Triboracyclobutanides: Four‐Membered Two‐Electron Aromatic Compounds with Fluctuating Skeletal Bonds

Abstract

Among the series of isoelectronic four-membered, twoelectron aromatic compounds 1±3 (Scheme 1), to date derivatives of the dicationic and uncharged skeletons 1 [1] and 2, [2±4] respectively, have been realized experimentally. The barriers of their ring inversions, which are a measure of the stabilization of the folded over the planar rings, have not been determined experimentally for any example. We report herein the synthesis, crystal structure, and the ring inversion barrier of the anion 3 a (Scheme 2), and show, based on line shape analyses of its temperature-dependent NMR spectra, that its skeletal bonds fluctuate via a distorted triboratetrahedrane anion 6 a (see Scheme 3) as transition state. B3LYP computations [5] on model molecules support this proposal. The unsymmetrically substituted triboracyclobutanide 3 a is obtained by reaction of lithium in diethyl ether with the symmetrically substituted derivative 4, which is accessible from 5 [6] and dichloro(trimethylsilylmethyl)borane (Scheme 2). Attempts to detect the symmetrically substituted compound 3 b by NMR spectroscopic monitoring of the reaction of 4 with lithium naphthalenide in [D 8 ]THF at À80 8C proved unsuccessful. The constitutions of 3 a and 4 are in accord with their NMR data ( Table 1) and are confirmed unambiguously by their crystal structure determinations [7] ( Figure 1).

trans‐Selective Insertional Dihydroboration of a cis‐Diborene: Synthesis of Linear sp3‐sp2‐sp3‐Triboranes and Subsequent Cationization

“…These are significantly downfield-shifted from 2-Pyr owing to the electron-withdrawing nature of the aryl versus the electrondonating nature of the amino substituent. Comparison with other literature-known amino-and aryl(diboryl)boranes (d(R 2 NB(BX 2 ) 2 ) % 50-62 ppm, [2,17] d(ArB(BX 2 ) 2 ) % 70-85 ppm) [18] shows that the central boron nuclei of 2-R are unusually deshielded, that is, particularly electron-poor. This was confirmed by density functional theory (DFT) calculations at the OLYP/TZ2P level of theory on 2-Mes in the gas phase, which gave negative Hirshfeld charges of À0.093 for B1 and B3 and a positive charge of 0.050 for B2 ( Figure 3).…”

A practical synthesis of novel α-dibromoalkyl- and trimethylsilylmethyl-aminoboranes and derivatives

“…Finally, compound 7 was obtained through reactions between compounds 6 and 5 and dry methanol. Since the boron atom of compound 7 is more electropositive, it can easily react with organoboron anions, and may be useful for preparing borepolyhedrals [7].…”

“…Iminium salts such as (Me 2 NCHBrB(NMe 2 ) 2 ) are stable carbocations with boryl substituents and could be used as starting reagents for aminoborylcarbenes [6]. Compound 7 is a borelectrophile that is especially useful in the conversion of cycloborane anions to borpolyhedral structures [7]. We studied these due to the importance of compounds 4 and 7, and obtained them under convenient reaction conditions.…”

Trans‐selektive Dihydroborierung eines cis‐Diborens durch Insertion: Synthese eines linearen sp3‐sp2‐sp3‐Triborans und anschließende Kationisierung

“…Die signifikante Tieffeldverschiebung der Signale im Vergleich zu 2-Pyr ist auf die elektronenziehenden Eigenschaften der Arylreste zurückzuführen, während die Aminogruppe einen elektronenschiebenden Effekt ausübt. Durch den Vergleich mit anderen literaturbekannten Amino-und Aryl(diboryl)boranen d(R 2 NB(BX 2 ) 2 ) % 50-62 ppm, [2,17] und d(ArB(BX 2 ) 2 ) % 70-80 ppm) [18] wird deutlich, dass die zentralen Borkerne in 2-R eine ungewçhnlich starke Entschirmung erfahren, also besonders elektronenarm sind. Dieser Befund wurde durch Dichtefunktionaltheorie-(DFT)-Rechnungen auf dem OLYP/TZ2P-Niveau anhand von 2-Mes in der Gasphase bestätigt, wobei negative Hirshfeld-Ladungen von À0.093 für B1 und B3 und eine positive Ladung von 0.050 für B2 erhalten wurden (siehe Abbildung 3).…”

“…Die Existenz zweier borgebundener Wasserstoffatome wurde mittels IR-Banden in einem Bereich von 2160 bis 2200 cm À1 bestätigt, die terminalen B-H-Schwingungen zugeordnet werden kçnnen. Hierbei handelt es sich um die ersten sp 3 -sp 2 -sp 3hybridisierten Triborane, während frühere Beispiele auf sp 2sp 2 -sp 2 - [2,17,18] oder sp 3 -sp 3 -sp 2 -Hybridisierung beschränkt sind. [11,13] Im Gegensatz zu Nçths Tris(aminoboryl)[3]ferrocenophan in dem das zentrale Boratom aus der B1B3Fe-Ebene herausgedreht ist, [20] liegen das Eisen-und alle drei Boratome von 2-R in einer Ebene.…”