Rhenium carboxylic acids

Reduction of Carboxylic Acids to Alcohols. In addition to the nonsupported catalysts mentioned for the hydrogenation of amides to amines, mthenium and rhenium on alumina can be used to reduce carboxyHc acids to alcohols. The conditions for this reduction are somewhat more severe than for most other hydrogenation reactions and require higher temperatures, >150° C, and pressures, >5 MPa (725 psi) (55). Various solvents can be used including water. 

Rhenium oxides are also useful in reduction of carboxylic acids (170" C, 3500 psig). Aromatic acids can be reduced to alcohols without ring saturation 3,4,S,6). Strongly synergistic effects were found on substituting half of the Re207 with rulhenium-on-carbon, and excellent results can be obtained al... 

Use of ruthenium catalysts [41] and rhenium heptoxide [42] is rare. Their specialty is reduction of free carboxylic acids to alcohols. 

Selective oxidation of N-1 of adenine derivatives is typically carried out with peracids <1998JOC3213>, but has also been achieved with hydrogen peroxide and catalytic methyltrioxorhenium (Scheme 10) <2000T10031>. The inclusion of pyridazine-2-carboxylic acid as a stabilizer for reactive rhenium peroxides led to increased yields. Caffeine did not react under these conditions. 

Broadbent et al. have found that the rhenium blacks prepared by reducing rhenium heptoxide are highly effective catalysts for the hydrogenation of carboxylic acids to alcohols at 150-170°C for monocarboxylic acids (eq. 10.3) and at 200-250°C for dicarboxylic acids (eq. 10.4) under 13.5-27 MPa H2.10,11 Rhenium heptoxide can be reduced to the active blacks in appropriate solvent (ethanol, 1,4-dioxane, acetic acid, or water) at 120-220°C and 15-21 MPa H2 for 1-2 h, or more conveniently, in situ in... 

Hydroxyazobenzne-2-carboxylic acid (HABA) [90] (red colored) Fluorescent amino acid incorporation Rhenium [91]... 

R,S)-N- 2-((3 -N -Propyl-(l"R-3"-j8-(4-fluorophenyl)tropane-2"-j8-carboxylic acid methyl ester))(2-mercaptoethyl)amino)acetyl)-2-aminoethanethiolato rhenium(V) oxide... 

Carboxylic acids are one of the most difficult of all functional groups to hydrogenate. They can be hydrogenated to the alcohol, however, over ruthenium 34 or rhenium heptoxide 35,136 t 150°-200°C and 300-400 atmospheres pressure. Activated Cu-Ba-CrO is also effective at 250°-300°C and 300-400 atmospheres (Eqn. 18.31). 3 A Re-Os bimetallic catalyst has been used to hydrogenate carboxylic acids at 100 -120°C and 20-100 atmospheres of hydrogen. High conversions to the alcohol were reported. 38... 

Rhenium catalysts. H. Smith Broadbent and co-workers have reported the preparation of a number of oxides of rhenium (RcbOt. ReOj, ReOa, ReO), which are effective hydrogenation catalysts, particularly for the reduction of carboxylic acids to primary alcohols. Kor the reduction of aromatic, olefinic. carbonyl, and nitro groups they are less aclivc than nickel or plutimim calalysts hence selective hydrogenation is possible. Bcn/ylic hydroxyl groups are stable to hydrogenolysis. 

Rhenium A hydrocarboxylation with high selectivity for anti-Markovnikov addition and predominant (Z)-enol ester product is mediated by ReBr(CO)5 (1 mol%, 110°C, 15 h) [168]. The 7i-activation mechanism proposed by the authors does not fit to the observed anti-Markovnikov selectivity. Iridium The precursor complex [ IrCl(cod) 2] (1 mol%) combined with P(OMe)3 (4 mol%) and Na2C03 (2 mol%) produces a catalyst that adds carboxylic acids to terminal alkynes (toluene, 100°C, 15 h) to give a mixture isomers with variable selectivities, although the Markovnikov product is usually formed in excess (ca 5 1) [169]. The complex []IrCl(cod) 2] (1 mol%) in the presence of Na2C03 (0.6 equiv) is also a catalyst for the transvinylation of vinylacetate with diverse alcohols [170]. 

Alcohols from carboxylic acids. Rhenium heptoxide, or rather rhenium black resulting from hydrogenation of the former, is the most effective catalyst for the reduction of carboxylic acids to alcohols. It is also effective in the reduction of carboxylic acid amides. On the other hand, it shows only moderate but definite activity toward many of the common organic functions reducible by Pt- or Ni-catalysts.—Gapric acid hydrogenated with rhenium heptoxide at 137°/173 atm. in dioxane-water n-decanol. Y ca. 100%. F. e., also with prehydrogenated RegOy, s. H. S. Broadbent et al., J. Org. Chem. 24, 1847 (1959). 

Haloform reactions are generally performed with halogens in the presence of hydroxide [251] or directly with hypohalites [252]. Alternative methods affording carboxylic acids from methyl ketones (or other enolizable substrates) include the aerobic oxidation in the presence of a catalytic amount of dinitrobenzene [253] with a base in a dipolar aprotic solvent such as DMF [254] or HMPT (hexamethylphospho-ric triamide) [255, 256] and the use of stoichiometric quantities of hypervalent iodide derivatives [95, 257] or nitrosylpentacyanoferrate [258]. Furthermore, metal catalysts can be used, and systems such as tert-butyl hydroperoxide in the presence of rhenium oxide [259], oxygen in combination with a copper complex [260], heteropolyacids [261] and Mn"/Co" systems [262] were found to be applicable. Finally, aryl ketones are selectively oxidized to aliphatic carboxylic acids by treatment with periodate [81] in the presence of ruthenium trichloride [263]. 

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