DOI: 10.1002/chem.201302179 Activation of Isocyanates and Carbon Dioxide by a Monomeric Aluminium Hydrazide as an Active Lewis Pair Frank Hengesbach, [a] Xing Jin, [b] Alexander Hepp, [a] Birgit Wibbeling, [b] Ernst-Ulrich Würthwein,* [b] and Werner Uhl* [a] Dedicated to Professor Bernt Krebs on the occasion of his 75th birthday Introduction The syntheses and properties of frustrated Lewis pairs (FLPs) have been the subject of tremendous interest in recent research. [1] These compounds exhibit Lewis acidic and basic centres in single molecules or in bimolecular sys- tems and show a remarkable and unprecedented propensity for the coordination and effective dipolar activation of small molecules, such as dihydrogen, carbon dioxide, alkynes, al- kenes, etc. Most systems are based on boron atoms as the Lewis acidic sites, which are usually activated by electron- withdrawing fluorinated aryl groups. Only recently were alu- minium–phosphorus compounds shown to form very effec- tive FLPs. [2–5] These have the considerable advantage of an inherently high Lewis acidity due to their aluminium atoms, which in most cases does not require activation by fluorinat- ed substituents. These compounds were applied in the coor- dination or activation of carbon dioxide, alkynes and al- kenes, in phase-transfer catalysis or in the catalytic dehydro- genation of amine–borane adducts. Interestingly, a similar reactivity was observed for dimeric Al À P compounds, [6] in which the opposite Lewis functionalities were quenched by adduct formation and for which the energetically unfavoura- ble dissociation of the dimeric formula units into the active monomeric molecules represented the initial step of the re- actions. These compounds were termed hidden or masked FLPs. Nitrogen atoms have been applied as Lewis basic cen- tres in few cases. [7] Only recently, we found easy access to active Al ÀN-based Lewis pairs by the hydroalumination of trimethylsilyl-substituted ynamines. [8] Al ÀN compounds in which the aluminium and nitrogen atoms were on the same sides of the resulting C =C double bonds were formed by a relatively weak intramolecular Al À N bonding interaction. Acute Al À C =C angles close to 908 verified a considerable ring strain in the central four-membered AlNC 2 heterocy- cles, and opening of the Al À N bonds gave active Lewis pairs that could activate the C ÀH bond of a terminal alkyne. Alu- minium hydrazides seemed to be interesting alternative Lewis pairs for an application in secondary activation proc- esses. These compounds have Lewis acidic aluminium atoms and lone pairs on two nitrogen atoms that may act as partic- ularly active Lewis basic functions. In this article, we report on the synthesis of a monomeric aluminium hydrazide and its reactions with carbon dioxide, isocyanates and an imidoyl chloride. Results and Discussion Synthesis of the monomeric aluminium hydrazide 4 : Di- ACHTUNGTRENNUNGalkylaluminium hydrazides, [R 2 Al À NH À NHR] 2 , usually Abstract: The monomeric aluminium hydrazide H 10 C 5 NÀNACHTUNGTRENNUNG(AltBu 2 ) ÀAd (4 ; Ad = adamantyl, NC 5 H 10 = piperidinyl) was obtained in high yield by hydroalu- mination of the corresponding hydra- zone derivative 1. Compound 4 has a strained AlN 2 heterocycle formed by a donor–acceptor bond between the b-ni- trogen atom of the hydrazide group and the aluminium atom. Opening of this bond resulted in the formation of an active Lewis pair that was able to cooperatively activate carbon dioxide or isocyanates. Insertion of the hetero- cumulenes into the Al À N bond selec- tively afforded a carbamate and two urea derivatives in high yield. In the first step, phenyl isocyanate gave the adduct 6, which has the oxygen atom coordinated to the aluminium atom and its central carbon atom bound to the nitrogen atom of the piperidine moiety. Adduct 6 represents a reason- ACHTUNGTRENNUNGable intermediate state for these activa- tion processes. The applicability of hy- droaluminated compounds, such as 4, in organic synthesis was demonstrated by the reaction with an imidoyl chlor- ide, which gave the corresponding ami- drazone derivative 9. Keywords: activation · aluminium · carbon dioxide · hydrazines · iso- cyanates [a] Dipl.-Chem. F. Hengesbach, Dr. A. Hepp, Prof.Dr. W. Uhl Institut für Anorganische und Analytische Chemie der Universität Münster, Corrensstrasse 30, 48149 Münster (Germany) E-mail : uhlw@uni-muenster.de [b] Dipl.-Chem. X. Jin, B. Wibbeling, Prof. Dr. E.-U. Würthwein Organisch-Chemisches Institut der Universität Münster Corrensstrasse 40, 48149 Münster (Germany) Chem. Eur. J. 2013, 19, 13901 – 13909  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 13901 FULL PAPER