Rhodium catalyzed isomerization

Allylamines have been used as nitrogen protective groups. They can be removed by isomerization to the enamine (t-BuOK, DMSO) or by rhodium-catalyzed isomerization. ... 

The rhodium-catalyzed isomerization can also be carried out with the chiral catalyst, Ru2Cl4(diop)3 (H2, 20-80°, 1-6 h, 47-90% yield). In this case, optically enriched enol ethers are obtained. ... 

Rhodium-catalyzed isomerization. Ru(cod)(cot) has been used to convert an allylamine into an enamine."... 

In conclusion, bidentate O-acylphosphite ligands were shown to be very useful modifyers for the rhodium catalyzed isomerizing... 

The rhodium-catalyzed isomerization of allylamine can be used for the deprotection of A-allyl protective groups (Scheme 7).19 20... 

A novel latent-active glycosylation strategy was reported. This strategy is based on a rhodium-catalyzed isomerization of substituted allyl glycosides followed by a Lewis acid-mediated glycosylation reaction (Scheme 46).76 77... 

Another rhodium-catalyzed isomerization, by Makino and Itoh [137], allows the conversion of enallenes 216 and 218 to either five-membered methylenecyclopentanes 217 or, under a CO atmosphere in dioxane, seven-membered alkylidenecyclo-heptenes 218 (Scheme 15.70). With one more carbon in the bridge (in 220), methy-lenecyclohexane 221 was accessible, but as much as 20 mol% of catalyst was necessary. A more recent paper also covers this reaction [138]. 

As for allylic ethers, there are relatively few reports of highly enantioselective rhodium-catalyzed isomerizations of allylic alcohols (for example, >80% ee). Relative to the other processes described above, reactions of allylic alcohols are more atom economical [6] since they obviate the need for a separate hydrolysis step (Eq. 5). 

We subsequently reported that altering the structure of the aryl groups of the tertiary phosphine can indeed produce a more effective catalyst [10]. Thus, replacement of Ph with o-Tol (PF-PPh2 -> PF-P(o-Tol)2) furnishes a ligand that provides improved enantioselectivity and yield for the rhodium-catalyzed isomerization of F-allylic alcohols (Tab. 4.1). 

Interestingly, whereas rhodium-catalyzed isomerizations of Z-aUylic alcohols generally afford higher enantioselectivities than the corresponding T-isomers when PF-PPh2 is employed as the ligand, the opposite trend is observed for PF-P(o-Tol)2- Thus, Rh(l)/PF-P(o-Tol)2 furnishes relatively modest enantioselectivities for isomerizations of Z-aUylic alcohols (compare Tab. 4.2 with Tab. 4.1). 

At this stage, we focused on the synthesis of a-silylketones 7 utilizing a rhodium-catalyzed isomerization of 6 to provide a new route to 4 and 5 (Scheme 6.3). As a result, neither 7 nor 8 was obtained from 6 in the presence of a catalytic amount of RhH(PPh3)4. However, when 9a (R=Ph) was present in 6a (R=Ph) as an impurity, 6a was isomerized in the presence of the rhodium catalyst, to afford the ketone 7a (R=Ph), thus providing the breakthrough for the studies outlined herein [4]. 

A few examples of vinylic to aryl rhodium migration have also been observed. One such example was discovered independently by two groups almost at the same time [77,78], In this reaction, aryl propargyl alcohols undergo a rhodium-catalyzed isomerization to furnish indanone products (Table 12). The reaction proceeds by a fairly complicated mechanism shown in Scheme 19. Thus, after base-promoted ligand... 

Scheme 19 Proposed mechanism for the rhodium-catalyzed isomerization with vinylic to aryl migration...

Table 12 Rhodium-catalyzed isomerization of aryl propargyl alcohols under rhodium migration conditions...

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

Andrieux, J., D. H. R. Barton, and H. Patin Rhodium-catalyzed Isomerization of Some Unsaturated Organic Substrates. J. C. S. Perkin I, 19T7, 359. 

Scheme 5.17 Simplified mechanism for n-regioselective rhodium-catalyzed isomerization-hydroformylation.

Although the authors did not speculate about the cyclization mechanism, it seems that the rhodium-catalyzed isomerization of the internal double bond is an important precondition. Under optimized conditions, both diastereomers were quantitatively formed and in the best case with a fourfold excess of the (17 ,27, 47, 5iJ)-diastereomer. 

Example one step of a tetrahydrofuranecarboxaldehyde synthesis involves a rhodium catalyzed isomerization of the double bond in a dioxacyclohep-tene ring ... 

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