Methylene conformations

Figure 11.4 PET, PTT and PBT chains viewed edge-wise, showing the effects of methylene conformations...

Two crystalline phases of PBT can appear under tension and relaxation the so called and a forms (Hall Pass, 1976) (see Fig. 6.1). The main difference in the molecular structure between the a and the forms lies in the methylene conformation ... 

The nature of the structures responsible for the various absorptions has been the subject of a plethora of spectroscopic research, both experimental and theoretical. Barnes and Fanconi have reviewed and recalculated the assignments of vibrations of PE and the n-alkanes (54). Inter-and intramolecular interactions in the perfect lattice are also included in the computations. The assigned bands relate to a perfect crystal of the all trans methylene conformation. 

The large sulfur atom is a preferred reaction site in synthetic intermediates to introduce chirality into a carbon compound. Thermal equilibrations of chiral sulfoxides are slow, and parbanions with lithium or sodium as counterions on a chiral carbon atom adjacent to a sulfoxide group maintain their chirality. The benzylic proton of chiral sulfoxides is removed stereoselectively by strong bases. The largest groups prefer the anti conformation, e.g. phenyl and oxygen in the first example, phenyl and rert-butyl in the second. Deprotonation occurs at the methylene group on the least hindered site adjacent to the unshared electron pair of the sulfur atom (R.R. Fraser, 1972 F. Montanari, 1975). 

Large annulenes tend to undergo conformational distortion, cis-trans isomerizations, and sig-matropic rearrangements (p. 40 and p. 100). Methylene-bridged conjugated (4n + 2)-ic cyclopolyenes were synthesized with the expectation that these almost planar annulenes should represent stable HOckel arenes (E, Vogel, 1970, 1975). 

Pyran, 2-methoxycarbonyltetrahydro-conformation, 3, 629 Pyran, 2-methoxytetrahydro-conformation, 3, 629 Pyran, methylene-synthesis, 3, 762 Pyran, 4-methylene-stability, 3, 762 X-ray studies, 3, 620 Pyran, 2-methylenetetrahydro- C NMR, 3, 586 Pyran, 2-methyltetrahydro-synthesis, 3, 776... 

In the third sequence, the diastereomer with a /i-epoxide at the C2-C3 site was targeted (compound 1, Scheme 6). As we have seen, intermediate 11 is not a viable starting substrate to achieve this objective because it rests comfortably in a conformation that enforces a peripheral attack by an oxidant to give the undesired C2-C3 epoxide (Scheme 4). If, on the other hand, the exocyclic methylene at C-5 was to be introduced before the oxidation reaction, then given the known preference for an s-trans diene conformation, conformer 18a (Scheme 6) would be more populated at equilibrium. The A2 3 olefin diastereoface that is interior and hindered in the context of 18b is exterior and accessible in 18a. Subjection of intermediate 11 to the established three-step olefination sequence gives intermediate 18 in 54% overall yield. On the basis of the rationale put forth above, 18 should exist mainly in conformation 18a. Selective epoxidation of the C2-C3 enone double bond with potassium tm-butylperoxide furnishes a 4 1 mixture of diastereomeric epoxides favoring the desired isomer 19 19 arises from a peripheral attack on the enone double bond by er/-butylper-oxide, and it is easily purified by crystallization. A second peripheral attack on the ketone function of 19 by dimethylsulfonium methylide gives intermediate 20 exclusively, in a yield of 69%. 

Even in the absence of flow, a polymer molecule in solution is in a state of continual motion set forth by the thermal energy of the system. Rotation around any single bond of the backbone in a flexible polymer chain will induce a change in conformation. For a polyethylene molecule having (n + 1) methylene groups connected by n C — C links, the total number of available conformations increases as 3°. With the number n encompassing the range of 105 and beyond, the number of accessible conformations becomes enormous and the shape of the polymers can only be usefully described statistically. 

Let us recall, for instance, the case of the crystalline form of s-PP [113], or of s-PS [114,115], which contain s(2/l)2 helices. In this kind of helix (Fig. 2) there are two non-equivalent sets of methylene carbons in the main chain, the first one on the chain axis and the second one far from the chain axis. While for the methylene on the axis there are two y-carbons (that is carbons separated by three bonds) in G conformation, for the methylene on the periphery of the helix there are two y-carbons in the T conformation. This generates the so called y-elfect, that is a shift difference between the resonances of the two methylene carbons (8.7 ppm for s-PP, 10 ppm for s-PS). 

It is hence easy to detect by this technique different polymorphic forms having different chain conformations. For instance, the a or p forms of s-PS (tram-planar chain conformations) present only a single methylene resonance at 48.1 ppm (vs.TMS), while the y form (helical conformation) presents two methylene resonances at 37.3 and 47.3 ppm (Fig. 20) [114]. 

Differences in the. solid-state NMR signals of crystalline forms having identical conformations have been also observed. For instance, well-crystallized a form samples of i-PP show splittings for the methyl (22.6, 22.1 ppm) and methylene resonances (45.2, 44.2 ppm) into two lines with relative intensities 2 1 [117,118]. These splittings have been interpreted in terms of the known crystalline packing of the a form, which is characterized by pairs of 3/1 helices of opposite handedness at closer distances (Fig. 10). This generates inequivalence between the carbons indicated as A and those indicated as B in Fig. 10 [117,118]. 

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