Reactions of N-Heterocyclic Silylenes with Covalent Azides

1. Reactions of N-Heterocyclic Silylenes with Covalent Azides Caroline Camic, Nicholas J. Hill Daniel F. Moser, and Robert West Organosilicon Research Center, University of Wisconsin-Madison, USA

2. Following the isolation of thermally stable N-heterocyclic silylenes (NHS) in the mid-1990’s, significant work has examined their behavior toward a range of organic, inorganic, and organometallic substrates. • Amongst the earliest reactions to be studied involved bulky covalent azide species RN3, a highlight of this work being the isolation of a base-stabilized silaketimine (NN)Si=NR. • We have recently sought to isolate a base-free silaketimine from the reaction of the NHS species 1 and 2 with various hindered covalent azides - this poster outlines the structural diversity observed in a range of NHS-azide adducts.

3. N-Heterocyclic Silylenes • Silicon analog of Arduengo carbene • Stable at room temperature (under N2!) • Much less reactive than transient silylenes • Stability enhanced by aromatic delocalization M. Haaf, A. Schmiedl, T.A. Schmedake, D.R. Powell, A. J. Millevolte, M. Denk and R. West, J. Am. Chem. Soc. 1998, 120, 2691

4. Saturated analog of silylene 1 • Similar synthesis and reaction profile to 1 • Equilibrium between 2, insertion product and tetramer • Exists in the solid state as red, diaminodisilyldisilene 2 M. Haaf, T.A. Schmedake, B. J. Paradise and R. West, Can. J. Chem., 2000, 78, 1526

5. Other Stable Silylenes Lappert 1995 Heinicke 1998 Kira 1998 Kira 1999 • 1, 2, 3, and 4 are indefinitely stable at room temperature • 5 and 6 are marginally stable • 6 is the first stable dialkyl substituted silylene

6. Reactions of NHS 1 with Bulky Azides Azidosilane: 29Si NMR δ– 46.9 ppm Silaterazoline: 29Si NMR δ – 53.4 ppm Silaketimine: 29Si NMR δ – 66.6 ppm M. Denk, R. K. Hayashi and R. West, J. Am. Chem. Soc., 1994,116, 10813; C. Camic, N. J. Hill, D. F. Moser and R. West, unpublished work.

7. Reactions of NHS 2 with Bulky Azides Silatetrazoline Major (ca. 90 %) product 29Si NMR δ – 53.5 ppm Azidosilane dimer Minor (ca. 10 %) product 29Si NMR δ – 55.6 ppm C. Camic, N. J. Hill, D. F. Moser and R. West, unpublished work.

8. Structure of a Silatetrazoline Distances (Å) and Angles (o) Si(1) - N(1-2) 1.690(3) - 1.693(1) Si(1) - N(3-6) 1.738(3) - 1.744(3) C(1) - C(2) 1.478(2) N(4) - N(5) 1.267(2) N(1) - Si(1) - N(2) 96.22(2) N(1) - Si(1) - N(6) 117.70(3) N(1) - Si(1) - N(3) 121.20(2) N(6) - Si(1) - N(3) 85.51(2) R = 0.053 @ 100 K

9. Structure of an Azidosilane Dimer Distances (Å) and Angles (o) Si(1) - N(1-2) 1.716(2) - 1.728(2) Si(1) - N(5-6) 1.741(2) - 1.757(1) C(1) - C(2) 1.508(1) N(1) - Si(1) - N(2) 94.74(2) Si(1) - N(6) - Si(2) 95.95(3) N(1) - Si(1) - N(6) 130.69(3) N(3) - Si(2) – N(6) 111.95(3) R = 0.065 @ 100 K

10. A Possible Mechanism ?

11. Conclusions • Wide range of structural motifs available from NHS – azide adducts • Base-free silaketimine not isolated, but appears to react further • With NHS 1, silaimine reacts with RN3via [2+3] cycloadditionto give silatetrazoline • With NHS 2, azidosilane dimer formed from dimerization of silaimine • or sequential insertion of RN3 into aminosilylsilylene • Bulkier azide substituents may stabilize a base-free silaketimine

12. Acknowledgements Chemistry Department UW - Madison Sponsors of the Organosilicon Research Center Further Silylene Literature M. Haaf, T. A. Schmedake and R. West, Acc. Chem. Res., 2000, 33, 704. B. Gehrhus and M. F. Lappert, J. Organomet. Chem., 2001, 617, 209.