Carbonylation precatalyst

Apart from catalysis with well-defined iron complexes a variety of efficient catalytic transformations using cheap and easily available Fe(+2) or Fe(+3) salts or Fe(0)-carbonyls as precatalysts have been pubhshed. These reactions may on first sight not be catalyzed by ferrate complexes (cf. Sect. 1), but as they are performed under reducing conditions ferrate intermediates as catalytically active species cannot be excluded. Although the exact nature of the low-valent catalytic species remains unclear, some of these interesting transformations are discussed in this section. 

After the precatalyst is completely converted to the active catalyst Xq, three steps are required to form the desired reduction product. The first step is the coordination of dehydroamino acid (A) to the rhodium atom forming adducts (Xi) and (Xi ) through C=C as well as the protecting group carbonyl. The next step is the oxidative addition of hydrogen to form the intermediate (X2). The insertion of solvent (B) is the third step, removing the product (P) from X2 and regenerating Xq. Hence, the establishment of the kinetic model involves these three irreversible steps. 

As shown in Fig. 13, a variety of metal carbonyls upon sonication will catalyze the isomerization of 1-pentene to cis- and tram-2-pentene (186). Initial turnover rates are about 1-100 mol 1-pentene isomerized/mol of precatalyst/hour, and represent rate enhancements of 102 5 over thermal controls (174). The relative sonocatalytic and photocatalytic activities of these carbonyls are in general accord. An exception is Ru3(CO)12, which is... 

Various methods have been used to convert precatalysts into the active species [7]. Ethylene can be easily displaced from the central atom of the corresponding complexes in solution, even at room temperature. CO-ligands in carbonyl complexes can conveniently be removed photochemically [8], Increasing the temperature is a further common method used to labilize precatalysts with respect to stabilizing ligands [9],... 

In Scheme 2 51, species 133 is formed from the precatalyst 132 and TifOPr )4. It is then converted to complex G upon addition of diethylzinc. Reaction between species G and an aldehyde furnishes intermediate E, which accomplishes the enantioselective addition of the nucleophile to the carbonyl group. Intervention of two molecules of Ti(OPr )4 releases the alkylated product, regenerates the active catalyst 133, and also completes the catalytic cycle. This cycle explains the fact that at least one equivalent of Ti(OPr )4 is required for an effective reaction. 

SEGPHOS [271, 272]. Using this complex as a precatalyst, transfer hydrogenation of 1,1-dimethylallene in the presence of diverse aldehydes mediated by isopropanol delivers products of ferf-prenylation in good to excellent yield and with excellent levels of enantioselectivity. In the absence of isopropanol, enantio-selective carbonyl reverse prenylation is achieved directly from the alcohol oxidation level to furnish an equivalent set of adducts. Notably, enantioselective ferf-prenylation is achieved under mild conditions (30-50°C) in the absence of stoichiometric metallic reagents. Indeed, for reactions conducted from the alcohol oxidation level, stoichiometric byproducts are completely absent (Scheme 13). 

The role of multicomponent ligand assembly into a highly enantioselective catalyst is shown in the enantioselective catalysis for the carbonyl-ene reaction (Table 8.9). The catalyst is prepared from an achiral precatalyst, Ti(0 Pr)4 and a combination of BINOL with various chiral diols such as TADDOL and 5-Cl-BIPOL in a molar ratio of 1 1 1 (10mol% with respect to the olefin and glyoxylate) in... 

The conjugate addition of carbonyl anions catalysed by thiazolium salts (via umpol-ung) that is fully operative under neutral aqueous conditions has been accomplished. The combination of a-keto carboxylates (157) and thiazolium-derived zwitterions (e.g. 160) in a buffered protic environment (pH 7.2) generates reactive carbonyl anions that readily undergo conjugate additions to substituted o /3-unsaturated 2-acylimidazoles (158) to produce (159). The scope of the reaction has been examined and found to accommodate various a-keto carboxylates and /3-aryl-substituted unsaturated 2-acylimidazoles. The optimum precatalyst for this process is the commercially available thiazolium salt (160), a simple analogue of thiamine diphosphate. In this process, no benzoin products from carbonyl anion dimerization were observed. The resulting 1,4-dicarbonyl compounds (159) can be efficiently converted into esters and amides by way of activation of the A-methylimidazole ring via alkylation.181... 

The same products are accessible by silver-catalyzed cycloisomerization of allenic ketones. Marshall and Bartley327 used AgNOs/silica gel in hexane to convert the allenic ketones 384 into the furans 386 with excellent yields (Scheme 112). Deuterium labeling experiments were interpreted in terms of the intermediate 385 which seems to arise from the coordination of silver catalyst to the allenic double bond distal to the carbonyl group. Again, gold precatalysts can be used with much lower catalyst loadings than their silver counterparts (see Section 9.12.4.3). 

The water-gas shift reaction has also been studied under high pH with metal carbonyls as catalysts. The catalytic cycle with Fe(CO)5 as the precatalyst is shown in Fig. 4.6. This reaction with low turnover is carried out at 130-180°C, under 10-40 bar of CO, with alkali metal hydroxide as a promoter. 

The mechanism of the Montedison reaction has been studied in some detail, and tentative mechanisms have been offered. The proposed catalytic cycle is shown in Fig. 4.11. The biphasic reaction medium consists of a layer of diphenyl ether and that of aqueous alkali. In the presence of alkali, the precatalyst Co2(CO)8 is converted into 4.7. The sodium salt of 4.7 is soluble in water but can be transported to the organic phase, that is, a diphenyl ether layer by a phase-transfer catalyst. The phase-transfer catalyst is a quaternary ammonium salt (R4N+X ). The quaternary ammonium cation forms an ion pair with [Co(CO)4]. Because of the presence of the R groups, this ion pair, [R4N]+[Co(CO)4], is soluble in the organic medium. In the nonaqueous phase benzyl chloride undergoes nucleophilic attack by 4.7 to give 4.40, which on carbonylation produces 4.41. The latter in turn is attacked by hydroxide ion transported from the aqueous phase, to the organic phase again by the phase-transfer catalyst. The product phenyl acetate and 4.7 are released in the aqueous phase as the sodium or quaternary ammonium salts. 

The carbonylation reaction in the Hoechst process involves the use of PdCl2(PPh3)2 as the precatalyst, a CO pressure of about 50 bar, and a temperature of about 130°C. It is performed in a mixture of an organic solvent and hydrochloric acid. The mechanism at a molecular level is not known with certainty. On the basis of the known chemistry of palladium, a speculative catalytic cycle is shown in Fig. 4.12. 

In the Shell process (SHOP) phosphine-modified cobalt-catalyzed hydrofor-mylation is one of the steps in the synthesis of linear alcohols with 12-15 carbon atoms (see Section 7.4.1). Two important characteristics of this reaction should be noted. First, the phosphine-modified precatalyst HCo(CO)3(PBu3) is less active for hydroformylation than HCo(CO)4 but more active for the subsequent hydrogenation of the aldehyde. In this catalytic system both hydroformylation and hydrogenation of the aldehyde are catalyzed by the same catalytic species. Second, the phosphorus ligand-substituted derivatives are more stable than their carbonyl analogues at higher temperatures and lower pressures (see Table 5.1). 

The Monsanto process, one of the most successful industrial homogeneous catalytic processes, uses a Rh complex and catalytic HI to carbonylate MeOH to MeC02H. A Rh precatalyst (almost any Rh complex will do) is converted into Rh(CO)2l2, the active catalyst, under the reaction conditions. The mechanism of the reaction involves three steps. In the first step, MeOH and HI are converted to Mel and H20 by an Sn2 mechanism. In the second step, Mel and CO are converted to MeCOI under Rh catalysis. In the third step, H2O (generated in the first step) hydrolyzes MeCOI to afford MeC02H and to regenerate HI. 

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