Coupling byproducts

Transmetallation of allyltributyltin with organolithium species has been used for the generation of allyllithium solutions free of the coupling byproducts which often result from reduction of allylic halides with lithium metal. These solutions may then be used directly for the preparation of Gilman reagents and other reactive modifications of the parent allyllithium. 

Using the same procedure Pennisi and Fetters prepared a series of asymmetric polybutadiene (PB) stars in which the third arm was of variable molecular weight [2]. It was found more efficient to add the living PB solution to the meth-yltrichlorosilane linking agent in order to reduce the formation of the coupled byproduct. Similar characterization techniques were also employed in this case. 

Asymmetric polyisoprene (PI) three-arm stars with variable length of the third arm were synthesized using the same method [8]. The reaction of the living PI chains with excess methyltrichlorosilane was performed at 5 °C. This low temperature was selected in an effort to minimize the coupled byproduct. Nevertheless the reduced steric hindrance of the PILi chain end in association with the low molecular weight of the polydienes used (Mn=5500 and 1100) led to the formation of an appreciable amount of the coupled byproduct, which was later separated by fractionation, with the excess of the last coupled arm, using a sol-vent-precipitant system. Pure products were finally isolated as evidenced by the molecular characterization techniques used (SEC, MO, SLS). 

The method takes advantage of the steric hindrance of the polystyryllithium living end, which in combination with the excess silane used for the linking reaction, reduces the possibility for the formation of the coupled byproduct. The reaction sequence was monitored by SEC and the reaction products were characterized by MO, differential laser refractometry and LALLS, revealing that well defined polymers were prepared. 

The method was also applied in the synthesis of asymmetric PBd75 and PI76 stars. When the molecular weight of the PI chain that was reacted with the excess silane was less than 5.5 x 103, the formation of the coupled byproduct could not be avoided, due to the low steric hindrance of the living PI chain end. The byproduct was removed by fractionation. The stars were characterized by SEC, MO, and LS, showing that the desired structures having low polydispersities were efficiently prepared. 

The main limitation in the broad apphcation of alkene cross-metathesis reactions in the synthesis of unsaturated compounds is the formation of self-coupling byproducts. Moreover, these impurities, frequently difficult to separate, are usually formed alongside the desired compound in an almost statistical ratio. Another issue associated with this kind of reaction is the stereocontrol of fhe newly formed double bond. 

Scheme 10.2 A Oxidative coupling byproducts in the CuAAC reactions catalyzed excess of the catalyst reactions with...

The coupled byproduct can be separated from the desired alcohol by taking advantage of the difference in solubilities of the hydrocarbon and the alcohol in ligroin. Ligroin, a nonpolar alkyl solvent, readily dissolves the nonpolar biphenyl, whereas the polar tertiary alcohol is much less soluble. 

It has already been described that a number of unsymmetrical tetrachalcogenafulvalenes can be prepared by a cross-coupling reaction of the corresponding l,3-dithiole-2-ones or 1,3-diselenoIe-2-ones, via triethyl phosphite, followed by column chromatography separation of the cross-coupled product (CCP) from the self-coupled byproducts (SCPs), if there is a considerable difference in the Rp values of the CCP and SCPs. For example, the Rf values of compounds (la -la ), (7b-7b), (8c-8c), (lOc-lOc), (12c-12c), (13a -13a ) and (29-29) are 0.75, 0.77, 0.55, 0.49, 0.25, 0.08, 0.05, respectively, on a silica gel plate using CH2CI2 as eluent, i.e., the polarity of the latter compounds is larger... 

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