Ferrocenyl carbene complexes

The preparation of a series of ferrocenyl carbene complexes [(CO)5CrC(X)-C5H4FeC5H5] (X = O - NMe4% OMe, OEt, NH2, NMc2, or NC4H8) has been reported. The presence of a ferrocenyl group adjacent to the carbene... 

Table 7-4. Formal electrode potentials vs. SCE) for the one-electron oxidation of the ferrocenyl-carbene complexes (l-R-C5H4)Fe C5H3(2 -R)[l -C(R ) = Cr(CO)5] ...

In the ferrocenyl-carbene complex shown in Scheme 7-6, the molybdenum fragment possesses a tetrahedral geometry. It has been reported that this complex onyl undergoes a ferrocene-centred oxidation at 4-0.52 V in THF solution [47]. 

Deprotonation of l-methyl-3-ferrocenylimidazolium tetrafluoroborate or iodide (98JOM(552)45) by lithium di-Mo-propylamide and subsequent reaction with W(C0)5-THF gives the carbene complex 107 and bis-carbene 108, even when excess W(CO)j THF is applied (99JOM(572)177). Numerous ferrocenyl benzimidazoles are known (97RCR613, 99JOM(580)26). 

These complexes can be isolated in some cases in others they are generated in situ from appropriate precursors, of which diazo compounds are among the most important. These compounds, including CH2N2 and other diazoalkanes, react with metals or metal salts (copper, palladium, and rhodium are most commonly used) to give the carbene complexes that add CRR to double bonds. Ethyl a-diazoacetate reacts with styrene in the presence of bis(ferrocenyl) bis(imine), for example, to give ethyl 2-phenylcyclopropane-l-carboxylate. Optically active complexes have... 

Despite their apparent steric bulk, both the Fc and bis-ferrocenyl modified carbene ligands could be successfully i Ued in the synthesis of mono- and bis-carbene complexes of mercury(II) [147], mngsten(0) [147], palladium(II) [147,183,192] andsilver(I) [191] (see Hgure 4.57). 

Naturally, it is possible to synthesise a similar ligand system without central chirality and in fact without the unnecessary methylene linker unit. A suitable synthesis starts with planar chiral ferrocenyl aldehyde acetal (see Figure 5.30). Hydrolysis and oxidation of the acetal yields the corresponding carboxylic acid that is transformed into the azide and subsequently turned into the respective primary amine functionalised planar chiral ferrocene. A rather complex reaction sequence involving 5-triazine, bromoacetal-dehyde diethylacetal and boron trifluoride etherate eventually yields the desired doubly ferrocenyl substituted imidazolium salt that can be deprotonated with the usual potassium tert-butylate to the free carbene. The ligand was used to form a variety of palladium(II) carbene complexes with pyridine or a phosphane as coligand. 

Like the chromium- and molybdenum-carbene complexes of the type illustrated in Scheme 7-3, the corresponding octahedral tungsten complexes exhibit only a one-electron oxidation, which is thought to be centered on both the ferrocenyl and the tungsten fragments. The redox potentials are summarized in Table 7-10. 

M = Cr, Mo, W) the cis-trans ratio of which depends on the metal. Similar results are obtained starting from methylcinnamate, diethylfumarate and diethylmaleate, but simple alkenes such as cyclohexene or tetramethylethylene fail to give cyclopropane. In the same way the ferrocenylcarbene complex (CO)5Cr[CFc(OMe)] (Fc = ferrocenyl) reacts at 136°C with dimethylfumarate to give the expected cyclopropane derivatives. The optically active carbene complex (CO)4Cr[C(OMe)Ph](PMePhPr) obtained from (—)(R)-methylphenylpropylphosphine reacts with diethylfumarate at 60°C to afford, in low yield, the optically active cyclopropane ... 

Palladium(ii) complexes of imidazoline-2-ylidenes with pendant 1- and 1,3-di-methylferrocenyl substituents, 78 and 79, were prepared by the reaction of the relevant benzimidazolium salt with Pd(OAc)2 and a crystal structure of 79 obtained. " In common with other related carbene complexes, the plane of the benzimidazole rings are twisted with respect to the palladium coordination plane the dihedral angle is 85.17(23)°. The closely related l-ferrocenyl-3-methyl-benzimidazolin-2-ylidene complex (in which there is no methyl linker between the N and the ferrocenyl group) has also been prepared by a similar route. " l-Ferrocenylmethyl-3-benzylimidazolidinium and 1-ferrocenyl-methyl-3-(2,4,6-trimethylbenzyl)imidazolidinium iodide salts were prepared and complexed to Pd by an in situ deprotonation route. " Cyclic voltammetry studies on the salts and the Pd complex show a number of redox processes, indicating that the ferrocenyl substituents are electronically isolated from the remaining molecular framework. 

Treatment of [PdCl2(COD)] with ferrocenyl bis-NHC (97) yielded complex (98), in which an isomerization cis/trans was observed (equation 12). It is worth noting that the free carbene was generated in situ using the strong base KN(SiMe3)2. ... 

Chloro(methoxy)- and chloro(phenoxy)carbenes, which are generated from the diazirine precursors, behave as ambiphiles in additions to alkenes, exhibiting high reactivities toward both electron-poor and electron-rich olefins. Methoxy(phenyl)- and ferrocenyl(methoxy)methylenes have been transferred in a stereospecific manner from transition metal complexes of these species to electron-deficient alkenes. Irradiation of benzocyclobutanedione with UV light induces a rearrangement of the cyclic a-diketone to 17, which has been trapped by alkenes in good yields " . Thermolysis of 18 gives rise to nucleophilic dimethoxycarbene, which has been intercepted by electron-deficient olefins or by styrene derivatives. 

A bidentate ferrocenyl-A-heterocychc carbene hgand precursor in the form of the bis-imidazolinium salt was deprotonated in situ with KHMDS and reacted with [PdCl2(cod)] to provide a bis(carbene)palladium(n) dichloride complex, which was stmcturally characterized (eq 100). ... 

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