Variable temperature XH NMR

Specifically, what was observed by examining a properly dideuterated variant of 114 by variable-temperature XH NMR is that the molecule is conformationally mobile and atropic at ambient temperature but freezes into a nearly equal mixture of the two diatropic rotamers,... 

Variable temperature xH-nmr studies of other hindered (in both ortho positions) triarylboranes have revealed 32> that these compounds are also mixtures of propeller-like stereoisomers on the nmr time scale at low temperatures, and that interconversion of these stereoisomers occurs with activation energies of ca. 14—16 kcal/mol (Table 3). These isomerizations are also interpretable in terms of two-ring flip mechanisms. 

This first example inspired the design of many other similar systems, most famously those investigated in the 1970s and 1980s by Mislow and Iwamura [19]. Later, the structurally complex but conceptually similar molecular turnstiles 3-5 (Fig. 2) were introduced by Moore [20]. Variable temperature XH NMR studies on the methylene protons, ffa and H, showed that the central aromatic ring of 4 spins rapidly on the NMR timescale at room temperature, while spindle rotation does not occur in turnstile 5 even at 85 °C due to steric constraints. 

The most complex pyridinophanes reported thus far are the stacked cyclophanes 76, 77, (Scheme VII) and 78 (Scheme VIII). The variable temperature XH NMR spectra of 76 indicate that the molecule undergoes a conformational inversion with a barrier of 11 kcal/mole, comparable to that of several previously considered cyclophanes (eg., 31, 40, 41, and 43) 27). 

The cyclohexene substructure resulting from Diels-Alder addition of acyclic dienes to fullerenes can adopt two boat conformations.50,387-389 If a new stereogenic center is generated during the cycloaddition, the boat-to-boat interconversion leads to conformational diastereoisomerism in each configurational stereoisomer.389,390 Such a case was studied by Zhang and Foote who performed VT (variable temperature) XH NMR and NOE measurements on... 

Variable-temperature XH- and 13C-NMR data suggest a trigonal-bipyramidal structure for XXI, with a methyl group and an oxygen atom in axial positions. The phenyl derivative has been obtained by the following reaction (76) ... 

A variable pressure oil pump was used in this distillation. Approximately 10 g of a volatile component, consisting mostly of hexamethyl-disiloxane, was obtained at room temperature (15 inn) before the forerun. The forerun contained the desired product and mineral oil from the n-butyl11thlurn solution. The pot residue was about 5 g. The submitters find the disilyl compound thus obtained is contaminated with a trace amount of mineral oil and 4-6% of a vinyl silane, probably 2-methyl-l-trimethylsiloxy-3-trimethylsilyl-2-propene. This impurity becomes quite significant if the reaction medium is less polar than the one described (e.g., too much hexane from n-butyllithium is allowed to remain behind). The spectral properties of the desired product determined by the checkers are as follows IR (neat) cm" - 2955, 1643, 1636, 1250, 1085, 885-830 XH NMR (chloroform-d, 90 MHz) 6 0.03 (s, 9 H, CHgSI(CH3)3), 0.14 (s, 9 H, 0S1(CH3)3), 1.50 (broad s, 2 H, CH2-Si(CH3)3), 3.93 (broad s, 2 H, CH2-0Si(CH3)3), 4.62 (m, 1 H, vinyl H), 4.92 (m, 1 H, vinyl H). 

Fig. 6. Variable-temperature 500 MHz XH NMR spectra (CDC13) of 14 in the region of the spindle methylene resonance at the indicated temperatures (reproduced with permission from [32])...

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