Vinyl radical structures

The rate of oxidation/reduction of radicals is strongly dependent on radical structure. Transition metal reductants (e.g. TiMt) show selectivity for electrophilic radicals (e.g. those derived by tail addition to acrylic monomers or alkyl vinyl ketones - Scheme 3.89) >7y while oxidants (CuM, Fe,M) show selectivity for nucleophilic radicals (e.g. those derived from addition to S - Scheme 3,90).18 A consequence of this specificity is that the various products from the reaction of an initiating radical with monomers will not all be trapped with equal efficiency and complex mixtures can arise. 

Aryl radicals are much more reactive than alkyl radicals (k ca. 2 x 106M-1 s 1), and rather more reactive than cyclopropyl radicals (ca. 8 x 107) or vinyl radicals (ca. 3 x 10s). Fluoroalkyl radicals are also about 100 times more reactive than alkyl radicals, because of contributions from structures Rf 6 - H -4+SnR3 to the transition state. Singlet-excited acetone is twice as reactive as triplet-excited acetone, which shows a similar reactivity to that of the /-butoxyl radical (k ca. 2 x 108M-1 s-1). 

Based on data from competition experiments, trapping of vinyl radicals occurs via a cr-type intermediate, which is lower in energy than the alternative jt-radical structure [55, 56], Stabilization of cr-radicals via hyperconjugation is small, which causes vinyl radicals to be more reactive than e.g. the methyl radical. /Z-Isomerization of a strained cr-vinyl radical proceeds with a rate constant k 3 x 108-1010 s-1 to provide the thermodynamically most favorable geometry [56],... 

The reactivity trend in terms of the ring-opening reaction for the series 43, 47, and 53 can be analyzed in terms of the geometries of imino and vinyl radicals. Most of the spin density in imino radicals is concentrated in the p-orbital on nitrogen, but vinyl radicals have a nonlinear, vp -like structure, where the spin density is in the i /j -orbital of the carbon. In the o-quinone diradicals 43,47, and 53, the / -orbitals of the nitrogens (accomodating the odd electrons) are better... 

Sequences in which addition precedes cyclization are not as straightforward to conduct as the reverse however, they are very important because a net annulation results (that is, a new ring is formed by the union of two acyclic precursors in one experimental step). The intermediate radical is differentiated from the other radicals provided that the cyclization reaction is rapid, but it can be difficult to differentiate the initial radical from the final radical. As illustrated in Scheme 57, this is particularly true in the tin hydride method because many different types of radicals react with tin hydride at similar rates. Reaction of (69) under standard radical addition conditions produces (70), which results from a sequence of addition/cy-clization/addition.233 That the last C—C bond is formed actually results from a lack of selectivity the initial and final radicals are not differentiated and they must undergo the same reaction. Of course, this lack of selectivity is of no consequence if the product contains the desired skeleton and the needed functionality for subsequent transformations. Such sequences are very useful for forming three carbon-carbon bonds, and they can also be conducted by Barton s thiohydroxamate method.234 Structural modifications are required to differentiate the initial and final radicals, and, as illustrated by the conversion of (71) to (72), phenyl groups can provide the needed differentiation (probably by retarding the rate of addition more than they retard the rate of hydrogen abstraction). Clive has demonstrated that phenyl-substituted vinyl radicals also provide the needed selectivity, as illustrated by the second example in Scheme 57.233... 

Vinyl radicals are one of the most reactive species among hydrocarbon radicals, and this example demonstrates a high chemical activity of PC in the processes of this type. This method was used to obtain substituted hydrocarbon radicals with different structures =Si-0-CH, sSi-CH2- CH(CH3), sSi-CH2- C(CH3)2, =Si-0-c- C6H6 [116], fluorine-substituted radicals, =Si-0-CH2- CF2, =Si-CF2- CF2, and =Si-0-CF2- CF(CF3) [117], and other organic radicals [9]. The 02, CO, C02, N20 (see Table 7.3), and S02 [118] molecules can act as modifiers of P. Of course, more complex structures can be designed, successively involving various molecules in the chemical modification of the surface site structure, for example,... 

Photolysis of 2-bromo-4,4-dimethyl-2-cyclohexenone only affords reduction, even in a nucleophilic medium343,344. Apparently, this substrate is structurally not suitable to form a vinyl cation. Formation of vinyl radical-derived products is also the main process for all vinylic halides, if their irradiation is performed in an apolar medium. Such photochemical reductive dehalogenation and especially dechlorination reactions have been extensively studied in the past, not in the least because of their importance as abiotic transformation of persistent polychlorinated environmental pollutants. Examples are the cyclodiene insecticides aldrin and dieldrin, which contain a vicinal dichloroethene chromophore. In recent... 

Case Studies Structures and Frequencies of Vinyl Radical in First Three Doublet Excited Electronic States... 

TABLE 6.16 Structures, and Adiabatic Excitation (AE) Energies (in eV) of the First Three Doublet Electronic Excited States of the Vinyl Radical. Bond Lengths are in A and Angles in Degrees... 

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