Rhenium aldehydes

It was later found that some aldehyde complexes of rhenium, [Re(NO) (PPh3) 0=C(Aryl)H (Tj-C5H5)]BF4, are also present in solution as a rapidly interconverting mixture of rf isomers and the if isomer.48,49 With the exception of solvent dependence, similar dependencies of the equilibrium constants on the aryl substituents and on the temperature were observed. The solvent dependence was found to be opposite, which, however, is easily understood when taking into account that the rhenium aldehyde complexes are cationic whereas the chromium and tungsten thio- and selenoaldehyde complexes are neutral. 

In conclusion, it can be noted that high valent transition metals seem perfectly capable of serving as effective Lewis acids. Many of the systems discussed here exhibit exceptional robusmess, stability and a propensity to form crystalline complexes. This would facilitate the task of crystallization and structural analysis, and one can imagine that transition metal complexes can be used as structural probes of Lewis acid-carbonyl interactions. In this vein, the first glimpse of the origins of Cram selectivity in a-chiral aldehydes may have been obtained from the crystal structure of a rhenium aldehyde complex. Lastly, the... 

Rhenium oxides have been studied as catalyst materials in oxidation reactions of sulfur dioxide to sulfur trioxide, sulfite to sulfate, and nitrite to nitrate. There has been no commercial development in this area. These compounds have also been used as catalysts for reductions, but appear not to have exceptional properties. Rhenium sulfide catalysts have been used for hydrogenations of organic compounds, including benzene and styrene, and for dehydrogenation of alcohols to give aldehydes (qv) and ketones (qv). The significant property of these catalyst systems is that they are not poisoned by sulfur compounds. 

Reduction of unsaturated aldehydes seems more influenced by the catalyst than is that of unsaturated ketones, probably because of the less hindered nature of the aldehydic function. A variety of special catalysts, such as unsupported (96), or supported (SJ) platinum-iron-zinc, plalinum-nickel-iron (47), platinum-cobalt (90), nickel-cobalt-iron (42-44), osmium (<55), rhenium heptoxide (74), or iridium-on-carbon (49), have been developed for selective hydrogenation of the carbonyl group in unsaturated aldehydes. None of these catalysts appears to reduce an a,/3-unsaturated ketonic carbonyl selectively. 

Only scare data is available in the literature on the application of rhenium containing mono- or bimetallic catalysts in the hydrogenolysis of esters to alcohols. Decades ago Broadbent and co-workers studied the hydrogenation of organic carbonyl compounds (aldehydes, ketones, esters, anhydrides, acids,... 

As can be seen in the scheme below, insertion reactions of aldehydes to the C-H bond of aromatic ketimines by using a rhenium catalyst provided benzo[c]furans via a mechanism involving consecutive steps of C-H bond activation, insertion of aldehyde, intramolecular nucleophilic cyclization, reductive elimination, and elimination of aniline <06JA12376>. 

Compound 146 was converted to 147 (relative stereochemistry is shown in Scheme 68) by addition of a series of aldehydes, and other isomers were also obtained which were easily purified. But attempts to liberate a dihydrofuran from 147 were unsuccessful, which is believed to be due to decarbonylation by rhenium.306 Alternative approaches can be envisaged, although the authors noted the difficulty in the preparation of the starting material as a serious barrier to this.306... 

C. Reactions of Tris(imido)rhenium(VII) with Aldehydes... 

Notably, as illustrated in Scheme 124, enantiofaces of aldehydes can be differentiated by a chiral rhenium template to result in stereoselective reaction with cyanide ion (297). The rhenium Lewis acid element forms stereoisomeric it complexes with aldehydes, which are convertible via... 

If the porous carbon cathodes are coated with copper or rhenium and an NH4C1-containing electrolyte is used, glycol aldehyde is obtained in the reduction of oxalic add S22). 

Platonov and co-workers have made a significant contribution to the study of alcohol dehydrogenation by means of rhenium catalysts (310). Rhenium disulfide was found to be efficient in promoting dehydrogenation of alcohols to aldehydes or ketones (acetone), and in the dehydrogenation of cyclohexanol to phenol and a small amount of cyclohexanone (308),... 

Prereduced rhenium heptoxide catalyst,65 especially the catalyst poisoned with pyridine, has been found to give high yields of unsaturated alcohols in the hydrogenation of unsaturated aldehydes (Table 5.2).66 A typical hydrogenation with the rhenium catalyst is shown in eq. 5.27. In the vapor phase hydrogenation of acrolein to allyl alcohol, the selectivity of rhenium catalysts has been found to be improved by poisoning with CO and CS2.67... 

TABLE 5.2 Hydrogenation of Unsaturated Aldehydes over Prereduced Rhenium Heptoxide Poisoned with Pyridine1 ... 

Supported oxo-rhenium catalysts in heterogeneous systems have also been reported, for example, the polystyrene-supported (catecholato)oxo-rhenium(VII) complexes (38), obtained from the reaction of polystyrene-supported catechol with [ReOCl3(PPh3)2], which catalyze alcohol oxidation to ketones or aldehydes with dimethylsulfoxide and epoxide... 

In contrast with the common activity of oxo-rhenium compounds in oxidation catalysis (see above), the Re -dioxo-complex [Re02l(PPh3)2] catalyzes the reductive hydrosilylation of aldehydes and ketones to give silyl ethers (reaction... 

Example 5.2. Hydroformylation of propene [2]. Hydroformylation converts an olefin to an aldehyde of next higher carbon number by addition of carbon monoxide and hydrogen. The reaction is catalyzed by dissolved hydrocarbonyl complexes of transition-metal ions such as cobalt, rhodium, or rhenium. The carbon atom of the carbon monoxide can attach itself to the carbon atom on either side of the olefinic double bond, so that two aldehyde isomers are formed. If the catalyst also has hydrogenation activity, the aldehydes are converted to alcohols and paraffin is formed as by-product. For propene and such a catalyst the (simplified) network is ... 

One means of stereoselective cleavage of biaryl lactones [53] is activation of the carbonyl group with a Lewis acid and subsequent attack with a chiral nucleophile. Conversely, activation can be effected with a chiral Lewis acid followed by attack of an achiral nucleophile. Complexation of a biaryl lactone to the chiral fragment [CpRe (NO)(PPh3)j then reduction with K(s-Bu)3BH (K-selectride) and ring opening of the intermediate rhenium lactolate gives the metalated aldehyde (dr = 75 25) which is converted to the alcohol without essential loss of optical purity (Sch. 6) [54]. 

Reactions of Phosphonium Ylides. - 2.3.1 Reactions with Carbonyl Compounds. This year we are able to report several variations of the traditional Wittig olefination which employ the addition of catalysts to effect the reaction. For example, Lebel et al. have reported a new salt-free process for the methyl-enation of aldehydes, in which the phosphorane is generated in situ from triphenylphosphine and a diazo precursor with either a rhodium- or rhenium-based catalyst (Scheme 6). It was found that the most effective combination of catalyst and diazo-compound were Wilkinson s catalyst [RhCl(PPh3)3] and... 

Vanhoye and coworkers [402] synthesized aldehydes by using the electrogenerated radical anion of iron pentacarbonyl to reduce iodoethane and benzyl bromide in the presence of carbon monoxide. Esters can be prepared catalytically from alkyl halides and alcohols in the presence of iron pentacarbonyl [403]. Yoshida and coworkers reduced mixtures of organic halides and iron pentacarbonyl and then introduced an electrophile to obtain carbonyl compounds [404] and converted alkyl halides into aldehydes by using iron pentacarbonyl as a catalyst [405,406]. Finally, a review by Torii [407] provides references to additional papers that deal with catalytic processes involving complexes of nickel, cobalt, iron, palladium, rhodium, platinum, chromium, molybdenum, tungsten, manganese, rhenium, tin, lead, zinc, mercury, and titanium. 

The catalysts 4-6 are employed at concentrations of 10 mol% (Mo) and 1-10 mol % (Re), with respect to the aldehyde. Too low a rhenium concentration favors the formation of ketazines. The reactions are normally conducted at room temperature (see below). 

Since the keto component reacts neither with the catalyst 5 (with or without phosphine) nor with the diazoalkane (exceptions below), it is likely to enter the catalytic cycle at the metal-carbene stage 8. If this proposal withstands future mechanistic studies, then the regioselectivity - oxygen to the oxophilic rhenium center-is reasonable by considering a metallacyclic intermediate 10 (eq. (6)). This hypothetical rhena-oxetane is assumed to eliminate the olefin. As a matter of fact, catalyst 5 has been detected from such precursors (e. g., 7a, b -1- diazoalkane f aldehyde) by gas chromatography [9, 10]. 

MTO proves to be an efficient and effective catalyst in this reaction when the dienophile is an a, -unsaturated ketone or aldehyde. It is especially active in water, usually with isolated yields >90%. Kinetic studies show that the reaction rate is proportional to the catalyst concentration. The desirability of 1 as a Diels-Alder catalyst stems from a combination of favorable properties the tolerance for many substrates, the inertness to air and oxygen, the use of aqueous medium, and the absence of product inhibition. The initial step appears to be the coordination of the carbonyl oxygen to the rhenium center. Steric crowding around rhenium inhibits reactions of the larger dienophiles [27]. 

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