Hexadiene complexes with iron

Hexadiene complexes with gold, 12 348 with iron, 12 263, 264 with platinum, 12 319 with rhodium, 12 297-299 with silver, 12 340 Hexafluorides, structures, 27 106-108 Hexafluoroacetone, 30 223-312, 44 317-318 metal complexes, 30 279-298 of cobalt, 30 286-287 of gold, 30 295... 

For a few combinations of less reactive dienes and dienophiles, transition metal catalyzed variants of the Diels Alder reaction have been developed. An example is the cycloaddition of an unpolar diene and an unactivated alkyne however, except when the reaction is catalyzed with iron, nickel, cobalt, or rho-dium(I) complexes, the temperature required often causes competing decomposition, even for the intramolecular version. [2] Wilkinson s catalyst [3] - tris(triphenylphosphane)rho-dium(I) chloride - frequently used for hydrogenations and for decarbonylations, permits the cyclization of 4 to the annelated cyclo-hexadiene 5 in excellent yield in only 15 minutes at 55 °C in trifluoroethanol as solvent (Scheme 2). [2c]... 

What appear to be perfluorodiene-Fe(CO)3 complexes have been obtained from the reaction of periluorodienes with iron carbonyls. Thus, as indicated in Eq. 6, the complex (LXVII) has been obtained from the reaction of either perfluoro-l,3-cyclohexadiene 81) or perfluoro-l,4-cyclo-hexadiene (55). The latter reaction involves a fluorine shift reminiscent of the hydrogen shift observed in the analogous reaction with 1,4-cyclo-hexadiene 43). 

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

The tricarbonyliron-coordinated cyclohexadienylium salts are readily available on a large scale by azadiene-catalyzed complexation of the corresponding cyclo-hexadienes with pentacarbonyliron [23] and subsequent hydride abstraction using trityl tetrafluoroborate [24]. Alkylation of methyl lithioacetate with the iron complex... 

Diastereoselective allylation of optically pure sulfinyl dienal complexes using tributyl allyltin can be obtained (Scheme 138). 2,4-Hexadien-1,6-dial iron tricarbonyl complex (88) undergoes nucleophilic addition reactions with diaUcylzincs in the presence of a catalytic amount of an optically active amino alcohol (Scheme 139). Very high enantio-and diastereoselectivity is observed. Related reactions of (88) with chiral allyl boronic esters give allylated alcohols in very high enantiomeric excess. 

The diene ligands transform to the cisoid form because of the thermodynamic stability of the complex. Because the bond distances of Cj—and C2—Cy in the butadiene ligand are 1.45 and 1.46 A, respectively, the double bond character between C2—C3 and the bond order alternation is still insignificant [43]. This may be due to the strong K-acidity of three carbonyl ligands which reduces n-back donation from iron to the butadiene ligand. (Cyclohexadiene)tricarbonyliron complexes are also noteworthy. Fe(l,3-cyclo-hexadiene)(CO)3 was prepared by the reaction of 1,3-cyclohexadiene with Fe(CO)5 [44] (eq (13)) and more stable Fe(l,4-cyclohexadiene)(CO)3 was also prepared [45]. 

A similar insertion of ethylene into a C—M bond of a w-butenyl complex seems to occur in the industrial synthesis of hexadienes from butadiene and ethylene, at least with rhodium and nickel compounds. These catalysts give tra f-l,4-hexadiene as the initial product 178). Cobalt and iron catalysts give the cis isomer 179,180), probably by a different mechanism. 

There exists a limitation to this direct method of synthesis in the case of acyclic dienes having m-alkyl substituents. Although these compounds may give rise to diene-Fe(CO)3 complexes, the ligand in the complex invariably involves a rearranged diene and, as of now, no m-substituted acyclic diene appears to have been prepared by the direct reaction. For example, 2,5-di-methyl-2,4-hexadiene (XVII) reacts with Fe(CO)5 to give trans-2,S-6x-methyl-l,3-hexadiene-iron tricarbonyl (XVIII) 41). Likewise, 4-methyl-1,3-pentadiene (XIX) gives ra r-2-methy 1-1,3-pentadiene-iron tricarbonyl... 

In 1958 Dauben and Honnen prepared the fluoroborate salt of cyclo-heptatrienyl-molybdenum tricarbonyl through reaction of cycloheptatriene-molybdenum tricarbonyl with triphenylmethyl fluoroborate (88), Fischer and Fischer (89) have shown that an analogous reaction between cyclo-hexadiene-iron tricarbonyl (LXX) and the triphenylmethyl cation leads to the formation of a salt of the cyclohexadienyliron tricarbonyl cation (LXXI), Eq. 9. In the cation (LXXI), and in the analogous complexes described below, it is assumed that there are five ip -type carbon atoms simultaneously bonded to the iron atom. 

Tricarbonyl(Ti -diene)iron complexes bearing a secondary hydroxy function in a-position are accessible in good diastereoselectivities and enantioselectivities via organocatalyzed enantioselective alkylation of the prochiral tricarbonyl(T -2,4-hexadien-1,6-dial)iron with dialkylzinc reagents (Scheme 4-135). ... 

The trans isomer of 1,4-hexadiene is one of the required monomers for EPDM rubber. Although iron-, cobalt-, and nickel-based Ziegler-type catalysts can codimerize butadiene and ethylene, the selectivity to the desired trans isomer is low. A soluble rhodium complex can, however, catalyze the dimerization with high selectivity to the trans isomer. 

In the absence of a carbonyl compound, the electroreduction leads to the formation of hexadiene, thus implying the formation of a Jt-allyl iron complex formed by addition of Fe on allyl acetate. Certainly, this is the Jt-allyl iron complex that reacts with carbonyl compounds leading to homoallylic alcohols. 

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