Heteroconjugation

Isobe and coworkers407 found an interesting diastereoselective heteroconjugate addition of methyllithium to 314. The stereochemical control was considered to be determined at the stage of the intermediate 315. Since methyllithium is considered to be coordinated strongly with the methoxyethoxymethoxyl (OMEM) group, the methyl anion would attack the -carbon of the olefin only from one side, as shown below. 

Reiter, Y., and Fishelson, Z. (1989) Targeting of complement to tumor cells by heteroconjugates composed of antibodies and of the complement component C3b. J. Immunol. 142, 2771. 

Accelerations of the rates due to an additive P are explained as electrophilic catalysis by the heteroconjugate NuH+P, while a second-order term in the concentration of P can be obtained if the relative basicities of Nu and P are such that P can compete with Nu for removal of the proton from I followed by electrophilic catalysis by the homoconjugate PH+P. 

Clearly, these isotope effects could also be explained on the basis of the dimer nucleophile or homo/heteroconjugate mechanisms. 

Finally, useful stereoselectivities have been recorded for the heteroconjugate addition of organometallic reagents to 1-silyl substituted vinyl sulfones.22 The synthesis of such sulfones can be achieved starting from phenylthioacetylene.6 1 11 23 The synthesis of the dicobalt hexacarbonyl complex24 and the polymerization of phenylthioacetylene25 have been described. 

This isomerization was used in the heteroconjugate addition to the acyclic system. Therefore, the substituted olefin 72, in which the double bond is conjugated with both sulfone and silicon atoms, undergoes a diastereoselective addition of CHsLi. The resulting lithium alcoholate is quantitatively converted into the silyl ether dianion 73 and the addition of deuterium oxide afforded the functionalized product 74 in excellent yield (equation 16. 

Formation constant (Kf) of the heteroconjugated complex (1 1 complexation of analyte-anion) is given by... 

Okada, T., Non-aqueous capillary electrophoretic separation of Bronsted acids as heteroconjugated anions, /. Chromatogr. A, 771, 275-284,1997. 

Kuldvee, R., Vaher, M., Koel, M., and Kaljurand, M., Heteroconjugation-based capillary elctrophoretic separation of phenolic compounds in acetonitrile and propylene carbonate. Electrophoresis, 24,1627-1634, 2003. 

Miller, J. L., Shea, D., and Khaledi, M. G., Separation of acidic solutes by nonaqueous capillary electrophoresis in acetonitrile-based media. Combinated effects of deprotonation and heteroconjugation, ]. Chromatogr. A, 888,251-266, 2000. 

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 ... 

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... 

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. 

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