Organometallic compounds in organic synthesis. Part 10. Preparations and some reactions of tricarbonyl-1,3- and -1,4-dimethoxycyclohexa-1,3-dieneiron and related compounds: the preparation of the tricarbonyl-3-methoxycyclohexadienyliumiron cation

Abstract

Tricarbonylcyclohexadienyliumiron cations can be regarded as synthetic equivalents, depending on the reaction sequence, either of specific aryl cations or, in the case of methoxy-derivatives, of cations derived from cyclohex-2-enones. An important series can be entered through the symmetrical 3-methoxy cation (32, R¹= R²= H, R³= OMe) made efficiently for the first time from tricarbonyl-1,3-dimethoxycyclohexa-1,3-dieneiron (18). It is synthetically equivalent to a meta-methoxy-benzene cation or to a 5-cation of cyclohex-2-enone. Another series of aromatic equivalents can be defined based on nucleophilic reactions at the 1-position of derivatives of tricarbonylcyclohexa-2,4-dienoneiron (33), and efficient syntheses of (33) and its 3-methoxy (35) and 4-methoxy (37) derivatives are described. The precursors for these reactions require the efficient complexation of the 1,3-or 1,4-dimethoxycyclohexadienes, which can be carried out using the conjugated, but not the unconjugated, 1,4-dienes where loss of OMe occurs. Related processes are described in preparations and uses of complexes of some 1-morpholinocyclohexa-1,3-dienes. Some mechanisms are discussed.