Reactions under High Pressure

In organic synthesis, chemical transformation with a negative volume of activation (AVI) can be improved by application of high pressure, leading in many cases to an increase of the yield, an enhancement of the reaction rate, and an improvement of the selectivity (chemo-, regio- and stereoselectivity). This is especially true for cycloadditions with AVt values of approximately -50 cm3 mol 1 [1]. 

Domino Reactions in Organic Synthesis. Lutz F. Tietze, Gordon Brasche, and Kersten M. Gericke Copyright 2006 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-29060-5 

It is assumed that the reaction is initiated by a radical bromine abstraction to give 10-13, which after carbon monoxide insertion undergoes a rapid 5 -exo cycliza-tion onto the hydrazone moiety. The two diastereomeric hydrazinyl cyclopentanones 10-16 and 10-17 are formed with good yields, though with low stereoselectivity. 

Entry Substrate 10-12 R Product 10-16/10-17 Ratio cis/trans Yield [%] 

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The following account refers primarily to commercial apparatus suitable for conducting hydrogenations under pressime the apparatus can, of coimse, be employed for other reactions under high pressures, but slight modifications of experimental procedime wiU probably be necessary. 

The parent TMM precursor (1) reacts with tropones (117) to give reasonable yields of the bridged [4.3.1]decanones (118) [43]. Various substituted TMMs also cycloadd to tropone with regioselectivity similar to that of the corresponding [3+2] cases [20, 43]. Addition of MesSnOAc as a co-catalyst also leads to yield improvement [16]. In the case of a phenyl-substituted tropone and a methyl-TMM, performing the reaction under high pressure favors the formation of kinetic products (119) and (120) over the thermodynamic product (121) [11]. 

Electrochemical methods 78,79 and reactions under high pressure have also been investigated.80 Due to the high resonance stabilization of the macrocyclc, the formation of the phthalocyanine is strongly exothermic. Nevertheless, a high thermal activation and therefore usually a high temperature is necessary. 

The chemical yield of the classical Henry reaction is not always good and depends on steric factors thus, highest yields are obtained when nitromethane is used. Performing the reaction under high pressure (9 kbar, 30 °C) with tetrabutylammonium fluoride catalysis19 enlarges the scope of the reaction dramatically. Thus, addition of nitropropane to 2-methylcyclohexanone, which is not reactive under the classical conditions, was achieved in 40 % yield. Improved yields... 

Vinylnaphthalenes give Diels-Alder reactions more easily than styrenes and have been used to synthesize steroid-like compounds. 2-Vinylnaphthalene (61) is less reactive than 1-vinylnaphthalene (62) (Figure 2.7) it requires drastic conditions to undergo Diels-Alder reaction and the yields are low. Better results can be obtained by carrying out the reaction under high pressure (Chapter 5). Some Diels-Alder reactions of 1-vinylnaphthalene (62) are summarized in Scheme 2.23. 

Scheme 10.4. Domino HWE/Michael reaction under high pressure.

Table 18. Intramolecular Diels-Alder reactions under high pressure of furfuryl-substituted alkylidenecyclopropanes 195...

Because the dipolar cycloaddition is a bimolecular process, there exists the opportunity to accelerate the reaction under high pressure (232,233). The reaction... 

For a review of the kinetics of reactions in solution at high pressures, see le Noble Prog. Phys. Org. Chem. 1967,5. 207-330. For reviews of synthetic reactions under high pressure, sec Matsumoto Sera Uchida Synthesis 1985,... 

The reaction rate is primarily determined by the enthalpy of activation (A// ), which is usually the case in square planar nucleophilic substitution reactions. Of greater importance, so far as a dissociative versus an associative mechanism is concerned, are the entropies and volumes of activation, AS and AY, respectively. Note that the values are negative for both the fct and the steps. The observed decrease in entropy is what we would expect for a mechanism in which two particles come together to give an activated complex. The volume of activation is determined by doing the reaction under high pressure ... 

Optical cells can also be used to investigate the kinetics of radical polymerization reactions under high pressure by means of the rotating sector method. Again, the apparatus is presented in Chapter 4.3.4. An example of the method for the evaluation of individual rate constants in radical polymerization of ethylene is given below. 

The difficulties created by stopcocks and valves can usually be minimized. However, it is occasionally necessary to completely eliminate these sources of leakage and contamination by the use of break-seals and vacuum seal-offs. Typical situations in which sealed tube techniques are widely used are quantitative hydrolysis and oxidation reactions which require elevated pressures and temperatures, precise physical measurements on highly reactive organometallic compounds, long-term storage of reactive samples, and nonaqueous reactions under high pressure (for example, SO2 or NH3 at room temperature). Each piece of apparatus must be constructed to meet a specific need, so it is not possible to outline an apparatus which is of general use. Nevertheless, several examples will be presented here which serve to indicate the approach. 

A study by Hayashi et al. demonstrates that less reactive electron-rich aromatic aldehydes efficiently undergo Mannich reactions under high pressure induced by water freezing [8], For instance, in the Mannich reaction of p-anisaldehyde, 3,4-dimethoxybenzaldehyde or N-acetyl-(4-formyl)aniline, with acetone and p-anisidine, good yields (61-99%) and excellent enantioselectivities (92-97%) have been obtained under water-freezing high-pressure conditions while there is no reaction at room temperature at 0.1 MPa (Scheme 9.6). 

Scheme 9.6. Proline-catalyzed Mannich reaction under high pressure induced by water freezing.

Numerous Amaryllidaceae alkaloids include phenanthridine skeletons, one of whose constructive methods constitutes an IEDA strategy. In some cases, the functionality on the dienophile influences the stereochemistry of cyclo addition reactions under high-pressure conditions. For example, the reactions of ( )-buta-l,3-dienyl acetate (6) and the quinolin-2(lH)-ones 7 gave rise to different configurations in the products 8 and 9, depending on the functional groups at the 4-position of 7 (Scheme 3). These results reflect... 

As an extension of the successful Strecker reaction under high pressure [83], a heterocyclic version of this reaction was investigated by us but met with very limited success. For example, 1 equivalent of 1,2-diacetylbenzene (152) with 1,2-diaminobenzene (153) and TMSCN (2.4 equivalents) afforded 5,6,11,12-tetrahydro-6,11 -dimethyldibenzo[b,f][ 1,4]diazo-cine-6,ll-dicarbonitrile (154), albeit only in 17% yield in one step. A similar reaction of cyclohexane-1,2-dione (155) with 153 produced 1,2,3,4,5,10-hexahydrophenazine-4a,10a-dicarbonitrile (156) in 13% yield along with the aromatized product, 1,2,3,4-tetrahydrophenazine (157) (Scheme 42). However, similar attempts with other diketones, such as 2,5-hexanedione, 2,3-butadiene, and 9,10-phenanthrenequinone, met with failure, either giving a complex mixture of products or well known and commercially available product possessing a pyrazine skeleton [100]. 

Matsumoto K (1994) Synthetic applications of amino SfjAr reactions under high pressure. In Taniguchi Y, Senoo M, Hara K (eds) High pressure liquids and solutions. Elsevier, Amsterdam, pp 119-135... 

A dramatic increase of the TON is observed in the Heck reaction under high pressure. A value of 770 000 was achieved in the coupling reaction of iodobenzene with 2,3-dihydropyrrole under 8kbar at 100 °C. High pressure seems to stabilize the catalyst [12]. 

The study of pyridine-piperidine reactions under high pressure conditions has given much information concerning the kinetics of HDN, but these results are however complicated by alkyl transfer (disproportionation) reactions, and thus the possibility of using such reactions as an easy test for determination of mechanism and as a catalyst probe is partly excluded. The study of polycyclic amines (quinoline, etc.) for the same purpose is limited by the complexity and the number of different possible routes, but is a very interesting test reaction for an overall study of catalytic activity or selectivity toward HDN in industrial conditions. Because no disproportionation occurs and the numbers of products and routes are reasonable, the studies of pyridine-piperidine and alkylpyridine-alkylpiperidine HDN under normal H2 pressure and low amine pressure (< lOTorr) are very powerful test reactions both for mechanism determination and catalyst study, although these conditions are far removed from those of industrial practice. 

Some older examples of the application of high pressure in hetero Diels-Alder reactions are found in excellent reviews and books on this topic [552-561]. In addition a few hetero Diels-Alder reaction under high pressure have already been mentioned in the foregoing chapters. 

Matsumoto, K., Ilasimoto, S Otani, S., Amita, F ., and Osugi, J., Mannich reaction under high pressure. Dimethylaminomethy-lation of ketones with bis(dimcthyl-amino)methanc under mild conditions, Syntlc Commun., 14, 585, 1984. 

Independent of the impurity the solid state structure is influenced by the preparation high crystallinity (< )-60%) by reaction in bulk under normal pressure or v uum combined with additional heating amorphous products by bulk reaction under high pressure or reaction in solution Polymers with uniform end groups are prepared from TCB, m i67) pmdA >. 

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