Rate Enhancement under High Pressure Conditions

Rate Enhancement under High-Pressure Conditions 

The rate of Diels-Alder reactions can be increased by the application of high pressure. An early example by Dauben and Kozikowski demonstrated that methyl 1-pentadienoate reacted with 126 to give 127 in 88% yield. 27 Applications of high-pressure techniques to organic synthesis are enumerated in an excellent review by Matsumoto 28 as well as in his monograph. 29 jiie second part of this review 29 deals with high-pressure pericyclic reactions, including Diels-Alder reactions. 

The thermodynamic properties of solutions are well known, and the rate of a reaction can be expressed in terms of the activation volume, AV  

Reactions in which the molecularity number (number of molecules) decreases when starting materials 

Reactions that proceed via cyclic transition states Claisen and Cope rearrangements (sec. 11.12.D 

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C. Rate Enhancement under High-Pressure Conditions... 

A remarkable effect on rate enhancement under high pressure is known [40]. The enantiomerically pure isoquinoline 55 was obtained from the bromide 54 in 70% yield and with high diastereoselectivity (16 1) under pressure (10 kbar). The yield was lower than 10% under normal pressure and even under Jeffery s conditions [41],... 

Success of the reactions depends considerably on the substrates and reaction Conditions. Rate enhancement in the coupling reaction was observed under high pressure (10 kbar)[l 1[. The oxidative addition of aryl halides to Pd(0) is a highly disfavored step when powerful electron donors such as OH and NHt reside on aromatic rings. Iodides react smoothly even in the absence of a... 

Thiophene (30) does not usually undergo Diels-Alder reaction under thermal and Lewis acid catalysed conditions but it has been reported that under high pressure (2.0 GPa) it reacts with maleic anhydride (31) in reasonable yield (47%) (Scheme 7.8). It was recently observed that the reaction yield is strongly improved (77%) if the cycloaddition reaction is carried out in perfluorohexane at 0.8 GPa the fluorophobic effect plays a crucial role in rate enhancement... 

B.xiv.d. High-Pressure Conditions, A wide range of liquid phase organic reactions have been reported to be promoted under high pressure.Pressure can be expected to cause a rate enhancement when the activation volume, that is, the difference of the volume of the transition state and the starting materials, is less than zero. This can easily be understood to be the case in addition reactions. However, a Pd-catalyzed Heck process consists of a number of reaction steps and, consequently, to predict the net effect of pressure in such a reaction is much more difficult. Nevertheless, rate enhancements and increased lifetime of the catalyst have been reported. 

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