Crystal structure of bis{(S)-1-[2-(diphenylphosphanyl)ferrocenyl]-(R)-ethyl}ammonium bromide dichloromethane monosolvate

During the synthesis of an FeBr2 complex with the PNP ligand (R,R,SFc,SFc)-[Fe2(C5H5)2(C38H35NP2)] (1), single crystals of the dichloromethane monosolvate of the Br− salt of the protonated ligand 1H+ were obtained serendipitously, i.e. [Fe2(C5H5)2(C38H36NP2)]Br·CH2Cl2. The crystal structure of 1H·Br·CH2Cl2 was determined by single-crystal X-ray diffraction. The mean bond lengths in the ferrocene units are Fe—C = 2.049 (3) Å and C—C = 1.422 (4) Å within the cyclopentadienyl rings. The mean C—N bond length is 1.523 (4) Å. The interplanar angle between the two connected cyclopentadienyl rings is 49.2 (2)°. One ferrocene moiety adopts a staggered conformation, whereas the other is between staggered and eclipsed. The Br− ions and the CH2Cl2 molecules are located in channels extending along <100>. One ammonium H atom forms a hydrogen bond with the Br− ion [H...Br = 2.32 (4) Å and C—H...Br = 172 (3)°]. The second ammonium H atom is not involved in hydrogen bonding

{(R,SFc,SFc)-2&amp;#8242;&amp;#8242;-Bromo-2-[1-(dimethylamino)ethyl-&amp;#954;N]-1,1&amp;#8242;&amp;#8242;-biferrocene}trihydridoboron

The title structure, [Fe2(C5H5)2(C14H19BBrN)], contains a chiral and asymmetrically 2,2&amp;#8242;&amp;#8242;-disubstituted biferrocene designed as precursor for enantioselective non-C2-symmetric biferrocenyldiphosphine catalysts. The mean bond lengths in the biferrocene unit are Fe&amp;#8212;C = 2.048&amp;#8197;(10)&amp;#8197;&amp;#197; and C&amp;#8212;C = 1.427&amp;#8197;(8)&amp;#8197;&amp;#197; within the cyclopentadienyl rings. The B&amp;#8212;N bond lengths of the BH3 protected amine is 1.631&amp;#8197;(3)&amp;#8197;&amp;#197;. The interplanar angle between the two connected cyclopentadienyl rings is 54.29&amp;#8197;(8)&amp;#176; and the corresponding Fe&amp;#8212;Cg&amp;#8212;Cg&amp;#8212;Fe torsion angle is &amp;#8722;52.5&amp;#176;. The conformation of the molecule is stabilized by an intramolecular C&amp;#8212;H...Br interaction

Walphos versus Biferrocene-Based Walphos Analogues in the Asymmetric Hydrogenation of Alkenes and Ketones

Two representative Walphos analogues with an achiral 2,2″-biferrocenediyl backbone were synthesized. These diphosphine ligands were tested in the rhodium-catalyzed asymmetric hydrogenation of several alkenes and in the ruthenium-catalyzed hydrogenation of two ketones. The results were compared with those previously obtained on using biferrocene ligands with a <i>C</i>₂-symmetric 2,2″-biferrocenediyl backbone as well as with those obtained with Walphos ligands. The application of one newly synthesized ligand in the hydrogenation of 2-methylcinnamic acid gave (<i>R</i>)-2-methyl-3-phenylpropanoic acid with full conversion and with 92% ee. The same ligand was used to transform 2,4-pentanedione quantitatively and diastereoselectively into (<i>S</i>,<i>S</i>)-2,4-pentanediol with 98% ee

Ruthenium Complexes of Phosphino-Substituted Ferrocenyloxazolines in the Asymmetric Hydrogenation and Transfer Hydrogenation of Ketones: A Comparison

Three novel routes have been developed for the synthesis of ferrocenyl-based phosphino-oxazolines in which the phosphino unit is attached to a ferrocenylmethyl or a ferrocenylethyl side chain. In two of the routes the phosphino-substituted ethyl side chain was built up diastereoselectively. Ruthenium complexes of the type [RuCl₂PPh₃(L)] of 12 bidentate phosphine-oxazoline ligands were synthesized, characterized, and tested in the transfer hydrogenation of acetophenone. For the best performing complexes a total of 12 additional ketones were screened in transfer hydrogenations and hydrogenations under transfer hydrogenation conditions. Two catalyst precursors in particular delivered products with an enantiomeric excess of up to 98% in transfer hydrogenations and 99% ee in hydrogenations. The transfer hydrogenation results obtained with all novel ligands were compared to those of two well-established FOXAP ligands. Furthermore, a qualitative comparison with the hydrogenation data was carried out. In both cases surprising similarities in product enantiomeric excess and product absolute configuration were found. Attempts were made to rationalize some of the observed features by considering a transition-state model. The molecular structures of one synthesis intermediate, two catalyst precursors, and two corresponding acetonitrile complexes were studied by X-ray diffraction

Iron(II) Complexes Containing Chiral Unsymmetrical PNP′ Pincer Ligands: Synthesis and Application in Asymmetric Hydrogenations

Four new chiral PNP′ pincer ligands with a scaffold consisting of a planar chiral ferrocene and a centro chiral aliphatic unit were synthesized and characterized. Treatment of anhydrous FeBr₂(THF)₂ with 1 equiv of the unsymmetrical chiral PNP′ pincer ligands afforded complexes of the general formula [Fe­(PNP′)­Br₂]. In the solid state these complexes adopt a tetrahedral geometry with the PNP′ ligands coordinated in a κ²<i>P</i>,<i>N-</i>fashion, as shown by X-ray crystallography. These complexes react with CO in the presence of NaBH₄ to yield hydride complexes of the type [Fe­(PNP′)­(H)­(Br)­(CO)], which were isolated and tested as catalysts in the asymmetric hydrogenation of ketones. Enantioselectivities of up to 81% ee were obtained

Iron(II) Complexes Containing Chiral Unsymmetrical PNP′ Pincer Ligands: Synthesis and Application in Asymmetric Hydrogenations

Four new chiral PNP′ pincer ligands with a scaffold consisting of a planar chiral ferrocene and a centro chiral aliphatic unit were synthesized and characterized. Treatment of anhydrous FeBr₂(THF)₂ with 1 equiv of the unsymmetrical chiral PNP′ pincer ligands afforded complexes of the general formula [Fe­(PNP′)­Br₂]. In the solid state these complexes adopt a tetrahedral geometry with the PNP′ ligands coordinated in a κ²<i>P</i>,<i>N-</i>fashion, as shown by X-ray crystallography. These complexes react with CO in the presence of NaBH₄ to yield hydride complexes of the type [Fe­(PNP′)­(H)­(Br)­(CO)], which were isolated and tested as catalysts in the asymmetric hydrogenation of ketones. Enantioselectivities of up to 81% ee were obtained

Biferrocene-Based Diphosphine Ligands: Synthesis and Application of Walphos Analogues in Asymmetric Hydrogenations

A total of four biferrocene-based Walphos-type ligands have been synthesized, structurally characterized, and tested in the rhodium-, ruthenium- and iridium-catalyzed hydrogenation of alkenes and ketones. Negishi coupling conditions allowed the biferrocene backbone of these diphosphine ligands to be built up diastereoselectively from the two nonidentical and nonracemic ferrocene fragments (<i>R</i>)-1-(<i>N</i>,<i>N</i>-dimethylamino)­ethylferrocene and (<i>S</i>_Fc)-2-bromoiodoferrocene. The molecular structures of (<i>S</i>_Fc)-2-bromoiodoferrocene, the coupling product, two ligands, and the two complexes ([PdCl₂(L)] and [RuCl­(<i>p</i>-cymene)­(L)]­PF₆) were determined by X-ray diffraction. The structural features of complexes and the catalysis results obtained with the newly synthesized biferrocene-based ligands were compared with those of the corresponding Walphos ligands

Halide-Mediated <i>Ortho</i>-Deprotonation Reactions Applied to the Synthesis of 1,2- and 1,3-Disubstituted Ferrocene Derivatives

The <i>ortho</i>-deprotonation of halide-substituted ferrocenes by treatment with lithium tetramethylpiperidide (LiTMP) has been investigated. Iodo-, bromo-, and chloro-substituted ferrocenes were easily deprotonated adjacent to the halide substituents. The synthetic applicability of this reaction was, however, limited by the fact that, depending on the temperature and the degree of halide substitution, scrambling of both iodo and bromo substituents at the ferrocene core took place. Iodoferrocenes could not be transformed selectively into <i>ortho</i>-substituted iodoferrocenes since, in the presence of LiTMP, the iodo substituents scrambled efficiently even at −78 °C, and this process had occurred before electrophiles had been added. Bromoferrocene and certain monobromo-substituted derivatives, however, could be efficiently <i>ortho</i>-deprotonated at low temperature and reacted with a number of electrophiles to afford 1,2- and 1,2,3-substituted ferrocene derivatives. For example, 2-bromo-1-iodoferrocene was synthesized by <i>ortho</i>-deprotonation of bromoferrocene and reaction with the electrophiles diiodoethane and diiodotetrafluoroethane, respectively. In this and related cases the iodide scrambling process and further product deprotonation due to the excess LiTMP could be suppressed efficiently by running the reaction at low temperature and in inverse mode. In contrast to the low-temperature process, at room temperature bromo substituents in bromoferrocenes scrambled in the presence of LiTMP. Chloro- and 1,2-dichloroferrocene could be <i>ortho</i>-deprotonated selectively, but in neither case was scrambling of a chloro substituent observed. As a further application of this <i>ortho</i>-deprotonation reaction, a route for the synthesis of 1,3-disubstituted ferrocenes was developed. 1,3-Diiodoferrocene was accessible from bromoferrocene in four steps. On a multigram scale an overall yield of 41% was achieved. 1,3-Diiodoferrocene was further transformed into symmetrically 1,3-disubstituted ferrocenes (1,3-R₂Fc; R = CHO, COOEt, CN, CHCH₂)