Observations and extensions

In parallel with the development of the heterolysis of b-substituted alkyl radicals, a rearrangement reaction was observed and extensively studied in organic solvents. This rearrangement was first noted for b-(acyloxy)alkyl radicals (Scheme 5) by Surzur et al. [48] and, later, for b-(phosphatoxy)alkyl radicals by the Crich and Giese groups [49,50]. 

CsNiCl3-type). Table 6 summarizes results on electronic spectra. The three spin-allowed transitions are usually observed and extensive vibrational contribution has been observed in single crystal spectra.120,121 Intense spin-forbidden transitions may also be observed122 due to exchange interactions in the solid phase. The diiodide123,124 and the complex halides121 show trigonal distortion. 

The [3+2] cycloreversion of bicyclo[ z.3.0]alkan-3-on-2-yl-l-oxonium ylides (m = 3-6) to alkenyloxyketenes is observed and extensively studied <2004JOC1331>. In particular, rhodium(ll)-catalyzed intramolecular reaction of diazomethyl oxepan-2-yl ketone 13 generates a bicyclo[5.3.0]decan-3-one-l-oxonium-2-ylide 14, which undergoes sigmatropic and stereospecific [3+2] cycloreversion to form hept-6-enyloxyketene 15. The latter is trapped by MeOH to form the corresponding ester 16 (Scheme 5). 

However, the C-4 substitution of the 1-oxabutadiene system with strong electron-donating substituents (—OR, —NR2) does serve to increase the nucleophilic character of the a,/3-unsaturated carbonyl compound to the extent that stepwise, addition-cyclization [4 + 2] cycloadditions, often in competition with [2 + 2] cycloadditions, with reactive electrophilic olefins, e.g., ketenes121-127 or sulfenes,127-130 have been observed and extensively investigated (Scheme 7-II). For early investigations, prior reviews should be consulted.5... 

Several cases of dielectric, hydrodynamic, and flexoelectric instabilities and domain structures have been observed and extensively studied in CLCs. Their appearance depends on the initial orientation of molecules, the physical parameters of the material, and the applied electric field. In CLCs with positive dielectric anisotropy Ae > 0, an electric field applied along the helix axis of a planar (Grandjean) texture can induce a two-dimensional spatially periodic deformation which has the form of a square grid [96], The period and threshold voltage of this field-induced instability depend on the elastie constants, the dieleetric anisotropy, and the sample thickness [97],... 

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