Cyclohexadiene iron tricarbonyl complex

The dimerization of thioformylketene was investigated by B3LYP and G3MP2B3 methods. The 4 + 4-pathway has the lowest energy barrier and calculations suggest that the reaction is pseudopericyclic.231 The stereospecific intramolecular 4 + 4-cycloaddition reaction between cyclohexadiene iron tricarbonyl complex and appended dienes (198) generates cyclooctadiene tricyclic adducts (199) (Scheme 56).232 The first example of an asymmetric intermolecular 4 + 4-photocycloaddition reaction in solution between 9-cyanoanthracene and chiral 2-methoxy-l-naphthamides has been reported. The frozen chirality is effectively transferred to the optically active product.233... 

Dienes coordinated to iron tricarbonyl do not undergo Diels-Alder reactions and a number of interesting annulation reactions of groups attached to iron tricarbonyl diene complexes have been developed. Cyclohexadiene iron tricarbonyl complexes having a pendant alkene undergo annulation reactions to form spirocychc compounds (Scheme 160). Related reactions of allylic thioesters afford spirocychc thialac-tones. 

The oxidative cyclization of chiral 2-pyrrolidino-l-ethanol derivatives is shown in the reaction of 251 with trimethyl-amine iV-oxide and a substoichiometric amount of cyclohexadiene iron tricarbonyl to produce the corresponding oxazolopyrrolidine ring 252. The mechanism of this reaction is unknown. Both amine oxide and iron complex are essential for the reaction (Equation 39) <2005TL3407>. 

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-... 

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. 

Triphenylcarbenium salts (PhsC ) readily abstract a hydride from V cyclo-hexadiene iron tricarbonyl complexes to form cationic tj -cyclohexadienyl salts, the hydride being removed from one of the methylene groups adjacent to the V-diene unit. When the cyclohexadiene ligand is monosubstituted, or has two or more different functionaUties attached, a mixture of regioisomers may potentially result. For example, hydride abstraction from 38 results in a 1.5 1 ratio of the two possible regioisomers 39 and 40. 

Diene)Fe(GO)3 complexes have been used to construct 2-azaspiro[5.5]undecane and tricyclic ring systems. Intramolecular coupling reactions between diene-Fe(GO)3 complexes and a pendant olefinic group or pendant alcohols have been studied.The cyclization of allylic thioester-functionalized cyclohexadiene iron tricarbonyl... 

Iron pentacarbonyl and l-methoxy-l,4-cyclohexadiene react as shown by Birch and oo-workera, but in dibutyl ether this solvent has been found superior. The tricarbonyl(methoxy-l,3-cyclohexadiene)iron isomers undergo hydride abstraction with triphenylmethyl tetrafluoro-borate to form the dienyl salt mixture of which the 1-methoxy isomer is hydrolyzed by water to the cyclohexadienone complex. The 2-methoxy isomer can be recovered by precipitation as the hexafluoro-phosphate salt. By this method the 3-methyl-substituted dienone complex has also been prepared from l-methoxy-3-methylbenzene. The use of the conjugated 1-methoxy-1,3-cyclohexadiene in Part B led to no increase in yield or rate and resulted chiefly in another product of higher molecular weight. An alternative procedure for the dienone is to react tricarbonyl(l,4-dimethoxycyclohexadiene)iron with sulfuric acid. ... 

Tricarbonyliron-coordinated cyclohexadienylium ions 569 were shown to be useful electrophiles for the electrophilic aromatic substitution of functionally diverse electron-rich arylamines 570. This reaction combined with the oxidative cyclization of the arylamine-substituted tricarbonyl(ri -cyclohexadiene)iron complexes 571, leads to a convergent total synthesis of a broad range of carbazole alkaloids. The overall transformation involves consecutive iron-mediated C-C and C-N bond formation followed by aromatization (8,10) (Schemes 5.24 and 5.25). 

Over the past 15 years, we developed three procedures for the iron-mediated carbazole synthesis, which differ in the mode of oxidative cyclization arylamine cyclization, quinone imine cyclization, and oxidative cyclization by air (8,10,557,558). The one-pot transformation of the arylamine-substituted tricarbonyl(ri -cyclohexadiene) iron complexes 571 to the 9H-carbazoles 573 proceeds via a sequence of cyclization, aromatization, and demetalation. This iron-mediated arylamine cyclization has been widely applied to the total synthesis of a broad range of 1-oxygenated, 3-oxygenated, and 3,4-dioxygenated carbazole alkaloids (Scheme 5.24). 

In the quinone imine cyclization of iron complexes to carbazoles, the arylamine-substituted tricarbonyl(ri -cyclohexadiene)iron complexes 571 are chemoselectively oxidized to a quinone imine 574 prior to cyclodehydrogenation. This mode of cyclization is particularly applicable for the total synthesis of 3-oxygenated tricyclic carbazole alkaloids (Scheme 5.25). 

Alkenes in (alkene)dicarbonyl(T i-cyclopentadienyl)iron(1 +) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, t974 A. J. Pearson, 1987). Tricarbon-yl(r)5-cyclohexadienyl)iron(l +) cations, prepared from the T 4-l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(ti4-l,3-cyclohexadiene)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). 

Chemoselective oxidation of 4-methoxyanilines to quinonimines can be achieved in the presence of tricarbonyl(ri4-cyclohexadiene)iron complexes. This transformation has been used for the synthesis of carbazoles via intermediate tricarbonyliron-coordinated 4b,8a-dihydrocarbazol-3-one complexes (Scheme 1.24) [57]. 

Using cationic tricarbonyl(q5-cyclohexadienyl)iron complexes as starting materials, different synthetic routes to a large number of carbazole alkaloids have been developed [51, 58, 67]. The first step is an electrophilic substitution of a substituted arylamine using the cyclohexadienyliron complex and provides the corresponding 5-aryl-substituted cyclohexadiene-iron complexes (Scheme 1.29). 

Principal Component Analysis (PCA) has proved a powerful statistical method to distiguish the contributions of multiple effects on a particular complex. The data for tricarbonyl(cyclohexadiene)iron (5) produces the PCA plot shown in Fig. 7.7d, in which values for pure solvents lie at the corners, data for binary mixtures, define the edges, and three-solvent mixtures give points that lie inside the boundaries [45]. Although so far demonstrated for single complexes and up to three solvents, the combination of data from two differentially responding complexes (see Fig. 7.7c) in a multi-carbonylmetal approach should give exceptionally clear measures of properties of hydrophobic and hydrophilic environments when exploited in bioprobe applications. 

A mechanism for the stereospecific monodeuteration of tricarbonyl(l-car-bomethoxycyclohexa- ,3-diene)iron [Eq. (129)] is suggested in Scheme 11. This mechanism is consistent with all the available data on stereospecific deuterations of variously substituted tricarbonyl(cyclohexadiene)iron complexes (A. J. Birch, B. J. Chauncy, and D. J. Thompson, unpublished results, 1975) and with the large D/H (deuterium/hydrogen) isotope effect, i.e., ratedetermining protonation at Fe (Whitesides and Nielan, 1975). It should be noted that the principle of microscopic reversibility may not strictly apply in... 

Cyclic Dienes - A number of optically active tricarbonyl iron ic-complexes with unsymmetrical cyclohexadiene and cyclohexadienyl ligands have been prepared from diols available by the oxidation of arenes by Pseudomonas putida. The circular dichroism spectra of the metal complexes were shown to provide an empirical guide for the assignment of absolute configuration from chiroptical data for this class of compounds. 

Bridged bicyclo[3.2.1]octenes and bicyclo[3.3.1]nonenes are obtained when this reaction is carried out in an intramolecular mode. The nucleophiles are generated in appropriately functionalized side chains at C5 of the tricarbonyl( n -cyclohexadiene)iron complexes. Acyclic (Ti -l,3-butadiene)Fe(CO)3 complexes with fimctionalized side chains at the terminal position of the diene ligands provide fused bicyclo[3.3.0]octanones and bicyclo[4.3.0]nonanones (Scheme 4-121). Up to four new stereogenic centers are established in a controlled way in the course of this reaction. ... 

Tricarbonyl(Ti -cyclohexadienylium)iron tetrafluoroborate was described first by Fischer in 1960. The required tricarbonyl(T -cyclohexadiene)iron complex can be conveniently obtained in high yield from cyclohexa-1,3-diene and pentacarbonyliron using catalytic amounts of a 1-azabuta-1,3-diene (see Section 2.5.1). Hydride abstraction from the tricarbonyl(ri -cyclohexadiene)iron complex using triphenylmethyl tetrafluoroborate affords the Ti -dienyliumiron complex salt almost quantitatively (Scheme 4-1... 

In the following, some recent examples of nucleophilic addition to (Ti -cyclohexadiene)iron complexes are presented. Cyanide can be added regioselectively at the Cl terminus of a tricarbonyl(T -4-methoxycyclohexadienylium)iron salt using trimethylsilyl cyanide as a cyanide source. The method has been exploited for the... 

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