Cyclisation reaction catalysis

Key Words Ethylene oxide, Ethylene, Epoxidation, Silver, Cl promotion, Cs promotion. Promotion, Selectivity, Oxametallacycle, Adsorption, Desorption, Chemisorption, Activation energy, Ag-O bond. Reaction mechanism, Oxidation, Cyclisation, Heterogeneous catalysis, Selective oxidation, Eletrophilic oxygen. Nucleophilic oxygen. Subsurface O atoms, Ag/a-A Oj catalyst. 2008 Elsevier B.V. 

The initial spark for proline catalysis was provided independently and simultaneously by two groups in 1971. Hajos and Parrish on the one hand (Scheme 5.1), and Eder, Sauer and Wiechert (Scheme 5.2) on the other developed an asymmetric aldol cyclisation of triketones such as 1 to bicyclic allq l ketones 2. In the former report, (S)-proline was applied at 3 mol%, a low organocatalyst loading, even to date. The quantitative cyclisation reaction was completed in the reasonable time of 20 h, and provided the product in 93.4% ee. Dehydration to enone 3 completed the synthesis of a valuable building block in steroid chemistry. 

Cyclisation of unsaturated substrates is a field extensively studied in the context of homogeneous gold catalysis. Cyclisation reactions proceed with high atom economy and a number of gold-promoted methodologies have been developed at room temperature. 

Cooperative catalysis using cinchona alkaloid derivatives in combination with metals such as silver have also been widely developed. On the basis of this concept, Escolano et al. have disclosed an enantioselective domino Michael-cyclisation reaction. This formal [3 + 2] cycloaddition occurred between isocyanoacetates and enones in the presence of a combination of a chiral hifunctional cinchona alkaloid, such as cupreine, and AgNOs to provide the corresponding chiral 2,3-dihydropyrroles in low to high yields and... 

Domino Michael-cyclisation reaction catalysed by chiral cinchona alkaloid catalysis and silver catalysis. 

On the other hand, another cooperative catalysis approach was developed by Oh and Kim with a highly diastereo- and enantioselective domino aldol-cyclisation reaction occurring between aldehydes and methyl a-isocyanoacetate. The process employed a combination of a chiral cobalt complex derived from brucine amino diol and an achiral thiourea. The reaction was applicable to a range of aliphatic, aromatic and heteroaromatic aldehydes, providing the corresponding chiral oxazolines in good yields and diastereoselectivities of up to >90% de combined with good to excellent enantioselectivities of up to 98% ee, as shown in Scheme 7.12. 

Scheme 7.47 Domino isomerisation-protonation-6-enrfo-trig cyclisation reaction catalysed by chiral phosphoric acid catalysis and ruthenium catalysis.

Scheme 7.61 Tandem Michael-acetalisation-cyclisation reaction catalysed by chiral amine catal) is and gold catalysis.

Scheme 7.72 Tandem a-aminooigrlation or -amination-reductive cyclisation reaction catalysed by chiral amine catalysis and palladium catalysis.

Several bisphosphonates bearing a substituted isoxazolidine ring (731), have been synthesised in good yields by a direct 1,3-dipolar cyclisation reaction of tetraethylvinyhdene-l,l-bisphosphonate (730) and substituted nitrone (729), under microwave catalysis, in the absence of solvent (Scheme 185). ... 

Another cyclisation involving 4-amino-6-methyl-3-thioxo-l,2,4-tiiazin-5-one 82 is the reaction with propargyl bromide followed by palladium(II) catalysis to yield a [1,2,4] triazino[3,4-f ][l,3,4]thiadiazine 83, or with phenacyl bromide to give 84 <96MI03 96CA(125)195597 96MI04 96CA(125)247767 >. 

A facile route to l,3,4-oxadiazepin-2-ones has been developed by Komatsu et al. involving acid-catalysed cyclisation of carbazate derivatives derived from the reaction of N, A -di-tert-butyldiaziridinone and a-hydroxy ketones with BF3.Et20 catalysis <00H(52)541>. 

This author is perfectly aware that he could add very little to the work done by these workers if an attempt was made to focus on intramolecular catalysis phenomena or on the relevance to cyclisation of available models of chain conformation and chain dynamics instead, the aim will be the presentation of a general treatment of the subject, namely, one that includes the cyclisation of very short chains as well as that of very long chains of, say, 100 atoms or more. With a subject as vast as this, an encyclopaedic review would be a hopeless task. Therefore, the subject will be treated in a systematic and critical way, with more concentration on reaction series with regular and wide variations in structure, rather than on scattered examples. The aim will be to show that the field of intramolecular reactions is a mature area in which the merging of concepts from both physical organic chemistry and polymer chemistry leads to a unified treatment of cyclisation rates and equilibria in terms of a few simple generalisations and theories. 

A more recent paper by Cunningham and Schmir242 on the hydrolysis of 4-hydroxybutyranilide XL in neutral and alkaline solution suggests that intramolecular nucleophilic attack by the neighbouring hydroxyl group is followed by bifunctional catalysis by phosphate or bicarbonate buffers of the conversion of the tetrahedral intermediate to products. A quantitative comparison was made between the effects of buffer on the hydrolysis of 4-hydroxybutyranilide and on the hydrolysis of 2-phenyliminotetrahydrofuran, since both reactions proceed via identical intermediates. The mechanism suggested243 states that the cyclisation of the hydroxyanilide ion yields an addition intermediate whose anionic form may either cleave to products or revert to reactant and whose neutral form invariably gives aniline and butyrolactone, viz. 

Another advantage of this approach is that we can now use electrophilic substitution on the pyrrole to add the rest of the molecule. So the secondary benzylic alcohol 106 might well cyclise to 105 with Lewis acid catalysis as the cation will be reasonably stable and the reaction is intramolecular. But the Friedel-Crafts alkylation to give 107 will not succeed as the cation would be primary. 

For ethane-1,2-diol the diester C2 is in equilibrium with the reactants, and its decomposition to the reaction products is rate limiting and not subject to acid-base catalysis. When the concentrations employed are such that C2 is present in appreciable concentration, the mixed-order kinetics described in section 1.3.2 are observed. Second-order kinetics (for the overall reaction) can arise in three ways (a) C2 in equilibrium, but its concentration negligible, (b) formation of C2 from Ci rate limiting, and the latter in equilibrium with the reactants but present in very low concentration, and (c) formation of Q rate-limiting. For pinacol in the range pH 2-10 alternative (a) cannot be operative because general acid-base catalysis is observed. The most likely step to be subject to catalysis is the formation of C2 from Cl, i.e. alternative (b), because this is a cyclisation and a base (B) could well facilitate reaction by removal of the C-OH proton, viz. 

The amino nitrile 9 fulfils all these hopes. It is easily made, it can be alkylated to give 10, and the anion from removal of its second proton gives a clean Michael reaction to form 11. Finally, the amino-nitrile functionality is easily hydrolysed at room temperature and neutral pH (unlike so many such compounds see chapter 14) with Cu(II) catalysis to give the ene-dione 4 and hence ds-jasmone 1 by thermodynamically controlled cyclisation in weak base.1 This process of adding a new five-membered ring, not necessarily to give a cyclopentenone, is often called cyclo-pentannelation and there are now many such methods.3... 

Intramolecular reactions are faster because AS - the entropy of activation (the probability of the reactant groups meeting) - is high and fastest when the reaction is a cyclisation (corresponding to intramolecular nucleophilic catalysis), which may be particularly favorable enthalpically. The simple measure of efficiency is the effective molarity (EM), the (often hypothetical) concentration of the neighboring group needed to make the corresponding intermolecular process go at the same rate [36]. It is simply measured, as the ratio of the first order rate constant of the intramolecular reaction and the second order rate constant for the (as far as possible identical) intermolecular process. In some convenient cases both reactions can be observed simultaneously, (Scheme 2.15) [37], and EM = ki/k2 measured di-... 

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