With rhodium dihydrogen complexes

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). 

The facility with which dihydrogen activation occurs in these A-frame and face-to-face bis-dppm-bridged rhodium and iridium complexes is striking when considering the scarcity of reported examples for such activations with d d compounds of different structure. [12]... 

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. 

Scheme 8.20 Difference in interaction of carbon dioxide and dihydrogen with rhodium and iridium pincer complexes.

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. 

Metal(II) species, homobimetallic complexes - During a rhodium insertion into H2(TPP) using [Rh(CO)2Cl]2 in HOAc/NaOAc, a paramagnetic rhodium(II) porphyrin was observed [57] which could be transformed into a hydridorho-dium(III) porphyrin with dihydrogen (path — p) and subsequently into its corresponding base, the anion [Rh(TPP)], which may be likewise regarded as... 

A clean formation of [Rh(OEP)]2 proceeds via thermolysis [269] or photolysis [273] with loss of dihydrogen from or autoxidation of the hydride RhH(OEP) (path p). The tetramesitylporphyrin complex, Rh(TMP) [61], does not dimerize at all due to the sterically hindrance created by the two ortho-methyl groups of each phenyl ring (see Ru(TMP) ), however, the meta-methyl groups of the rhodium(II) derivative prepared from tetra-kis(3,5-xylyl) porphyrin [H2(TXP)] do not prevent dimerization, and the complex is isolated as a dimer [Rh(TXP)] 2 which dissociates (path — q) prior to chemical reactions. Photolysis of RhMe(TMP) [274] (path r) is another suitable access to Rh(TMP) [271]. 

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