Monoradical

Other postulated mechanisms for spontaneous initiation include electron transfer followed by proton transfer to give two monoradicals, hydrogen atom transfer between a charge-transfer complex and solvent,110 and formation of a di radical from a charge-transfer complex, JJ[Pg.111]

F. R. Mayo (private communication) has found evidence that thermal polymerization of styrene may actually be of a higher order than second, i.e., about five-halves order. This would suggest a termolecular initiation step. Generation of a pair of monoradicals in this manner, i.e., from three monomer molecules, would be acceptable from the standpoint of energy considerations. 

If reactions other than those considered so far did not take place to an appreciable extent, the average degree of polymerization would be directly related to the kinetic chain length v. Assuming termination by combination of radicals, as is indicated by experiments previously cited, each polymer molecule formed in a monoradical-initiated polymerization would consist of two kinetic chains grown from two other-... 

Thermal initiation makes an appreciable contribution to the polymerization rate for styrene at very low initiator concentrations, as we have pointed out earlier. Since the rate Rp includes contributions from thermal as well as from catalytic initiation, the second term in Eq. (36) remains valid provided the thermal initiation involves monoradicals. Diradical initiation, if it occurred, would introduce a deviation, since it produces no chain ends. 

The short lifetimes of carbon-centered monoradicals are generally reduced in the case of diradicals due to their propensity to form covalent bonds. It has been suggested that stable diradicals may be observable from highly strained bicyclic molecules where the TS for inversion is a diradical. Unfortunately, only persistent diradicals have been obtained in this way. Akin to this approach, in a recent attempt to generate the oxyallyl diradical, Sorensen and co-workers synthesized two substituted bicyclobutanones hoping to stretch and homolytically break the central bond using bulky substituents, which would also stabilize the diradical. Though the bicyclobutanones did not yield the desired oxyallyl derivative, the X-ray structures showed... 

The formed biradical can form trioxide. This trioxide is unstable and rapidly decomposes back to the biradical [4], Then the biradical rapidly reacts with the C—H bond and forms the peroxyl monoradical. 

A diradical is an atom or molecule containing two impaired electrons. The properties of diradicals are for the most part like those of monoradicals. They are paramagnetic and show paramagnetic resonance absorption. Although they are very reactive chemically, this is not a reliable criterion for the diradical state. Spectroscopically the diradical will probably be a triplet state if a double bond structure coupling the two electrons is geometrically possible. But when the two electrons are fairly well isolated from each other the state is probably a double doublet, like two independent radicals. 

In the presence of ferrous salts the hydroperoxide decomposition goes by way of an oxygen monoradical intermediate, with different migration aptitudes. Para nitrophenyl rather than phenyl migrates under the radical conditions. Although the oxygen cation resembles the carbonium ion both chemically and in charge type, it may also be a diradical ... 

Addition of the Grignard reagent, generated in situ from (375), to nitrone (373) or to 2,5-dimethyl-l-pyrroline-A-oxide, affords biradical (379) or nitrone containing monoradical (380). Furthermore, (380) can be transformed into biradical (381) and triradical (382) (Scheme 2.165) (620). 

Homolytic cleavage of b—d bond gives a diradrical intermediate. If the centers bearing the unpaired elections in a diradical are well separated (for example by a long carbon chain) there will be little interaction between them and each will act as an independent monoradical function. Usually the centers are close enough for molecular interaction and it eliminates their reaction as separate chemical entities. 

The main products initially formed, dimers (2a), will undergo further slow coupling steps. At the given potentials, dimers (2a) will immediately be oxidized to the corresponding monoradical cations. Again, the monoradical cations can be found in different mesomeric structures. The most reasonable notation has the positive charge next to the donor... 

The idea of stacking monoradicals with S = 1/2 and di- or poly-radicals with 5 > 1 in mixed crystals was proposed by Buchachenko (Buchachenko, 1979). If the chemical structures are almost identical, the two component radicals will have a chance to stack in alternation and the two spin sublattices having S = 1/2 and S> 1, respectively, will be formed with opposite orientation. 

The major product of the keto olefin cyclizations often corresponds to what one would predict, assuming the intermediate ketyl behaves like the corresponding monoradical (Eq. 34). For example, given an option between a 5-exo and a 6-endo-trig cyclization, the former predominates in radical cyclizations [51], and constitutes the exclusive cyclization path in the electrochemical counterpart, 110 - 112 [47]. In addition, the stereochemical outcome parallels that of the radical... 

The principle side reaction corresponds to reduction of the carbonyl without cyclization. For example, reduction of 6-methylhept-6-en-2-one (113) leads to a 12% yield of alcohol 114 no cyclized adduct 115 is produced. Were the intermediate to behave precisely like the monoradical, one would have anticipated that the presence of the methyl group on the alkene would have slowed the rate of 5-exo-trig cyclization to a point where closure to form 115, the product of a 6-endo-trig cyclization, would have dominated it did not. 

The matrix isolation and spectroscopic characterization of m- and p-benzynes and their derivatives have been reported. Fourier transform ion cyclotron resonance mass spectrometry has been employed to investigate the reactivity of w-benzyne biradical with a pyridinium charge site in the 5-position. The chemical properties of w-benzyne in the gas phase differ from those of the monoradical and... 

The EPR Spectroscopic D Parameter of Localized Triplet Drradicals as Probe for Electronic Effects in Benzyl-type Monoradicals (Adam,... 

Our choice was the two series of dendritic polymers 5 and 6, depicted in Figure 4, which have all their open-shell centers (or trivalent carbon atoms) sterically shielded by an encapsulation with six bulky chlorine atoms in order to increase their life expectancies and thermal and chemical stabilities. Indeed, it is very well known that the monoradical counterpart of both series of polyradicals, the perchlo-rotriphenyl methyl radical, shows an astonishing thermal and chemical stability for which the term of inert free radical was coined. The series of dendrimer polymers 5 and 6 differ in the nature and multiplicity (or branching) of their central core unit, N, as well as in their branch-juncture multiplicities, N Thus, series 5 has a hyperbranched topology with = 3 and = 4, while dendrimer series 6 has a lower level of branching with = 3 and = 2, and the topology of a three-coordinated Cayley tree. 

The relative rates of reaction of the singlet TMM derivative 14b with a series of alkenes (32) parallel those of a conjugated diene with the same alkenes in Diels-Alder reactions. These relative rates also are well correlated by the frontier orbital model for a concerted reaction. The absolute rates of the biradical cycloadditions are many orders of magnitude greater than those of the model dienes. The relative rates of the alkenes in the cycloadditions of the triplet biradical 14b, on the other hand, follow the reactivity order of their addition reactions with monoradicals. 

The cage effect can be a source of great frustration in matrix isolation studies of monoradicals, because such species are usually formed by homolysis from closed-shell compounds, and hence any radical generated in situ is invariably accompanied by another radical that will be trapped in the same matrix cage. 

Of course, biradicals can also be generated by a sequence of two cleavage reactions such as described above for monoradicals, for example, from diiodides or bis(acylperoxides). Several examples of this strategy can be found in Chapter 16 in this volume. 

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