Pyridine-based catalysts

While the 2,6-bis(imino)pyridine ligand frame has continued to lead the way, the past 10 years has also seen the development of alternative ligand sets that can act as compatible supports for iron and cobalt ethylene oligomerisation/polymerisation catalysts (Table 4) [46, 50, 181, 182, 183, 184, 185, 186, 187, 188, 189, 190, 191, 192,193,194,195,196,197,198,199,200, 201,202,203,204,205, 206,207,208, 209] active catalysts based on bimetallic iron and cobalt precatalysts have also started to emerge (Table 5) [54, 210, 211, 212, 213, 214, 215], In the main, these systems show lower activities than the prototype bis(imino)pyridine-based catalysts, although several of these systems have significantly started to approach their catalytic performances (e.g. 65/MAO [46, 188], 69/MAO [191,192,193], 70/MAO [194], 71/MMAO [195]). 

The observant among you may also have noticed that the (weak—pyridine) base catalyst in this reaction works very slightly differently from the (strong—hydroxide) base catalyst in the hemiacetal-forming reaction on p. 279 one removes the proton after the nucleophile has added the other removes the proton before the nucleophile has added. This is deliberate, and will be discussed further in Chapter 13... 

Figure 59 Ethylene polymerization with phenoxy-pyridine-based catalysts.

Figure 5 (a) Development of pyridine-based catalysts (b) hydrogenation of selected model substrates and (c) selected application in total synthesis... 

Mukhopadhyay and Ray (2011) described the synthesis of 2-amino-5-alkyl-idenethiazol-4-ones from ketones, rhodanine, and secondary amines promoted by a silica-pyridine-based catalyst. Compounds containing a 2-amino-5-arylidene-l,3-thiazol-4(5H)-one moiety display a wide range of interesting pharmaceutical activities such as antiviral (Abdel-Ghani, 1999), antimicrobial (Soltero-Higgin et al., 2004), cardiotonic (Andreani et al., 1996), and anti-inflammatory (Nasr and Said, 2003) effects. The one-pot synthesis of thiazol-4-ones was optimized on a model... 

Mukhopadhyay, C., Ray, S. 2011. Synthesis of 2-amino-5-alkylidenethiazol-4-ones from ketones, rhodanine, and amines with the aid of re-usable heterogeneous sihca-pyridine based catalyst. Tetrahedron 67(41 ) 7936-7945. 

Samal M, Misek J, Stara IG, Stary I (2009) Organocatalysis with azahelicenes the first use of helically chiral pyridine-based catalysts in the asymmetric acyl transfer reaction. Collect Czech Chem Commun 74 1151... 

Tetrahydrofurfuryl alcohol reacts with ammonia to give a variety of nitrogen containing compounds depending on the conditions employed. Over a barium hydroxide-promoted skeletal nickel—aluminum catalyst, 2-tetrahydrofurfur5iarnine [4795-29-3] is produced (113—115). With paHadium on alumina catalyst in the vapor phase (250—300°C), pyridine [110-86-1] is the principal product (116—117) pyridine also is formed using Zn and Cr based catalysts (118,119). At low pressure and 200°C over a reduced nickel catalyst, piperidine is obtained in good yield (120,121). 

Pyridine, 3-(dimethylamino)-amination, 2, 236 methylation, 2, 342 nitration, 2, 192 iV-oxide synthesis, 2, 342 Pyridine, 4-(dimethylamino)-in acylation, 2, 180 alkyl derivatives pK, 2, 171 amination, 2, 234 Arrhenius parameters, 2, 172 as base catalysts, 1, 475 hydrogen-deuterium exchange, 2, 286 ionization constants, 2, 172 methylation, 2, 342 nitration, 2, 192 iV-oxide synthesis, 2, 342... 

In 1960 Rapoport and his co-workers found that some 2,2 -biquinoline is formed when quinoline w as used as a solvent for dehydrogenations in the presence of palladiuin-on-carbon catalyst, and they showed that several related bases (including pyridine) gave 2,2 -biai yls when refluxed at atmospheric pressure with a 5% pal-ladium-on-carbon catalyst. With a pyridine-to-catalyst ratio of 10 1, 11% conversion of pyridine to 2,2 -bipyridine was observed after heating for 24 hr. 

Fig. 3 Suggested mechanisms for the reaction of cellulose with tosyl chloride in the presence of pyridine (Py) and/or triethylamine (EtsN) base catalyst. Redrawn from [147]...

Suitable reagents for derivatizing specific functional groups are summarized in Table 8.21. Many of the reactions and reagents are the familiar ones used in qualitative analysis for the characterization of organic compounds by physical means. Alcohols are converted to esters by reaction with an acid chloride in the presence of a base catalyst (e.g., pyridine, tertiary amine, etc). If the alcohol is to be recovered after the separation, then a derivative which is fairly easy to hydrolyze, such as p-nltrophenylcarbonate, is convenient. If the sample contains labile groups, phenylurethane derivatives can be prepared under very mild reaction conditions. Alcohols in aqueous solution can be derivatized with 3,5-dinitrobenzoyl chloride. 

The stereochemistry of reduction by homogeneous catalysts is often controlled by functional groups in the reactant. Delivery of hydrogen occurs cis to a polar functional group. This behavior has been found to be particularly characteristic of an iridium-based catalyst that contains cyclooctadiene, pyridine, and tricyclohexylphosphine as ligands, known as the Crabtree catalyst 6 Homogeneous iridium catalysts have been found to be influenced not only by hydroxy groups, but also by amide, ester, and ether substituents.17... 

The introduction of metal alkyls to an MAO-activated bis(imino)pyridine iron catalyst has also been the subject of a number of studies. Both AlMe3 and AlEt3 have been added to 5a/MAO-based polymerisation catalysts leading to polyethylene displaying a bimodal distribution similar to that observed using 5a/M AO,... 

Figure 1.13 Generation of rhodium-based supramolecularcatalysts by assembly of pyridine/hydroxypyridine pairs (a) Self-assembly modes of pyridine-based phosphines, (b) Alkene hydroformylation with supramolecular rhodium-diphosphine catalysts (c) CAChe minimized 3D structure ofthe rhodium-diphosphine complex (other ligands from the metal omitted for clarity).

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