Iron catalysis addition

Recently, Fu and coworkers have shown that secondary alkyl halides do not react under palladium catalysis since the oxidative addition is too slow. They have demonstrated that this lack of reactivity is mainly due to steric effects. Under iron catalysis, the coupling reaction is clearly less sensitive to such steric influences since cyclic and acyclic secondary alkyl bromides were used successfully. Such a difference could be explained by the mechanism proposed by Cahiez and coworkers (Figure 2). Contrary to Pd°, which reacts with alkyl halides according to a concerted oxidative addition mechanism, the iron-catalyzed reaction could involve a two-step monoelectronic transfer. 

Oxidative addition generally increases the oxidation state of the metal by two units and, based on the common oxidation states of iron, leads from iron(0) to iron(II) or iron(-II) to iron(0). The former represents the most widespread system for iron catalysis in organic synthesis but the latter also has enormous potential for applications (see Section 1.4). 

Iron catalysis is experiencing a renaissance. In 2007, Bolm described a selective Nl-arylation using FeCls as the catalyst and K3PO4 as the base [218]. The coupling reaction was facilitated by the addition of 20 mol% of DMEDA as a chelating agent. 

The dienol phosphates are known to be less reactive and more stable than the corresponding dienic iodides or bromides. However, under iron catalysis, the oxidative addition step is easier than with palladium or nickel catalysts. This methodology circumvents the conventional use of dienyl iodides or bromides. Since these reagents are known to be sensitive and prone to polymerization, this strategy constitutes a substantial improvement for the synthesis of stereodefined conjugated dienes. 

Easy-to-handle arylboronic compounds can also be reacted in a Suzuki-Miyaura-like fashion with nonactivated alkyl halides using iron catalysis (Scheme 4-239). Two novel iron complexes with sterically hindered diphosphane ligands have been developed for this transformation. Additionally, a magnesium cocatalyst is required. For the mechanism, action of the redox couple Fe(III)/Fe(II) is discussed. This requires the intermediate formation of an alkyl radical species as displayed in Scheme 4-238. " ... 

A chlorination of styrene carried out at 50 C and in the presence of iron trichloride led to a detonation after having incorporated 10% of the chlorine. The presence of iron trichloride or even the use of steel reactors is sufficient to make styrene chiorination hazardous. Various authors assume that the danger comes from the catalysis of the styrene decomposition by iron sait rather than the cataiysis of the addition of chlorine by this Lewis acid. 

Silvery metal, that can be cut with a knife. Terbium alloys and additives are widely used in optoelectronics to burn CDs as well as in laser printers. The pronounced magnetostriction (Joule effect) makes "terfenol-D" (terbium-dysprosium-iron) indispensable in sonar technology. The physics of the element appears to be more interesting than its chemistry, in which it is rarely used in catalysis. 

Catalysis by various low-valent metalloporphyrins of the type already depicted in Section 3.7.2 (see reference lb for a precise list) is represented in Figures 4.3 and 4.4 for several cyclic and acyclic 1,2-dibromides. A striking example of the contrast between redox and chemical catalyses is shown in Figure 4.3a, with fluorenone anion radical on the one hand and iron(I) octaethylporphyrin on the other. Starting with the oxidized, inactive form of the catalyst, in each case—the active form is produced at a reversible wave. Addition of the same amount of 1,2-dibromocyclohexane triggers a catalytic increase in the current that is considerably less in the first... 

Fig. 21. Detailed mechanism for HO-1 catalysis. In 1, oxygenation and electron transfer forms the ferric (Fe +)-peroxy complex. Steric factors and H-bonding help to bend the peroxide toward the a-meso-heme position for regio-selective hydroxylation. One proposed mode of forming verdoheme is shown in part 2. A key part of step 2 is the resonance structures between Fe + and Fe +/radical, which enable the porph3rrin ring to be oxygenated. Although the mechanism shown does not require any reducing equivalents (176), there remain experimental inconsistencies on the requirement of an additional electron in step 2. However, reduction of the verdoheme iron is necessary to prepare the substrate for step 3, verdoheme to bihverdin.

Proteins containing iron-sulfur clusters are ubiquitous in nature, due primarily to their involvement in biological electron transfer reactions. In addition to functioning as simple reagents for electron transfer, protein-bound iron-sulfur clusters also function in catalysis of numerous redox reactions (e.g., H2 oxidation, N2 reduction) and, in some cases, of reactions that involve the addition or elimination of water to or from specific substrates (e.g., aconitase in the tricarboxylic acid cycle) (1). 

The reasoning which led the author to make this first shot in the dark regarding the usefulness of combinations of solid compounds as ammonia catalysts was as follows If we assume that a labile iron nitride is an interminate in the catalytic ammonia synthesis, every addition to the iron which favors the formation of the iron nitride ought to be of advantage. In other words, the hypothesis was used that surface catalysis acts via the formation of intermediate compounds between the catalyst and one or more of the reactants. An experimental support for this theory was the fact that a stepwise synthesis via the formation and successive hydrogen reduction of nitrides had been carried out with calcium nitrides (Haber), and cerium nitrides (Lipski). Later, the author found molybdenum nitride as being the best intermediate for such a stepwise synthesis. 

---