Rhodium dihydrogen complex

When cyclobutanone is treated under dihydrogen with a catalytic amount of a rhodium(I) complex containing a bidentate diphosphine ligand, the ring opened alcohol 89 is produced in goodyield [46,47]. The oxidative addition intermediate 87 is hydrogenated to give aldehyde 88, which is further reduced to the alcohol 89. 

To effectively model the asymmetric hydrogenation reaction, we must look at the mechanism carefully. The first step involves the displacement of solvent and the coordination of the enamide to produce the two diastereomers (Fig. 3) (17-20). It appears as though the enamide-coordinated diastereomers are in rapid equilibrium with each other through the solvento species (Fig. 4). This square planar rhodium(I) cation is then attacked by dihydrogen to form an octahedral rhodium(III) complex (Fig. 4). Hydrogen then inserts into the Rh-C bonds, and the product is reductively eliminated (Fig. 4). From a molecular mechanics standpoint we have three entities to model the square planar rhodium(I) solvento species and the two intermediates (square pyramidal dihydrogen complex and the octahedral dihydride). 

Figure 4 Attack of dihydrogen on the two diastereomers of [(S,S-CHIRA-PHOS)Rh(MAC)]+. Notice that the two diastereomers of [(S,S-CHIRAPHOS) Rh(MAC)]+ are in equilibrium via the solvento species. After hydrogen attacks the square planar rhodium(l) complex, an octahedral rhodium(lll) dihydride is formed. (Redrawn from Ref. 32.)...

Several pentacoordinate monohydrido complexes can add an additional, small, neutral ligand to form complexes of the type [RhHX2 L2 L ]. The dihydrido complexes are conveniently prepared by the oxidative addition of dihydrogen to rhodium (I) complexes dissolved in inert organic solvents. Chlorinated... 

However, the reaction of H2 with the rhodium thiolate complex proceeds via a different pathway (Scheme 9). For this reaction, a ri -dihydrogen complex is first formed. Then, the H—H bond is heterolytically splitted via a four-center transition state under the cooperation of the Rh center and thiolate ligand. The free energy barrier for the rate-determining step (the formation of the dihydrogen complex) is 15.2 kcal/mol in the solvent. 

Following this observation, a general approach for the synthesis of pincer-type methylene arenium compounds was developed (Scheme 3.4). Upon reaction of the methyl rhodium (or iridium) complexes 5 with a slight excess of triflic acid, dihydrogen (not methane ) was evolved to form the methylene arenium complexes 4a.11 Thus, the methylene arenium form is clearly preferred over the benzylic M(III) form, in which the positive charge is localized at the metal center. 

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