Tricarbonyl iron complexes isomerization

Tricarbonyl)iron complexes of 1,2-diazepines do not show the rapid isomerization found in their azepine counterparts (Scheme 41) 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 (CHEC 5.18.2.1) (81ACR348). 

Azepine derivatives form a diene complex with (tricarbonyl)iron, leaving the third of the double bonds uncomplexed. If the 3-position is substituted, two different such complexes are possible and are in equilibrium, as seen in the 11 NMR spectrum. An ester group in the 1-position of the complex can be removed by hydrolysis to give an NH compound that, in contrast to the free 1 //-azcpinc, is stable (Scheme 82). The 1-position can then be derivatized in the manner usual for amines. The same (tricarbonyl)iron complex can, by virtue of the uncomplexed 2,3-double bond, serve as a dienophile with 1,2,4,5-tetrazines. The uncomplexed N-ethoxycarbonylazepine also adds the tetrazine, but to the 5,6-double bond. Thus, two isomeric adducts can be synthesized by using or not using the complex (Scheme 83). 

Diene)tricarbonyliron complexes have found use as synthons for the preparation of functionalized dienes. Substituted 4-vinylcyclohexene derivatives are isomerized by pen-tacarbonyliron into a mixture of conjugated cyclohexadiene tricarbonyl iron complexes . When the 4-vinyl cyclohexene 90 was refluxed with 1.2 equivalents of fclCOjs in di-n-butyl ether, a 3 1 mixture of cyclohexadiene isomers 91 and 92 was acquired in 75% overall yield (equation 48). 

Iron complex-catalyzed isomerization of allylic alcohols was found during the studies on the hydrolysis of 7t-allyliron tricarbonyl salts. The isomerization of allyl alcohol to propionaldehyde was observed on heating in the presence of Fe(CO)s (Equation (9)).30... 

Iron carbonyls have been used in stoichiometric and catalytic amounts for a variety of transformations in organic synthesis. For example, the isomerization of 1,4-dienes to 1,3-dienes by formation of tricarbonyl(ri4-l,3-diene)iron complexes and subsequent oxidative demetallation has been applied to the synthesis of 12-prostaglandin PGC2 [10], The photochemically induced double bond isomerization of allyl alcohols to aldehydes [11] and allylamines to enamines [12,13] can be carried out with catalytic amounts of iron carbonyls (see Section 1.4.3). 

Chiral tricarbonyl(butadienyl)iron complexes are easily accessible by resolution and show excellent diastereofacial selectivities with a variety of reagents55. The tricarbonyl(trienyl)iron complex 2 and methyl diazoacetate (copper bronze catalysis) give only two cis 7/m -isomeric cyclopropanes 5< These can be separated by column chromatography, and each diastereomer transformed into methyl traits- or 7.r-2-formyl-3.3-dimcthylcyclopropanecarboxylate (hemica-ronic aldehyde) by destructive cleavage of the diene complex auxiliary. The enantiomeric excess in these compounds is above 90%. 

A third pathway leads via the quinone imine intermediates 38 to 3-hydro-xycarbazoles 41 (mode C in Scheme 12) [97, 98, 108, 109]. Oxidation of the complexes 36 with manganese dioxide afforded the quinone imines 38, which on treatment with very active manganese dioxide undergo oxidative cyclization to the tricarbonyl(ri" -4b,8a-dihydrocarbazol-3-one)iron complexes 39. Demetalation of 39 with trimethylamine iV-oxide and subsequent aromatization lead to the 3-hydro-xycarbazoles 41. The isomerization providing the aromatic carbazole system is a... 

The tetra-cA-cycIononatetracne 241 is unstable and easily rearranges at 23 °C (t /2 50 min) to the isomeric d.v-8,9-dihydroindcne 242 (equation 77)89. It is interesting, however, that the iron(III) tricarbonyl complex of tetraene 241 is stable for many days at room temperature and isomerizes to the Fe-complex of 242 only upon heating in octane at 101 °C89. The principle of stabilization of the reactive multiple bonds with metal carbonyl complexes is well-known in modem organic synthesis (e.g. see the acylation of enynes90). 

Dihydromesitylene likewise gives a 1,3 complex (34) and 1,4-cyclohexadiene gives 1,3-cyclohexadieneiron tricarbonyl. 1,5-Cyclo-octadiene on treatment with catalytic quantities of Fe(CO)g gives 1,3-cyclooctadiene (35), as the iron tricarbonyl complex is probably not very stable and is continuously displaced by fresh 1,5-diene until isomerization is complete. 

Dienes form very stable complexes with a variety of metal caibonyls, particularly Fe(CO)s, and the neutral V-diene metal carbonyl complexes are quite resistant to normal reactions of dienes (e.g. hydrogenation, Diels-Alder). However, they are subject to nucleophilic attack by a variety of nonstabilized carbanions. Treatment of -cyclohexadiene iron tricarbonyl with nonstabilized carbanions, followed by protonolysis of the resulting complex, produced isomeric mixtures of alkylated cyclohexenes (Scheme 15).24 With acyclic dienes, this alkylation was shown to be reversible, with kinetic alkylation occurring at an internal position of the complexed dienes but rearranging to the terminal position under thermodynamic conditions (Scheme 16).2S By trapping the kinetic product with an electrophile, overall carbo-... 

A similar type of reaction has been observed in the reactions of iron-caibene complexes with 1,3-dienes. In this case the direction of reductive elimination in the metal hydride intermediate corresponding to (193) is constrained to that which generates a conjugated 1,3-diene however, two isomeric products are also obtained from this reaction which are epimers about the face of the diene to which the iron tricarbonyl group is attached. This reaction produces highly functionalized 1,3-dienyl complexes of iron in high yield under relatively mild conditions and will likely play a role in the development of the... 

Cyclopropene provides isomeric tricycles upon reaction with cyclobutadiene generated from its iron tricarbonyl complex (equation 54) but this thermal reaction is more likely to... 

The primary process is the formation of (5), which can be isolated but very easily decomposes to (6) and 1.5-cyclooctadiene. Absorption of another light quantum converts (5) - -(7), in a third photochemical step (7) can be isomerized to 1.3-cyclooctadiene iron tricarbonyl (8), which can be prepared directly from 1.3-cyclooctadiene, too. Reaction (7)- - (8) comprises an example of the photochemical rearrangement of a photochemically formed metal carbonyl complex. Such reactions occur quite frequently and can be of preparative value (see section F. 2,3). In (6) the diene is acting as a bridging ligand, other cases of this type of coordination are found in 164,228). 

The photochemical isomerization of 1.5-cyclooctadiene iron tricarbonyl (7) to 1.3-cyclooctadiene iron tricarbonyl (8) mentioned in section E 3 is an example of ligand isomerization by formal shift of C=C bonds 284>. No deuterium is incorporated in the complex if the isomerization is carried out in a D 2-atmosphere. It appears not impossible, that the rearrangement occurs by a skeletal reorganization involving cleavage of C—C bonds and not hydrogen shift 284>. 

The reaction of 1,2-diphenylcyclobutadiene (generated in situ by oxidation of its iron tricarbonyl complex) with /j-benzoquinone yields adduct 1-A as the exclusive product. A completely analogous structure is obtained using maleimide as the dienophile. However, with the more reactive dienophiles tetra-cyanoethylene and dicyanomalemide, two isomeric adducts of type 1-B and 1-C are found in a 1 7 ratio in each case. Discuss these results and explain how they relate to the issue of a square versus a rectangular structure for the cyclobutadiene ring. 

The seven-membered cyclic ketone, tropone, C7H7O, might be expected to form complexes like those of cycloheptatriene. Only the iron carbonyl complexes have been studied in any detail. Troponeiron tricarbonyl is one of the products of the reaction between acetylene and Fe2(CO)9 131a) this reaction, and the properties of the complex, have been summarized by Pettit and Emerson (118). Reaction betvreen Fc3(CO)i2 and phenylacetylene gives two isomeric complexes of formula (2,4,6-triphenyltropone)Fe(CO)3, which are assigned structures (VIII) and (IX) IS). 

The initial stage involves the formation of a highly reactive anion Fe(CO)4 , which interacts with isolated double bonds, followed by double bond isomerization and the formation of n-complexes with the iron tricarbonyl residues. The final product is composed of the ri" -(butadienyl)irontricarbonyl units with both trans-trans- and cis-trans-tetramethylene groups. Since the iron tricarbonyl complexes with two noncon-jugated double bonds are unstable, no complexes between two polymer chains are formed. The reaction of iron carbonyls with low-molecular-weight nonconjugated dienes is accompanied by the double bond migration. 

The 3c-nmR spectrum of the thermal reaction product between 1,7-octadiene and FeCCO) (Fig. M) is complex and consists of 24 principal peaks which can only be accounted for in terms of the formation of a minimum of the six isomeric octadienyl iron tricarbonyl compounds shown in Fig. 2. While we have not unequivocally assigned each of these peaks, the eleven peaks at high field (14-40 ppm) are due to -CH and -CH2 carbons. The six peaks at (53-66 ppm) are mainly derived from the 1,4 carbons of the diene and the six peaks at 81-93 Ppm may be attributed to the 2,3 carbons of the diene. The low field doublet at 212 and 213 ppm derives from the carbonyl carbons and is indicative of two distinct carbonyl environments. 

Phenylacetylene and Fe3(CO)i2 in inert hydrocarbon solvents at 80°C give, as the main products isomeric triphenyltropone iron tricarbonyl complexes. The triphenyltropone ligand can be derived from three acetylene molecules and one carbonyl group (cf. the formation of cyclopenta-dienone complexes p 229). The X-ray structure of one isomer 9.6, shows that the tropone ring is attached to the iron atom by a diene system, the third double bond being bent out of the plane of the diene carbon atoms, away from the iron (cf. structure of GgH8Fe(CO)3 p 166). 

---