Synthesis of Ferrocene Bridgedbis(2-Indenyl) Ligands

Abstract

An optimised synthesis of 1,1¢-bis(2-indenyl)ferrocene in 47% yield from the Pd(0)-catalysed cross coupling between 1,1¢-zincated ferrocene and 2-bromo-indene is described along with the formation of (2-indenyl) ferrocene in 40% yield. The analogous synthesis of planar chiral derivatives gave either pure N, 1¢-bis(2-indenyl)ferrocenyl]ethylamine (26%) dependent upon the amount of Pd(0) catalyst used.The design and development of catalytic enantioselective reactions is an important area of synthetic chemical research. Organometallic systems have featured prominently in this area as they can be made to possess well-defined three dimensional structures. 1 More specifically chiral ansa-metallocenes, also known as bridged metallocenes, have found wide use as catalysts that effect enantioselective bond forming processes. 2 The formation of chiral ansa-metallocenes can be complicated by the formation of meso and asymmetric metallocene isomers. 2 With the incorporation of a chiral bridging group it is possible to selectively form a single stereoisomer. An example of the use of this strategy is Bosnich's Ti[(S)-biphenacene]Cl 2 complex (1) 3 which has been used in asymmetric Lewis acid-catalysed Diels-Alder reactions. 4 We wish to extend these types of systems by investigating the effect planar chirality can impose on the arrangement of indenyl systems around a Lewis acid metal centre. This paper describes our first steps towards synthesising planar chiral metallocene ligands for assay in asymmetric catalysis.We decided initially to synthesise a novel range of planar chiral ansa-bis(indenyl) based metallocenes of type 2 as this ligand system would not suffer from the potential formation of meso isomers as the more complicated pseudo-C 2 symmetric 3 could (Figure 1). The corresponding tetrahydroindenyl complexes could be prepared by standard hydrogenation if necessary. An important feature of our systems is that they contain the readily available N,N-dimethyl-1-ferrocenylethylamine unit as the chiral bridging group which has proved highly effective for the synthesis of chiral ligands in asymmetric catalysis. 5 This linker contains both planar and central elements of chirality which we wish to harness in our new ligand system. Figure 1An attractive retrosynthesis for planar chiral bis(2-indenyl)ligand in 2 followed from the related work by Bosnich et al. 3 The indenyl groups could be built onto the cyclopentadienyl rings by reaction of the bis-Grignard of orthobis(chloromethyl) benzene (5) with N,N-dimethyl-2,1¢-bis(methylcarboxyl)ferrocene 6 followed by dehydration. Scheme 1We decided first to conduct model studies with ferrocene. The dimethyl ester 7 was synthesised by treating the corresponding known diacid 6 with SO 2 Cl 2 in MeOH (quant.). 7 Treatment of 7 with the bis-Grignard reagent 5, 8 gave the bis-carbinol 8 in 44% yield after trituration with MeOH (Scheme 1). Dehydration with p-TsOH followed by recrystallation from toluene/methanol gave the pure 1,1¢-bis(2-indenyl)ferrocene (9) in 17% unoptimised yield.With the ...

Charge Transfer Properties of Multi(ferrocenyl)trindenes

“…Then, by refluxing a toluene solution of bromohydrins with p -toluenesulfonic acid a mixture of 4,7-dihydro- and 6,7-dihydro-2,5,8-tribromo-1 H -trindenes (TdHBr 3 ) was obtained. Subsequently, the Negishi reaction of excess of FcZnCl (Fc = ferrocenyl), TdHBr 3 , and PdCl 2 (PPh 3 ) 2 catalyst in THF at 25 °C for 22 h gave the (multi)ferrocenyl complexes 1 – 3 , which were separated by preparative TLC on silica gel and elution with 4/1 hexane/CH 2 Cl 2 (see the Experimental Section). In Scheme only the syn , syn , syn configuration is shown but any combination of syn / anti configurations is possible.…”

Synthesis of planar chiral ferrocenyl 1,3-diamines and 1,3-amino ethers

“…The use of a Curtius rearrangement to introduce the desired amino substituent 21 was thwarted by our inability to form the precursor amino amide from direct treatment of lithiated 2 with TMS-isocyanate 22 or via the amino acid with solid CO 2 . 23 Our attention briefly turned to the introduction of an ortho-hydroxy group into 2. The synthesis of 2-(N,N-dimethylaminomethyl)ferrocenol had been reported from the corresponding iodide by treatment with AcOH and Cu 2 O.…”

Selective Syntheses of Planar-Chiral Ferrocenes

“…2- or 2,1′-dilithiated ferrocenes derived from Ugi’s amine or related compounds have been treated with a wide range of electrophiles, giving phosphines, − ,− ,− ,− alcoholes and thioles, , aldehydes, − carboxylic acids and derivatives thereof (e.g., imines, oxazolines, esters, or amides), − thio- and selenoethers, ,,,,− and dithiocarbamates . Furthermore, transition-metal-mediated homo- and cross-coupling reactions have been performed with derived 2-halo- and 2-lithioferrocenes, furnishing aryl- and alkynyl-substituted ferrocenes − as well as biferrocenes. − In addition, diferr...…”