Highly regioselective benzylic deprotonation of some (eta(6)-tetralin)- and (eta(6)-trans-octahydroanthracene)Cr(CO)(3) derivatives: Is the regioselectivity stereoelectronically controlled?"

The regioselectivity of benzylic deprotonation of a number of conformationa lly restricted (arene)Cr(CO)(3) complexes has been examined in order to asc ertain whether stereoelectronic effects play a role in such reactions. The complexes rac-5, rac-6, rac-7, rac-8 and rac-9 were diastereoselectively sy nthesized from the corresponding Ct-symmetric ligands (2,3-disubstituted tr ans-1,2,3,4-tetrahydronaphthalene and trans-1,2,3,4,4a,9,9a, 10-octahydroan thracene derivatives). Due to the desymmetrization caused by the Cr(CO)(3) complexation, all four benzylic protons could be distinguished by H-1 NMR a nd 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/[D-6]DMSO), De protonation (n-butyllithium)/deuteration (D2O or CF3CO2D) experiments revea led a very high, unforeseen regioselectivity in the cases of rac-5 and rac- 7, while the other substrates showed a low selectivity (rac-8) 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 r esults clearly rule out the notion that the regioselectivity is due to the preferred abstraction of pseudoaxially oriented benzylic hydrogen atoms. Th e crystal structures of rac-1, rac-5 and rac-7 suggest a possible link betw een the preferred conformation of the Cr(CO), tripod and the regioselectivi ty of the benzylic deprotonation. In analogy to a commonly accepted picture often used to explain the regioselectivity of nucleophile additions to (ar ene)Cr(CO)(3) complexes, it was anticipated that those benzylic positions w hich are activated by an eclipsed CO ligand should be preferentially deprot onated (kinetically controlled). This (new) stereoelectronic model was corr oborated by experiments using complexes rac-60 and rac-62, which were regio selectively 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 d eprotonation in (arene)Cr(CO)(3) complexes, at least in conformationally un ambiguous situations where no obvious electronic effects are operative.