PHIP Detection of a Transient Rhodium Dihydride Intermediate in the Homogeneous Hydrogenation of Dehydroamino Acids

Role of the reaction intermediates in determining PHIP (parahydrogen induced polarization) effect in the hydrogenation of acetylene dicarboxylic acid with the complex [Rh (dppb)]+ (dppb: 1,4-bis(diphenylphosphino)butane)

“…8 Transient dyhidride intermediates are extremely elusive and they have never been observed, except in one case, using parahydrogen with an unusually reactive complex at low temperature. 10 Nevertheless, hyperpolarization attained on products is often much lower than that theoretically predicted, 5 even with this kind of catalyst. In this work it is shown how the level of polarization obtained on the hydrogenated product is also influenced by the structure and lifetime of these reaction intermediates, in fact an intermediates stability change due to the interaction with different solvent molecules leads to different hyperpolarization level.…”

Para‐hydrogen enrichment and hyperpolarization

“…2(b), right, it can be seen that the enhancement of all lines is in the ratio 2 ⌬/3␦Ј, which can be of the order of 10 4 : the enhancement that is thus obtained has been widely applied to the study of hydrogenation mechanisms, and very low abundant species, such as reaction intermediates, become observable by means of parahydrogen (9,(32)(33)(34)(35)(36)(37) . The lines (with enhanced intensities) corresponding to transitions starting from ␣␣ will appear in emission, whereas for transitions reaching ␤␤, enhanced lines appear in absorption.…”

Modular P‐Chirogenic Aminophosphane‐Phosphinite Ligands for Rh‐Catalyzed Asymmetric Hydrogenation: A New Model for Prediction of Enantioselectivity

“…Naturally, much work has been focused on the origin of the enantioselectivity of such reactions, and for a long time it was considered that the selectivity does not stem from the relative stabilities of the substrate-rhodium complexes, but from their rate constants for the oxidative addition of dihydrogen (unsaturated route). [36][37][38][39][40][41][42][43][44] However, recent experimental and computational data have provided support for a second reaction pathway involving a fast equilibrium of the dihydride-substrate complexes 3, followed by the stereodetermining migration insertion step [35,[45][46][47][48][49][50][51][52] (Scheme 2). Finally, the stereoselectivities of the unsaturated and the dihydride pathways are related, even though in the first, substrate coordination occurs before the oxidative addition of dihydrogen, and in the second, substrate coordination occurs after dihydrogen addition.…”