Rhenium carboxylates

Although the structure of the CO2 adduct, which is an important intermediate in the photocatalysis of the rhenium complexes, has not yet been confirmed, several proposals have been made. As candidates, a C02-bridged binuclear complex (CO)3(dmb)Re—CO2—Re(dmb)(CO)3 (16, dmb = 4,4 -dimethyl-2,2 -bipyridine) and a rhenium carboxylate complex Re(bpy) (COlsCCOOH) (17) were synthesized. 

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... 

Rhenium(VI) complexes, 4,194 alkoxides, 4,196 amides, 4,194 amines, 4,199 carboxylates, 4,199 dimethylformamide, 4,199 dioxane, 4,198 halides, 4,195,199 2-hydroxypyridine, 4,199 imides, 4,194 magnetic behavior, 1,271 mixed sulfur-nitrogen compounds, 4,196 N heterocycles, 4,199 nitrides, 4,194 oxide halides, 4, 195 oxoanions,4,196 pyridine, 4,199 sulfates, 4,198 sulfur compounds, 4,196 tellurates, 4,198... 

A greater tendency of rhenium complexes (compared to technetium analogues) to expand their coordination numbers has been invoked to rationalize the stronger interaction of the perrhenate ion with carboxylate ligands. This association has been suggested as a possible cause of the different quantitative biodistribution and excretion characteristics of pertechnetate and perrhenate perrhenate is accumulated in thyroid to a lesser extent and renally excreted more rapidly than pertechnetate [6]. 

An pFl-dependent coordination isomerism has been observed for the tumor targeting rhenium(V) 0x0 complex [ReO(DMSA)2] (DMSA = 2,3-mercaptosuccinate). In solution the crystallo-graphically characterized syn,endo-isomQx (87a) slowly isomerizes into the anti- (87b) and the iyn,exo-isomers (87c) which has consequences for the biodistribution of the potential pharmaceutical (Scheme 10). The conversion rate decreases with increasing pH suggesting an acid catalyzed reaction. The syn,exo-comy> ex is favored in alkaline solutions (pH > 8.4) which can be understood in terms of the repulsions between the deprotonated carboxylic groups and the 0x0 ligand. 

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. 

The conclusion that variations in the coupling of vibrational modes in these dimers is not responsible for the entire frequency shift is an important point in that it justifies the search for a chemical rationale of the bonding differences in Mo2Xf and Mo2(02CR)4. The authors suggest that 6 bonds may make an important bonding contribution in rhenium dimers and molybdenum carboxylates to account for Fmm near 4.5 mdyne A-1. Furthermore, it is proposed that such a 6 component would probably not contribute significantly in the molybdenum halides where is near 3.5 mdyne A"1, possibly... 

Table 11 Acidities of rhenium Fischer carbene complexes and intrinsic rate constants for their deprotonation by amines and carboxylate ions in 50% MeCN-50% water (v/v) at 25°Ca,b...

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