[51508-58-8] · C11H11F6FeO4P · Tricarbonyl(4-methoxy-1-methylcyclohexadienyl)iron Hexafluorophosphate · (MW 408.04)
Solubility: easily sol MeCN, MeNO2; difficult in nonpolar solvents; partially sol H2O.
Form Supplied in: yellow powder.
Preparative Methods: easily prepared from 4-methylanisole by Birch reduction, followed by complexation with Pentacarbonyliron and hydride abstraction.
Handling, Storage, and Precautions: is stable to air and moisture. Potent alkylating agent. Toxicity unknown.
Despite the stability of this iron-bound carbocation, it is highly reactive toward nucleophiles. The cation regiospecifically reacts with a wide range of nucleophiles, including cyanide, malonate, b-diketone enolates, and silyl ketene acetals, to give synthetically useful products. The presence of the iron results in addition almost exclusively at the methyl terminus of the dienyl system (the less electron-rich terminus) on the face of the ligand opposite to the metal to give the substituted diene complexes.2 Nucleophiles react at C-1 or C-5, but do not attack at C-2, C-3, or C-4.3
Morpholine and hydroxide ion add at the methylated terminus C-1 of the diene system,4 in preference to the electronically deactivated terminus C-5 (eq 1), while treatment with Sodium Borohydride gives addition of H- at both termini (eq 2).4 A possible explanation is partial attack on carbonyl to give a formyl complex with transfer of hydride directly to the carbon system.5
A wide range of carbon nucleophiles react with the cation under mild conditions to give the corresponding C-C bonded product. Nitrile adducts can be obtained with Sodium Cyanide6 or with Cyanotrimethylsilane (eq 3).7,8 The product obtained from reaction of analogs of the complex with sodium malonate can be used for the synthesis of spirocyclic compounds.9a,10 Reaction of the Reformatsky reagent BrZnCH2CO2Me leads to a mixture of alkylation products and triene.2
Alkylation with sodium or potassium enolates has potential in organic synthesis.1,11,12 The reaction intermediates with enolates of keto esters derived from cyclopentanones, cyclohexanones,13 and tetralones have been used for the synthesis of C-nor-D-homosteroids,1b,12 D-homosteroids,12 12,13-epoxy-14-methoxytrichothecene,11a quassinoids,14 and 6-keto steroids.13,15 The cation reacts with the potassium enolate of methyl 2-oxocyclopentanecarboxylate to give a quantitative yield of the two diastereoisomers (eq 4) (one of these has been transformed to trichothecane), thus leading to the controlled formation of two contiguous quaternary centers.11
Simple lithium enolates of ketones and esters have proven too basic, causing deprotonation of the cation to give a triene complex instead of the desired addition products. Reaction of the cation with silylketene acetals gives high yields of the desired products (eq 5). Most of the reactions occurred with very high regioselectivity.16
A number of tin enolates react cleanly with the cation in cases where the use of lithium enolate or silyl enol ethers is problematic. This new C-C bond forming reaction allows diastereoselective construction of proximate quaternary centers and provides a key step in a short synthesis of (±)-trichodiene (eq 6),18 (±)-trichodermol,19 3,14-dihydroxytrichothecenes,20 and (±)-12,13-epoxytrichothec-9-ene.18
Decomplexation and hydrolysis of the enol ether generates the 4-substituted cyclohexenone, making the starting complex the synthetic equivalent of a cyclohexenone g-cation.1 Illustrative of this are syntheses of polycyclic natural compounds, especially terpenes and steroids (eq 7).1c,16,21,22
Induction23 of asymmetry during the complexation of 1-methoxy-4-methylcyclohexadiene and reaction24 of the resulting complex with triphenylmethyl tetrafluoroborate provides a convenient route to partially resolved cation. The complete resolution of the cation by selective crystallization of enantiomers has been achieved.25
Reaction of racemic cation with a chiral carbanion occurs enantioselectively.26,27 The chiral intermediates are converted into optically active natural steroids.26a
Florida State University, Tallahassee, FL, USA