On the reactivity of tricarbonyl(1–4-η-cyclohepta-1,3,5-triene)iron derivatives: C–C bond formation of tricarbonyl(cycloheptatrienide)irons with 2-chlorotropone

Abstract

Tricarbonyl(1–4-η-cyclohepta-1,3,5-triene)iron derivatives [(RC₇H₇)Fe(CO)₃](R = H, OMe, CN, and Ph)(4a–d) have been prepared and a study made of the nucleophilic attack of 2-chlorotropone by their corresponding anions. The tricarbonyl(cycloheptatrienide) iron and tricarbonyl(methoxycycloheptatrienide)iron undergo the reaction to give tricarbonyl[1–4-η-7-(2-oxocyclohepta-1,3,5-trienyl)cyclohepta-1,3,5-triene]iron (7a) and tricarbonyl[1–4-η-6-methoxy-7-(2-oxocyclohepta-1,3,5-trienyl)cyclohepta-1,3,5-triene] iron (7b), respectively. In contrast, tricarbonyl (cyanocycloheptatrienide)iron reacted with 2-chlorotropone to afford tricarbonyl[1–4-η-6-cyano-7-(2-oxocyclohepta-1,3,5-trienyl)cyclohepta-1,3,5-triene]iron (7c) and tricarbonyl[1–4-η-5-cyano-7-(2-oxocyclohepta-1,3,5-trienyl)cyclohepta-1,3,5-triene] iron (9c) in a ratio of (7c)/(9c)=1:5.6. Similarly, tricarbonyl(phenylcycloheptatrienide)iron with 2-chlorotropone also affords two products, tricarbonyl[1–4-η-6-phenyl-7-(2-oxocyclohepta-1,3,5-trienyl)cyclohepta-1,3,5-triene]iron (7d) and tricarbonyl[1–4-η-2-phenyl-7-(2-oxocyclohepta-1,3,5-trienyl)cyclohepta-1,3,5-triene]iron (10d), in a ratio of (7d)/(10d)= 3 : 1. The selective formation of (7b) and the product ratios of (7c) : (9c) and (7d) : (10d) are discussed on the basis of the electronic and steric factors of the substituent in the formally unco-ordinated allyl anion on the cycloheptatrienide ring. The stereochemistry of the tropone nucleus at C-7 is deduced to be exo to the [Fe(CO)₃] entity on the basis of ¹H n.m.r. spectral results. The products (7a, b, d) are easily decomplexed by trimethylamine oxide to give 7-(2-oxocyclohepta-1,3,5-trienyl)- and 1-substituted 7-(2-oxocyclohepta-1,3,5-trienyl)cyclohepta-1,3,5-triene derivatives in good yields.