Synthesis of 5,1′-alkynyl- and 5,1′-alkenyl-1,3-cyclohexadiene-tricarbonyliron complexes using aluminum reagents

Abstract

Dimethylaluminum acetylides react with (η 5-cyclohexadienyl)tricarbonyliron fluoroborate ( 1) to give the corresponding C(5) alkynyl substituted complexes in high yields without competing reduction products. Using this method, phenylethynyl and trimethylsilylethynyl groups have been added to the cationic complex at C(5). In the latter case, the yield can be improved substantially if tris(trimethylsilylethynyl)-aluminum is used instead. Desilylation of this complex yields the 5-ethynyl complex. Hydrogenation with Lindlar's catalyst yields the corresponding alkenyl complex. Oxidative removal of the Fe(CO) 3 moiety yields the organic ligand in near quantiative yields. Use of aluminum reagents as alkynyl anion synthons in reactions with 1 is a far superior method compared to the use of either lithium acetylides which give complicated mixtures of products or 1-trimethylsilylalkynes which are unreactive.