Relative rates, relative activation parameters and substituent effects of lithium-metalloid exchange reactions

Abstract

The relative reactivity of heteroatomic compounds (PhM; M = SeR, SnR3, TeR, I) towards BuLi was estimated by competitive experiments. The rates of exchange reactions in THF increase in the order I > Te >> Sn >> Se with a ratio of 1000:300:15: 1 at -70 "C when R =Bu. Me,SnPh underwent exchange much faster than Bu,SnPh, suggesting that the exchange reaction was largely affected by steric hindrance. The evidence that Bu,SnPh and BuTePh react with BuLi at almost the same rate in the presence of HMPA and that the relative exchange rates of Li-Te/Li-Sn increase in less polar solvents such as Et10 and hexane indicates that coordination of solvents towards lithium plays an important role in exchange reactions. Relative activation parameters obtained from competitive reactions of PhTeBu and PhSnBu, with BuLi showed that Li-Te exchange was favored over Li-Sn exchange by an enthalpy factor. Hammett treatment of Li-Te exchange of ArTeBu with BuLi in THF showed a substantially large positive reaction constant ( p ) of 3.4, indicating that the anion charge was largely distributed on the leaving group in the transition state.

What’s Going on with These Lithium Reagents?

“…38 Although the correlation is poor overall, there is a reasonable correlation within a single column of the periodic table, providing some support that all proceed through ate complex intermediates. 39 These data show why the Li/I, Li/Te and Li/ Sn exchanges are the most reliable and least complicated by side reactions since they are over 4 orders of magnitude faster at 0 °C than the next fastest, the Li/Br exchange. 35a,36a At the temperatures where these reactions are normally done (−78 °C), the difference is much larger.…”

The Role of Ate Complexes in Halogen(Metalloid)–Metal Exchange Reactions: A Theoretical Study

“…[12b] Furthermore, the order of the recently measured [42] relative rates of the exchange reactions of PhϪY with BuLi in THF (PhSnBu 3 << PhTeBu < PhI) is exactly opposite to that of the calculated formation (Table 3). In contrast, the trends in the computed stabilities ∆E X within rows of elements seem to be inconsistent at first glance with the experimental observation that the disubstituted methanes of the type IϪCH 2 ϪSnR 3 (R ϭ Ph, n-Bu, or Me) preferentially exchange their iodine atoms.…”

Efficient Synthesis of 1,4‐Bis‐heteroatom‐substituted Tetraselanylbenzenes via 1,4‐Dilithiation of Hexaselanylbenzene and Investigation on Their Electronic Properties

“…Using hexakis(phenylselanyl)benzene ( 1 ) as a precursor, we focused on the transmetalation reactions with organolithium reagents in order to introduce heteroatom functionalities into compound 1 . Although the lithium–selenium exchange reaction is generally slow, compared with other transmetalation reactions, for the preparation of hexa‐substituted benzenes containing different heteroatom substituents, starting from compound 1 has three advantages: first, compound 1 can be easily prepared on a large scale and is stable toward atmospheric oxygen and moisture. Second, hexaselanylbenzene would react with organolithium reagents to generate the corresponding lithiated species, caused by its low‐lying LUMO (Figure ).…”