Theoretical Studies On Reaction Mechanisms Foriron-catalyzed Two Coupling Reactions

Coupling reactions catalyzed by the iron catalyst play an important role inorganocatalysis chemistry. In this paper, the reaction mechanisms of the two types ofiron-catalyzed coupling reactions have been explored using density functional theorymethods. The reaction mechanism and some major information such as geometriesand energies of the reactants, intermediates, transition states, and products areobtained.Our calculated results are consistent with and provide a reliableinterpretation for the experimental observations. Knowledge gained frominvestigating thesis mechanism will contribute to the design and synthesis of moreefficient environmentally friendly catalysts and also provide useful information forfurther reaction development in this expanding area of research.The maincontributions are summarized as follows:1. FeCl3-catalyzed cross-coupling reaction of alcohols with alkenesThe mechanism of the FeCl3-catalyzed cross-coupling reaction of alcohols withalkenes has been investigated using the density functional theory (DFT). Allcalculations were performed in liquid phase. The structures of intermediates andtransition states are computed and analyzed in detail. The calculations show that theentire catalytic cycle consists in three steps: i) H-abstraction, ii) free radical addition,and iii) hydrogen transfer. The rate-limiting step in the whole catalytic cycle is thehydrogen abstraction step. Only the quartet potential surface is likely to play anessential role in this cross-coupling reaction. The alpha-C(sp3)-H bond of thephenylpropanol is cleaved in a homolytic fashion.Moreover, we justify the change ofthe oxidation state of iron along the overall reaction pathway on the basis of the computed natural population atomic (NPA) charges and spin densities. Our calculatedresults are consistent with and provide a reliable interpretation for the experimentalobservations that suggest the cross-coupling reaction occurs by a radical mechanism.2. Iron-catalyzed olefin synthesis by direct coupling of alkenes with alcoholsThe mechanism of the iron-catalyzed olefin synthesis reaction has beeninvestigated using density functional theory (DFT) calculations at the B3LYP/6-31G*(LANL2DZ for Fe) level of theory. The catalytic performances of the FeCl3and FeCl2in different spin states for the olefin synthesis reaction have been studied in detail.Our calculations suggest:(1) The overall catalytic cycle includes three basic steps:dehydration, electrophilic addition, and deprotonation. The electrophilic addition isthe rate-limiting step in the whole catalytic cycle, which is consistent with theexperimental prediction.(2)(E)-alkene is the main product in the olefin synthesisreaction.(3)The FeCl3and FeCl2catalyzed reactions are found to proceedpredominantly on the sextet and quintet spin state surfaces, respectively.(4) Thegeometry, mechanism, and the overall reaction barrier of the olefin synthesisreactionare very similar when FeCl2and FeCl3areused as catalysts.