Temperature influence rearrangement

Temperature influence on isomer ratio in Fries rearrangement s. 18, 827... 

Clearly the temperature at which the reaction occurs exerts a major influence on the product composition To understand why an important fact must be added The 1 2 and 1 4 addition products interconvert rapidly by allylic rearrangement at elevated tempera ture m the presence of hydrogen bromide Heating the product mixture to 45°C m the presence of hydrogen bromide leads to a mixture m which the ratio of 3 bromo 1 butene to 1 bromo 2 butene is 15 85... 

The irradiations are in general performed under purified, oxygen-free nitrogen in a cylindrical irradiation apparatus with a water-cooled central sleeve into which the lamp is introduced and efficient cooling is used to keep the temperature between 10° and 0° C. Temperatures in the range of 0° to 80° do not influence the photolysis step as demonstrated in the case of a 6j -nitrite, but increase the rate of rearrangement of the nitroso derivative to the oxime. 

In the course of studying the bromination reactions of the bicyclic systems we noticed that the reaction temperature has a dramatic influence on the product distribution. Increasing of the temperature gives non-rearranged reaction products (refs. 1,2). For this reason, we submitted 1 to high temperature bromination. To a solution of 1 in decalin at 150 C was added a hot solution of bromine in decalin in one portion. The colour of bromine disappeared immediately. After silica gel chromatography followed by fractional crystallization we isolated four products 2-6 in yields 8, 35, 37, and 9 % respectively. The structure of these compounds has been elucidated on the basis of spectral data by iH NMR and NMR experiments and by comparison with those reported in the literature. Symmetrical endo-c/5-isomer 6 has been observed for the first time. Studies concerning the mechanism of syn-addition show that the syn-adduct can arise either from direct... 

Thus the earlier failure to observe migratory preference with p-substituted phenyl groups in triarylmethylazides results from the fact that the presence of the p-substituent would have no influence on the ground state conformation of the azides. In support of this hypothesis is the observation that the decomposition and rearrangement of (27) becomes increasingly statistical as the temperature is raised (higher energy conformations become more populated) ... 

We have seen that 1,2-H migrations in singlet carbenes may be affected by (e.g.) the participation of carbene precursor excited states, QMT, stabilization of the hydride shift transition state by polar solvents, and temperature. Here, we consider our third principal theme, the effect of substituents on the kinetics of carbenic rearrangements. We first examine the influence of bystander and spectator substituents (as defined in Eq. 22) on 1,2-H rearrangements of alkyl, alkylchloro, and alkylacetoxycarbenes. 

Kinetic data on the influence of the reaction temperature on the enantioselectivity using chiral bases and prochiral alkenes revealed a nonlinearity of the modified Eyring plot [16]. The observed change in the linearity and the existence of an inversion point indicated that two different transition states are involved, inconsistent with a concerted [3+2] mechanism. Sharpless therefore renewed the postulate of a reversibly formed oxetane intermediate followed by irreversible rearrangement to the product. 

The influence of the oxime geometry on the base-promoted rearrangement is pointed out for compounds 29 (85T5181). Thus, when treated with aqueous potassium hydroxide at room temperature, Z isomers 29Z (R = H, -oxyiminomethyl), arising as cycloadducts of the dimer or trimer of fulminic acid and norbornene, rearrange into 30. On the contrary, the E isomer 29E remains unchanged under similar conditions of aqueous bases (Scheme 6). 

At higher temperatures the tertiary. chloride undergoes a rearrangement under the influence of CuCl giving a chloride with a conjugated diene system. This isomer has a considerably higher refractive index. 

We now have to consider the so-called irregular rearrangements . These involve dissociation by breaking a P —X bond to give a P+ X ionic structure, and have been studied by Westheimer (open chain phosphoranes) (76JA179) and Ramirez (cyclic phosphoranes) (71APO(9)25). Polar solvents, acid catalysis and even change of temperature can influence the equilibrium. 

For naphthoxyacetophenones [e.g., ketone (85)], PPA gives 2-phenyl-naphtho[2,l-6]furan (86), whether at 80° or at 132°, This shows the influence not only of the temperature, but also of the structure of the starting acetophenone,241 on the rearrangement process. 

---