Radical-based reactions

A very useful radical-based reaction has been developed that can be applied to alcohols. As seen in chapter 4, conversion of an alcohol such as 73 to a thionocarbonate (74), followed by treatment with tributyltin hydride under radical conditions, gives cleavage to the C—O bond to give, in this case, 75. This example is take from Beak s synthesis of (-)-dihydropinidine, and is an example of a transformation called the Barton deoxygenation or the Barton-McCombie reaction. ... 

SCHEME 10 The unexpected radical-based reactions of the tricyclic iodide 57—formation of the D-ring galanthamine isomer 63. 

D.W. Jenkins, S.M. Hudson, Review of vinyl graft copolymerization featuring recent advances toward controlled radical-based reactions and illustrated with chitin/chitosan trunk polymers, Chem Rev. 101 (11) (2001) 3245-3274. 

SAM-dependent iron sulfur proteins can catalyze a great variety of radical-based reactions that have in common the production of 5 -deoxy-adenosine, methionine and an oxidized protein or substrate radical (Fig. 11.10) [80,81]. AH these enzymes contain an unusual [Fc4S4] cluster and the CxxxCxxC sequence motif but, otherwise, share very low amino add sequence homology [82]. In most cases, the cleavage of SAM is irreversible and stoichiometric but some examples are known... 

All wood chemical components (lignin, cellulose, hemicellulose, and extractives), which contain internal chemical groups such as carboxyl, carbonyl, aldehyde, unsaturated double bonds, phenolic hydroxyl, and external entities such as fats, waxes, and metal ions, are capable of absorbing sunlight or UV light. The absorbed energy can cause the dissociation of bonds in the molecules of the wood constituents. Similar to thermoplastics, the photodegradation of wood also involves a serials of radical-based reactions. 

The effects of an added solvent on radical-based reactions are generally quite small but it has been observed that the rates of copper-based ATRP reactions increase in the presence of polar solvents and water. ... 

Molecular oxygen, O2, readily reacts with free radicals, and since free radicals play a dominant role in the radiolysis of polymers, O2 can significantly affect radiation-induced chemical alterations. For instance, it enhances the radiation-induced degradation of most polymers. Linear polymers, including polyethylene, polypropylene, polystyrene and poly(vinyl chloride), that crosshnk in the absence of oxygen undergo predominantly main-chain scission in its presence. As a typical example, a free-radical-based reaction mechanism proposed for the oxidative degradation of polyethylene is shown in Scheme 5.16. 

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