Tetraenes, with iron

The three reported preparations " of tricarbonyI(cyclo-octatetraene)iron involve the interaction of cycloocta-tetraene with iron pentacarbonyl. Although the yields arc good, considerable amounts of CgHg[Fe(CO)3]2 and CgHsFe2(CO)7 are often obtained as well. The reaction betw cen cyclooctatetraene and triiron dodecacarbonyl takes place more rapidly and gives a product of better purity. 

Thus they treated the cisltrans mixture obtained by isomerization with iron pentacarbonyl and after decomposition of the complex with ferric chloride obtained the desired all-trans-tetraene ester (4) in 51% yield. The remaining steps in the synthesis involved lithium aluminum hydride reduction (80% yield) and Mn02 oxidation (52% yield). 

Because of the success with iron cyclam, our search for other non-heme iron complexes involved a survey of cyclam like ligands including HMCD (5,7,7,12,14,14-hexamethyl-l,4,8,ll-tetraazocyclodec-4,l 1-diene), TIM (2,4,9,10-tetramethyl-l,4,8,ll-tetraazocyclodec-l,3,8,ll-tetraene), TDO (1,4,8,ll-tetraazocyclodecane-5,7-dione), etc. Under the same reaction conditions as for iron cyclam, the iron complexes of these other ligands also proved to be efficient catalysts for olefin epoxidation with PhIO and MCPBA, but not for H2O2. These results further support the notion that the three oxidants behave differently in their reactions with these non-heme iron complexes. 

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). 

Synthesis of the parent homotropone 34 was achieved starting from tricarbonyl(cycloocta-tetraene)iron complex 32 via protonation and formation of the bicyclo[5.1.0]octadienylium cation complex 33. Nucleophilic addition of hydroxide and oxidation was followed by oxidative decomplexation with cerium(IV). ... 

In 2011, Grubbs and coworkers reported the reaction of CAAC C with bis-(cyclooctatetraene)iron, Fe(cot)2 [301]. Earlier work had shown that NHCs react with Fe(cot)2 to give tri- and tetrametallic iron clusters [302]. However, in the case of CAAC C, slow [4+1] cycloaddition with the iron-coordinated cycloocta-tetraene gave the product shown in Figure 15.52 over two days. The differing reactivity when compared to NHCs is attributed to the enhanced nucleophilicity and electrophilicity of CAACs. 

Various reports on the synthesis and reactivity of iron complexes of trienes and tetraenes have been reported in the past 10 years. Reactivity studies of various triene and tetraene complexes predominate in the recent literature. (77-Triene)Fe(GO)3 complexes have been reported to undergo osmylation reactions with OSO4 to yield the corresponding glycols (Scheme 29). Subsequent reaction with NaI04 in THF/H20/23°C affords (dienal)Fe(GO)3 complexes. 

A sequence of nucleophilic additions can be performed in cationic (ri -cycloocta-tetraene)(ti -cyclopentadienyl)iron complexes. The first nucleophile adds anti to the Cp-iron fragment to provide an (Tl -cyclooctatrienyl)-Cp-iron complex. This species can be protonated forming a cationic (T -cyclooctatriene)-Cp-iron complex that in turn can be reacted with a second nucleophile. After protonation and demetalation, cis-5,1-disubstituted cycloocta-1,3-dienes are obtained (Scheme 4-182). 

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