-Triboranes and Subsequent Cationization

The reaction of aryland amino(dihydro)boranes with dibora[2]ferrocenophane 1 leads to the formation 1,3-transdihydrotriboranes by formal hydrogenation and insertion of a borylene unit into the B=B bond. The aryltriborane derivatives undergo reversible photoisomerization to the cis-1,2-μ-H-3hydrotriboranes, while hydride abstraction affords cationic triboranes, which represent the first doubly base-stabilized B3H4+ analogues. Unlike carbon, whose ability to form long and stable homonuclear chains is the basis of organic polymer chemistry, electron-deficient boron has a strong tendency to oligomerize in the form of stable non-classical clusters, in which three-centertwo-electron bonding predominates, especially within oligoboron hydrides. In contrast, classical oligoboranes of the form BnRn+2, in which each boron atom is sp-hybridized, are particularly prone to ligand scrambling and hydrolysis unless stabilized by electron-donating amino or alkoxy substituents, as exemplified by the commercially available diboranes(4) B2(NMe2)4, B2Pin2 (Pin = pinacolato), B2Cat2 (Cat = catecholato) and B2Neop2 (Neop = neopentyl glycolato). In order to enforce electronprecise B-B bonding in oligoboranes, therefore, Lewis bases are commonly used to electronically saturate the boron centers. The ubiquity of hydroborane and diborane reagents in organic synthesis has fueled the search for new synthetic routes to a greater variety of electron-precise diand oligoboron hydrides. The reductive coupling of N-heterocyclic carbene (NHC)-stabilized (NHC)BX2R (R = Br, Ph) precursors, for example, provided access to neutral diand tetrahydrodiboranes of the form [(NHC)R'HB–BHR'(NHC)] (R' = H, Ph), whereas that of [ArBH2]2 diborane(6) precursors yielded [ArH2B–BH2Ar] 2– dianions which were in turn converted via double hydride abstraction to neutral dihydrodiboranes(4). Milder routes to diboranes with terminal B-H bonds include the dehydrocoupling of boranes, selective dimethylamino-hydride exchange at B2N2C2 heterocycles, [9] the spontaneous transfer hydrogenation of diborenes with Me2NH·BH3, [10] or the insertion of a borylene into a B-H bond at a boron cluster. Electron-precise 1-hydrotriboranes were obtained via the uncatalyzed hydroboration of 1,2-diheteroaryldiborenes with HBCat (Scheme 1a). Use of 9-borabicyclo[3.3.1]nonane (9BBN) instead of HBCat led to a B3 arachno cluster, presumably due to the greater electron-deficiency at boron in 9-BBN. More recently, the double hydroboration of a diboryne to a 2,3dihydrotetraborane, followed by hydride abstraction, yielded the first cationic 2,3-μ-hydrotetraborane (Scheme 1b). In this work we report a new strategy for the selective formation of doubly base-stabilized trans-1,3-dihydrotriboranes by dihydroboration of a strained cis-diborene, resulting in the formal hydrogenation of, and insertion of a borylene moiety into, the B=B bond. Furthermore, we study the photoisomerization and cationization of these species (Scheme 1c). Scheme 1. Atom-efficient methods for the synthesis of electron-precise oligoboron hydrides. While studying the stoichiometric transfer hydrogenation of the ferrocene-bridged diborene 1 with Me2NH·BH3, we observed, beside the expected 1,2-dihydrodiborane (δ11B = –18.0 ppm), a second product (δ11B = 88.6, –29.6 ppm, 1:2 ratio), which we deemed to result from the reaction of 1 with the dehydrocoupling byproduct Me2N=BH2. [10] Similarly, the reaction of 1 with 1 equiv pyrrolidinoborane (PyrBH2) in C6D6 at 60 °C overnight resulted in quantitative formation of the triborane 2-Pyr (Scheme 2), which shows two broad B NMR resonances at 87.8 (sp-B) and –28.8 U. Schmidt, L. Werner, Dr. M. Arrowsmith, Dr. A. Deissenberger, A. Hermann, Dr. A. Hofmann, Dr. S. Ullrich, Prof. Dr. H. Braunschweig Institut für Anorganische Chemie and Institute for Sustainable Chemistry & Catalysis with Boron Julius-Maximilians-Universität Wüzburg Am Hubland, 97074 Würzburg (Germany) E-mail: h.braunschweig@uni-wuerzburg.de Dr J. D. Mattock, Dr A. Vargas Department of Chemistry, School of Life Sciences University of Sussex Brighton BN1 9QJ, Sussex (UK) Supporting information for this article is given via a link at the end of the document.