Applications of poly(α-olefin)s as solvents in organometallic chemistry

Abstract

Poly(α-olefin)s (PAOs) that can be used as alternative solvents and solvent systems for catalysis and synthesis can substitute for conventional alkane solvents like pentane, hexane, and heptane for storage and in reactions of reactive organometallics. In this work, we show that organolithium reagents including polymer-supported diphenyllithium and triaryllithium reagents have extended stability as suspensions or as solutions in PAOs just as is the case for alkyllithium reagents. These highly colored species are stable in PAO solvents for months. PAOs can also be used as safer solvents for dialkylmagnesium reagents. In PAO solvents, these dialkylmagnesium reagents behave in reactions with electrophiles like benzaldehyde, benzophenone, and carbon dioxide the same as they do in heptane. PAOs can also be used as recyclable solvents in hydroboration/oxidation of alkenes when products are separated from the PAO by distillation or extraction. In contrast to PAO solutions of alkyllithium or dialkylmagnesium reagents, the PAO in a solution of trihexylborane in PAO did not itself inflame on exposure to a propane torch flame. While the alkylborane did burn briefly with a green flame, that flame due to the borane self-extinguishes after the borane is consumed. The PAO does not itself catch fire. Finally, we examined the recyclability and reactivity of PAO solvents containing a polyisobutylene (PIB) bound analog of LDA in enolate chemistry. Our results show that lithium PIB-tert-butylamide in PAO behaves like LDA or PIB-tert-butylamide in THF or heptane in formation of trimethylsilyl enol ethers from ketones with the difference being that PIB-tert-butylamide in a PAO solution is a fully recyclable reagent/solvent system.