A.S. Abd-El-Aziz

Studies on nucleophilic substitution reactions with η6-o-dichlorobenzene-η5-cyclopentadienyliron hexafluorophosphate

Abstract Reaction of η 6 - o -dichlorobenzene-η 5 -cyclopentadienyliron hexafluorophosphate (IPF 6 ) with an excess of phenol or p -thiocresol in the presence of K 2 CO 3 could give disubstitution of both chloro groups of I, while a similar reaction with one equivalent of the nucleophile, and under conditions of high dilution, monosubstitution of only one of the chloro groups of I could be obtained. Similarly, di- or monosubstitution could be brought about under appropriate conditions with benzyl or methyl alcohol as the source of the nucleophile. While no reaction could take place between IPF 6 and aniline, a reaction did occur between IPF 6 and o -anisidine ( o -methoxyaniline), but only the monosubstitution product was obtained, even in the presence of an excess of o -anisidine. Similar results of monosubstitution were observed with other nucleophiles containing the NH 2 group, including NH 3 , NH 2 NH 2 , CH 3 NH 2 and C 6 H 5 CH 2 NH 2 . These findings are consistent with the reported differences in yields when IPF 6 was treated with two nucleophilic groups (OH, SH and/or NH 2 ) located in the 1,2-positions of a benzene ring to give CpFe complexes of heterocyclic systems related to 9,10-dihydroanthracene with two hetero-atoms at the 9,10-positions [15]. Reactions were also carried out between IPF 6 and the carbanion-enolate anion derived from acetylacetone, α-benzoylacetophenone, diethyl malonate or ethyl acetoacetate. In these cases, only monosubstitution of one of the chloro groups of I was observed, leading to the formation of a CC bond. A possible explanation for the formation of only monosubstitution products in reactions with N- or C-containing nucleophiles is discussed.

Studies on nucleophilic substitution reactions with cyclopentadienyliron complexes of some chloroarenes and nitroarenes and syntheses of substituted arenes by demetallation of the substitution products

Abstract Nucleophilic substitution reactions with the cyclopentadienyliron (CpFe) complex of m - or p -dichlorobenzene with carbanion nucleophiles derived from ethyl acetoacetate (EAA), dibenzoylmethane (DBM) or diacetylmethane (DAM) were found to give only monosubstitution as previously observed for the CpFe complex of o -dichlorobenzene. Reaction of the CpFe complex of 2,6-dimethylchlorobenzene (XIVa) or 2,6-dimethylnitrobenzene (XIVb) with nucleophiles derived from ammonia, dimethylamine, n-butylamine, pyrrolidine, ethanol, phenol, o -thiocresol and EAA all gave S N Ar products, without significant steric hindrance. However, no reaction was observed in the treatment of XIVa or XIVb with DBM, suggesting that only with a bulky nucleophile such as that derived from DBM were steric hindrance effects sufficiently large to prevent an S N Ar reaction with XIV, a or XIVb. Pyrolytic sublimation of the various S N Ar products was found to cause decomposition in some cases, but in most instances, demetallation took place giving rise to substituted arenes. New substituted arenes prepared in this way in the present work included RC 6 H 4 CH(COC 6 H 5 ) 2 , with R = p -CH 3 , o -Cl, m -Cl or p -Cl, and 2,6-(CH 3 ) 2 C 6 H 3 Y, with Y = CH 3 (CH 2 ) 3 NH, C 2 H 5 O or o -CH 3 C 6 H 4 S.

Nucleophilic Aromatic Substitution Reactions in Cyclopentadienyliron Complexes of Chloroarenes and Nitroarenes

Abstract n6-Chloroarene or nitroarene-n5-cyclopentadienyliron hexafluorophosphates react readily with CF3CH2OH/K2CO3 or with piperidine or piperazine to give nucleophilic aromatic substitutions with Cl or NO2 as the leaving group, further demonstrating an activation for SNAr reactions by complexation to the CpFe+ moiety. Similar nucleophilic aromatic substitution reactions can also occur with carbanion-enolate anion nucleophiles derived from acetylacetone, diethyl malonate and ethyl acetoacetate, leading to the formation of C-C bonds. Pyrolytic sublimation of these SNAr reaction products could liberate the free substituted arene ligands, such as the synthetically useful diethyl phenylmalonate and ethyl α-phenylacetoacetate.

Arylation of Diethyl Malonate via Nucleophilic Substitution Reactions with Cyclopentadienyliron Complexes of Chloroarenes

Abstract Reaction of an η6-chloroarene-η5-cyclopentadienyl-iron hexafluorophosphate with diethyl malonate in the presence of K2CO3 in THF/DMSO gave a corresponding SN-Ar product, η6-diethyl arylmalonate-η5-cyclopentadienyliron hexafluorophosphate, which, upon pyrolytic sublimation, resulted in the liberation of the arylated malonic ester. Such esters prepared in the present work included RPhCH(COOEt)2, with R = o-, m- or p-CH3, 2,6-(CH3)2, or o-, m- or p-Cl2.

Electrochemical studies of η5-cyclopentadienyliron hexafluorophosphates of η6-substituted arenes and η6-heterocycles

Abstract Polarographic half-wave potentials for two reduction steps of 49 cyclopentadienyliron complexes of substituted arenes or heterocycles in dimethylformamide were determined. A fast cyclic voltammetry (10–40 V/s) was used to study the electron transfer kinetics of some of these complexes. After correction for the double layer effects, the rate constants of all the complexes studied show that the transfer of electrons in the second reduction step occurs significantly faster than in the first one. This was interpreted as a result of greater delocalization of electrons in the 20-electron complexes of iron compared to the 19-electron complexes.

Studies on some mono- and di-substitution reactions with cyclopentadienyliron complexes of o-, m- and p-dichlorobenzenes

Abstract Reaction of the cyclopentadienyliron (CpFe) complex ofo-,m- orp-dichlorobenzene (Ia, Ib or Ic, respectively) with an excess of n-butylamine gave only monosubstitution of one of the two chloro groups, while a similar reaction carried out in the presence of some added HOAc resulted in disubstitution of both chloro groups. On the other hand, in the reaction of Ia, Ib or Ic with one equivalent or with an excess of pyrrolidine, respectively, mono- or di-substitution took place. These observation support the previous suggestion that, under the basic conditions of the reaction, deprotonation of the monosubstitution product from a reaction with a primary amine would render the second chloro group incapable of undergoing another nucleophilic substitution, while for a reaction with a secondary amine, there could be no deprotonation with the monosubstitution product, thus allowing disubstitution to occur. When Ia, Ib or Ic was treated with an excess of NaCN in DMF for up to 4 days, only monosubstitution was observed, a pure product being obtained with Ia, while with Ib or Ic, the product was contaminated with a large amount of starting material. When the same reaction of Ia, Ib or Ic with NaCN was carried out for 30 min and then worked up without any added aqueous NH4PF6, cyclohexadienyl complexes from the nucleophilic addition of a cyanide ion were obtained. Under similar reaction conditions but with the reaction time extended to 2 days, Ia could also give rise to products derived from monosubstitution and cyanide addition. Mechanistic implications of these results are discussed.

Arylation of diethyl alkylmalonates: synthetic route to diethyl alkyl(substituted aryl)malonates with the aid of temporary arene complexation by the cyclopentadienyliron moiety

Nucleophilic substitution of chlorine in the ironcyclopentadienyl hexafluorophosphates of chlorobenzene, isomeric chlorotoluenes and dichlorobenzenes (1) with the anions generated from diethyl alkylmalonates (2) and (3) leads to the formation of ironcyclopentadienyl complexes of diethyl alkyl(substituted phenyl)malonates (4) and (5). An excess of the anion employed in reactions with the m- and p-dichlorobenzene complexes leads to substitution of both chlorine atoms and formation of isomeric phenylenedimalonate complexes (8). Malonyl cations (4) and (5) possessing a chloro substituent on the complexed phenyl ring provide the possibility of further modifications of phenyl ring substituents via substitution [complex (10)] or addition reactions [complexes (12)]. The described complexes have been demetallated giving diethyl alkyl(substituted phenyl)malonates in >50% overall yield from the cations (1). All the complexes and compounds have been fully characterized. This approach to the synthesis of the diethyl alkyl(substituted phenyl)malonates which are intermediates in the synthesis of important biologically active barbiturates is easy, efficient, and currently the most general in the area.

Recovery of Arenes Containing Keto or Ester Groups by the Electrochemical Reduction of Their Respective Cyclopentadienyliron Complexes

Abstract Electrochemical reduction of the cyclopentadienyliron hexafluorophosphates of benzophenone, o-chlorophenyldibenzoylmethane and diethyl ethylphenylmalonate led to the liberation of arenes without reduction of keto or ester groups in over 85% yields.

Preparative Electrochemical Liberation of Arenes and Heterocycles from Their Respective Ironcyclopentadienyl Complexes

Abstract Electrolysis of arene and heterocycle cyclopentadienyliron hexafluorophosphates led to the liberation of arenes or heterocycles in 69–90% yield. The procedure described gives yields comparable with or superior to the yields obtained from pyrolytic sublimation used previously.