lowering effect of the increased aromaticity of 3 - -S. …”
An ab initio study of six carbon-to-carbon identity proton transfers is reported. They refer to the benzenium ion/benzene (C6H7(+)/C6H6), the 2,4-cyclopentadiene/cyclopentadienyl anion (C5H6/C5H5(-)), and the cyclobutenyl cation/cyclobutadiene (C4H5(+)/C4H4) systems and their respective noncyclic reference systems, that is, [structure: see text], [structure: see text] and [structure: see text]. For the aromatic C6H7(+)/C6H6 and C5H6/C5H5(-) systems, geometric parameters and aromaticity indices indicate that the transition states are highly aromatic. The proton-transfer barriers in these systems are quite low, which is consistent with a disproportionately high degree of transition-state aromaticity. For the antiaromatic C4H5(+)/C4H4 system, the geometric parameters and aromaticity indices indicate a rather small degree of antiaromaticity of the transition state. However, the proton-transfer barrier is higher than expected for a transition state with a low antiaromaticity. This implies that another factor contributes to the barrier; it is suggested that this factor is angle and torsional strain in the transition state. The question whether charge delocalization at the transition state might correlate with the development of aromaticity was also examined. No such correlation was found, that is, charge delocalization lags behind proton transfer as is commonly observed in nonaromatic systems involving pi-acceptor groups.
“…According to the PNS, these results imply that the development of aromaticity is ahead of proton transfer at the transition state. The study of a third reaction, eq 5, yielded Δ(O) < Δ(S) . However, a detailed analysis 19,20 revealed that the observed reactivity order is an artifact stemming from other factors that mask the Δ-lowering effect of the increased aromaticity of 3 - -S. …”
An ab initio study of six carbon-to-carbon identity proton transfers is reported. They refer to the benzenium ion/benzene (C6H7(+)/C6H6), the 2,4-cyclopentadiene/cyclopentadienyl anion (C5H6/C5H5(-)), and the cyclobutenyl cation/cyclobutadiene (C4H5(+)/C4H4) systems and their respective noncyclic reference systems, that is, [structure: see text], [structure: see text] and [structure: see text]. For the aromatic C6H7(+)/C6H6 and C5H6/C5H5(-) systems, geometric parameters and aromaticity indices indicate that the transition states are highly aromatic. The proton-transfer barriers in these systems are quite low, which is consistent with a disproportionately high degree of transition-state aromaticity. For the antiaromatic C4H5(+)/C4H4 system, the geometric parameters and aromaticity indices indicate a rather small degree of antiaromaticity of the transition state. However, the proton-transfer barrier is higher than expected for a transition state with a low antiaromaticity. This implies that another factor contributes to the barrier; it is suggested that this factor is angle and torsional strain in the transition state. The question whether charge delocalization at the transition state might correlate with the development of aromaticity was also examined. No such correlation was found, that is, charge delocalization lags behind proton transfer as is commonly observed in nonaromatic systems involving pi-acceptor groups.
“…Intrinsic barriers for deprotonation of ketone 1 (X = S) by amines and hydroxide ion were found to be higher than for 1 (X = O) at 25 1C. 1 These data were found to be consistent with a higher intrinsic barrier for the formation of a more aromatic anion 2 (X = S) implying that the development of aromaticity lags behind proton transfer at the transition state. Rate constants for the reversible deprotonation of methylnitroacetate (MeO 2 CCH 2 NO 2 ) by a series of bases were determined in water and 50:50 DMSO:water at 20 1C.…”
“…Another system, eq. 7, yielded somewhat ambiguous results stemming from complications due to resonance effects in 9H-X [22]; however, a detailed analysis revealed that here again the greater aromaticity of the sulfur derivative increases k o relative to the oxygen derivative [21].…”
The question as to what extent aromaticity in a reactant or product is expressed in the transition state of a reaction has only recently received serious attention. Inasmuch as aromaticity is related to resonance, one might expect that, in a reaction that leads to aromatic products, its development at the transition state should lag behind bond changes as is invariably the case for the development of resonance in reactions that lead to delocalized products. However, recent experimental and computational studies on proton transfers from carbon acids suggest the opposite behavior, i.e., the development of aromaticity at the transition state is more advanced than the proton transfer. The evidence for this claim is based on the determination of intrinsic barriers that show a decrease with increasing aromaticity. According to the Principle of Nonperfect Synchronization (PNS), this decrease in the intrinsic barrier implies a disproportionately large amount of aromatic stabilization of the transition state. Additional evidence for the high degree of transition state aromaticity comes from the calculation of aromaticity indices such as HOMA, NICS, and the Bird Index. Possible reasons why the degree to which aromaticity and resonance are expressed at the transition state is different are discussed.
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