REACTIONS ON SP2 TYPE UNSATURATED SYSTEMS

P. Deslongchamps, Reactions on sp2 type unsaturated systems, in Stereo-Electronic Effects in Organic Chemistry, Pergamon Press, Oxford, 1983, p. 252. 

Reactions on SP2 Type Unsaturated Systems Considering a conformationally rigid cyclohexenone such as 76, an attack by a nucleophile with stereoelectronic control on the top face yields the boat-like enolate ion 77 whereas that on the bottom face gives the chairlike enolate ion 78. The second process should therefore be favored as suggested by Toromanoff (31). 

Reactions on SP2 Type Unsaturated Systems Very few studies have been carried out on the stereochemistry of the Michael reaction. However, Abramovitch and Struble (36) have found that compound 101 was the main product when diethyl sodiomalonate (free of ethoxide ion and ethanol) was added to 4- t-butyl-1-cyano cyclohexene (99) in boiling toluene. This result can be rationalized by axial attack on 99 to give first 100 having a chair-like conformation which is then transformed into 101 by internal trapping (see arrow). However, when the addition of diethyl malon-ate anion was carried out in ethanol under thermodynamically controlled conditions, product 103 with an equatorial malonate group was obtained, presumably via the twist-boat intermediate 102. 

Reactions on SP2 Type Unsaturated Systems 211 The situation is however different when the carbonyl group is part of a ring as in oxonium ion 9. The attack from the bottom face of leads to a chair intermediate 10 while that from the top face leads to a twist-boat 11. The attack from the bottom face of 9 is therefore strongly preferred for stereoelectronic and steric reasons. 

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