Interpretation cleavage reactions

Most recent publications by Hartshorn and his collaborators introduce a note of caution with regard to the mechanistic interpretation of results obtained in BF3-catalyzed epoxide cleavage reactions. They point out the marked dependence of the course of the reaction on the solvent used, either... 

Saeva FD (1990) Photoinduced Electron Transfer (PET) Bond Cleavage Reactions. 156 59-92 Sahni V (1996) Quantum-Mechanical Interpretation of Density Functional Theory. 182 1-39 Sakai S,see Inokuma S (1994) 172 87-118 Sandanayake KRAS, see Bissel RA (1993) 168 223-264... 

The cleavage reaction of Equation 23-2 reveals other useful generalizations. Whatever its source, a parent molecular ion, M+, has one unpaired electron and is properly described as an odd-electron ion (a radical cation). When a parent molecular ion fragments, it does so homolytically, as shown in Equation 23-2, and produces a radical and an ion in which the electrons are paired—an even-electron ion. The m/e value of an even-electron ion is an even number for any elemental composition of C, H, O in combination with an odd number of nitrogens. These generalizations are summarized in Table 23-2 and can be useful in the interpretation of mass spectra, as illustrated by Exercises 23-4 and 23-5. 

There are many cleavage reactions of the phenothiazine heterocycle which may be interpreted as substitutions at the carbon atom adjacent to sulfur. These reactions depend to a great extent upon the oxidation state of sulfur they occur with phenothiazines and phenothiazine-5-oxides, but not with the 5,5-dioxides. In structure proofs such cleavage reactions are of particular importance, because they often lead to diphenylamines with known structimes. 

To the extent that studies have been done, most of the results can be interpreted on the basis that siloxycarbene formation is a rapid reversible process in which the short-lived siloxycarbene must be trapped quickly by a kinetically acidic reagent. Competitive with this is a slower Norrish type I cleavage reaction, which only predominates when there is no good trapping agent present to siphon off the simultaneously formed siloxycarbene (equation 67). 

Bromocyclopropane has been used as a heavy-atom quencher of singlet states. The advantage of bromocyclopropane is that the C—Br cleavage reaction, which leads to alkyl radical and bromine atom formation and results in difficulties in the quantitative interpretation of the heavy-atom quenching experiment, is inefficient because cyclopropyl radicals are formed only with difficulty. This technique has been used to show that 1,8-divinylnaphthalene (8) undergoes intramolecular photocycloaddition to give (9) and (10) via the triplet state.114... 

Figure 16.12. Mass spectra of a series of sec-butyl compounds. Major features of the spectra can be interpreted in terms of simple initial cleavage reactions. All are atlOeV ionizing energy.

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