Molecular Radical Chain Initiators for Ambient‐ to Low‐Temperature Applications

Abstract

An overview of methodsf or the initiation of radical chain reactions by specific initiator compounds, which generate radicals, is given. These can be utilized to initiate any kind of radical chain reactionb yt ransforming substrates into the desired radical intermediates. Azo initiators,p eroxides, nitroxides, trialkylboranes, dialkyl zinc compounds, and type Ip hotoinitiators are discussed, as well as methods of redox-and sonochemical initiation. Methods of direct radical formation from the substrates, such as photoredoxc atalysis or high-energyi rradiation, are not included. The focus of this review lies on rather "low" temperatures in the range of 50 8Cd own to À78 8C, which can be useful to achievem ore selectiver eactions. Illustrative applications of such radical chain initiatorsi navariety of reactions are discussed, including stereoselective ones and polymerizations.Scheme1.Generalmechanism for radical chain reactions transforming substrates (S) into product (P) via intermediate radicals (iR), being initiatedw ith radicals generated from molecularinitiatorsX -Y.Scheme2.Classification of radicalgeneration from initiator molecules X-Y (D:thermal energy;h n:photochemically;)))): sonochemically).Scheme19. Applications of type-I photoinitiators in organics ynthesis.Scheme20. Initiation of athiol-enereaction with N-phenyltriazolinedione 40.Scheme21. Sonochemical initiation of radical reactions.

Molecule‐Induced Radical Formation (MIRF) Reactions—A Reappraisal

“…The O−O bond energy of 48 amounts to Δ H 298 (R19)= +181.3 kJ mol −1 , again a value significantly higher than many of the MIRF reactions in Tables and . In practical terms the applicability of peroxide initiators is often described through the temperature required for a decay half‐life of 10 h. For 48 this parameter amounts to 73 °C …”

Molekül‐induzierte Radikalbildung – eine Neubewertung

“…In praktischer Hinsicht wird der Einsatzbereich von Peroxidinitiatoren oft durch die Temperatur definiert, bei der die Halbwertszeit des Zerfalls 10 h beträgt. Für 48 beträgt diese Temperatur 73 °C …”

Alkylboranes in Conventional and Controlled Radical Polymerization

“…Owing to the Lewis acid character of boron atom, the neutral organoborane readily reacted with a heteroatom‐centered radical to form a Lewis acid–base complex, further providing an alkyl radical by β‐fragmentation . The autoxidation of organoboranes was a classic example to give an alkyl radical, which made trialkylborane as a universal radical initiator under a wide range of reaction temperature and solvents …”

“…The mechanism of trialkylborane autoxidation was not yet fully understood, but several important steps shown in Scheme are generally accepted . In the initiation process, trialkylborane reacted with triplet oxygen in a homolytic substitution (S H 2) reaction to generate peroxyboryl radical and alkyl radical.…”

“…14 The autoxidation of organoboranes was a classic example to give an alkyl radical, which made trialkylborane as a universal radical initiator under a wide range of reaction temperature and solvents. 4,15 The utilization of boron reagents in organic radical chemistry was comprehensively reviewed. 4 Concerning the polymer chemistry, the organoborane/oxygen system as radical initiators in radical polymerizations was also nicely summarized.…”