| Abstrakt: |
The regioselectivity of benzylic deprotonation of a number of conformationally restricted (arene)Cr(CO)3 complexes has been examined in order to ascertain whether stereoelectronic effects play a role in such reactions. The complexes rac-5, rac-6, rac-7, rac-8 and rac-9 were diastereoselectively synthesized from the corresponding C2-symmetric ligands (2,3-disubstituted trans-1,2,3,4-tetrahydronaphthalene and trans-1,2,3,4,4a,9,9a,10-octahydroanthracene derivatives). Due to the desymmetrization caused by the Cr(CO)3 complexation, all four benzylic protons could be distinguished by 1H NMR and were assigned in all cases by a combination of H,H-COSY spectra and the observation of H/D exchange at both exo positions (tert-BuOK/[D6]DMSO). Deprotonation (n-butyllithium)/deuteration (D2O or CF3CO2D) experiments revealed a very high, unforeseen regioselectivity in the cases of rac-5 and rac-7, while the other substrates showed a low selectivity (rac-6) or could not be deuterated at all under these conditions (rac-8, rac-9). In the case of rac-5, the regioselectivity of the deprotonation was further confirmed by acylation (AcCl) or alkylation (MeI) of the lithiated intermediate. These results clearly rule out the notion that the regioselectivity is due to the preferred abstraction of pseudoaxially oriented benzylic hydrogen atoms. The crystal structures of rac-1, rac-5 and rac-7 suggest a possible link between the preferred conformation of the Cr(CO)3 tripod and the regioselectivity of the benzylic deprotonation. In analogy to a commonly accepted picture often used to explain the regioselectivity of nucleophile additions to (arene)Cr(CO)3 complexes, it was anticipated that those benzylic positions which are activated by an eclipsed CO ligand should be preferentially deprotonated (kinetically controlled). This (new) stereoelectronic model was corroborated by experiments using complexes rac-60 and rac-62, which were regioselectively deprotonated at the predicted position. In summary, it has been shown for the first time that the preferred conformation of the Cr(CO)3 tripod may have a directing influence on the regioselectivity of benzylic deprotonation in (arene)Cr(CO)3 complexes, at least in conformationally unambiguous situations where no obvious electronic effects are operative. |
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