Influential Role of Ethereal Solvent on Organolithium Compounds: The Case of Carboranyllithium

The influence of ethereal solvents (diethyl ether (Et2O), tetrahydrofuran (THF) or dimethoxyethane (DME)) on the formation of organolithiated compounds has been studied on the 1,2-C2B10H12 platform. This platform is very attractive because it contains two Cc?H adjacent units ready to be lithiated. On would expect that the closeness of both Cc?H units would induce a higher resistance of the second Cc?H unit being lithiated following the first lithiation. However, this is not the case, which makes 1,2-C2B10H12 attractive to get a better understanding of the ethereal solvent influence on the lithiation process. The formation of carboranyl disubstituted species has been attributed to the existence of an equilibrium in which the carboranyl monolithiated species disproportionates into dilithium carborane and pristine carborane. The way Li+ binds to Cc in the carboranyl fragment and how the solvent stabilizes such a binding is paramount to drive the reaction to the generation of mono- and disubstituted carboranes. In fact, the proportion of mono- and disubstituted species is a consequence of the formation of contact ion pairs and, to a lesser extent, of separated ion pairs in ethereal solvents. All ethereal solvents generate contact ion pairs in which a large degree of covalent Cc?Li(solvent) bonding can be assumed, according to experimental and theoretical data. Furthermore, Et2O tends to produce carboranyllitium ion pairs with a higher degree of contact ion pairs than THF or DME. It has been determined that for a high-yield preparation of monosubstituted 1-R-1,2-C2B10H11, in Cc?R (R=C, S or P) coupling reactions, the reagent type defines which is the most appropriate ethereal solvent. In reactions in which a halide is generated, as with ClPPh2 or BrCH2CH?CH2, Et2O appears to produce the highest degree of monosubstitution. In other situations, such as with S8, or when no halide is generated, THF or DME facilitate the largest degree of monosubstitution. It has been shown that upon the self reaction of Li[1,2-C2B10H11] to produce [LiC4B20H22]- the nucleophilicity of the carboranyllithium can even be further enhanced, beyond the ethereal solvent, by synergism with halide salts. The mediation of Li+ in producing isomerizations on allyl substituents has also been demonstrated, as Et2O does not tend to induce isomerization, whereas THF or DME produces the propenyl isomer. The results presented here most probably can be extended to other molecular types to interpret the Li+ mediation in C?C or other C?X coupling reactions.