Toluene, rotational isomers

The structure and chiral properties of the enolate intermediate were then investigated. Treatment of 40 with KHMDS (1.1 equiv) in toluene-THF (4 1) at —78°C for 30 minutes followed by t-butyldimethylsilyl (TBS) triflate gave Z-enol silyl ether 54 and its -isomer 55 in respective isolated yields of 57% and 27%.30 In the lH NMR spectra of both 54 and 55, methylene protons of the MOM groups appeared as AB quartets, which indicates restricted rotation of the C(l)-N bonds. The rotational barrier of the C(l)-N bond of the major Z-isomer 54 was determined to be 16.8 kcal/mol at 92°C by variable-temperature NMR measurements in toluene- (400 MHz 1 H... 

As mentioned earlier, initiator 2 has rotational isomers in which the alkylidene substituent is turned either towards the second multiple bond (5y ) or away from it anti)-, eqn. (11). In this case the equilibrium position is very much in favour of the syn rotamer K= 1450 in toluene at 25°C), making it difficult to detect the anti rotamer in routine spectra. However, the equilibrium can be displaced by UV irradiation (366 nm) of the solution for several hours at — 80°C to yield a mixture containing about 33% of the anti rotamer as determined from the resonances syn, 6 12.11,7ch = 120.3 Hz anti, 5 13.30, Jch= 153.3 Hz (Oskam 1992, 1993a). On adding 0.33 equivalents of 2,3-bis(trifluoromethyl)norbomadiene to this solution and running the spectrum again at — 80°C it is found that the anti rotamer has been completely consumed, giving the syn first-addition product, eqn. (12), while the syn rotamer has scarcely reacted at all. It is estimated that the... 

Stationary state measurements (Longworth and Bovey, 1966, Bokobza et al., 1977) as well as measurements of the transient behavior of the excited states (De Schryver et al., 1982a) were reported. Longworth and Bovey (1966) published the uncorrected fluorescence spectra of meso Al and racemic Al at 298 and 77 K. At 77 K the spectra are, within experimental error, identical and resemble the spectrum of toluene at 77 K. At 298 K excimer emission is detected for meso Al and racemic Al. The ratio of the intensity of the excimer emission (Id, at 313 nm) over the intensity of the locally excited state emission (Im, at 283 nm) equals 2.45 for meso Al and 0.74 for racemic Al. The difference in excimer forming capacity is explained based on the NMR data reported above (Bovey et al., 1965). The meso isomer starting from the TG/GT form reaches the TT conformation via an easy bond rotation, leading to a full overlap of the phenyl chromophores. The racemic isomer is predominantly TT (75%) with about 25% of GG present at room temperature. 

However, before moving on to LSCE materials, we will discuss briefly the isomerisation of azophenols in both isotropic and liquid-crystalline environments. Specifically, 4-(5-hexenyloxy)-4 -hydroxyazobenzene (AZO-OH) has been chosen as a representative example since it will be further used as a comonomer in LSCEs. The cis isomer of AZO-OH exhibits relaxation times of 311 and405 ms in ethanol and acetonitrile at 298 K, respectively (Fig. 18.8b). Otherwise, its isomerisation rate is more than 1000-fold slower when it is dissolved in toluene at the same temperature (t = 23 min). In both ethanol and acetonitrile, the thermal back reaction of AZO-OH proceeds through a solvent-assisted tautomerisation to yield a hydrazone-like intermediate. Subsequently, a rotation around the N-N bond of the hydrazone-like intermediate can occur regenerating thereby the thermodynamically stable trans isomer. Such an intermediate cannot be promoted when the azo dye is dissolved in toluene. 

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