Herrn Prof. Dr. V . Prelog zum 70. Geburtstag gewidmet (4. VI. 76)Summary. ESR. studies are reported for the radical anions of 5,6-didehydro-and 5,6,11,12tetradehydro-dibenzo[a,e]cyclooctene (I11 and IV, resp.), in addition to that of dibenzo[a, e]cyclooctene (I [) itself, the spectrum of which has been reexamined. Comparison of the proton and 13C coupling constants for I1 . @, I11 . 0 and IV . 0 indicates that the thrcc radical anions do not differ greatly in their electronic and molecular structures. This statement implies that I1 . 0 should also be substantially planar, i.e., the tub-shaped eight-membered ring in I1 is expected t o flatten on passing from the neutral molecule t o its radical anion. Support for postulating such a change in geometry, analogous t o that encountered with the parent cyclooctatetraene (I), is provided by INDO calculations.
ESR. spectra of the radical anion (I⊖) produced from dimethyl‐phenyl‐phosphine (I) both by electrolysis and reaction with alkali metals have been studied upon variation of temperature. The coupling constant assigned to the 31P nucleus depends strongly on temperature, whereas the coupling constants attributed to protons do not exhibit such a dependence. The π‐spin populations at the benzene ring of I⊖ give evidence ‐ in accordance with other experimental data [1] [2] – that the dimethylphosphino substituent is electron‐attracting. This effect is thought to be due mainly to P ← Cπ delocalization, which is analogous to the SiCπ interaction in trimethylsilyl‐substituted π‐systems [3].
The ESR. spectrum previously [4] ascribed to I⊖ is shown to arise from a secondary radical. The formation and structure of this radical are briefly discussed.
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