Vinyl arenes, hydroformylation

Rhodium-Catalyzed Asymmetric Hydroformylation of Vinyl Arenes. .. 41... 

Abstract This chapter presents the latest achievements reported in the asymmetric hydroformylation of olefins. It focuses on rhodium systems containing diphosphites and phosphine-phosphite ligands, because of their significance in the subject. Particular attention is paid to the mechanistic aspects and the characterization of intermediates in the hydroformylation of vinyl arenes because these are the most important breakthroughs in the area. The chapter also presents the application of this catalytic reaction to vinyl acetate, dihydrofurans and unsaturated nitriles because of its industrial relevance. 

The first report to use diphosphite ligands in the asymmetric hydroformylation of vinyl arenes revealed no asymmetric induction [46]. An important breakthrough came in 1992 when Babin and Whiteker at Union Carbide patented the asymmetric hydroformylation of various alkenes with ee s up to 90%, using bulky diphosphites 2a-c derived from homochiral (2R, 4R)-pentane-2,4-diol (Scheme 4) [17]. Their early results showed that (a) bulky substituents are required at the ortho positions of the biphenyl moieties for good regio- and enantio-selectivity and (b) methoxy substituents in the para positions of the biphenyl moieties always produced better enantio-selectivities than those observed for the corresponding ferf-butyl-substituted analogues. 

Over the years, several authors have developed new diphosphite ligands with binaphthyl, spiro, pyranoside, and macrocyclic backbones for asymmetric hydroformylation of vinyl arenes with low-to moderate success (ee s from 36% to 76%) [48-58]. 

Perfluoroalkyl-substituted BINAPHOS ligand 29c was also developed for the asymmetric hydroformylation of vinyl arenes in SCCO2. With this ligand regio-and enantio-selectivity (ee s up to 93.6%) were high without the need for hazardous organic solvents [63,64],... 

The asymmetric hydroformylation of functionalized aliphatic alkenes is generally more difficult than the hydroformylation of vinyl arenes. The rhodium-catalyzed hydroformylation of vinyl acetate (36) yields 2- and 3-acetoxypropanals, 37 and 38, with high chemoselectivity. Ethyl acetate and acetic acid can also be found as by-products. One of the potential applications of vinyl acetate hydroformylation is the production of enantiopure propane 1,2-diol (Scheme 6). 

Rhodium(I) complexes with l,3-dimethylimidazolin-2-ylidene ligands were used in the hydroformylation of olefins. However, the activity and selectivity toward formation of branched versus linear aldehyde cannot compete with rhodium-phosphine systems. " Similar catalyst systems with the sterically more demanding l,3-dimesitylimidazolin-2-ylidene give higher branched/linear ratios for vinyl arenes (95 5), but the turnover frequency is still low compared to established systems [Eq. (52)]. ... 

The asymmetric hydroformylation of vinyl arenes can provide a route to the preparation of the profen class of drugs. Naproxen and ibuprofen, two examples in the profen class, are NSAIDs on the market.50... 

Unmodified rhodium catalysts are readily formed in seCOa from simple precursor complexes such as [(CO)2Rh(acac)j, [(cod)Rh(hfacac)], or [Rh( (CO)i6] [33. The resulting rhodium carbonyl species are highly active in this medium for a range of substrates including simple olefins, vinyl arenes and polar substrates such as aUyl acetate. Especially the reaction rates for internal C=C bonds are remarkably higher than those observed in liquid organic solvents under typical hydroformylation conditions (Scheme 12.13). 

The BINAPHOS system has also been modified by using the perfiuoroalkyl-substituted derivative of (R,S)-B1NAPH0S (see Chapter 10) in the rhodium catalyzed hydroformylation of vinyl arenes in liquid or supercritical carbon dioxide as solvent This system leads to similar catalytic activity and the same level of enantiocontrol as the rhodium-(R,S)-BINAPHOS system in organic solvents [48]. 

A similar synthesis strategy was employed to construct high-weight catalysts carrying multiple catalytic centers in the outer core of the dendrimer [137]. The catalyst depicted below is characterized by 16 catalytic centers. It was found to be more active in the hydroformylation of vinyl arenes than its synthetic precursor carrying only four catalytic units. The ratio of branched to linear aldehydes ranged from 36 1 to 39 1 at >99% conversion. By simple filtration, the dendrimeric catalyst was separated from the product. Even the 10th hydroformylation cycle proceeded without loss of activity and selectivity. 

Hydroformylation of Styrene and Other Aromatic Olefins Crudden and coworkers [76] tested the NHC-rhodium complex la in the hydroformylation of various vinyl arenes (CO/H2 = 1 1, approximately 70 bar 1 mol% of precatalyst, 60 "C, benzene). As expected, preferentially branched aldehydes were formed in high yields (85-95%, h/l = 94 6-98 2). However, the reactivity was low (TOF = 7 h ). Similar results were obtained with precatalyst lb. Addition of varying amounts of PPhj slightly increased the efficiency but did not change the pronounced iso-regioselectivity. Interestingly, under these conditions the catalyst derived from lb proved to be more active and selective than RhCl(CO)(PPh3)2 which was used for comparison. 

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