Endocyclische S N ‐Reaktionen am gesättigten Kohlenstoff? Vorläufige Mitteilung

Fig. 1 illustrates a definition of the terms endocyclic and exocyclic SN ‐reactions, referring to intramolecular nucleophilic substitution processes that occur by an SN 2 ‐analogous mechanism. Crossing experiments show that the methyl transfer I → II (see scheme 1) does not follow the formally appealing mechanism of the endocyclic SN ‐process III; the reaction proceeds intermolecularly under all conditions investigated. Kinetic measurements indicate that the methyl transfer XI → XII (see scheme 3) occurs in a similar fashion. This behaviour is believed to follow from the preference of tetrahedral carbon for backside attack by the nucleophile in SN 2 ‐reactions. The general experience, according to which intramolecular reaction paths over cyclic transition states with ring sizes of 5 or 6 are preferred to their intermolecular counterparts, is not to be extrapolated to SN 2 ‐reactions at tetrahedral carbon.