Formation of Hydrogen‐bonded Carbanions as Intermediates in Hydron Transfer between Carbon and Oxygen

Abstract

This chapter is divided into three sections: Proton transfer from carbon acids to methoxide ion; proton ttransfer from methanol to carbanion intermediates; proton transfer associated with methoxide promoted dehydrohalogenation reactions. Proton Transfer from Carbon Acids to Methoxide IonKnowledge of equilibrium pK a values of carbon acids is important for an understanding of organic chemistry; however, they are not always reliable indicators of relative rates for proton transfer reactions. Ritchie [1] predicted in 1969 that carbon acids whose conjugate bases have localized charge will show "kinetic acidities" greater than their thermodynamic acidities. This is illustrated by the weaker acid pentafluorobenzene-t [PFB-t], pK a = 25.8 [2], with a methanolic sodium methoxide catalyzed protodetritiation rate, k = 2.57 · 10 -2 M -1 s -1 [3], that is 15 times faster at 25 C than that for 9-phenylfluorene-9-t [9-PhFl-9-t], pK a = 18.5 and k = 1.54 · 10 -3 M -1 s -1 [4]. Primary kinetic isotope effects [PKIE] can be used to determine C-H bond breaking in the rate limiting steps of any reaction mechanism. Normal hydrogen isotope effects are largely due to zero-point energy differences of the stretching frequencies of C-H vs. C-D bonds. If only stretching frequencies are considered the value for k H /k D would be 6.2 for the C-i H bond. With allowance for bending vibrations this could result in a k H /k D = 10, k H /k T = 27 and k D /k T = 2.7 at 25 C [5]. Melander [6] and Westheimer [7] suggested that lower values of k H /k D could be due to residual zero-point energy in an asymmetric transition structure. For this reason the magnitude of k H /k D was often used to assign early or late transition structures for hydron-transfer reactions. Smaller values due to asymmetric transition structures should still obey the Swain-Schaad relationship [8], k H /k T = (k H /k D ) 1.442 . The k D /k T = 1.0 associated with the exchange reactions of PFB i H suggest that the C-H bond is not broken in the rate limiting step and this differs significantly from the k D /k T of 2.54 measured from the reactions of 9-PhFl-9-i H.

Abnormal Michael Reaction: Condition-Dependent Product Distributions in the Michael Reaction of Monoalkyl-Substituted Malonates

“…On the basis of these findings, it is reasonable to hypothesize that the presence of a proton source is associated with the formation of the abnormal product, which requires a proton shift that could potentially be facilitated by a proton source present in the reaction media . To confirm this hypothesis, we conducted reactions using NaH in THF or Et 2 O, which do not generate any proton sources, thereby resulting in no formation of the abnormal product regardless of the concentrations of reactants (entries 13–16).…”

“…The fact that 21 is always dominant indicates that the rate of formation of the malonate anion D by a proton shift in C is slower than that of the elimination of a malonate anion in C , contrary to expectations based on the highly acidic character of a malonate proton. It is well-known that the rates of proton transfer involving carbon acids are much slower than those between electronegative atoms because of the increased energy requirements for internal geometric reorganizations on going from the acid to a resonance-stabilized conjugate base. , Therefore, the findings that abnormal product 23 is obtained only when using NaOEt and NaOBu t lead us to assume that the alcohols, resulting from proton abstraction, participate in the proton shift in C , for example, in the manner shown in Figure . The assumption can explain the concentration-dependent formation of the abnormal product 23 and its increase in the presence of an excess of ethanol.…”

NMR Study of CHN Hydrogen Bond and Proton Transfer in 1,1-Dinitroethane Complex with 2,4,6-Trimethylpyridine

“…The ability of a CH group to form C–H···X bonds often follows the trend C(sp)–H > C(sp 2 )–H > C(sp 3 )–H, , but introducing electronegative substituents could significantly change it. CH acidity can also be significantly changed, for example, the p K a for (CN) 3 CH is 5.13 and that for (CN) 2 CH 2 is 11.19. , After the proton transfer, complexes of the type C – ···HX + could be formed, i.e., zwitterionic pairs with carbanions. − On the one hand, the C – ···HX + hydrogen bond might be a local energy minimum along the proton transfer pathway, as, for example, it was demonstrated by calculations for the reaction of C 6 H 5 (CF 3 ) 2 CH with MeO – . On the other hand, after the proton transfer, a significant redistribution of electron density in the acid molecule can occur, decreasing the proton-accepting ability of the carbon atom and making the C – ···HX + form energetically unfavorable. − Besides, the C – /HX + ion pair might be solvent-separated or even completely dissociated in a polar solvent. , …”

“…10−16 On the one hand, the C − •••HX + hydrogen bond might be a local energy minimum along the proton transfer pathway, as, for example, it was demonstrated by calculations for the reaction of C 6 H 5 (CF 3 ) 2 CH with MeO − . 17 On the other hand, after the proton transfer, a significant redistribution of electron density in the acid molecule can occur, decreasing the proton-accepting ability of the carbon atom and making the C − •••HX + form energetically unfavorable. 18−22 Besides, the C − /HX + ion pair might be solvent-separated or even completely dissociated in a polar solvent.…”