ChemInform Abstract: A Highly Regioselective Epoxide Ring Opening Reaction by Triorganylborane via Electrochemical Method.

Abstract

We recently described that triorganylboranes can be used as organyl group transfer agents to the carbonyl compounds by implementing an electrochemical procedure that uses copper as a sacrificial anode. Secondary and tertiary alcohols are readily obtained in good yields by performing the electrochemical reaction in an undivided cell with a platinum electrode and a copper sacrificial anode. This methodology has a great attention since of its enormous potential synthetic utility. The reaction can offer the wide scopes for the further application to various functional groups similar to Grignard reagents derived from alkyl halides. Therefore, we now report the epoxide ring opening reaction by the electrochemical organyl transfer of triorganylboranes at constant negative reduction potential (-2.6 V~-3.0 V) in an undivided electrolytic cell using a sacrificial metallic anode. Especially, of particular interest is the regioselectivity for the epoxide ring opening reaction by electrochemical tool. In addition, these electrochemical epoxide ring opening reactions were compared with those of corresponding Grignard reactions. For most purposes, triorganylboranes, such as tributylborane and tri-sec-butylborane, adapted in this experiment would be derived from hydroboration of alkenes, however triphenylborane could not be prepared by hydroboration in this case Grignard reagent would be utilized. First of all, a variety of degassed solvents, such as DMF (entry 5), THF (entry 6), Py (entry 7), and acetonitrile (AN) (entry 8), sacrificial anodes, such as Cu (entry 5), Mg (entry 9), Al (entry 10), Zn (entry 11), and Fe (entry 12), and electrolytes, such as tetrabutylammonium tetrafluoroborate (Bu4NBF4) (entry 5), tetrabutylammonium perchlorate (Bu4NClO4) (entry 13), tetrabutylammonium hexafluorophosphate (Bu4NPF6) (entry 14), tetrabutylammonium chloride (Bu4NCl) (entry 15), tetrabutylammonium bromide (Bu4NBr) (entry 16), tetrabutylammonium iodide (Bu4NI) (entry 17), and lithium bromide (LiBr) (entry 18) with trisec-butylborane and styrene oxide were examined to find the combination which afforded the best yield. As a result, it revealed DMF as a solvent, Cu sacrificial anode and Bu4NBF4 as a supporting electrolyte gave the best yield to the corresponding alcohol (entry 5). Additionally, we found that the reaction time decreased with increasing their stainless steel cathode surface area from 4 h at 1 cm (entry 19) to 1.5 h at 4 cm (entry 5) with increasing of yield from 74% to 89%, and by addition of bases, such as potassium tert-butoxide (entry 5), sodium ethoxide (entry 20), sodium hydroxide (entry 21), and sodium 2,6-di-tert-butyl-4-methylphenoxide (entry 22), especially among them, by addition of potassium tert-butoxide the efficiency significantly enhanced, resulting in the reduction of reaction potential from -3.2 V to -2.6 V. Moreover, it revealed that yields of electrochemical epoxide ring opening reactions were markedly influenced by addition of Lewis acid, such as BF3 · etherate, and the electrochemical reaction of 2b by 1b in the presence of BF3 · etherate afforded the exclusively corresponding less substituted alcohol in 89% yield (entry 5) whereas in absence of BF3 · etherate gave in 57% yield (entry 23) under same circumstance. We also found that there was no significant difference in yield between Pt and stainless steel cathode in the electrochemical epoxide ring opening reaction with 1b, however, it should be noticed that the regioselectivity significantly depends upon the choice of cathode; Stainless steel furnished the exclusive product 3 with extremely high regioselectivity (entry 5), whereas Pt gave two products, 3 and 4, with somewhat low regioselectivity (entry 24) which was,