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
Borabenzene is stabilized in an extremely mild fashion in the matrix‐isolated borabenzene–nitrogen adduct 4, which was identified by IR spectroscopy. In general, boraarenes and boraethenes can only be isolated when the electron deficiency on boron is satisfied by a donor. Compound 4 is formed via 3 upon flash pyrolysis of 1 in N2 as carrier gas.
Auf äußerst milde Weise stabilisiert ist Borabenzol 3 im matrixisolierten Borabenzol‐Stickstoff‐Addukt 4, das sich IR‐spektroskopisch nachweisen ließ. Generell gilt, daß Boraarene und Boraethene nur zu fassen sind, wenn die Elektronenlücke am Bor durch einen Donor gefüllt ist. 4 bildet sich via 3 bei der Blitzpyrolyse von 1 in N2 als Trägergas.
The O-vinylketoximes 2 were synthesized from ketoximes 1 and acetylene in superbase systems in good to excellent yields. Their thermal stability was investigated.O-Vinylketoximes, precursors in the synthesis of pyrroles from ketoximes and acetylenes, 1 contain a highly reactive NO-vinyl group and attract much attention as promising monomers and synthons. However, they are potentially explosive and hence for their safe and proper application in organic synthesis and polymer chemistry, the knowledge of their thermal stability is crucial. Meanwhile, no data of such a kind were so far published.Direct acetylene vinylation of a ketoxime, namely acetoxime 1a, was first reported 20 years ago. 2 The reaction was carried out under atmospheric pressure at 110°C, the yield of O-vinylacetoxime being 10%. Later, the yield was improved up to 78% by carrying out the reaction in an autoclave (80-90°C, initial acetylene pressure 15 atmospheres) and by use of fractional vinylation. 3 However, this procedure turned out to be poorly reproducible.A series of vinylketoximes 2 was unselectively prepared by using superbase systems MOH-DMSO (M = Li, Na, K) under acetylene pressure, 3-12 with the help of indirect methods using halogen derivatives. 13,14 The synthetic conditions and yields of 2 are presented in Table 1.The goal of the present work was to develop high-performance and reliable synthetic routes to O-vinylketoximes of accessible alkyl-and arylmethylketones and to undertake their stability investigation.Using vinylation of acetoxime 1a (R 1 = Me, R 2 = R 3 = H) as a model, the effect of the reaction parameters on the yield and purity of O-vinylketoximes under both atmospheric and elevated pressure has been examined. The results of acetoxime vinylation under atmospheric pressure are presented in Table 2. The runs were carried out in a flow system; the O-vinylacetoxime 2a formed was brought out with the acetylene flow into a trap cooled to -78°C. The formation of undesirable (in this case) pyrroles 3 and 4 during O-vinylacetoxime synthesis 1 was suppressed by water additives. 2,3 At the same time, it is known that the presence of water in the alkali-DMSO system decreases its basicity 15 and, consequently, the ketoxime-acetylene reaction rate. Indeed, variation of the reaction conditions has shown the yields of O-vinylacetoxime 2a to increase with decreasing the concentration of water and increasing the amount of alkali loading (Table 2). A significant growth (from 30% to 42%) of the yield of 2a with increasing the amount of alkali (Table 2, runs 3-5) or with reducing the acetoxime concentration (Table 2, run 6) is observed under anhydrous conditions. The synthesis of oxime 2a under the conditions of runs 3-5 may be considered as a preparative one, but, though experimentally simple, it suffers from shortcomings. First, a considerable portion of the initial acetoxime transforms to the pyrroles 3a and 4a. Second, a loss of O-vinylacetoxime 2a is observed owing to its high volatility in the presence of acetylene; in the trap a sol...
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