Tetrahydrofuran , basicity solvent effects

Since Lewis base additives and basic solvents such as tetrahydrofuran are known to deaggregate polymeric organolithium compounds, (21,23,26) it was postulated that ketone formation would be minimized in the presence of sufficient tetrahydrofuran to effect dissociation of the aggregates. In complete accord with these predictions, it was found that the carbonation of poly(styryl)lithium (eq. 9), poly(isoprenyl)-lithium, and poly(styrene-b-isoprenyl)lithium in a 75/25 mixture (by volume) of benzene and tetrahydrofuran occurs quantitatively to produce the carboxylic acid chain ends (8 ). 

The correlation of these mechanisms with solvent polarity (Lewis basicity) is strongly supported by a study of the solvent effect (% E) on the EjZ product ratio of equation (95) at 25 °C dioxane (100), THF (100), THF +AlCl, (100), 2,5-dimethyl-tetrahydrofuran (55), EtjO (60), Et20 +AICI3 (60), O-Pr) (25) . The addition of a crown ether raised the yield in /-Pr20 to 70% E, presumably by facilitating dissociation and the ionic route a drop in reaction temperature to —25 °C lowered the yield in EtjO to 45% E, presumably by facilitating association and the aggregate route . ... 

The role of the solvent may be important in anionic-coordinated polymerizations. Firstly it can play a role in the preparation of the initiator. It is known that alcoholysis reactions are solvent dependant. Secondly, it can interfere by its coordination strength on the initiator. In the case of the polymerization of propylene oxide with zinc alcoholates Ishimori etal [27] have shown that a retardation effect was observed when weakly basic solvents such as tetrahydrofuran, diethyl ether or dioxane are used. Triethylamine inhibits the polymerization almost completely. 

T, must be taken. Stability decreases with increasing atomic number of the group-IIA element and aryl are more stable than alkyl. Donors exert a stabilizing effect and, therefore, solvents such as tetrahydrofuran (THE) or pyridine (PY) are expedient for preparation and handling them. Diethyl ether is not sufficiently basic to impart adequate stability. The reactivity can be correlated with ionic character in the Pb-group-IIA element bond, the organolead moiety being anionic. 

N Az-butyllithium in hexane added dropwise to a soln. of trimethylsilylacetylene in tetrahydrofuran at 0° under N2, after 30 min ZnCl2 added at the same temp., stirred for 10 min at room temp., the solvent removed, the residue treated with a soln. of startg. (phenylthio)lactam in dry xylene under N2, and heated at 140° with stirring for 30 min - product. Y 80%. Reaction is also possible under both weakly basic (using (R —C = C)2Zn) and acidic (using R —C = C —ZnCl) conditions. F.e., also effect of... 

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