Tricarbonyliron complexes

H-Azepine derivatives form a diene complex with tricarbonyliron, leaving uncomplexed the third of the double bonds. If the 3-position is substituted, two different such complexes are possible, and are in equilibrium, as seen in the NMR spectrum. An ester group in the 1-position of the complex can be removed by hydrolysis, to give an NH compound which, in contrast to the free 1/f-azepine, is stable. The 1-position can then be derivatized in the manner usual for amines (Scheme 22). The same tricarbonyliron complex can, by virtue of the uncomplexed 2,3-double bond, serve as the dienophile with 1,2,4,5-tetrazines. The uncomplexed N-ethoxycarbonylazepine also adds the tetrazine, but to the 5,6-double... 

Tricarbonyliron complexes of 1,2-diazepines do not show the rapid isomerization found in their azepine counterparts (Scheme 22) the iron forms a diene complex with the C=C double bonds in the 4- and 6-positions. The chemistry of the 1,2-diazepine complexes is similar to that of the azepine complexes (Section 5.18.2.1) (81ACR348). 

Azepine-1-carboxylic acid, methyl ester, tricarbonyliron complex X-ray, 7, 494 <70JCS(B)1783) 4//-Azepine-2-carboxylic acid, 6,7-diphenyl-, methyl ester... 

IH-Azepine, 1-methoxy carbonyl-cycloaddition reactions, 7, 522 with nitrosobenzene, 7, 520 tricarbonyliron complex acylation, 7, 512-513 conformation, 7, 494 tricarbonylruthenium complex cycloaddition reactions, 7, 520 1 H-Azepine, l-methoxycarbonyl-6,7-dihydro-synthesis, 7, 507... 

Another short protocol for preparation of 3 was recently presented by Knolker and Reddy, who devised a short sequence involving a double iron-mediated arylamine cyclization as the key step (Scheme 19). Thus, the reaction of m-phenylenediamine (140) with the tricarbonyliron-complexed cyclohexadienyl cation 141 yielded the complex 142, which was eventually transformed into indolo-[2,3-()]carbazole (3) via cyclization and dehydrogenation (98TL4007 00T4733). 

The light-induced reaction of pentacarbonyliron with oxepin or 2,7-dimethyloxepin results in the formation of small quantities (3-5 %) of a tricarbonyliron complex of the seven-mem-bered heterocycle.253,251 The main products are benzene (o-xylene) and phenol (2,6-dimethyl-phenol). When 1-benzoxepin is treated with pentacarbonyliron, the tricarbonyliron complex is obtained in 22% yield.254... 

The greater lability of 1-benzothiepin 1-oxides, compared to the parent compounds, may lead to differences in chemical behavior. Thus, treatment of the tricarbonyliron complex of 1-benzo-thiepin 1-oxide (8, X = SO) with ammonium cerium(IV) nitrate in acetone at — 30 °C leads, with the loss of sulfur monoxide, to naphthalene. In contrast, the iron ligand can be removed selectively from the corresponding 1-benzothiepin by ammonium cerium(IV) nitrate.23 92 For the synthesis of 1-benzothiepin 1-oxide, see Section 2.1.4.1,... 

In accordance with FMO theory predictions,273 C2 —C4is the preferred modeofcycloaddition of tricarbonyliron and -ruthenium complexes of methyl l//-azepine-l-carboxylate with ethenetetracarbonitrile,222,274 hexafluoroacetone,222 and 2,2-bis(trifluoromethyl)ethene-l,l-dicarbonitrile 222 however, with ethenetetracarbonitrile, tricarbonyl[f/4-l-(ethoxycarbonyl)-1/f-azepine]iron(0) (1) yields a 1 6 mixture of the predicted C2 —C4 exo-adduct 2 and the C2 — C7 [6 + 2] 7i-cycloadduct 3,222 the latter heing formed by rearrangement of the former.274 Mixtures of the two adducts are also obtained with the tricarbonyliron complexes of 3-acetyl-l//-azepine and its l-(ethoxycarbonyl) derivative.274... 

In sharp contrast to the uncomplexed l//-azepine, which yields a C4 — C5 adduct, the tricarbonyliron complex of ethyl 1 W-azepine-l-carboxylate with dimethyl l,2,4,5-tetrazine-3,6-dicar-boxylate furnishes the C2 —C3 adduct 4 in excellent yield.273 Likewise, cycloaddition with 2,3,4,5-tetrachlorothiophene 1,1-dioxide yields adduct S.131... 

Nitrile oxides react with cycloheptatriene and its tricarbonyliron complex to give mixtures of adducts. In particular, for the complex, these adducts are 84, 85 (regioisomers at the uncomplexed double bond) and bisadduct 86. The regiose-lectivity of the reactions of cycloheptatriene is similar to that of the reactions of its tricarbonyliron derivative (246). 

The diastereoselective formation of dienol tricarbonyliron complexes on treating rf-2,4-pentadienal)Fe(CO)3 with functionalized zinc-copper reagents has been investigated (equation 49)66. Cyano-substituted complexes undergo intramolecular nucleophilic additions when treated with lithium diisopropylamide (LDA) as shown in equation 50. 

Acylation of terminal alkenes by ( j4-2,4-pentadienoyl chloride)Fe(CO)3 proceeds in high yield in the presence of Lewis acid catalysts67. As shown in equation 51, the reaction generally produces a mixture of ( j4-diene)tricarbonyliron complexes of the /J-chloroketone and /Ly-unsaturated ketone. 

More recently, an environmentally benign method using air as oxidant has been developed for the oxidative cyclization of arylamine-substituted tricarbonyl-iron-cyclohexadiene complexes to carbazoles (Scheme 19). Reaction of methyl 4-aminosalicylate 45 with the complex salt 6a affords the iron complex 46, which on oxidation in acidic medium by air provides the tricarbonyliron-complexed 4a,9a-dihydrocarbazole 47. Aromatization with concomitant demetalation by treatment of the crude product with p-chloranil leads to mukonidine 48 [88]. The spectral data of this compound are in agreement with those reported by Wu[22j. 

The procedure is also applicable to acetoxy-substituted arylamines, thus broadening the scope of the carbazole synthesis (Scheme 22). Reaction of the complex salt 6a with the 5-acetoxyarylamines 56 affords the tricarbonyliron-complexed 4a,9a-dihydrocarbazoles 57, which on demetalation and subsequent catalytic dehydrogenation [100] give the 4-acetoxycarbazoles 58. Removal of... 

Due to the extensively represented oxidative behaviour of the carbenium ions as hydride abstractor or one-electron oxidant [157], attempts were made to employ the carbocations as reagents. Recently the enantioselective outcome in a hydride transfer reaction was reported [158, 159]. The abstraction of the exo hydrogen atoms from the tricarbonyliron complex 57 resulted in a yield up to 70% and enantiose-lectivity of 53% (Scheme 62) [158]. 

The two key steps for the construction of the carbazole framework by the iron-mediated approach are, first, C-C bond formation by electrophilic aromatic substitution of the arylamine with the tricarbonyliron-complexed cyclohexadienyl cation and, second, C-N bond formation and aromatization by an oxidative cyclization. Application of this methodology provides murrayanine (9) and koenoline (8) in three steps and 15%, and in four steps and 14% overall yield, respectively, starting from the commercial nitroaryl derivative 601 (573,574) (Scheme 5.33). 

Electrophilic substitution of 3-methoxy-4-methylaniline (655) by the complex 663 leads to the molybdenum complex 664. Oxidative cyclization of complex 664 with concomitant aromatization using activated commercial manganese dioxide provides 2-methoxy-3-methylcarbazole (37) in 53% yield (560). In contrast, cyclization of the corresponding tricarbonyliron complex to 37 was achieved in a maximum yield of 11 % on a small scale using iodine in pyridine as the oxidizing agent (see Scheme 5.49). 

Electrophilic substitution at the arylamine 709 using the complex salt 602, provided the iron complex 725 quantitatively. Sequential, highly chemoselective oxidation of the iron complex 725 with two, differently activated, manganese dioxide reagents provided the tricarbonyliron-complexed 4b,8a-dihydrocarbazol-3-one (727) via the non-cyclized quinone imine 726. Demetalation of the tricarbonyliron-complexed 4b,8a-dihydrocarbazol-3-one (727), followed by selective O-methylation, provided hyellazole (245) (599,600) (Scheme 5.70). 

Electrophilic aromatic substitution of the arylamine 780a using the iron-complex salt 602 afforded the iron-complex 785. Oxidative cyclization of complex 785 in toluene at room temperature with very active manganese dioxide afforded carbazomycin A (260) in 25% yield, along with the tricarbonyliron-complexed 4b,8a-dihydro-3H-carbazol-3-one (786) (17% yield). The quinone imine 786 was also converted to carbazomycin A (260) by a sequence of demetalation and O-methylation (Scheme 5.86). The synthesis via the iron-mediated arylamine cyclization provides carbazomycin A (260) in two steps and 21% overall yield based on 602 (607-609) (Scheme 5.86). 

The total synthesis of carbazomycin D (263) was completed using the quinone imine cyclization route as described for the total synthesis of carbazomycin A (261) (see Scheme 5.86). Electrophilic substitution of the arylamine 780a by reaction with the complex salt 779 provided the iron complex 800. Using different grades of manganese dioxide, the oxidative cyclization of complex 800 was achieved in a two-step sequence to afford the tricarbonyliron complexes 801 (38%) and 802 (4%). By a subsequent proton-catalyzed isomerization, the 8-methoxy isomer 802 could be quantitatively transformed to the 6-methoxy isomer 801 due to the regio-directing effect of the 2-methoxy substituent of the intermediate cyclohexadienyl cation. Demetalation of complex 801 with trimethylamine N-oxide, followed by O-methylation of the intermediate 3-hydroxycarbazole derivative, provided carbazomycin D (263) (five steps and 23% overall yield based on 779) (611) (Scheme 5.91). 

Construction of the carbazole framework was achieved by slightly modifying the reaction conditions previously reported for the racemic synthesis (641,642). The reaction of the (R)-arylamine 928 with the iron complex salt 602 in air provided by concomitant oxidative cyclization the tricarbonyliron-complexed 4b,8a-dihydro-9H-carbazole (931). Demetalation of the complex 931, followed by aromatization and regioselective electrophilic bromination, afforded the 6-bromocarbazole 927, which represents a crucial precursor for the synthesis of the 6-substituted carbazole... 

Acylation of limonene at the disubstituted double bond is favoured by a factor of 2.3 over reaction at the trisubstituted double bond using acetyl hexachloroan-timonate. Mixed alkylcuprate alkylation of tricarbonylcyclohexadienyliron salts has been used to synthesize the a-phellandrene tricarbonyliron complex. Dichlorocarbene addition to limonene in the presence of 1,4-diazabicy-clo[2,2,2]octane is almost 100% stereoselective at the trisubstituted double bond (no yield given) (cf. Vol. 6, p. 31) in contrast to dibromocarbene addition to carvone (Vol. 7, p. 34), dichlorocarbene addition to the carveols is not regio-specific. ... 

The azepine-carbonyliron complex is also attacked at C-3 by the 2-oxyallyl cation (98) to give initially the charged intermediate (99), and finally either the furanoazepine (100) or the ketone (97 Me2CHCOCH2 in place of COR), as their tricarbonyliron complexes... 

The facility of 1H-azepines to form transition metal carbonyl complexes was realized soon after they were first synthesized. Variable temperature HNMR studies on the tricarbonyliron complex formed either by photolysis of 1-ethoxycarbonyl-l//-azepine with tricarbonyliron in THF, or by heating the azepine with nonacarbonyldiiron in hexane, demonstrated that it undergoes rapid reversible valence tautomerism and that there is considerable restricted rotation about the N—CO bond (B-69MI51600). The molecular geometry of the complex has been determined by X-ray analysis (see Section 5.16.2.2). 

Carbonylation of 2,3-homo-l//-azepines has been effected by means of their metal and carbonyl complexes and provides a useful route to a variety of isomeric azabicyclo-nonadienones. For example, the tricarbonyliron complex with carbon monoxide at 80 °C and 160 atm yields the 9-oxo-2-azabicyclo[3.3.l]nona-3,7-diene (167) (57%) or the 9-oxo-2-azabicyclo[3.2.2]nona-3,6-diene (168) (60%) depending on the exo or endo configuration of the tricarbonyliron complex. A third isomer, namely ethyl 7-oxo-9-azabicyclo[4.2.1]nona-2,4-diene-9-carboxylate (169), is formed on heating (125 °C) the azepine with carbon monoxide under pressure in the presence of the rhodium carbonyl complex [Rh(CO)Cl2] (78CB3927). 

Acylation. Some of the problems inherent in the extension of the iV-imide route to the preparation of IH- 1,2-diazepines with a variety of N-substituents (80) can be avoided by using a procedure involving the alkylation or acylation of the tricarbonyliron complex (79) of the unstable parent lH-diazepine (76CC844). 

As with many other unsaturated seven-membered heterocyclic systems 1,2-diazepines form organometallic complexes of various types. The stable tricarbonyliron complexes of Iff-1,2-diazepines, e.g. (79), have been much studied and found useful in synthesis (81ACR348), and it has been recently shown that 317- 1,2-diazepines (5) and analogous benzodiazepines form complexes of a quite different type (105) involving the azo group only (81TL353). 

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