Double bond switch

Application of symmetry rules also yields important results for the class of reactions known as sigmatropic shifts, in which a double bond switches position as an atom or a group migrates ... 

Solution As shown, the double bond is ruptured making the oxygen end negative and single bonded. The double bond switches to become a carbon-to-carbon double bond. As indicated in subsequent reactions, this flip-flopping of the double bond continues until the formation of chloroform. Ans... 

The methyl substitution at a-position leads to an increase of the reactivity of styrene during polymerization as well as EDA-complex formation. However, the methyl substitution in p-position achieves an opposite effect. The strengthened complex formation connected with a further increase of the HOMO is faced with a drastically decreased polymerization rate. This can be explained by the well known steric effect of group hindrance around the p-C-atom under attack 72), as well as the polarity switch in the vinyl double bond. The p-C-atom in the p-methyl styrene possesses a... 

In addition to the sensitivity to ring structure, slight alterations of the hydrocarbon chain may lead to dramatic differences in electro-optic performance in chiral compounds [43]. For example, in some electroclinic Sm-A materials it has been reported that if the chain is shortened or if a double bond is localised at the end of the hydrocarbon chain, the tilt angle, electroclinic coefficient, and switching time are significantly suppressed [43]. 

There have been examples of sonochemical switching in homogeneous reactions. The decomposition of lead tetraacetate in acetic acid in the presence of styrene at 50 °C generates a small quantity of diacetate via an ionic mechanism. Under otherwise identical conditions sonication of the mixture gives 1-phenylpropyl acetate predominantly through an intermediate methyl radical which adds to the double bond (Scheme 3.8) [55,56]. These results are in accord with the proposition that radical processes are favoured by sonication. 

Kekul6 proposed that the structure for benzene resonated between two alternate structures in which the position of the double and single bonds switched positions. 

Kekule proposed that the structure for benzene resonated between two alternate structures in which the position of the double and single bonds switched positions. In the figure, benzene is depicted as changing back and forth between two structures in which the position of the double bonds shifts between adjacent carbon atoms. The two structures are called resonance structures. 

In a trans unsaturated fat, the orientation of the double bond is switched such that the carbon chain... 

The reducing equivalents temporarily stored in N AD(P)H are utilized in a number of ways, all of which lead to biosynthesis of essential molecules and/or oxidative degradation of metabolites. Examples range from the simple reduction of a substrate for biosynthetic purposes (e.g. the steroid reductase mediated hydrogenation of the isolated double bond of desmosterol to give cholesterol, Table 1) (71MI11000) to complex electron transport chains that are switched on by the transfer of the electrons of N AD(P)H to the next electron carrier of the chain. These multienzyme systems are used for a number of purposes (see below). 

The geometrical isomers and enantiomers of the overcrowded alkenes 15-18 can readily be separated using chiral HPLC. Recently, an asymmetric synthesis of overcrowded alkenes has been developed, involving chirality transfer from an axial single bond to an axial double bond (Scheme 8).32 This methodology is particularly attractive for preparation of larger quantities of enantiomerically pure chiral switches based on overcrowded alkenes. The orientation of the two xanthylidene moieties is dictated by a binaphthol template. After a coupling step and separation of the diastereomers, the bi-xanthylidene is obtained with 96 % e.e. after removal of the template. 

Photochemical E —> Z isomerization produces a sterically strained state in which the methyl substituents are positioned in close proximity. The strain is released when the methyl groups and the naphthyl moieties switch their positions. This second step is followed by back-isomerization from the Z to the E form of the double bond and again steric hindrance provides an energetically unfavorable state which can reach an energetically more favorable state by a second flip of the sub-... 

Unimolecular dynamics of smaller clusters has also been studied. The HF dimer provides a particularly interesting system because it involves a highly quantal degenerate rearrangement consisting of a concerted double hydrogen-bond switch (Quack and Suhm 1991 Truhlar 1990). 

As the azocinoindole 40 constitutes the tricyclic substructure of the indole alkaloid apparicine , we attempted to improve the cyclization yield. Satisfactorily, the regioselectivity was completely switched to the 8-endo mode when the alkene acceptor was substituted at the internal position by a bromine atom. Thus, cyclization of selenoester 43 led to the desired target 40 as the only reaction product in 75% yield. Clearly, the bromine atom not only sterically prevented the competitive 1-exo attack, but also benefited the cyclization by activation of the double bond. It should be noted that similar halogen-controlled 8-endo cyclizations are known in the literature, but involving amidyl-type radicals <06OL2647>. 

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