Diradicals carbenes

The ntility of the experimental methods are illnstrated in this chapter by considering their applications to the stndy of reactive molecules, including radicals, car-benes and diradicals, carbynes and triradicals, and even transition states. These are provided in Section 5.4, which inclndes resnlts for representative bond dissociation energies and an extensive list of thermochemical results for carbenes, diradicals, carbynes, and triradicals. Section 5.5 provides a comparison and assessment of the resnlts obtained for selected carbenes and diradicals, whereas spectroscopic considerations are addressed in Section 5.6. 

TABLE 5.3. Enthalpies of formation and BDEs for formation of carbenes, diradicals, carbynes, and triradicals. ... 

The strain energy of cyclopropene is about 53 kcal/mol. The energy of a normal C—C bond is about 83 kcal/mol. If all the cyclopropene strain were concentrated in one C—C bond, then only ca. 30 kcal/mol extra energy would be required to break the bond. Indeed, cyclopropenes thermally open to vinylcarbenes with activation energies of 30—40 kcal/mol i9.38-40) (Table 3). The carbene-diradical resonance hybride 19 can also be formed from appropriately substituted vinyl-diazoalkanes (20) 41,42). 

The rate constant of the triplet carbene with a typical quencher such as O2 and 1,4-cyclohexadiene can be employed as a more quantitative measure of the reactivity. However, neither ko nor chd appreciably correlated with the D value (Table 2). Presumably, simple linear correlations with spin delocalization factors may not be expected for the reaction of triplet carbenes (diradicals) to form the corresponding monoradicals since the extents of the delocalization of unpaired electrons should be different between the two states. An additional... 

Wentmp, C. (2011) Nitrenes, carbenes, diradicals, and ylides. Interconversions of reactive intermediates. Acc. Chem. Res., 44, 393-404. 

By far the main part of the studies involves aromatic azides, however. These are the object of unrelenting interest. During the period considered, some reference works have been published, including a wide-scope review on the detection and role of intermediates, another one on pyr-idineazides and an account on the relation between nitrenes, carbenes, diradicals and zwitterions in the thermal and photochemical reaction operating in the decomposition of aromatic and heterocyclic azides. A recent, nice example illustrates the variety of paths followed. Thus, tetra-zolo[l,5-a]quinoxaline and tetrazolo[5.1-c]quinazoline equilibrate thermally with the corresponding azides. 

Photofragmentation of chloromethyldiazirine in the presence of several gases was assumed to involve C—N bond cleavage to form a diradical, which undergoes a second C—N bond cleavage to form the carbene (8OMI508OI). 

There is much evidence that the mechanism" of the 1-pyrazoline reactions generally involves diradicals, though the mode of formation and detailed structure (e.g singlet vs. triplet) of these radicals may vary with the substrate and reaction conditions. The reactions of the 3 f-pyrazoles have been postulated to proceed through a diazo compound that loses N2 to give a vinylic carbene." ... 

Singlet-triplet Energy Splittings in Carbenes and Diradicals 229... 

Different methods have been developed for the generation of carbene and diradical negative ions. One of the most commonly used approaches involves the reaction of an organic substrate with atomic oxygen ion, O , to form water by H2 abstraction (Eq. 5.7). "... 

This reaction can proceed by 1,1-proton abstraction to form a carbene radical anion, but can also occur by l,n-abstraction to form the negative ion of a diradical. Thus, reaction of O with methylene chloride results in the formation of CCI2 (Eq. S.Sa), reaction with ethylene gives vinylidene radical anion, H2CC (Eq. 5.8b), and the reaction with acetonitrile gives the radical anion of cyanomethylene, HCCN (Eq. 5.8c) Investigations of these ions have been used to determine the thermochemical properties of dichlorocarbene, CCI2, vinylidene, and cyanomethylene. ... 

Finally, in some cases diradical negative ions can even be generated directly npon ionization of appropriate precnrsors. For example, nitrene and carbene anions can be formed by El of organic azides, diazo-compounds, and diazirines, whereas Branman and co-workers have reported the formation of oxyallyl anions by El of flnorinated acetyl componnds (Eq. 5.12). ... 

Alternative negative ion-based methods for measuring carbene and diradical enthalpies of formation have been developed, which can give BDEs indirectly. A common approach for this involves the use of halide affinity measurements. The relationship between enthalpy of formation and halide affinity is illustrated by Eq. 5.14. 

Enthalpies of formation and BDEs for the formation of carbenes and diradicals measured by using negative ion approaches are included in Table 5.3. 

The methods for generating these types of ions are the same as those described above for diradicals and carbenes. Eor example, the reaction of dichloromethane with O leads to the formation of CCl2. Thus, Jesinger and Squires have used CID of halocarbene anions to determine the thermochemical properties of carbynes (Eq. 5.16). ... 

From the point of view of both synthetic and mechanistic interest, much attention has been focused on the addition reaction between carbenes and alkenes to give cyclopropanes. Characterization of the reactivity of substituted carbenes in addition reactions has emphasized stereochemistry and selectivity. The reactivities of singlet and triplet states are expected to be different. The triplet state is a diradical, and would be expected to exhibit a selectivity similar to free radicals and other species with unpaired electrons. The singlet state, with its unfilled p orbital, should be electrophilic and exhibit reactivity patterns similar to other electrophiles. Moreover, a triplet addition... 

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