Organometallic free radical

Since the g-values of organic and organometallic free radicals are usually in the range 1.9-2.1, the free electron value is a good starting point for describing the experiment. 

Electron spin resonance spectroscopy has made a considerable impact on the study of organometallic free radical chemistry. Four significant developments may be identified (i) the introduction of steady-state photoly-... 

Reviews Relevant to Organometallic Free Radical Chemistry... 

Laser flash photolysis of [CpM(CO>3]2 (M = W, Mb, and Cr) provides a convenient source of CpM(CO)3, an organometallic free radical with 17 valence electrons. It is a transient and highly reactive species. Depending on the circumstances and the other reagents present, the radical will dimerize, undergo halogen and hydrogen atom abstraction reactions, and electron transfer reactions. With tetramethyl-phenylenediamine, there is a cyclic process of electron transfer steps, the net result of which is the catalyzed disproportionation of the metal radical. 

To achieve stability, these reactive entities may undergo any number of reactions. The purpose here is to enumerate what these reactions are and the mechanisms by which they occur. Moreover, an attempt is made to show how the organometallic free radicals are analogous to organic radicals, which are in general more familiar. The 17e organometallic species are isolobal with alkyl radicals which are 7e spedes, also one electron shy of a closed shell. 

Organic and organometallic free radicals structure, kinetics and mechanism... 

In Section 6.21 we listed three main methods for polymerizing alkenes cationic, free-radical, and coordination polymerization. In Section 7.15 we extended our knowledge of polymers to their stereochemical aspects by noting that although free-radical polymerization of propene gives atactic polypropylene, coordination polymerization produces a stereoregulai polymer with superior physical properties. Because the catalysts responsible for coordination polymerization ar e organometallic compounds, we aie now in a position to examine coordination polymerization in more detail, especially with respect to how the catalyst works. 

Photoinitiation of Free Radical Polymerization by Organometallic Compounds... 

Two types of organometallic photoinitiators for free radical vinyl polymerization are considered (1) transi-... 

Beside these free radical reactions of sulfur dioxide, its electrophilic reactions generating sulfinates with organometallic compounds453,454 or sulfinic acids with arenes under Friedel-Crafts conditions455 are well known. To complete these three-component syntheses, the sulfinates prepared first are transformed to sulfones by reactions with appropriate electrophiles, discussed earlier in this chapter, i.e. equation 82. 

It might be mentioned that matters are much simpler for organometallic compounds with less-polar bonds. Thus Et2Hg and EtHgCl are both definite compounds, the former is a liquid and the latter is a solid. Organocalcium reagents are also known, and they are formed from alkyl halides via a single electron transfer (SET) mechanism with free-radical intermediates. "... 

It is unlikely that a single mechanism suffices to cover all conversions of organometallic compounds to alkyl halides. In a number of cases the reaction has been shown to involve inversion of configuration (see p. 762), indicating an Se2 (back) mechanism, while in other cases retention of configuration has been shown, implicating an Se2 (front) or SeI mechanism. In still other cases, complete loss of configuration as well as other evidence have demonstrated the presence of a free-radical mechanism. ... 

Investigations of the cobalt-carbon bond energies in organometallic cobalt porphyrins continue to attract interest, originally because of their similarity to coenzyme B 2, and more recently because of their role in the catalysis of free radical... 

A discussion of ligand exchange reactions of organometallic compounds associated with oxidation-reduction processes leading to free-radical formation will be found in Volume 14 (Free-radical polymerization). 

The Ir11 dimer [Ir(oep)]2 (oep = octaethylporphyrin) has been prepared in low yield by photolysis of (oep)IrCH3 in C6D6.473 This preparation has been improved by Chan et al.474, as shown in Reaction Scheme 24, where TEMPO = 2,2,6,6-tetramethyl-l-piperidinyloxy, free radical. The dimer undergoes several organometallic reactions, including oxidative addition of alkyl C 11 bonds and alkene insertions.475... 

The Period 1930-60. Following the work of Paneth (J ), there was a long period of active work on free radicals, mostly of importance in organic chemistry but with some relevance to organometallic chemistry. The radicals were generated by thermal, photochemical or discharge... 

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