Herman Ho-Yung Sung

Synthesis of rhenabenzenes from the reactions of rhenacyclobutadienes with ethoxyethyne.

Treatment of Na[Re(CO)5 ] with RCCCO2 Et (R=phenyl, naphthalen-1-yl, phenanthren-9-yl and pyren-1-yl) followed by reaction with acetyl chloride and ethanol afforded the rhenacyclobutadienes Re{-C(R)C(CO2 Et)C(OEt)}(CO)4 . Reactions of these rhenacyclobutadienes with HCCOEt produced rhenabenzenes Re{-C(R)C(CO2 Et)C(OEt)CHC(OEt)}(CO)4 . Except for R=Ph, new rhenacyclobutadienes with pendant alkenyl substituents Re{-C(R)C(C(OEt)CH(CO2 Et))C(OEt)}(CO)4 were also isolated from these reactions. The NMR spectroscopic and X-ray structural data, as well as the aromatic stabilization energy (ASE) values suggest that the rhenabenzenes are aromatic, with extensive delocalized π character.

Role of diastereomeric solid-solution disorders in limiting resolution for spatially similar enantiomers: case studies using spiroborate anions

The classical method of resolution through diastereomeric salt formation has long been established. [1] However detailed structure determination of isolated solids is frequently ignored, since the most important issue is the %ee (enantiomeric excess) present in solid (or solution) phase. Recently we described the use of a spiroborate anion [B(Man)2] (Man = mandelate) for the efficient resolution of a variety of chiral cations with %ee in excess of 90% found in the first isolation step.[2] However certain challenging cations for which the spatial overlap between enantiomers is high may not give such excellent %ee although a single phase solid is isolated. This is due to the formation of a diastereomeric solid-solution in which the enantiomeric pair is disordered at the cation site. Study of possible disorder modes is of interest and importance in overcoming this issue. For simple amines with a single chiral carbon possessing C-H group our studies have discovered four distinct disorder modes. Two such disorders are shown for the sec-butylammonium and a-methylbenzylammonium salts of [B(TarNHPh2)2] derived from the diol diphenyltartramide. Using an expanded set of spiroborate resolving anions and crystallization conditions are both important for improving the chances of more optimal resolutions.

Resolution of SPINOL by cinchona alkaloids and application of high-throughput screening by solvent-assist grinding

SPINOL (1,1’-spirobiindan-7,7’-diol) is a useful chiral ligand used in asymmetric catalysis.[1] Originally prepared itself by a chiral catalysis, it was later resolved using an inclusion complex technique pioneered by Toda.[2] Thus the N-benzylcinchonidinium chloride or bromide salt could co-crystallize with SPINOL.[3] The use of enantiomeric neutral cocrystals for resolution was also well established, thus we also attempted to resolve SPINOL by screening for co-crystal formation. This included using the neutral alkaloid bases from the Cinchona family. Solvent-assisting grinding was applied in screening of diastereomeric solid formation with N-benzylcinchonidinium bromide and cinchonidine. Other than the reported ‘’inclusion complex’’, a new inclusion complex with a solvate, which resolved the opposite hand of (R)-SPINOL, was discovered when screening with N-benzylcinchonidinium bromide. Also a 1:1 co-crystal with cinchonidine itself was readily formed and found to contain predominantly R-SPINOL. These phases were characterized by single X-ray diffraction, and the optical purity was determined by chiral HPLC. A single recrystallization improved the % ee to the required 99% level. The isolation of both hands is facilitated since recrystallization of the chiral SPINOL from the supernatant solution can rapidly afford an enantiopure sample since SPINOL forms a conglomerate when crystallizing from methanol.