Platinum-Catalyzed Hydroformylation

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

As early as 1972 Parshall described the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate melts [1]. [NEt4][SnCl3], the ionic liquid used for these investigations, has a melting point of 78 °C. Recently, platinum-catalyzed hydroformylation in the room-temperature chlorostannate ionic liquid [BMIM]Cl/SnCl2 was studied in the author s group. The hydroformylation of 1-octene was carried out with remarkable n/iso selectivities (Scheme 5.2-13) [66]. 

Much less is known concerning the platinum-catalyzed hydroformylations. However, a reasonable catalytic cycle can be constructed (Scheme 3) from the available information on the generation and reactions of many of the intermediate complexes shown.6,8,9,15 The ability of platinum to catalyze hydroformylation reactions while palladium is not a good catalyst could be due to the ability of platinum to achieve the +4 oxidation state more readily. 

Thus the enantiomeric excesses obtained in the platinum-catalyzed hydroformylation reactions of certain alkenes were achieved at a level (75-85% ee) necessary for facile enrichment to optically pure compounds, presenting an opportunity to establish this reaction as a viable asymmetric synthesis of aldehydes. 

In all examples available, geometrically similar antipodes react preferentially in the case of mono- and di-substituted ethylenes respectively, the three exceptions in Table 5 being due to the fact that substrates with similar geometry have different notations (Fig. 5). No observable kinetic resolution is achieved in the platinum-catalyzed hydroformylation of 3-methyl-1-pentene whereas a slight enantiomer discrimination is observed in the case of 2,4-dimethyl-1-pentene. 

Ru clusters can be used in a phosphonium salt melt [25] and the platinum-catalyzed hydroformylation can be performed in cblorostannate ionic liquids which serve as the solvent and (via the anion) as the catalyst activator at the same time [12, 26]. 

A similar effect was attributed to the SnClg" ligand in platinum-catalyzed hydroformylation. Because of its inherent trans effect, SnCl3 activates the Pt-H bond and thus facilitates its insertion into the olefin [6]. The same, but less pronounced effect was found by quantum chemical calculations for the migratory insertion of CO into the Pt-alkyl bond [7]. 

The range of olefins screened in platinum-catalyzed hydroformylation is rather narrow. As seen already above, mostly styrene or 1-olefins were investigated in mechanistic studies with the aim of establishing the structure of catalytic intermediates or to find structure-activity-regio/stereoselectivity relationships. Usually, Pt/Sn catalysts operate under rather mild conditions (10-100 bar syngas, 50-130 °C) [53]. Pt/S ratios of up to 2000 1 have been realized. 

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