Adn-E mechanism

In some cases both the silicon and oxygen atoms of the siloxane bonds may simultaneously act as the reactive surface sites (the processes according to Adn.E mechanism). 

The stereochemistry of nucleophilic vinylic substitution via the AdN-E mechanism ranges from retention of configuration to stereoconvergence. For SnV 1-type reactions, often partial inversion has been observed. The inversion has been attributed to nucleophilic attack on an ion pair intermediate.As illustrative examples, results of the acetolysis of the vinyl triflates la-c in the presence of sodium acetate are given in Scheme The fraction of inversion ranges from 40 to 80%, and is smaller the more stable the intermediate cation /3-alkyl substitution... 

The tetrahedral mechanism, often also called addition-elimination (AdN-E), takes place with much less facility than with carbonyl groups, since the negative charge of the intermediate must be borne by a carbon, which is less electronegative than oxygen, sulfur, or nitrogen ... 

An attempt to study resolved (( )-18) as a probe for the detailed mechanism of the Adn—E vinylic substitution reaction has been complicated by intervention of a competing reaction route this is believed to involve a competing (ElcB elimination-addition, for which antiperiplanar orientation of H and Cl is not a requirement.7 a-Deuterated (ca 50%) E- and Z-substitution products (which do not themselves exchange deuterium) are obtained on reaction with MeS in 9 1 CD3CN-D2O but no incorporation of deuterium in unreacted ((/r)-18) occurs and neither does isomerism to ((Z)-18) precede elimination. 

The trifluoromethylation and pentafluoiophenylation described above are considered to proceed via the addition-elimination mechanism (AdN-E) shown in Scheme 5. The attack of the Rp anion upon the C=C or ON double bond gives the anionic intermediate 16, which converts the Rp-substituted products by releasing the fluoride anion on the carbon connected with the Rp group. 

Instead of performing the one step bimolecular SN2 reaction, alkenes react via two closely related bimolecular pathways. The first of these is called the tetrahedral mechanism and proceeds via a negatively charged intermediate. This mechanism is sometimes called the addition/elimination reaction, which is given the label Adn/E. This alternative name is unfortunate, because the other pathway is called the addition/elimination mechanism and proceeds via a readily detectable neutral intermediate. This latter mechanism will be considered in the chapter on sequential addition/elimination reactions. In this book, in an attempt to reduce the confusion, we will call the mechanism that proceeds via an anionic intermediate the tetrahedral mechanism, and reserve the name addition/elimination mechanism for the mechanism that proceeds via a neutral species. 

AdN-E Tetrahedral mechanism of nucleophilic addition followed by elimination. 

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