Conformation of radicals

Some other interesting observations regarding free radicals in these systems are noteworthy. In many instances, multiple conformations of radicals are found at lower but not higher temperatures. This indicates that the radicals exist in shallow energy wells at low temperature this phenomenon was observed very early, in the 4 K ENDOR investigation of radical formation in amino acids.23 Unlike the process in DNA. In which it is well understood that the thymine anion radical protonates at C6 to form T(C6)H-, in the crystalline state there is a not clear link between pyrimidine electron adducts and H-addition radicals. We finally note that a deuterium isotope effect of protonation/deprotonation processes was found in cytosine.HCl and 2 -deoxycytidine.HCl, as evidenced by a lower propensity for these processes to occur in partially deuterated systems than in predated ones. 

It has been observed that addition of Lewis acids to the free radical allylation improved the chemical yield [101]. When substrates with a chiral auxiliary were subjected to free radical allylation in the presence of a Lewis acid, the desired allylated products were obtained with high stereoselectivity [94 d]. In these reactions the Lewis acid plays a pivotal role in fixing the conformation of radical intermediates. Recently Sibi indicated that an elevated reaction temperature accelerated inversion of the stereochemistry of the radical-centered carbon giving rise to greater diastereoselectivity (Scheme 12.39) [102]. When enantiomerically pure Lewis acids were employed as chiral auxiliaries enantioselective free radical allylation of sulfones [103] and oxazolidinones [104] were realized. In the latter reaction two contiguous chiral centers were generated successfully in a single operation with excellent stereoselectivity via tandem C-C bond formation both enantiomers can be se-... 

Figure 5. Ground state and transition state conformations of radical 7...

McLachlan calculations of spin densities for various conformations, discussion of conformation of radical anion from 2-methyl-l,l -dinaphthylketone. ) da/d7-=2-10- TK-. ") Similar data obtained for corresp. radical with SOjCDj instead of SCDj substituents. ... 

An anchor, as defined above, contains stable molecules, conformers, all pairs of radicals and biradicals formed by a simple bond fission in which no spin re-pairing took place, ionic species, and so on. Figure 1 shows some examples of species belonging to the same anchor. Thus, an anchor is a more general and convenient temi used in the discussion of spin re-pairing. 

Alkyl derivatives of 1,3-butadiene usually undergo photosensitized Z-E isomerism when photosensitizers that can supply at least 60 kcal/mol are used. Two conformers of the diene, the s-Z and s-E, exist in equilibrium, so there are two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-E and s-Z conformers. Theoretical calculations suggest that at their energy minimum the excited states of conjugated dienes can be described as an alkyl radical and an orthogonal allyl system called an allylmethylene diradical ... 

The Coleman-Fox two state model describes the situation where there is restricted rotation about the bond to the preceding unit (Scheme 4.3). If this is slow with respect to the rate of addition, then at least two conformations of the propagating radical need to be considered each of which may react independently with monomer. The rale constants associated with the conformational equilibrium and two values of Pirn) are required to characterize the process. 

It is known that the penultimate unit influences the conformation of both model radicals and propagating radicals.32 3 Since addition requires a particular geometric arrangement of the reactants, there are enthalpic barriers to overcome for addition to take place and also potentially significant effects on the entropy of activation. Comparisons of the rate constants and activation parameters for homopropagation with those for addition of simple model radicals to the same monomers also provide evidence for significant penultimate unit effects (Section 4.5.4). 

Fig. 8-6. Conformation of phenyldiazenyl radicals with the N=N group rotating about the C-N bond axis (after Suehiro et al., 1986).

Ph4P8fi forms orange needle-like air-sensitive crystals which were characterized by X-ray diffraction. The anions are cycHc and adopt a chair-conformation of C2 h symmetry. These rings can be considered to be composed of two 83 units connected by two extremely long bonds of 263 pm. The E8R spectrum of Ph4P86 at 115 K (g 2.056/2.036/2.003) is different from those of 82 and 83 and it is possible that the 8e radical is identical to the species which many authors have taken for 84 (see above). 

Direct esr evidence for the intermediacy of radical-cations was obtained on flowing solutions of Co(III) acetate and a variety of substituted benzenes and polynuclear aromatics together in glacial acetic acid or trifluoroacetic acid solution . A p value of —2.4 was reported for a series of toluenes but addition of chloride ions, which greatly accelerated the reaction rate, resulted in p falling to —1.35. Only trace quantities of -CH2OAC adducts were obtained and benzyl acetate is the chief product from toluene, in conformity with the equation given above. 

Entry 5 is an example of the use of fra-(trimethylsilyl)silane as the chain carrier. Entries 6 to 11 show additions of radicals from organomercury reagents to substituted alkenes. In general, the stereochemistry of these reactions is determined by reactant conformation and steric approach control. In Entry 9, for example, addition is from the exo face of the norbornyl ring. Entry 12 is an example of addition of an acyl radical from a selenide. These reactions are subject to competition from decarbonylation, but the relatively slow decarbonylation of aroyl radicals (see Part A, Table 11.3) favors addition in this case. 

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