Homogeneous catalysis and chemical industry application 3
2 (a) B. Cornils, W. A. Herrann, Eds., Catálisis Homogénea Aplicada umi Compuesto Organometálico reheve, VCH, Weinhein; (b) L. A. Oro, E. Sola, Eds., Fundamentos ha Aplicaciones Catálisis Homogénea rehegua, Zaragoza, Spanjë, 2000.
Objectives 9
Pd-catalysed methoxycarbonylation of ethene 15
- Mechanism 18
- Background of ethene carbonylation 20
Lucite International is one of the largest manufacturers of methyl methacrylate (MMA) in the world. The two key reagents in the ACH pathway are hydrogen cyanide and acetone, as shown in Scheme 2.3.5. MMA is then obtained by the hydrolysis of this addition product with sulfuric acid in the presence of methanol.
In addition, the reactants and by-products generated in the Alpha process are less toxic than those of the ACH pathway.7. Both methoxycarbonylation and copolymerization of ethylene consist of the following steps: a) initiation, b) propagation and c) termination. In the carbomethoxy mechanism (B)1,15, the migratory insertion of CO into a Pd-OMe (the nucleophilic attack of the methanol on a coordinated CO has also been proposed) leads to the formation of a methoxycarbonyl complex.
In the late 1990s, I.C.I synthesized a novel bidentate phosphine, 1,2-bis(di-tert-butylphosphinomethyl)benzene (6) (known as Alpha ligand) which was used in the synthesis of methyl propionate. The catalytic system was formed by combining the alpha ligand/palladium dibenzylideneacetone in the presence of methane sulfonic acid.
Results and discussion 23
Reaction of (46-49) with ethene 35
Conformational dynamic process in (73) 38
Reaction of complex 73 with vinyl acetate 39
Experimental Section 43
The solvent was then removed under vacuum to give a white crystalline solid, which was then isolated in the glove box. To this solution was added BunLi (2.5 M in hexanes, 179 mmol), and then it was stirred at room temperature for 1 h. This was then added to a solution of the corresponding cis-1,2-(dibromomethyl)cycloalkyl (82 mmol) in THF (200 mL) dropwise, and then the mixture was kept overnight at room temperature.
The solution was then dried under vacuum and the residue suspended in diethyl ether (400 mL). The solvent was then removed under vacuum and the residue was quenched with a solution (degassed with N 2 for 20 min) of potassium hydroxide in water (21 g, 200 mL, 651 mmol). The synthesis of 27 was carried out from mmol) according to the general procedure previously described.
THF (50 mL) was then added and the resulting solution was stirred overnight at room temperature. Complex (41) was synthesized from 26 (500 mg, 1.30 mmol) according to the general procedure previously described. The reaction mixture was then stirred at room temperature for twenty minutes and then methanesulfonic acid was added by syringe to obtain palladium(II) complexes [Pd(O3SCH3)(L-L)]O3SCH3.
MeOH (3 mL) was then added and the resulting solution was stirred for 20 min at room temperature. Trifluoroacetic acid (0.480 mmol) was then added and the reaction mixture was then stirred for an additional 20 minutes. The solution was then transferred to a sapphire tube and filled with 10 bar of ethene at 353 K for 20 minutes.
P,P-ligands in Pd-catalysed aminocarbonylation and double-carbonylation
Scope 67
Pd-catalysed double-carbonylation of aryl iodides 71
- Mechanism 134
- Asymmetric hydrogenation of unfunctionalised
The reactivity of the aryl halide decreases in the order of PhI > PhBr >> PhCl. The presence of substituents on the phenyl rings modifies the reactivity of the aryl halides. The selectivity for the double carbonylation of aryl halides is strongly influenced by the nature of the amine used.
Having demonstrated that DBU is the suitable base for the Pd-catalyzed double carbonylation of aryl iodides with the Pd/29 catalytic system, other parameters affecting the chemoselectivity of the reaction were studied. Nature of base: very strong effect on chemoselectivity as only DBU gave high selectivity to -ketoamide products. This experiment showed that the presence of carbon monoxide favors the formation of palladium complex 182.
An NMR study of the catalytic system Pd/29 in the presence of DBU was performed. In the case of (R)-methylbenzylamine (entry 4), the selectivity was improved when the reaction Entry Nucleophile Product Temp. 1H NMR spectrum of the reaction of PdCl2(DBU)2 with 1-iodo-4-methoxybenzene under atmospheric CO pressure.
In the following sections, some of the best results to date in the field of asymmetric hydrogenation of non-functionalized olefins are described. The stereogenic center of the oxazoline unit has a considerable influence on the enantioselectivity of the reaction. The formation of an alcohol-imidazoline intermediate is necessary for the introduction of the phosphite moiety into the ligand framework.
An optimization of the reaction conditions was carried out with iridium complexes containing ligand 316 in the asymmetric hydrogenation of E-1,2-diphenylpropene (Table 4.4). Therefore, the asymmetric hydrogenation of trisubstituted olefins strongly depends on the nature of the substituent on the nitrogen atom of the ligand. When the catalytic precursor was formed in situ by the reaction of the iridium dimer in the presence of stoichiometric amount of ligand 316 , no conversion was obtained (entry 3).
Once the reaction conditions were optimized, a family of phosphino-imidazoline ligands was screened (Table 4.8). These results indicate that the acidic proton of the imidazoline ring can be abstracted in the presence of substrate to give a protonated 2-.
Ir-catalysed hydrogenation of imines 143
- Mechanism 144
- Asymmetric hydrogenation of imines. Scope 148
Results and discussion 155
- Synthesis of phosphino-imidazoline ligands 155
- Asymmetric hydrogenation of unfunctionalised
- Asymmetric Hydrogenation of Imines 168
Because of the excellent results published with phosphite-oxazoline ligands (see section 4.1.1.2), we were interested in phosphite-imidazoline ligands and their use in asymmetric hydrogenation reactions. (1R,2R)-N1-benzyl-1,2-diphenylethane-1,2-diamine (331) was previously synthesized from (1R,2R)-1,2-diphenylethane-1,2-diamine and benzyl chloride in the presence of a base. The P,N ligands and iridium complexes prepared in the previous section 4.2.1 and 4.2.2 were synthesized for use as catalysts in the asymmetric hydrogenation of unfunctionalized olefins and imines.
Optimization of reaction conditions in the asymmetric hydrogenation of E- 1,2-diphenylpropene with [Ir(COD)316]BArF complex (341).a. Under these conditions, the most efficient catalytic system in the asymmetric hydrogenation of E -1,2-diphenylpropene was the isolated complex 345 , which contains ligand 325 with an alkyl chain on the nitrogen atom of the imidazoline ring. Optimization of reaction conditions in the asymmetric hydrogenation of N-(-phenylethylidene)aniline with complex 341 or Ir/316.a.
Since the introduction of cyclohexyl substituents on the imidazoline ring improves enantioselectivity, several electron donating groups were introduced at the nitrogen atom. However, no effect was observed by introducing a benzyl or an alkyl chain into the ligands bearing a phenyl group in the imidazoline ring (322 and 325) did not produce any effect on enantioselectivity. The phosphino or phosphite-imidazoline ligands have been tested in the hydrogenation of 2-methylquinoline under the aforementioned conditions (Table 4.10).
Various additives were tested with the catalytic system formed by Ir/319 in the asymmetric hydrogenation of 287a-c. Various sources of iodide have been tried, such as NBu4I, which is widely used as an additive in the asymmetric hydrogenation of imines.89. Furthermore, two different quinoline derivatives (287b and 287c) were hydrogenated with the Ir/319 catalytic system in the presence of iodine in toluene under 40 bar hydrogen pressure.
Summarizing the results obtained in the Ir-catalyzed asymmetric hydrogenation of imines, we can conclude that the cationic complexes [Ir(COD)L]BArF were very active in the hydrogenation of N-(- phenylethylidene)aniline, but not enantioselective. The introduction of substituents at the nitrogen atom of the imidazoline ring did not improve the catalytic results. Introduction of electron-donating groups to the nitrogen atom of the imidazoline ring produced racemic amines.
