Electrocyclic addition

Eastman, R. H., 158, 166 Eaton, P. F., 460 Eigen, M., 80 Eisenberg, W., 125 Electrocyclic addition, 46 Electrocyclic reaction rules, 339 Electrocyclic reactions, 402,408 4n-examples, 408 (4n + 2)-examples, 410 Electron impact spectroscopy, triplet energy, 220-223 Electronic energy transfer, 267 Electronic integral, 21 Electronic transitions /-a ,16 n -Mr, 16... 

Mn nmr spectroscopy the authors concluded that the crown ether-separated ion pair prevails in solution. At 25°C, the complex reacts slowly (half life 48 h) with benzene. The complex reacts readily with olefins to give the potassium salts of the corresponding acids in high yields. The authors proposed a mechanism, in which the first step is a [2 + 4] electrocyclic addition as... 

Various electrocyclic additions, with subsequent extrusion of a small molecule have been used to construct pyridines addition to oxazoles is one of these. 

It is important to recognize the distinction between the reaction of the singlet carbene (e.g., CH2) formed directly either thermally or photochenucally and triplet carbene (e.g./ CH2) formed from either (a) the singlet state by crossing over to the triplet, or (b) sensitized excitation. Singlet carbene undergoes electrocyclic addition with retention of stereochemistry. On the contrary, triplet carbene adds as a diradical and loss of stereochemistry (Scheme 6.47). The stereochentical evidence lends credence to the presumed path for the addition. 

The addition of ozone (O3) to alkenes to give a primary ozonide (molozonide), which rearranges to an ozonide and eventually leads, on reduction, to carbonyl compounds (aldehydes and/or ketones), has already been mentioned and the reaction itself is shown in Scheme 6.11. However, it is important to recognize that this is only one example of a 4th- 2n electrocyclic addition and that orbital overlap for many sets of these reactions dictates their courses as well. Thus, to show the similarity of some of these dipolar 3 -f 2 addition reactions Equations 6.53-6.56 are provided. Although any alkene might be used as an example, (Z)-2-butene is used in each to emphasize that aU of them occur with retention of stereochemistry and, in the first (Equation 6.53), the reaction with ozone to form the primary ozonide (molozonide) is presented again (i.e., see Scheme 6.11). In a similar way, with a suitable azide, R-N3, readily prepared from an alkyl halide (Chapter 7), the same alkene forms a triazoline (Equation 6.54) and with nitrous oxide (N2O) the heterocycle (Chapter 13) cis -4,5-dimethyl-A -l,2,3-oxadiazoline (ds-4,5-dihydro-4,5-dimethyl-l,2,3-oxadiazole) (Equation 6.55). Finally, with a nitrile oxide, such as the oxide derived from ethanenitrile (acetonitrile [CH3ON]), the same alkene yields a different heterocycle, the dihydroisoxazole, 3,4,5-trimethyl-4,5-dihydroisoxazole (Equation 6.56). 

---