PAPER www.rsc.org/dalton | Dalton Transactions The reactions of dialkylgallium hydrides with tert-butylethynylbenzenes—a systematic investigation into the course of hydrogallation reactions Werner Uhl,* Michael Claesener, Sima Haddadpour, Beate Jasper and Alexander Hepp Received 26th September 2006, Accepted 13th November 2006 First published as an Advance Article on the web 23rd November 2006 DOI: 10.1039/b614003c The reactions of bis- and tris(tert-butylethynyl)benzenes with dialkylgallium hydrides afforded two different types of products. 1,4-Di(tert-butylethynyl)benzene and dialkylgallium hydrides R 2 GaH bearing relatively small substituents (R = Et, nPr) gave the expected addition products with each C≡C triple bond inserted into a Ga–H bond. The intact GaR 2 groups are attached to those carbon atoms which are in a-position to the benzene rings, and intermolecular Ga–C interactions led to the formation of one-dimensional coordination polymers. In contrast secondary reactions with the release of the corresponding trialkylgallium derivatives GaR 3 (R = Et, nPr, iPr, CH 2 tBu, tBu) were observed for all hydrogallation reactions involving the trisalkyne 1,3,5-tris(tert-butylethynyl)benzene. A similar reaction was observed upon treatment of the 1,4-bisalkyne with a dialkylgallium hydride bearing a relatively bulky substituent (R = neopentyl). Cyclophane type molecules are formed in all these cases with two or three gallium atoms in the bridging positions between both benzene rings. Introduction Hydroalumination and hydrogallation reactions are well-known procedures for the reduction of unsaturated organic compounds such as alkenes or alkynes. 1,2 In many cases the structures of the organometallic products were derived from the constitution of hydrolysis products only, and molecules were postulated in which intact ER 2 groups were attached to the carbon atoms of the reduced C 2 moieties. In some recent investigations on hydroalumi- nation or hydrogallation reactions with alkynes we found that the reaction courses are more complicated than those suggested in the literature and that unprecedented molecular structures resulted. The simple addition products, R ′ –(R 2 E)C=C(H)–R ′′ (E = Al, Ga), were isolated only when trimethylsilyl substituted alkynes 3 or sterically highly encumbered dialkylaluminium hydrides 4 were employed. In other cases very fast condensation reactions occurred with the release of the corresponding trialkylelement derivatives, and the simple addition products could not be detected even as intermediates by NMR spectroscopy. Accordingly, the hydroa- lumination of dialkylaluminium alkynides yielded carbaalanes, which established a new class of compounds and possess clus- ters formed by aluminium and carbon atoms [e.g. an Al 8 C 5 cluster in (AlMe) 8 (CCH 2 C 6 H 5 ) 5 (H)]. 5 Gallium alkynides gave heteroadamantane type molecules, (GaR) 6 (CCH 2 R ′ ) 4 , containing coordinatively unsaturated gallium atoms and a localized bonding situation. 6 Compounds possessing two or more coordinatively unsaturated aluminium atoms are very effective chelating Lewis- acids. 7 We were able to generate such a compound by the twofold hydroalumination of di(tert-butyl)butadiyne only recently. 8 A per- sistent butadienyl cation resulted in that case (1, Scheme 1) because Institut f¨ ur Anorganische und Analytische Chemie, Universit¨ at M ¨ unster, D- 48149, M ¨ unster, Germany. E-mail: uhlw@uni-muenster.de Scheme 1 of the very effective coordination of the hydride counterion by two aluminium atoms. Owing to experiences gained in our group in recent investiga- tions hydrogallation reactions proved to be more selective than This journal is © The Royal Society of Chemistry 2007 Dalton Trans., 2007, 417–423 | 417 Downloaded on 19 October 2010 Published on 23 November 2006 on http://pubs.rsc.org | doi:10.1039/B614003C View Online