The combinatorial approach to asymmetric hydrogenation: phosphoramidite libraries, ruthenacycles, and artificial enzymes

Prediction of enantioselectivity in rhodium catalyzed hydrogenations

Using the Q2MM method, new molecular mechanics parameters were developed to perform initial screening of a chiral library to focus the experimental screening for the rhodium catalyzed hydrogenation of enamides. Computational predictions agree very well with experimental data.

Rh-catalyzed enantioselective conjugate addition of arylboronic acids with a dynamic library of chiral tropos phosphorus ligands

A library of 19 chiral tropos phosphorus ligands, based on a free-to-rotate (tropos) biphenol unit and a chiral P-bonded alcohol (11 phosphites, 1-P(O)(2)O to 11-P(O)(2)O) or secondary amine (8 phosphoramidites, 12-P(O)(2)N to 19-P(O)(2)N), were screened, individually and in combinations of two, in the rhodium-catalyzed asymmetric conjugate addition of arylboronic acids to enones and enoates. High enantioselectivities (up to 99 % ee) and excellent yields were obtained in the addition to either cyclic or acyclic substrates. The flexible biphenolic P ligands outperformed the analogous rigid binaphtholic P ligands. Variable-temperature (31)P NMR studies revealed that the biphenolic ligands are tropos even at low temperature. Only below 190 K was a coalescence observed; upon further cooling, two atropisomers were detected. The Rh homocomplexes ([Rh(L(a))(2)](+)) were also studied: in general, a single doublet (P-Rh coupling) was observed in the case of the biphenolic phosphite ligands, over the temperature range 380-230 K, demonstrating their tropos nature in the rhodium complexes even at low temperatures. On the other hand, the phosphoramidites showed different behaviors depending on the structure of the ligand and on the nature of the rhodium source. The spectrum at 230 K of the mixture of [Rh(acac)(eth)(2)] (eth=C(2)H(4)) with phosphite 6-P(O)(2)O and phosphoramidite 19-P(O)(2)N (the most enantioselective ligand combination in the conjugate addition reaction) revealed the presence of four homocomplexes (total approximately 40 %: [Rh{6-P(O)(2)O}(2)], [Rh{(aR)-19-P(O)(2)N}(2)], [Rh{(aS)-19-P(O)(2)N}(2)], [Rh{(aR)-19-P(O)(2)N}{(aS)-19-P(O)(2)N}]) and one heterocomplex, [Rh{6-P(O)(2)O}{(aR)-19-P(O)(2)N}] (approximately 60 %) In the heterocomplex, the biphenol-derived phosphite is free to rotate (tropos) while the biphenol-derived phosphoramidite shows a temperature-dependent tropos/atropos behavior (coalescence temperature=310 K).

Synthesis of solution-phase phosphoramidite and phosphite ligand libraries and their in situ screening in the rhodium-catalyzed asymmetric addition of arylboronic acids

Herein, we report the automated parallel synthesis of solution-phase libraries of phosphoramidite ligands for the development of enantioselective catalysts. The ligand libraries are screened in situ in the asymmetric rhodium-catalyzed addition of arylboronic acids to aldehydes and imines. It is shown that the described methodology results in the straightforward discovery of leads for highly efficient enantioselective catalysts.