Rhodium catalyzed isomerization, aldehyde

The present rhodium catalyzed isomerization reaction provides a convenient access to chiral terpenoid enamines and aldehydes. Its synthetic utility toward such natural products as... 

There are two important rhodium-catalyzed transformations that are broadly used in domino processes as the primary step. The first route is the formation of keto carbenoids by treatment of diazo keto compounds with Rh11 salts. This is then followed by the generation of a 1,3-dipole by an intramolecular cyclization of the keto carbenoid onto an oxygen atom of a neighboring keto group and an inter- or intramolecular 1,3-dipolar cycloaddition. A noteworthy point here is that the insertion can also take place onto carbonyl groups of aldehydes, esters, and amides. Moreover, cycloadditions of Rh-carbenes and ring chain isomerizations will also be discussed in this section. 

It has been repored (86) that these same three isomeric aldehydes were formed from allyl acetate by a reaction catalyzed by rhodium trichloride and iron pentacarbonyl. Reaction proceeded at 135°C under 300 atm of H2/CO, but the product composition was not specified. 

In 2000, we demonstrated that the planar-chiral phosphaferrocene PF-PPhj is a useful ligand for rhodium-catalyzed asymmetric isomerizations of several allylic alcohols, providing the first catalyst system that furnishes the target aldehyde in >60% ee (Eq. 6) [7]. It appears that, in order to obtain high enantiomeric excess (>0% ee), the olefin should bear a relatively bulky substituent (for example, Pr Eq. 6). 

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

The C5 aldehyde intermediate is produced from butadiene via catalytic oxidative acetoxylation followed by rhodium-catalyzed hydroformylation (see Fig. 2.30). Two variations on this theme have been described. In the Hoffmann-La-Roche process a mixture of butadiene, acetic acid and air is passed over a palladium/tellurium catalyst. The product is a mixture of cis- and frans-l,4-diacetoxy-2-butene. The latter is then subjected to hydroformylation with a conventional catalyst, RhH(CO)(Ph3P)3, that has been pretreated with sodium borohydride. When the aldehyde product is heated with a catalytic amount of p-toluenesulphonic acid, acetic acid is eliminated to form an unsaturated aldehyde. Treatment with a palladium-on-charcoal catalyst causes the double bond to isomerize, forming the desired Cs-aldehyde intermediate. 

In the BASF process the 1,2-diacetate is the substrate for the hydroformylation step. It can be prepared either directly via oxidative acetoxylation of butadiene using a selenium catalyst or via PtCl4-catalyzed isomerization of the 1,4-diacetate (see above). The latter reaction affords the 1,2-diacetate in 95% yield. The hydroformylation step is carried out with a rhodium catalyst without phosphine ligands since the branched aldehyde is the desired product (phosphine ligands promote the formation of linear aldehydes). Relatively high pressures and temperatures are used and the desired branched aldehyde predominates. The product mixture is then treated with sodium acetate in acetic acid to effect selective elimination of acetic acid from the branched aldehyde, giving the desired C5 aldehyde. 

An enantiomerically pure aldehyde, (lR,2R,3R)-2,7,7-trimethylbicyclo[3.1.1]hep-tane-2-aldehyde, is produced from a-pinene by rhodium-catalyzed hydroformylation [79, 80]. Initially, reaction with ferrocene under acidic conditions leads to a 1 1 mixture of diastereoisomeric cations, but on standing for a few hours at room temperature, isomerization by rotation around the ferrocene — cationic carbon bond to the thermodynamically more stable cation (with configuration (R) at the cationic center) occurs (Fig. 4-11). An enantiomerically pure amine is available by trapping of this cation by azide and reduction [75]. Analogously, the isomeric aldehyde with the bicyclo [2.2.1] heptane structure is formed by hydroformylation of a-pinene with cobalt catalysts [79, 80] and was used as the starting material for an isomeric series of chiral amines [75]. 

Rhodium(I)-catalyzed isomerization of allylic amines is also good route to P-substituted aldehydes [89]. 

The hydroformylation of conjugated dienes such as 1,3-butadiene, isoprene, and 1,3-pentadiene gives mixtures of regioisomers, isomerized aldehydes, and dialdehydes depending on the conditions and catalysts used. The reaction of 1,3-butadiene provides 1,6-hexanedial and has relevance to nylon produc-The reaction of 1,3-cyclohexadiene catalyzed by a rhodium complex... 

In order to eliminate the possibility for in situ carbene formation Raubenheimer et al. synthesized l-alkyl-2,3-dimethylimidazolium triflate ionic liquids and applied these as solvents in the rhodium catalyzed hydroformylation of l-hejEne and 1-dodecene [178]. Both, the classical Wilkinson type complex [RhCl(TPP)3] and the chiral, stereochemically pure complex (—)-(j7 -cycloocta-l,5-diene)-(2-menthyl-4,7-dimethylindenyl)rhodium(i) were applied. The Wilkinson catalyst showed low selectivity towards n-aldehydes whereas the chiral catalyst formed branched aldehydes predominantly. Hydrogenation was significant with up to 44% alkanes being formed and also a significant activity for olefin isomerization was observed. Additionally, hydroformylation was found to be slower in the ionic liquid than in toluene. Some of the findings were attributed by the authors to the lower gas solubility in the ionic liquid and the slower diffusion of the reactive gases H2 and CO into the ionic medium. 

A phosphite-modified calixarene with unsubstituted hydroxyl groups was used as a ligand in 1-hexene hydroformylation catalyzed by rhodium complexes [224], The reaction was carried out at a synthesis gas pressure of 6.0 MPa and 160 °C. Rh(acac)(CO)2 was a catalyst precursor. In 3 h, the conversion of the initial alkene virtually reached its theoretically predicted value the yield of aldehydes was 80-85%, and the normal-to-isomeric aldehyde ratio was approximately 1 1. Some similar phosphites 83 were also studied as components of catalytic systems for 1-octene hydroformylation [225]. It was shown that the nature and steric volume of substituent R have no essential effect on the main laws of the process. For example, the conversion was 80-90% at a selectivity with respect to nonanal of about 60% in all cases. The regioselectivity with respect to nonanal was considerably increased to 90-92% by using the chelate biphosphite 84 [220]. 

Industrially, the rhodium-catalyzed hydroformylation is normally operated at about 100°, at pressures up to 50 atm and in the presence of a large excess of added phosphorus ligand, it can be carried out in molten PPhs. Under these conditions, a terminal olefin can be converted in over 90% yield to linear aldehyde. By-products include branched aldehydes as well as small amounts of alkanes and isomerized olefins. Advantages over the more conventional cobalt catalysts include lower temperatures and pressures, higher ratios of linear to branched products, and less hydrogenation of aldehyde products to alcohols. 

It is well known that the main goal in rhodium-catalyzed hydroformylation of unsaturated, especially vinyl, substrates, concerns the control of reaction regioselectivity, i.e. of the regioisomeric ratio b l (branched to linear) between the isomeric aldehydes produced. 

Landis and coworkers [44] investigated the rhodium-catalyzed AHF of isomeric 1,2-disubstituted enol esters and enamides with Rh[(S,5, 5 )-Bisdiazaphos]. In most cases, with the exception of styrenyl enamides (see below), ratios of a/p >99 1 were noted. Enol aroyl esters gave the corresponding saturated aldehydes in 92-99% ee. The reaction with related enamides was found to be slower than transformation of enol esters (Scheme 4.70). Therefore, with the former, for complete conversion within 1 day, a higher catalyst loading was required. 

The ionic compounds 1,2,3-trimethylimidazolium triflate and l-ethyl-2,3-dimethylimidazolium trifiate and the coordination compound (3-butylimidazole) triphenylboron were used as solvents for biphasic rhodium-catalyzed hydroformylation of 1-hexene and 1-dodecene. High conversions with varying linear/branched aldehyde ratios were observed. Compared to the conventional solvent toluene, similar turnover numbers, but a higher tendency toward isomerization and hydrogenation, were found [96]. 

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