Rhodium carbonyl elimination

Rhodium carbonyl complexes also catalyze the cascade cyclization/hydrosilylation of 6-dodecene-l,l 1-diynes to form silylated tethered 2,2 -dimethylenebicyclopentanes. For example, reaction of ( )-85 with dimethylphenylsilane catalyzed by Rh(acac)(CO)2 in toluene at 50 °G under GO (1 atm) gave 86a in 55% yield as a single diastereomer (Equation (56)). Rhodium-catalyzed caseade cyclization/hydrosilylation of enediynes was stereospecific, and reaction of (Z)-85 under the conditions noted above gave 86b in 50% yield as a single diastereomer (Equation (57)). Rhodium(i)-catalyzed cascade cyclization/hydrosilylation of 6-dodecene-1,11-diynes was proposed to occur via silyl-metallation of one of the terminal G=G bonds of the enediyne with a silyl-Rh(iii) hydride complex, followed by two sequential intramolecular carbometallations and G-H reductive elimination. ... 

Because of our previous success In applying Fourier-transform infrared spectroscopy to the study of the rhodium carbonyl clusters under high pressures of carbon monoxide and hydrogen 2. A, we have applied the same technique and equipment in this work. 3. The temperature has been kept In all these experiments below 200° with maximum pressures of 832.0 atm to maximize the trend towards fragmentation of clusters. The absence of bases, e.g., salts or amines, in the systems under evaluation in this work was desirable to eliminate the ambiguity that would result from the enhancement of the fragmentation of clusters by carbon monoxide In a basic medium. . ... 

Structure 4 is an intermediate for manufaeturing vitamin A (Scheme 2). The annual demand for vitamin A is about 3000 tons. Major producers are BASF, Hoffmann-La Roche and Rhone-Poulenc Animal Nutrition [55]. At an early stage in the synthesis BASF and Hoffmann-La Roche are using a hydroformylation step to synthesize 4 starting from l,2-diacetoxy-3-butene (5) and 1,4-di-aeetoxy-2-butene (6), respectively [56, 57]. The selectivity toward the branched product in the BASF process is achieved by using an unmodified rhodium carbonyl catalyst at a high reaction temperature. The symmetry of 6 in La Roche s process does not lead to regioselectivity problems. Elimination of acetic acid and isomerization of the exo double bond (La Roche) yields the final product 4 in both processes. 

The initial step is oxidative addition of the acid chloride to the dissociated form of the complex (LVI) to form the acyl complex (LXV). Aryl migration (reverse insertion) affords a supposed intermediate (LXVI) which can eliminate CO to form (LXVII), which in turn loses aryl halide to regenerate the catalyst. Alternatively (LXVI) can lose aryl halide (reductive elimination) to form a rhodium carbonyl (LXVIIl). It is thought that this path is... 

Aldehydes undergo a decarbonylation reaction by action of a transition metal complex [17]. For example, benzaldehyde was decarbonylated with Wilkinson complex to furnish benzene along with a rhodium carbonyl complex 47 (Scheme 7.14) [17e,f]. Oxidative addition of the aldehydic C-H bond to rhodium, migratory deinsertion of CO, and reductive elimination operate in sequence for the decarbonylation reaction. Decanal was also decarbonylated to furnish a mixture of nonane and nonene, which were produced via the alkylpalladium intermediate 48 (Scheme 7.15) [17d]. 

Union Carbide Corporation was working on a low-pressure hydroformylation process as early as 1967, and used a rhodium carbonyl catalyst modified with a triphenyl phosphine ligand. " The ligand dramatically improved the normal/iso-aldehyde ratio of the product when compared to the free metal carbonyl. Reduced operating temperatures and pressures, low by-product formation, and the elimination of metallic rhodium deposition were also achieved. By 1975, a plant was operating using this catalyst, and the process was being licensed for the hydroformylation of propylene. 

Polymer-supported catalysts incorporating organometaUic complexes also behave in much the same way as their soluble analogues (28). Extensive research has been done in attempts to develop supported rhodium complex catalysts for olefin hydroformylation and methanol carbonylation, but the effort has not been commercially successful. The difficulty is that the polymer-supported catalysts are not sufftciendy stable the valuable metal is continuously leached into the product stream (28). Consequendy, the soHd catalysts fail to eliminate the problems of corrosion and catalyst recovery and recycle that are characteristic of solution catalysis. 

Historically, the rhodium catalyzed carbonylation of methanol to acetic acid required large quantities of methyl iodide co-catalyst (1) and the related hydrocarboxylation of olefins required the presence of an alkyl iodide or hydrogen iodide (2). Unfortunately, the alkyl halides pose several significant difficulties since they are highly toxic, lead to iodine contamination of the final product, are highly corrosive, and are expensive to purchase and handle. Attempts to eliminate alkyl halides or their precursors have proven futile to date (1). 

Rhodium(i) complexes are excellent catalysts for the 1,4-addition of aryl- or 1-alkenylboron, -silicon, and -tin compounds to a,/3-unsaturated carbonyl compounds. In contrast, there are few reports on the palladium(n) complex-catalyzed 1,4-addition to enones126,126a for the easy formation of C-bound enolate, which will result in /3-hydride elimination product of Heck reaction. Previously, Cacchi et al. described the palladium(n)-catalyzed Michael addition of ArHgCl or SnAr4 to enones in acidic water.127 Recently, Miyaura and co-workers reported the 1,4-addition of arylboronic acids and boroxines to a,/3-unsaturated carbonyl compounds. A cationic palladium(n) complex [Pd(dppe)(PhCN)2](SbF6)2 was found to be an excellent catalyst for this reaction (dppe = l,2-bis(diphenyl-phosphine)ethane Scheme 42).128... 

Although detailed mechanistic studies are not reported, the postulated mechanism for the reductive cyclization of allenic carbonyl compounds involves entry into the catalytic cycle via silane oxidative addition. Allene silylrhodation then provides the cr-allylrhodium hydride A-18, which upon carbometallation of the appendant aldehyde gives rise to rhodium alkoxide B-14. Oxygen-hydrogen reductive elimination furnishes the hydrosilylation-cyclization product... 

The reductive elimination/oxidative addition is of practical importance in catalytic cycles, for example the rhodium/methyl iodide catalysed carbonylation of methanol. In organic synthesis the palladium or nickel catalysed cross-coupling presents a very common example involving oxidative addition and reductive elimination. A simplified scheme is shown in Figure 2.19 [17],... 

The spectroscopic and kinetic data from this reaction indicated the existence of a long sought catalytic reaction topology, bimetallic catalytic binuclear elimination. The kinetic data provided a linear-bilinear form in organometallics [95]. One term represented the classic unicyclic rhodium catalyzed hydroformylation and the other represented the attack of manganese hydride carbonyl on an acyl rhodium tetracarbonyl species. A representation of the interconnected topology is shown in Figure 4.12. 

With the long chain a-diazo ketone. 6-diazo-7-tridecanone, 1,5-insertion could proceed with placement of the carbonyl outside the ring, or included in the ring. In fact, only the product 7, from the first of these two cyclization modes, is observed67. The alternative cyclopentane 9 is not formed. As with the a-diazo ester, the relative proportion of 1,2- and 1,5-products depends on the rhodium carboxylate employed. Throughout these studies, it has been observed that the olefin 8, obtained from 1,2-elimination, is cleanly Z-configured67 68. 

To that end, Kinnunen and Laasonen model the reductive elimination pathways from the anionic acetyltriiododicarbonyl rhodium and iridium anions, and from the acetyldiiodotri-carbonyl iridium neutral using the B3LYP functional in combination with an unpolarized... 

Support-bound transition metal complexes have mainly been prepared as insoluble catalysts. Table 4.1 lists representative examples of such polymer-bound complexes. Polystyrene-bound molybdenum carbonyl complexes have been prepared for the study of ligand substitution reactions and oxidative eliminations [51], Moreover, well-defined molybdenum, rhodium, and iridium phosphine complexes have been prepared on copolymers of PEG and silica [52]. Several reviews have covered the preparation and application of support-bound reagents, including transition metal complexes [53-59]. Examples of the preparation and uses of organomercury and organo-zinc compounds are discussed in Section 4.1. 

---