Synthesis and Characterization of Cyclic Silenolates

[1] T. Guliashvili, I. El-Sayed, A. Fischer, H. Ottosson, Angew. Chem., Int. Ed. 2003, 42, 1640. [2] J. Ohshita, S. Masaoka, Y. Masaoka, H. Hasebe, M. Ishikawa, Organomet. 1996, 15, 3136. [3] H. Stueger, B. Hasken, M. Haas, M. Rausch, R. Fischer, A. Torvisco, Organometallics 2014, 33, 231. [4] R. Fischer, T. Konopa, S. Ully, J. Baumgartner, C. Marschner, J. Organomet. Chem. 2003, 685, 79. [5] S. C. Nyburg, A. G. Brook, F. Abdesaken, G. Gutekunst, W. Wong-Ng, Acta Cryst. 1985, C41, 1632.


Introduction
Although silenes have been known for more than forty years, the synthesis and characterization of these compounds are still a challenging endeavor.Based on earlier work by Ottosson 1 and Ohshita 2 the previously unknown cyclic silenolates 2a-c and 3a-b have been synthesized by the reaction of acylcyclohexasilanes with one or two equivalents of KOtBu. 3,4The nature of the anions (silenide or silenolate) and consequently the outcome of subsequent reaction steps largely depends on the substituent R at the carbonyl function.Detailed X-ray-and NMR-analysis of 2a-c and 3a-b corroborate this deduction.

Synthesis and Characterization of Cyclic Silenolates
Michael Haas, Roland Fischer, Ana Torvisco and Harald Stüger Institute of Inorganic Chemistry, Graz University of Technology, Stremayrgasse 9/V, 8010 Graz, Austria

Cyclic Silenolates
Our new cyclic acylsilanes 1a-c reacted cleanly with 1.05 or 2.1 eq. of KOtBu to give the corresponding cyclic silenolates 2a-c and the dianionic species 3a,b, respectively.
Upon heating to 50 °C 2b undergoes a hitherto unknown 1,4-trimethylsilyl-migration and cleavage of the cyclohexasilane cycle to form the anion 4 which could be trapped with MeI to give an enantiomeric mixture of 5 from which the (S)-enantiomer could be crystallized.Bis-silanolates could not be obtained because the silenolate intermediate 7 formed in the reaction of 1,4-bis-acylcyclohexasilane 6 with 1.05 to 2.1 eq. of KOtBu as the primary product immediately rearranges to the carbanionic species 8 in a related process: For 2a-c and 3a,b two resonance structures can be drawn: one with the negative charge residing predominately on the silicon atom (I), while in the other one (II) it is located on the oxygen atom. 29Si NMR analysis suggests increasing contributions of structure II for the aromatic compounds 2b,c and 3b as indicated by a marked low field shift of the Si 1 signal relative to the adamantyl derivatives 2a and 3a (Table 1).Similar trends were reported earlier by Ohshita et al. 2

Acknowledgement
The Austrian Science Fund (FWF) and NAWI-Graz shall be gratefully acknowledged for financial support.

Cyclic Silenes and Germenes
If the silenolates 2a-c and 3a,b were reacted with chlorosilanes either silenes or new acylsilanes were formed depending on the nature of R attached to the carbonyl C: With aromatic R-groups the silenes 9b,c and 10b were obtained while aliphatic R-groups gave rise to the formation of the new acyl silanes 9a and 10a.
With this powerful synthetic strategy we also achieved the synthesis of the exocyclic germenolates 12a,b and the first exocyclic germene 13b: