Carbon-to-Carbon Identity Proton Transfer from Allene, Ketene, Ketenimine, and Thioketene to Their Respective Conjugate Anions in the Gas Phase. Anab InitioStudy

Abstract

Gas-phase acidities of CH2=C=X (X = CH2, NH, O, and S) and barriers for the identity proton transfers (X=C=CH2 + HC triple bond C-X- right harpoon over left harpoon -X-C triple bond CH + CH2=C=X) as well as geometries and charge distributions of CH2=C=X, HC triple bond C-X- and the transition states of the proton transfer were determined by ab initio methods at the MP2/6-311+G(d,p)//MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. The acidities were also calculated at the CCSD(T)/6-311+G(2df,p) level. A major objective of this study was to examine how the enhanced unsaturation of CH2=C=X compared to that of CH3CH=X may affect acidities, transition state imbalances, and intrinsic barriers of the identity proton transfer. The results show that the acidities are all higher while the barriers are lower than for the corresponding CH3CH=X series. The transition states are all imbalanced but less so than for the reactions of CH3CH=X.

Transition State Asymmetry in C–H Bond Cleavage by Proton-Coupled Electron Transfer

“…Reactions with imbalanced transition states have long been discussed in the physical organic literature. In particular, Bernasconi has argued that this is a very common situation, as enunciated in his Principle of Nonperfect Synchronization (PNS). ,,,,, Others have developed similar ideas in different formalisms, including Marcus, Kresge, , More O’Ferrall, Jencks, Grunwald, Guthrie, ,, and Bordwell . One classic example is the deprotonation of nitroalkanes, where C–H bond cleavage is more pronounced at the TS than the electronic delocalization of the π system. , …”

Proton Transfer to and from Carbon in Model Reactions

“…1.2A). This has led to the determination of the Marcus intrinsic barriers for a variety of proton transfer reactions by experiment and through calculations [58][59][60][61][62][63][64][65]. 1.2A) [46,50,51], carbocation-nucleophile addition [52], bimolecular nucleophilic substitution [53,54] and other reactions [55][56][57] by assuming that their reaction coordinate profiles may also be constructed from the intersection of parabolas that describe the reactant and product states.…”

One-step preparation of novel 1-(N-indolyl)-1,3-butadienes by base-catalysed isomerization of alkynes as an access to 5-(N-indolyl)-naphthoquinones

“…This mechanism is inspired in the accepted route for alkyne isomerizations in basic media, which occur by a series of equilibria that involve resonance-stabilized carbanions. 9 The initial N -substitution product 8, forms in basic medium a resonance-stabilized propargyl carbanion (9). Allenic carbanions are particularly stable, and subsequent protonation gives the allene 10.…”