Radical cations fragmentation

Scheme 2,2. Radical-cation fragmentation following the cyclization of substrate 33...

Further functionalizations are obtained via the electron transfer— radical cation fragmentation pathway a typical example is side-chain nitration by irradiation of methyaromatics with tetranitromethane. Aromatics form charge-transfer complexes with C(N02)4 irradiation leads to electron transfer and fragmentation of the C(N02)4 radical anion to yield the triad [Ar + C(NO)J N02], followed by combination between the arene radical cation and the trinitromethanide anion. Thus, cyclohexadienes are formed that generally eliminate and rearomatize at room temperature yielding ring-functionalized products [234] (Sch. 21). 

There are two general types of fragmentation reactions. In one type the radical cation fragments to a neutral molecule and a new radical cation. This process is especially favorable when the neutral product is a small, stable molecule. For example, the loss of water from the molecular ion of alcohols is very facile. For this reason the M+ peak is very small for primary and secondary alcohols, and it is usually undetectable for... 

Using electron ionization, the molecular radical cation is formed in the source. This radical cation fragments into a radical and a cation with an even number of electrons or, through rearrangements or multiple steps, into a neutral molecule and a new odd-electron cation. The latter are often easily recognized in the spectrum because their mass is even in the absence of a nitrogen atom. 

RADICAL CATION FRAGMENTATION REACTIONS IN ORGANIC SYNTHESIS... 

Arnold s demonstration" that oxocarbenium ion intermediates can be formed through homobenzylic ether radical cation fragmentation reactions shows that mild oxidizing conditions can be used to prepare important reactive intermediates. Scheme 3.2 illustrates a critical observation in the development of an explanatory model that allows for the application of radical cation fragmentation reactions in complex molecule synthesis. In Arnold s seminal work, cleavage of the benzylic carbon-carbon bond in substrate 1 is promoted by 1,4-dicyanobenzene (DCB) with photoirradiation by a medium-pressure mercury vapor lamp. With methanol as the solvent, the resulting products were diphenylmethane (2) and formaldehyde dimethyl acetal (3). 

Cationic radicals are much less stable and noticed prominently in mass spectroscopy. When a molecule in gas phase is subjected to electron ionization, one electron is abstracted by the electron beam to create a radical cation. This species represents the molecular ion or parent ion, which on fragmentation gives a complex mixture of ions and uncharged radical species. For example, the methanol radical cation fragments into a methyl cation CFl and a hydroxyl radical. Secondary species are also generated by proton gain (M -F 1) and proton loss (M — 1). 

Aromatic radical cations containing -XH substituents, e.g., in the simplest case, phenol, aniline and toluene, can deprotonate to form the corresponding neutral radicals. This is the simplest type of radical cation fragmentation reaction. Thus, the stability of these radical cations in solution is also reflected by the pK (reaction 4). 

Detection of radical cation fragments in modern mass spectrometers has been refined to the point where it is possible to determine the mass of a fragment to four or five decimal places. In other words, a molecular ion may be determined as 100.1251, or better. With this level of accuracy, a molecular formula may be obtained directly from the molecular ion. However, the mass used for this determination requires clarification. In the periodic table, the atomic mass of carbon is shown to be 12.0107 amu, hydrogen is 1.00794 amu, oxygen is 15.9994 amu, and nitrogen is 14.00674 amu. Interestingly, chlorine is 35.4527 amu and bromine is 79.904 amu. These are the masses of the elements as found in nature. However, carbon in nature is not just carbon-12 ( C), but rather a mixture of three isotopes and... 

Maruyama T, Suga S, Yoshida J (2005) Radical addition to cation pool . Reverse process of radical cation fragmentation. J Am Chem Soc 127 14702-14703... 

Steric interactions in the cyclic transition structures for a hydrogen transfer result in different abundance of the butene radical cation fragment, obtained from the diastereoisomeric unsaturated alcohols erythro-10 and threo-10. The higher energy of the transition state of erythro-10 due to the steric interaction of the methyl and R groups, results in a lower abundance of the fragment, as compared with the stereoisomeric threo-9 (see Scheme 9). 

The radical cation fragmentation involves assistance by cerium ammonium nitrate (0.15 mM), the photochemical precursor of the oxidant used in this case, the nitrato radical. 

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