The mechanism of asymmetric hydrogenation catalyzed by a new effective catalyst, viz., a rhodium complex of (S,S)-1,2-bis(tert-butylmethylphosphino)ethane (BisP*), has been studied by multinuclear NMR. Hydrogenation of the precatalyst [Rh(BisP*)(nbd)]BF4 (8) at −20 °C in deuteriomethanol affords solvate complex [Rh(BisP*)(CD3OD)2]BF4 (9), which is, in turn, hydrogenated at −90 °C producing equilibrium amounts (20% at −95 °C) of [RhH2(BisP*)(CD3OD)2] (10)the first observable dihydride of a Rh(I) complex with a diphosphine ligand. Dihydride 10 is in equilibrium with 9 and dihydrogen, which was studied in the temperature interval from −100 to −50 °C, yielding thermodynamic parameters ΔH = −6.3 ± 0.2 kcal M-1 and ΔS = −23.7 ± 0.7 cal M-1 K-1. The hydrogenation of 9 is stereoselective:  two isomers 10a and 10b are produced in a ratio 10:1. Use of HD for the hydrogenation of 9 yields the isomers with deuterium cis and trans to the phosphine in a ratio 1.3 (±0.1):1. The thermodynamic parameters of the equilibrium between 9, 10 d , and HD are ΔH = −10.0 ± 0.4 kcal M-1 and ΔS = −20.3 ± 1 cal M-1 K-1. Dihydride 10 reacts with the substrate 12 at −90 °C, yielding the monohydride intermediate 17a. The same product is obtained when 13 is hydrogenated at −80 °C. At temperatures above −50 °C monohydride intermediate 17a undergoes reductive elimination, affording the hydrogenation product 15 in equilibrium with the product−catalyst complex 16 in which the catalyst is η6-coordinated to the phenyl ring of the product. The experimental data require that the dihydride mechanism is operating in the case of asymmetric hydrogenation catalyzed by 9. This, in turn, suggests that the enantioselective step is the migratory insertion in a dihydride intermediate 18.

This account brings together the recent experimental and computational data on the mechanism of Rh-catalyzed asymmetric hydrogenation of activated double bonds. Two alternative reaction pathways (unsaturated and dihydride) are compared. It is suggested that the differences in these mechanisms are not primarily important for stereoselection, since they join in a single pathway before stereoselection occurs. This approach was used to rationalize the present discrepancies in the prediction of the sense of enantioselection for the P-stereogenic ligands and the ligands with backbone chirality.

Both enantiomers of 2,3-bis(tert-butylmethylphosphino)quinoxaline (QuinoxP*), 1,2-bis(tert-butylmethylphosphino)benzene (BenzP*), and 1,2-bis(tert-butylmethylphosphino)-4,5-(methylenedioxy)benzene (DioxyBenzP*) were prepared in short steps from enantiopure (S)- and (R)-tert-butylmethylphosphine-boranes as the key intermediates. All of these ligands were crystalline solids and were not readily oxidized on exposure to air. Their rhodium complexes exhibited excellent enantioselectivities and high catalytic activities in the asymmetric hydrogenation of functionalized alkenes, such as dehydroamino acid derivatives and enamides. The practical utility of these catalysts was demonstrated by the efficient preparation of several chiral pharmaceutical ingredients having an amino acid or a secondary amine component. A rhodium complex of the structurally simple ligand BenzP* was used for the mechanistic study of asymmetric hydrogenation. Low-temperature NMR studies together with DFT calculations using methyl α-acetamidocinnamate as the standard model substrate revealed new aspects of the reaction pathways and the enantioselection mechanism.

The reaction of 2-alkynyl-1-methylene azide aromatics 1 with iodine gives 1,3-disubstituted 4-iodoisoquinolines 3, and the treatment of 1 with a gold-silver combined catalyst affords isoquinolines 4. On the other hand, TfOH-catalyzed reaction of 1 produces 1,3-dipolar cycloadducts, triazoles 5. Computations reveal that non-symmetrical and slightly non-symmetrical coordinations between the triple bond and electrophiles (cationic Au and iodonium species) are prone to give the isoquinolines, while symmetrical coordination with electrophiles (a Brønsted acid and Au(I)) affords the triazoles. Keeping this background material in mind, the reactions through alkyne activation with electrophiles are surveyed. In most cases, products having similar structural frameworks were obtained through alkyne activation with Brønsted acids, iodine and gold complexes; the difference is whether H or I is incorporated in the final products. However, in a few cases, different reactivities and product structures were observed between those three reagents and catalysts.