Radical initiating conditions

Polymerization through the vinyl function in pentachloro(vinyloxy)-cyclotriphosphazene can be accomplished under radical initiation conditions. TTie presence of moisture and (vinyloxy)phosphazenes with degrees of substitution >1 must be avoided because the former can effect chain transfer and the latter results in the formation of cross-linked materials. 

Studies of the homopolymerization kinetics that we carried out were curious and appeared to be greater than half-order in initiator and the polymerizations were sluggish under radical initiated conditions. The reason for this was cleared up by the excellent and precise homopolymerization kinetic studies of George and Hayes, who clearly demonstrated the rate was essentially first order in both initiator and monomer.39,40,41 What could cause such a rate law Could the iron center (formally Fe(II)) contribute Was a redox reaction involved that would be essentially impossible for organic monomers like styrene The observed rate law r = A M 11 [ 1111 stands in sharp contrast to the normal half-order in initiator concentration found in most vinyl addition polymerizations. Apparently, first-order chain termination had occurred rather than classic bimolecular termination. Indeed, the iron atom was playing a key role. 

CIDNP. Radical pairs have been detected by CIDNP in the reactions of various phosphanes with l-nitro-l-alkenes. CIDNP spectra were also observed during the dimerization and trimerization of allylphosphine under photochemical and free radical initiation conditions. ... 

The Wohl-Ziegler reaetion is the reaction of an allylic or benzylie substrate with Wbromosuccinimide (NBS) under radical initiating conditions to provide the corresponding allylic or benzylie bromide. Conditions used to promote the radical reaction are typically radical initiators, light and/or heat carbon tetrachloride (CCI4) is typically utilized as the solvent. 

A proposal for the mechanism of the Fukuyama indole synthesis is proposed as shown below. Treatment of the isonitriles 1 with tributyltin hydride and a catalytic amount of AIBN, affords a-stannoimidoyl radical 2, followed by cyclization, to give radical 16. It was found that the substrates bearing radical-stabilizing groups at the P-position gave indoles 3 in excellent yield after tautomerization, and acidic workup. Similarly, when thioamide derivatives such as 2-alkenylthioanilide 18 are subjected to radical-initiating conditions, radical 5 or imidoyl radical species are formed, which then undergo radical cyclization to furnish 2,3-disubstiuted indoles 6. 

Autoxidation (Section 8.7) Autoxidation involves reaction of a CH bond, especially an allylic one, with oxygen under radical initiation conditions. The primary product is a hydroperoxide. The mechanism involves a radical chain process in which resonance-delocalized allylic radical intermediates react with molecular oxygen to give a peroxy radical that continues the radical chain. 

Since the first work reported in 1987, more than a dozen papers on Pd-catalyzed hydrostannation of alkynes have been reported. As detailed below, Pd catalysts significantly accelerate hydrostannation and hydrogermation of alkynes. Thus, the reaction now proceeds at or below room temperatures. However, the selectivity aspects remain unpredictable and often problematical, with three isomers—a, p-trans, and 13-cis— being formed in a capricious manner. The isomer profiles for Pd-catalyzed hydrostannation are often substantially different from those observed under either uncatalyzed or radical-initiated conditions. When all of the seemingly random results are sorted out, however, the following potentially useful generalizations may be presented as a set of guidelines. 

Under radical initiation conditions, typically peroxides, hypervalent iodine reagents can be homolytically cleaved to iodine-centered radicals. These iodine centered radicals abstract a hydrogen atom from a labile benzylic C—H bond to yield a resonance-stabilized benzylic radical. At this point in the mechanism, researchers seem divided on the next step. Some propose a second single electron transfer (SET) to form a benzylic carbocation, ° which undergoes ionic reactions to form product. Others suggest radical combination to form an alkyl halide or organic peroxide which reacts further under the reaction conditions to form product. 

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