Stabilization of Diborane(4) Derivatives by tert‐Butyl Groups: The First Tetraalkyldiborane(4)

Abstract

Boron compounds of type (alkyl)2B—B(alkyl)2, which have been sought for more than 40 years, have been independently synthesized by two research teams almost simultaneously and by the same pathway. Stepwise replacement of the methoxy groups of (1) by alkyl groups gave the tetraalkyldiboranes (4), (5) (RMe), and (6) (R=tBuCh2). Tetra‐tert‐butyldiborane could not be prepared, while the silylborane (7), RSiMe3, could.

Deprotonation ofB,B′-Di-tert-butyl-B,B′-difluorenyldiborane(4) and the Structure of the Di-tert-butyl-difluorenylidenediborate(2−) Anion

“…Indeed, di-tert-butyl-difluorenyldiborane(4) (2-H 2 ) could be readily prepared by the stepwise substitution of the bromine atoms of dibromodi-tert-butyldiborane (4) [7] by the fl Ϫ anion. Unexpectedly, however, MeO substitution of (tBu) 2 B 2 (OMe) 2 , which has been successfully employed for the preparation of the tetraalkyldiborane(4) derivatives (tBu) 2 B 2 (tBu)R (R ϭ Me, [5] CH 2 SiMe 3 , [5] CH 2 CMe 3 [6] ) and (tBu) 2 B 2 (CH 2 CMe 3 ) 2 , [6] did not work in the present case. No reaction occurred between (tBu) 2 B 2 (OMe) 2 and fluorenyllithium.…”

“…This group is known [5,6] to be sufficiently bulky to prevent redox disproportionation, which occurs in diborane (4) compounds that lack adequate electronic shielding. Indeed, di-tert-butyl-difluorenyldiborane(4) (2-H 2 ) could be readily prepared by the stepwise substitution of the bromine atoms of dibromodi-tert-butyldiborane (4) [7] by the fl Ϫ anion.…”

“…The 11 B-NMR resonance of 2-H 2 at δ( 11 B) ϭ 102.5 is indicative of a tetraalkyldiborane(4) compound. [5,6] However, only a single, albeit broad resonance at δ( 11 B) ϭ 92.9 is observed for the two different boron atoms of 3, this chemical shift being intermediate between those of the resonances of the corresponding symmetric species 2-H 2 [δ( 11 B) ϭ 102.5] and (tBu) 2 B 2 Br 2 [δ( 11 B) ϭ 88.0]. [7] The asymmetric structure of 3 is, however, reflected in its 1 H-and 13 C-NMR spectra, which show discrete signals due to the two different tert-butyl groups.…”

Singular Metal Activation of Diboron Compounds

“…Organodiboron compounds, R 2 B-BR 2 , are stable only when sterically demanding R groups, such as t-Bu, CH 2 -t-Bu, and mesityl, are protecting the B atom. [24][25][26] On the contrary, tetra(amino)diborons 21,27,28 and tetra(alkoxy)diborons 21,29-32 show a remarkable stability due to the extensive π-type overlap with the neighboring nitrogen or oxygen atoms which stabilize the boron atom. The concomitant consequences are lower reactivity against unsaturated substrates, but the resulting organoboron compounds become generally air and water stable and amenable to chromatography.…”

Syntheses and Reactions of a Stable 1,2‐Dichloro‐1,2‐diborolane and Aromatic Tetraboranes

“…The mixtures of products obtained obviously contained no compounds featuring a BϪB(SiMe 3 )C moiety, which would result in a 11 B NMR signal for a strongly deshielded boron center. [17] Reactions of 1b with K 2.8 Na in n-pentane at room temperature yielded the nonclassical dichlorotetraborane(6) cis-11 (Scheme 4). [5,18,19] As seen from its classical isomer 11*, there are only six electrons to connect the four centers of the B 4 ring of cis-11.…”