Selective Syntheses of Planar-Chiral Ferrocenes - ACS Publications

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Selective Syntheses of Planar-Chiral Ferrocenes Dieter Schaarschmidt and Heinrich Lang* Inorganic Chemistry, Institute of Chemistry, Faculty of Sciences, Technische Universität Chemnitz, 09107 Chemnitz, Germany ABSTRACT: Planar-chiral ferrocenes are widely applied in catalytic asymmetric transformations in scientific research as well as in industry. A plethora of different methodologies have been developed to access these molecules with a high degree of regio- and stereoselectivity. The aim of this contribution is to give a comprehensive overview of this topic. The synthesis of 1,2- and 1,3-substituted ferrocenes by electrophilic aromatic substitution, ortho-directed metalation, kinetic resolution, and desymmetrization is discussed. Advantages and disadvantages are highlighted.

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INTRODUCTION Ferrocene (Fe(η5-C5H5)2, FcH) is often regarded as an ordinary aromatic compound; however, it exhibits some special features not present in common aromatics such as benzene. Ferrocene, for instance, undergoes electrophilic aromatic substitutions 3 × 106 times faster than benzene, which is due to the partial negative charge of the cyclopentadienyl ligands.1 The presence of oxidizing electrophiles results in the decomposition of ferrocene, and hence the synthesis of nitroferrocene is much more elaborate than that of nitrobenzene.2 Ferrocene as a ligand backbone offers more degrees of freedom than benzene does, as different conformations of the two cyclopentadienyls may facilitate the ligand to reduce steric strain, which is excellently documented for 1,1′-bis(diphenylphosphino)ferrocene (dppf).3 The most obvious difference, admittedly, is the three-dimensional nature of ferrocene, resulting in the planar chirality of 1,2- and 1,3heterodisubstituted ferrocenes (Figure 1). Planar-chiral ferrocenes have become an integral part of a chemist’s tools dealing with catalytic asymmetric transformations in academia as well as in industry. Probably the most impressive example in this context is the highly efficient Ir/Xyliphoscatalyzed enantioselective imine hydrogenation depicted in Scheme 1, which is actually the largest scale known

Scheme 1. Enantioselective Imine Hydrogenation Catalyzed by Ir/Xyliphos6

enantioselective catalytic process. The imine obtained is an intermediate in the synthesis of the herbicide (S)-metolachlor produced by Ciba-Geigy/Syngenta in a volume >10000 tons per year.6 This review is intended to give a comprehensive overview on the synthesis of planar-chiral ferrocenes. In order to avoid too large overlaps with the excellent monographs and reviews published so far,7 this contribution will, whenever possible, be limited to aspects that were not covered before. For this reason the applications of planar-chiral ferrocenes in asymmetric catalysis will not be discussed, as up to date overviews can be found elsewhere.8 Within this review the stereodescriptors “Rp” and “Sp”, determined according to Schlögl,4 are used to denote planar chirality. In order to avoid ambiguity with phosphorus chirality centers, “RX” and “SX” are used to identify noncarbon chirality centers, whereby X is replaced by the chemical symbol of the respective element: e.g., “S” or “P”. For chiral compounds possessing several stereogenic elements, the following order is used: central > axial > planar > helicity. For clarity “E” is used as Special Issue: Ferrocene - Beauty and Function

Figure 1. Enantiomeric 1,2-heterodisubstituted ferrocenes from two different perspectives and assignment of the stereodescriptor according to Schlögl4 (priority X > Y).5 © XXXX American Chemical Society

Received: June 17, 2013

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dx.doi.org/10.1021/om400564x | Organometallics XXXX, XXX, XXX−XXX

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established the absolute configuration of (+)-6b applying Horeau’s method.16 Therefore, (+)-6b was converted into (+)-exo-8 by reduction with Li[AlH4], esterification with acetic anhydride, and subsequent solvolysis in aqueous acetone solution (Scheme 4).17 Treatment of (+)-8 with racemic 2-

an abbreviation for an electrophile introduced at the ferrocene backbone subsequent to lithiation. The stereodescriptor for such ferrocenes was assigned by assuming that the priority of “E” is equal to that of PPh2.

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ELECTROPHILIC AROMATIC SUBSTITUTION Soon after the discovery of ferrocene by Pauson and Kealy in 1951,9 Woodward and co-workers classified it as an aromatic compound and synthesized the first derivatives thereof by acylation of ferrocene in the presence of AlCl3.10 If 1- or 1,1′substituted ferrocenes are applied in such electrophilic aromatic substitutions, planar-chiral ferrocenes may be formed. The major drawback of this approach is, however, the fairly low selectivity toward the exclusive substitution of either an α or β proton. Rinehart et al. have shown that the monoacetylation of 1,1′dialkylferrocenes gives mixtures of the β-acetyldialkylferrocene and the α-acetyl isomer, with the first being the major product.11 Later on, Rosenblum and Woodward reported on the same behavior for the acetylation of ethylferrocene.12 Not surprisingly, the selectivity toward the formation of a 1,3-substituted product increases with increasing steric bulk of the alkyl substituents (Scheme 2); however, the isolation of a pure product is tedious,

Scheme 4. Determination of the Absolute Configuration of (+)-8 applying Horeau’s Method18

phenylbutanoic anhydride gave scalemic 2-phenylbutyric acid, with the levorotatory acid being formed in excess, which corresponds to an S-configured carbon atom in (+)-8 and consequently an Rp configuration of the chiral plane in (+)-6b and (+)-8 (Scheme 4).18 The applicability of Horeau’s method for metallocenes was unambiguously proven in 1971 at the example of carboxylic acid 9 (Scheme 5). McPhail and Sim were able to get single crystals of

Scheme 2. Monoacetylation of 1- and 1,1′-Alkylferrocenes13 Scheme 5. Conversion of (+)-10 into (+)-9 and 11−16, Demonstrating That the Absolute Configuration of Ferrocenes Can Be Determined by Horeau’s Method18,22

requiring an elaborate chromatographic workup or repeated recrystallizations. Therefore, an intermolecular electrophilic aromatic substitution is rather unsuited for the straightforward synthesis of a planar-chiral ferrocene. The situation is quite different if the electrophile is already covalently bonded to the ferrocene. For Friedel−Crafts reactions of ω-ferrocenylaliphatic acids, for instance, a high degree of selectivity toward either an intramolecular homo- or heteroannular or an intermolecular acylation is observed, depending on the chain length of the aliphatic acid.14 Whereas 4a exclusively forms the heteroannular-bridged ferrocene 5a (87%), elongated 4b,c give the homoannular cyclized planar-chiral products 6 (63−83% (b), 28% (c)) (Scheme 3). In contrast, the formation of oligomers 7 (m = 2, 3) is observed starting from εferrocenylcaproic acid (4d; 11%) (Scheme 3). In 1959 Thomson achieved the resolution of 6b by treatment with (−)-menthydrazide (l-menthyl-N-aminocarbamate) and isolated 8 mg of (+)-6b after acid-mediated cleavage of the hydrazone.15 A few years later Schlö gl and co-workers

the quinidine salt of (−)-(Sp)-919 and determined the absolute configuration by means of Bijvoet’s anomalous-dispersion method with respect to the absolute stereochemistry of the alkaloid quinidine.20 Schlögl and co-workers in turn synthesized

Scheme 3. Friedel−Crafts Reaction of ω-Ferrocenylaliphatic Acids (n = 2 (a), 3 (b), 4 (c), 5 (d))a,14

a

TFAA = trifluoroacetic anhydride; PPA = polyphosphoric acid. B


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(+)-9 starting from (+)-1-methylferrocene-3-carboxylic acid (10), confirming its configuration as Rp.21 The ferrocene (Rp)(+)-10 was converted into γ-ferrocenylbutyric acid (11) as well, which upon intramolecular acylation gave ketones (−)-12 and (+)-13, respectively (Scheme 5).18,22 The assignment of the absolute configuration of these two ketones is possible in consideration of the configuration of (Rp)-(+)-10 and the fact that (+)-13 gives upon Clemmensen reduction an achiral ferrocene, whereas (−)-12 does not.18,22 If (−)-12 is reduced with Li[AlH4] to the secondary alcohol endo-16, Horeau’s method can be applied. The results were in agreement with the absolute configuration of (−)-12 and endo-16 (Scheme 5). Comparison of the optical rotary dispersion of (−)-12 and strongly related (+)-6b allows the conclusion that (+)-6b is Rpconfigured as already reported earlier.18,22 The synthesis, derivatization and resolution of 2, 3, 6, and closely related compounds along with the determination of their absolute configuration has already been reviewed and will therefore not be discussed here.23 The application of chiral, enantiopure γ-ferrocenylbutyric acids in an intramolecular Friedel−Crafts reaction in 1965 is most likely the first example of a diastereoselective synthesis of a planar-chiral ferrocene (Scheme 6).24 Schlögl and co-workers

Surprisingly, the formation of the two epimeric ferrocenes 23− 26 (de 20−78%) was observed in each case (Scheme 7).25 In Scheme 7. Diastereoselective Friedel−Crafts Acylation of (S)2-Methyl-2-phenyl-4-ferrocenylbutyric Acid and (R)-3Methyl-3-phenyl-4-ferrocenylbutyric Acid, Respectively25

addition, for both acids the diastereomers with the phenyl moiety in an endo position (24 and 25) are formed in excess, which is for 21 in contrast to the results of Schlögl, as cyclization of related (+)-17 gives only the exo isomer (−)-18 (Scheme 6). This contradiction cannot be explained by epimerization of either the asymmetric carbon atom or the chiral plane after cyclization, as 23−26 are formed in optically pure form, and there is no evidence in the literature for CCp−Fe bond cleavage in the presence of TFAA or TFA (TFA = trifluoroacetic acid).26 Consequently, the diastereoselectivity of the Friedel−Crafts acylation is either strongly temperature dependent or the replacement of a proton for a methyl group in the substrate impacts the conformation of the transition state severely. Heteroannular cyclization of 1,1′-disubstituted ferrocenes is another possible synthesis route for the preparation of planarchiral ferrocenes. As already mentioned earlier, β-ferrocenylpropionic acids form heteroannular-bridged [3]ferrocenophanes in high yields,14 being therefore predestined for this task. For example, in the presence of TFAA propionic acid 27 gives the two racemic ketones 28 and 29 in a 1.0:3.9 ratio (Scheme 8),27

Scheme 6. Diastereoselective Friedel−Crafts Acylation of (S)2-Phenyl-4-ferrocenylbutyric Acid and rac-3-Phenyl-4ferrocenylbutyric Acid24

Scheme 8. Heteroannular Cyclization of β-(1′Methylferrocenyl)propionic Acid29

prepared (S)-2-phenyl-4-ferrocenylbutyric acid ((+)-17) by Friedel−Crafts acylation of ferrocene with racemic phenylsuccinic anhydride, Clemmensen reduction of the ketone, and subsequent resolution with (−)-1-phenylethylamine (Scheme 6). Cyclization with trifluoroacetic anhydride afforded highly diastereoselectively (>95% de) (−)-18. In the case of 3-phenyl-4ferrocenylbutyric acid (19) an enantiopure starting material was not accessible by resolution; however, intramolecular cyclization yielded only one pair of enantiomers, (±)-(20) (Scheme 6). The diastereoselectivity in both transformations can be rationalized by analyzing the transition states of the intramolecular ring closures, as the lowest energy barrier is expected for an antiperiplanar conformation of the phenyl and ferrocenyl moieties. In 1975 des Abbayes and Dabard reported on the cyclization of (S)-2-methyl-2-phenyl-4-ferrocenylbutyric acid (21) and (R)-3methyl-3-phenyl-4-ferrocenylbutyric acid (22), applying almost the same reaction conditions as Schlögl, with the exception of increasing the reaction temperature from 0−25 to 40 °C.25

which could be resolved by treatment with (−)-menthydrazide.28 The preferred substitution of a proton in a position β to the methyl group in 27 is in good agreement with the regioselectivity observed for the intermolecular acylation of 1- and 1,1′alkylferrocenes (Scheme 2). In 1997 Richards and co-workers reported on a diastereoselective heteroannular cyclization by a Friedel−Crafts reaction. Diacid 30, obtained in high optical purity by addition of silyl enol ethers to ferrocenyl methyl ethers,30 forms upon conversion to the diacid chloride in presence of SnCl4 as Lewis acid 1,2,1′-substituted 31 as well as the epimeric [3]ferrocenophanes 32 and 33 (Scheme 9).31 The cyclization is highly regioselective toward an attack at the β position; however, the diastereoselectivity is rather low ((Sp)-32:(Rp)-33 = 1:1.5). C


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Scheme 9. Diastereoselective Heteroannular Cyclization of βFerrocenylpropionic Acids 30 and 33a,31

producing cyclic secondary amine rac-39, which is subsequently methylated by H2CO/HCO2H (Scheme 10). If enantiomerically pure (R)-1-ferrocenyl-2-propylamine (40) is applied in this transformation, the ring closure proceeds highly diastereoselectively, giving only the secondary amine (R,Sp)-41 (Scheme 11).33 Scheme 11. Pictet−Spengler Reaction of (R)-1-Ferrocenyl-2propylamine with Formaldehyde33

Bonini and co-workers extended this transformation to the chiral β-aminoethyl ferrocenyl sulfides 42a−c, forming 3substituted ferroceno[f ][1,4]thiazepines 44 and 45 in 40−75% de.34 Only in the case of i-Pr-substituted 42c was the formation of the desired N,N-dimethylated β-aminoethyl ferrocenyl sulfide 43c observed (Scheme 12).34

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a Legend: (i) (1) PCl3, (2) SnCl4, (3) H2O, (4) DCC, MeOH; (ii) (1) Et3SiH, TFA, (2) LiOH, (3) TFAA, (4) Et3SiH, TFA.

ORTHO-DIRECTED METALATION Metalation, in particular lithiation, is undoubtedly the most frequently applied methodology for the derivatization of the ferrocene backbone. Whereas dilithiations can easily be performed in n-hexane with n-butyllithium in the presence of tmeda (tmeda = N,N,N′,N′-tetramethylethylenediamine),35 selective monolithiations are more tedious. There exist excellent synthesis protocols to exclusively access monolithioferrocene.36 Planar-chiral ferrocenes may be obtained if mono- or 1,1′disubstituted ferrocenes are treated with strong bases. If isopropylferrocene (46), for example, is reacted with n-BuLi in diethyl ether and subsequently functionalized by addition of trimethylchlorosilane, a mixture of the four differently substituted ferrocenes 47−50 is obtained (Scheme 13).37 At a 1:1 molar ratio of 46 to n-BuLi mono- and dilithiation occurs almost equally (60:40). Deprotonation occurs in the order 1′ > 3 ≫ 2 due to (i) the higher number of protons at the unsubstituted cyclopentadienyl ring and (ii) the preference of lithiation in a position β to the isopropyl substituent for minimization of steric interactions. The product ratio changes dramatically if an ortho-directing group (ODG) is present.38 This was shown for the first time by Benkeser et al. in 1961 with the example of diphenylferrocenylcarbinol (51) (Scheme 14).39 If 51 is deprotonated with an excess of n-BuLi in diethyl ether at ambient temperature and subsequently treated with dry ice or methyl iodide, solely 1,2disubstituted ferrocenes rac-52a,b are obtained (Scheme 14). Lithiation at the unsubstituted cyclopentadienyl or at one of the phenyl rings has not been observed. Shortly afterward Slocum et al. introduced with N,Ndimethylaminomethylferrocene (53) another ODG,40 which has from then on repeatedly been used for the synthesis of ferrocene-based molecules.41 In diethyl ether/n-hexane mixtures ferrocene 53 gives the 1,2-disubstituted derivative rac-54.40 For very long reaction times or in the presence of an excess of n-BuLi the 1,2,1′- or 1,1′-substituted ferrocenes rac-55 and 56 are formed in trace amounts. If the solvent is changed to a thf/nhexane mixture, the lithiation of 53 is less selective, providing mixtures of 54−56 with 1,2,1′-substituted 55 as the major

Reduction of the keto unit in 33 and hydrolysis of the ester functionality furnishes a carboxylic acid, which upon treatment with TFA gives in excellent regio- and diastereoselectivity after ketone reduction the [3](1,1′)[3](3,3′)-ferrocenophane 34 (Scheme 9). In addition to carboxylic acids or derivatives thereof, ferrocenylamines are other suitable starting materials for the synthesis of planar-chiral ferrocenes, as demonstrated in 1958 for the first time.32 Lednicer et al. intended to transform 2ferrocenylethylamine (35) into the corresponding dimethylated tertiary amine applying the conditions of the Eschweiler−Clark modification of the Leuckart−Wallach reaction, whereby homoannular bridged rac-36 was obtained (Scheme 10).32 Scheme 10. Pictet−Spengler Reaction of 2Ferrocenylethylamine and Formaldehyde32

Upon transformation of rac-36 into its methiodide and subsequent β elimination in the presence of potassium amide, 1,2-disubstituted vinylferrocene rac-37 was isolated, proving the identity of heterocycle 36 (Scheme 10).32 The formation of rac36 can be understood by considering that a Pictet−Spengler reaction took place. The intermediately formed N-methylene derivative 38 is not reduced by formic acid but protonated. The iminium species electrophilically attacks the ferrocene backbone, D


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Scheme 12. Synthesis of Planar-Chiral 3-Substituted 2,3,4,5-Tetrahydroferroceno[f ][1,4]thiazepines34

formation of heterocycle rac-36 by transformation of the β elimination product rac-37 into the corresponding alcohol- and nitrile-functionalized ferrocenes (Scheme 10).32 If 53 or rac-54 is treated with methyl iodide, trimethylamine can be replaced by a nucleophile, e.g. CN−, according to an SN1 mechanism (Scheme 16). The intermediately formed α ferrocenyl carbocations have been examined in detail, and their stability has been ascribed to direct Fe−Cα interactions.44,45

Scheme 13. Lithiation and Subsequent Silylation of Isopropylferrocenea,37

Scheme 16. Synthesis of Nitrile rac-57 by a Nucleophilic Substitution Reaction40

a Legend: (i) first n-BuLi, Et2O, ambient temperature, 30 h, (2) ClSiMe3, 0 °C.

Scheme 14. Ortho-Directed Lithiation of Diphenylferrocenylcarbinol (51)a,39

Weissensteiner and Widhalm utilized such a nucleophilic substitution reaction for the resolution of racemic 2-N,Ndimethylaminomethyl-1-haloferrocenes 58 (Scheme 17).46 Ferrocene 53 was converted into rac-58 via ortho lithiation and subsequent halogenation. Quaternization of the nitrogen atom with methyl iodide and treatment with (1R,2S)-ephedrine yielded a 1:1 mixture of diastereomeric ferrocenes 59 and 60, which could be separated by column chromatography.46 Transformation into the methiodide and reaction with dimethylamine gave enantiomerically pure (Sp)- and (Rp)-58 (Scheme 17).47 The concept of ortho-directed metalation has been extended to other functional groups, as summarized in Chart 1.48−61 Depending on the ODG, different metalation protocols have to be applied with a variation of base (e.g., LDA, n-BuLi, sec-BuLi, tBuLi, if necessary, while adding tmeda or KO-t-Bu), solvent (nhexane, Et2O, thf), and reaction temperature (−78 °C to reflux). The 1,1′-disubstituted ferrocenes 62b and 64b, respectively, were dilithiated and then treated with ClPPh2 to give in both cases the appropriate chiral 1,1′,2,2′-tetrasubstituted products in ca. 60% yield. The reaction is highly diastereoselective, as only for 62b was the formation of the meso compounds (3%)

a Legend: (i) (1) n-BuLi, Et2O, ambient temperature, 36 h, (2) CO2, HCl (a), MeI (b).

product (Scheme 15). Shortly thereafter Rockett and co-workers showed that a replacement of the NMe2 unit in 53 for pyrrolidine or piperidine does not affect the selective lithiation in an ortho position.42 The broad application of 53 in ferrocene chemistry cannot only be attributed to its selective ortho lithiation but is also due to the straightforwardness of a nucleophilic displacement of the NMe2 moiety. This reactivity has been recognized as early as 195743 and has, for instance, been used to indirectly prove the

Scheme 15. Ortho-Directed Lithiation of N,N-Dimethylaminomethylferrocene (53)40

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iation at the cyclopentadienyl and the phenyl ring as well as 2fold lithiation of the second cyclopentadienyl (Scheme 18), as evinced from NMR spectroscopy and single-crystal X-ray diffraction.60 Nevertheless, this unusual lithiation behavior, which is not unprecendented in the literature,62 does not represent a restriction for the application of aryl ethers as ODGs. Lang and Schaarschmidt have shown that the desired mono- and diphosphines rac-71 and rac-72 can be synthesized in analytically pure form in good yields by adjusting the n-BuLi:69 ratio.60 The metalation of ferrocenes with bimetal bases (“ate” bases, e. g., TMPMgCl·LiCl, (TMP)3CdLi, or (TMP)2Zn·2LiCl (TMP = 2,2,6,6-tetramethylpiperid-1-yl)) has gained considerable interest in the last few years. This methodology has been applied for the mono- and polymetalation of unsubstituted ferrocene;63 however, it offers great advantages over alkyllithium bases with regard to the compatibility with sensitive functional groups, such as esters and nitriles.64 Knochel and co-workers used this methodology for the transformation of ferrocenyl carboxylic acid derivatives 65 into 1,2-disubstituted rac-294 by metalation with TMPMgCl·LiCl and reaction with an electrophile (Scheme 19).65 Repeated ortho-directed metalation allowed for the synthesis of 1,2,3-trisubstituted as well as 1,2,3,4-tetrasubstituted derivatives. In 2010 Krishna and Mongin applied an in situ generated mixture of Zn(TMP)2 and LiTMP for the orthodirected zincation of 65, accessing iodoferrocenes rac-294 in excellent yields (Scheme 19).66 In addition, C,C cross-coupling reactions could successfully be performed at zincated 65 in the presence of catalytic amounts of (dppf)PdCl2. The metalation of ester 65f with the mixed lithium−cadmium base (TMP)3CdLi and subsequent reaction with iodine afforded depending on the ferrocene:base ratio rac-294f and 295, demonstrating the facile dideprotonation of substituted ferrocenes by bimetal bases (Scheme 20).66 Diastereoselective Ortho-Directed Metalation. Ugi’s Amine and Related Compounds. Ortho-directed metalations at chiral ferrocenes give as a matter of principle diasteromeric products, which may be separated by chromatography or crystallization. If the source of chirality is in proximity to the ferrocene backbone, deprotonation may occur in high

Scheme 17. Synthesis and Resolution of 2-N,NDimethylaminomethyl-1-haloferrocenes 5846

observed.50,59 Another diastereoselective transformation has been reported by Ito and co-workers starting from 61.49 Orthodirected metalation, iodination, and subsquent homocoupling yielded exclusively (Rp,Rp)- and (Sp,Sp)-biferrocenes, which could be resolved by (+)- and (−)-dibenzoyltartaric acid. Breit synthesized rac-2-diphenylphosphinoferrocene carboxylic acid starting from 65a and resolved both enantiomers by employing the diacetonide of D-glucose.54 The vast majority of the ferrocenes depicted in Chart 1 furnish 1,2-substituted ferrocenes in reasonable yields and high regioselectivity. However, for 64a, 66, and 69 the formation of a 1,2,1′-substituted ferrocene is always observed.53,56,60 Moreover, 69 can under specific conditions be converted into the four different phosphines rac-71−74 involving ortho-directed lith-

Chart 1. Mono- and Disubstituted Ferrocenes Applied in Ortho-Directed Metalations48−61

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Scheme 18. Lithiation and Subsequent Phosphanylation of 6960

Upon treatment with n-BuLi and subsequent reaction with dry ice, amino acid (Rp)-76a was obtained, which gave (Rp)-methyl2-methylferrocene carboxylate (76b) in 24% yield and 94% optical purity after quaternization with methyl iodide, reduction with sodium amalgam in water, and esterification with diazomethane (Scheme 21). Only shortly thereafter did Ugi and coworkers raise doubts regarding the diastereoselectivity of the lithiation of 75, as in their hands only 67% optical purity had been achieved.68 At the same time Ugi and co-workers introduced (R)- and (S)N,N-dimethyl-1-ferrocenylethylamine (77, commonly known as Ugi’s amine) as the starting material for diastereoselective orthodirected metalations, which proved to be a significant breakthrough in ferrocene chemistry (Scheme 22).69,70 Resolution of (±)-77 with (R)-(+)-tartaric acid furnishes both enantiomers in 80−90% yield. Ferrocene 77 is readily deprotonated by n-BuLi in diethyl ether at 27 °C, forming after treatment with an electrophile (R,Sp)-78 or (S,Rp)-78 in 92% de (Scheme 22). The diastereomer formed to minor extent can be removed by either column chromatography or crystallization. The stereochemical course of the ortho-directed metalation has unambiguously been proven by X-ray crystallography and is in accordance with a transition state, minimizing the steric interaction between the C-methyl group and the ferrocene backbone.71,72 Not unexpectedly, (S)-77 can be lithiated at both cyclopentadienyl rings in the presence of tmeda, thus giving 1,2,1′trisubstituted ferrocenes with no interference in the diastereoselectivity.73,74 Likewise, lithiation of 1,1′-disubstituted (R,R)81 yields upon reaction with ClPPh2 (R,R,Sp,Sp)-82 as a single diastereomer (Scheme 23).75−77 As already outlined for 53 and related compounds,32 the NMe2 moiety in α-aminomethylferrocenes can straightforwardly be displaced by a nucleophile upon quaternization (Schemes 16 and 17). Consequently, this is equally applicable to Ugi’s amine and derived planar-chiral ferrocenes; however, due to the presence of an α-methyl substituent in 77 a stereochemical feature is added. It has been shown that the nucleophilic displacement of the NMe2 unit proceeds with full retention of configuration. This can be rationalized by considering that in the course of the substitution reaction the intermediately formed carbocations

Scheme 19. Ortho Functionalization of Ferrocenyl Carboxylic Acid Derivatives by Bimetal Basesa,65,66

a

E = I, allyl, C(O)Ph, CO2Et, CO2-t-Bu, ...

Scheme 20. Mono- and Dideprotonation of Ester 65f66

diastereoselectivity, forming, at best, only one diastereomer. Thus, such a ortho-directed metalation makes a resolution after the synthesis of the 1,2-disubstituted ferrocene, as demonstrated for rac-58 (Scheme 17), redundant.46 However, it has to be kept in mind that the synthesis of the starting material of a diastereoselective ortho-directed metalation possibly requires a resolution if it cannot be synthesized by a catalytic enantioselective transformation or if the use of a chiral pool reagent is not possible. In 1969 Aratani et al. reported for the first time on a diastereoselective ortho metalation with the example of (S)-1ferrocenylmethyl-2-methylpiperidine (75), which is accessible by the reaction of the methiodide of N,N-dimethylaminomethylferrocene (53) with (S)-2-methylpiperidine in 70% yield.67

Scheme 21. Diastereoselective Ortho-Directed Lithiation of (S)-1-Ferrocenylmethyl-2-methylpiperidine (75)67

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Scheme 22. Diastereoselective Ortho-Directed Lithiation of (R)-N,N-Dimethyl-1-ferrocenylethylamine (77)69,70

Scheme 23. Diastereoselective Ortho-Directed Lithiation of (S)-N,N-Dimethyl-1-ferrocenylethylamine (77) and (R,R)1,1′-Bis(1-N,N-dimethylaminoethyl)ferrocene (81)73−77

Chart 2. Chiral Ferrocenes with a NMe2 Moiety Applied in Diastereoselective Ortho-Directed Lithiations77,80−110

are stabilized by direct Fe−Cα interactions and hence the nucleophile can only attack exo to ferrocene. If weak nucleophiles (e.g., MeOH instead of NaOMe) which favor a long lifetime of the carbocation are applied, epimerization at the stereogenic carbon atom is observed. In the absence of a nucleophile the methiodides yield vinylferrocenes 84 (Scheme 24).45,69,78 In addition, Knochel and co-workers developed a protocol for the functionalization at the α-carbon atom with electrophiles upon reductive lithiation with lithium naphthalenide.79 In the last few decades it has been found that Ugi’s approach is applicable to related substrates, revealing almost the same stereoselectivities regarding ortho-directed lithiation and nucleophilic substitution of the amine moiety. This includes utilization of N,N-dimethyl-1-ferrocenylalkylamines and -benzylamines (85),77,80−96 homoannularly bridged N,N-α-dimethylamino1,2-tetramethylene ferrocenes (86),97−101 heteroannularly bridged [3] (87,100,102−105 88106,107)- and [5]ferrocenophanes (89)108,109 as well as bisferrocenyldiamines (90)110 (Chart 2). The enantiopure starting materials 85−90 have been made

available among others by resolution,97,102,106 asymmetric reductions of ferrocenyl ketones77,80,82,94,96,108,111,112 and imines,105,113 asymmetric alkylation/arylation of ferrocenealdehydes,114,115 and the use of enantiomerically pure cyclopentadienes or cyclopentadienyl ligands.88,99,116,117 It should be noted that ferrocenophanes 87−89 exhibit a reversed selectivity toward ortho-directed lithiation in comparison to Ugi’s amine. Whereas (S)-77 yields upon lithiation and subsequent phosphanylation (S,Rp)-78 as the major product, bridged (S)-87 gives, due to the fixed conformation, exclusively (S,Sp)-91 on applying the same reaction conditions (Scheme 25).102 As a matter of fact, dilithiation of 87−89 results in the formation of two chiral planes. Due to the absence of an ODG in proximity, lithiation of the CH2-bounded C5H4 moiety is less regioselective as well as stereoselective, resulting in the formation of four isomeric diphosphines 92−95 (Scheme 25).118 In the case of benzylferrocenes 85 (e.g., R = Ph) a deviating behavior regarding ortho-directed metalation as well as the nucleophilic substitution reaction at the α-carbon atom has been

Scheme 24. Nucleophilic Substitution Reactions at Planar-Chiral Ferrocenes Derived from Ugi’s Amine45,69,78

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Scheme 25. Comparison of the Ortho-Directed Metalation of Ugi’s Amine (S)-77 and Its [3]Ferrocenophane Analogue (S)87102,118

Scheme 27. Ortho-Directed Metalation of α-MethoxySubstituted Ethyl-121 and Benzylferrocene82,122

observed. Ferrocene (R)-85a gives upon treatment with t-BuLi and subsequent reaction with an electrophile ferrocene (R,Sp)96, whereas bromo-substituted (R)-85b affords (R,Rp)-97 (Scheme 26).119 The reversion of configuration of the chiral Scheme 26. Ortho-Directed Metalation of (R)-85a and (R)85b, Respectively119

decreases the diastereoselectivity of the ortho-directed lithiation. Kumada and co-workers observed with this substrate the formation of two diastereomeric 1,2-substituted ferrocenes in a 1:1 ratio.124 Ugi’s amine has been applied as the starting material in asymmetric synthesis in dozens of publications. There are at least three reasons for this remarkable success story: (i) both enantiomers of 77 are readily accessible, (ii) the ortho ithiation proceeds highly diastereoselectively, and (iii) the ODG can be substituted in a straightforward manner essentially without epimerization. 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,73−76,83−87,89−93,125−171 alcoholes and thioles,172,173 aldehydes,174−176 carboxylic acids and derivatives thereof (e.g., imines, oxazolines, esters, or amides),177−185 thio- and selenoethers,133,143,146,147,186−190 and dithiocarbamates.191 Furthermore, transition-metal-mediated homo- and cross-coupling reactions have been performed with derived 2-halo- and 2-lithioferrocenes, furnishing aryl- and alkynyl-substituted ferrocenes192−194 as well as biferrocenes.195−198 In addition, diferrocenes are accessible with a dichalcogenide bridge upon treatment with chalcogenides and subsequent oxidation.94,140,199−203 The introduction of a nucleophile at the α carbon is usually achieved by starting from the corresponding acetate, which is quantitatively accessible upon treatment of the tertiary amine with excess acetic anhydride or, more conveniently, directly from

plane is due to Fe−Li interactions as well as a dimeric arrangement of the organolithium fragments, which is supported by DFT calculations.119 Regarding the nucleophilic substitution reaction, it has been found that for benzylferrocenes such as 96 and 97, depending on the configuration of the α-carbon atom and the chiral plane as well as the presence of an ortho substituent at the ferrocene and/or the benzene moiety the displacement of the NMe2 group proceeds with either full retention or full inversion of configuration rather than partial epimerization.96,120 If the NMe2 functionality in (R)-77 is replaced by OMe, directed metalation proceeds much less regio- and stereoselectively, leading to a mixture of 1,2-, 1,3-, and 1,1′disubstituted ferrocenes in ratios depending on the solvent and reaction time, as shown by Ugi and co-workers.121 The asymmetric induction was estimated to be only about 10% of that of the corresponding amine.121 In contrast, the group of Knochel has shown that methoxy-directed ortho lithiation at αmethoxybenzylferrocenes, such as (R)-101, results almost exclusively in the formation of (R,Sp)-102 (Scheme 27).82,122 The same holds true for the corresponding alcohols, as demonstrated by Ueberbacher et al.123 The introduction of a methylene spacer between ferrocene and CHMeNMe2 in (R)-77 I


Organometallics

Review

the amine in glacial acetic acid. In this way, the NMe2 moiety has been replaced by ammonia and primary and secondary amines74,128,129,131,134,135,137,148,155,158,197,204,205 as well as phosphines,74,138,147,157,172,181,183,184,193,196,198,206 including heterocyclic species,87,128,129,140,141,146,154,179,184,185,190,207−210 azides for 1,3-dipolar cycloadditions,166 carbon nucleophiles,84,85,143 alcohols, 74,76 and thioles 147,161,211 as well as carboxylates125,130,142,177 and thiocarboxylates,132 giving upon hydrolysis the corresponding alcohols or thioles. Primary and secondary amines obtained can be transformed into imines,89,91−93,139,147,151,168,188 amidines,90,149 and amides152 upon treatment with carbaldehydes/ketones,212 formamides, or acid chlorides/anhydrides. Primary and secondary amines as well as alcohols give with chlorophosphines phosphoramidites,150,153,156,160 phosphites,153 phosphinamidites,145,159,189 and phosphinites.153 Twofold substitution with one nucleophile produces diferrocenes which are bridged through the α-carbon atoms.140,213,214 Elimination of the NMe2 entity can be achieved either by quaternization with methyl iodide in the absence of a nucleophile or by treatment with a chloroformate, giving only planar-chiral alkenylferrocenes or, upon hydrogenation, the corresponding alkylferrocenes.73,74,85,130,141,163,174,175,182,215,216 Excellent overviews on ligands for asymmetric catalysis based on Ugi’s approach, including but not limited to the Josiphos, BoPhoz, Walphos, Taniaphos, Ferriphos/Mandyphos and TRAP ligand families, can be found elsewhere (Chart 3).217

The synthesis of 1,2,1′-trisubstituted ferrocenes starting from Ugi’s amine is a well-established procedure in synthetic organometallic chemistry, as outlined earlier. However, the approach is almost exclusively realized by reacting 2-fold lithiated 77 with 2 equiv of one electrophile. It may be advantageous at least for some applications, e.g. the covalent attachment of ferrocenylphosphines to any support bridging the gap between homogeneous and heterogeneous catalysis, to introduce two different electrophiles at the heretofore mono- and unsubstituted cyclopentadienyl rings. In the 1980s Butler and Cullen showed that starting from Ugi’s amine ansa-ferrocene 103 is accessible by a 2-fold lithiation and subsequent treatment with Cl2PPh (Scheme 28). Ring opening with phenyllithium and consecutive hydrolysis afforded the two isomeric ferrocenylphosphines 106 and 78 in a ratio of 85:15 (Scheme 28).218 Introduction of a second electrophile instead of hydrolysis would provide the desired 1,2,1′-heterotrisubstituted ferrocenes. Another approach was presented by Togni and co-workers in the late 1990s starting from dibromo-substituted 107.219 Selective metal−halogen exchange with n-BuLi in n-hexane and subsequent phopshanylation resulted in the formation of 108 in 58% yield in addition to recovering 24% unreacted starting material (Scheme 28). The second bromine substituent can be exchanged for various electrophiles, which has been used for the immobilization of Josiphos ligands on polymers, dendrimers, and inorganic materials as well as the synthesis of ferrocenes soluble in water and ionic liquids, respectively.219 Both reactivities, the selective ring opening of 103 as well as the selective metal−halogenexchange in 107, can be explained by a higher thermodynamic stability of a species with the lithium atom being in proximity to the stabilizing NMe2 moiety: i.e., the increased stability of 104 in comparison to 105. Acetals and Aminals. As a matter of fact, ferrocenecarboxaldehyde cannot be applied in an ortho-directed lithiation without prior transformation into an acetal or hemiaminal. Essentially, this does not represent a general limitation, as aldehydes can usually be protected as well as deprotected in almost quantitative yields. If chiral alcohols or amines are used for this purpose, the resulting acetals or hemiaminals may be ortho-functionalized diastereoselectively, providing after aqueous/acidic workup only planar-chiral ferrocenecarboxaldehydes. This approach was realized by Kagan and co-workers in 1993.220 Ferrocenecarboxaldehyde (109) was converted into dimethylacetal 110; transacetalization with (S)-1,2,4-butanetriol gave 1,3-

Chart 3. Josiphos, BoPhoz, Walphos, Taniaphos, Ferriphos/ Mandyphos, and TRAP Ligand Familiesa217

a

R, R′, R″, R‴ = singly bonded organic group.

Scheme 28. Selective Heterodifunctionalization of Ugi’s Amine218,219

J


Organometallics

Review

Scheme 29. Synthesis of the Enantiopure Acetal (S,S)-112a,220

a

p-TsOH = 4-toluenesulfonic acid.

Scheme 30. Diastereoselective Ortho-Directed Lithiation of (S,S)-112 and the Pentamethyl Derivative (S,S)-115a,220,223

a

E = SiMe3, PPh2, Sn(n-Bu)3, I, ...

Scheme 31. Ortho-Directed Lithiation of (S,S,S,S)-118224

exclusively (S,S,S,S,Sp,Rp)-121, which gives upon acetal cleavage an achiral meso-aldehyde (Scheme 31).224 More than one decade after the introduction of chiral acetal (S,S)-112 into asymmetric ferrocene chemistry, Zhang and Ikeda presented 2-ferrocenyl-(S,S)-bis(methoxymethyl)-1,3-dioxolane (122), an enantiopure acetal derived from L-tartaric acid.225 Surprisingly, treatment with n-, sec-, or t-BuLi did not result in an ortho lithiation but yielded varying amounts of carbinol 123 (up to 54% ee) and the β-elimination product 124 (Scheme 32). Obviously, a six-membered cyclic acetal is required for an adequate coordination of lithium in an ortho position.226

dioxane (S,S)-111 in 85% yield along with other isomeric acetals, which could be separated by recrystallization from toluene (Scheme 29). Subsequent O-methylation gave (S,S)-112 in quantitative yield. If (S)-1,2,4-butanetriol is converted into (S)-4methoxybutane-1,3-diol prior to transacetalization with 110, no isomeric acetals are formed, resulting in a slight increase of the overall yield to 92%.221 Deprotonation of (S,S)-112 proceeds with t-BuLi highly regioand diastereoselectively (de > 98%), yielding after treatment with an electrophile (S,S,Sp)-113 (Scheme 30).220 In constrat to 2ferrocenyl-1,3-dioxane 64a no competitive lithiation at the unsubstutited cyclopentadienyl ring occurs,53 indicating that the pendant methoxymethyl moiety in 112 significantly enhances the ortho-directing effect. Hydrolysis of the acetal in the presence of p-TsOH gives almost enantiomerically pure solely planarchiral aldehydes (Sp)-114.220 The chiral auxiliary cleaved in this course can be recovered as (S)-4-methoxybutane-1,3-diol and hence can be reused for transacetalization with 110.222 The diastereoselectivity of the ortho-directed metalation is not affected by the presence of substituents at the heretofore unsubstituted cyclopentadienyl ring such as in pentamethylferrocene (S,S)-115 (Scheme 30).221,223 The situation was quite different if chiral ferrocenyl bis-acetal (S,S,S,S)-118 was used as starting material for an ortho-directed metalation (Scheme 31).224 Methyllithium gave the best results for the monolithiation of the 1,1′-disubstituted ferrocene (up to 69% yield), however, only poor diastereoselectivities could be achieved (de < 36%). Interestingly (S,S,S,S,Rp)-119 is formed in excess. Dilithiation occurs to a lesser extent (8% yield), providing

Scheme 32. Reaction of 2-Ferrocenyl-(S,S)bis(methoxymethyl)-1,3-dioxolane (122) with Butyllithium To Give 123 and 124, Respectively225

For the application of planar-chiral formylferrocenes and compounds derived thereof to asymmetric catalysis it may be advantageous to access both enantiomers. Acetals derived from (S)-1,2,4-butanetriol give Sp-configured ferrocenes (Scheme 30). Consequently, the use of (R)-1,2,4-butanetriol allowed for the isolation of the other enantiomer with the same degree of stereoselectivity, which was experimentally confirmed reK


Organometallics

Review

Scheme 33. Synthesis of (Rp)-114 and (Sp)- and (Rp)-126 Starting from (S,S)-112a,228,229

a

Conditions: (a) (1) IPPh3C7H15, KO-t-Bu, (2) p-TsOH; (b) (1) p-TsOH, (2) IPPh3C7H15, KO-t-Bu, (3) Li[AlH4], (4) MnO2.

cently.227 Even though both enantiomers of 1,2,4-butanetriol are supplied commercially, the R enantiomer is less available. Probably for this reason, other routes were developed to access Rp-configured formylferrocenes. A common methodology, which is not limited to Kagan’s approach but may be applied to all stereoselective ortho-directing functionalizations, characterized by a high degree of regio- and stereoselectivity, is the socalled “silyl” or “silicon trick”. SiMe3-substituted (S,S,Sp)-113a, obtained by the reaction sequence shown in Scheme 30, can be ortho-deprotonated with t-BuLi; subsequent reaction with an electrophile results in the formation of 1,2,3-tribsubstituted 125 (Scheme 33). Cleavage of the acetal as well as the SiMe3 group is possible by hydrolysis in the presence of acid and fluoride, thus yielding (Rp)-114 (Scheme 33).228 Recently, Schmalz and coworkers presented another smart approach, leading to both enantiomers of 2-(oct-1-en-1-yl)formylferrocene starting from (S,S)-112.229 Ferrocene (Rp)-126 was obtained in consecutive reactions by ortho-directed lithiation of acetal (S,S)-112, introduction of a formyl functionality by treatment with DMF, Wittig olefination, and subsequent acetal cleavage. The synthesis of the Sp enantiomer started by introduction of a methoxycarbonyl substituent ortho to the acetal, which was subsequently hydrolyzed to the aldehyde and gave upon Wittig olefination the desired alkenylferrocene. Reduction of the ester and subsequent oxidation with MnO 2 produced (S p )-2-(oct-1-en-1-yl)formylferrocene (126) (Scheme 33). The synthesis of 1,2,1′-heterotrisubstituted ferrocenes is likewise possible. This can be readily achieved by starting from 1′-substituted formylferrocene according to Kagan’s approach, if the functionality is stable under the reaction conditions applied for ortho-directed metalation. A methodology utilizing 1,2disubstituted formylferrocenes 114, giving rise to a wider range of functionalities to be introduced at the second cyclopentadienyl ring, was presented by Manoury and Balavoine, originally developed for the 1′-functionalization of formylferrocene.230 Ferrocene 114 was treated with lithium N-methylpiperazide, generated in situ by deprotonating N-methylpiperazine with tBuLi, forming the hemiaminal anion as a temporary protecting/ directing group. Further addition of t-BuLi resulted in lithiation at the 1′-position, giving after reaction with an electrophile and cleavage of the hemiaminal by hydrolysis 1,2,1′-trisubstituted

127 (Scheme 34).231 The regioselectivity of the deprotonation in 1′- vs 2-position is >98:2. In the case of formylferrocene typically Scheme 34. Synthesis of Planar-Chiral 1,2,1′Heterotrisubstituted Ferrocenes230

a ratio of 90:10 is achieved. The presence of an ortho substituent obviously enhances the regioselectivity, most likely due to steric hindrance.230 Chiral hemiaminal anions facilitate the diastereoselective synthesis of 1,2,1′-tri- and 1,2,1′,2′-tetrasubstituted diformylferrocenes, which is, as pointed out earlier, a challenge that had not yet been overcome using chiral acetals. 1,1′-Diformylferrocene (128) and the lithium salt of (S)-1-(2-pyrrolidinylmethyl)pyrrolidine form a chiral hemiaminal dianion, which may be deprotonated with t-BuLi once or twice. Subsequent treatment with an electrophile and hydrolysis of the hemiaminal provides (Rp)-129 and (Rp,Rp)-130, respectively (Scheme 35).232 The yields are at best ≤30%, due to partial decomposition of ferrocenes under the reaction conditions applied. Trisubstituted 129 as well as tetrasubstituted 130 can, however, be isolated almost enantiomerically pure (up to 99% ee).232 Ferrocene aminal (R,R)-131, obtained by the reaction of ferrocenecarbaldehyde and (R,R)-2,2′-bipyrrolidine in 70% yield, was applied in a diastereoselective ortho-directed metalation by Alexakis et al. Lithiation with sec-BuLi and subsequent treatment with 1,2-diiodoethane gave after hydrolysis in the presence of hydrochloric acid (Sp)-2-iodoferrocenecarbaldehyde (133) in 22% yield and 70% ee (Scheme 36).233 Since Kagan’s seminal work, several publications appeared dealing with the synthesis of enantiopure planar-chiral formylferrocenes carrying different substituents at the 2- and 1′-positions as well as the conversion of these molecules by different approaches, giving a plethora of chiral ferrocenes, which L


Organometallics

Review

Scheme 35. Synthesis of Planar-Chiral 1,2,1′-Tri- and 1,2,1′,2′-Tetrasubstituted Diformylferrocenes (Rp)-129 and (Rp,Rp)-130a232

a

E = Me, SiMe3, PPh2, ...

Ferrocenyl-4-tolyl sulfoxide (134) and its tert-butyl congener 136 (Scheme 37) have almost exclusively been used as starting materials in this field of chemistry. Both 134 and 136 are accessible as SS and RS enantiomers, respectively, and yield 1,2disubstituted ferrocenes with excellent diastereoselectivity (de ≥ 96%).243 However, both ferrocenes differ considerably in the applicable bases for ortho-directed lithiation. Whereas tert-butylsubstituted 136 can be selectively deprotonated with n-BuLi, tBuLi, or (2,4,6-triisopropylphenyl)lithium, compound 134 requires LDA or sterically hindered (2,4,6-triisopropylphenyl)lithium,244 as n-BuLi gives a mixture of products and t-BuLi leads to the formation of tert-butyl 4-tolyl sulfoxide due to nucleophilic attack at sulfur.236,241,245 Sulfoxides such as 138, bearing protons α to sulfur, do not give the appropriate 1,2-disubstituted ferrocenes upon treatment with strong bases, even if the base is used in excess (Scheme 38).238

Scheme 36. Synthesis of (Sp)-2-Iodoferrocenecarbaldehyde (133) According to Alexakis et al.233

have been applied among others in asymmetric catalysis and material science. A concise review covering this field of chemistry can be found elsewhere.228 For other examples see ref 234. Sulfoxides. Chiral sulfoxides were the first non-carbon centralchiral ODGs applied in ferrocene chemistry. Already in 1991 Kagan reported on the asymmetric synthesis of (RS)-ferrocenyl4-tolyl sulfoxide (134) according to the so-called Andersen method by reacting monolithioferrocene with commercially available (RS,1S)-menthyl-4-tolylsulfinate, proceeding with inversion of configuration at sulfur.235 A short time later the diastereoselective ortho-directed lithiation of 134 with LDA at −78 °C was described to provide the 1,2-disubstituted ferrocenes 135 almost diastereomerically pure (de ≥ 98%) in yields ≥80% (Scheme 37).236

Scheme 38. Undesired Lithiation of 138 at the Carbon Atom α to Sulfur238

Scheme 37. Synthesis of Planar-Chiral 1,2-Substituted Ferrocenyl Sulfoxidesa,235,236 The stereochemical course of the reaction is not affected by a second lithiation at the unsubstituted cyclopentadienyl ring, e.g. for the synthesis of diphosphine (SS,Sp)-140 (Scheme 39),236 or the presence of inert substituents.246 Moreover, as already observed in the case of Ugi’s amine, 1,1′-bis(tert-butylsulfinyl)ferrocene (141) gives upon dilithiation and subsequent phosphanylation (RS,RS,Rp,Rp)-142 diastereoselectively in reasonable yields (Scheme 39).247 Scheme 39. Dilithiation of (SS)-136 and (RS,RS)-141236,247

a

E = Me, SiMe3, PPh2, Sn(n-Bu)3, ...

Since then, other enantiopure ferrocenyl sulfoxides have been accessed by the reaction of monolithioferrocene with sulfinates236−238 and thiosulfinates239 carrying other chiral nucleofuges (e.g., diols or carbohydrate derivatives) as well as the enantioselective oxidation of ferrocenyl sulfides.240,241 A general overview on the asymmetric synthesis of chiral sulfoxides can be found elsewhere.242 M


Organometallics

Review

cenes are the nucleofuges in these reactions, which give upon treatment with a wide range of electrophiles only planar-chiral ferrocenes 147, typically with >99.5% ee (Scheme 42).243,251,252

The sulfur atoms in 135 and 137 can be oxidized to give sulfones 143 by treatment with peroxy acids236,241,247,248 or reduced (e.g., by addition of NaI/TFAA, HSiCl3, or Li[AlH4]) to give sulfides 144239,241,245,246,248−250 with the chiral plane as the sole source of chirality (Scheme 40).

Scheme 42. Transformation of 1,2-Disubstituted Ferrocenyl Sulfoxides into Only Planar-Chiral Ferrocenes by Sulfoxide− Lithium Exchangea,243,251,252

Scheme 40. Transformation of 1,2-Disubstituted Ferrocenyl Sulfoxides into Only Planar-Chiral Sulfides and Sulfonesa,236,239,241,245−250

a a

E = Me, SiMe3, PPh2, Sn(n-Bu)3, ...

E = Me, SiMe3, PPh2, Sn(n-Bu)3, ...; E′ = CHO, CO2Me, PCy2, ...

As the sulfinyl moiety is completely removed, it is commonly referred as traceless ODG. This approach can furthermore be exerted for the synthesis of 1,3-substituted ferrocenes or for the resolution of planar-chiral ferrocenes.253 Some examples of planar-chiral ferrocenes accessible by the sulfoxide route are depicted in Chart 4.249,251,254−257 Reviews covering the synthesis and application of sulfur-containing chiral ferrocenes can be found elsewhere.243,258 For other examples see refs 259 and 260.

The sulfinyl moiety differs in comparison to other ODGs in two aspects. To the best of our knowledge, there is no example in the literature describing the conversion of 135 or 137, synthesized by ortho-directed lithiation, into a homoannular trisubstituted ferrocene bearing two substituent ortho to the sulfinyl group, which is, however, a common feature of other ODGs, as demonstrated earlier by the example of (S,S)-112 (Scheme 33). Probably, the conformation necessary to stabilize a lithium at C-5 in 135 or 137 is energetically too unfavored, as in this case the sterically demanding t-Bu or p-Tol group interferes with the ferrocene moiety (Scheme 41). This can only be avoided

Chart 4. 1,2-Substituted Planar-Chiral Ferrocenes Synthesized According to the Sulfoxide Route249,251,254−257

Scheme 41. Synthesis of a Trisubstituted Ferrocene and Inversion of Configuration at Sulfur by Reduction and Subsequent Reoxidation Starting from 137a,236,241

Oxazolines. Oxazoline-containing ligands play an important role in asymmetric catalysis.261 The first report on 2-ferrocenyl2-oxazolines appeared in 1982. Schmitt et al. synthesized 5substituted 2-ferrocenyl-2-oxazoline 149 by acid-catalyzed rearrangement of 2-substituted 1-ferrocenoyl aziridines, obtained by condensation of ferrocenoyl chloride and the respective aziridine. Compound 149 could be regioselectively lithiated at the ortho position to produce after subsequent reaction with an electrophile racemic, planar-chiral ferrocene 150 in about 70% yield (Scheme 43).262 The asymmetric variant of the reaction described before was not explored until 1995. At that time Richards, Sammakia, and Uemura independently reported on diastereoselective orthodirected lithiations of 4-substituted 2-ferrocenyl-2-oxazolines.263−265 These oxazolines are typically synthesized either by condensation of cyanoferrocene and chiral β-amino alcohols in the presence of ZnCl2265,266 or by a two-step synthetic process involving the preparation of N-(2-hydroxyethyl)ferrocenoyl amides, e.g. accessible through Weinreb transamidation or by the reaction of ferrocenoyl chloride with β-amino alcohols, and subsequent cyclization under Appel conditions or by tosylation or mesylation of the alcohol functionality (Scheme 44).263,267−273 1,1′-Bis(oxazolinyl)ferrocenes can be prepared likewise starting from 1,1′-disubstituted ferrocenes.274−276 Both the S and R enantiomers of ferrocene 151 are accessible, due to

a

Conditions: (i) magnesium monoperoxyphthalate; (ii) (1) n-BuLi, (2) ClSn(n-Bu3); (iii) diisobutylaluminium hydride (DIBAL-H), Li[AlH4]; (iv) m-chloroperoxybenzoic acid (m-CPBA).

by inversion of the configuration at sulfur or oxidation of sulfur to the respective sulfone. The applicability of the latter approach has been demonstrated by Kagan for the synthesis of (Sp)-145 (Scheme 41).236 Regarding an inversion of configuration at sulfur in 137 there is a report of Hua et al. reducing (RS,Rp)-137 to only planar-chiral 144, which gives upon oxidation with m-CPBA exclusively (SS,Rp)-146. Ferrocene 146, however, has not been subjected to a further ortho-directed lithiation (Scheme 41).241 The second important distinction from other ODGs is that ptolyl-substituted 135 is nucleophilically attacked at sulfur by tBuLi or phenyllithium.243,251,252 Substituted monolithioferroN


Organometallics

Review

Scheme 43. Synthesis of 2-Ferrocenyl-5,5-dimethyl-2-oxazoline and Its Ortho-Directed Lithiationa,262

a

E = Me, SiMe3.

Scheme 44. Synthesis of Enantiopure 4-Substituted 2-Ferrocenyl-2-oxazolinesa

a

Conditions: (i) PPh3, CCl4, NEt3; (ii) p-TsCl or MeSO2Cl, NEt3.

the availability of both enantiomers of the respective β-amino alcohols. The diastereoselectivity of the deprotonation of enantiopure 2-ferrocenyl-2-oxazolines 151 strongly depends on the applied base and the solvent as well as the substituent R at C-4.263−268 It has been found that exceptionally high selectivities (dr >500:1) can be achieved with n- or sec-BuLi/tmeda in n-hexane or diethyl ether as solvent.277 In course of the deprotonation the alkyllithium reagent is coordinated to the oxazoline nitrogen,278 favoring an anti rather than a syn orientation to the substituent at C-4 (Scheme 45). This equilibrium is affected by steric

hindrance; thus, sterically demanding alkyllithium reagents and sterically demanding substituents at the oxazoline increase the diastereoselectivity toward the formation of (S,Sp)-152 (Scheme 45).279 The highly diastereoselective lithiation of 2-ferrocenyl-2oxazolines, the availability of both (R)- and (S)-151, and the possibility to temporarily block one α-position of the ferrocene backbone with a silyl moiety (cf. “silyl” or “silicon trick”) make all four diastereomeric ferrocenes (S,Sp)- and (R,Rp)-152 and (S,Rp)- and (R,Sp)-153 selectively accessible.263,267,268,280−284 It is noteworthy that, to the best of our knowledge, there is no example in the literature that ferrocene 151 has been applied in a 2-fold lithiation to access 1,2,1′-trisubstituted ferrocenes in one step. Such ferrocenes have been described, yet they were synthesized by starting from 1,1′-disubstituted ferrocenes.285 Ortho-directed lithiation of 1,1′-bis(oxazolinyl)ferrocenes is in the case of both mono- and dilithiation less diastereoselective.274,275,286−289 The highest selectivity obtained so far was reported by Richards and co-workers for the dimethylation of (S,S)-154, giving (S,S,Rp,Rp)-155 and its epimer (S,S,Rp,Sp)-156 in a 10:1 ratio (Scheme 46).288 2-Oxazolines are protecting groups for carboxylic acids, and hence it should be possible to convert 2-ferrocenyl-2-oxazolines such as 152 into only planar-chiral ferrocenecarboxylic acids or derivatives thereof. This is typically achieved by applying the reaction conditions developed by Meyers.290 Ferrocene 152 gives ester 157 by treatment with TFA in the presence of water and subsequent acylation with acetic anhydride. The latter compound in turn yields carboxylic acid 158a upon hydrolysis or

Scheme 45. Diastereoselectivity of Ortho-Directed Lithiation of (S)-151a279

a

E = Me, SiMe3, PPh2, SePh, ...

Scheme 46. Diastereoselective Functionalization of (S,S)-154a,288

a

Conditions: (i) (1) sec-BuLi, Et2O, −78 °C; (2) MeI. O


Organometallics

Review

Scheme 47. Conversion of Central- and Planar-Chiral Ferrocene 152 into Only Planar-Chiral 158a

a

E = Me, SiMe3, PPh2, SePh, ...

Scheme 48. Synthesis of Planar-Chiral Ferrocenyl Ketones (Sp)-162 Applying Ender’s SAMP-Hydrazone Methoda,309

a

Conditions: (i) O3 or TiCl3 or SnCl2 or Cr(OAc)2 or VCl2; E = Me, SiMe3, PPh2, I, ...

Scheme 49. Synthesis of Central- and Planar-Chiral Ferrocenes 167 Applying Ender’s SAMP-Hydrazone Methoda,311

a

E = PPh2, SMe, Si-Pr; E′ = SMe, PPh2, P(i-Pr)2, SePh, ...

ester 158b upon transesterification (Scheme 47).281,288,291−293 The thus available only planar-chiral ferrocenes have been used for the preparation of other valuable ferrocenes and ferrocenecontaining molecules,294−296 including oxazolines without a stereogenic center at C-4282,297,298 and amides.299−302 Ito and co-workers chose another approach to access ferrocenecarboxylic acids, which was based on N-methylation and subsequent hydrolysis of the oxazolinium ion obtained.303,304 If the oxazolinium ion is reduced before hydrolysis, ferrocene aldehydes are formed, which has been demonstrated by Sammakia et al.264,305 Reviews covering the syntheses as well as applications of 2ferrocenyl-2-oxazolines, including ferrocenyloxazolinylphosphines (FOXAPs), in asymmetric catalysis can be found elsewhere.297,306 For other examples see ref 307. Hydrazones and Imines. A direct route to planar-chiral ferrocenyl ketones was presented by Enders et al. in 1997, applying his SAMP/RAMP hydrazone method.308 Hydrazone (S)-160, accessible by the reaction of ferrocenyl ketone 159 with SAMP/AlMe3 (SAMP = (S)-1-amino-2-methoxymethylpyrrolidine), can be diastereoselectively ortho-lithiated with n-BuLi (up to 98% de). Subsequent reaction with an electrophile and oxidative (O3) or reductive (TiCl3, SnCl2, Cr(OAc)2, or VCl2) cleavage of the hydrazone yielded only planar-chiral ferrocenyl ketones (Sp)-162 with up to 96% ee (Scheme 48).309 Likewise,

1,1′-bisbenzoylferrocene can be converted into 1,2,1′-tri- and 1,2,1′,2′-tetrasubstituted ferrocenes with the same degree of stereoselectivity. The latter ferrocenes are synthesized in two consecutive reaction sequences; thus, two different electrophiles can easily be introduced. 310 It is noteworthy that the diastereoselectivity of the lithiation of hydrazone (S)-160 (R = Ph) strongly depends on the reaction temperature. Whereas at −70 °C (S,Sp)-161 is obtained in 97% de, its Rp epimer is preferentially formed at 0 °C (14% de) and ambient temperature (47% de).309 Ferrocenyl ketones 163 bearing acidic protons at the αposition have been investigated as well.311 The corresponding hydrazones 164 are deprotonated at the side chain with LDA; intermediately formed aza enolates give upon reaction with an electrophile in up to ≥96% de ferrocenes 165 (Scheme 49). Subsequent treatment with t-BuLi resulted in the ortho-directed lithiation at the ferrocene backbone to furnish after subsequent addition of a second electrophile planar-chiral 166 with the same degree of stereoinduction as for the functionalization at the side chain. Cleavage of the hydrazone is achieved by highly diastereoselective reduction with catecholborane to the respective hydrazines, which can be transformed into centraland planar-chiral ferrocenes 167 (≥96% ee) by protonation in the presence of a hydride source (Scheme 49).311 Li[ClO4] is used in both metalation processes as additive in order to ensure a P


Organometallics

Review

Scheme 50. Synthesis of Planar-Chiral Diferrocenyl Ketones (Sp)-170a,312

a

Conditions: (i) O3 or TiCl3 or SnCl2 or MnO2; E = Me, SiMe3, PPh2, SMe, ...; E′ = Me, SiMe3, SMe, Si-Pr, PPh2, ...

high E/Z ratio with respect to the CN double bond for the introduction of the first electrophile E as well as to avoid side chain deprotonation in the course of the second functionalization.311 It has to be mentioned that the E and Z isomers of 160 and 165 (only the E isomer is depicted in Schemes 48 and 49) differ regarding which ortho proton at the ferrocene backbone is abstracted, thus providing different epimers with respect to the planar chirality. Furthermore, the Z isomers were found to be less reactive, resulting in lower yields and the necessity of stronger bases, which has been attributed to a preference for a fivemembered chelate system (for E) rather than a six-membered system (for Z).309,311 The SAMP-hydrazone method can be extended to the synthesis of planar-chiral diferrocenyl ketones, starting from ferrocene 159 (Scheme 50). Hydrazone (S)-168 gives not unexpectedly (S,Sp)-169 in ≥96% de, which yielsd upon hydrazone cleavage only planar-chiral monosubstituted diferrocenyl ketones (Sp)-170 almost enantiomerically pure (97−99% ee) (Scheme 50).312 In contrast, the attempted introduction of a functionality E′ at the second ferrocenyl moiety, subsequent to the synthesis of hydrazone (S,Sp)-169, is, in addition to being highly diastereoselective (≥96% de), less regioselective, forming ferrocene 171 or 172 or mixtures thereof depending on the nature of the substituent E (Scheme 50).312 For instance, in case of E = PPh2·BH3 171 is formed exclusively, whereas for E = C(OH)Ph2 ferrocene 172 is the only product of this transformation. Mixtures of both regioisomers are obtained by starting from SMe-substituted 169. The regio- and diastereoselective (typically ≥96% de) synthesis of diferocenyl ketones bearing ortho substituents at both sandwich moieties was finally achieved by ortho-directed lithiation of ketone (Sp)-170 with sec-BuLi/tmeda and subsequent treatment with the electrophile E′+ (Scheme 51). It should be highlighted that this transformation is likely the first example of a solely planar-chiral compound to induce the formation of a second chirality plane in the same molecule.312 A different approach for the stereoselective synthesis of planarchiral acetylferrocenes has recently been reported by Top and coworkers.313 In this study, acetylferrocene (174) was converted into the enantiopure imine (S)-175 by applying readily available

Scheme 51. Diastereoselective Transformation of Only Planar-Chiral Diferrocenyl Ketone (Sp)-170a,312

a

E = SMe, Si-Pr, CHPh2, ...; E′ = Me, PPh2, SMe, ...

β-amino alcohols (Scheme 52). The ortho-directed metalation with t-BuLi proceeded in ≥90% de. Hydrolysis of the imine finally gave only planar-chiral (Rp)-177.313 P-Stereogenic Ortho-Directing Groups. The P(O)R2 moiety in ferrocenylphosphine oxides 61 serves as an ODG for the metalations at the ferrocene backbone, which has been discussed before.49 In 2000, Widhalm, van Leeuwen, and co-workers reported on the use of the enantiopure only P-stereogenic ferrocenylphosphine oxide (RP)-178 in the ortho-directed metalation.314 The necessary starting materials have been synthesized using ephedrine315 as a chiral auxiliary.316 Treatment of (RP)-178 with (N,N-diisopropylamido)magnesium bromide at room temperature gave upon addition of iodine planar-chiral ferrocenes (RP,Sp)-179 in 50−94% de depending on the steric demand of the substituent R (Scheme 53). Enantiopure material could be obtained by column chromatography. Iodoferrocenes (R P ,S p )-179 were further converted into 2,2′-bis(diarylphosphino)-1,1′-biferrocenyls 180 by Ullmann coupling and stereoselective reduction of the phosphine oxides using HSiCl3 as reducing agent (Scheme 53).314 Xiao and co-workers successfully applied an 1,3,2-oxazaphospholidine 2-oxide derived from (−)-ephedrine as a chiral ODG.317 Ferrocene 181 is deprotonated by t-BuLi to give upon treatment with an electrophile (4S,5R,RP,Rp)-182 as the only diastereomer (>99% de) (Scheme 54).317 Recently, the synthesis of the P-epimer of 181 has been reported by the same group, which can be transformed to planar-chiral ferrocenes with the same degree of stereoinduction however, this process yields the corresponding Sp isomer.318 The authors suggest the possible assistance of the PO oxygen atom in the deprotonation at the Q


Organometallics

Review

Scheme 52. Diastereoselective Synthesis of 2-Substituted Acetylferrocene (RP)-177 with an Intermediately Formed β-Methoxy Imine as Chiral Directing Groupa313

a

E = SiMe3, SiMe2Ph, I, ...

Scheme 53. Synthesis of Planar- and P-Stereogenic Ferrocenylphosphines (SP,SP,Rp,Rp)-180314

chirality. Treatment with hydrogen chloride forms the corresponding PCl2-substituted and upon subsequent reduction with Li[AlH4] the PH2-substituted ferrocenes, which may be reacted with nucleophiles or electrophiles, respectively. The transformation of ferrocene (S,S)-183 into phospholane/diarylphosphino Kephos 185 and OH-Taniaphos 186 reveals the versatility of this approach (Scheme 55).319 After submission of this review Jurkschat and co-workers reported on the diastereoselective ortho-directed metalation of a chiral ferrocenylphosphonic diamide in 76% de.320 Other Ortho-Directing Groups. In addition to the ferrocenes discussed earlier, there are a couple of examples of chiral ferrocenes (Chart 5) applied in diastereoselective ortho-directed metalations that do not fit in either of the preceding subsections and will therefore be summarized here. Already in 1995 Ganter and Wagner reported on the synthesis of (S)-(2-(methoxymethyl)pyrrolidin-1-ylmethyl)ferrocene (187) and its conversion into planar-chiral ferrocenes carried out by ortho-directed lithiation with sec-BuLi at −78 °C, which proceeds with 98% de in favor of (S,Sp)-198.321 The ODG can be displaced by means of a nucleophilic substitution reaction at the α-carbon atom, which is a characteristic reactivity of (ferrocenylmethyl)amine derivatives (Scheme 56).321,322 Later on, Marinetti and co-workers extended Ganter’s methodology to the 1,1′-disubstituted ferrocene (S,S)-188, which serves as the starting material for the synthesis of planar-chiral 2-phospha[3]ferrocenophanes (Sp,Sp)-201 (Scheme 56).323 The structurally related ferrocenes (R,R)-189 and (R)-190, which are based on

Scheme 54. Diastereoselective Ortho-Directed Lithiation of (4S,5R,RP)-3,4-Dimethyl-2-ferrocenyl-5-phenyl-1,3,2oxazaphospholidine 2-Oxidea,318

a

E = Me, SiMe3, PPh2, ...

metallocene backbone as well as in the stabilization of the intermediately formed lithioferrocene. Such an assistance would serve as an explanation for the reverse stereochemistry of 181 and its P-epimer. Recently, Lotz, Pugin, and co-workers demonstrated the utility of bis(2-(methoxymethyl)pyrrolidin-1-yl)phosphine as an ODG, which has no stereogenic phosphorus atom but is based on (S)or (R)-2-(methoxymethyl)pyrrolidine as the source of chirality.319 Ferrocene (S,S)-183 undergoes ortho-directed lithiation with sec-BuLi at −30 °C, yielding after treatment with ClPPh2 the sandwich compound (S,S,Sp)-184 as a single diastereomer in 85% yield (Scheme 55).319 Beneficial features of this synthetic methodology are, in addition to the excellent stereochemical induction, the accessibility of both enantiomers 184 starting either from (S,S)- or (R,R)-183 and the possibility of further functionalization of the ODG after establishment of the planar

Scheme 55. (S,S)-Bis(2-(methoxymethyl)pyrrolidin-1-yl)ferrocenylphosphine for the Diastereoselective Synthesis of PlanarChiral Ferrocenes319

R


Organometallics

Review

Chart 5. Chiral Ferrocenes Applied in Diastereoselective Ortho-Directed Lithiations

combination with t-BuLi. Applying only one of the two bases resulted either in no or in a considerably less selective deprotonation. Reactions with different ferrocene to base ratios indicate that the formation of mixed aggregates of LDA and tBuLi is not sufficient to explain these results but that the formed lithioferrocene may be involved in an aggregation process as well. In contrast to this, not only did the pentamethyl and pentaphenyl derivatives show excellent diastereoselectivities for an orthodirected lithiation with t-BuLi but also addition of LDA as described before decreased the selectivity of the reaction.325,326 N-Ferrocenyl-substituted 192, based on L-proline, has been investigated in detail by Metallinos et al. recently.327 Ferrocenes 202 are formed in ≥90% de by treatment with t-BuLi and subsequent addition of an electrophile (Scheme 57). If the epimer of 192 at C-1 is applied, an Sp-configured ferrocene is obtained with the same degree of stereoinduction.327 Treatment of 202 with p-TsOH gave rise to only planar-chiral N-ferrocenyl imidazolones 203. Furthermore, ferrocene 202 could be converted into primary aminoferrocenes 204 by complete cleavage of the ODG (Scheme 57).327 With the example of ferrocenylmethylamine (Sa)-193, Widhalm et al. demonstrated that a stereocontrolled establishment of planar chirality (80% de) is possible starting from a solely axial-chiral ferrocene derivative.328 Weissensteiner and coworkers in turn have shown the utility of the O-methylephedrine

Scheme 56. Synthesis of Planar-Chiral Ferrocenes (Sp)-199 and (Sp,Sp)-201 Utilizing (S)-2-(Methoxymethyl)pyrrolidin1-ylmethyl as the ODGa,321,323

a

E = SiMe3, Br, I, ...

chiral pyrrolidine derivatives, have been investigated by Guiry and co-workers as well as Vasse and co-workers.324 The group of Peters synthesized 2-ferrocenyl imidazolines 191 as well as their 1′,2′,3′,4′,5′-pentamethyl and -phenyl derivatives and studied their metalation behavior.325,326 High diastereoselectivities (up to 96% de) were achieved using LDA in

Scheme 57. Ortho-Directed Lithiation of N-Ferrocenyl-Substituted 192a,327

a

Conditions: (i) (1) K2CO3, (2) Na[BH4], (3) KOH; E = B(OH)2, SiMe3, Sn(n-Bu)3, ... S


Organometallics

Review

moiety in ferrocenylmethylamine (1R,2S)-194 as ODG, allowing the synthesis of 1,2-disubstituted ferrocenes in up to 98% de.329 Ephedrine is a relevant chiral auxiliary for the resolution of planar-chiral ferrocenes. This has been demonstrated by the example of 2-N,N-dimethylaminomethyl-1-haloferrocenes (58) before (Scheme 17).46 Carboxamides derived from ferrocenecarboxylic or 1,1′ferrocenedicarboxylic acid and chiral amines have been applied in ortho-directed functionalizations by Dimitrov and co-workers. Camphor-based ferrocenes 195 as a representative example gives upon lithiation with sec-BuLi/tmeda and subsequent reaction with an electrophile planar-chiral ferrocenes with up to 99% de.330 Krishna, Mongin, and co-workers deprotonated sugarderived ferrocenecarboxylic esters, such as 196 (Chart 5), with mixed lithium−cadmium and lithium−zinc bases to yield 1,2disubstituted ferrocenes with up to 92% de.331 The chiral ferrocenyl sulfoximine (SS)-197 was introduced by Bolm and co-workers.332 Ortho-directed lithiation proceeds highly diastereoselectively, as only one diastereomer has been observed, which is attributed to the coordination of lithium by the sulfoximine oxygen atom. Substitution at the second cyclopentadienyl ring or at the benzene moiety of the tosyl group was identified as a side reaction.332 Enantioselective Ortho-Directed Metalation. The synthesis of ferrocenes with planar chirality as the sole source of chirality is, from an atom-economical point of view, most conveniently achieved by an enantioselective metalation. The first attempts regarding this matter date back to 1969. Aratani et al. performed a dilithiation of isopropylferrocene (1d) with nBuLi in the presence of (−)-sparteine.333 After reaction with ClSiMe3 1,3,1′-trisubstituted 205 was isolated in 80% yield along with 5% of isomeric ferrocenes and 15% recovered starting material (Scheme 58). The enantiomeric excess of ferrocene 205 was determined to be only 3%.67

Scheme 59. Enantioselective Ortho-Directed Lithiation of 61a with Lithium Bis((R)-1-phenylethyl)amide334 and of 65b with Me2(PEA)2ZnLi2335

Scheme 60. Enantioselective Ortho-Directed Lithiation of 53 with n-BuLi in the Presence of (R,R)-N,N,N′,N′Tetramethylcyclohexanediamine336

lective deprotonation of the sulfonylferrocene FcSO2-t-Bu led to a racemic product, as already observed by Simpkins and Price.334 Snieckus and co-workers achieved a breakthrough in enantioselective ortho-directed lithiations of ferrocenes in the same year.337 Ferrocenecarboxamide 65b could be deprotonated with n-BuLi/(−)-sparteine, yielding after reaction with an electrophile almost enantiomerically pure (up to 99% ee) planar-chiral ferrocenes (Rp)-208 (Scheme 61).337 In addition, further transformations of these ferrocenes, e.g. the reduction of the carbonyl group, subsequent substitutions at the second cyclopentadienyl ring, or cross-coupling reactions with iodosubstituted ferrocenes, were shown to proceed as expected for related compounds (Scheme 61).337 This approach has been applied to the synthesis of planar-chiral ferrocenes derived from 65b by others.338 The asymmetric mono- and dilithiation of 1,1′-disubstituted ferrocenecarboxamide 212 was closely investigated by Jendralla and Paulus as well as Snieckus and co-workers in the following years.339,340 Treatment with n-BuLi/(−)-sparteine and subsequently with an electrophile resulted in the formation of 1,2,1′trisubstituted (Rp)-213 in 71−97% ee (Scheme 62). It was found that the use of toluene as solvent was crucial for a high degree of enantioinduction. Solvents with higher coordinating abilities (Et 2O, t-BuOMe) afforded (Rp)-213 only in moderate enantioselectivities (59−70% ee).340 The direct synthesis of 1,2,1′,2′-tetrasubstituted ferrocenes has been achieved by applying sec-BuLi/(−)-sparteine as base. However, this procedure preferentially yielded meso-214 in up to 92% de (Scheme 62).339,340 In contrast, if trisubstituted (Rp)-213 is deprotonated with n-BuLi/(−)-sparteine, ferrocene (Rp,Sp)-215 is obtained in up to 98% de almost enantiomerically pure (91− 99% ee) (Scheme 62).339,340 In 2003 Snieckus broadened the substrate scope of enantioselective ortho-directed metalations with the introduction of N-cumyl-N-ethylferrocenecarboxamide (216). Ferrocene 216 can be converted into planar-chiral (Rp)-217 using n-BuLi/ (−)-sparteine as base in reasonable yields and high enaniose-

Scheme 58. Metalation of Isopropylferrocene (1d) with nBuLi/(−)-Sparteine To Give 20567

Since than, there has been no scientific contribution to this field for 25 years. In 1995, Price and Simpkins reported on the asymmetric metalation of FcP(O)Ph2 (61a) with lithium bis((R)-1-phenylethyl)amide to form the silylated ferrocene (Rp)-206 in 95% yield and 54% ee (Scheme 59).334 In contrast, ferrocenes FcSO2Ph and FcC(O)N(i-Pr)2 (65b) were found to give racemic products.334 For the latter ferrocene Hilmersson, Mongin, and co-workers recently demonstrated that the use of chiral lithium−metal bases strongly influences the stereoselectivity.335 Treatment of 65b with 2 equiv of Me2(PEA)2ZnLi2 (PEA = bis((S)-1-phenylethyl)amide) allows for the isolation of (Sp)-290b in 97% yield and 86% ee (Scheme 59). Almost at the same time Uemura investigated the orthodirected lithiation of ferrocenylmethylamines such as 53 with nBuLi in the presence of chiral amines (Scheme 60). The best results were obtained for the use of (R,R)-N,N,N′,N′tetramethylcyclohexanediamine, giving aldehyde (Rp)-207 in 41% yield and 80% ee (Scheme 60).336 Attempted enantioseT


Organometallics

Review

Scheme 61. Enantioselective Ortho-Directed Lithiation of Ferrocenecarboxamide 65ba,337

a

E = Me, SiMe3, PPh2, I, ...

ation, sterically less crowded (Rp)-218 gave the 1,2,3trisubstituted 219 as the only isolable product. Consequently, the silyl trick may be applied to synthesize (Sp)-218 (94% ee) as shown in Scheme 63, circumventing the unavailability of (+)-sparteine.341 The introduction of the (+)-sparteine surrogate (1R,2S,9S)(+)-11-methyl-7,11-diazatricyclo[7.3.1.02,7]tridecane342 into ferrocene chemistry by O’Brien and his group provided an alternative access to the other enantiomer of 217.343 Whereas the use of (−)-sparteine yields (Sp)-217 (E = Me, 96% ee), (+)-sparteine surrogate furnishes (Rp)-217 (E = Me, 92% ee).343 Furthermore, the same group was able to show that the chiral diamines do not have to be applied in a stoichiometric amount. If only 0.4 equiv of (−)-sparteine or (+)-sparteine surrogate is used, neither the yield nor the enantioselectivity for ferrocene 217 is substantially affected.343,344 Metallinos and Dudding extended the methodology of enantioselective ortho-directed lithiation onto BF3-activated aminoferrocenes such as 68.345 An extensive screening of several organolithium bases and chiral diamines as well as additives provided conditions for the highly enantioselective synthesis of (Sp)-220 as well as of its enantiomer (Scheme 64).345 The application of chiral bases in diastereoselective orthodirected metalations is a promising approach if neither the diastereoselective lithiation in the absence of a chiral base nor the enantioselective deprotonation of similar achiral substrates provides satisfactory stereoselectivities. This applies, for example, to the ortho-directed metalation of ferrocenylsulfonates 221, as

Scheme 62. Enantioselective Ortho-Directed Lithiation of 1,1′-Ferrocenedicarboxamide 212a,339,340

a

E = Me, SiMe3, PPh2, I, ...

lectivities (88−96% ee); subsequent decumylation by refluxing in 2,2,2-trifluoroethanol gave the secondary carboxamides (Rp)218 (Scheme 63).341 It was shown that these amides serve as suitable starting materials for the synthesis of tertiary amides, amines, and esters.341 However, even more important is the changed metalation behavior regarding the introduction of a third substituent. Whereas tertiary carboxamide (Rp)-208 exclusively yielded 1,2,1′-substituted ferrocenes upon metal-

Scheme 63. N-Cumyl-N-ethylferrocenecarboxamide (216) for the Enantioselective Synthesis of (Rp)-218 and (Sp)-218a341

a

E = Me, SiMe3, PPh2, I, ... U


Organometallics

Review

Scheme 64. Synthesis of (Sp)- and (Rp)-220 by Enantioselective Ortho-Directed Lithiation of BF3-Activated Aminoferrocenes 68345

shown by Metallinos and Snieckus (Scheme 65).346 Indeed, the combination of a chiral substrate and (−)-sparteine improves the

Scheme 66. HLADH-Catalyzed Oxidation of Ferrocenyl Alcohol rac-224 and Reduction of Aldehyde rac-225348

Scheme 65. Stereoselective Ortho-Directed Metalation of Ferrocenylsulfonates 221b,346

chiral349−355 and homo- and heteroannular-bridged356−359 as well as planar- and central-chiral ferrocenes356,358,360,361 have been made accessible in excellent enantiomeric purity. Furthermore, this approach is applicable to scalemic ferrocenes for a further enantioenrichment.104,355 The kinetic resolutions of aldehyde 226,349 esters 228356 and 231,358 and alcohols 229357 and 232354 are depicted in Scheme 67 as representative examples. A more detailed discussion of this topic can be found in a review published by Alba and Rios.362 Nonenzymatic Kinetic Resolutions. The nonenzymatic kinetic resolution of planar-chiral ferrocenes applying chiral reagents or catalysts has been considerably less developed than its enzymatic counterpart. The first example of a nonenzymatic kinetic resolution appeared as late as 2003. Uemura and coworkers applied binaphthyl-based selenoxide 233 as stoichiometric oxidant for the resolution of planar-chiral ferrocenylphosphine 234 (Scheme 68).363 Using phenol as an additive, phosphine oxide (Rp)-235 is formed in 13% ee at 52% conversion; the recovered phosphine is consequently enriched in the Sp enantiomer (29% ee) (Scheme 68).363 Three years later Moyano and co-workers presented the first catalytic nonenzymatic kinetic resolution of a planar-chiral ferrocene by the use of a Sharpless asymmetric dihydroxylation.364 2-Substituted vinylferrocenes, such as 236, give upon dihydroxylation ferrocene 237 with excellent diastereoselectivity (>95:5 dr) (Scheme 69).364 Depending on the quinoline derivative used as the chiral auxiliary, either (Rp)- or (Sp)-236 is preferentially converted. Ferrocene 237 and recovered starting material 236 could be isolated in moderate to excellent enantiomeric purity (Scheme 69).364 At the same time Ogasawara and Takahashi reported on a asymmetric ring-closing metathesis for the kinetic resolution of 1,1′-diallylferrocene 238 by applying axial-chiral molybdenum carbene 239 as catalyst (Scheme 70).365 Excellent enantiomeric purities of ansa-ferrocene (Rp)-240 and recovered starting material (Sp)-238 are achieved for ferrocenes bearing a less reactive methallyl group at the monosubstituted cyclopendienyl

a

(Rp)-222 is formed as major diastereomer. bE = CHO, SiMe3, PPh2, ...

diastereoselectivity in case of 221c significantly.346 In contrast to that, its enantiomer 221b favors the formation of different diastereomers in the absence or presence of (−)-sparteine with almost the same degree of stereoinduction, representing a mismatched case of the configuration of (1S)-menthyl and (−)-sparteine.346

■

KINETIC RESOLUTIONS In the preceding sections the enantio- and diastereoselective synthesis of at best only one enantiomer of a planar-chiral ferrocene has been described. All of these syntheses rely on the discrimination between two enantio- or diastereotopic protons at substituted ferrocenes: thus, the selective establishment of a chiral plane. The reverse approach, the preparation of racemic 1,2- or 1,3-disubstituted planar-chiral ferrocenes followed by discrimination between these two enantiomer, will be the subject of this section. Resolution can be achieved (i) by the use of chiral resolving agents, examples of which have been discussed in the first two sections of this review, (ii) by HPLC, equipped with chiral stationary phases,347 and (iii) by kinetic means, applying enzymes, chiral catalysts, or reagents. Enzymatic Kinetic Resolutions. To our knowledge, the first enzymatic kinetic resolution of a planar-chiral ferrocene was described by Yamazaki et al. in 1989.348 Oxidation of alcohol 224 with NAD+ or reduction of aldehyde 225 with NADH in presence of the enzyme horse liver alcohol dehydrogenase (HLADH) yielded at 50% conversion preferentially (Sp)-225 and (Sp)-224, respectively (Scheme 66).348 Since then several enzymes have been applied for the kinetic resolution of planar-chiral ferrocenes, mainly by the reduction of aldehydes, the esterifications of alcohols, or the hydrolysis of esters. In this way, 1,2- and 1,3-substituted solely planarV


Organometallics

Review

Scheme 67. Enzymatic Kinetic Resolution of Planar-Chiral Ferrocenes 226−232349,354,356−358

244 could be recovered by prolonging the reaction time to 65 h.367 Rios, Moyano, and co-workers reported in 2009 on the first organocatalytic kinetic resolution of a planar-chiral ferrocene.368 The ferrocenecarbaldehyde rac-247 gave in an aldol reaction with acetone in the presence of L-proline as organocatalyst the aldol reaction product (R,Sp)-248 and the aldol condensation product (Rp)-249 in similar yields and enantiomeric purities (Scheme 73). Recovered (Rp)-247 was obtained in moderate enantiomeric enrichment (73% ee).368 Recently, the kinetic resolution of planar-chiral diene 250 by an asymmetric Diels−Alder reaction with the enantiopure dienophile (SS)-251 was demonstrated by Latorre et al.369 Depending on the reaction temperature and the diene:dienophile ratio, ferrocene (S,Sp,P)-252, formed from the cycloaddition product by sulfenic acid elimination, and recovered (Rp)-250 could be isolated almost enantiomerically pure (Scheme 74). The configuration of tetracyclic (S,Sp,P)-252 and the preferred conversion of (Sp)-250 are in agreement with the proposed model for cycloadditions with p-tolylsulfinylquinones.369

Scheme 68. Kinetic Resolution of Planar-Chiral Ferrocenylphosphine 234363

ring (R = Me). Furthermore, the metathesis reaction has to be conducted under dilute conditions (0.005 mol L−1 238) to prevent homodimerization.365 Just recently, the same group extended this methodology to 1,2-diallylferrocenes 241 (Scheme 71).366 Typically, the recovered starting material is obtained almost enantiomerically pure (91−99% ee), whereas the cyclized ferrocene 243 is at best isolated in 87% ee. Another transition-metal-catalyzed process for a kinetic resolution of a planar-chiral ferrocene was reported by Patti and Pedotti in 2010.367 The racemic ferrocenyl ketone 244 gives in the presence of the chiral ruthenium catalyst 245 in a transfer hydrogenation central- and planar-chiral ferrocene (R,Sp)-246 in up to 96% ee (Scheme 72). Almost enantiomerically pure (Rp)-

■

DESYMMETRIZATION Prochiral ferrocenes, i.e. compounds bearing the same substituent at the 1- and 2-/3-positions, can be converted into

Scheme 69. Kinetic Resolution of Planar-Chiral Vinylferrocene 236 by a Sharpless Asymmetric Dihydroxylationa,364

a

Conditions: (i) K3[Fe(CN)6], K2CO3, K2OsO2(OH)4). (DHQD)2PYR = hydroquinidine-2,5-diphenyl-4,6-pyrimidinediyl diether; (DHQ)2PYR = hydroquinine 2,5-diphenyl-4,6-pyrimidinediyl diether. W


Organometallics

Review

Scheme 70. Kinetic Resolution of Planar-Chiral 1,1′-Diallylferrocene 238 by an Asymmetric Ring-Closing Metathesis365

Scheme 71. Kinetic Resolution of Planar-Chiral 1,2-Diallylferrocene 241 by an Asymmetric Ring-Closing Metathesis366

Scheme 72. Kinetic Resolution of Planar-Chiral Ferrocenyl Ketone 244 by an Asymmetric Transfer Hydrogenation367

Scheme 74. Kinetic Resolution of Planar-Chiral Diene 250 by an Asymmetric Diels-Alder Reactio.369

Scheme 75. Desymmetrization of 1,2-Diformyl- (253) and 1,2-Bis(hydroxymethyl)ferrocene (255) by Enzymatic Transformations370,372

planar-chiral ferrocenes, if a reagent selectively interacts with only one of these two enantiotopic substituents. Desymmetrizations for the synthesis of planar-chiral ferrocenes were achieved for the first time in the late 1980s by enzymatic transformations at 1,2-homodisubstituted ferrocenes such as 1,2-diformylferrocene (253), 3 7 0 , 3 7 1 1,2-bis(hydroxymethyl)ferrocene (255),370,372 and 1,2-bis((methylthio)methyl)ferrocene.373 The corresponding planar-chiral ferrocenes are obtained in good yields and excellent enantioselectivities (Scheme 75). In recent years the desymmetrization of achiral ferrocenes with the aid of chiral catalysts has been demonstrated as well. In 2007 the groups of Shintani and Hayashi synthesized planar- and central-chiral (R,Rp)-258 by a 1,4-addition of phenylboronic acid to ferrocene 257 in presence of 8 mol % of rhodium.374 The conjugate addition proceeded with high diastereoselectivity

(94:6 dr) and yielded the chiral bis ketone almost enantiomerically pure (Scheme 76).

Scheme 73. Kinetic Resolution of Planar-Chiral Ferrocenecarbaldehyde 247 by an Organocatalytic Aldol Reaction368

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MISCELLANEOUS SYNTHESES OF 1,2-DISUBSTITUTED FERROCENES The aim of this section is to summarize different syntheses of planar-chiral ferrocenes, which have not been included in the preceding sections in order to provide a better overview. Siegel and Schmalz demonstrated in 1997 the enantioselective synthesis of homoannularly bridged planar-chiral ferrocenes by a carbene insertion into a CCp−H bond.377 Diazo ketones 264 and 265 gave with copper(I) triflate and an enantiopure bis(oxazoline) ligand (5 mol %) cyclization products 266 and 267 in good chemical yields and in enantiomeric purities of 78 and 62% ee, respectively (Scheme 79).

Scheme 76. Desymmetrization of Ferrocenobenzoquinone 257 by an Asymmetric 1,4-Addition374

Kündig and co-workers used a chiral palladium complex to differentiate between the two enantiotopic bromine substituents in ferrocene 259 (Scheme 77).375 Subsequent to the oxidative

Scheme 79. Enantioselective Cyclizations of Diazo Ketones 264 and 265a,377

Scheme 77. Desymmetrization of Dibromoferrocene 259a,375

a

Review

DABCO = 1,4-diazabicyclo[2.2.2]octane.

addition of the aryl halide to [Pd(0)], hydrogenolysis with Li[BH4] yielded planar-chiral ferrocene (Rp)-260 in 66% ee (Scheme 77). Higher enantioselectivities have been achieved for related ruthenium and Cr(CO)3 complexes.375 Furthermore, the establishment of planar chirality at chiral 1,2homodisubstituted ferrocenes can be accomplished by means of a desymmetrization with achiral reagents, as demonstrated by Fukuzawa et al.376 Ammonolysis of (R,R)-ferrocenyldiacetate 261 yielded (R,R,Sp)-262 as sole diastereomer (Scheme 78). The

a

The absolute configuration of the products (266 or ent-266 and 267 or ent-267) was not established.

Activation of C−H bonds of substituted ferrocenes in the presence of stoichiometric or catalytic amounts of transition metals, e.g., palladium, ruthenium, or iridium, has been used for the synthesis of planar-chiral ferrocenes.378 Functionalized ferrocenes have been obtained by treatment with alkynes,379 alkenes,380 arenes,381 arylboronic acids,382 carbon monoxide,383 and phosphides.384 Gu, You, and co-workers, for example, converted ferrocene 53 into planar-chiral 1,2-disubstituted 268 in up to 99% ee by means of a palladium-catalyzed direct arylation with arylboronic acids (Scheme 80).382 N-Bocprotected amino acids served as chiral auxiliary and air as oxidant for this highly selective transformation.

Scheme 78. Desymmetrization of (R,R)-261 by Ammonolysis To Give (R,R,Sp)-262376

Scheme 80. Enantioselective Arylation of N,NDimethylaminomethylferrocene (53) with 3-Tolylboronic Acida,382

reason for this reactivity is that for steric reasons only one of the acetate groups is positioned exo to ferrocene, which is substituted considerably more quickly than the corresponding acetate moiety in an endo position. Considering that nucleophilic substitutions at the α-carbon atom proceed with retention of configuration, as discussed earlier, solely intermediate (R,R,Sp)263 can be formed. Intramolecular transfer of the acetyl group finally afforded ferrocene 262 in 89% yield (Scheme 78). The intermediately formed amino acetate 263 could be isolated from the reaction mixture by shortening the reaction time.376

a

TBAB = tetrabutylammonium bromide; DMA = N,N-dimethylacetamide.

Bonini et al. and Butenschön and co-workers utilized an anionic rearrangement of ortho-lithiated S-ferrocenyl phosphorodiamidothioates and ferrocenyl triflates, respectively, for the synthesis of 1,2-disubstituted ferrocenes.385,386 The P−S to P−C rearrangement reported by Bonini et al. was performed among others at enantiopure ferrocene (R,R)-269, which undergoes ortho-directed lithiation with t-BuLi highly diasterY


Organometallics

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eoselectively.385 The rearrangement product (R,R,Sp)-270 was isolated in 55% yield as the only diastereomer (Scheme 81).385

obtained by ortho-directed lithiation of monobromoferrocene. Subsequent addition of an electrophile gives racemic planarchiral ferrocene 279 in up to 88% yield (Scheme 83). Replacing 2,2,6,6-tetramethylpiperidine by an enantiopure amine may allow the enantioselective synthesis of 1,2-disubstituted ferrocenes starting from 63c.388

Scheme 81. Anionic Rearrangements for the Synthesis of Planar-Chiral Ferrocenes385,386

Scheme 83. Conversion of 1,1′-Dibromoferrocene (63c) into 1,2-Disubstituted rac-279a,388

a

E = PPh2, Br, I.

Furthermore, the synthesis of planar-chiral ferrocenes has been achieved by [3 + 2] and [4 + 3] cycloadditions, as demonstrated by Toma389 and Zanotti.390

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1,3-DISUBSTITUTED FERROCENES The selective synthesis of 1,3-substituted ferrocenes is a considerably less explored research field in metallocene chemistry. As outlined before, 1,3- or 1,3,1′-substituted ferrocenes are in principle accessible by acylation of alkylferrocenes (Scheme 2); however, the separation from the 1,2-isomers formed to a lesser extent is tedious, requiring repeated recrystallization or elaborate chromatographic workup. So far, only a few routes have been reported for the direct synthesis of 1,3-substituted ferrocenes. In 1994 Compton and Rauchfuss accessed 1,3,1′-trisubstituted 280 through dilithiation of tert-butylferrocene (1e) and treatment with excess sulfur (Scheme 84).391 The corresponding synthesis of a 1,3,1′trisubstituted ferrocene starting from isopropylferrocene (1d) had already been demonstrated in 1969 by Aratani et al. (Scheme 58).67 Another highly regioselective meta lithiation was presented by Brown and co-workers in 2004.392 Ferrocenyl sulfide 281 gives upon lithiation with sec-BuLi and subsequent reaction with an electrophile 1,3-disubstituted 282 in up to 94% regioselectivity (Scheme 84).393 The asymmetric deprotonation of 281 in the presence of (−)-sparteine, however, proceeded with low enantioselectivity.392 At the same time Plenio’s group synthesized borylated ferrocenes exerting an iridium-catalyzed C−H activation.394 1,1′-Dibromoferrocene (63c) gives exclusively the 1,3,1′-trisubstituted rac-283 in 81% yield (Scheme 84).394 Ortho-directed metalation can serve as a detour method to synthesize 1,3-substituted ferrocenes involving the preparation of 1,2,3-trisubstituted derivatives and cleavage of the substituent at C-2. Two synthetic routes may give 1,2,3-trisubstituted ferrocenes, which differ in whether the second ortho-directed metalation proceeds ortho to ODG1 or ODG2 (Scheme 85). Consequently, following route A, cleavage of ODG1 leads to the

The S−O to S−C rearrangement (anionic thia-Fries rearrangement) presented by Butenschön proceeds by deprotonation of ferrocenyl triflate 271 with LDA in almost quantitative yield; however, efforts to perform this reaction enantioselectively have been unsuccessful so far. The use of Simpkins’ base instead of LDA gave, for instance, only slightly enantioenriched 272 (10% ee) (Scheme 81). It is worth noting the double thia-Fries rearrangement of 273, which gives meso-274 as the only diastereomer (Scheme 81).386 Jäkle, Manners, and co-workers observed upon treatment of 1,1′-bis(trimethylstannyl)ferrocene (275) with Cl2BR via an unexpected rearrangement reaction the formation of 1,2-and 1,3disubstituted 276 and 277, whereas only trace amounts of the 1stannyl-1′-borylferrocene 278 could be detected (Scheme 82).387 The rearrangement reaction is believed to proceed through an electrophilic attack of the boron halide followed by a proton migration and subsequent elimination of ClSnMe3. The composition of the product mixture and the rate of this reaction are governed by the electronic and steric properties of the boron electrophile. Sterically demanding substituents R enhance the formation of the 1,3-disubstituted 277 (276:277:278 = 87:10:3 (R = Cl), 55:45:− (R = C6F5)), whereas the rate of the reaction decreases with a decreasing Lewis acidity of Cl2BR (R = Ph, 2,4,6C6H2Me3).387 Another conversion of a 1,1′- into a 1,2-disubstituted ferrocene was reported by Butler.388 Metal−halogen exchange at 1,1′-dibromoferrocene (63c) with 1 equiv of n-BuLi followed by the addition of 2,2,6,6-tetramethylpiperidine resulted in its isomerization to a 1,2-disubstituted ferrocene by a protonation− deprotonation equilibrium, which represents the product

Scheme 82. Synthesis of Disubstituted 276−278 Starting from 1,1′-Bis(trimethylstannyl)ferrocene (275)387

Z


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Scheme 86. Synthesis of 1,3-Substituted rac-286395

Scheme 84. Synthesis of Ferrocenes 280, 282, and 283 Featuring a 1,3-Disubstituted Cyclopentadienyl Ringa,391,392,394

second ortho-directed lithiation to proceed solely adjacently to ODG2; consequently ODG2 has to have a stronger orthodirecting effect than ODG1. This shall be illustrated by two examples presented by Top and Jaouen399 as well as Weissensteiner and co-workers397 (Scheme 87). Both ferrocenes (R,Sp)-78 and (S,S,RS,Sp)-113 are exclusively deprotonated at the ortho position of bromide and sulfinyl, giving the respective 1,2,3-trisubstituted compounds in high yields.397,399 Exclusive lithiation adjacent to ODG2 can also be ensured by temporary or permanent protection of ODG1.398 Bromoferrocene (R,Rp)-198 yields by treatment with LiTMP, reaction with DMF, and subsequent reduction predominantly the undesired (R,Rp)-291.398 Temporary protection of the tertiary amine as the BH3 adduct (R,Rp)-292, however, reverses the regioselectivity of the ortho-directed lithiation and gives rise to trisubstituted (R,Sp)-293 (Scheme 88), which can be transformed into a 1,3substituted ferrocene by lithium−bromine exchange and subsequent hydrolysis.398

a Conditions: (i) [Ir(OMe)(cod)]2, 4,4′-di-tert-butyl-2,2′-bipyridine, bis(pinacolato)diboron. cod = (Z,Z)-1,5-cyclooctadiene.

target molecule, whereas according to route B ODG2 has to be removed. Furthermore, this methodology offers the possibility to obtain the substituted ferrocenes stereoselectively by applying chiral ODGs. The necessity to replace one ODG by a proton in the final reaction step restricts the range of directing groups to choose from to chloride, bromide, and sulfinyl. To our knowledge, the only example for the synthesis of a planar-chiral 1,3-substituted ferrocene according to route A dates back to 1974.395 Slocum et al. prepared rac-286 by ortho-directed lithiation of 1-chloro-2-methylferrocene (rac-284), treatment with benzophenone, and subsequent dechlorination with sodium (Scheme 86).395 In the case of bromoferrocenes there have been reports by Butler and co-workers regarding the unexpected 2,5dilithiation of 1,1′-dibromoferrocene, which has been used to access symmetric 1,3-substituted ferrocenes.52,396 Sulfinyl as an ODG, however, does not allow for the straightforward synthesis of a homoannular trisubstituted ferrocene. As discussed earlier for the example of (SS,Sp)-134 (Scheme 41) the introduction of a third substituent by a second ortho-directed metalation requires either the oxidation of the sulfoxide to the sulfone or inversion of configuration at sulfur by reduction and consecutive oxidation.236,241 Although the feasibility of this methodology has in principle been demonstrated as early as 1993, no planar-chiral 1,3-substituted ferrocene has to our knowledge yet been synthesized that way. Several racemic or enantiopure 1,3-disubstituted ferrocenes have been accessed with respect to route B in the past decade using Br and S(O)-t-Bu or S(O)-p-Tol as ODG2.361,397−400 A high-yielding synthesis requires a high regioselectivity of the

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CONCLUSION This review summarizes different approaches for the regio- and stereoselective synthesis of planar-chiral ferrocenes, mainly focusing on electrophilic aromatic substitution and orthodirected metalation for the functionalization of monosubstituted as well as kinetic resolution and desymmetrization of disubstituted ferrocenes. Applying these methodologies, ferrocenes featuring diverse substitution patterns, including homoand heteroannular-bridged compounds, are accessible in a highly diastereo- or enantioselective manner. In case of diastereoselective ortho-directed lithiation diverse directing groups are described coordinating to lithium via nitrogen or oxygen. An asymmetric substituted atom (e.g., carbon, sulfur, or phosphorus) serves in most cases as the source of chirality; however, examples exerting axial- and planar-chiral ODGs (ortho-directing group) have been reported as well. Through the use of enantiomeric starting materials as well as innovative synthesis strategies, such as the “silyl” or “silicon trick”, it is quite possible to obtain all four diastereomers of a central- and planar-chiral ferrocene in almost enantiomerically pure form. Solely planarchiral compounds are available either by a diastereoselective approach and subsequent cleavage of the directing group or by an enantioselective ortho-directed lithiation. For the latter process at least two challenges remain: (i) broadening of the substrate scope and the range of chiral bases and (ii) improvements

Scheme 85. Two Routes for the Synthesis of 1,3-Disubstituted Ferrocenes Exerting Ortho-Directed Metalation and Cleavage of One ODG

AA


Organometallics

Review

Scheme 87. Synthesis of Enantiopure 1,3-Substituted Ferrocenes (R,Sp)-288 and (S,S,Sp)-290a,397,399

a

LiTMP = lithium 2,2,6,6-tetramethylpiperazide.

Scheme 88. Synthesis of 1,2,3-Trisubstituted Ferrocenes (R,Rp)-291 and (R,Sp)-293398

regarding the use of chiral additives just in a substoichiometric amount. In general, stereoselective syntheses applying catalytic amounts of a chiral auxiliary offer great potential irrespective of whether achiral, prochiral, or chiral ferrocenes are intended to be converted. It will be of interest to see if these methodologies will reach levels comparable to the broad application of chiral ODGs in diastereoselective ortho-directed metalations.

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fellowship of the Fonds der Chemischen Industrie. In 2011 he joined the group of Prof. Dr. John F. Hartwig at UC Berkeley (USA) as a visiting graduate student researcher.

AUTHOR INFORMATION

Corresponding Author

*E-mail for H.L.: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Heinrich Lang studied chemistry at the University of Constance, where he graduated in Chemistry (Diploma, 1982; Ph.D., 1985) under the supervision of G. Huttner. Afterward he spent two years as a postdoctoral fellow (DFG) at MIT with D. Seyferth. He joined the Faculty at the Ruprecht-Karls-University of Heidelberg in 1988, where he received his Habilitation in 1992. From 1992 until 1996 he held the position of a Privat-Dozent as a Heisenberg fellow (DFG) at the same university. Since 1996 he has been a full professor (holding the Chair of Inorganic Chemistry) at TU Chemnitz. The call to TU Kaiserslautern in 2003 was not taken. He is a member of scientific and editorial advisory boards and has published 570 peer-reviewed papers and received honors and awards. Since 2012 he has been vice president of TU Chemnitz for Research and Young Scientists. His research interests are in organometallic and metal-organic chemistry, electron transfer studies, inkjet printing, gas-phase deposition, spray coating of conductive/semiconductive patterns/layers, and homogeneous catalysis. He is also

Dieter Schaarschmidt studied chemistry at the TU Chemnitz, where he received his Diploma in 2009 for his work on planar-chiral ferrocenylphosphines under the supervision of Prof. Dr. Heinrich Lang. Currently he is working in the same research group toward his Ph.D. on atropselective Suzuki−Miyaura couplings, financed by a AB


Organometallics

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(13) In the case of ethylferrocene acetylation occurs at the unsubstituted cyclopentadienyl ring as well: product ratio 1,1′:1,2:1,3 = 1.0:0.55:1.7. For details see ref 12. (14) (a) Rinehart, K. L., Jr.; Curby, R. J., Jr. J. Am. Chem. Soc. 1957, 79, 3290−3291. (b) Rinehart, K. L., Jr.; Curby, R. J., Jr.; Gustafson, D. H.; Harrison, K. G.; Bozak, R. E.; Bublitz, D. E. J. Am. Chem. Soc. 1962, 84, 3263−3269. (15) Thomson, J. B. Tetrahedron Lett. 1959, 6, 26−27. (16) For Horeau’s method see, for example: Horeau, , A. In Stereochemistry: Fundamentals and Methods; Kagan, , H. B., Ed.; Thieme: Stuttgart, Germany, 1977,; Vol. 3,; Chapter Determination of the Configuration of Secondary Alcoholsby Partial Resolution, pp 51−−94. (17) For the epimerization of 8 see, for example: Hill, A. E.; Richards, J. H. J. Am. Chem. Soc. 1961, 83, 4216−4221. (18) (a) Schlögl, K.; Falk, H. Angew. Chem. 1964, 76, 570. (b) Falk, H.; Schlögl, K. Monatsh. Chem. 1965, 96, 266−275. (19) For the synthesis and resolution of 9 see, for example ref 11 and: Westman, L.; Rinehart, K. L., Jr. Acta Chem. Scand. 1962, 16, 1199− 1205. (20) Carter, O. L.; McPhail, A. T.; Sim, G. A. J. Chem. Soc. A 1967, 365−373. (21) Falk, H.; Schlögl, K. Monatsh. Chem. 1971, 102, 33−36. (22) Haller, G.; Schlögl, K. Monatsh. Chem. 1968, 99, 2044−2057. (23) (a) Schlögl, K. Pure Appl. Chem. 1970, 23, 413−432. (b) Schlögl, K. J. Organomet. Chem. 1986, 300, 219−248. (24) Falk, H.; Schlögl, K. Monatsh. Chem. 1965, 96, 1065−1080. (25) des Abbayes, H.; Dabard, R. Tetrahedron 1975, 31, 2111−2116. (26) For the racemization of optically active ferrocenes see, for example: (a) Slocum, D. W.; Tucker, S. P.; Engelmann, T. R. Tetrahedron Lett. 1970, 11, 621−624. (b) Bauer, K.; Falk, H.; Lehner, H.; Schlögl, K.; Wagner, U. Mon. Chem. 1970, 101, 941−943. (c) Falk, H.; Lehner, H.; Paul, J.; Wagner, U. J. Organomet. Chem. 1971, 28, 115− 124. (d) des Abbayes, H.; Dabard, R. J. Organomet. Chem. 1973, 61, C51−C54. (e) Roman, E.; Astruc, D.; des Abbayes, H. J. Organomet. Chem. 1981, 219, 211−220. (27) Falk, H.; Hofler, O.; Schlögl, K. Mon. Chem. 1969, 100, 624−648. (28) For the synthesis of planar-chiral [3]ferrocenophanes see also: (a) Schlögl, K.; Seiler, H. Tetrahedron Lett. 1960, 1, 4−8. (b) Rinehart, K. L., Jr.; Bublitz, D. E.; Gustafson, D. H. J. Am. Chem. Soc. 1963, 85, 970−982. (29) The apparently low yields for the formation of 28 and 29 (cf. related 5a is isolated in 87% yield, Scheme 3) are due to the fact that the reported values correspond to four steps. (30) Locke, A. J.; Gouti, N.; Richards, C. J.; Hibbs, D. E.; Hursthouse, M. B. Tetrahedron 1996, 52, 1461−1472. (31) (a) Locke, A. J.; Richards, C. J.; Hibbs, D. E.; Hursthouse, M. B. Tetrahedron: Asymmetry 1997, 8, 3383−3386. (b) Locke, A. J.; Richards, C. J. Organometallics 1999, 18, 3750−3759. (32) (a) Lednicer, D.; Lindsay, J. K.; Hauser, C. R. J. Org. Chem. 1958, 23, 653−655. (b) Lednicer, D.; Hauser, C. R. J. Org. Chem. 1959, 24, 43−46. (33) Ratajczak, A.; Zmuda, H. Bull. Acad. Pol., Ser. Sci. Chim. 1977, 25, 35−38. (34) Bernardi, L.; Bonini, B. F.; Comes-Franchini, M.; Fochi, M.; Mazzanti, G.; Ricci, A.; Varchi, G. Eur. J. Org. Chem. 2002, 2776−2784. (35) Rausch, M. D.; Ciappenelli, D. J. J. Organomet. Chem. 1967, 10, 127−136. (36) (a) Sanders, R.; Mueller-Westerhoff, U. T. J. Organomet. Chem. 1996, 512, 219−224. (b) Guillaneux, D.; Kagan, H. B. J. Org. Chem. 1995, 60, 2502−2505. (37) Benkeser, R. A.; Bach, J. L. J. Am. Chem. Soc. 1964, 86, 890−895. (38) For ortho-directed metalation see, for example: (a) Snieckus, V. Chem. Rev. 1990, 90, 879−933. (b) Haag, B.; Mosrin, M.; Ila, H.; Malakhov, V.; Knochel, P. Angew. Chem. 2011, 123, 9968−9999. (39) Benkeser, R. A.; Fitzgerald, W. P.; Melzer, M. S. J. Org. Chem. 1961, 26, 2569−2571. (40) Slocum, D. W.; Rockett, B. W.; Hauser, C. R. J. Am. Chem. Soc. 1965, 87, 1241−1246.

interested in topics such as hybrid materials and nanomaterials, including metal and metal oxide nanoparticles and their embedding in polymers, and is researching the field of photovoltaics.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. D.S. is grateful to the Fonds der Chemischen Industrie for a Chemiefonds fellowship. We thank Dr. Alexander Hildebrandt and Dipl.Chem. Frank Strehler for their great help in the preparation of this paper and Dipl.-Chem. Marcus Korb for fruitful discussions.

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REFERENCES

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Organometallics

Review

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Organometallics

Review

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Review

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Organometallics

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Organometallics

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