Phenyllithium forms a mixture of tetramer and dimer in ether. Complete conversion to dimeric solvates is achieved by the addition of THF, dioxolane, DME, or TMEDA in near stoichiometric amounts. The addition of 2,5-dimethyltetrahydrofuran favors dimer, but tetramer is still detectable at 14 equiv of cosolvent. PMDTA converts PhLi to monomer in ether. In THF, PhLi is a mixture of dimer and monomer. Addition of TMEDA forms a series of complexes, but the dimer/monomer ratio is essentially unaffected. PMDTA and HMPA form monomeric PhLi stoichiometrically. HMTTA and DMPU also result in monomer formation but several equiv are required. 12-Crown-4 shows no spectroscopically detectable complexation of PhLi in THF. All of the cosolvents tested increase the reactivity of PhLi in THF in a test metalation reaction:  HMPA and 12-crown-4 show the largest effects, PMDTA is intermediate, and HMTTA and TMEDA result in the least activation. In two selectivity tests, HMPA and 12-crown-4 show a substantially lower selectivity than the other cosolvents. We postulate that a contribution from a highly reactive separated ion pair (SIP) intermediate is responsible for the lower selectivity.

This paper is dedicated to Professor Dieter Seebach in honor of his 65th birthday 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 (> 10 9 ) 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.Introduction. ± The lithium-metalloid exchange reaction is the mildest and most general procedure for the preparation of organolithium reagents. The lithium-bromine [1] [2a] and lithium-iodine exchanges have been the most popular, but the reaction applies to many of the main-group third-, fourth-, and fifth-row elements. Tin [3a] [4], selenium (first studied by Seebach and Peleties [3b], and tellurium [3c] have been used extensively. The reaction fails with second-row CÀM bonds like those of chlorides, sulfides, and phosphines, except in exceptional circumstances, e.g., when there are no protons that can be metallated, when an unusually stable carbanion is being prepared, or when a strained ring is being cleaved [5] [6].Hypervalent ate complexes such as 1 ± 3 have been spectroscopically characterized and are likely intermediates in the degenerate phenyl-phenyl Li/I [7a] [7b] [8] [9], Li/Te [7b] [7c] [10a], and Li/Sn [7d] [11] [12] exchange reactions. Ate complexes of third-and even second-row metalloids Se, P, and Si can be detected in favorable structures, e.g., when the aryl groups are heavily substituted with electronegative halogen atoms (4, see [10b]) or when o,o'-biphenyldiyl ligands are present (5, see [13] and 6, see [14a]).