Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
How valid is the proof of existence of intermediates with pentacoordinated C-atoms in aliphatic SN2-reactions?Retention of configuration in the preparative conversion of cis-3-ethoxycyclobutylbrosylate (4) with sodium iodide and with lithium bromide in boiling acetone for 5 days to cis-3-ethoxycyclobutyliodide (6) and bromide (8), respectively, has been published by Ugi et al. to be the basis of a proof that species with pentacoordinated C-atoms exist sufficiently long during aliphatic SN2-reactions that they can undergo ligand reorganizations.It is now shown that the conditions used lead to a thermodynamic control, inasmuch as the cis- and trans-halides 6 and 7, as well as 8 and 9 isomerize to ∼2:1 equilibrium mixtures. Furthermore it is shown that the reaction of kinetic control occurs with inversion, inasmuch as the trans-: cis-halide ratio during the course of the reaction starts with high values and falls slowly to the equilibrium.Thus the nucleophilic substitutions on the examined four-membered ring system do not contravene previous experience with other aliphatic and alicyclic systems, so that the existence of pentacoordinated carbon species as intermediates in aliphatic SN2-reactions cannot be considered as proven.
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