Symmetrical oligomers

SI O Donoghue, M Nilges. Calculation of symmetric oligomer stiaictures from NMR data. In R Krishna, IF Berliner, eds. Modern Techniques m Protein NMR, vol. 17 of Biological Magnetic Resonance. New York Kluwer Academic/Plenum, pp 131-161, 1999. 

S. I. O Donoghue and M. Nilges, Calculation of Symmetric Oligomer Structures from NMR Data , p. 131... 

Volpe et al.270 prepared the symmetric oligomer of MMA by initiation with lithium and sodium enolates derived from methyl isobutyrate in THF followed by termination with methyl iodide. They then produced a 13C NMR spectrum of the mixture of three of six possible stereoisomers of the hexamer (n — 4) ... 

In a similar manner, the coalescence temperature for the methyl groups of the tetramer was determined as 4°C. AG was calculated as 12.7 kcal/mol which was 3.7 kcal/mol smaller than that for the pentamer. The hexamer showed a total of seven signals with narrow linewidth due to two methyl groups and six methine protons even at 70°C, indicating that the rate of helix sense reversal is much slower than the rate for the pentamer. This suggests the possibility that the symmetrical oligomers over the pentamer level may be optically resolved at room temperature based entirely on conformational asymmetry. This was confirmed by the chiral HPLC technique using (+)poly(triphenylmethyl methacrylate) as a stationary phase.282... 

A symmetric oligomer chain elongation approach was introduced by Tour et al. [168-170]. This group developed a bidirectional approach to the solid phase synthesis of oligo phenylene ethynylenes (Scheme 7.8). 

Calculation of Symmetric Oligomer Structures from NMR Data... 

Figure 11.1. Number N of conformers of a symmetric oligomer chain with two possible orientations for each additional repeat unit n.

Fig. 4.16 Representation of supramolecular substitution reaction between nonsymmetric oligomer 39 and symmetric oligomer 40. The H NMR spectra at different temperatures indicated that the complexation induced nonsymmetry. Reprinted with the permission from Ref. [35]. Copyright 2008 American Chemical Society...

---