Trialkylboranes chain transfer reactions

Lin W, Niu H, Chung TC, Dong JY (2010) Borane chain transfer reaction in olefin polymerization using trialkylboranes as chain transfer. J Polym Sci A Polym Chem 48 3534... 

Oxidation of star polymethylene 6 affords 3 equiv. of a-hydroxy-co-(4-methoxyphenyl) polymethylene 5 with DP = n. The strict correspondence between stoichiometry and DP in these examples is consistent with a rapid initiation and the absence of termination or chain transfer reactions in the polymerization of ylide 1. The absence of a termination step is reasonable, since the living end of this polymer is a trialkylborane, which is stable under the reaction conditions. Dimethyl sulfoxide (DMSO), a byproduct that accumulates during the reaction does not inhibit the initiator/catalyst. 

Sect. 3.2). However, this approach is limited to the few trialkylboranes that are easily available and cheap since only one of the three alkyl group is transferred. By using a triethylborane as a chain transfer reagent, the reaction could be extended to alkyl iodides as radical precursors. Bertrand [94,114] and Naito [95,97] reported both the use of triethylborane for the tin-free addition of alkyl iodides to imines. A typical example for a tentative of asymmetric addition to a glyoxylate imine is depicted in Scheme 50 (Eq. 50a). More recently additions to isatin imines were reported (Eq. 50b) as well as addition to 2H-aziridine-3-carboxylates by Lemos [100] and Somfai [101] (Eq. 50c). 

In the preceding chapters it was noted that trialkylboranes are useful radical initiators as well as an efficient source of alkyl radicals. Organoboranes can also be used as chain transfer reagents. This approach is used when the direct reaction between the radical precursor and the radical trap cannot proceed (Scheme 6.7). Alkyl radicals generated from the organoboranes are not involved in product formation, but they produce the radicals leading to products. For this purpose, an extra step such as an iodine atom transfer or a... 

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