Iron carbenoids

The use of chiral iron-carbenoid complexes in the preparation of optically active cyclopropanes has been described. ... 

The synthesis of sulfones has been performed by iron-catalysed decomposition of sulfonylhydrazones. As commonly accepted for the related Bamford Stevens reaction, the reaction is thought to proceed through generation of a diazocompound by base-mediated thermal decomposition of the sulfonylhydrazone, leading to an intermediate iron carbenoid (65), which forms the sulfone (66) when trapped by the sulflnate anion released in the first step (Scheme 9). 

Catalytic Wittig-type and Doyle-Kirmse reactions have been achieved using diazo compounds as carbenoid sources in the presence of ferrate complex Bu4N[Fe(CO)3(NO)] as catalytic species this result highlights for the first time the potential of the electron-rich iron complex for activating diazo compounds to iron carbenoids. 

Interestingly, [Ee(F20-TPP)C(Ph)CO2Et] and [Fe(p2o-TPP)CPh2] can react with cyclohexene, THF, and cumene, leading to C-H insertion products (Table 3) [22]. The carbenoid insertion reactions were found to occur at allylic C-H bond of cyclohexene, benzylic C-H bond of cumene, and ot C-H bond of THF. This is the first example of isolated iron carbene complex to undergo intermolecular carbenoid insertion to saturated C-H bonds. 

Bis(phosphoranimine) ligands, chromium complexes, 5, 359 Bis(pinacolato)diboranes activated alkene additions, 10, 731—732 for alkyl group functionalization, 10, 110 alkyne additions, 10, 728 allene additions, 10, 730 carbenoid additions, 10, 733 diazoalkane additions, 10, 733 imine additions, 10, 733 methylenecyclopropane additions, 10, 733 Bisporphyrins, in organometallic synthesis, 1, 71 Bis(pyrazol-l-yl)borane acetyl complexes, with iron, 6, 88 Bis(pyrazolyl)borates, in platinum(II) complexes, 8, 503 Bispyrazolyl-methane rhodium complex, preparation, 7, 185 Bis(pyrazolyl)methanes, in platinum(II) complexes, 8, 503 Bis(3-pyrazolyl)nickel complexes, preparation, 8, 80-81 Bis(2-pyridyl)amines... 

Further restrictions to the scope of the present article concern certain molecules which can in one or more of their canonical forms be represented as carbenes, e.g. carbon monoxide such stable molecules, which do not normally show carbenoid reactivity, will not be considered. Nor will there be any discussion of so-called transition metal-carbene complexes (see, for example, Fischer and Maasbol, 1964 Mills and Redhouse, 1968 Fischer and Riedel, 1968). Carbenes in these complexes appear to be analogous to carbon monoxide in transition-metal carbonyls. Carbenoid reactivity has been observed only in the case of certain iridium (Mango and Dvoretzky, 1966) and iron complexes (Jolly and Pettit, 1966), but detailed examination of the nature of the actual reactive intermediate, that is to say, whether the complexes react as such or first decompose to give free carbenes, has not yet been reported. A chromium-carbene complex has been suggested as a transient intermediate in the reduction of gfem-dihalides by chromium(II) sulphate because of structural effects on the reaction rate and because of the structure of the reaction products, particularly in the presence of unsaturated compounds (Castro and Kray, 1966). The subject of carbene-metal complexes reappears in Section IIIB. 

Although the dirhodium catalysts are the most commonly used today, other metals are still being explored for carbenoid C-H insertion [35], These include copper [36-45], silver [46, 47], iron [48-50], gold [40], and magnesium [51, 52], and most will be discussed in context, particularly in Sect. 3.1. 

Very high endo or cis selectivities have been reported for the reactions of olefins with phenyldiazomethane in the presence of a cationic iron complex11. a-Selenobenzyllithium compounds are carbenoids which transfer the phenylcarbene moiety to olefins with considerable trans preference12. 

Figure 18 shows the X-ray crystal structure of Sn(OEP)Fe(CO)4 The tin atom is pentacoordinated by the four nitrogen atoms ((Sn-N) = 2.187(3) A) and the iron atom (Sn-Fe = 2.492(1) A, Sn-Fe-C(52) = 179.1(1)°) is in the axial position. As expected, the tin-iron bond is short and represents the first example of a carbenoid metal-metal bond in metalloporphyrin chemistry. Furthermore, the large out-of-plane distance of the porphyrin central metal (A4N = 0.818(9) A) is in good agreement with a formal Sn oxidation state. The iron atom has a pseudo Csv symmetry and the average iron carbonyl distance is 1.754(5) A. 

In addition, carbenoid insertion of aliphatic C-H bond can be achieved in the presence of iron(III) porphyrin as catalyst. A benzylic C-H insertion product was obtained along with the arene C-H insertion product in a ratio of about 5 1 when p-xylene was used as substrate and solvent (eq 57). 

The reaction of carbenes or carbenoids with olefins to form cyclopropanes represents the most prominent example for [2+1] cycloaddition reactions. Formation of cyclopropanes from intermediate (carbene)iron complexes has been first observed by Pettit et al. by treating a methoxymethyliron complex with tetrafluoroboric acid. Later on reagents containing stoichiometric amounts of iron complexes have been developed to achieve cyclopropanation of olefins (cf. Section 2.2.4.b). Such reagents are (carbene)iron complexes or appropriate precursors forming these complexes in situ. The first catalyti-cally active iron-containing carbene transfer complex has been described by Hossein et al. in the form of [(Cp)Fe(CO)2(THF)] BF4. It allows the formation of cyclopropanes by reaction of ethyl diazoacetate with styrene or a-methylstyrene (Scheme 4-296). 

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