Recent advances in boron-nitrogen chemistry - II

Nöth, Heinrich; Geisberger, Gilbert; Linti, Gerald; Loderer, Dirk; Rattay, W.; Salzbrenner, E.

doi:10.1351/pac199163030351

Cited by 18 publications

(8 citation statements)

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“…However, the fact that donor groups destabilize the eight-membered ring with respect to the four-membered ring suggests that interesting oligomerization chemistry might be observed in systems such as [(strong donor)BNR] 2 . In this regard, we note the interesting recent syntheses by Meller and co-workers of [(RO)BNR] 2 and those by Nöth and co-workers of [(R 2 N)BNR] 2 …”

Section: Discussionmentioning

confidence: 63%

Dimerization of Diboradiazacyclobutadienes To Form Tetraboratetraazacyclooctatetraenes: Computational Study of Boron−Nitrogen Ring Formation and Ring Opening

and

Gailbreath

2001

Organometallics

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Computational studies of the dimerization of the diboradiazacyclobutadienes (HBNH) 2 and (MeBNMe) 2 to form the tetraboratetraazacyclooctatetraenes (RBNR) 4 suggest that the preferred pathway involves "face-to-face" dimerization of the four-membered ring, followed by asynchronous, one-step scission of the two transannular BN bonds to form the eightmembered monocyclic product. Detection of the proposed intermediates in either system is unlikely, as they lie in shallow potential energy wells. That in certain situations the fourand eight-membered rings establish equilibria is consistent with the comparatively small energy difference between (MeBNMe) 2 and (MeBNMe) 4 .Iminoboranes RBtNR′ prefer to oligomerize rather than react with other molecules. 1-3 Paetzold 4 and coworkers, among others, 5 have examined in detail the relationship between the size of R/R′ and the degree of oligomerization, finding in broad terms an inverse correlation, as one would expect. However, inexplicable subtleties exist, and these are readily seen in the behavior of the iminoborane tetramers, 6 here termed tetraboratetraazacyclooctatetraenes. 7 For example, NMR studies show that equilibria between the iminoborane dimers (diboradiazacyclobutadienes) and the tetramers can occur at or somewhat above room temperature for R ) Me, R′ ) tBu 8 and for R ) R′ ) iPr 9 but not for R ) nPr, R′ ) iPr, R ) R′ ) iBu, or several other combinations. 10 The mechanism by which the equilibria are established remains lightly studied. Paetzold proposed the mechanism shown in Scheme 1. 4 The diboradiazacyclobutadiene 1 dimerizes to the tricyclic "Dewar cyclooctatetraene" analogue 2. This undergoes sequential breakage of the internal transannular BN bonds to form first fused bicyclic 3 and then the product tetraboratetraazacyclooctatetraene 4. All steps proceed reversibly, so that equilibria between the four species can occur. Experimentally, however, only the end points 1 and 4 were observed, suggesting that 2 and 3, if they are intermediates, lie in very shallow potential energy wells.Close inspection of the mechanism suggests several issues meriting computational examination. First, does the diboradiazacyclobutadiene dimerization occur "edgeto-edge", to give a staircase-like tricycle, or "face-to-face", to give a tub-like tricycle? Can these tricycles equilibrate, possibly through decomposition back to the fourmembered rings and then recombination? Second, need the internal bond breaking occur stepwise, or can it occur synchronously (or nearly so)? Third, given that rather strong bonds (ca. 88-90 kcal mol -1 ) 11,12 must break to convert the tricyclic intermediate to the bicyclic intermediate and monocyclic product, why are the proposed intermediates 2 and 3 so difficult to observe? Are the wells they lie in really so shallow?We report here an examination of these questions through calculations of the structures and energetics of the parent perhydrogen and permethyl homologues represented in Scheme 1 and the transition states connecting them. We find that t...

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Section: Discussionmentioning

confidence: 63%

Dimerization of Diboradiazacyclobutadienes To Form Tetraboratetraazacyclooctatetraenes: Computational Study of Boron−Nitrogen Ring Formation and Ring Opening

and

Gailbreath

2001

Organometallics

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“…In contrast to boron clusters exhibiting nonclassical multicentre bondings, classical two-center-two-electron bonds (2c,2e) be- tween boron atoms are relatively rare and well-defined chains with more than two boron atoms are confined to only some few compounds. [37][38][39][40][41][42][43] The problem of an uncontrolled chain length formation becomes evident by obtaining unselective low-yield product mixtures, mostly difficult to separate. [43] One of the more promising synthetic methods to electron precise species thus far has involved diboration or hydroboration of diborenes, or diborynes, as demonstrated by Braunschweig and Kinjo.…”

Section: With a Chain Structurementioning

confidence: 99%

“…[37][38][39][40][41][42][43] The problem of an uncontrolled chain length formation becomes evident by obtaining unselective low-yield product mixtures, mostly difficult to separate. [43] One of the more promising synthetic methods to electron precise species thus far has involved diboration or hydroboration of diborenes, or diborynes, as demonstrated by Braunschweig and Kinjo. [44][45][46] Very recently, Braunschweig et al reported a remarkable cationic planar tetraborane with a hydrogen atom bridging the central B 2 entity (Scheme 5) exhibiting characteristics of a protonated diborene and a nonclassical three-center-two-electron B-H-B bond.…”

Section: With a Chain Structurementioning

confidence: 99%

Chemistry of Dicationic Diboranes

2020

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Cationic monoboranes are commonly associated with elusive and highly electrophilic compounds used in modern organic synthesis. By contrast, only a few cationic diboranes are known and their chemistry comparatively underdeveloped. This review highlights some aspects of these species and their reactivity focusing primarily on especially stable guanidinate‐bridged cations investigated by our group. Contrary to the intuitive presumption, sp2‐hybridized cationic diboranes discussed herein serve not exclusively as Lewis acids, but also as two electron donors capable for reduction of organic π‐acidic substrates. The latter proves them both as versatile synthetic reagents and valuable building blocks for the synthesis of remarkably stable macrocyclic structures with interesting electronic properties. The high tendency for formation of nonclassical multicenter bonding reflects in the dimerization behavior of cationic diborane species presented herein, leading to highly electron‐deficient tetraboranes with unprecedented aromatic structures.

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“…1 The complexity of its allotropic modifications, the borides, the boranes (boron hydrides), the metalloboranes, the carboranes (among them the strongest of the superacids), 2 the metallocarboranes, the boron halides, the borates (boron-oxygen compounds) and, finally, the boron-nitrogen compounds provide examples of the various bonding motifs exhibited by this element. The boron-nitrogen compounds have received considerable attention for many decades because they present a very versatile chemistry, 3,4 and because they bear interesting similarities and dissimilarities with their hydrocarbon counterparts. Based on the type of bonds formed, they can be subdivided into five classes: amineboranes derived from H 3 BNH 3 (the analog of ethane); aminoborane H 2 BQNH 2 (the analog of ethylene) and its derivatives; the iminoboranes derived from HBRNH (the analog of acetylene); borazine B 3 N 3 H 6 (the analog of benzene) and substituted borazines; and boron nitrides (hexagonal and cubic, the analogues of graphite and diamond).…”

Section: Introductionmentioning

confidence: 99%

Unusual substituent effects on the bonding of iminoboranes

Mó

Yáñez

Pendás

et al. 2007

Phys. Chem. Chem. Phys.

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Substituent effects on iminoboranes XBNH, HBNX and XBNX (X = H, CH3, NH2, OH, F) have been analyzed in the framework of the NBO, AIM and ELF approaches, using B3LYP/6-311++G(d,p) optimized geometries and electron densities. Boron-substituted derivatives, XBNH, are more stable than the corresponding nitrogen-substituted isomers HBNX, with the energy difference increasing as the electron withdrawing character of the substituent increases. The BN linkage is not much affected by N-substitution, but it is significantly altered when the substituent is attached to the boron atom in both XBNH and XBNX series of compounds. Moreover, substituent effects on the structures of iminoboranes are opposite those observed for the corresponding isoelectronic acetylene derivatives. The ELF analysis indicates that electron-withdrawing substituents enhance the localization of electrons in a torus around the CC or the BN axis. As a result, although electron density is depleted at the bcp, the bond does not necessarily become weaker, since density increases around the periphery, a phenomenon named the "hole" effect. The dissimilarities between acetylene and iminoborane derivatives are primarily a consequence of the significant distortion of this torus in the latter, due to the large difference between the electronegativities of B and N, which leads to a large contribution of the X-B=:N-Y resonance structure in some cases. The "hole" effect is reflected in a reasonable correlation between the Laplacian of the electron density at the bcp and the BN bond length.

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Recent advances in boron-nitrogen chemistry - II

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Cited by 18 publications

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Dimerization of Diboradiazacyclobutadienes To Form Tetraboratetraazacyclooctatetraenes: Computational Study of Boron−Nitrogen Ring Formation and Ring Opening

Dimerization of Diboradiazacyclobutadienes To Form Tetraboratetraazacyclooctatetraenes: Computational Study of Boron−Nitrogen Ring Formation and Ring Opening

Chemistry of Dicationic Diboranes

Unusual substituent effects on the bonding of iminoboranes

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