Mechanistic aspects of the isomerization of Z-vinylic tellurides double bonds in the synthesis of potassium Z-vinyltrifluoroborate salts

Abstract

Through direct transmetalation reaction of Z-vinylic tellurides with nBuLi was observed the unexpected isomerization of double bonds leading to potassium E-vinyltrifluoroborates salts in low to moderate yields. Using EPR spin trapping experiments the radical species that promoted the stereoinversion of Z-vinylic organometallic species during the preparation of potassium vinyltrifluoroborate salts was identified. The experiments support the proposed mechanism, which is based on the homolytic cleavage of the TenBu bond.

Synthesis of cis-vinyltrimethylstannanes and cis-vinylpinacolboronates in a two-step highly regio and stereoselective process

“…Recently, a mechanistic study of the synthesis of potassium trans-vinyltrifluoroborate salts, from cis-vinyltellurides via cisvinylboronates, has been reported by Stefani et al 30 In view of the historically characteristic cis-selectivity of the tellurium-lithium exchange, a free radical pathway has been proposed to account for the trans-selectivity of this system (Scheme 3). It was suggested that the loss of stereochemistry was caused by a n-butyl radical 11, generated from the homolytic cleavage of dibutyl telluride 7, caused by lithium species present in the reaction medium.…”

“…A variety of alkynes (i.e., 1) ( Table 1, [25][26][27][28][29][30][31][32][33][34] were treated with dibutyl ditelluride 2 and sodium borohydride under standard conditions 21,22 to give cis-vinyltellurides (i.e., 3) in 58-94% yield (Table 1, 35-43) (Scheme 1). For all conjugated alkynes, the cis-vinyltellurides were obtained with excellent regio-and stereo-selectivity.…”

Method in the Madness - Methodology from Total Synthesis

“…A report that appeared while we were investigating this area described the synthesis of trans-vinyltrifluoroborate salts from cis-vinyltellurides via the intermediacy of cisvinylboronates. 11 The authors attributed this change in alkene geometry to the heterolysis of the cis-vinylboronate to generate a vinyl radical, which isomerised and was subsequently trapped as the trans-trifluoroborate salt. We questioned whether a similar mechanism was in play in our system, resulting in the regeneration of the vinyltelluride.…”