Si = C Double Bonds DOI: 10.1002/anie.200700067 Stable Cyclic Silenes from Reaction of Disilenides with Carboxylic Acid Chlorides** Iulia Bejan, Denis Güclü, Shigeyoshi Inoue, Masaaki Ichinohe, Akira Sekiguchi, and David Scheschkewitz* Dedicated to Professor Armin Berndt on the occasion of his 70th birthday Since first evidenced as intermediates by Gusel)nikow and Flowers 40 years ago, [1] silenes, that is, compounds with Si =C bonds, have attracted continuous interest. [2] The stable silene 1 (Scheme 1) reported by Brook et al. gave further impetus to the field. [3] Its unexpectedly long Si =C bond prompted an intense discussion, which was resolved by calculations. [4] The term “reversed polarity” was coined: the p-donating siloxy group in 1 reduces the polarity of the Si = C bond significantly, thereby enhancing its stability and increasing the bond length. Shortly thereafter Wiberg et al. reported silene 2, which is free of p-donating substituents and accordingly exhibited the expected shorter Si =C bond. [5] Other isolable donor-free silenes were subsequently reported by the groups of Apeloig and Kira. [6] Recent studies by Ottosson et al. focusing on silenolate 3 and other silenes bearing substituents with stronger p-donating ability than that of the siloxy group in 1 revealed a decreasing bond order for Si =C accompanied by increased pyramidalization of the silicon center. [7] Silenes of type 1 had been prepared by thermal and photochemical rearrangement of suitable acyl silanes, easily accessible from reactions of acid chlorides with silyl anions. [8] In the last years a few disilenides, that is, disila analogues of vinyl anions became available by efforts of both of our groups. [9] In light of a number of successful applications of 4a as an unsaturated, nucleophilic reagent, [10] we attempted the preparation of acyl disilenes by reaction of 4a,b (Scheme 2) with acid chlorides. In reactions of 4a with 1-adamantoyl and pivaloyl chloride, however, the expected (and likely intermediate) acyl disilenes 5a, b were not detected even at 193 K. This instability is in marked contrast to the carbon analogues, a,b- unsaturated ketones. Instead, the four-membered cyclic silenes 6a, b were formed quantitatively as indicated by NMR and UV/Vis spectroscopic data (Scheme 2, see the Supporting information). [11] The reaction of disilenides with acid chlorides turns out to be quite general. Thus, 4b readily reacts with 1-adamantoyl chloride to yield the cyclic silene derivative 6c. 29 Si NMR spectroscopy reveals almost identical chemical shifts for the tri- and the tetracoordinated silicon nuclei of both 6a and 6b. Curiously, in the case of 6a the signal at higher field (d = 17.5 ppm) is assigned to the Si atom of the double bond. It is considerably shielded compared to the corresponding signal of 1 (d = 41.4 ppm). [3c,12] The ring carbon atoms exhibit chemical shifts (6a : d = 213.4 ppm, 6b : d = 214.6 ppm) that are very similar to that found for 1 (d = 214.2 ppm [3c] ). The Si–C coupling of the double bond in 6b is distinctly smaller than that in 1 (6b : 1 J(C,Si) = 67.3 Hz), 1: 1 J(C,Si) = 84.4 Hz [3c] ). These observations can be rationalized by the increase in p character of the endocyclic bonds due to ring strain. As a consequence the orbitals hosting the remaining electron density at silicon should get higher Scheme 1. Open-chained silenes 1–3 (Ad = 1-adamantyl). Scheme 2. 4a :R = Tip = 2,4,6-iPr 3 C 6 H 2 ; 5a, 6a :R = Tip, R’ = tBu; 5b, 6b :R = Tip = 2,4,6-iPr 3 C 6 H 2 ,R’ = 1-adamantyl; 4b, 5c, 6c : R = SiMetBu 2 ,R’ = 1-adamantyl. [*] Dipl.-Chem. I. Bejan, D. Güclü, Dr. D. Scheschkewitz Institut für Anorganische Chemie Julius-Maximilians-Universität Würzburg Am Hubland, 97074 Würzburg (Germany) Fax: (+ 49) 931-888-4623 E-mail: scheschkewitz@mail.uni-wuerzburg.de Homepage: http://www-anorganik.chemie.uni-wuerzburg.de/ Dipl.-Chem. S. Inoue, Dr. M. Ichinohe, Prof. Dr. A. Sekiguchi Department of Chemistry Graduate School of Pure and Applied Sciences University of Tsukuba Tsukuba, Ibaraki 305-8571 (Japan) [**] Funding by the DFG (Sche 906/3-1), the Fonds der Chemischen Industrie and the Otto-Röhm-Gedächtnisstiftung is gratefully acknowledged. D.S. thanks Prof. H. Braunschweig for generous support and Dr. R. Bertermann for NMR spectroscopy. A.S., S.I. and M.I. acknowledge funding by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, and Culture of Japan (Nos. 16205008, 16550028, 17655014, 18037008, 18039004), JSPS Research Fellowships for Young Scientists (SI), and COE (Center of Excellence) Program. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Angewandte Chemie 3349 Angew. Chem. Int. Ed. 2007, 46, 3349–3352 # 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim