Iron complexes ligand

Aminoboranes have been used as ligands in complexes with transition metals (66) in one instance giving a rare example of two-coordinate, non-t/ transition-metal complexes. The molecular stmcture of the iron complex Fe[N(Mes)B(Mes)2]2 where Mes = is shown in Figure 1. The... 

In the same manner, the S-monoxidized iron complex can be oxidized with a second equivalent of 3-chloroperoxybenzoic acid and subsequent irradiation to give the stable 1-benzothiepin 1,1-dioxide in 76% yield.23 (For removal of the iron ligand, see also Section 2.2.1.). 

Que and coworkers reported on a similar monomeric iron complex, formed with the BPMEN ligand but without acetic acid [128]. This complex was able to epoxidize cyclooctene in reasonably good yield (75%), but at the same time a small amount of the ris-diol (9 %) was formed. This feature observed with this class of complexes has been further studied and more selective catalysts have been prepared. Even though poor levels of conversion are often obtained with the current... 

Similar effects are observed in the iron complexes of Eqs. (9.13) and (9.14). The charge on the negatively charged ligands dominates the redox potential, and we observe stabilization of the iron(iii) state. The complexes are high-spin in both the oxidation states. The importance of the low-spin configuration (as in our discussion of the cobalt complexes) is seen with the complex ions [Fe(CN)6] and [Fe(CN)6] (Fq. 9.15), both of which are low-spin. 

The reaction in Eq. (9.34) is also faster because the bpy ligand is a strong field ligand and there is no longer any need for electronic rearrangement upon change in oxidation state. The process is now comparable to those discussed earlier for low spin iron complexes. 

Ligands of type 48 were synthesized by the cyclization reaction of diamines with dithioaldehydes. Iron complexes formed with those structures led, however, to active but weakly enantioselective catalysts. The best results were... 

The catalytic cycle was devised relying on a ligand exchange between the iron complex and a chlorosilane, regenerating the iron (111) halide due to the more oxophilic character of the silicon (Scheme 34). 

Abstract Organic syntheses catalyzed by iron complexes have attracted considerable attention because iron is an abundant, inexpensive, and environmentally benign metal. It has been documented that various iron hydride complexes play important roles in catalytic cycles such as hydrogenation, hydrosilylation, hydro-boration, hydrogen generation, and element-element bond formation. This chapter summarizes the recent developments, mainly from 2000 to 2009, of iron catalysts involving hydride ligand(s) and the role of Fe-H species in catalytic cycles. 

This chapter treats iron complexes with Fe-H bond(s). An H ligand on a transition metal is named in two ways, hydride and hydrido. The term hydrido is recommended to be used for hydrogen coordinating to all elements by lUPAC recommendations 2005 [1], However, in this chapter, the term hydride is used because it has been widely accepted and used in many scientific reports. 

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

An iron complex-catalyzed enantioselective hydrogenation was achieved by Morris and coworkers in 2008 (Scheme 13) [49]. Reaction of acetophenone under moderate hydrogen pressure at 50°C catalyzed iron complex 12 containing a tetradentate diimi-nodiphosphine ligand in the presence of BuOK afforded 1-phenylethanol with 40% conversion and 27% ee. 

An iron complex-catalyzed asymmetric hydrosilylation of ketones was achieved by using chiral phosphoms ligands [68]. Among various ligands, the best enantios-electivities (up to 99% ee) were obtained using a combination of Fe(OAc)2/(5,5)-Me-Duphos in THF. This hydrosilylation works smoothly in other solvents (diethylether, n-hexane, dichloromethane, and toluene), but other iron sources are not effective. Surprisingly, this Fe catalyst (45% ee) was more efficient in the asymmetric hydrosilylation of cyclohexylmethylketone, a substrate that proved to be problematic in hydrosilylations using Ru [69] or Ti [70] catalysts (43 and 23% ee, respectively). 

Interestingly, the activity of the corresponding cobalt catalyst possessing a pincer-type ligand is higher than that of the iron complex. In addition, the cobalt complex also acts as a catalyst in asymmetric mtermolecular cyclopropanations. 

Very recently, Hammarstrom, Ott, and coworkers found that the catalytic activity was significantly increased (up to 200 TON based on the iron complex) when a diiron complex having a 3,6-dichlorobenzene-1,2-dithiolate ligand (Fig. 10) instead of a benzylazadithiolate ligand was used as a WRC (see ref. [64] in [232]). 

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