Rhodium benzylic alcohols

Because of the complexity of the rhodium-catalyzed reduction of benzaldehyde to benzyl alcohol with CO and H20, it is not possible to fully elucidate the mechanism of catalytic reduction given the extent of the kinetic studies performed to date. However, the results do allow us to draw several important conclusions about the reaction mechanism for benzaldehyde hydrogenation and several related reactions. 

Vinylogs of benzylic alcohols, e.g. cinnamyl alcohol, undergo easy saturation of the double bond by catalytic hydrogenation over platinum, rhodium-platinum and palladium oxides [39] or by reduction with lithium aluminum hydride [609]. In the presence of acids, catalytic hydrogenolysis of the allylic hydroxyl takes place, especially over platinum oxide in acetic acid and hydrochloric acid [39]. 

However, ruthenium, rhodium, and rhodium-platinum catalysts have been found to be highly effective for the selective hydrogenation of these benzyl-oxygen compounds without loss of the oxygen functions. Thus, benzyl alcohol is hydrogenated to cyclohexanemethanol in high yield over ruthenium dioxide with addition of a small amount of acetic acid (eq. 11.35).114... 

In contrast to benzyl alcohol, a-substituted benzyl alcohols, benzyl ethers, and aryl ketones may be successfully hydrogenated over rhodium and rhodium-platinum catalysts to give the corresponding saturated products in high yields, as shown in eqs. 11.38-11.41. In the hydrogenations shown in eqs. 11.38 and 11.39, no racemization took place D-mandelic acid afforded D-hexahydromandelic acid in 94% yield and meso- and dl-2,3-dicyclohexyl-2,3-butanediol were obtained in 93 and 94% yields, respectively, by hydrogenation of the corresponding diphenyl compounds. 

Alcohols can be dehydrogenated to carbonyl compounds by exposure to a catalytic amount of a rhodium(I) complex under phase-transfer conditions. This reaction is particularly useful for benzylic alcohols such as 1-phenylethanol (50) which gave acetophenone (51) in 78% yield using chlorodicarbonylrhodium(I) dimer as the metal catalyst and benzyltrieth-ylammonium chloride or Aliquat 336 as the phase-transfer catalyst (52). 

Hydrogenation of benzyl alcohols. Stocker found that hydrogenolysis is completely avoided by use of rhodium on alumina. Under very mild conditions,... 

Benzyl alcohols. After transmetaUation arylboiDnic acids are converted into aryl-rhodium species that are nucleophilic toward various aldehydes. A very active catalyst (1) is that derived from Rh2(OAc)4, l,3-bis(2,6-diisopropylphenyl)imidazolium chloride and r-BuOK. ... 

The reduction of 2- benzyl(methyl)amino]-l-[3-(benzyloxy)phenyl]ethanone was carried out with the same in situ rhodium/phosphane catalyst to give (S)-2-[benzyl(methyl)amino]-l-[3-(benzyl-oxy)phenyl]ethanol in 85 % ee44 (Table 6). This reaction constitutes the enantioselective step in a synthesis of (tf)-phenylephrine hydrochloride, another /1-receptor-stimulating agent having a chiral benzylic alcohol group44. 

Several syntheses are available to the 13,14-dihydroprostaglandins, some of which are metabolites of the E and F series. The first of these routes [143, 144] started from the formyl derivative (LVII) of the enol ether of cyclo-pentan-l,3-dione which on reaction with ethyl 6-bromosorbate and tri-phenylphosphine followed by selective catalytic reduction afforded the ester (LVIII). A second formylation followed by elaboration with n-hexanoyl-methylenetriphenylphosphonium chloride 1 to the ketone (LIX) which on reduction of the exocyclic double bond and acid-catalysed solvolysis in benzyl alcohol afforded the benzyl ether (LX) and its isomeric enol ether. Reduction with lithium tri-t-butoxyaluminium hydride to the corresponding 15-hydroxy-compound and palladium-charcoal catalysed hydrogenolysis followed by prolonged catalytic hydrogenation with rhodium-charcoal led to ( )-dihydro-PGEi ethyl ester. 

A mild intramolecular fluorocyclization reaction of benzylic alcohols and amines was developed, using commercially available Selectfluor to trigger electrophilic cyclizations to afford fluorinated dihydrobenzo[c]furans (14CC13928). A novel rhodium-catalyzed carbonylative [3 + 2 + 1] cycloaddition of alkylidenecyclopropanes was developed for the synthesis of phenols with benzo[c]furan species (140L4352). 

Metal and Metal Oxide Catalysts. Only a few examples are known on application of metal and metal oxide catalysts for carbonylation reaction. For example, it has been reported that benzyl alcohol could be carbonylated with CO to phenylacetic acid (with 100% selectivity) in the presence of rhodium metal Rh(0) (72). Methyl formate was prepared by treatment of CH3OH with CO in the presence of alkali earth metal oxide catalysts. The oxide catalysts are preferably CaO catalysts supported on oxides of group Ila (except Ca), Ilb, Ilia, or IVa metals. CH3OH was autoclaved with CaO/ZnO under 50 atm of CO at 180°C for 2 h to give 9.7% of methyl formate at 99.9% selectivity (73). 

The great advantage of rhodium catalysts is their high selectivity, which is > 99%, based on methanol. Even in the presence of hydrogen, no hydrogenation products such as methane, acetaldehyde or ethanol are observed, in contrast to cobalt-based catalysts. In addition, the high activity allows the use of metal concentrations as low as 10 M [10]. Besides methanol, a variety of other alcohols can be submitted to the rhodium-catalyzed carbonylation and more detailed data are known for ethanol [17] and benzyl alcohol [18]. 

Secondary benzylic alcohols are often oxidized to the corresponding ketones under the palladium-catalyzed arylation conditions. In contrast, a unique rhodium-catalyzed arylation of diarylmethanols having a 2-(2-pyridiyl) moiety... 

The system using the trimethylphosphine-substituted rhodium(I) complex RhCl(CO)(PMe3)2 has been studied in more detail. The photocarbonylation of benzene at 1 atm of CO in the presence of this complex gives benzaldehyde, along with other carbonylation products such as benzyl alcohol and benzophenone. Alkanes can also be photocarbonylated to aldehydes under mild conditions in the presence of RhCl(CO)(PMe3)2 with a high selectivity for carbonylation at the terminal position ... 

Figure 7 (A) Suggested reaction sequence for rhodium-catalyzed hydroacylation of C=C bonds via benzylic alcohol, (B) simultaneous hydroacylation and hydrogenation of PBD, (C) hydroacylation of PBD starting from symmetric diols, and (D) simultaneous hydroacylation and hydrogenation of PBD starting from substituted aldehydes.

An interesting system for the catalytic aerobic oxidation of benzyl alcohol was developed by Albrecht and co-workers. A porphyrin rhodium(iii) complex [(IMe)2Rh(TPP)]Cl (TPP = meso-tetraphenyl porphyrin) with two apical NHC ligands led to porphyrin distortion and dearomatization. This coordi-natively saturated complex was catalytically active in the oxidation of benzyl alcohol. It was demonstrated that its catalytic activity was imparted by NHC dissociation, as facile cleavage of the Rh-NHC bond was promoted by the strong trans effect of the distal NHC ligand and by the distorted and partially dearomatized porphyrin. 

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