Sind Zwischenstufen mit pentakoordinierten C‐Atomen bei aliphatischen S N 2‐Reaktionen nachgewiesen??

How valid is the proof of existence of intermediates with pentacoordinated C‐atoms in aliphatic S N 2‐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 S N 2‐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 S N 2‐reactions cannot be considered as proven.