Nucleophilic additions to cyclopentadienyliron complexed substituted benzenes hindered at the ortho positions with one or two methyl groups

Abstract

Reaction of NaBH 4 with CpFe complexed 2-Ch 3C 6H 4X, with X = NO 2, CN, SO 2PhCH 3- p or Cl (I, II, III or IV, respectively) resulted in the exo-addition of the hydride ion a C(2), C(3), C(4), C(5) and C(6), with reaction at C(6) to give the ortho-6 adduct as the major product. Similarly, reaction of the cyanide ion with I, II or III (treatment of CN − with IV resulted in substitution to give II) or the acetonyl anion with I, II, III or IV, gave only o-addition, with the ortho-6 adduct as the sole or predominant product. These results clearly suggested a steric hindrance effect by the 2-methyl group, with the C(6) position, the unhindered o-position to the electron-withdrawing substituent X, as the preferred site of reaction to give the major product. Analogous reactions of NaBH 4 with CpFe complexed 2,6-(CH 3) 2C 6H 3X, with X = NO 2, CN, SO 2PhCH 3- p or Cl (V, VI, VII or VIII, respectively) resulted in hydride additions at the o-, m- and p-positions. Since both o-positions at C(2) and C(6) were hindered by methyl groups, relatively greater amounts of m- and p-products were formed, with the m-adduct obtained in the greatest amount. In the addition of the cyanide ion to V, VI or VII, or the acetonyl anion to V, VI, VII or VIII, the o-adduct was the dominant product, but formation of some m- and p-products were also observed. Considering the relative bulkiness of the cyanide ion, the acetonyl anion and the borohydride ion, it was concluded that other factors beside steric effect, such as electronic and free valency effects, as well as the nature of the substituents and the nature of the nucleophile, all could play a role in giving rise to the overall product distribution in these nucleophilic addition reactions.