Addition-fragmentation mechanisms

Compounds with a thiocarbonyl a to the S-S bond such as the dithiuram (e.g. 8f2Al and xanthogen disulfides (e.g. 9)M have transfer constants that are much higher than other disulfides. In part, this may be due to the availability of another mechanism for induced decomposition (Scheme 6.9) involving addition to the C S double bond and subsequent fragmentation. Thiocarbonyl double bonds are very reactive towards addition and an addition-fragmentation mechanism has been demonstrated for related compounds (Section 6.2.3.5). 

Most monosulfides generally have very low transfer constants. Exceptions to this rule are allyl sulfides (Section 6,2.3.2) and thiocarbonylthio compounds such as the trithiocarbonatcs and dithiocstcrs (Section 9.5.3) that react by an addition-fragmentation mechanism. 

In the case of allyl peroxides (12 X= CH2, A=CH2, BO),1 1 1 intramolecular homolytic substitution on the 0-0 bond gives an epoxy end group as shown in Scheme 6.18 (1,3-Sn/ mechanism). The peroxides 52-59 are thermally stable under the conditions used to determine their chain transfer activity (Table 6.10). The transfer constants are more than two orders of magnitude higher than those for dialkyi peroxides such as di-f-butyl peroxide (Q=0.00023-0.0013) or di-isopropyl peroxide (C =0.0003) which are believed to give chain transfer by direct attack on the 0-0 bond.49 This is circumstantial evidence in favor of the addition-fragmentation mechanism. 

Other transfer agents which react with propagating species by an addition-fragmentation mechanism include the thionc derivatives (81-83) and RAFT... 

For allyl acetate a significant deuterium isotope effect supports the hydrogen abstraction mechanism (Scheme 6,31).183 Allyl compounds with weaker CTT-X bonds (113 X=SR, S02R, Bi etc.) may also give chain transfer by an addition-fragmentation mechanism (Section 6.2.3). 

Certain monomers may be able to act as reversible deactivators by a reversible addition-fragmentation mechanism. The monomers are 1,1-disubstituted and generate radicals that are unable or extremely slow to propagate or undergo combination or disproportionation. For these polymerizations the dormant species is a radical and the persistent species is the 1,1 -disubstituted monomer. 

Addition-cured model systems, of silicone networks, 22 569 Addition-curing silicones, 22 35 Addition-fragmentation mechanism, of chain- transfer (CT) agents, 23 383-384... 

Recently, it was found that the common CT agent a-methylstyrene dimer operates by an addition-fragmentation mechanism (Figure 24.4). [10]... 

Figure 24.4 Addition-fragmentation mechanism for the chain-transfer activity of a-methylstyrene dimer...

Common error alert Abstraction ofC (e.g., X + R3C Y-> X CR3 + Y) does not occur in free-radical reactions. An addition-fragmentation mechanism can be drawn for an apparent atom abstraction of C(sp2) or C(sp). If your mechanism requires an abstraction of C(sp3), it is almost certainly incorrect. Even abstraction of O is very rare. 

C=N An addition-fragmentation mechanism is much more / reasonable. 

Unsaturated groups, however, can shift by a two-step addition-fragmentation mechanism via a discrete cyclopropane radical intermediate. Phenyl groups are especially good at undergoing radical 1,2-shifts 1,2-shifts of acyl, alkenyl, and, to a lesser extent, alkynyl and cyano groups are also seen. 

Heavy atoms such as Br and Se can also undergo a radical 1,2-shift by an addition-fragmentation mechanism. An intermediate that features a heavy atom with a nonet of electrons intervenes. First-row elements cannot accommodate a nonet of electrons like heavy elements can, so the addition-fragmentation mechanism is disallowed for them. 

Problem 5.8. Draw an addition-fragmentation mechanism for the following reaction. 

Alkenes undergo electrophilic substitution reactions by the same addition-fragmentation mechanism as do arenes. Alkenylsilanes and -stannanes are especially good substrates for electrophilic substitution reactions because the carbo-... 

Atom abstraction of Se and other heavy atoms may also take place by an addition-fragmentation mechanism involving a nine-electron intermediate. 

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