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Die Synthese von Pyridin-Borabenzol (8) gelingt, wenn aus 1 -Methoxy-6-(trimethylsilyl)-l-bora-2,4-cyclohexadien (7) in Gegenwart von Pyridin bei 60°C Methoxytrimethylsilan abgespalten wird. Die gelbe Verbindung zeigt eine Charge-Transfer-Bande bei 472 nm. Die Rontgenstrukturanalyse sowie die Protonensignale von 8 belegen den aromatischen Charakter des Borabenzols. Pyridin-und Borabenzolring sind um 43.3" gegeneinander verdreht. Pyridin-2-Boranaphthalin (14a), dessen Herstellung durch Abspaltung von Chlortrimethylsilan in Gegenwart von Pyridin aus 2-Chlor-i ,2-dihydro-I -(trimethylsilyl)-2-boranaphthalin (13) erfolgt, bildet tiefrote Kristalle. Seine Charge-Transfer-Bande bei 486 nm kommt ebenso wie diejenige in 8 durch einen ubergang aus dem HOMO des Boraaren-Molekulteils in das LUMO des Pyridinteils zustande. Die geringe Verdrillung der beiden Ringsysteme in 14a von 8.1 O erleichtert den Elektronentransfer. Die Konjugation der beiden Molekulteile in 14a fuhrt gegenuber 8 zu einer Verkurzung der B-N-Bindung von 155.8 auf 151.5 pm. Im Gegensatz zum roten 14a ist Triethylamin-2-Boranaphthalin (14b) farblos. Synthesis and Structure Investigation of Pyridine-Borabenzene and Pyridine-2-BoranaphthaleneThe synthesis of pyridine-borabenzene (8) succeeds when methoxytrimethylsilane is eliminated from l-methoxy-6-(trimethylsilyl)-l-bora-2,4-cyclohexadiene (7) in the presence of pyridine at 60°C. The yellow compound shows a charge transfer band at 472 nm. The X-ray structure analysis as well as the proton NMR signals of 8 prove the aromatic character of the borabenzene. Pyridine and borabenzene ring are twisted by 43.3". Pyridine-2-boranaphthalene (14a), whose synthesis occurs by elimination of chlorotrimethylsilane from 2-chloro-l,2-dihydro-l -(trimethylsilyl)-2-boranaphthalene (13) in the presence of pyridine, forms deep red crystals. Its charge transfer band at 486 nm results like that in 8 by a transition from the HOMO of the boraarene part of the molecule into the LUMO of the pyridine part. The small twist of the two ring systems in 14a of 8.1" facilitates the electron transfer. The conjugation of both parts of the molecule in 14a leads to a shortening of the B-N-bond compared with 8 from 155.8 to 151.5 pm. In contrast to the red 14a, triethylamine-2-boranaphthalene (14 b) is colourless.Obwohl die Bor-Kohlenstoff-Doppelbindung nicht der klassischen Doppelbindungsregel widerspricht, sind bis heute weder das Borabenzol selbst, noch andere Boraarene bekanntgeworden. Herberich 1) schreibt dem Borabenzol 1 (Borini), Borixinz), Borinina)) einen hochreaktiven 0 VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1985
Preparation, Reactions, and Structure of tert-Butyl(tert-buty1imino)boraneThe iminoborane t BuB = Nt Bu (1) is formed from the aminoborane Cl(t Bu)B-N(t Bu)SiMe, by the elimination of Me,SiCI in the gas phase at 530°C. Compound 1 slowly dimerizes to the diazadiboretidine [tBuBNtBu], (2). In a way typical for iminoboranes, the borane 1 undergoes a? ethyloboration with BEt,, a [2 + 31-cycloaddition with PhN,, and an azidosilation with Me3SiN3. The crystal and molecular structure of 1 and 2 are described; there is a linear central CBNC-chain in 1 with a BN-bond distance of 125.8 pm; the ring-skeleton of 2 deviates from planarity by the influence of the bulky ring ligands. The structural properties of the iminoborane 1 are comparable to those of the isoelectronic alkyne fBuC=CtBu (3). and the properties of the cyclodimer 2 are comparable to those of the isoelectronic cyclobutadiene [t BuC = Ct Bu], (4). The strength of the triple bond decreases from 3 to 1 in a similar manner as by going from N, to CO. Darstellung und Reaktionen von tert-Butyl(tert-buty1imino)boran (1)Aus dem Aminoboran Cl(tBu)B -N(tBu)SiMe,, das aus dem Chlorboran C1,BtBu und dem Lithiumamid LiN(tBu)SiMe, zuganglich ist, laBt sich in der bekannten 0
The 1‐azonia‐2‐boratanaphthalenes (NH)(BX)C8H6 can be synthesized from 2‐aminostyrene and the dihaloboranes XBHal2 (1‐4: X = Cl, Br, iPr, tBu). Further derivatives (NH)(BX)C8H6 are obtained from 1 by replacing Cl by alkoxy or alkyl groups [5‐8: X = OMe, OtBu, Me, (CH2)3NMe2]. The hydrolysis of 1 gives a mixture of the bis(azoniaboratanaphthyl) oxide [(NH)BC8H6]2O (9) and the hydroxy derivative (NH)[B(OH)]C8H6 (10). The diboryl oxide 9 crystallizes in the space group C2/c. The lithiation of 4 at the nitrogen atom gives [NLi(tmen)](BtBu)C8H6 (11), which upon reaction with the diborane(4) B2Cl2(NMe2)2 yields the 1, 2‐bis(azoniaboratanaphthyl)diborane B2[N(BtBu)C8H6]2(NMe2)2 (12). The 2‐chloro‐1‐methyl‐4‐phenyl derivative (NMe)(BCl)C8H5Ph (13) of the parent (NH)(BH)C8H6 can be synthesized from the aminoborane BCl2(NMePh) and phenylethyne. Substitution of Cl in 13 gives the derivatives (NMe)(BX)C8H5Ph [14‐20: X = N(SiMe3)2, Me, Et, iBu, tBu, CH2SiMe3, Ph] and the reaction of 13 with Li2O affords the bis(azoniaboratanaphthyl) oxide [(NMe)BC8H5Ph]2O (21). The reaction of 16 or 19 with [(MeCN)3Cr(CO)3] yields the complexes [{(NMe)(BX)C8H5Ph}Cr(CO)3] (22, 23: X = Et, CH2SiMe3), in which the chromium atom is hexahapto bound to the homoarene part of 16 or 19, respectively. The complex 23 crystallizes in the space group P21/c. Upon reaction of the phenols para‐C6H4R(OH) with the aryldichloroboranes ArBCl2 and subsequent condensation of the products with phenylethyne, the 1‐oxonia‐2‐boratanaphthalenes O(BAr)C8H4RPh with R in position 6 and Ph in position 4 are formed (24‐26: Ar = Ph, R = H, Me, OMe; 27‐29: Ar = C6F5, R = H, Me, OMe). The azoniaboratanaphthalenes 1‐23 were characterized by NMR methods.
Lbstracf; -New perspectives in boron-nitrogen chemistry result from molecules RB4X with two-coordinate boron atoms (iminoboranes) and from boranes NxByHz with nitrogen atoms of coordination number 4 , 5, or 6 in the skeleton (azaboranes). -The structure, stabilization, and reactivity of iminoboranes RB-NR and the isoelectronic alkynes RCECR are compared. Iminoboranes are building blocks of molecules (RBNR)n (n = 2 , 3 , 4 , . 0 ) which are isoelectronic with cyclobutadiene, benzene, Dewar benzene, cyclooctatetraene, or polyacetylene. Acyclic and cyclic additions to the triple bond of RB-NR are described with a stress on transition metal complexes as reaction partners. -The formation of new azaboranes is reported, which are more or less organo-derivatives of m -N B 3 H 6 , e -N z B 3 H 7 , m-NzB4H6, grachno-NB6His-, ~-N B i o H i 3 , and u -N B i i H i z . NEW PERSPECTIVES IN THE LIGHT OF A CLASSIFICATION OF BORON NITROGEN CHEMISTRYNumerous boron compounds being notorious examples for electron-deficiency, a useful classification of boron chemistry may start from distinguishing between classically and non-classically bonded boron atoms. In this context, the attributes "classical" and "nonclassical" mean that the two-center-two-electron approximation either describes the skeletal bonding situation sufficiently well or does not, respectively; delocalized s bonds, as in boron fluoride or borazine, or hyperconjugation, as possibly in trialkylboranes, are not in consideration for the purpose of this classification. The general distinction between a molecular and a solid-state structure may be superimposed, establishing four classes of compounds: classical molecular species (e.g. triethylborane, ether-trifluorborane, etc.) and classical solid-state, species (e.g. boron oxide, colemanite etc.), on the one hand, and non-classical molecular species (e.g. diborane, dicarbadodecaborane etc.) and non-classical solid-state species (e.g. elementary boron, calcium hexaboride etc.), on the other hand. Classical boron compounds are structurally governed by three-and four-coordinate, non-classical boron compounds by four-, five-, and six-coordinate boron atoms.The diversity of traditional boron nitrogen chemistry is dominated by classical molecules. The basic types are represented by aminoboranes, XzB-NRz, and amine-boranes, X3B-NR3, with the coordination numbers 3 and 4 , respectively. The diversity comes from the variation of the ligands X and R, comprising chains like XzB-NR-BX2 or RzN-BX-NRz or even larger BN aggregates, comprising also cyclic aggregates of BN units, the most common ones being benzene-analogous borazines, and comprising, moreover, ions of the type [RsN-BXz-NR3]+ etc. -Classical solids in BN chemistry seem to be restricted to boron nitride until quite recently. -Non-classical molecules in BN chemistry are restricted to those that contain one or more nitrogen atoms in the electron-deficient skeleton. Such azaboranes have been investigated for more than two decades, e.g. the species NBsHis-, NCzBsHii, NBsHi3, NBsHi...
Iminoboranes RB=NR', isoelectronic with akynes, cyclooligomerize to give B-N rings (RBNR'), that are either analogues of cyclobutadienes (n = 2) or benzenes or Dewar benzenes (n = 3) or cyclooctatetraenes (n = 4), depending on the ligand set R/R' and on conducting the oligomerization either thermally or catalytically. Cyclodimers and cyclotetramers may either undergo reversible interconversions or cyclotetramers are formed irreversibly from cyclodimers. The (4+2) cycloaddition of cyclodimers and iminoboranes yields cyclotrimers. Cyclotrimers may thermally be converted into cyclodimers, and poly(iminoboranes), isoelectronic with polyalkynes, can thermally be depolymerized into cyclotrimers. &-Cluster derivatives of N2BqH6, arachno-cluster derivatives of N2B3H7, four-membered rings N~B Z R~R ' , or five-membered rings N2B&R' are isolated, when three-membered rings NB2R3, isoelectronic with cyclopropenyl cations, either dimerize or add aminoboranes H2B=NR'z or insert nitrenes R'N (from R'N3) or insert iminoboranes R'B=NR into the B-B bond, respectively.
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