Radical triethyl borane

Synthesis of 12- and 13-membered sulfur-containing lactones by homolytic macrocyclization of mercaptoacetic esters and alkynes has been investigated [95S307]. Reaction of the mercaptoester 245 with alkynes using triethyl borane radical initiation gave the macrolactones 247 and 248 in low yield [95TL2293], Remote asymmetric induction is observed during the cyclization. 

This substitutes very well for most tin hydrides in many reactions. Roberts, on the other hand, has demonstrated that thiols act as polarity reversal catalysts for hydrogen-atom transfer from organosilanes to alkyl radicals.185 The use of triethyl borane and oxygen as radical initiators166 has allowed the reaction to be carried out at much lower temperatures and hence mechanistic elucidations have been possible.187... 

Radical reactions of substrate 27 were initially conducted at low temperature in toluene with triethyl borane as initiator and tributylstannane as reducing agent. Under these conditions no conversion was detected. Increasing the temperature to ambient temperature led to 99% yield and a diastereomeric ratio (d.r.) of 47/53 in favour of the cis compound cis-29. Reactions in the presence of the chiral complexing agent... 

The spin trapping method has also been applied to the detection and identification of intermediate primary radicals produced from initiator systems such as triethyl-borane/oxygen benzoyl peroxide/N,N-dimethylaniline and N-chlorosuccin-imide/p-toluenesulfonic acid , and provided important information on the generation of free radicals in initiator systems. 

Polymerization may be brought about both by anionic and free radical methods. Typical anionic polymerizations are carried out at -78°C in anhydrous diethyl ether with dimethyl formamide as initiator. Number average molecular weights for the polymer are typically in the range 300000-400000. Free radical polymerization is carried out effectively using triethyl borane/oxygen as the initiator. Similar temperatures to those used in anionic polymerization are preferred. 

A synthetic method for the introduction of an ethynyl group in cyclic structures is based on an intramolecular silicon-tethered radical cyclization with iodine transfer (Scheme 25.8). For example, starting from alkyne 10, a triethyl borane-initiated process allows a radical formation and cyclization to give the alkenyl radical 11. An iodine atom is abstracted from the starting material to give the alkenyl iodide 12, thus, propagating the chain. Treatment of 12 with tetra-/j-butylammonium fluoride (TBAF) results in elimination to furnish 13 in good yields. 

Beilstein Handbook Reference) Borane, triethyl- Boron ethyl Boron triethyl BRN 1731462 EINECS 202-620-9 HSDB 897 Triethylborane Triethylborine Triethylboron. Used as an initiator of radical reactions. Liquid mp = -93° bp = 95° d = 0.700 ... 

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