Rh/Diop catalytic system

For instance, when a-[2H]-styrene is hydroformylated with the Rh/(—)-DIOP catalytic system, the two reaction products obtained have almost identical optical purity and opposite absolute configuration (Fig. 2)51). Therefore, in this case, the enantiomeric excess measured indicates both the type (the re-re enantioface reacts preferentially) and extent of enantioface discrimination ( 15%) occurring during the reaction. 

When more than one isomer is formed during the reaction an enantiomer discrimination can take place also with quantitative transformation of the substrate (Scheme 1, reaction 4). This is the case in the hydroformylation of 3-phenyl-1-butene with a Rh/(—)-DIOP catalytic system 14) where 4-phenylpentanal arises preferentially... 

Fig. 4. Enantiofaces preferentially attacked in asymmetric hydroformylation of 1,1-disubstituted ethylenes with the Rh/(—)-DIOP catalytic system...

The results concerning the enantiomeric excess for 1-pentene show that quadrant Q 2 is preferred to quadrant Q t, as predicted both for Pt/(—)-DIOP and Rh/ (—)-DIOP catalytic systems. Furthermore, with Pt/(—)-DIOP, quadrant Q2 is preferred to quadrant Qt, as predicted, whereas with Rh/(—)-DIOP, quadrant Qt is preferred to quadrant Q2, in agreement with the results obtained with aliphatic 1,1-disubstituted ethylenes. Comparing the results obtained with a-[2H]-styrene and with 2-phenyl-1-propene it appears that the phenyl group prefers quadrant Qt (as the n-propyl group in 1-pentene) when 2H is in quadrant Q2 however, if a methyl group is present, steric repulsion is minimized when methyl occupies quadrant Qt and the phenyl group is in quadrant Q2. 

Table 14. Correlation between enantioface-discriminating and enantiomer-discriminating hydroformylation with Rh/(—)-DIOP catalytic systems"...

The Rh/Diop catalytic system is one of the fastest catalyzed gas-liquid asymmetric hydrogenations. A (R,S)-Cy-Cy-Josiphos ligand behaves almost as good as the Diop ligand and provides abetter enantioselectivity of 75% (Josiphos family of ferrocenyl diphosphine ligands cy cyclohexyl). The latter is the most active of the Josiphos family (88% conversion). The reproducibility of the data obtained has been checked with the Rh/Diop catalytic system. For more than five tests, the mean deviation was 2% for conversions and less than 1% for the enantiomeric excess that proved the reliability of this new microdevice. 

On the contrary, no influence of the metal atoms is apparent when (Z)- or (E)-2-butene are used as substrate with Rh/(—)-DIOP and Pt/(—)-DIOP catalytic systems. An attempt to rationalize the above results is presented in Section 4. 

In the case of aliphatic or alicyclic olefins only norbornene hydroformylation with Rh/(—VDIOP does not fit into the picture. The contrasting results between (Z)-2-butene and bicyclo[2.2.2]oct-2-ene obtained with Co/(—)-DIOP and Ru/ (—)-DIOP catalytic systems have been attributed to extensive isomerization of (Z)-2-butene to (E)-2-butene during hydroformylation16>. In the case of the only phenyl-substituted substrate investigated, the prediction of the unsaturated carbon atom preferentially formylated is not correct. This type of exceptions is found also for styrene, as it will be discussed later. 

The simplest case which can be used to explore the factors influencing the difference in the energies of the diastereomeric transition states, which determine asymmetric induction, is the hydroformylation of (Z)-2-butene with the Rh/(—)-DIOP or Pt/ (—)-DIOP catalytic system. In this case, the asymmetric induction cannot be connected with enantioface discrimination in the step leading to the Tt-complex because this olefin has no enantiofaces 68). In the first step of the reaction it is assumed that a Tt-complex is formed by interaction between substrate and catalyst. This Tt-complex, depending on its geometry, can exist in two different conformations arising from the rotation of the olefin around the metal-olefin Tt-system-bond axis. 

As shown in the asymmetric hydroformylation of 1- and 2-butene with Rh/(--)-DIOP or Pt/(—)-DIOP catalytic systems, it seems likely that asymmetric induction occurs mainly in the step in which the n-olefin complexes, which are assumed to be formed in the first reaction step, are transformed into the corresponding metal-alkyl intermediate 15). 

The geometry of the complex seems to be of paramount importance in the determination of repulsive interactions in the different quadrants around the metal. The most significant examples are given by the deuteroformylation of monosub-stituted ethylenes in which the aliphatic substituent occupies in the model of the transition state (Fig. 6) preferentially quadrant Q2 in the Pt/(—)-DIOP catalytic system but quadrant Qt in the Rh/(—)-DIOP catalyst although, according to our model, the chirality at the metal atom, as determined by the hydroformylation of (Z)-2-butene, is the same. 

The fact that with (Z)-2-butene opposite prevailing chiralities are obtained in hydroformylation, using Rh/(—)-DIOP or Pt/(—)-DIOP catalytic systems, or in hydro-alkoxycarbonylation, using the Pd/( )-DIOP catalytic system, can be explained according to the model assuming that the metal atoms have opposite chiralities in the two cases. 

The bis-DIOP complex HRh[(+)-DIOP]2 has been used under mild conditions for catalytic asymmetric hydrogenation of several prochiral olefinic carboxylic acids (273-275). Optical yields for reduction of N-acetamidoacrylic acid (56% ee) and atropic acid (37% ee) are much lower than those obtained using the mono-DIOP catalysts (10, II, 225). The rates in the bis-DIOP systems, however, are much slower, and the hydrogenations are complicated by slow formation of the cationic complex Rh(DIOP)2+ (271, 273, 274) through reaction of the starting hydride with protons from the substrate under H2 the cationic dihydride is maintained [cf. Eq. (25)] ... 

The investigation of platinum(II)-chiral olefin complexes has shown that, when the diastereomeric equilibrium is reached, which diastereoface of the olefin is preferentially bound to the metal depends on the type of chirality of the olefin used61-63. When an optically active asymmetric ligand is present in the complex and a racemic olefin, is used, one diastereoface will be preferred for complexation and correspondingly one of the antipodes is preferentially complexed61 63). Let us suppose that with a certain catalytic system (e.g., Rh/(—)-DIOP), the re-re enantioface of a prochiral a-olefin reacts preferentially. With the same catalytic system the same face of all a-olefins, including the racemic a-olefins, is expected to react preferentially. However, when a racemic olefin is used, two diastereomeric transition states (e.g. a and b in Fig. 11) can form for each of the transition states shown in Fig. 7, depending on which one of the antipodes of the racemic monomer approaches the catalyst. 

A similar process occurred when the double bond bore a carboxylic function. Reduction of a-acylaminoacrylic acids with the catalytic system (— )-(diop)-Rh(I) allowed an asymmetric hydrogenation with optical yields ranging from 70 to 80%192 (Scheme 131). 

Close attention has been devoted in recent years to the homogeneous catalytic reduction of N-acylaminoacrylic acids with the aid of chiral Rh-complexes. In some cases exceptionally high optical yields have been achieved in these reactions. Phosphine-Rh catalysts of the DIOP type, i. e. with chiral carbon skeleton, have been used 108, 109, 133,142,145, 146, 172, 193, 194, 234), as have catalysts with phosphine oxide ligands 409), ferrocenyl-phosphine-Rh complexes 171), bisphosphine-Rh complexes with a chiral pyrrolidine ring 3, 4), systems with chiral phosphines 216—221) and bisphosphines (222), or with a chiral P- and C-skeleton 130). 

---