Photoinduced nucleophilic substitution of iodocubanes with arylthiolate and diphenylphosphanide ions. Experimental and computational approaches

A new synthetic route to modify the cubane nucleus is reported here. Methyl-4-iodocubane-1-carboxylate (1) and 1,4-diiodocubane (2) were employed as reagents to react with arylthiolate and diphenylphosphanide ions under irradiation in liquid ammonia and dimethylsulphoxide. The reactions proceed to afford thioaryl- and diphenylphosphoryl- cubane derivatives in moderate to good yields. It is also found that the monosubstituted product with retention of the second iodine is an intermediate compound. Mechanistic aspects are supported by DFT calculations.


Test of different solvent´s models
Since the reactions were carried out in liquid ammonia but this solvent was not available in g09, the solvent employed was methanol (as in previous works from the group). To check the differences arisen from different solvent approximations the energies of 3 molecules were computed with methanol, DMSO and liquid ammonia build using parameters available in literature. Almost no differences were observed until the 4 th decimal of the energy (~0.1 to 0.3 kcal/mol). With this small energy we considered that the results obtained in the study of the mechanism would not significantly change in either (DMSO, NH3, or methanol used instead of NH3)

Calculation of the redox potentials of phenylsulphide and dipheylphosphide anions
The overall calculated reaction is presented in the thermodynamic cycle shown in scheme 1 and eq. 1.
In previous works, this equation has been employed for the calculation of reduction. [1][2][3][4] In some cases the ΔΔ ( ) 0 was calculated over the gas phase geometries, in other cases different solvent models were employed to improve its quality and hence the 0 . In the present work the 0 was directly calculated from the optimized geometries in solvent by employing the IEF-PCM continuum solvation model, 5 including the thermal corrections and the changes in enthalpy and entropy.

S43
Finally, the red 0 is related to the redox potential, 0 , by the Faraday's equation (2) where n is the number of electrons transferred (in this case is equal to 1) and F is the Faraday constant (96485 C mol -1 or 23.061 kcal mol -1 V -1 ). According to these equations, the Redox potential of both molecules was calculated:

Hypothetical mechanism involving formation of 1,4-dehydrocubane A
We propose that 1,4-diiodocubane 2 receives an electron from the nucleophile (ArS -/Ph2P -) in a photoinduced process through a dissociative-ET pathway leading directly to 4-iodocubyl radical 17. This intermediate can afford 1,4-dehydrocubane A by homolytic dissociation (with formation of iodine radicals) or by ET (with formation of iodide ions). Another possibility is its reaction with the anion, which is in excess, to afford the radical anion of the mono-iodosubstituted cubane 13 (Scheme 1-SI). Calculations indicated that the preferred reaction is the coupling with the S45 nucleophile to allow a typical SRN1 pathway. Relevant energetic factors (ΔGº) of the three mechanistic possibilities are presented in the next Table 1-SI. 6,7