Rhodium-Catalyzed Transformations

SCHEME 536 Rh-catalyzed cascade synthesis of 2,3-diphenylindole from P,P-diphenylstryryl azide. 

SCHEME 537 Rh-catalyzed cascade synthesis of indoles 55 from azides 54. 

SCHEME 5.38 Rh-catalyzed cascade synthesis of 3-nitroindoles 57 from azides 56. 

SCHEME 5.39 Rh-catalyzed multicomponent synthesis of isoqninolinium salts 58. 

SCHEME 5.40 Mechanism proposed for the formation of isoquinolinium salts. 

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. 

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. 

---

The rhodium-catalyzed transformation of allenyne 16 provides complementary re-gioselectivity to that obtained with molybdenum. For example, whereas the internal olefin of the allene is engaged in the molybdenum-catalyzed reaction, it is the allene s... 

In 1961 Heck and Breslow presented a multistep reaction pathway to interpret basic observations in the cobalt-catalyzed hydroformylation.28 Later modifications and refinements aimed at including alternative routes and interpreting side reactions.6 Although not all the fine details of hydroformylation are equally well understood, the Heck-Breslow mechanism is still the generally accepted basic mechanism of hydroformylation.6,17,19,29 Whereas differences in mechanisms using different metal catalysts do exist,30 all basic steps are essentially the same in the phosphine-modified cobalt- and rhodium-catalyzed transformations as well. 

Davies HML, Walji AM (2005) In Evans PA (ed) Modern rhodium-catalyzed transformations. Wiley, New York, p 301... 

The organosilicon reagents also participate in the rhodium-catalyzed reaction with a./S-unsaturated carbonyl compounds, imines, and alkynes (eq 4). The use of optically active diene ligands allows the rhodium-catalyzed transformation to proceed in an enantioselective manner. The reaction can be performed on a gram scale, and the cyclic silyl ether can also be recovered by distillation of a crude product. 

---