Diels-Alder rearrangements

After in the foregoing chapter thermodynamic properties at high pressure were considered, in this chapter other fundamental problems, namely the influence of pressure on the kinetic of chemical reactions and on transport properties, is discussed. For this purpose first the molecular theory of the reaction rate constant is considered. The key parameter is the activation volume Av which describes the influence of the pressure on the rate constant. The evaluation of Av from measurement of reaction rates is therefor outlined in detail together with theoretical prediction. Typical value of the activation volume of different single reactions, like unimolecular dissociation, Diels-Alder-, rearrangement-, polymerization- and Menshutkin-reactions but also on complex homogeneous and heterogeneous catalytic reactions are presented and discussed. 

Irradiation of the triazoline (70) in methanol gave the ester (71) via the ketene (72), formed by a retro-Diels-Alder rearrangement as shown in Scheme An analogous cycloreversion has been reported in azatricyc-... 

Diels-Alder rearrangement from flavone and flavolone [160,161] and, finally, molecular transpositions occurring in sulfur-containing compounds [163],... 

Cycloaddition reactions of two-co-ordinate phosphorus compounds described this year includes the [2 + 2] cycloaddition of an iminoborane to aminoimino-phosphines to give 1,3,2,4-diazaphosphaboretidines (125). In an attempt to prepare hydrazino-bis-phosphaalkenes (126) as new candidates for hetero-Cope rearrangements, the tricyclic compounds (127) were obtained. The result is explained by a Diels-Alder rearrangement of (126), as shown. 

Multicenter fragmentations encompass eliminations, retro Diels-Alder, rearrangement, and other reactions involving the cleavage of more than one bond and the concurrent formation of one or more new bonds. 

In the case of PS, even subtle properties of the reachon could be revealed, e.g., a Diels-Alder rearrangement product due to thermal inihahon as well as a chain scission product due to the MALDI condihons. °... 

The two main rearrangements observed in El are the McLafferty rearranganent and the reaction known as the retro Diels-Alder rearrangement. The ions issued from these rearrangements are easy to identify in mass spectra based on the nitrogen rule (Section 9.6.3). 

C-jHg. A slightly coloured liquid, b.p. 90 C, obtained by a Diels-Alder reaction between cyclopcntadiene and ethyne at approximately 150°C. At temperatures in excess of 450°C it rearranges to cycloheptatriene (tropilidene). 

Benzilic acid rearrangement Benzoin reaction (condensation) Blanc chloromethylation reaction Bouveault-Blanc reduction Bucherer hydantoin synthesis Bucherer reaction Cannizzaro reaction Claisen aldoi condensation Claisen condensation Claisen-Schmidt reaction. Clemmensen reduction Darzens glycidic ester condensation Diazoamino-aminoazo rearrangement Dieckmann reaction Diels-Alder reaction Doebner reaction Erlenmeyer azlactone synthesis Fischer indole synthesis Fischer-Speior esterification Friedel-Crafts reaction... 

Apart from the thoroughly studied aqueous Diels-Alder reaction, a limited number of other transformations have been reported to benefit considerably from the use of water. These include the aldol condensation , the benzoin condensation , the Baylis-Hillman reaction (tertiary-amine catalysed coupling of aldehydes with acrylic acid derivatives) and pericyclic reactions like the 1,3-dipolar cycloaddition and the Qaisen rearrangement (see below). These reactions have one thing in common a negative volume of activation. This observation has tempted many authors to propose hydrophobic effects as primary cause of ftie observed rate enhancements. 

Dramatic rate accelerations of [4 + 2]cycloadditions were observed in an inert, extremely polar solvent, namely in5 M solutions oflithium perchlorate in diethyl ether(s 532 g LiC104 per litre ). Diels-Alder additions requiring several days, 10—20 kbar of pressure, and/ or elevated temperatures in apolar solvents are achieved in high yields in some hours at ambient pressure and temperature in this solvent (P.A. Grieco, 1990). Also several other reactions, e.g, allylic rearrangements and Michael additions, can be drastically accelerated by this magic solvent. The diastereoselectivities of the reactions in apolar solvents and in LiClO EtjO are often different or even complementary and become thus steerable. 

The transition state for the first step of the Claisen rearrangement bears much m common with the transition state for the Diels-Alder cycloaddition Both involve a con certed six electron reorganization... 

Other methods for the preparation of cyclohexanecarboxaldehyde include the catalytic hydrogenation of 3-cyclohexene-1-carboxaldehyde, available from the Diels-Alder reaction of butadiene and acrolein, the reduction of cyclohexanecarbonyl chloride by lithium tri-tcrt-butoxy-aluminum hydride,the reduction of iV,A -dimethylcyclohexane-carboxamide with lithium diethoxyaluminum hydride, and the oxidation of the methane-sulfonate of cyclohexylmethanol with dimethyl sulfoxide. The hydrolysis, with simultaneous decarboxylation and rearrangement, of glycidic esters derived from cyclohexanone gives cyclohexanecarboxaldehyde. 

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