Regioselectivity in the nucleophilic functionalization of dibenzofuran, dibenzothiophene and xanthene complexes of Mn(CO) 3+

Abstract

The cationic complexes[(η 6-DBF)Mn(CO) 3]BF 4, [(η 6-DBT)Mn(CO) 3]BF 4 and[(η 6-XAN)Mn(CO) 3]PF 6 (DBF = dibenzofuran, DBT = dibenzothiophene, XAN = xanthene) are prepared. These species react with carbon-based nucleophiles to form exo-substituted cyclohexadienyl complexes. Addition is found exclusively at carbons 1 and 4. The C(1)-functionalized regioisomer predominates, especially for the DBF and XAN complexes, for which 90 to > 95% selectivity is found with stabilized nucleophiles. Preference for C(1) is greatly reduced for unstabilized magnesium and lithium nucleophiles. The origin of this selectivity is discussed. Loss of the aromatic ring from the cationic complexes in the presence of CH 3CN is found to be extremely facile for the DBF and DBT complexes owing to increased ease of ring slippage. Second-order rate constants for the displacement reactions with CH 3CN are4.6 ± 0.6 × 10 −5M −1,s −1 (DBF complex, 25°C),4.4 ± 1.0 × 10 −5,M −1,s −1 (DBT complex, 25°C) and about 1.0 × 10 −5,M −1,s −1 (XAN complex, 60°C). A crystal structure is presented for the hydride adduct of the DBF complex,[(η 5-C 12H 9O)Mn(CO) 3]: space group P2 1/c, a = 15.301(2), b = 6.4422(12), c = 12.988(2)A˚, β = 90.888(13)°, V = 1280.1(3)A˚ 3, Z = 4, D calc = 1.599Mg m −3, R = 0.0410 and wR2 = 0.0945 on 2446 reflections with F 2 > 4.0 σ(F 2).