P-stereogenic phosphines

Scheme 21. Example of deboronation of P-stereogenic phosphine-boranes...

In recent years, the research group of Imamoto has been very active in the area of C2-symmetric P-stereogenic phosphine ligands [87]. Two such ligands,... 

Preparation. - Supercritical nitrous oxide has been shown to oxidise phosphines to the related phosphine oxides under mild conditions, allowing a simple isolation of products. Oxidation of precursor phosphines by hydrogen peroxide is the final step in the synthesis of the chiral functionalised phosphine oxides (219) and (220). A novel resolution procedure for the preparation of P-stereogenic phosphine oxides is afforded by the reactions of racemic chiral tertiary phosphines with an optically pure camphorsulfonyl azide, followed by separation of the diastereoisomeric phosphazenes, and acid... 

Catalytic symmetric synthesis of P-stereogenic phosphines by cross-coupling of secondary phosphines with benzyl or other alkyl halides was promoted by chiral ft and Ru complexes [112-114]. The key step is believed to be the nucleophilic attack of a metal-phosphido complex on a free electrophile the background reaction of the unactivated nucleophilic substrate is much slower. The origin of enantioselec-tivity, as in the Pd-catalyzed asymmetric cross-couplings described above, is the interconversion of diastereomeric phosphido complexes, whose speciation and relative rates of nucleophilic attack determine the product ratio. In the case of ft ((R./ )-Me-DuPhos)(Ph)(PMels), as with the Pd analog in Scheme 43 above, the major product phosphine was formed from the major diastereomeric phosphido complex (Scheme 63) [112-113]. 

Molybdenum-catalyzed enantioselective olefin metathesis reactions of P-stereogenic phosphinates (597) which led to formation of five-, six-, and seven-membered P-heterocycles (698) in up to 98% ee has been reported by Hoveyda and co-workers (Scheme 148). ° ... 

Scheme 10 P-Stereogenic //-phosphinate 24 as versatile P-stereogenic building block...

For the last few year the catalytic asymmetric synthesis of tertiary phosphines has attracted the attention of many chemists. Interesting results were published in many articles and reviews [109-115]. Catalyzed by transition metals, asymmetric phosphination of secondary, racemic phosphines with aryl halides or triflates to prepare a tertiary P-stereogenic phosphines with control of the stereochemistry at the phosphorus atom is shown in Scheme 62. 

One of the routes leading to P-stereogenic phosphines is electrophilic substim-tion at the phosphorus atom of secondary phosphines, as a result of asymmetric catalysis in which a catalyst activates a phosphorus nucleophile or a carbon electrophile, creating an asymmetric environment, i.e., creating preference for one of Si or Re face sides at the reactive center [103-113]. Upon reaction with chiral metal complexes, racemic secondary phosphines are converted into diaste-reomeric metal-phosphide complexes A or B, which interconvert rapidly through the inversion at phosphorus. If the equilibrium A B is faster than the reaction of A or B with an electrophile E, then P-stereogenic phosphines 196, in which pyramidal inversion is slow, can be formed enantioselectively. The product ratio in this dynamic kinetic asymmetric transformation depends both on and on the rate constants ks and (Scheme 63). 

Reichl KD, Ess DH, Radosevich AT (2013) Catalyzing pyramidal inversion configurational lability of P-stereogenic phosphines via single electron oxidation. J Am Chem Soc 135 9354-9357... 

Glueck DS (2008) Applications of P NMR spectroscopy in development of M(Duphos)-catalyzed asymmetric synthesis of P-stereogenic phosphines (M=Pt or Pd). Coord Chem Rev 252 2171-2179... 

Glueck DS (2007) Metal-catalyzed asymmetric synthesis of P-stereogenic phosphines. Synlett 2627-2634... 

Scriban C, Glueck DS (2006) Platinum-catalyzed asymmetric alkylation of secondary phosphines enantioselective synthesis of P-stereogenic phosphines. J Am Chem Soc 128 2788-... 

Blank NF, McBroom KC, Glueck DS, Kassel WS, Rheingold AL (2006) Chirality breeding via asymmetric phosphination. Palladium-catalyzed diasteieoselective synthesis of a P-stereogenic phosphine. Organometallics 25 1742-1748... 

Figure 1.2 Examples of P-stereogenic phosphines resolved between 1961 and 1966.

The optical yields of 9 were low, but proved that enantioselective catalytic homogeneous hydrogenation was feasible. The extremely low enantioselectivity with the phosphine of entry 5 (Table 1.1) was justified by the location of the stereogenic centres further away from the metal in comparison to P-stereogenic phosphines. 

Knowles and co-workers reported that DiPAMP, the most well-known P-stereogenic phosphine, gave 12 with an outstanding (at that time) 96% ee. With this phosphine many other enamides, enol esters and similar compounds could be hydrogenated in high ee. This work can be considered a milestone in the field of enantioselective homogeneous catalysis and have been profusely cited. William S. Knowles was awarded (along with Ryoji Noyori and Barry Sharpless) the Nobel Prize in Chemistry for the year 2001 for his contributions to enantioselective catalysis. ... 

Phosphine oxides were the classic precursors to P-stereogenic phosphines until they were (partially) supplanted by phosphine boranes. Here some of the methods of oxidation and reduction of phosphines (Scheme 1.6) are briefly... 

Finally, Section 2.5 describes the use of stoichiometric Pd-templated synthesis of certain P-stereogenic phosphines. 

This types of resolution were originally described in 1971 by Otsuka and coworkers with 15 (R = R = Me) and since then they have been applied to numerous racemic P-stereogenic phosphines. In spite of an early use of complex 17 (R = R = Me), 15 or 16 are more typically used. A limitation of the method is that the optimum complex for the resolution depends on the exact phosphine being resolved. Naphthyl complex 16 is frequently a better resolving agent than 15, but also more expensive. ... 

Scheme 2.18 One-pot synthesis of P-stereogenic phosphinous acid boranes.

The first example of a resolution of a free secondary P-stereogenic phosphine was reported by Wild and co-workers.They found that the diastereomers of menthylmesitylphosphine (92) could be separated by fractional recrystallisation of acetonitrile solutions of the nearly 1 1 epimeric mixture of 92. The key to their success was the presence of sodium acetylacetonate (0.04%) as proton scavenger. The optical purity of the recovered solids and of the mother liquor was easily monitored by because, remarkably, the resonances of the... 

Scheme 2.37 Preparation of pyrrolidine-based P-stereogenic phosphines.

Many syntheses of chiral ferrocenes start from the well-known Ugi s amine, and P-stereogenic phosphines are not an exception (Scheme... 

Table 2.15 P-stereogenic phosphines prepared by Pd-promoted Diels-Alder reactions.

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