Norbomene hydroformylation

Figure 2. Conversion and selectivity of the norbomene hydroformylation as a function of the Ru content of the system (data from [21]).

Because the norbomene double bond of dicyclopentadiene is more strained, its selective hydrofbrmyl-ation can be achieved under mild reaction conditions (entry 9). Under more vigorous reaction conditions both double bonds are hydroformylated. The exo faces of both rings are the least hindered, accounting for the selectivity. 

Hydroformylation of norbomene proceeds slowly in spite of the expected reactivity of the strained double bond. Racemization of the product aldehyde does not occur under the reaction conditions. The hydroformylation of methyl methacrylate gives only one isomer, which is a useful chiral synthon. Dimethyl itaconate also can be hydroformylated using (12) as the chiral ligand to give aldehyde product in 82% ee accompanied by hydrogenated ester.64... 

The hydroformylation of styrene in triethyl orthoformate is slower than that observed in benzene, but a 98% ee is obtained, since racemization of the product acetal does not occur. Hydrolysis of the acetal to the aldehyde can be accomplished without racemization. A number of other substrates are hydroformylated in the presence of triethyl orthoformate. The reactions are slower, but with all substrates tried except norbomene, enantiomerically pure products can be obtained. 

The synergistic effect described can also be used to increase the selectivity of a catalytic system. Hydroformylating norbomene instead of cyclohexene, catalytic cycles operate according to Scheme 2 [21]. In this case, besides the aldehyde, a lactone is formed in a side reaction. Adding the Ru species Ru3(CO)i2 to the system increases not only the norbomene conversion but also the selectivity of the system to aldehydes. For a fixed number of cobalt atoms, the norbomene conversion as well as the aldehyde selectivity of the system is plotted as a function of the quotient n Jnco in Figure 2. In this case, too, the synergistic effect is pronounced for the addition of small amounts of the bifunctional Ru species only. 

Scheme 2. Catalytic cycle of the hydroformylation of norbomene (according to [21]).

The enantioselective hydrocyanation of alkenes has the potential to serve as an efficient method to generate optically active nitriles, as well as amides, esters, and amines after functional group interconversions of the nitrile group. As in asymmetric hydroformylation, asymmetric hydrocyanation requires control of both regiochemistry and stereochemistry because simple olefins tend to generate achiral terminal nitrile products. The hydrocyanation of norbomene will give a single constitutional isomer and was studied initially. However, modest enantioselectivities were obtained, and the synthetic value is limited. ... 

Because the widespread use of 2-oxo-norbomyl carboxylic acid as a building block for pharmacologically active molecules, Pfizer (New York, NY) recently reported on a multikilogram-scale hydroformylation of norbomene using the l,l -bis-diphenylphosphino-ferrocene (dppf) ligand to provide an exclusively cxo-aldehyde as a racemic mixture, needed in a dmg development campaign. " ... 

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