Rhodium complexes hydrolysis

Complexation of (124) and (125) with [ Rh(COD)Cl 2] in the presence of Si(OEt)4, followed by sol-gel hydrolysis condensation, afforded new catalytic chiral hybrid material. The catalytic activities and selectivities of these solid materials have been studied in the asymmetric hydro-gen-transfer reduction of prochiral ketones and compared to that of the homogeneous rhodium complexes containing the same ligands (124) and (125) 307... 

Organotrialkoxysilanes (ArSi(OR)3) were used as organometallic reagents without fluoride additives (Scheme 56).144,144a ArSi(OR)3 was easy to use because of its higher air and moisture stability. Oi and co-workers believed that hydrolysis of the trialkoxysilanes to generate silanetriols was likely occurring prior to transmetallation of the cationic rhodium complex. 

An interesting asymmetric transformation is the asymmetric conjugate addition to a-acetamidoacryhc ester 30 giving phenylalanine derivative 31, which has been reported by Reetz (Scheme 3.10) [10]. The addition of phenylboronic acid 2m in the presence of a rhodium complex of l,T-binaphthol-based diphosphinite ligand 32 gave a quantitative yield of 31 with up to 11% enantiomeric excess. In this asymmetric reaction the stereochemical outcome is determined at the hydrolysis step of an oxa-7r-aUylrhodium intermediate, not at the insertion step (compare Scheme 3.7). 

The reaction mechanism proposed for the addition of organostannanes [29] is similar to that for organoboronic acids. An example of the reaction of methyl vinyl ketone 42 is outlined in Scheme 3.15. The catalytic cycle involves a cationic rhodium complex G, phenylrhodium H, and oxa-n -allylrhodium I. Stannyl enol ether 44 is formed by the reaction of oxa-n -allylrhodium I with Me3SnBF4, which upon hydrolysis gives the ketone 43. The lower yields in the absence of water were explained by the further reaction of 44 with methyl vinyl ketone 42. The rapid hydrolysis with water may prevent such oligomerization. 

Transition metal catalysts and biocatalysts can be combined in tandem in very effective ways as shown by the following example (Scheme 2.21). An immobilized rhodium complex-catalyzed hydrogenahon of 46 was followed by enzymatic hydrolysis of the amide and ester groups of 47 to afford alanine (S)-9 in high conversion and enanhomeric excess. Removal of the hydrogenation catalyst by filtration prior to addition of enzyme led to improved yields when porcine kidney acylase 1 was used, although the acylase from Aspergillus melleus was unaffected by residual catalyst [23]. 

Chromium alkenyl Fischer carbenes have been shown to undergo a 3 + 2-cyclization with allenes under Rh(I) catalysis and a CO atmosphere, yielding 2-alkylidenecyclo-pentanone (54) after acidic hydrolysis.46 Reactions with electron-rich allenes are carried out with a neutral rhodium complex whereas electron-poor allenes require a... 

Step (1) involves the formation of methyl iodide, which then reacts with the rhodium complex Rh(I)L by oxidative addition in a rate-determining step (2) to form a methylrhodium(III) complex. Carbon monoxide is incorporated into the coordination sphere in step (3) and via an insertion reaction a rhodium acyl complex is formed in step (4). The final step involves hydrolysis of the acyl complex to form acetic acid and regeneration of the original rhodium complex Rh(I)L and HI. Typical rhodium compounds which are active precursors for this reaction include RhCl3, Rh203, RhCl(CO)(PPh3)2, and Rh(CO)2Cl2. 

All of the carbonato cobalt(III) complexes reported here are reddish in color and extremely soluble in water. The rhodium complex is pale-yellow, whereas the iridium salt is virtually white they are both soluble in water. Treatment with dilute acid immediately gives the corresponding aqua complex with evolution of carbon dioxide. The characterization and the mechanistic details of acid hydrolysis of these complexes have been reported.3,4,11... 

For asymmetric hydrosilylation of ketones a rhodium complex coordinated with the ferrocenyl(dimethyl)phosphine 3b has been reported to be more effective than other ferrocenylphosphines to give optically active alcohols (up to 49% ee) after hydrolysis (Scheme 2-49) [7]. 

This mechanism consists of several steps (1) oxidative addition of hydrosilane to the rhodium(I) complex (2) and (3) coordination and insertion of the ketone into the rhodium-silicon bond to form a diastereomeric a-silyloxyalkylrhodium intermediate (4) reductive elimination of alkoxysilane as a primary product and (5) hydrolysis of the alkoxysilane yielding an optically active alcohol. Hydrosilylation of prochiral ketones by prochirally disubstituted silanes leads to asymmetry on the silicon atom as well as on the carbon atom and, in the presence of chiral rhodium complexes, results in optically active monohydrosilanes (eq. (5)) [2] ... 

This two-step synthesis of ketones has been improved from the aldehyde into one-step synthesis with the cocatalyst system of the rhodium complex and 2-amino-3-picoline, which reacts with an aldehyde to give an aldimine in situ. The ketimine produced is easily converted to a ketone by in situ hydrolysis with H20, which is formed in the step of condensation of the aldehyde with the amine (Eq.56) [125]. 

---