ortho-effect of hexacarbonyldicobaltacetylene substituents of propargyl type in aromatic systems

Abstract

In this work we analyzed the features of cobaltpropargylation of some aromatic systems (ferrocene, five-membered nitrogen-containing heterocycles, thiophenes) with μ,η 2 -hexa-carbonyldicobalt complexes of acetylenic alcohols of the propargyl type.The cobalt-propargylation reaction proceeds under the influence of μ,η 2 -hexacarbonyldicobalt complexes of acetylene alcohols of the propargyl structure in the presence of boron trifluoride diethyl etherate. The latter generates in situ a superstable carbenium cation Co 2 (CO) 6 (R)C≡CC + (R 2 ), where R are hydrogen atoms or different organic substituents. Such cations are sufficiently active electrophilic reagents [1-3].It has been shown [2] that the μ,η 2 -hexacarbonyldicobalt-2-propynol complex I reacts with ferrocene in the presence of BF 3 . OEt 2 to form the ferrocene derivative monosubstituted in the cyclopentadiene ring. With a four-fold excess of this complex the cobaltpropargylation occurs also in the second cyclopentadienyl ring to give the disubstituted product in a low yield.The carbazole is cobalt-propargylated with propynol complex I primarily at the HN nitrogen atom followed by substituting at the atoms C 3 and C 6 of carbazole skeleton [4]. The progenitor compound, pyrrole, is cobalt-propargylated with this complex on the α-carbon atom to form 2-(μ,η 2 -hexacarbonyldicobalt-2-propynyl)pyrrole [3]. An attempt to introduce the second hexacarbonyldicobalt moiety into HNposition failed, although the cationic character of the reagent formed in the presence of BF 3 . OEt 2 from the complex I did not exclude such a possibility.1,2,4-Triazole reacts with the propynol complex I exclusively at the nitrogen atom HN 1 to give 1-(μ,η 2 -hexacarbonyldicobalt-2-propynyl)-1,2,4-triazole [3]. The subsequent cobaltpropargylation also does not proceed, despite the fact that the electron density on the C 5 atom should remain sufficient for the electrophilic attack on it.The C-substituted tetrazoles are cobaltpropargylated at the nitrogen atoms N 1 or N 2 to form primarily the N 2 -monosubstituted products [3]. The unsubstituted 1H-tetrazole gives one N-cobaltpropargylated product, whose assignment to the N 1 -or N 2 -substitution is impossible. However, N,N-bis(μ,η 2 -hexacarbonyldicobalt-2-propynyl)carbamide was detected as a byproduct [3]. Its formation can be represented as the result of biscobaltpropargylation of tetrazole on the N 1 -and N 4 -positions, followed by the oxidative degradation of the ring, which loses aromaticity due to the introduc-tion of the second cobaltpropargyl moiety into the ring. This analysis may indicate the benefit of the orthoeffect of cobaltpropargyl substituent preventing the entry of the second cobaltpropargyl moiety into the ortho-position relative to the just entered one. Obviously, in the examined aromatic systems this substituent exhibits a deactivating effect in subsequent reactions of electrophilic substitution, manifesting the electron-withdrawing properties (in the case of the ferrocene cobaltpropargylation) and providing the st...