Heteroconjugation reaction

The characteristics of acid-base reactions in dipolar aprotic solvents, compared to those in dipolar amphiprotic solvents, are the easy occurrence of homo- and heteroconjugation reactions [2, 3, 5]. However, before discussing the homo- and heteroconjugations, we first discuss the solvent effects on the acid dissociation constants in dipolar aprotic solvents. 

In dipolar aprotic solvents, the dissociation process of some acids (HA) is complicated by homo- and heteroconjugation reactions. In the homoconjugation reaction, some part of the A ions formed by the dissociation of HA [Eq. (3.18)] reacts with undissociated HA to form HA2, as shown by Eq. (3.19) ... 

I11 the heteroconjugation reaction, A reacts with an acid other than HA. If we express the acid by HR, the heteroconjugation reaction can be shown by Eq. (3.20) and AHR is formed ... 

Oil the other hand, the paH of picric acid/picrate buffers in AN are not much influenced by water and alcohols of up to 0.5 M, because, in the buffers, homo-and heteroconjugation reactions do not occur appreciably [12]. In protophilic DMSO, the influences of hydrogen bond donors and acceptors on the paH of buffers are smaller than in protophobic AN. DMSO solvates to hydrogen bond donor HR, and suppresses the heteroconjugation, while DMSO suppresses the influence of a hydrogen bond acceptor by competing with it. 

An additional source of TiCl4 sequestration could be tiie zwitterions acting as chain carriers in these systems. It is conceivable that the following heteroconjugation reaction might take place and contribute to the stabilisation of the charge separation, e.g. ... 

The term heteroconjugation is justified only when the acid HR is much weaker than HA, otherwise the reaction HR+A R +HA more or less occurs. The... 

Further examples of gas-phase measurements which reveal the influence of stepwise solvation of homo- and heteroconjugated anionic nucleophiles, A (AH)n and A (BH)n, on the kinetics of Sn2 discplacement reactions with halomethanes can be found in referenees [475, 486], In all cases, solvation of the reaetant anion with up to three solvent moleeules leads to a decrease in the rate constants by at least three orders of magnitude. 

Epoxides can also be accessed asymmetrically using hypervalent iodine reagents in combination with imidazolidinone catalysts 78 (Scheme 30). The methodology developed by MacMillan et al. includes participation of hypervalent iodine reagent in a 1,4-heteroconjugate addition reaction for the organocatalytic, asymmetric epoxidation of a,p-unsaturated aldehydes 77. This organocatalytic reaction allows for the enantioselective formation of epoxides 78 from a wide array of electronically and sterically diverse a,p-unsaturated aldehydes [92]. 

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