Claisen rearrangement activation parameters

RATE COEFFICIENTS AND ACTIVATION PARAMETERS FOR THE INDIVIDUAL STEPS OF THE CLAISEN REARRANGEMENT AT 170 °C... 

The generation of unsaturated bis(sulfines) by a dithio-Claisen rearrangement is discussed in Sect. 2.6. The intermediacy of these products is proposed to take place in nature from unsaturated thiosulfinates formed in onion and garlic, and to be the source of zwiebelanes [91-93]. Studies by Block and his group produced a great deal of information about this fascinating chemistry. Activation parameters as low as 15.5 kcal/mol for AH+ have been measured. 

On heating the neat sample or a solution in a variety of solvents, 2-allyloxythiazole (125) undergoes a thermal rearrangement to N-allyl-A-4-thiazoline-2-one (126) in excellent yield (Scheme 64) (283). Deuterium labeling reveals the complete inversion or the allylic moiety in the rearrangement. First-order rate law is found, and activation parameters show a negative entropy in accord with that measured in most of Claisen rearrangements. 

Overall rate coefficients and activation parameters for the Claisen rearrangement of predominantly ort/io-substituted allyl aryl ethers are given in Table 26. There are some changes in the solvent medium for these rearrangements, but as seen later this should not greatly affect the comparisons. 

ACTIVATION PARAMETERS FOR SOME RING AND CHAIN SUBSTITUTED /S-ALKYLALLYL ARYL ETHERS IN THE ClAISEN REARRANGEMENT ... 

Volumes of activation for the Claisen rearrangement of allyl phenyl ether in various solvents are given in Table 35. This parameter was calculated according to... 

Jackson, D.Y. Liang. M.N. Bartlett, P.A. Schultz, P.G. Activation parameters and stereochemistry of an antibody-catalyzed Claisen rearrangement. Angew. Chem., Int. Ed. 

The activation parameters for the CM-catalyzed and uncatalyzed Claisen rearrangement are listed in Table 1.2 [20, 21, 26, 42]. For the uncatalyzed reaction, the activation barrier (AG ) is 24.5 kcal/mol. Chorismate mutase is able to reduce the activation barrier by 7-10 kcal/mol. Table 1.2 shows that the rate acceleration is due to a reduction in the entropy of activation to near zero and a decrease in the enthalpy of activation by about 5 kcal/mol the only exception is the BsCM-catalyzed reaction for which there is a significant unfavorable AS. However, the reliability of these data has been called into question [44], and it was suggested [44] that both the substrate binding and product leaving are expected to show large solvent compensation effects involving AH and AS [45, 46). 

Figure 21.10 Eyring plot to determine activation parameters of the uncatalyzed Gosteli-Claisen rearrangement of (Z)-1 to ( )-2 obtained by temperature-dependent measurements using stereoselective ocRGC between 70 and 120°C.

To obtain the experimental activation parameters and AS of the catalytic asymmetric Gosteli-Claisen rearrangement catalyzed by R,R)-5, the incomplete conversion of (Z, )-3 to 4a and 4b was investigated by temperature-dependent measurements between 65 and 85 °C. Best agreement in the calculation of... 

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