The Role of Ate Complexes in the Lithium‐Sulfur,Lithium‐Selenium and Lithium‐Tellurium Exchange Reactions

Hypervalent ate complexes are presumptive intermediates in the metal‐halogen, metal‐tellurium, and related exchange reactions. The effect of o,o′‐biphenyldiyl vs. diphenyl substitution on formation of tellurium ate complexes was studied by a kinetic technique and by NMR spectroscopy. Only a modest increase in the association constant (K_ate) was measured. When Li/M exchanges of o,o′‐biphenyldiyl sulfides and selenides were made intramolecular by means of a m‐terphenyl framework ( 12‐S, 12‐Se, 21 ), enormous increases (>10⁹) in the rate of Li/S and Li/Se exchange were observed compared to acyclic models. Apparently, these systems are ideally preorganized to favor the T‐shaped geometry of the hypervalent intermediates. For the selenium systems, ate complex intermediates ( 20‐Se, 26 ) were detected spectroscopically in THF‐ or THF/HMPA‐containing solutions. A DNMR study showed that Li/Se exchange was substantially faster than exchange of the lithium reagents with the ate complex. Therefore, these ate complexes are not on the actual Li/Se exchange pathway.