Hydroformylation- decarboxylative

In a continuation of this work, Breit and Smejkal (43) showed that when a-p unsaturated carboxylic acids are exposed to hydroformylation conditions in the presence of supramolecular catalyst 5, the reaction takes a completely unexpected path, yielding a product corresponding to a decarboxylative hydroformylation (Scheme 1, bottom). Under standard hydroformylation conditions, moderate activity for the hydrogenation of the double bond was observed, but no aldehyde product was detected (Scheme 1, top). 

This principle was extended to the tandem decarboxylative hydroformylation-hydrogenation reaction ofa,P Unsaturated carboxylic acids (Scheme 5.59) [77]. 

Scheme 5.59 Decarboxylative hydroformylation-hydrogenation tandem reaction of a,p-unsaturated carboxylic acids.

Rhodium-catalyzed decarboxylative hydroformylation has been used to generate a substrate for a Knoevenagel condensation with malonic acid. This interesting sequence results in two carbon homologation of carboxylic acids (Scheme 26.17) [97]. 

In a similar way, carbocycles having a quaternary center could be obtained from acyclic unsaturated 1,3-dicarbonyl compounds [206]. Other combinations are the domino hydroformylation/Wittig olefmation/hydrogenation described by Breit and coworkers [207]. The same group also developed the useful domino hydroformyla-tion/Knoevenagel/hydrogenation/decarboxylation process (Scheme 6/2.14) [208] a typical example is the reaction of 6/2-66 in the presence of a monoester of malonic acid to give 6/2-67 in 41 % yield in a syn anti-ratio of 96 4. Compounds 6/2-68 and 6/2-69 can be assumed as intermediates. 

Scheme 6/2.14. Domino hydroformylation/Knoevenagel/hydrogenation/decarboxylation process.

These considerations now provide a guideline for the development of other potential catalysts for the use of CO + H2O in the hydroformylation of olefins. If the catalyst is to function in the same manner as just described for Fe(CO)s, then a minimum requirement is that the system form a metal carbonyl which will be readily attacked by a weak base to form an anion analogous to 1. A weak base is essential because CO2 is an inevitable by-product, and only the carbonate salts of weak bases regenerate the base and CO2 upon heating. Thus, if the system is to be catalytic in base as well, then clearly only a weak base can be used. This would appear to be the critical requirement, for the literature indicates that metallocarboxylic acids readily decarboxylate (12), and the final step in Reaction 6, the protonation of a hydridometalcarbonyl anion, would seem to offer no problem provided the catalyst system was not in a highly basic medium. 

A final decarboxylation step is also part of a hydroformylation-decarboxylative Knoevenagel (= Doebner-Knoevenagel) reaction sequence (Scheme 5.144)... 

Scheme 5.144 Hydroformylation-decarboxylative Knoevenagel (Doebner-Knoevenagel) reaction and typical reaction products.

Scheme 5.160 Tandem hydroformylation-decarboxylation-hydrogenation reaction.

Besides isomerization of the olefinic substrates, some other side reactions may complicate the hydroformylation of fatty acids. Such reactions are decarboxylation and decarbonylation (see also Chapter 8) under formation of saturated or unsaturated compounds reduced by one carbon atom (Scheme 6.88) [37]. For example, at high temperatures and long reaction times, the formed aldehydes can undergo dehydrocarbonylation (a). Subsequent hydrogenation produces saturated fatty acids [25, 38]. This reaction sequence may lead to the false conclusion that hydrogenation of the starting olefin has taken place. The same products can suffer decarbonylation (b). On the other hand, decarboxylation of formyl carboxyl acids produces aldehydes (c). 

---