A Theoretical Study on the Complete Catalytic Cycle of the Hetero‐Pauson–Khand‐Type [2+2+1] Cycloaddition Reaction of Ketimines, Carbon Monoxide and Ethylene Catalyzed by Iron Carbonyl Complexes

Abstract

The [2+2+1] cycloaddition reaction of 1,4-diazabutadienes, carbon monoxide and ethylene catalyzed by iron carbonyl complexes produces pyrrolidin-2-one derivatives. Only one of the two imine moieties is activated during the catalysis. The mechanism of this cycloaddition reaction is studied by density functional theory at the B3LYP/6-311++G(d,p) level of theory. In accordance with experimental results, a [(diazabutadiene)Fe(CO)(3)] complex of square-pyramidal geometry is used as the starting compound S of the catalytic cycle. Based on experimental experience, the reaction with ethylene is considered to take place before any interaction with carbon monoxide. According to the computational results, the reaction does not proceed by ligand dissociation followed by addition of ethylene and subsequent intramolecular activation steps but by the approach of an ethylene molecule from the base of the square-pyramidal complex. This reaction yields an intermediate I(4) in which ethylene is coordinated to the iron centre and a new C-C bond between ethylene and one of the imine groups is formed. The insertion of a terminal carbon monoxide ligand into the metal-carbon bond between ethylene and iron produces the key intermediate I(7). The reaction proceeds by metal-assisted formation of a lactam P. The catalytic cycle is closed by a ligand-exchange reaction in which the diazabutadiene ligand substitutes P with reformation of S. This reaction pathway is found to be energetically favored over a reductive elimination. It leads to the experimentally observed heterocyclic product P and a reactive [Fe(CO)(3)] fragment.

Regioselectivity in Iron‐Catalyzed [2+2+1] Cycloadditions: A DFT Investigation of Substituent Effects in 1,4‐Diazabutadienes

“…The addition of ethylene (B) to the cyclic (1,4-diazabutadiene)Fe(CO) 3 complex I 1 from the base of the square-pyramidal coordination sphere of iron was shown to be the rate-determining step of the whole reaction sequence. [16] In addition, these elementary steps produce the new CÀC bond between ethylene and the competing iminecarbon atoms, and thus determine the regioselectivity of the complete cycloaddition (Scheme 1). This reaction can also be interpreted as a [3+2] cycloaddition reaction in which a metal is involved leading to a new ferra-pyrrolidine ring and thus is closely related to the formation of titanacyclobutenes.…”

“…The B3 LYP functional has previously been found to be of a suitable theoretical level for the study of the interactions of transition metals with ligands, especially with CO. [26] Suitable starting geometries concerning the coordination sphere of the iron atom were taken from the calculation of the complete catalytic cycle of this cycloaddition reaction. [16] Stationary points were rigorously characterized as minima or transition states according to the number of imaginary modes by applying a second-order derivative calculation (vibrational analysis). [27] Visualization of the reactive mode in the transition structures was used to support the assignments of the pertaining minimum structures.…”

“…[15] The complete catalytic cycle of this formal cycloaddition reaction has been calculated at a high level of theory. [16] A simplified catalytic cycle is depicted in Scheme 2. The cycle is initiated by the formation of the [(1,4-diazabutadiene)Fe-(CO) 3 ] complex A, in which the metal center displays a square-pyramidal coordination sphere.…”

A Computational Study on the Role of Chiral N‐Oxides in Enantioselective Pauson–Khand Reactions

“…Density functional and DFT/MM calculations have been previously shown to be a useful tool to gain mechanistic understanding and explain the enantioselectivity of a variety of catalytic systems. [8,9] Computational studies on the PKR have been performed both for the most common dicobaltbased catalysts [10,11] and for alternative catalysts based on iron, [12] rhodium, [13] and ruthenium. [14] However, to our knowledge, the only previous theoretical work on enantioselective PKR dealt with a reaction based on the chiral catalyst [Co 2 A C H T U N G T R E N N U N G (BINAP)(CO) 6 ].…”

Recent advances in the iron-catalyzed cycloaddition reactions

“…A hetero-Pauson-Khand-type [2+2+1] cycloaddition reaction of ketimines, carbon monoxide and ethylene under catalytic amount of Fe 2 (CO) 9 was reported by Imhof and co-workers [52]. The reaction exclusively took place at the imine functional group adjacent to the oxazine oxygen atom (Scheme 24), however, only 55% of 1,4-diazabutadiene 23 was converted to spirolactam 24 and the reaction scope was rather limited.…”