sp²–sp³diboranes: astounding structural variability and mild sources of nucleophilic boron for organic synthesis

Abstract

Despite the widespread use of organoborane reagents in organic synthesis and catalysis, a major challenge still remains: very few boron-centered nucleophiles exist for the direct construction of B-C bonds. Perhaps the most promising emerging solution to this problem is the use of sp(2)-sp(3) diboranes, in which one boron atom of a conventional diborane(4) is quaternised by either a neutral or anionic nucleophile. These compounds, either isolated or generated in situ, serve as relatively mild and convenient sources of the boryl anion [BR2](-) for use in organic synthesis and have already proven their efficacy in metal-free as well as metal-catalysed borylation reactions. This Feature article documents the history of sp(2)-sp(3) diborane synthesis, their properties and surprising structural variability, and their burgeoning utility in organic synthesis.

Easy access to nucleophilic boron through diborane to magnesium boryl metathesis

“…Both of these factors limit the broader synthetic utility of such anionic boron species. A more operable approach to the realization of nucleophilic boron has been derived from the quaternisation of one of the three-coordinate boron centres of a diborane(4) molecule by a neutral or anionic nucleophile (for example, 5 and 6 )434445. Although the resultant adducts may act as viable surrogates for boron nucleophiles under (metal- and metal-free) catalytic conditions, these processes do not necessarily take place through the explicit generation of anionic boryl derivatives.…”

Homolytic Cleavage of a B−B Bond by the Cooperative Catalysis of Two Lewis Bases: Computational Design and Experimental Verification

“…[2] Many transition-metal complexes are known to be able to effectively cleave the B À Bb ond of diborane and catalyze the borylation or diboration of unsaturated organic substrates. [5] Recently,t he Lewis adducts of sp 2 -sp 3 diborane compounds have been reported for the borylation or diboration of unsaturated substrates. [5] Recently,t he Lewis adducts of sp 2 -sp 3 diborane compounds have been reported for the borylation or diboration of unsaturated substrates.…”

“…[3,4] On the other hand, transitionmetal-free catalytic systems for activating the BÀB bond of diborane compounds have also attracted much attention. [5][6][7][8][9] It has been shown that diborane compounds became asource of nucleophilic boryl moiety,w hen one of the boron atoms of the diborane was tetracoordinated to as trong base (Scheme 1a). [5][6][7][8][9] It has been shown that diborane compounds became asource of nucleophilic boryl moiety,w hen one of the boron atoms of the diborane was tetracoordinated to as trong base (Scheme 1a).…”

“…When B 2 (pin) 2 (2)w as added to aT HF solution of 4-cyanopyridine,aspectrum at g = 2.00319 (298 K) was obtained, as shown in Figure 2. Interestingly,i nt he chemistry of frustrated Lewis pairs (FLP) [15] or chemistry containing Lewis adduct of sp 2 -sp 3 diboranes, [5][6][7][8][9] the Lewis bases are always regarded as an electron donor. Interestingly,i nt he chemistry of frustrated Lewis pairs (FLP) [15] or chemistry containing Lewis adduct of sp 2 -sp 3 diboranes, [5][6][7][8][9] the Lewis bases are always regarded as an electron donor.…”

A mild carbon–boron bond formation from diaryliodonium salts

“…This approach is reminiscent of previous alkoxide base mediated reactions developed by some of us 18, 19 and others. 20.21 A subsequent screening between various diphenyliodonium salts 1a-d and bis(pinacolato)diboron (2a) was undertaken and for the acetate derivative 1d confirmed the correctness of the initial hypothesis (Scheme 2). In agreement with previous experience, 18,19 methanol was identified as the best solvent.…”