Afrooz Zirakzadeh

Enantioselective Transfer Hydrogenation of Ketones Catalyzed by a Manganese Complex Containing an Unsymmetrical Chiral PNP′ Tridentate Ligand

Manganese complexes of the types [Mn(PNP′)(Br)(CO)2] and [Mn(PNP′)(H)(CO)2] containing a tridentate ligand with a planar chiral ferrocene and a centro chiral aliphatic unit were synthesized, characterized, and tested in the enantioselective transfer hydrogenations of 13 ketones. The catalytic reactions proceeded with conversions up to 96 % and ee values up to 86 %. The absolute configuration of all products was determined to be (S). Notably, the presence of dihydrogen (up to 20 bar) did not affect the reduction. On the basis of DFT calculations, preliminary mechanistic details including the origin of the (S) selectivity are presented. The molecular structure of [Mn(PNP′)(Br)(CO)2] was studied by X‐ray diffraction.

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 (R)-1-(N,N-dimethylamino)ethylferrocene and (SFc)-2-bromoiodoferrocene. The molecular structures of (SFc)-2-bromoiodoferrocene, the coupling product, two ligands, and the two complexes ([PdCl2(L)] and [RuCl(p-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.

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 C2-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 (R)-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 (S,S)-2,4-pentanediol with 98% ee.

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

The ortho-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 ortho-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 ortho-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 ortho-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 ortho-deprotonated selectively, but in neither case was scrambling of a chloro substituent observed. As a further application of this ortho-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).

Synthesis, coordination behavior and structural features of chiral iron( ii ) PNP diferrocene complexes

Five new chiral PNP ferrocene ligands with either an imine or amine nitrogen coordination site were synthesized. Only the imine type ligands formed Fe(II) complexes with the general formula [Fe(PNP)X2] (X = Cl, Br). In the solid state these complexes adopt a tetrahedral geometry with the PNP ligand coordinated in a κ2P,N-fashion with the one pendant-arm and the other not coordinated, as determined by X-ray crystallography and Mossbauer spectroscopy. The complexes are paramagnetic with a quintet ground state. In solution there is an equilibrium between [Fe(κ3P,N,P-PNP)X2] and [Fe(κ2P,N-PNP)X2] complexes. Boronation of the non-coordinated arm shifts the equilibrium towards the four-coordinate complex [Fe(κ2P,N-PNPBH3)Br2]. DFT calculations are consistent with the experimental results and indicate that the experimentally observed κ2 isomer is thermodynamically the most stable. In a CO atmosphere, [Fe(PNP)(CO)2Br]Br was formed rather than [Fe(PNP)(CO)Br2].

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

The absolute structure of (R,R,S Fc,S Fc)-[Fe2(C5H5)2(C38H36BrNP2)]·Br·CH2Cl2 has been determined by X-ray single-crystal diffraction.