A redox-active diborane platform performs C(sp³)–H activation and nucleophilic substitution reactions

Abstract

Targeted C(sp3)–H activation or nucleophilic substitution reactions have been achieved through the interaction of a diborane dianion with haloalkanes.

Reactivity of a Tetra(o‐tolyl)diborane(4) Dianion as a Diarylboryl Anion Equivalent

“…In the last decades,m any B = Bd ouble-bond species were synthesized. [3a,b] Theh ydrogen-and carbon-substituted diborane (4) dianion C was also synthesized by areduction of the Eind-substituted boron difluoride. Thes ynthesis of aminosubstituted diborane(4) dianions A and B was reported with stabilization through coordination of the amino groups to the lithium cation.…”

“…These compounds can be generally classified as neutral diborenes [2] or dianionic diboranes (4), [3] in which the boron atom has an oxidation state of one.T he number of the latter compounds was relatively limited in comparison to that of the former compounds ( Figure 1a). [3c] All-carbon-substituted diborane(4) dianions D and E were synthesized by ar eduction of tetraaryldiborane (4) [3d] and ah ydrogenbridged diarylborane dimer,r espectively. [3a,b] Theh ydrogen-and carbon-substituted diborane (4) dianion C was also synthesized by areduction of the Eind-substituted boron difluoride.…”

“…[3c] All-carbon-substituted diborane(4) dianions D and E were synthesized by ar eduction of tetraaryldiborane (4) [3d] and ah ydrogenbridged diarylborane dimer,r espectively. [4] In this transformation, the boron-containing cyclic system in E can be considered as two equivalents of the diarylboryl anion G. [5] Considering the Wanzlick equilibrium between tetraaminoethylene and two equivalents of Nheterocyclic carbene (Figure 1c), [6] one-half of ad iborane (4) dianion having aB = Bdouble bond could be considered to be the synthetic equivalent of boryl anion. Asole exception for the reactivity of diborane(4) dianion was recently reported in the reaction of E with 1,2-dibromoethane to give B-alkylated derivatives F with ar earrangement of the boron-containing six-membered ring to afive-membered ring ( Figure 1b).…”

“…[9] DFT calculations suggested the high reactivity of 1 toward these molecules would originate from al ow-lying LUMO consisting of two vacant p-orbitals on two boron atoms and ar eactive BÀBs ingle bond contributing to the HOMO. [7] Considering as imple structure and scalable synthesis of 1,o ne can expect that the dianion of 1 would have av ariety of reactivity in comparison to other diborane (4) dianions.H erein, we describe the synthesis of tetraaryldiborane(4) dianions by reduction of 1,adetailed study on their photophysical and electrochemical properties,and their reactivity as two equivalents of diarylboryl anion, toward electrophiles. Thelithium salt of diborane(4) dianion 2 was obtained as adark-red solid in 98 %yield by the two-electron reduction of tetra(o-tolyl)diborane(4) 1 with metallic lithium in toluene/ THF (Scheme 1).…”

“…Thel ithium salt 1 crystallized as ac ontact ion pair with two THF molecules and an inversion center located at the midpoint of the two boron atoms.T he lithium cation was coordinated by one THF molecule and two ipso carbon atoms in o-tol groups [Li1ÀC1:2 .244(3) ,L i1ÀC8*:2.277 (3) ]. Thes tructure of the magnesium salt 3 was also ac ontact ion pair and the magnesium cation was bound to B 2 (o-tol) 4 2À anion in h 2fashion. As aresult, all four methyl groups of the o-tol groups pointed to the opposite side of the magnesium cation.…”