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

Abstract

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]).

Differing views of the role of selenium in thioredoxin reductase

“…In this reaction, exchange with S occurs only to a small extent such that attack at Se is ∼10 4 times faster than attack at S (Reich et al 2002). In an experiment to show that attack of the butylate anion occurs preferentially at Se, (Seebach et al (1977) used phenylseleno(phenylthio)methane (PhSeCH 2 SPh) for the exchange reaction and found that the selenoether was the product (Scheme 3).…”

Selenium as an Electron Acceptor during the Catalytic Mechanism of Thioredoxin Reductase

“… 6 , 26 This model is also supported by recent theoretical and experimental studies that show that attack at the selenium in a selenosulfide is highly favored over attack at the sulfur in thiol/disulfide exchange reactions. 27 , 28 The high electrophilicity of selenide relative to that of sulfide has been recognized in the field of chemistry 29 but is a largely unrecognized property in the biochemical literature.…”

High‐Yield Lithiation of Azobenzenes by Tin–Lithium Exchange

“…This signal was assigned to ate complex 12 , on the basis of a literature precedent 41. 57 At −103 °C, the relative intensity of the signal for the ate complex remained constant, whereas the signal due to the starting material continued to disappear, and tetramethylstannane formed at the same rate (based on the percentage change of the integral of each species). The signal corresponding to the tin ate complex could be observed at temperatures of up to −88 °C, but when the reaction mixture was warmed further to −70 °C, the signal could no longer be observed.…”