Hydroformylation rhodium-catalyzed, alkenes

Pronounced rate and selectivity enhancements had previously been observed for Ni(0)-catalyzed cyclodimerizations as well as rhodium-catalyzed alkene hydroformylation reactions. For related references see (Ni) van Leeuwen, P.W.N.M. Roobeek, C.F. Tetrahedron 1981, 37, 1973. (Rh) van Leeuwen,... 

Abstract Recent advances in synthetic aspects of the rhodium-catalyzed hydroformylation of alkenes are reviewed. Emphasis is given to practical improvements, efficient new catalysts for regioselective and enantioselective hydroformylation, and to applications of the reaction in organic synthesis. Furthermore, new developments in directed hydroformylation are covered as well as new approaches toward efficient hydroformylation catalysts employing the concept of self-assembly. 

Breit B (2007) Directed Rhodium Catalyzed Hydroformylation of Alkenes. Top Organomet Chem, published online... 

The second rhodium-catalyzed route which is widely used in connection with domino processes is that of hydroformylation. This by itself is a very important industrial process for the formation of aldehydes using an alkene and carbon monoxide. Finally, rhodium catalysts have also been used in this respect. 

Although most of the reports that have appeared since 1980 on hydroformylation of alkenes focus on rhodium catalysts, alkene hydroformylation catalyzed by Ptn complexes in the presence of Sn11 halides has been the object of great interest and platinum can be considered as the second metal in hydroformylation.77-79... 

Arylphosphines in rhodium catalyzed hydroformylation reactions exchange an aryl group for an alkyl, principally linear alkyl, corresponding to the alkene being hydro-formylated to give an alkyldiarylphosphine [22](see Equation 2.5). 

Although early catalysts were based on cobalt, nowadays, rhodium catalysts are preferred because they require lower pressure and afford higher chemo- and regioselectivity [1,2]. In recent years, extensive research into the production of only linear aldehydes has provided impressive results. The application of phosphines with a wide bite angle in the rhodium catalyzed hydroformylation of terminal alkenes enable the regioselectivity to be almost totally controlled [3]. Branched selective hydroformylation, al-... 

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). 

Recently, and for the first time, it has been shown that high levels of diastereocontrol may be realized in the rhodium(l)-catalyzed hydroformylation of acychc alkenes. In one example, acetals 10 afford aldehydes 11 with superior diastereoselectivity (Scheme 5.4) [4]. This result was attributed to a strong conformational bias in the substrate, as shown. Evidence for this conformational bias was secured by 2D-NOESY NMR experiments and MM3 force field calculations. When the experiment was repeated with R=H, the diastereoselectivity was lost, lending further support to the model. 

Wittig yhdes have been shown to be compatible with hydroformylation conditions, and may thus be used in a domino reaction sequence such as from 16a to 38 (Scheme 5.15) [20]. When an a-unsubstituted ylide is employed, the resulting alkene undergoes in-situ rhodium-catalyzed hydrogenation in a triple tandem reaction to convert 10 a to 39. Several other examples were reported establishing the generality of this domino reaction sequence. 

Arya et al. 50) immobilized similar dendrimers for the alkene hydroformylation on polystyrene (PS) beads, using solid-phase synthesis (58—60). The rhodium-catalyzed hydroformylation of styrene was investigated, and recycling experiments... 

Scheme 8.1 Rhodium-catalyzed hydroformylation of alkenes leading to linear (L) and branched (B) aldehydes and isomerized (IS) olefins.

Despite some differences, the mechanism of rhodium-catalyzed hydroformylation is very similar to that of the cobalt-catalyzed process.39-42 Scheme 7.1 depicts the so-called associative route which is operative when the ligand is in excess. Rhodium metal and many Rh(I) compounds serve as precursor to form21,22—in the presence of triphenylphosphine, CO and H2—the active species [RhH(CO)(PPh3)3] (5). At high CO partial pressure and low catalyst concentration without added PPh3, the [RhH(CO)2(PPh3)] monotriphenylphosphine complex instead of 6 coordinates the alkene and participates in the so-called dissociative route.21,39... 

Iridium complexes show very limited activity as hydroformylation catalysts compared with those of rhodium. [IrCl(CO)(PPh3)2] (43) has been shown to hydroformylate terminal alkenes but much higher temperature and pressure are required than with rhodium.348 349 Reviews on rhodium-catalyzed hydroformylation are available.350,351... 

An example is the rhodium catalyzed hydroformylation reaction, which is an industrially important homogenous catalytic process [3]. In contrast, it is amazing that such an important transition-metal catalyzed C/C bond-forming process has been employed only rarely in organic synthesis [4]. Part of the reason stems from the difficulty in controlling stereoselectivity. Even though some recently developed chiral rhodium catalysts allow for enantio- and diastereoselective hydroformylation of certain specific classes of alkenes [5, 6], only little is known about the diastereoselective hydroformylation of acyclic olefins [7, 8]. 

---