Linear dimerization functionalization

The linear dimerization of terminal alkynes via mthenacyclopentatrienes was also carried out in the presence of the cationic [Cp Ru(CH3CN)3]PF6 complex and functionalized 1,3-dienes were formed by nucleophilic addition of alcohols [7] or water [8, 9] [Eqs. (5) and (6)]. Further transformations can produce furans or quinoline derivatives. 

While linear dimerization of dienoic esters can also be accomplished with nickel-AMP systems, other functionalized dienes undergo little or no conversion. The reaction of methyl hexa-2,4-dienoate, 81, furnishes diastereomeric trienoic diesters (82) in high yields (equation 43). 

Because lactic acid has both hydroxyl and carboxyl functional groups, it undergoes iatramolecular or self-esterificatioa and forms linear polyesters, lactoyUactic acid (4) and higher poly(lactic acid)s, or the cycUc dimer 3,6-dimethyl-/)-dioxane-2,5-dione [95-96-5] (dilactide) (5). Whereas the linear polyesters, lactoyUactic acid and poly(lactic acid)s, are produced under typical condensation conditions such as by removal of water ia the preseace of acidic catalysts, the formation of dilactide with high yield and selectivity requires the use of special catalysts which are primarily weakly basic. The use of tin and ziac oxides and organostaimates and -titanates has been reported (6,21,22). 

The growth of the polymer chains (polymerization) may be described by a condensation reaction between the anhydride and amine functions as depicted here for the formation of a linear dimer  

On the other hand, earlier calculations by Frisch et al. [94] with somewhat larger basis sets had indicated that the relative amounts of zero-point vibrational energy in the cyclic and linear dimers can reverse upon small changes in basis set, e.g. addition of diffuse functions. Furthermore, the use of the harmonic approximation on a surface as flat as this one is dubious, so the question remains incompletely resolved. 

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