A Theoretical Study of Substituent Effects on Allylic Ion and Ion Pair S_N2 Reactions

Abstract

An ab initio study of ionic and ion pair displacement reactions involving allylic systems has been carried out at the RHF/6-31+G* level. The geometries and natural charges show the absence of conjugative stabilization in the ionic transition states, thus differing from traditional explanations. The high reactivity of allyl halides is explained by electrostatic polarization of the double bond. Substituent effects were also studied; in general, electron-withdrawing groups lower the barriers of the ionic S(N)2 reactions but increase the barriers of the ion pair reactions. The allylic reactions are compared with related benzylic systems. Hammett correlations give rho of opposite sign for the ionic and ion pair displacement reactions, in agreement with some experimental results.

Effect of Allylic Groups on SN2 Reactivity

“…26b If conjugative π delocalization is significant then, as Streitwieser suggests, the C α –C β bond should become shorter in the transition states. 25 This in fact does occur for all reactions, but only by 0.05 Å or less, at least for the reactions of the nonallylic substrates; see Table 4. We conclude that for these all-hydrocarbon R groups the reactant-state polarity is of less importance than it is in such highly activated polar systems as acetonyl chloride and cyanomethyl chloride (and more modestly expressed by para -substituted benzyl halides), 26a but that transition-state polarity is critical.…”

“…Rather, the Streitwieser electrostatic polarization model in which the unsaturated C α –C β bond offers some stabilization of the transition state provides a satisfactory conceptual framework. 25 …”

Computational Study of Reactivity and Transition Structures in Nucleophilic Substitutions on Benzyl Bromides

“…[14] This pathway has also been taken into consideration in other cases. [3,[15][16][17] In this paper we report on DFT computations relating to nucleophilic substitutions of benzyl bromides with the charged Br -and the uncharged pyridine as nucleophiles. Optimized structures of the reactants and TSs and also the activation parameters were calculated for the reactions of compounds containing e-d and e-w substituents in the substrate or in the nucleophile, in different solvents and in vacuo.…”

Synthesis of carbasugar-containing non-glycosidically linked pseudodisaccharides and higher pseudooligosaccharides

“…bmanno-Configured 5a-carba-1,2-epoxides (1 and 2) were opened It is noteworthy that nitrogen atoms bridging two secondary carbons in N-linked pseudodisaccharide structures tend to be resistant towards acylation. stereospecifically and with excellent regioselectivity by carbohydrate amines (3)(4)(5)(6)(7)(8)(9) to give good yields of a-manno-configured imino-linked pseudodisaccharides and pseudotrisaccharides (10-15; Table 1). 10,11 Tribenzylated (1) and unprotected (2) epoxides have both been used.…”