NMR and Conformational Studies

NMR spectroscopy has also proven useful in studying rates of C-C bond rotation in allyl and pentadienyl species. The allyl systems are simpler to analyze since only one allyl conformation is possible (ignoring substituent orientations), whereas the pentadienyl 

As noted earlier, 7H and 13C nmr studies are also indispensible for determining pentadienyl anion conformations in solution. Generally, these studies involve coordinat- 

To illustrate one application, one can observe that XIV, the S conformation below, is characterized by six different J values - four across the two external C-C bonds (two cis 

Reasonably similar conclusions have also been reached regarding phenyl or trimethyl-silyl substituted pentadienyl anions. Thus, 1,3,5-triphenylpentadienyllithium, XV, has been observed to exist in two conformations at room temperature, the more abundant of which demonstrated collapse of its H nmr resonances on cooling, and therefore was assigned to be a mixture of two comparably stable S forms. The other conformer was formulated as W . The S W ratio was determined to be ca. 2.1 1, and both S S and S W interconversions were observed, the latter requiring more energy74.  

Various thermally less stable Zn(C5H7)2L2 complexes (L = (CH3)3NO, THF) were also quite fluxional to well below room temperature, but again incorporation of a chelating ligand such as TMEDA led to freezing out of the terminally a-bonded spectra. Even so, however, at ca. 90° some of the spectra demonstrated fluxionality. While Zn(C5H7)(Cl)(TMEDA) exists entirely in the W conformation, Zn(C5H7)2(TMEDA) and Zn(3-QH9)2(TMEDA) each contained substantial (20%) quantities of the S con- 

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N. E. Nifantiev, A. S. Shashkov, G. M. Lipkind, and N. K. Kochetkov, Synthesis, NMR, and conformational studies of branched oligosaccharides. 7. Synthesis and l3C NMR spectra of 2,3-di-O-glycosyl derivatives of methyl a-L-rhamnopyranoside and methyl a-D-mannopyrano-side, Carbohydr. Res., 237 (1992)95-113. 

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