Radicals halogen bridging

Some of the steps in the above sequence of reactions are reduction steps in which the transition metal is reduced to a low valency state possessing unfilled ligand sites. The reduction steps are very important as the low-valency transition metal species are believed to be the real catalysts or precursors of real catalysts. For heterogeneous catalysts, the reactions are, in fact, more complicated than those shown above. Radicals formed in these reactions may be removed by different processes such as combination, disproportionation, or reaction with solvent. Unlike heterogeneous catalysts, the soluble catalysts appear to have well defined structures. For example, the soluble catalyst system that is obtained by the reaction of triethyl aluminum and bis(cyclopentadienyl)titanium dichloride is known by elemental and X-ray analysis to have a halogen-bridged structure (I) ... 

Unusual behavior results from d7r-p7r interactions between open-shell metal ions and organic radicals as illustrated by the family of 3-D metals [Cu(R R -DCNQI)2] (DCNQI = 7V,A -dicyanoquinonediimine) (Figure 14). The halogen-bridged binuclear-metal mixed-valence complex, [Pt2(dta)4l] (dta = dithioacetate) (1-D linear chain Scheme 5a), exhibits metallic conduction above room temperature, representing the first example of a metallic halogen-bridged 1-D transition-metal complex. ... 

Bromination of an optically active form of the corresponding chloro compound (l-chloro-2-methylbutane) also results in an optically active product, and retention of configuration. It may be that an actual bridged radical is formed, but a somewhat less concrete interaction seems more likely, as halogenation with the more reactive chlorine is found to lead wholly to racemisation. 

The simple most systems displaying the structural motive described in Scheme 1 carry a halogen atom in /3-position to the radical center. An early controversy arising from stereochemical experiments deals with the equilibrium structure of these types of radicals.5,6 The stereochemical control observed in some of these reactions suggests that the halogen is either asymmetrically or symmetrically bridging the radical center, in particular if X = Br or I (Scheme 2). 

Halogen and thiol migration, and radical bridging 29 Diradicals in the vinylcyclopropane rearrangement 30 Trimethylenemethane 30... 

The 17e metal-based radical Ta 3(CO)4(dppe) was formed via hydrogen atom extraction from TaIH(CO)4(dppe).659 In solution, this radical abstacted halogen atoms from many organic halides RX to give TaIX(CO)4(dppe). Ta°(CO)4(dppe) existed in solution as an equilibrium mixture of monomer and dimer [(dppe)(CO)3Ta](/i2-CO)2. The latter is the form stable in the solid state. While no crystal structures are available, DFT calculations indicated a stable pseudo-octahedral stereochemistry for the monomer and several possible (/i2-CO)2 structures for the dimer.656 Each of the latter featured linear, semi-bridging carbonyls supporting a weak, delocalized Ta—Ta interaction. 

The issue of the role of bridged radicals in the stereochemistry of halogenation has recently been examined computationally and a new interpretation offered. The structure, rotational barriers, and for halogen atom abstraction for P-haloethyl radicals were studied. For the reactions where X=C1 or Br, the halogen atom abstraction reaction shows a preference for a trans TS. 

Single-electron oxidation of the metalloporphyrins 9 (e.g. by halogens or electrochemically) leads to the formation of radical cations. These undergo addition with nucleophiles at a methine bridge and, after acid demetalation, yield monosubstituted porphyrins 10 [18] ... 

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