CH Tmw
Figure 4.5. Diastereomeric alkene complexes with DIPAMP-Rh One can try to figure out from the pictures how two diastereomeric intermediates are formed. It is more convenient to adopt a formal approach. We can explain the formation of enantiomers when a metal even a bare Ag ion would do co-ordinates to our alkene substrate, and equally so the formation of diastereomers in Figure 4.5. Ligand intermezzo. A large series of asymmetric ligands have been developed most of which having the asymmetric...
Electrophilic catalysts
The earliest catalytic application of C-H bond activation and functionalisation is that of methane using platinum chlorides as the catalyst and oxidising reagent. The exchange of hydrogen atoms in arenes with D2O was discovered in 1967 by Garnett and Hodges 21 and the exchange with alkanes two years later by Shilov and co-workers 22 . The effective functionalisation of methane involves platinum II 23 as the activating metal complex and platinum IV as the oxidising species to generate methyl...
R Bbf
Figure 14.11. Examples of asymmetric epoxidation Chiral trans-cyclohexane-1,2-diamine was used as the bridging diamine and this led to a successful catalyst 14 , often referred to as Jacobsen's catalyst Figure 14.12 . The oxidising agent is household bleach diluted and buffered at high pH. The substrate is dissolved in dichloromethane and the reaction is carried out as a two-phase reaction, the same procedure as is commonly used in metalporphyrin oxidation. The extra t-butyl groups compared to...
Xantphos ligands tuneable bite angles
Readily available backbones for tuneable wide bite angles turned out to be those based on xanthene and related compounds Figure 8.10 . The diphosphines derived from these cover a range of angles from 102 to 121 or 126 for cyclic substituents . Key feature of these ligands is the oxygen ether atom in the bridge that prevents metallation and that in neutral rhodium hydrides does not participate in the coordination to the metal. By changing the fragment in the back of the molecule Q, C2, N, S the...
Jacobsen asymmetric ringopening of epoxides
Symmetric epoxides such as cyclohexene oxide would be an interesting source of chiral compounds if we were able to open the epoxide ring in an enantioselective fashion. Epoxides will bind to Lewis acids and the use of chiral Lewis acids might lead to enantioselective ring-opening. Nugent reported that chiral Lewis acids of zirconium complexes with S,S,S-triisopropylamine catalysed the enantioselective ring-opening of rneso-epoxides by azidotrimethylsilane with an e.e. of 93 for the product of...
Terephthalic acid
Introduction. The production of terephthalic acid 1,4-benzenedicarboxylic acid has several interesting features. First, it is one of the examples of a homogeneous, radical-catalysed oxidation with the use of dioxygen and cobalt salt initiators. Secondly, it is an example of a catalyst product separation involving a filtration of the product from the liquid that contains the catalyst. Crystallisation on such a huge scale is not very attractive, but the low solubility of phthalic acid in many...
Crosscoupling reaction
As mentioned above Section 13.1 , the coupling of carbocations and carb-anions or their formal precursors was of little practical value until the transition-metal-catalysed cross-coupling became available. The cross-coupling reaction has found wide application in organic synthesis both in the laboratory and in industry. Here we will present only the main scheme and use palladium as the example. Figure 13.21 gives the general scheme of the palladium catalysed cross-coupling reaction. Note that...
Ethene trimerisation
Another, highly selective oligomerisation reaction of ethene should be mentioned here, namely the trimerisation of ethene to give 1-hexene. Worldwide it is produced in a 0.5 Mt y quantity and used as a comonomer for ethene polymerisation. The largest producer is BP with 40 market share utilizing the Amoco process, formerly the Albemarle Ethyl Corporation process. About 25 is made by Sasol in South Africa where it is distilled from the broad mixture of hydrocarbons obtained via the...
Borylation of alkanes
Activation of alkane C-H bonds is much easier than one might have thought before 1980. Substitution of the carbon atom after its addition to a low-valent metal complex, however, remains a difficult task and an important target. The key issue is to find a reagent that can transfer a functional group or atom in such a way that the reactive metal complex is retained. Alkenes and alkynes are candidates as we have seen above, but the products of such reactions are alkenes via dehydrogenation of...
The Murai reaction
An efficient catalytic addition of aromatic carbon-hydrogen bonds to alkenes was developed by Murai and co-workers 18 . The aromatic compound usually contains a functionality such as a ketone or imine and the activation takes place ortho to this group. Thus the Murai reaction is more akin to the older C-H activation in ligands coordinated to the activating metal than to the alkane arene activation described in the previous sections. The conversion is catalytic, though, and the number of...
L Mav
The important difference between the insertion mechanism 2.2 and the migration mechanism 2.3 is the following. In the insertion mechanism carbon monoxide inserts into the metal methyl bond and the acyl bond formed takes the position of the methyl group, i.e. the c-bonded fragment retains its position trans to P . In the migration mechanism the methyl group migrates to the co-ordinated carbon monoxide and now the resulting acetyl group occupies the position cis to P . There is convincing...
The adiponitrile process
The transition metal catalysed addition of HCN to alkenes is potentially a very useful reaction in organic synthesis and it certainly would have been more widely applied in the laboratory if its attraction were not largely offset by the toxicity of HCN. Industrially the difficulties can be minimised to an acceptable level and we are not aware of major accidents. DuPont has commercialised the addition of HCN to butadiene for the production of adiponitrile ADN, NC CH2 4CN , a precursor to...
Process scheme Monsanto process
The rate equation for the Monsanto process under process conditions reads v k . Rh I2 CO 2- . CHbI The two catalyst components are rhodium and iodide, which can be added in many forms. A large excess of iodide may be present. Rhodium is present as the anionic species RhI2 CO 2 . Typically the rhodium concentration is 10 mM and the iodide concentration is 1.5 M, of which 20 occurs in the form of salts. The temperature is about 180 C and the pressure is 50 bar. The methyl iodide formation from...
Polyketone
Thermoplastics with high-performance properties are in increasing demand 2 . The present products e.g. aromatic polyketones, polyesters, polyamides, polyacetals, polyolefin specialities span a wide range of performance and production costs. Relevant properties comprise strength, toughness, wear resistance, chemical resistance, heat resistance, UV stability, etc. Applications include a variety of automotive components, gears, fittings, containers, fibres, packaging, etc. In the last decades much...
Asymmetric hydroxylation of alkenes with osmium tetroxide
About a decade after the discovery of the asymmetric epoxidation described in Chapter 14.2, another exciting discovery was reported from the laboratories of Sharpless, namely the asymmetric dihydroxylation of alkenes using osmium tetroxide. Osmium tetroxide in water by itself will slowly convert alkenes into 1,2-diols, but as discovered by Criegee 15 and pointed out by Sharpless, an amine ligand accelerates the reaction Ligand-Accelerated Catalysis 16 , and if the amine is chiral an...
Wilkinsons catalyst
Homogeneously catalysed hydrogenation reactions are only of minor importance from an industrial viewpoint with the notable exception of asymmetric hydrogenation. A large part of today's general knowledge on homogeneous catalysis has been derived from the early studies on hydrogenation. Undoubtedly the most popular homogeneous catalyst for hydrogenation is Wilkinson's catalyst, RhCl PPh3 3, discovered in the sixties 1 . The reaction mechanism, its dependence on many parameters, and its scope...
The Wacker reaction
Acetaldehyde is the product of the Wacker process. At the end of the fifties oxidation of ethene to ethanal replaced the addition of water to acetylene, because the acetylene coal-based chemistry became obsolete, and the ethene petrochemistry entered the commercial organic chemicals scene. The acetylene route involved one of the oldest organometallics-mediated catalytic routes started up in the 1920s the catalyst system comprised mercury in sulfuric acid. Coordination of acetylene to mercury II...
Hydrogen transfer reactions of alkanes
Hydrocarbons that might be compatible with electron-rich organometallic complexes capable of C-H activation are alkenes. A potential reaction therefore is dehydrogenation of alkanes to alkenes. Thermodynamics require high temperatures and low hydrogen pressures otherwise the process is energetically uphill. The stoichiometric reaction was discovered for cycloalkanes and iridium complexes L2IrH2 with the use of hydrogen acceptors such as t-butylethene 11 Figure 19.9 . Stoichiometric reactions...
Info Sud
a Data from reference 12 . Reaction conditions Styrene Ni 28.5, HCN Ni 17.5, Ni 73.3 mM, T 60 C, t 18 h. Yields are based on HCN. Maximum yields based on styrene are 61 . a Data from reference 12 . Reaction conditions Styrene Ni 28.5, HCN Ni 17.5, Ni 73.3 mM, T 60 C, t 18 h. Yields are based on HCN. Maximum yields based on styrene are 61 . Moloy studied the effect of the bite angle on the stoichiometric reductive elimination of RCN from R CH2Si CH3 3 from palladium complexes P2Pd R CN from a...
Asymmetric Isomerisation
An important application of an isomerisation is found in the Takasago process for the commercial production of - menthol from myrcene. The catalyst used is a rhodium complex of BINAP, an asymmetric ligand based on the atropisomerism of substituted dinaphthyl Figure 5.5 . It was introduced by Noyori 1 . BINAP has been extensively used for the asymmetric hydrogenation, transfer hydrogenation and isomerisation of double bonds using both ruthenium and rhodium complexes. Figure 5.5. The two...