Rhodium-acyl complex

For these systems, mixtures of [HRh(CO)3L] and [HRh(CO)2L2] were observed in the absence of alkene. On addition of 1-octene, rapid scan IR spectroscopy revealed partial conversion to a mixture of isomeric rhodium acyl complexes, confirmed by HP NMR. 

The reaction was also performed with to enable identification of the rhodium-acyl resonance in NMR. All resonances obtained in the NMR spectrum are very broad at room temperature. The resonances sharpened when the temperature was decreased (Figure 6.13). The NMR spectrum at 223 K (Spectrum 3a, Figure 6.13) showed two rhodium-acyl resonances (d = 230.0, 227.2 ppm) and three different rhodium carbonyl resonances (5 = 194.7,193.7 and 190.6 ppm), indicating that we are dealing with two different rhodium-acyl complexes. 

The structure of the two rhodium-acyl complexes was elucidated using COSY90 spectra, selective decoupling of the phosphorus resonances, and HMQC spectra. Spectrum 3d (Figure 6.13) shows the complete simulation of the NMR spectrum obtained at 223 K. The NMR spectrum... 

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. 

There is also direct spectroscopic evidence for the formation of a rhodium acyl complex as an intermediate during methanol carbonylation over RhNaX. Scurrell 213) showed that infrared bands characteristic of the complex CH3CORh(III)ICOL were formed at 2085 and 1710 cm 1 after exposure of RhNaX to CO and CH3I at 100°C. 

In the rhodium-catalyzed carbonylation of methanol via methyl iodide, acetyl iodide is formed by reductive elimination from an anionic rhodium acyl complex [T14] ... 

Several pentacoordinate rhodium-acyl complexes were observed in the phosphorus diamide-modified 1-hexene hydroformylation by using in situ P, and ° Rh NMR spectroscopy. The major rhodium-acyl complex, CH3(CH2)5(C=0)RhL2(C0)2 was found to have a trigonal bipyramidal structure... 

The reaction of ethyl methallylic amine under HAM conditions in common organic solvents may lead via intramolecular hydrocarboxylation to the corresponding lactam 2 as a result of the fast nucleophilic attack of the nitrogen atom at the carbonyl group in the intermediate rhodium-acyl complex (Scheme 5.98) [35a, 82]. This path could be suppressed by running the reaction in supercritical... 

Rhodium(I) complexes are effective reagents and/or catalysts for the decarbonylation of acyl halides and aldehydes 9 11,34,195,230,231,236). The compound Rh(PPh3)3Cl, especially, has received considerable attention. The first step in such reactions involves oxidative addition to Rh(I) of the organic molecule, exemplified by the following ... 

In 2004 Caporali investigated the hydroformylation of 1-hexene and cyclohexene using HRh(CO)(PPh3)3 [61]. The collected data indicated that the rate-determining step in the hydroformylation cycle depends upon the structure of the olefin. With an alpha-olefin like 1-hexene, the slowest step seems to be the hydrogenolysis of the acyl rhodium complex. In the presence of cyclohexene as a model for an internal olefin, the rate-determining step is the reaction of the olefin with the rhodium hydride complex (intermediate II in Fig. 6). 

However, considerable amounts of 2,3-dihydrofuran 50 and tetrahydro-furan-2-carbaldehyde 53 were present because of an isomerization process. The isomerization takes place simultaneously with the hydroformylation reaction. When the 2,5-dihydrofuran 46 reacts with the rhodium hydride complex, the 3-alkyl intermediate 48 is formed. This can evolve to the 2,3-dihydrofuran 50 via /3-hydride elimination reaction. This new substrate can also give both 2- and 3-alkyl intermediates 52 and 48, respectively. Although the formation of the 3-alkyl intermediate 48 is thermodynamically favored, the acylation occurs faster in the 2-alkyl intermediates 52. Regio-selectivity is therefore dominated by the rate of formation of the acyl complexes. The modification of the phosphorus ligand and the conditions of the reaction make it possible to control the regioselectivity and prepare the 2- or 3-substituted aldehyde as the major product [78]. As far as we know, only two... 

Complexes 6 undergo the second migratory insertion in this scheme to form the acyl complexes 7. Complexes 7 can react either with CO to give the saturated acyl intermediates 8, which have been observed spectroscopically, or with H2 to give the aldehyde product and the unsaturated intermediates 3. The reaction with H2 involves presumably oxidative addition and reductive elimination, but for rhodium no trivalent intermediates have been observed. For iridium the trivalent intermediate acyl dihydrides have been observed [29], The Rh-acyl intermediates 8 have also been observed [26] and due to the influence of the more bulky acyl group, as compared to the hydride atom in 2e and 2a, isomer 8ae is the most abundant species. 

In addition to the catalysts listed in Table 2, several rhodium(I) complexes of the various diphosphines prepared by acylation of bis(2-diphenylphosphinoethyl)amine were used for the hydrogenation of unsaturated acids as well as for that of pyruvic acid, aUyl alcohol and flavin mononucleotide [59,60]. Reactions were mn in 0.1 M phosphate buffer (pH = 7.0) at 25 °C under 2.5 bar H2 pressure. Initial rates were in the range of 1.6-200 mol H2/molRh.h. 

Garland et al. have developed a powerful method for the reconstruction of individual pure component spectra from complex catalytic mixtures [20]. Using this band-target entropy minimization (BTEM) protocol, he was able to identify the mononuclear rhodium acyl intermediate in the hydroformylation reaction of 3,3-dimethylbut-l-ene starting from Rh4(a-CO)9(p-CO)3 as catalyst precursor [21]. In addition to the catalyst precursor and the more stable decomposition product... 

The presence of 4e as the predominant species during the catalysis is also in accord with the observed kinetic behavior of this catalyst with 1-octene and styrene as the substrates. The observation of this saturated acyl rhodium complex is in line with the positive dependence of the reaction rate on the hydrogen concentration and the zero order in alkene concentration. It was concluded previously that this saturated acyl complex is an unreactive resting state [18]. Before the final hydro-genolysis reaction step can occur, a CO molecule has to dissociate in order to form... 

Similarly, highly selective hydrogen reductions of rhodium-acyl intermediates are known, such as in hydroformylation reactions. Rhodium phosphine complexes are highly selective with little further reduction of products to alcohols (19). 

The mechanism that has been proposed for the decarbonylation of acid chlorides by chlorotris (triphenylphosphine) rhodium (I) involves acyl- and alkyl- or arylrhodium complexes (2). Three of the intermediates in the catalytic cycle, II, III, and IV (see Reaction 3), can be isolated in this unique reaction. Both acyl complexes (II) and alkyl... 

In an attempt to resolve this question of stereochemistry and also to determine whether or not the decarbonylation of an acid chloride containing a f3 hydrogen takes place stereospecifically, erythro- (XI) and fhreo-2,3-diphenylbutanoyl chlorides (XII), obtained by the reaction of the known acids (13, 14) with oxalyl chloride, were synthesized. The reaction of these acid chlorides (see Reaction 8) with chlorotris( triphenyl-phosphine) rhodium gave the corresponding acyl complexes of type lib [R = C6H5CH(CH3)CH(C6H5)]. Decarbonylation of the erythro- cy complex in benzene at 30 °C gave a 90% yield of frans-a-methylstilbene while decarbonylation of the threo-acyl complex under similar reaction... 

The five-coordinate acyl complex may not be mononuclear. Using a rhodium complex in presence of a diamine, alkenes were converted to alcohols according to reaction (84),... 

Equations 2c and 2d show the acyl-alkyl migration and reductive elimination steps, respectively. There is good evidence that this same mechanistic scheme applies to the decarbonylation of aldehydes (see Equation set 2, X = H), although in this case reaction intermediates have not been isolated (3, 5, 9, 18). Additionally, evidence exists that the rate-determining step is oxidative addition for aldehyde decarbonylation (see Equation 2b, X = H) (3, 9, 18). Several recent reports have shown that for some special aldehydes, oxidative addition of the carbonyl-hydrogen bond indeed does occur using rhodium(I) complexes (8,19). In these studies a stable chelate was formed after oxidative addition that enabled isolation and characterization of the products (8, 19). 

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