Z. Naturforsch. 2020; 75(1–2)b: 63–71 Werner Uhl*, Katja Martinewski, Julia Silissa Bruchhage, Alexander Hepp, Marcus Layh, Fabian Dielmann and Paul Mehlmann Cooperative activation of azides by an Al/N-based active Lewis pair – unexpected insertion of nitrogen atoms into C–Si bonds and formation of AlCN 3 heterocycles https://doi.org/10.1515/znb-2019-0138 Received August 19, 2019; accepted September 16, 2019 Abstract: The active Lewis pairs (ALPs) 2,6-Me 2 H 8 C 5 N– C(H) = C(SiMe 3 )–AlR 2 (1a: R = t Bu, 1b, R = i Bu) have strained AlC 2 N heterocycles and relatively weak Al–N bonds. They react readily with a series of organic azides R′N 3 [R′ = Ph, CH 2 C 6 H 4 (4- t Bu), t Bu, SiMe 3 , CH 2 Ph] by cleavage of the hete- rocycles and addition of the azides with their α-N atoms to the Al atom. The Al–N interactions result in an activation of the azide groups which insert into the C–Si bonds of the vinyl groups with their terminal γ-N atoms. Compounds with approximately planar five-membered AlCN 3 hetero- cycles and intact N 3 groups are formed in highly selective reactions. Keywords: active Lewis pair; aluminum; azides; heterocy- cles; substrate activation. Dedicated to: Professor Arndt Simon on the occasion of his 80 th birthday. 1 Introduction Investigations into the reactivity of frustrated Lewis pairs (FLPs) are an important topic in current research. FLPs have coordinatively unsaturated Lewis acidic and basic centers, and caused by this specific functionality they show a unique cooperative behaviour in stoichiometric and catalytic transformations. These highly promising materials are considered as attractive alternatives or complements for transition metal catalysts. The major- ity of FLPs is based on systems with B and P atoms as Lewis acids and Lewis bases [1–4], but it has recently been shown that Al/P based systems are also highly efficient in various transformations [5–10]. Al atoms possess an inherently high Lewis acidity, which makes an activation by electron-withdrawing substituents, as required in B based systems, unnecessary. A class of compounds closely related to the Al/P FLPs contain Al or Ga and N atoms. Weak bonding interactions between the Lewis acidic metal and the Lewis basic N atoms result in strained four- [11–14] or three-membered heterocyles [15–18]. Cleavage of the relatively weak donor-acceptor bonds in the presence of suitable substrates results in a reactivity similar to that of FLPs. Due to the lack of frustration these compounds were named active Lewis pairs (ALPs) [11]. The exceptional and variable reactiv- ity of these ALPs has been demonstrated by their reac- tions with isocyanates, nitriles, terminal alkynes, carbon dioxide, carbodiimide and other substrates [11–18]. Par- ticularly noteworthy is their capability to oligomerize suitable monomeric starting materials such as cyana- mides [12]. Typical examples of Al/N based active Lewis pairs are shown in Scheme 1 (1, A). Organic azides, R′N 3 , represent a particularly inter- esting class of substrates, which react with monomo- lecular B/P or Al/P FLPs to yield heterocycles with one (B) [9, 19–22], two (C) [23] or three (D) [24] N atoms of the azide groups included in the rings (Scheme 1). A related reaction of [Ph 3 C][B(C 6 F 5 ) 4 ] and F 5 C 6 N 3 led in the pres- ence of P(o-tol) 3 and Ph 3 SiH to [(Ph 3 Si)(F 5 C 6 )N–N=N– P(o-tol) 3 ][B(C 6 F 5 ) 4 ] as an acyclic analogue of compound D [25]. Organic azides are highly reactive species that are frequently explosive but of importance for the synthesis of nitrogen containing heterocycles and amines [26–29]. They loose dinitrogen when exposed to higher temper- atures, light, pressure or in the presence of transition metal catalysts. The highly reactive six-electron nitrene species formed as intermediates found application for the synthesis of amines [26–29]. Some FLP azide adducts were similarly found to release dinitrogen at elevated temperatures or after irradiation with UV light, but the reactive nitrenes could not be utilised synthetically and were usually trapped instead by the FLPs to form ther- mally stable adducts (E, Scheme 1) [21–23]. A B/P FLP *Corresponding author: Werner Uhl, Institut für Anorganische und Analytische Chemie der Universität Münster, Corrensstraße 30, 48149 Münster, Germany, e-mail: uhlw@uni-muenster.de Katja Martinewski, Julia Silissa Bruchhage, Alexander Hepp, Marcus Layh, Fabian Dielmann and Paul Mehlmann: Institut für Anorganische und Analytische Chemie der Universität Münster, Corrensstraße 30, 48149 Münster, Germany