1,3-dienes iron tricarbonyl complexes

Chiral diene—iron tricarbonyl complexes were acylated using aluminum chloride to give acylated diene—iron complexes with high enantiomeric purity (>96% ee). For example, /ra/ j -piperjdene—iron tricarbonyl reacted with acyl haUdes under Friedel-Crafts conditions to give l-acyl-l,3-pentadiene—iron tricarbonyl complex without any racemization. These complexes can be converted to a variety of enantiomericaHy pure tertiary alcohols (180). 

The double bond transposition could also be achieved by the conversion of an intermediate for PGA2 synthesis into a 1,3-diene iron tricarbonyl complex from which PGC2 was synthesized in four steps. The Fe(CO)3 diene complex which survived the Wittig reaction was cleanly removed by Collins reagent in the subsequent step (Ref. 10). 

An alternative preparation giving access to anfi-hydroxyvinyl compounds uses diastereo-selective nucleophilic addition to a chiral diene-iron tricarbonyl complex, followed by iodocarbamation and ring opening (Scheme 20)J75l... 

D. Diene-Iron Tricarbonyl Complexes Dibromocarbene Insertion. 206... 

Diene-iron tricarbonyl complexes undergo C—H bond insertion reac-... 

Investigations into the synthesis and reactivity of -diene iron tricarbonyl complexes are among the most active areas of research in iron organometalhc chemistry. A nnmber of excellent reviews are available for the reader seeking greater detail than can be provided here. ... 

The complexation of chiral dienes leads to the formation of diastereomeric j -diene iron tricarbonyl complexes. An early example was actually carried out for the protection of the diene unit found in the steroid B-ring of ergosteryl acetate (equation 53) neither the stereochemical assignment nor the diastereomer ratio were estabhshed. Similarly, the tricarbonyl iron complex of calciferol was also prepared. ... 

The use of cyclic and acyclic diene iron tricarbonyl complexes (see Diene Complexes) in organic synthesis of... 

Diene iron tricarbonyl complexes are prepared by thermal or photochemical reaction of conjugated dienes with iron pen-tacarbonyl in the presence of TMANO, triiron dodecacarbonyl, ()]" -benzylidenacetone)iron tricarbonyl, diiron nonacarbonyl, or diiron nonacarbonyl absorbed on silica gel in the absence of solvent. The latter method is particnlarly usefiil for the preparation of complexes from polar electron-rich dienes and heterodienes. A reductive complexation of cycloheptatrienes using iron tricarbonyl and sodium borohydride to give cyclo-heptadiene iron tricarbonyl has been developed (Scheme 126). 

Unsymmetrically substituted diene iron tricarbonyl complexes are formed as racemic mixtures of compounds having planar chirality. Optically pure diene iron tricarbonyl complexes can been prepared by classical chromatographic separation of diastereomeric mixtures of a-methylbenzylamine and ephedrine salts, and of semioxa-mazones and oxazohdines derivatives. Direct asyimnetric complexation to cychc dienes can be achieved 73% ee using iron pentacarbonyl in the presence of a chiral l-aza-1,3-butadiene. 

Diene complexes containing alkene or diene substituents undergo Diels Alder reactions in good yields. Hetero-Diels Alder reactions have also been reported. Chirahty transfer is observed upon reaction of chiral diene iron tricarbonyl complexes. Reaction of the chiral complex (101) with cyclopentadiene in the presence of a Lewis acid give (102) with a relatively high chirahty transfer from the metal complex (Scheme 162). 

The chemistry of diene iron tricarbonyl complexes described above has been in a number of total syntheses. An iterative stereospecific 1,3-migration of the iron tricarbonyl moiety was used to prepare compounds with multiple chiral centers. An example of one iteration can be seen in Scheme 168. Ester hydrolysis of (105) and protection of the resulting alcohol gives (106). Reduction of the nitrile with DIBAL-H followed by olefination furnished (107). Treatment of (107) with a base resulted in the migration of iron toward the nitrile to give (108). The uncomplexed double bond can... 

Hydride abstraction from dienyl tricarbonyl iron complexes furnishes cationic dienyl tricarbonyl iron complexes. For example, reaction of the diene-iron tricarbonyl complex (115) with triphenyhnethyl hexafluorophosphate followed by trimethylsilyl cyanide furnished with excellent regio- and stereoselectivity a new diene iron tricarbonyl complex (116) (Scheme 170). Excellent regio- and stereoselectivity is seen upon reaction of the cationic complex (116) with trimethylsilyl cyanide (TMS-CN) (Scheme 170). Reduction of the nitrile affords a spirocyclic lactam complex. Intramolecular cyclization of in situ formed enols furnishes spirocyclic compounds again with excellent stereoconfrol (Scheme 171). An interesting example of hydride transfer from a cyclohexadiene ring to a pendant aldehyde followed by nucleophilic addition is seen in Scheme 172. 

Cyclic 1,3-diene iron tricarbonyl complexes eliminate hydrogen on electron impact to give predominant odd electron ions with iron bonded to an aromatic system 57) These same molecules eliminate hydrogen and iron on photolysis to give aromatic hydrocarbon products. 

Cyclobutadiene-iron tricarbonyl is a pale, yellow solid, m.p. 26°C., which exhibits a single, sharp NMR absorption at r 6.09. In common with other diene-iron tricarbonyl complexes, the material displays appreciable thermal stability, as well as a pronounced resistance to further replacement of the CO ligands several hours treatment with triphenylphosphine in refluxing toluene leaves the complex unaffected. 

Substituted methylenecyclopropanes react with diiron nonacarbonyl to give trimethylenemethane and 1,3-diene iron tricarbonyl complexes . Theoretical analysis of the former... 

The preparation of some (polyfluorocyclohepta-l,3-diene)iron tricarbonyl complexes [104] is reported, and parameters are given in Table IV. (55) The appearance of the geminal fluorines at C5 and C 7 as an AB quartet when the adjacent carbon (C4 or Cl) carries a fluorine, but as a singlet when the vinylic neighbour is hydrogen is noteworthy. Since a crystallographic study shows that the ring is puckered across the line C1-C4, (56) this must be due to accidental equivalence. 

A simple synthesis of some 1,1-dialkyIcyclopentadienes (234) has used the 1,3-dipolar addition of diazo-alkanes to 3,4-dichlorocyclobutene. The resulting pyrazo-lines were converted quantitatively into the bicyclo[2,l,0]pentanes (235) by photo-chemically induced loss of nitrogen sodium-naphthalene dechlorination gave the cyclopentadienes, by way of the bicyclo[2,l,0]pentenes. Dehalogenation of (235) with Fe2(CO)g gave the diene-iron tricarbonyl complexes (236) directly. 

Gycloheptatrienes in protic solvents are reported to react with Fe(GO)s and a catalytic amount of NaBH4 to produce ( 7" -l,3-diene)iron tricarbonyl complexes. Pearson and Ghidu have demonstrated that stereospecific cyclization of iron tricarbonyl diene complexes with pendant alkenes and arenes proceeds via protonation of a double bond vicinal to the iron tricarbonyl diene moiety. This methodology has been used to diastereoselectively produce polycycles from iron tricarbonyl-stabilized pentadienyl carbocations. " ... 

Reactions of azadienes and dienamides, carrying chiral substituents, are repotted295 to react with [Fe2(CO)9] to afford (T -diene) iron tricarbonyl complexes with good to excellent diasteroselectivity, depending on the nature of the chiral auxiliary employed. Complete selectivity was observed for the dienamide prepared from (5)-2-(diphenylhydroxymethyl)pyrrolidine and sorbic acid, leading to a chiral non-racemic diene complex, the stereochemistry of which was determined by single crystal X-ray methods. 

III. Preparation and Properties of Diene-Iron Tricarbonyl Complexes... 

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