Borinate radicals

Most of the living radical polymerizations using organic radicals as regulating persistent species involved nitroxides. Exceptions are triphenylmethyl and other carbon-centered radicals in the early work of Otsu and Braun.24,25 More recently, Chung showed that borinate radicals 10 formed by the thermal cleavage of in situ generated alkyl boryl peroxides (Scheme 31) can be employed to control methacrylate... 

The alkoxy radicals are very reactive and initiate radical polymerizations readily On the other hand, the borinate radicals are stabilized by the empty p-orbitals of boron through back-donating electron density and are too stable to initiate polymerizations. During the polymerization, the borinate radicals may form weak and reversible bonds with the growing chains. 

The pending stable polymeric radicals associated with dormant borinate radicals in the SPS copolymer (111) are very useful in the preparation of SPS graft copolymers. The alkoxyl radical (C-O ) is active in initiating polymerization of functional monomers. On the other hand, the borinate radical (B-O ) is too stable to initiate polymerization due to the back donating of electrons to... 

The peroxide 179 dissociates in the presence of a monomer giving rise to alkoxyl (CO-) and borinate (BO-) radicals, but the latter are believed to be too stable to initiate polymerization. It should be mentioned that the molecular weight continuously increases throughout the process implying the pseudo-living mechanism for chain growth. After the completion of the process borane residue is completely oxidized into diol <2004MM6260>. Thus, the 8-boraindane molecule not only initiates the polymerization, but also is a precursor to two functionalities in the polymer chain. 

Four studies of the thermal decomposition of borine carbonyl have been made1-4. In the absence of a radical scavenger, Burg1 proposed a two-step mechanism... 

The oxidation of cholesteryl esters and low-density lipoproteins by free radicals has been reviewed.228 The use of bis(pentafluorophenyl)borinic acid as a strong Lewis acid allows efficient Oppenauer oxidation of allylic and benzylic alcohols using Bu CHO as oxidant.229 Saturated alcohols were only slowly oxidized and this allowed selective conversion of allylic alcohols in the presence of saturated alcohols. 

When, however, diborane passes with hydrogen through a tube at 200-250° the stable pentaborane, B Hg, is obtained. Kinetic studies suggest the changes involve a radical mechanism with perhaps borine, BH3, as an intermediate, though direct evidence of this entity is wanting. 

Chung and coworkers have reported on the use of stable borinate or boroxyl radicals e.g. 114) to mediate radical polymerization. " Methacrylates (MMA) and acrylates (trifluoroethyl acrylate) have been polymerized at ambient temperature to yield polymers with relatively narrow molecular weight distributions.The method has been used to prepare block copolymers and polyolefin graft copolymers,... 

Homolytic cleavage of the carbon-boron bond of a trialkylborane can be promoted by oxygen. The so-formed alkyl radical can be used in synthesis (for radical reactions, see Section 4.1). Indeed, triethylborane in air can be used to generate radicals from precursors such as alkyl iodides or selenides. Hydroboration followed by addition of an a,(3-unsaturated aldehyde or ketone leads to transfer of an alkyl group from the boron atom via an alkyl radical intermediate. The reaction takes place by addition of the alkyl radical to the conjugated system to form an enol borinate, hydrolysis of which gives the aldehyde or ketone product (5.37). 

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