By Boschke F.L.

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X~tl -C3H7 o 1820 1630 1858 1527 1859 1553 190 1850 i620/1660(C=0) 1830 1620/1660(C=0) 1770 1605/1640(C=0) 19! fulvenes 190 and 188 carbonyl absorption is found at 1630 and 1680 cm -1, respectively; however, C=O groups in anthrone (1670 cm - 1 ) and methyl cyanoacetate (1770 cm -1 ) absorb at markedly higher frequency. This may point to an enhanced single-bond character for the exocyclic C=O group in the triafulvenes, which supports the idea of "cyclopropenium enolate" participation in the triafulvene resonance hybrid, as shown in 192a/192b: O Ole 192a 192b 4.

35 r146); 202: - 1 . 2 0 r 173)) shows the cyclopropenone system to exhibit a far stronger deshielding effect ( " 2 p p m ) than the cyclopropene moiety, but weaker than a cyclopropenium ring system. The 13C_ H coupling constants o f methyl (213 Hz 23)) and phenyl (216 Hz ss)) cyclopropenone are in the order o f those obtained for cyclopropene vinylic p r o t o n s (200/201:218 Hz]221 Hz 174)) and reflect an s-contribution o f more than 40 % in the carbon hybrid orbital o f the vinyl C - H bond.

Table 8. 92) UV comparison of some diaryl cyclopropenones R = phenyl R =p-toly R=p-anisyl 297 308 342 R =p-chloro phenyl 310 Cis-stilbene Diphenyl eyelopropenium cation 171) 306 285 297 315 296 300 220 nm 240 256 227 nm 233 nm 236 227 nm 277 224 202 nm 293 nm In diaryl cyclopropenones, the UV data are not very characteristic and resemble those of diaryl cyclopropenones and stilbenes as well as diaryl cyclopropenium cations, as shown in Table 8. 06 (cyclohexane)] which was assigned tentatively to an intramolecular charge-transfer.

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