C barriers

Class C barriers are made of noncombustible materials and are not rated to provide any smoke, flame or temperature passage restrictions. 

Further band-shape work (35) has been performed on malonic ester derivatives 9 (R = H R, R2 = —(CH2) —, with n increasing from 2 to 5). The C=C barriers showed a steady decrease from AG > 23.2 kcal/mol for n = 2 to 14.6 kcal/mol for n — 5, whereas the C—N barriers were less dependent on the ring size. An exception was the aziridine derivative, for which the C—N barrier was <7.3 kcal/mol. 

Shvo and Belsky (42) have also studied a methylthio analog of 23, R = Me (24). Here the C=C barrier is above the DNMR region (>27.5), that is, at least... 

Ketene mercaptals (27) have rather high C=C barriers (Table 5), and only those with quite strongly electron-accepting groups fall within the region accessible to the DNMR method (45-49). As an example, the C=C barrier for 27... 

Systems with one amino group and one alkylthio group as donors, commonly known as ketene /V,S-aminals, are sterically rather similar to the 1-ethyliden-amino analogs like 9 and 10 (R = Me). However, comparison of the C=C barrier in 10, where R = R = R2 = Me (14.8 kcal/mol Table 3), with that of 31 (7.4 kcal/mol Table 6) shows that the MeS group must be much more... 

Triafulvene compounds (41) with acceptor groups for X and Y have been studied notably by Eicher et al. (55,56), who found C=C barriers from 14 kcal/mol upward (Table 9). The barrier-lowering effect of the acceptor groups seems to fall in the same order as for ketene mercaptals and aminals, but the capacity of the cyclopropenium ring to stabilize the transition state seems to be less than that of the (MeS)2C group in ketene mercaptals. The barrier is quite sensitive... 

Comparison of C=C barriers in Table 10 with those in Tables 3 and 4 shows that the cyclopentadiene ring has a moderate capacity for stabilizing a dipolar transition state, being slightly less efficient than two COzR groups or one CN and one C02R group. 

As with other ketene aminals, inclusion of the donor groups in 5- or 6-membered rings (43 to 45) lowers the C=C barriers (60). The data for 44 and 45 are difficult to reconcile with a high C=C barrier for 42. 

S, or NR. In the transition state to C=C rotation, pyrylium, thiopyrylium, or pyridinium ions are formed, which should cause a considerable lowering of the C=C barriers compared to those in systems lacking this delocalization possibility. This is clearly demonstrated by the fulvenes 64 and (5 (57). The barrier... 

Five-membered ring compounds of the type 68, where A and B represent donor groups, have been studied in some instances. Berg (81) found C=C barriers in... 

The low C=C barriers in push-pull ethylenes compared to the 6S.S kcal/ mol in ethylene show that die effects of delocalization on the tr-electron energy in the transition state must be much greater than the effects in the ground state— that is, the important substituent effects on the barriers must occur in the transition state. Besides, an effect that improves delocalization in the ground state would be barrier raising, if it were not accompanied by an at least equal stabilization of the transition state. 

Attempts have been made to correlate the effects of acceptor groups on the C=C barriers by Hammett substituent constants. Since the important interaction is concerned with the delocalization of a negative charge, the cr scale should be more appropriate than the normal a scale (136). However, in the ctp scale, CN is more efficient than COMe, which is contrary to experience, and therefore Shvo et al. preferred the ctr = ctp - o, scale (78 see also Sect. II-C-5). Similarly, in a series of ketene mercaptals, a reasonable correlation was found... 

In the first DNMR studies of push-pull ethylenes, a strong effect of solvent polarity on the C=C barriers was noted. Thus Kende et al. (64) found AG = 18.0 kcal/mol for 46a in N,/V-dimethylformamide (dielectric constant e = 38) and 19.4 kcal/mol in Ph20 ( = 4). Similar observations have been made by many other workers, and they have been seen as a strong support for a zwitterionic transition state. Kessler et al. (140) observed reasonably linear correlations between AG for two ketene aminals and the solvent polarity parameter T (141) with variations in AG of ca. 2.5 kcal/mol over T values between 25 and 46. Similarly, Shvo et al. (78) found linear correlations between log km and the polarity parameter Z (141) for three compounds from Table 12. 

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