Dedicated to Professor Albert Eschenmoser on the occasion of his 85th birthdayChiral diphosphines are the most frequently used ligands in asymmetric catalysis. [1] In contrast, chiral secondary phosphine oxides (SPOs) are little explored as ligands. While their chemical and physical properties are well known, their use in asymmetric catalysis is still in its infancy. [2] SPOs are stable molecules which exist in equilibrium between two tautomeric forms: [3] the preferred pentavalent phosphine oxide and the trivalent phosphinous acid. When two different substituents are attached to the phosphorus atom, a configurationally stable, P-chiral group results which can coordinate to metals either through the phosphorus atom or through the oxygen atom.To date, only a few examples of asymmetric catalytic reactions with chiral SPOs have been described.[2] Ph-(tBu)P(O)H, a monodentate P-chiral SPO gave approximately 80 % ee in the palladium-catalyzed allylic alkylation, [4] while over 90 % ee was obtained with P-chiral diamino phosphine oxides.[5] In asymmetric hydrogenation, Rh and Ir complexes of monodentate chiral SPO ligands gave only moderately active and selective catalysts (ee values up to 85 %). [2c, 6] We thought that these somewhat disappointing results might be due to an insufficient affinity of SPOs for Rh, Ir, or Ru centers, the typical metals used in asymmetric catalytic hydrogenations. Our idea was therefore to combine an SPO with a PR 2 substituent which should not only lead to stronger coordination to the metal center but also should give better defined complexes. To avoid cumbersome resolution procedures [2c, 6, 7] we used either a chiral backbone or a chiral substituent, so that the chiral SPO unit could be built up in diastereoselective reactions (Scheme 1).Herein we present results for selected members of two SPO-P ligand families based on a chiral ferrocenyl backbone and a menthyl substituent, respectively (Scheme 1). The first approach leads to ligands structurally similar to the well known Josiphos [8] (therefore called JoSPOphos) while the second gives menthyl derivatives (called TerSPOphos since other terpene moieties are feasible). Both ligand families are modular, allowing the ligand properties to be tuned by the choice of the R and R' groups. First tests showed that these novel ligands give excellent enantioselectivities and high turnover numbers for the hydrogenation of a variety of functionalized alkenes.Two routes were developed for the preparation of the JoSPOphos ligands (Scheme 2). In route 1 the phosphine group was introduced before the SPO group, starting from (R)-N,N-dimethyl-1-[(S)-2-bromoferrocenyl]ethylamine (3), obtained by lithiation/bromination of the (R)-Ugi amine.[9]The dimethylamino group was exchanged for the desired PR 2 group to give ferrocenyl phosphine bromides 4 with retention of configuration. JoSPOphos ligands 1 a-d were obtained by treating 4 a or 4 b with BuLi at low temperature, subsequent addition of the chosen dichlorophosphine, and finally hydrolysis with water...
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