[33678-01-2] · C9H7BF4FeO3 · Tricarbonyl(cyclohexadienyl)iron Tetrafluoroborate · (MW 305.82)
Physical Data: mp 193 °C (dec).
Solubility: readily sol H2O, DMSO, DMF, MeNO2; difficult in nonpolar solvents.
Form Supplied in: yellow solid; widely available.
Preparative Methods: readily obtained from tricarbonylcyclohexadieneiron by hydride abstraction with trityl fluoroborate in methylene chloride followed by precipitation of the salt with ether. Salts of this type are invariably yellow microcrystalline involatile solids.
Handling, Storage, and Precautions: this solid is stable to air and moisture.
As might be expected from its cationic nature, the dienyl-Fe(CO)3 complex reacts with nucleophiles to give neutral diene complexes.1a The cation possesses sufficient stability towards water to be recrystallized from it and yet, on the other hand, conveniently reacts with nucleophiles stronger than water to afford addition products with attack occurring at carbon to give 5-substituted 1,3-cyclohexadiene-Fe(CO)3 derivatives. The reaction is usually very clean and is regiospecific (terminal C atom) and stereospecific (opposite Fe).1c
Reaction of the cation with anions RO- (R = H, Me, CMe3, SiMe3) gives the corresponding complexes as well as dimeric products in yields depending on RO- and the reaction conditions.2c,3 The (5-exo-halogenocyclohexa-1,3-diene)Fe(CO)3 can be prepared from reaction of the cation with Potassium Fluoride, KBr, or KCl in the presence of 18-Crown-6.4 In contrast, iodide ion ultimately gives the product from addition at iron and displacement of carbon monoxide.5
Addition of a wide variety of anilines and pyridines to the cation forms novel C- or N-alkylation products, depending on the substituents and conditions (eq 1).6 Electron-withdrawing substituents (4-NO2) on the arene favor N-alkylation whereas electron-donating substituents (3-OMe, 3-NR2) favor C-alkylation.
The N-alkylation products may undergo dissociation to the cation upon treatment with acid.7 Amides1a,8 form adducts with the cation when the reaction is carried out at higher temperature. Thiourea forms the S-linked adducts.6c,9 Attack of nucleophiles such as nitrosoarenes,10a nitrosoalkanes,10b phosphorus,11 Acetonitrile,12 and thiocyanate13 also give the corresponding 1,3-diene 5-exo-substituted products.
Nucleophilic addition of phosphite and phosphines to the cation has led to a variety of novel phosphorus adducts (eq 2).14 Reaction of an electron-rich transition metal p-complex could readily give stereo- and regioselective C-C bond formation (eq 3).15
Addition of metal carbonyl anions [such as Re(CO)5-, Mn(CO)5-, (h5-(C5H5W(CO)3)-] to the cation gives heterobimetallic h1:h4-hydrocarbon-bridged complexes, and addition of organometallic nucleophiles [M(CO)42- (M = Os, Ru)] leads to the heterotrimetallic h1:h4-hydrocarbon-bridged complexes.16 Hydride reduction of the cation favors simultaneous attack at the ring carbon atom and Fe(CO)3 group.17 The cation is reduced efficiently by Sodium Borohydride18 and Tri-n-butylstannane.14b Dissolving metal reductions lead to dimerization.14b
Although it is impossible to achieve direct alkylation in good yield with Grignard or alkyllithium reagents,19 alkylation of the cation can be achieved using organoboron, -zinc, -cadmium, and -copper reagents,1c,20 enolates,18,21 allylstannane,22 enamines,23 and electron-rich p-systems.24
The cation reacts with a wide variety of aromatic compounds.1a,23,25 Activated arenes, ArMMe3 (M = Si or Sn, Ar = 2-furyl, 2-thienyl, substituted benzene), are more reactive towards the cation than Ar-H compounds.25 Dimethylaluminum acetylides react with the cation to give the corresponding alkynyl substituted complexes in high yield without competing reduction products (eq 4).26 Trialkylalkynylborates also react in a stereo- and regiospecific manner to yield novel complexes.27 Tetraphenylborate serves as a biphenyl anion synthon (eq 5).28 The reaction of 2-halogenocycloheptadienone enolate with the cation gives 2-substituted tropone (eq 6).29
Complexes resulting from nucleophilic addition to the cation can usually be subjected to oxidative reaction conditions which will remove the metal and oxidize the resultant cyclohexadiene to a substituted aromatic compound.1c The regio- and stereoselective reactions of the cation with arylamines (eq 7) show excellent potential for application in organic synthesis. A procedure of consecutive iron-induced C-C and C-N bond formation has been developed. The synthesis of various highly substituted dihydrocarbazole and aromatic carbazole derivatives6c,30 involves electrophilic aromatic substitution of an arylamine, followed by oxidative cyclization to the carbazole derivatives.30
Florida State University, Tallahassee, FL, USA