Se2 reactions

As far as investigated77, most reactions of the allyllithium-sparteine complexes with electrophiles proceed antarafacially, either as SE2 or anti-SE2 reactions. As a working hypothesis it is assumed that the bulky ligand obliterates the Lewis acid properties of the lithium cation. 

The reason for these discrepancies may arise from the very important discovery that a single acidity function cannot describe the protonation of indicator bases of even the same charge type12-19 and Katritzky et al.19 have emphasised that this renders the attempt by Bunnett20 to obtain a more satisfactory diagnosis of the A-l, A-2, and A-SE2 reactions by the use of w and w values less reliable than it at one time appeared to be. 

The differences in rate for the two positions of naphthalene show clearly that an additional-elimination mechanism may be ruled out. On the other hand, the magnitude of the above isotope effect is smaller than would be expected for a reaction involving rate-determining abstraction of hydrogen, so a mechanism involving significant internal return had been proposed, equilibria (239) and (240), p. 266. In this base-catalysed (B-SE2) reaction both k and k 2 must be fast in view of the reaction path symmetry. If diffusion away of the labelled solvent molecule BH is not rapid compared with the return reaction kLt a considerable fraction of ArLi reacts with BH rather than BH, the former possibility leading to no nett isotope effect. Since the diffusion process is unlikely to have an isotope effect then the overall observed effect will be less than that for the step k. ... 

The decarboxylation of 2,4-dimethoxybenzoic acid by 2.5-8.0 M sulphuric acid at 60.5 °C has also been studied and division of the first-order rate coefficient by the molar concentration of sulphuric acid gave constant values (2.94+0.25 x 10 6) throughout the acid range so that the A-SE2 reaction was again indicated643. 

Both the Se2 (front) and Se2 (back) mechanisms are designated DeAe in the lUPAC system. With substrates in which we can distinguish the possibility, the former mechanism should result in retention of configuration and the latter in inversion. The reaction of allylsilanes with adamantyl chloride and TiCU, for example, gives primarily the antiproduct via a Se2 reaction. When the electrophile attacks from the front, there is a third possibility. A portion of the electrophile may assist in the removal of the leaving group, forming a bond with it at the same time that the new C—Y bond is formed... 

It has been shown that in certain cases (e.g., Me4Sn + I2) the reactants in an Se2 reaction, when mixed, give rise to an immediate charge-transfer spectrum (p. 102), showing that an electron donor-acceptor (EDA) complex has been formed. In these cases it is likely that the EDA complex is an intermediate in the reaction. 

Fig. 9 Excess acidity plot against X for some A-SE2 reactions at 25°C a-methylstyrene hydration (open circles) 120,182 phenylacetylene hydration (open and closed squares) 118,160,183,184 cw-stilbene isomerization (closed circles) 120,185 and tritiated benzene T-exchange (open and closed triangles).117,147,186,187...

Aromatic substitution, a quantitative treatment of directive effects in, 1, 35 Aromatic substitution reactions, hydrogen isotope effects in, 2, 163 Aromatic systems, planar and non-planar, 1, 203 Aryl halides and related compounds, photochemistry of, 20, 191 Arynes, mechanisms of formation and reactions at high temperatures, 6, I A-Se2 reactions, developments in the study of, 6,63... 

In the second mechanism, the first and second steps are concerted. In the case of hydrolysis of 2-(p-nitrophenoxy)tetrahydropyran, general acid catalysis was shown470 demonstrating that the substrate is protonated in the rate-determining step (p. 259). Reactions in which a substrate is protonated in the rate-determining step are called A-Se2 reactions.471 However, if protonation of the substrate were all that happens in the slow step, then the proton in the transition state would be expected to lie closer to the weaker base (p. 259). Because the substrate is a much weaker base than water, the proton should be largely transferred. Since the Brpnsted coefficient was found to be 0.5,the proton was actually transferred only... 

Se2 reactions generally occur with retention of configuration. In some cases, this stereochemistry has been directly proved.142 It can also be deduced from observations such as the very easy substitutions which occur in neopentyl compounds (whose rear face is sterically blocked),143 and at bridging atoms.144 According to March,145 inversion of configuration only occurs when the leaving group side is severely hindered. 

Equation (113) allows us to arrive at an estimate of the importance of transfer effects for A-SE2 reactions in which the transferred hydrogen acquired by the product does not readily undergo isotopic exchange out of the product. Under such conditions the ratio of the first two terms in the first factor of equation (113) measures the ratio of protium to deuterium in the product, i.e. 

A-Se2 reactions (mainly alkylmereuric halide cleavages) which have been investigated by Kreevoy s group from a similar point of view (Williams and Kreevoy, 1968). The case of 2-dichloro-methylene-l,3-dioxolan (3, DMD, for short) (Gold and Waterman, 1968b) illustrates most of the interesting features of all these examples. 

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