Beiträge zur Chemie des Bors, 209. 1,2‐Diboran(4)diyl‐ und 1,3‐Triboran(5)diyl‐ditosylate und ‐bis(triflate): Elektronenpräzise Polyboran‐Derivate mit aktiven Austrittsgruppen

Abstract

Contributions to the Chemistry of Boron, 209. — 1,2‐Diborane(4)diyl and 1,3‐Triborane(5)diyl Ditosylate and Bis(triflate): Electron‐Precise Polyborane Derivatives with Active Leaving Groups Introduction of the tosylate or triflate group into dimethylamino polyboranes gives electron‐precise polyborane derivatives of high reactivity. Five representatives 1 – 4 of these types of compounds are reported. The RSO3 groups strengthen the B – N bond while they are good leaving groups as shown by alkylation reactions when using LiR compounds. The X‐ray structure of B2(NMe2)2(tos)2 (1) reveals C2 symmetry, and this also holds for B2(NMe2)2(CCPh)2 (7). While the two BNO planes in 1 include an angle of 80.8°, the two BNC planes in 7 are closer to orthogonality (84.6°). In contrast, the bis(9‐fluorenyl)diborane 10 reveals different structures in solution and the solid state. Experimental results correspond fairly well with MNDO calculations on model compounds B2(NH2)2X2, particularly with B – B and B – N bond lengths and conformations. They also suggest that nucleophilic substitutions at 1 – 4 may proceed by a dissoziative process.

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

“…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.…”

“…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. 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.…”

“…The lithium monoborate, containing the anion [tBu 2 B(fl)] Ϫ ( [4] Ϫ ) is the main product of the reaction between tBu 2 B(fl-H) (4-H) and one equivalent of LiNtBu-(SiMe 3 ) in tetrahydrofuran solution [Scheme 2, (5)]. The 11 B-NMR spectrum of the reaction mixture not only shows a signal due to this anion (δ ϭ 77.9), but further signals at δ( 11 B) ϭ 87.9 [tBu 2 B(fl-H)] and 54.0 [possibly tBu 2 BϪNtBu(SiMe 3 ), 8%] and minor signals in the region δ( 11 B) ϭ ϩ10.0 to 0.0 (tetracoordinated borates, 5%).…”

“…The 11 B-NMR spectrum of the reaction mixture not only shows a signal due to this anion (δ ϭ 77.9), but further signals at δ( 11 B) ϭ 87.9 [tBu 2 B(fl-H)] and 54.0 [possibly tBu 2 BϪNtBu(SiMe 3 ), 8%] and minor signals in the region δ( 11 B) ϭ ϩ10.0 to 0.0 (tetracoordinated borates, 5%). The lithium salt [Li(thf) 2 ] [4] was recovered as a 4:1 mixture with fluorenyllithium. All attempts to separate these compounds by crystallization were unsuccessful.…”

“…We have recently reported on the synthesis, molecular structures, [2] and synthetic potential [3] of the alkali and alkaline earth metal salts of bis(dimethylamino)difluorenyldiborane(4), [4] (Me 2 N) 2 B 2 (fl-H) 2 (fl-H ϭ fluorenyl ϭ C 13 H 9 ). A common feature of these compounds is a bent metallocene-type structure, in which the metal cations (Li, K, or Ca) are complexed by the two fluorenylidene ligands of the [(Me 2 N) 2 B 2 (fl) 2 ] 2Ϫ unit.…”