Nicholas K. Roberts

The origins of the enantioselection in asymmetric catalytic hydrogenation of amino-acid precursors

Comparison of c.d. spectra confirms that the isolated diastereomer of the initial catalyst-substrate adduct in the [Rhchiraphos]+-catalysed [chiraphos =(2S, 3S)-2,3-bis(diphenylphosphino)butane] hydrogenation of ethyl (Z)-α-acetamidocinnamate is also the major diastereomer in solution; this supports the conclusion that the prevailing chirality of the product has its origins in the minor diastereomer.

Resolutions using metal complexes. Resolution of (RR,SS)-ortho-phenylenebis(methylphenylarsine) using internally diastereoisomeric palladium complexes

The dissymmetric chelating agent (RR,SS)-ortho-phenylenebis(methylphenylarsine) has been resolved into its enantiomers by fractionally crystallising internally diastereoisomeric palladium complexes containing the ditertiary arsine and optically active ortho-metallated dimethyl(α-methylbenzyl)amine. The full procedure is described and constitutes the first satisfactory route to optically active ditertiary arsines containing asymmetric arsenic atoms. The optically active ditertiary arsines, α(589 nm, CH2Cl2)± 95°, are air-stable crystalline solids, m.p. 75–76 °C, and the absolute configuration of the enantiomer with a positive rotation at the sodium D line (589 nm) has been established as RR by a single-crystal X-ray determination of the structure of the internally diastereoisomeric palladium complex containing (S)(–)-dimethyl(α-methylbenzyl)amine. The meso diastereoisomer undergoes a halide-ion assisted epimerisation in acidic methanol solution and may be converted, under suitable conditions, into the less soluble racemic diastereoisomer thereby effecting a second-order asymmetric transformation between the diastereoisomeric ditertiary arsines.

Partial asymmetric synthesis of substituted trans-2,3-diaryloxirans using optically active arsonium ylides

Ylides derived from optically active arsonium salts react with aromatic aldehydes to give trans-2,3-diaryl-oxirans in high chemical yields and with optical purities of between 4·7 and 38%.