Asymmetric Phospha‐Diels–Alder Reaction: A Stereoselective Approach towards P‐Chiral Phosphanes through Diastereotopic Face Differentiation

Abstract

Old principles, new chemistry: a facile stereoselective synthesis of P-chiral phosphanes is described in which the principle of enantiotopic and diastereotopic face differentiation used in carbon chemistry is extended to a phosphorus atom in a planar environment, that is, 2H-phospholes.

Reaction of Phospholes with Aldimines: A One-Step Synthesis of Chelating, Alpha-C2-Bridged Biphospholes

“…A certain number of bicyclic phosphines have already been prepared with these ideas in mind. 2 In this context, the synthesis of α-C 2 -bridged biphospholes is obviously interesting because it can offer an access to chelating biphosphines with two nonracemizable chiral phosphorus centers. Unfortunately, until now, only one such biphosphole has been described 3 and its lengthy multistep synthesis precludes its use for further developments.…”

P-chiral 1-phosphanorbornenes: from asymmetric phospha-Diels–Alder reactions towards ligand design and functionalisation

“…This system seemed to be suitable for stereoselective Diels-Alder reactions for two reasons: the fast equilibrium between 2b and 7 provides the intermediate 2Hphosphole 3b, which is then available for an additional cycloaddition reaction. 6 Furthermore, a detailed explanation of the observed selectivities was given which was supported by DFT-calculations. However, treating 2b/3b with 4 at −30 °C (addition of SiMe 3 Cl to a solution of 1 in THF at −30 °C, 30 min equilibration, addition of 4, 16 h at −30 °C) and subsequent filtration over silica gave 5 (characterised by 31 P{ 1 H} NMR spectroscopy).…”

“…Furthermore, 3b seems to react in a highly stereoselective manner in cycloaddition reactions, since only one of its dimers is formed at low temperatures. 6 A closer look at the energetic profile of the reaction ( Figure 3) gave further insight into the course of the reaction. Therefore, direct trapping of a 2H-phosphole, generated from the corresponding protonated or silylated 1Hphosphole, with 4 did not yield the desired 1-phosphanorbornene.…”

“…7b, 7d Increasing the temperature shifts the equilibrium towards 2b. 6 The yields and selectivities were good to high and the absolute configuration of the obtained isomers was completely determined by NOE NMR spectroscopy and X-ray crystallography ( Table 1). This system seemed to be suitable for stereoselective Diels-Alder reactions for two reasons: the fast equilibrium between 2b and 7 provides the intermediate 2Hphosphole 3b, which is then available for an additional cycloaddition reaction.…”

“…1 Today, the most widely applied ligands in asymmetric transition metal catalysis carry their optical information in the chiral backbone of the ligand. 6 Even though these compounds with a planar-coordinated phosphorus atom in a five-membered ring are generated in situ, they are readily accessible and their chemistry is well understood. Nevertheless, tremendous efforts in recent years have furnished novel synthetic pathways towards P-chiral phosphanes and triggered a comeback of this class of ligands.…”