Iron nitrene

Keywords Carbene Catalysis Complex Iron Nitrene... 

Iron-nitrene/imido complexes are proposed to be the reaction intermediates in nitrogen group transfer reactions. The nitrene group can be transferred to organic substrates. Aziridination and amination are the well-known nitrogen atom/group... 

In analogy with the corresponding known iron-oxo complexes, an iron nitrene complex (L Fe=NR) has been proposed as a reactive intermediate in these processes for both catalytic systems described above [176]. 

Iron Catalysts Direct amidation of C—H bonds presents an efficient method for C—N bond formation. A combination of simple air-stable FeCl2 and NBS has been successfully used in the amidation of benzylic sp3 C—H bonds (Equation 11.16) [38], The reactions are insensitive to atmospheric moisture and oxygen. Neither a dried solvent nor an inert atmosphere is required. An iron-nitrene intermediate has been proposed [39]. The carbene insertion of benzylic C—H bonds provides the final products. 

Mansuy, D., P. Battioni, and J.P. Mahy (1982). Isolation of an iron-nitrene complex from the dioxygen and iron porphyrin dependent oxidation of a hydrazine. J. Am. Chem. Soc. 104,4487-4489. 

In 2014, Maldivi and Latour used DESI-MS, optical, and Mossbauer experiments to identify an ArlNTs adduct of the di-iron species shown in Scheme 26 [90]. This compound loses Arl to provide a fleeting intermediate iron nitrene complex that could only be detected by the sensitive DESI-MS technique. This intermediate rapidly reacts with thioethers or hydrogen atom donor (2,4-di-tert-butylphenol). 

Scheme 26 Formation and reactivity of iron nitrene species...

A variety of sulfonamides and carboxamides have been introduced at the benzylic position using this methodology in reasonably good yields. NBS plays the role of an oxidant as well as a free radical initiator. The reaction is proposed to involve the generation of an iV-bromocarboxamide or iV-bromosulfonamide as the initial intermediate that then reacts with iron salt eventually forming the reactive iron-nitrene complex that can aminate the benzylic position. 

The porphyrin ligand can support oxidation states of iron other than II and III. [Fe(I)Por] complexes are obtained by electrochemical or chemical reduction of iron(II) or iron(III) porphyrins. The anionic complexes react with alkyl hahdes to afford alkyl—iron (III) porphyrin complexes. Iron(IV) porphyrins are formally present in the carbene, RR C—Fe(IV)Por p.-carbido, PorFe(IV)—Fe(IV)Por nitrene, RN—Fe(IV)Por and p.-nittido, PorFe(IV)... 

An early synthesis of pyrido[3,4-6]quinoxalines involved cyclization by strong heating of o-aminoanilinopyridinamine derivatives, e.g. (418) to give (419) (49JCS2540). In a related reaction, o-nitroanilinopyridines (420) were cyclized to pyrido-[2,3-6]- or -[3,4-6]-quinoxa-lines (421) by reduction with iron(II) oxalate, probably via a nitrene intermediate (74JCS(P1)1965). 

The synthesis of metal-eoordinated 1-azirines and the reaetions of azirines indueed by metals have opened a new area in the ehemistry of this small ring heteroeyele. Many of the reaetions eneountered bear resemblanee to previously diseussed thermally and photo-ehemieally indueed reaetions of 1-azirines. The reaetion of a series of diiron enneaearbonyls in benzene results in eoupling and insertion to give diimine eomplexes and ureadiiron eomplexes as well as pyrroles and ketones (76CC191). A meehanism for the formation of these produets whieh involves initial 1,3-bond eleavage and generation of a nitrene-iron earbonyl eomplex as an intermediate was proposed. 

Aziridines are versatile intermediates in organic synthesis and commonly found in bioactive molecules. The transition metal-catalyzed nitrene transfer to alkenes is an attractive method for the synthesis of aziridines [7]. In 1984, Mansuy and coworkers reported the first example of an iron-catalyzed alkene aziridination in which iron porphyrin [Fe(TTP)Cl] was used as catalyst and PhINTs was used as nitrene source [30]. Subsequently, the same authors demonstrated that [Fe(TDCPP) (CIO4)] is a more efficient and selective catalyst than [Fe(TTP)Cl] (Scheme 20). 

Modification of [Fe(TTP)Cl] due to intramolecular nitrene insertion to iron-nitrogen bond... 

A more practical, atom-economic and environmentally benign aziridination protocol is the use of chloramine-T or bromamine-T as nitrene source, which leads to NaCl or NaBr as the sole reaction by-product. In 2001, Gross reported an iron corrole catalyzed aziridination of styrenes with chloramine-T [83]. With iron corrole as catalyst, the aziridination can be performed rmder air atmosphere conditions, affording aziridines in moderate product yields (48-60%). In 2004, Zhang described an aziridination with bromamine-T as nitrene source and [Fe(TTP)Cl] as catalyst [84]. This catalytic system is effective for a variety of alkenes, including aromatic, aliphatic, cyclic, and acyclic alkenes, as well as cx,p-unsaturated esters (Scheme 28). Moderate to low stereoselectivities for 1,2-disubstituted alkenes were observed indicating the involvement of radical intermediate. 

In 1982, Breslow and coworkers reported the first example of iron-catalyzed nitrene C-H bond insertion [29]. They used [Fe(TTP)] as catalyst and PhINTs as nitrene precursor to achieve C-H bond amination of cyclohexane. However, the product yield was low (around 10%). Subsequently, the same authors found that iminoio-dane 7 derived from 2,5-diisopropylbenzenesuIfonamide underwent intramolecular C-H amination efficiently with [Fe(TPP)Cl] as catalyst at room temperature, giving the insertion product in 77% yield (Scheme 29) [85]. 

The synthesis of sulfoximides and sulfimides has attracted considerable attention in recent years due to the potential utility of these compounds as efficient auxiliaries and chiral ligands in asymmetric synthesis (reviews [86-88]). Transition metal-catalyzed nitrene transfer to sulfoxides and sulfides is an efficient and straightforward way to synthesize sulfoximides and sulfimides, respectively. Bach and coworkers reported the first iron-catalyzed imination of sulfur compounds with FeCl2 as catalyst and B0CN3 as nitrene source. Various sulfoxides and sulfides were... 

Nitrene addition to alkenes can be aided by the nse of a transition metal, such as copper, rhodium, ruthenium, iron, cobalt, etc. NHC-Cu catalysts have been used in nitrene addition. For example [Cu(DBM)(IPr)] 147 (DBM = dibenzoyl-methane) was successfully employed in the aziridination of aliphatic alkenes 144 in presence of trichloroethylsulfamate ester 145 and iodosobenzene 146 (Scheme 5.38) [43]. 

Copper catalyzes the decomposition of sulphonyl azides in benzene very slowly. When methanesulphonyl azide was boiled under reflux in benzene solution in the presence of an excess of freshly reduced copper powder, some decomposition occurred to give methanesulphonamide and azide was recovered 78>. Transition metal complexes have been found to exert a marked effect upon the yields of products and isomer ratios formed in the thermal decomposition of methanesulphonyl azide in methyl benzoate and in benzotrifluoride 36>. These results will be discussed in detail in the section on the properties of sulphonyl nitrenes and singlet and triplet behaviour. A sulphonyl nitrene-iron complex has recently been isolated 37> and more on this species will be reported soon. 

In 1987, Nitta reported the formation of an unexpected vinylketene complex from the reaction of an azido-substituted cyclopropene with diiron nonacarbonyl.104 They had previously investigated the chemical behavior of the complexed nitrene intermediates that result from the reaction of organic azides and iron carbonyls113 and were interested in replicating the thermal isomerization of 3-azido-l,2,3-triphenylcyclopropene (163) into 4,5,6-triphenyl-l,2,3-triazine using a metal carbonyl-promoted re-... 

A variety of pyrrole ring closure reactions are conveniently formulated as proceeding via nitrene intermediates, although it is doubtful whether a free nitrene is involved. Pyrolysis of o-nitrobiphenyls with iron(II) oxalate (61T(16)80), or reduction under milder conditions with triethyl phosphite (65JCS4831) or tris(trimethylsilyl) phosphite (79TL375), leads to the carbazole, as does the pyrolysis or photolysis of 2-azidobiphenyls (Scheme 3) (75JA6193). 

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