Linear dimerization functionalization

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). 

An alternative method is to examine the relative rates of formation of circles and linear dimers by the ligase as a function of DNA length. Early experiments043) using this method yielded a = 7.1 x 10 12 dyn-cm.recent study using what should be a superior protocol yields a static torsion constant in the range a = (5.9 0.6) x 10 12 dyn-cm.<221)... 

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 ... 

Kaiser et al. have reported a general entry for the selective synthesis of dimeric macrocycles like cyclostellettamines and for polymeric natural products [41]. It uses the Zincke reaction by which it is possible to control the number of units in a 3-alkylpyridinium polymer. As summarized in Fig. (33), the reaction of the free amine 89 with the Zincke salt 88 gives the dimer 90 (route b) which, after terminal amine deprotection and DNB functionalization at the A-pyridine centre, gives the cyclic dimer, as in the synthesis of cyclostellettamine B. Otherwise, compound 90 furnishes both the protected dimer 91 and the free linear dimer, which, refluxed together in butanol, give the linear tetramer (route c). By the same iterative sequence, the linear octamer was obtained from the tetramer, and from the latter the hexadecamer. 

Reis et al. report theoretical studies of the urea250 and benzene251 crystals. Their calculations start from MP2 ab initio data for the frequency-dependent molecular response functions and include crystal internal field effects via a rigorous local-field theory. The permanent dipolar fields of the interacting molecules are also taken into account using an SCF procedure. The experimental linear susceptibility of urea is accurately reproduced while differences between theory and experiment remain for /2). Hydrogen bonding effects, which prove to be small, have been estimated from a calculation of the response functions of a linear dimer of urea. Various optoelectronic response functions have been calculated. For benzene the experimental first order susceptibility is accurately reproduced and results for third order effects are predicted. Overall results and their comparison with studies of liquid benzene show that for compact nonpolar molecules environmental effects on the susceptibilities are small. 

Following the introdnction of a directional hydrogen bonding potential function into MM3, the parameterization of the force field for the ammonia dimer was undertaken anew . Three conformers were considered, namely 28, 29 and a bifurcated structure 31, and were calcnlated ab initio at the 6-31G level. The results (after corrections for Basis Set Superimposition Error BSSE) favor the linear dimer over the cyclic one by 0.4 kcalmol and yield dimerization energies of —2.49, —2.09 and —0.62 kcalmol for 28, 29 and 31, respectively. A comparison of force field (original MM3 and MM3 with the directional hydrogen bonding function) and ab initio resnlts for the three ammonia... 

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. 

The lactic acid molecule has a hydroxyl and an acid functional group, which may result in intermolecular and intramolecular esterification reactions. The first step is the formation of a linear dimer (lactoyl lactic acid). This condensation reaction can proceed to higher oligomers and is promoted by removal of water. Also a cyclic dimer, lactide, is formed in small amounts. Lactide can be formed by intramolecular esterification of lactoyl lactic acid or by breakdown of higher oligomers. All reactions are equilibrium reactions (Figure 1.2). 

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. 

Brown and Lin reported a quantitative method for methanol based on its effect on the visible spectrum of methylene blue. In the absence of methanol, the visible spectrum for methylene blue shows two prominent absorption bands centered at approximately 610 nm and 660 nm, corresponding to the monomer and dimer, respectively. In the presence of methanol, the intensity of the dimer s absorption band decreases, and that of the monomer increases. For concentrations of methanol between 0 and 30% v/v, the ratio of the absorbance at 663 nm, Asss, to that at 610 nm, Asio, is a linear function of the amount of methanol. Using the following standardization data, determine the %v/v methanol in a sample for which Agio is 0.75 and Ag63 is 1.07. 

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). 

For equilibration processes, one must synthesize both oligomers and what are termed dimers, or disiloxanes. Our primary interest is in the utilization of these functional oligomers for the synthesis of both linear block or segmented copolymers, and also surface modified, oughened networks such as the epoxy and imide systems (3-27). The generalized structure of the oligomers of interest is shown in Scheme 1. 

Various polyphosphates are effective sequestering agents under appropriate conditions. The best known of these is sodium hexametaphosphate (10.14), the cyclic hexamer of sodium orthophosphate. Further examples are the cyclic trimer sodium trimetaphosphate (10.15), as well as the dimeric pyrophosphate (10.16), the trimeric tripolyphosphate (10.17) and other linear polyphosphates (10.18). All of these polyanions function by withdrawing the troublesome metal cation into an innocuous and water-soluble complex anion by a process of ion exchange as shown in Scheme 10.7 for sodium hexametaphosphate. Hence these compounds are sometimes referred to as ion-exchange agents. 

---