Penny A. Chaloner

The mechanism of asymmetric homogeneous hydrogenation. Solvent complexes and dihydrides from rhodium diphosphine precursors

Abstract Bicyclo [2,2,1]hepta-2,5-diene and cycloocta-1,5 diene(biphosphine)rhodium tetrafluoroborates react with hydrogen at 1 atmosphere in methanol or other polar solvents. The initial product may be either a solvated dihydride or a sol- vate; depending on phosphine structure the equilibrium between these two species varies widely. Dihydrides are normally the stable product when the ligand is a monophosphine although ( o -methoxyphenyl)methylphenylphos- phine is an exception. cis -Chelating biphosphines normally form solvate com plexes with no affinity for hydrogen. R -Phenyl bis-diphenylphosphinoethane falls into this category, but the 31 P NMR spectra of its complexes demonstrate an equilibrium between monomeric and dimeric species, and addition of triethylamine gives rise to a trimer. trans -Chelating biphosphines show more variable behaviour, and in the case of bis-1,5-diphenylphosphinopentane, a number of complexes, including one requiring C—H activation, are observed.

Enantioselective addition of diethylzinc to benzaldehyde in the presence of ephedrine derivatives

Abstract Diethylzinc reacts with benzaldehyde derivatives to yield 1-aryl-1-propanols. In the presence of ephedrine and its derivatives an optical yield of up to 80% is obtained for reaction at room temperature without special precautions.

Catalytic asymmetric alkylation of aldehydes by diethylzinc in the presence of N-iso-propylephedrine; dependence of enantioselectzvity on Et2Zn:RCHO

Abstract The enantioselective alkylation of a range of aldehydes by diethylzinc in the presence of N-isopropylephedrine has been investigated. Both the yield and the enantioselectivity of the reaction was found to depend on the ratio of Et 2 Zn to the aldehyde.

The mechanism of asymmetric hydrogenation catalysed by rhodium complexes of chiral pyrrolidinobiphosphines

Abstract A wide range of modes of coordination of dehydroamino acids and itaconic acid derivatives to rhodium complexes of pyrrolidinobiphosphines may be discer

New dithiolate-bridged rhodium complexes

Addition of a dithiol ligand HS(CH2)nSH (n= 2–4) to [{Rh(µ-OMe)(cod)}2](cod = cycloocta-1,5-diene) afforded dithiolate-bridged complexes [{Rh2[µ-S(CH2)nS](cod)2}x](x= 1 or 2). These diene complexes react with carbon monoxide to give the binuclear tetracarbonyl complexes [Rh2{µ-S(CH2)nS}(CO)4]. The crystal structures of [Rh2{µ-S(CH2)nS}(cod)2](n= 2 or 3) have been determined by X-ray diffraction methods. A study of the electronic structure of the complexes by analysis of the topology of the charge density shows that in spite of a zero formal bond order between the rhodium atoms a direct metal–metal interaction may be characterized.

Rhodium(I) and iridium(I) complexes of Ph2P(S)CH2P(S)Ph2, bis(diphenylphosphino)methane disulfide

Abstract Rhodium(I) and iridium(I) complexes of the ligand bis(diphenylphosphino)methane disulphide (dppmS 2 ) have been prepared and characterised. The structures of [Rh(dppmS 2 )(cod)][ClO 4 ] and [Rh(dppmS 2 (CO) 2 ][ClO 4 ] were established in diffraction studies, the latter complex adopting an interesting pseudo-dimeric structure in the solid state.

Mixed biphosphine complexes of palladium(0) ; the structure of {bis(diphenylphosphino)ethane}-{bis(dicyclohexylphosphino)ethane} palladium(0)

Abstract A range of mixed biphosphine complexes of palladium(0) has been prepared and characterized by 31P NMR spectroscopy. The complexes disproportionate in solution at rates which depend on the nature of the ligands. The structure of [Pd(dcpe)(dppe)] has been established in an X-ray diffraction study.

New mono- and bis(pentafluorophenyl)palladium(II) complexes with iminophosphine ligands. Crystal structure of [Pd(C6F5)(SC6H5)(o-Ph2PC6H4–CHNiPr)]

Abstract The synthesis of new mono- and bis(pentafluorophenyl) derivatives of palladium(II) with the mixed-donor bidentate ligands o-Ph2PC6H4–CHNR (RN–P) has been achieved. The new complexes of general formula [Pd(C6F5)2(RN–P)] [R=Me (1), Et (2), nPr (3) iPr (4), tBu (5), Ph (6), NH–Me (7)] and [Pd (C6F5)Cl(RN–P)] [R=Me (8), iPr (9), NH–Me (10)] have been, respectively, prepared by reaction between the labile precursors cis-[Pd(C6F5)2(PhCN)2] or [{Pd(C6F5)(tht)(μ-Cl)}2] (tht=tetrahydrothiophene) and the corresponding iminophosphines. Complex (9) undergoes metathetical exchange of chloride with anionic monodentate ligands when reacting with alkaline salts, giving the complexes of formula [Pd(C6F5)X(iPrN–P)] [X=Br (11), I (12), CN (13), SCN (14), SC6H5 (15), p-SC6H4Me (16), p-SC6H4NO2 (17), OMe (18)]. Furthermore, when the chloropentafluorophenyl complex (9) was treated with silver trifluoromethanesulfonate in the presence of tertiary phosphines, the cationic derivatives [Pd(C6F5)(L)(iPrN–P)](CF3SO3) [L=PEt3 (19), PMe2Ph (20), PMePh2 (21)] were obtained in good yield. The new complexes were characterized by partial elemental analyses and spectroscopic methods (IR, 1H, 19F and 31P NMR). The molecular structure of complex (15) has been determined by a single-crystal diffraction study, showing that the iminophosphine act as chelating ligand with coordination around the palladium atom slight distorted from the square-planar geometry.

Intermediates in asymmetric hydrogenation : The structure and 31P NMR spectra of rhodium enamide complexes containing 1R, 2R-trans-1,2-bis (diphenylphosphinomethyl) cyclo buta

Abstract Hydrogenation of bicyclo[2.2.1]heptadiene-1R,2R-( trans -1,2- bis (diphenylphosphinomethyl)cyclbutane rhodium ( I ) tetrafluoroborate in methanol gives a solvent adduct which reacts with N-acyldehydroamino acids and their esters to give air-unstable scarlet to yellow complexes. Effects of structure variation in the enamide on the 31 P NMR spectra of these species are reported and discussed with reference to (a) the equilibrium between methanol and enamide complexes; (b) the ratio of diastereomeric enamide complexes formed; (c) the temperature dependence of this ratio; (d) the rate of complexation equilibria and (e) the structure of enamide complexes, which are normally bidentate with binding via the olefin and amide groups. In certain cases the complex may be terdentate and E -enamides bind through the olefin and carboxyl groups. Each mode of binding gives rise to characteristic 31 P NMR spectra with regard to P-P and P-Rh coupling constants.

Crystal growth and hydration effects in morpholinium 4-hydroxybenzoate (M4HB)

The growth of a large single crystal of the new non-linear optical salt morpholinium 4-hydroxybenzoate (M4HB) is reported. Growth of crystals from dry ethanol, or 2-propanol, yields the non-centrosymmetric form belonging to space group Cc (No. 9). This is the desirable structure since it exhibits second harmonic generation (SHG). Attempts to improve solubility by using a water : ethanol co-solvent result in the total loss of SHG activity due to the formation of morpholinium 4-hydroxybenzoate dihydrate in the centrosymmetric space group P2 1 /c (No. 14).