Oxygen in non-polar solvents

Allyltrialkylstannanes react with singlet oxygen in non-polar solvents to give all three types of product (hydrogen-ene, metalloene, and cyclisation equation 9-32 see Section 14.5),68-70 but with one or more electronegative ligands on the tin, only the metalloene... 

If we Investigate the Influence of the solvent, It Is necessary to take Into account the nature of the species In the medium. In non-polar solvent we have aggregated speclesi when we add tetramethylenediamine (THEDA) we destroy the aggregates, and we have an equilibrium between Ion pairs and complexed species.In polar solvent, we have another equilibrium between the Ion pairs, the solvated Ion pairs and loose Ion pairs.11 If we compare the oxidation results (Table IV), we observe that for similar oxygen concentration, the coupling yield and the hydroperoxide functionality Increase like the lonlclty of the C-Ll linkage. 

Lithium ester enolates are extremely important in polymer chemistry as initiators and active centers of the anionic polymerization of acrylic and methacrylic monomers in polar solvents. Thus, HF-SCF studies, comparable to those mentioned above, were undertaken on monomeric methyl isobutyrate (MIB) enolate210,211. The overall conclusions on the aggregation and solvation trends are exactly the same, the bent rj3-0,C mode being preferred over the rj1-O planar one by ca 3.3 kcalmol-1. While the dimeric MIB enolate solvated by four molecules of THF was found to be the enthalpically most stable aggregate, the prismatic S6 unsolvated MIB hexamer was computed as the preferred structure in non-polar solvents (Scheme 55)212. In the latter case, the supplementary oxygen of the ester acting as a side-chain ligand for the lithium seems to explain this remarkable stability. 

Allylstannanes are stable towards allylic rearrangements up to ca. 100 °C in non-polar solvents, or at rather lower temperature in polar solvents.30-31 Photolytic sigmatropic rearrangement occurs in competition with homolysis, and, in the presence of oxygen as a radical trap, cinnamyltriphenylstannane gives a 1 9 mixture with the a-stannyl isomer.32... 

If the acid group is joined directly to the oxygen atom as an ester 6 we have the Fries rearrangement3 which can be controlled to give either the para or ortho products. In polar solutions such as nitrobenzene,4 the para product 5 is formed in high yield, e.g. R = Me, 75%, while in non-polar solvents or in the absence of solvent, the ortho product 7 is formed in similar yield, e.g. R = Me, 70%. 

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