Electrophilic character photochemically

Other tetraazapentalenes have been produced by photochemical or thermal decomposition of azidophenyl-l - and -2//-benzotri-azoles.339-341 Hall, Stephanie, and Nordstrom341 found that decomposition of the azide 259 in decalin at 170° led to a mixture of 260 and 261 in the ratio 1 9. The preference for cyclization to the position para to the methyl group to give 261 is consistent with the known electrophilic character of the intermediate nitrene. 

As already indicated, the chemistry of terminal borylene complexes is as yet almost unexplored. In addition to the photochemically induced borylene transfer, which was already discussed in Chapter 3.2, studies of the reactivity of terminal borylene complexes are restricted to two recent reports by Roper.147,148 The base-stabilized borylene complex [Os (=BNHC9H6N)Cl2(CO)(PPh3)2] (26) undergoes a reaction with ethanol to yield the ethoxy(amino)boryl complex [Os B(OEt)NHCgH6N Cl(CO) (PPh3)2] (35) according to Eq. (13) with a 1,2-shift of the quinoline nitrogen atom from the boron to the osmium center. The alcoholysis of 26 indicates that even the boron atom in base-stabilized borylene complexes displays some electrophilic character—a fact already predicted by a theoretical study.117... 

The reaction enthalpies for the photo-reduction of ketones and azoalkanes in n,ji states by the model hydrogen donors methanol and dimethylamine have been calculated. These data, together with a consideration of excited-state electrophilicity and the antibonding character of transition states, have been used to rationalize the nucleophilic character of singlet n,ji excited azoalkanes and the electrophilic character of correspondingly excited ketones. A theoretical model for the photo-reduction of carbonyl compounds, based on the assumption that hydrogen transfer involves electron transfer followed by proton transfer, has been developed. The model was able to describe the available kinetic data for the photo-reduction of o-benzoquinones and fluorenone in the presence of para substituted iV,iV-dimethylanilines and those for the photo-reduction of p-chloro-anil and 2,6-dichloro-l,4-benzoquinones in the presence of polymethylben-zenes. The photochemical properties of the triplet ji, n state of 5,12-naph-... 

The methyl radical is not very discriminating, and shows none of the electrophilic character of most of the other radicals discussed earlier in this chapter. To what extent steric effects enter into the determination of the rate is hard to decide, but they probably are not pronounced, since the relative reactivity series is not greatly different from that of hydrogen atom additions to the same series of olefins. The relative rates are given in Table 50. It should be noted that these are derived from the absolute rates, and are not the results of competition reactions. Accordingly, there may be some substantial errors in the individual values. A very large number of miscellaneous olefinic compounds have been studied and the rate parameters for the addition are given in Table 51. In many of these cases, the methyl radicals were produced by photochemical decarbonyla-tion of acetaldehyde. After the methyl radical adds to the olefin, the new... 

Moderate to good yields of spiro[2,4]hexatrienes (152) result from photochemical addition of diazocyclopentadienes to alkynes. Electron-rich acetylenes give better yields than electron-poor ones, providing further proof of the electrophilic character of cyclopentadienylidenes cf. p. 29). The spiro-compound (152) affords further adducts with reactive dienophiles, e.g. tetracyanoethylene gives the cyclopropane (153). 

The reactivity of pyridine is determined by its character as an electron-deficient aromatic azine system. Therefore, electrophiles should attack preferably at the N-atom, but also at the ring-C-atoms in SnAr reactions, while nucleophiles should attack at the ring carbons and undergo SNAr reactions. In analogy to benzene, thermal as well as photochemical valence tautomerizations can be expected. 

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