trans- and cis-1-Chloro-2-thiophenylcyclohexane C12H15ClS

Section 24.7. Intramolecular catalysis: anchimeric assistance.

Intramolecular catalysis follows similar mechanisms to intermolecular catalysis, but with the added bonus that the catalyst is already present near the reaction site, and the wait for the collision to allow the intermolecular reaction to take place is considerably reduced. In this case, the sulfur atom is acting as a nucleophilic catalyst, being a good nucleophile and the good leaving group. However, the SN2 mechanism has the constraint that the attacking nucleophile must approach from the back of the leaving group. For the trans-cyclohexane (left) this is possible (if both groups are axial, as in the model), and the sulfur can act as a catalyst, speeding up the reaction about 70,000 times over the unsubstituted chlorocyclohexane. For the cis-cyclohexane, the necessary configuration for the sulfur to become involved cannot be attained, and no catalysis is observed, the only effect being a steric one which slows down the substitution compared with the unsubstituted chlorocyclohexane. Such a neighboring group effect is called anchimeric assistance.


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File created Wed Mar 26 21:23:35 2003