Onium intermediate

Additions that Take Place Diastereoselectively as trans-Additions (Additions via Onium Intermediates)... 

In the second step of trans-additions, the onium intermediates react with a nucleophile. Such a nucleophile can be either an anion released from the reagent in the first step or a better nucleophile present in the reaction mixture. It opens the onium intermediate in an SN2 reaction with inversion. In this way, the trans addition product is produced stereoselectively (Figure 3.42). 

Carbenium ions become intermediates in two-step additions to alkenes when they are more stable than the corresponding cychc onium intermediates. With many electrophiles (e. g., Br2 and Cl2), this occasionally is the case. With others, such as protons and carbenium ions, it is always the case (Figure 3.51). 

Protons and carbenium ions always add to C=C double bonds via carbenium ion intermediates simply because no energetically favorable onium ions are available from a reaction with these electrophiles. An onium intermediate formed by the reaction of a proton would contain a divalent, positively charged H atom. An onium intermediate produced by the reaction of a carbenium ion would be a carbonium ion and would thus contain a pentavalent, positively charged C atom. Species of this type are generally rare, but an excellent example is the nor-bornyl cation (Figure 2.29). 

Cyclic "onium intermediates of the type (2) or (3) are believed to survive only until attacked by an anion or nucleophilic solvent molecule. The reaction leads specifically to the trans product. This is a consequence of the extra stability... 

Such reactions are not, however, indicative of cyclopentadienide character in the five-membered ring but alternatively may reflect conjugative interaction of the hetero-atom atom with the reacting site, leading to a stable onium intermediate ... 

Subsequent loss of a proton yields the Brqnsted acid HPFe. Catalytic curing of the epoxy resin proceeds through an onium intermediate ... 

Cyclization is likely to proceed via primary formation of an onium intermediate 22 by attack of E-X at the triple bond of 20, followed by cyclization via O-attack on 22 ( 23) and O-demethylation of the oxonium ion 23 to yield MeX and 21. 

Ester Dealkylation. Among the widest uses for TMS-I involves the mild cleavage of carboxylic esters under neutral conditions. The ester is treated with TMS-I to form an initial ox-onium intermediate which suffers attack by iodide (eq 5). The trimethylsilyl ester is cleaved with H2O during workup. Although the reaction is general and efficient, it is possible to accomplish selective cleavage according to the reactivity trend benzyl, r-butyl > methyl, ethyl, i-propyl. Neutral transesterification is also possible via the silyl ester intermediate. Aryl esters are not cleaved by TMS-1, however, since the mechanism involves... 

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