Rhodium carbene reactions ligand effects

Rh(Por)l (Por = OEP. TPP, TMP) also acts as a catalyst for the insertion of carbene fragments into the O—H bonds of alcohols, again using ethyl diazoacetate as the carbene source. A rhodium porphyrin carbene intermediate was proposed in the reaction, which is more effective for primary than secondary or tertiary alcohols, and with the bulky TMP ligand providing the most selectivity. ... 

Use of Rh2(OAc)4 suggested that there was no inherent selectivity attributable to the coordinated carbene or to rhodium(ll). However, modification of dirhodium(ll) ligands to imidazolidinones provided exceptional diastereocontrol, obtained by influencing the conformational energies of the intermediate metal carbene [19, 23], as well as high enantiocontrol. Representative examples of products from these highly selective intramolecular C-H insertion reactions with cyclic systems is given in Scheme 15.6. Additional examples of effective insertions in systems from which diastereomeric products can result are illustrated in processes of the synthesis of 2-deoxyxylolactone (Scheme 15.7) [64, 65]. Here the conformation of the reactant metal carbene that is responsible for product formation is 32 rather than 33. Other examples in non-heteroatom-bound systems (for example, as in Eq. 15) confirm this preference. 

Catalytic asymmetric cyclopropanations via carbene transfer to alkenes were reviewed by Singh and co-workers in 1997," Doyle and Protopopova in 1998," and mostly recently by Doyle in 2000." The reaction can be catalyzed by copper," rhodium," and other metals." Bis(oxazolines) are known to be among the most effective ligands for this cyclopropanation reaction (see Chapter 9). 

Chiral rhodium(II) oxazolidinones 5-7 were not as effective as Rh2(MEPY)4 for enantioseleetive intramolecular cyclopropanation, even though the sterie bulk of their chiral ligand attachments (COOMe versus /-Pr or C Ph) are similar. Significantly lower yields and lower enantiomeric excesses resulted from the decomposition of 11 catalyzed by either Rh2(4S-IPOX)4, Rh2(4S-BNOX)4, or Rh2(4R-BNOX)4 (Table 3). In addition, butenolide 12, the product from carbenium ion addition of the rhodium-stabilized carbenoid to the double bond followed by 1,2-hydrogen migration and dissociation of RI12L4 (Scheme II), was of considerable importance in reactions performed with 5-7 but was only a minor constituent ( 1%) from reactions catalyzed by Rh2(5S-MEPY)4. This difference can be attributed to the ability of the carboxylate substituents to stabilize the earboeation form of the intermediate metal carbene. 

Preliminary mechanistic studies by Fernandez, Peris, and Crudden provided evidence that, under smooth hydroformylation conditions, NHCs remain ligated in the coordination sphere of rhodium [55, 68]. A similar conclusion was drawn by Scholten and Dupont for hydroformylation in ionic liquids [69]. A serious problem sometimes faced is the reductive elimination of carbene ligand from the metal as imidazohum salt under the effect of H2, as found in hydrogenation reactions [70]. 

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