Phosphine ligands, optically active

A number of excellent reports which deal with synthesis of optically active phosphine ligands are available to date, and have been referenced in this chapter. Therefore it is not the intention here to overlap with them, but rather to describe recent advances in the field. Thus, this chapter is intended to serve as a review to the preparation of some efficient P-chirogenic compounds which have been developed over the past ten years, by either resolution or asymmetric synthesis. Considerable progress has been made in the preparation and use of P-stereogenic compounds. Use of newer methods is stressed here however an at-... 

For general application of these chiral ligands, see (a) Kagan, H. B. Chiral Ligands for Asymmetric Catalysis in Morrison, J. D. ed. Asymmetric Synthesis, vol. 5, Chap. 1, Academic Press, New York, 1985. (b) Kagan, H. B., Sasaki, M. Optically Active Phosphines Preparation, Uses and Chiroptical Properties in Hartley, F. R. ed. The Chemistry of Organo Phosphorous Compounds, John Wiley Sons, New York, 1990, vol. 1, Chap. 3. 

Figure 5.25 Schematic representation of the coordination of the complexes [Rh(NBD)(BDPBzPS03)] (51) and [Rh(NBD) (P-P)](0S02Cp3), where P-P = the optically active phosphine ligands BINAP (52a) and DIOP (52b).

Optical yields up to 17% and 25%, respectively, have been reached in the styrene hydroformylation in the presence of cobalt or rhodium catalysts using N-alkylsalicylaldimine or phosphines as asymmetric ligands. Furthermore the hydroformylation of aliphatic and internal olefins have been achieved using rhodium catalysts in the presence of optically active phosphines. With the same catalysts, cis-butene surprisingly undergoes asymmetric hydroformulation with optical yields up to 27%. On the basis of the results obtained for cis-butene and the asymmetric induction phenomena in dichlor(olefin)(amine)platinum( 11) com-... 

The initial study in this area employed the meso-1,3-dimethylallyl ligand.432 The chloride dimer was generated from 2-pentene, and a variety of optically active phosphine ligands were added to form the chiral bisphosphine complexes. Reaction of these allyl complexes with sodiodiethyl malonate resulted in optical yields in the range of 2-29% (equation 351). 

Hydrogenation catalysts. A number of new optically active phosphine ligands have been developed for selective enantioselective reduction of N=C bonds of amino acid precursors, including DPCB(l)1, the methylene homolog (2)2 of dipamp, and the novel ligand 3, in which the source of chirality is a Re atom.5 A ligand 4 has been used as a Rh(I) complex for hydrosilylation.4... 

Asymmetric reaction is one of the most exciting features of catalyzed hydroboration since optically active phosphine ligands are the chiral auxiliaries most extensively studied for metal-catalyzed reactions (Scheme 13).134 The chiral ligands used for asymmetric hydroboration of alkenes include BINAP,136 1 03-106,167-170 QUINAP,171-173 107-109,172,174-176 and BDPP.177,178... 

Asymmetric Hydrogenation Catalyst. Since catalytic asymmetric hydrogenation of a-acylaminoacrylic acid derivatives is an important process to afford a-amino acid derivatives, many rhodium complexes possessing optically active phosphine ligands have been developed in last two decades. [Rh (R)-(la) (cod)]Bp4 (2a) is one of the most efficient catalysts for this purpose with respect to activity and enantioselectivity. Besides (2a), several derivatives, which have various substituents on the nitrogen atom of the pyrrolidine ring, will be mentioned below. 

Similarly, enantioselective hydrophenylation of the alkene C=C double bond in norbomene and in norbomadiene has been performed by using a Pd-NORPHOS catalyst (eq 11). The use of other optically active phosphine ligands [eg., R,S)-BPPFOH (9)] generally afforded slightly higher chemical yields of hydrophenylated products with somewhat lower optical yields. ... 

The Homo Diels-Alder Reaction of Norbomadiene with Acetylenes. [2 + 2+2] Cycloadditions of dienes such as norbomadiene with the double bonds in 1,4-position are called homo Diels-Alder reactions. Using an in situ catalyst (consisting of Co(acac)3-Et2AlCl-bis(diphenylphosphino)ethane) the products obtained with monosubstituted acetylenes, such as phenyl, i-propyl-, n-butyl-, t-butyl-, and trimethylsilylacetylene, are 4-substituted deltacyclenes. - In the formation of the polycyclic deltacyclene skeleton, six new stereo centers are generated in one step. Thus enantiocontrol by using optically active phosphine ligands as cocatalysts allows the synthesis of optically active cycloadducts, as shown for the reaction of norbomadiene with phenylacetylene to give 4-phenyldeltacyclene (eq 1). 

In the pioneering studies of Homer et al. [57] and Knowles and Sabacky [58], chirally modified Wilkinson catalysts were introduced in the homogeneous enantioselective hydrogenation of prochiral olefins. To this end, in Wilkinson-type catalysts the triphenylphosphine ligand was replaced by the optically active phosphine ligands (-i-)-PMePr"Ph and H-PMePfPh, chiral at the phosphoms atom. 

Usually, a catalyst has to be synthesized or conditioned prior to its use in a catalytic reaction. However, there is an alternative to such an isolated or preformed catalyst, the so-called in-situ catalyst. The in-situ catalyst is prepared by mixing the transition metal compound (the procatalyst) and the ligand (the cocatalyst) in the solvent in which the reaction is to be carried out [79]. The use of in-situ catalysts is most appropriate in enantioselective hydrogenation reactions with Wilkinson-type catalysts. The optically active phosphines needed for optical induction have to be synthesized in multi-step syntheses [80, 81]. It is most convenient to combine them directly with the Rh-containing procatalysts. 

Two important industrial processes are based on biphasic systems the Shell higher olefin process (SHOP) [3] and the hydroformylation developed by Ruhrchemie/ Rhone Poulenc [4]. Prerequisite was the synthesis of water-soluble ligands, especially water-soluble phosphines. Scheme 1 shows a selection of optically active phosphines for asymmetric reactions under biphasic conditions. 

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