2- Butene asymmetric hydroformylation

Asymmetric hydroformylation of the three isomeric straight-chain butenes provided evidence that asymmetric induction in both rhodium- and platinum-catalyzed reactions occurs during and before the formation of the alkylmetal intermediate.66 67... 

Finally a very important fact arising from our results is the occurrence of the asymmetric hydroformylation in olefins having C2v symmetry like d -butene. In this case the two faces of the olefin are identical, and the catalyst is evidently able to dissymmetrize the double bond favoring the attack of hydrogen to the re carbon atom and of carbon monoxide to the si carbon atom. A very simple way of looking at this dissymmetriza-tion of the double bond is to assume that the face of the olefin attacked... 

To obtain information about the steps in which the asymmetric induction actually takes place, 1-butene, cis-butene, and trans-butene were hydroformylated using asymmetric rhodium catalyst. According to the Wilkinson mechanism, all three olefins yield a common intermediate, the sec-butyl-rhodium complex, which, if the asymmetric ligand contains one asymmetric center, must exist in the two diastereomeric forms, IX(S) and IX(R),... 

The complex [PtCl2(DIOP)] in the presence of SnCl2 has been used as catalyst for the asymmetric hydroformylation of various alkenes. With a series of butene and styrene derivatives very low optical yields were obtained. The best results were achieved with 2,3-dimethyl-l-butene which gave 15% optically pure 3,4-dimethylpentanal, and with a-ethylstyrene which gave 15% optically pure 3-phenylpentanoic acid after oxidation of the aldehyde.373... 

No substantial progress has been made in the field of cobalt-catalyzed asymmetric hydroformylation since our last review on this subject1S). Besides (+)-N-(l-phenyl-ethyljsalicylaldimine, which was originally used as asymmetric ligand6, a chiral catalyst formed in situ from HCo(CO)4 and (—)-DIOP has been employed 16) (Table 1). With the latter catalytic system, optical yields of 2.7% and 1.2% have been obtained in the case of (Z)-2-butene and of bicyclo[2,2,2]oct-2-ene, respectively. 

The first asymmetric hydroformylation with platinum catalysts was carried out42 using NMDPP as the asymmetric ligand. An optical yield of 9% was obtained in the hydroformylation of 2-methyl-l-butene to 3-methylpentanal. Subsequently, different types of olefins were asymmetrically hydroformylated using a catalytic system formed from [(—)-DIOP]PtCl2 and SnCl2 2 H20 in situ 42,45) (Table 4). 

The optical yield obtained starting with 1-butene at low conversion is the highest ever reported ( 47% at 60 °C) for asymmetric hydroformylation of an. aliphatic olefin, the (R) antipode of 2-methylbutanal predominating. 

In Table 6 the results concerning the asymmetric hydroformylation of 1-butene and of styrene with different catalytic systems are reported. When rhodium-containing catalytic systems are used in the presence of several diphosphine ligands, the face of the prochiral unsaturated carbon atom which is preferentially formylated is the same in both substrates for each chiral ligand. 

It was shown15) by asymmetric hydroformylation of linear butenes that asymmetric induction in hydroformylation occurs substantially before or during metal alkyl intermediate formation, which, according to the accepted hydroformylation mechanism 2I-57,58), is the second step in the catalytic process (see also Sect. 5.1). 

Fig. 8. Transition states determining asymmetric induction in asymmetric hydroformylation of (Z)-2-butene (Figures refer to the Rh/(—)-DIOP catalytic system)...

From the results of the asymmetric hydroformylation of the isomeric straight-chain butenes we have concluded that asymmetric induction in the case of Rh/(—)-DIOP catalytic complexes does not result either in carbon monoxide insertion or in the following steps described in Fig. 12 but either in the 7t-eomplex formation or the alkyl-complex formation 15) (Fig. 13). 

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). 

Takaya and Nozaki invented an unsymmetrical phosphin-phosphite ligand, (R,S)-BINAPHOS, which was used in the Rh(l)-catalyzed asymmetric hydroformylation of a wide range of prochiral olefins, with excellent enantioselectivities [120, 155]. A highly crosslinked PS-supported fR,S)-BINAPHOS(257)-Rh(I) complex was prepared and applied to the same reaction (Scheme 3.83) [156]. Using the polymeric catalyst, the asymmetric hydroformylation of olefins was performed in the absence of organic solvents. The reaction of cis-2-butene, a gaseous substrate, provided (S -methylbutanal with 100% regioselectivity and 82% ee upon treatment with II, and CO in a batchwise reactor equipped with a fixed bed. 

The most simple substrate for asymmetric hydroformylation is 1-butene leading to chiral 2-methylbutanal [iso (i) or branched product] and achiral pentanal [normal (n) or linear product]. The best result of 46.7 % ee (although with only 32 % aldehyde yield and a 96 4 ratio of n/i products) is obtained with PtCI2/SnCl2/Diop catalysts68. 

Scheme 4.96 Products of the asymmetric hydroformylation observed with isomeric butenes.

Important early work by Consiglio and Pino and Consiglio et al. on the asymmetric hydroformylation of butenes was published as early as 1982. In this work, the hydroformylation of 1-butene and ( ) and (Z)-2-butene was carried out under different conditions using different chiral complexes of rhodium and platinum. The highest enantiomeric excess of 47% was actually achieved using the platinum complex [((—)-DIOP)Pt(SnCl3)Cl] (DIOP = 2,2-dimethyl-4,5-bis(diphenylphosphinomethyl)-l,3-dioxolane) ... 

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