Triethylborane (Et3B)

Abstract

(A) Sibi and co-workers used triethylborane as the radical initiator in their enantioselective syntheses of butyrolactone natural products. Lewis-acid mediated radical addition to differentially protected fumarate 1 gave addition product 2 in 80% yield as a single diastereoisomer. This was further elaborated to afford the dibenzylbutyrolactone lignan (-)-arctigenin. Similar chemistry was used to synthesise the butyrolactone natural products (-)-enterolactone, (-)-isoarctigenin, (-)-nephrosteranic acid and (-)-roccellaric acid. 2 (B) Malacria and co-workers used triethylborane/O2 as the radical initiator in their tin-free method for the intramolecular addition of alkyl radicals to aldehydes and ketones.5 Efficient cyclisation of ω-iodoaldehydes was accomplished with 10 equivalents of triethylborane. ω-Iodoketones required 20 equivalents of triethylborane to ensure a good yield of the desired product. The use of triethylborane as a radical initiator reverses the selectivity observed in the cyclisation of enal precursors with AIBN/Bu3SnH, with cyclisation onto the carbonyl carbon preferred with triethylborane. This is attributed to the Lewis acidity of the borane. (C) Triethylborane has been used in the radical hydroindation of alkynes and alkenes.9 Alkenylindiums generated by this means can be employed in a one-pot cross-coupling with aryl halides or trapped with various electrophiles. The hydroindation of alkynes and subsequent palladium-catalysed cross-coupling proceeds with high (Z)-selectivity. The (Z)-alkenylindiums thus formed can also be trapped with various electrophiles, such as water and I 2, with retention of the alkenylindium stereochemistry. Hydrogallation proceeds with lower (Z)-selectivity. (D) Chandrasekhar and co-workers reported the first non-metal mediated tandem Michael-aldol reaction for the synthesis of α-alkyl-β-hydroxy ketone derivatives using triethylborane.15 (E) Tamaru and co-workers have used triethylborane as an efficient promoter of the palladium-catalysed allylation of active methylene compounds with unactivated allylic alcohols.16 The primary role of the borane is to activate the C-O bond of the allylic alcohol towards oxidative addition by the Pd0 species. (F) Tomioka and co-workers have investigated the initiator-dependent chemoselective addition of THF radical to aldehydes and imines in a three component reaction. Triethylborane preferentially afforded the THF adduct of the aldehyde. The use of dimethyl-zinc as initiator preferentially gave the THF adduct of the corresponding imine. The reason for this chemoselectivity is not explained. 10.