Phosphabenzenes shifts

The NMR spectra of pyridine, phosphabenzene, arsabenzene, and stibabenzene are quite similar in appearance (11,12). The y protons appear at lowest field partially as a result of remoteness from the heteroatom. The effect of the magnetic anisotropy of the heteroatom should decrease for the )3 and y protons. The chemical shift differences for the y protons in pyridine and arsabenzene are almost negligible. The low-field chemical shift value was interpreted in terms of the presence of ring current in the heteroatomic derivatives. 

Studies on benzo-fused analogs of 78 have also been reported. Solutions of unstable dibenzo[b,e]phosphabenzene (phosphaanthracene) (80, R = H)286 and dibenzo[6,d]phosphabenzene (phosphaphenanthrene) (81)287 have been prepared and examined by UV spectroscopy. The overall shape of the spectrum due to 80 (R = H) resembles that of anthracene rather than acridine, whereas that of 81 resembles the phenanthridine more closely than the phenanthrene spectrum. The absorptions of both 80 (R = H) and 81 show the typical strong bathochromic shifts relative to their nitrogen and carbocyclic analogs. 10-Phenyldibenzo[6, e]phospha-benzene (80, R = Ph)288 is more stable than its parent and shows similar UV absorption. 

Ashe has recently synthesized both arsabenzene (arsenin) (83 )283 and stibabenzene (antimonin) (84).66 The former is very air-sensitive while the latter polymerizes even at —80°. The NMR spectra of these, like that of phosphabenzene,283 show that the a-proton is highly deshielded and Ashe reports the following order of chemical shifts pyridine 5 8.1, phosphabenzene 8.6, arsabenzene S 9.3, and stibabenzene S 10.7. Significantly, the more remote /3- and 7-protons in each compound in the series are each at approximately the same chemical shift, which may imply an appreciable ring current for each compound. J23 = 11 Hz for both 83 and 84, which is greater than the 10 Hz found for phosphabenzene and 5.5 Hz for pyridine. 

In the UV spectra, 83 shows bands at 219 and 268 mu while 84 shows absorptions at 236 and 312 mji (cf. phosphabenzene Xmax 213 and 246 m/i). Ashe66 suggests that if the bands are due to tt —> ir transitions, then the shifts to longer wavelengths exhibited by the heteroaromatics with the heavier atoms may arise from weaker bonding in these compounds. 

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