Nucleophilic addition to tricarbonyliron(0) complexes of 1-aza-1,3-dienes and the production of pyrroles

Abstract

The synthesis of a number of novel tricarbonyl(1 -aza-I ,3-diene)iron(o) complexes and their reactions with methyl-lithium are described. Addition proceeds under mild conditions and, in general, occurs at a metal carbonyl ligand; this leads via an unprecedented reaction to the production of several novel pyrroles.Paper 9/03075A

The chemistry of (1-azabuta-1,3-diene)tricarbonyliron(0) complexes

“…For example, reaction of diironnonacarbonyl with 1-azabuta-1,3-dienes (14) and (16) derived from cinnamaldehyde and a-substituted benzylamines produces the diastereomeric complexes 15 and 17, respectively (Scheme 4) [4,9]. When the asubstituent R of the azabuta-1,3-diene is a methyl group, a 1:1 mixture of diastereoisomers is formed.…”

“…12), a mixture of E,E (C‚N) (12a) and E,Z (C‚N) (12b) geometrical isomers is usually formed. Complexation of the 1-azabuta-1,3-diene mixture (12) to the tricarbonyliron(0) moiety by reaction with Fe 2 (CO) 9 leads to complex 13 in which the E,E (C‚N) ligand is selectively coordinated to the tricarbonyliron(0) moiety (Scheme 3) [4]. Evidence for the stereochemistry of the ligand was obtained from NOE difference spectroscopy.…”

“…For complex 13, reaction with methyl lithium leads to attack at a co-ordinated carbonyl group and formation of an anionic iron acyl complex 87 [4,5]. After insertion of the acyl fragment into the co-ordinated 1-azabuta-1,3-diene, and the anionic azaallyl intermediate 88 is formed.…”

“…In the case of the E,Z (C‚N) complex, there is a significant interaction between the ortho protons of the N-phenyl and the proton at C-4 of the 1-azabuta-1,3-diene and this presumably prevents the complexation reaction. When the tricarbonyliron(0) moiety is removed from the complex by mild oxidation [(CH 3 ) 3 N + À O À ], the 1-azabuta-1,3-diene reverts to the original mixture of geometric isomers [4][5][6] and 12a and 12b are regenerated.…”

“…The reaction is quite generally applicable and a large variety of complexes have been prepared from stable, isolated precursor 1-azabuta-1,3-dienes [4][5][6][7]. In cases where isolation of the starting 1-azabuta-1,3-diene is problematic [e.g.…”