An Access to Base-stabilized Three-membered Silicon Heterocycles

The three-membered silacyclic ring compounds LSi[N(2)(Ph)(2)]tBu (1), LSi[HCN(Ph)(2)]tBu (2) and LSi[C(2)(Ph)(2)]tBu (3) were obtained by the treatment of base stabilized monoalkylsilylenes LSitBu (L = PhC(NtBu)(2)) with PhN=NPh, PhN=CHPh and PhC≡CPh. The reaction of PhN=NPh and PhC≡CPh with LSitBu shows a different reactivity pattern with base stabilized monochlorosilylene LSiCl. The arrangement of the three-membered ring (SiNN) in 1 is the first structurally isolated example of a siladiaziridine compound.

Three-membered ring compounds possessing higher coordinate group 14 elements especially silicon at the position adjacent to the heteroatom have received considerable attention because of their unique structures and also their wide application in synthetic chemistry. 1These three-membered ring compounds bearing a silicon atom are interesting for chemists, due to their high strain and novel bonding arrangement within the ring. 2 Moreover, these systems can be compared with epoxides and aziridines which have become an important class of compounds. 3,4Epoxides as well as aziridines are natural products and show a great number of transformations. 4Related reactions are expected with silicon containing three-membered rings.In the case of Brook rearrangement, the transition state has been proposed as a three-membered ring compound bearing a pentacoordinate silicon at the position adjacent to an oxygen. 5Further Nevárez and Woerpel reported on the dearomatization reaction which involves treatment of benzaldehyde with silaziridine with an aryl group either on carbon or nitrogen atoms, which is considered to be driven by the relief of the ring strain of the threemembered ring. 6Recently there has been emerging interest in the replacement of carbon atoms with silicons for developing new innovative drugs. 7Several bioactive silacycles have been reported.One example is sila-haloperidol which shows a different metabolism pattern when compared to that of the carbon analogue. 8However, due to the high ring strain of the threemembered ring system only a few silicon compounds with heteroatoms have been reported so far. 9 Compounds with silicon(IV) atoms are quite easy to access from Si(II) by the oxidative addition reaction.9a-e The divalent silicon species called silylenes were generated as short-lived intermediates in numerous thermal and photochemical reactions from organosilicon precursors. 10The existence of silylenes has been evidenced by chemical trapping and spectroscopic methods in the fluid phase or in low-temperature matrices.Initially the stable silylenes were isolated as N-heterocyclic silylene (NHSi) by West et al. in 1994. 11 One can compare the reactivities of NHSis with those of the N-heterocyclic carbenes (NHCs).3][14] In 2006, we reported the three coordinate stable chlorosilylene LSiCl (L = PhC(NtBu) 2 ) stabilized by an amidinato ligand in less than 10% yield using potassium as a reducing agent. 15Subsequently we reported the synthesis of LSiCl utilizing LiN(SiMe 3 ) 2 as a reducing agent to get higher yields without using hazardous reducing agents. 16Currently we are interested in the chemistry of silylenes and utilized them as σ-donor ligands for transition metal complexes, 17 oxidative addition reactions with organic substrates 18 and Lewis bases. 19Recently we synthesized the monoalkylsilylene LSitBu by a facile metathesis reaction treating LSiCl with LitBu. 20We observed a quite different reactivity pattern of LSitBu with N 2 O when compared with that of LSiCl.In the case of LSitBu we obtained [LSitBu(μ-O)] 2 containing a four-membered Si 2 O 2 ring. 20In contrast the reaction of LSiCl proceeds to the trimer [LSi(μ-O)Cl] 3 with a Si 3 O 3 six-membered ring.19b We presume that the difference in reactivity is due to the presence of the bulky tBu substituent attached to the silicon atom.This motivated us to explore the chemistry of LSitBu further and to compare its reactivity pattern with LSiCl.We treated LSitBu with PhNvNPh, PhNvCHPh and PhCuCPh leading to the products LSi[N 2 (Ph) 2 ]tBu (1), LSi[HCN(Ph) 2 ]tBu (2) and LSi[C 2 (Ph) 2 ]tBu (3).These three reactions yielded the three-membered silacyclic ring compounds.The reaction of PhNvNPh and PhCuCPh with LSitBu shows a different reactivity pattern when compared with that of LSiCl.The reaction of LSiCl with PhNvNPh resulted in the formation of the fused two five-membered unsymmetric rings formed by C-H bond activation and LSiHCl 2 elimination. 21The reaction of LSiCl with PhCuCPh proceeds in a 2 : 1 ratio to yield the fourmembered ring compound 16 while LSitBu yielded the silacyclopropene 3. It is of interest to mention that silylenes react with alkynes to form a variety of small membered silacycles. 9,22he formation of the SiNN three-membered ring in 1 is the first structurally isolated example of a siladiaziridine compound.
Compounds 1, 2 and 3 were obtained in good yields when LSitBu was treated with PhNvNPh, PhNvCHPh and PhCuCPh in a 1 : 1 ratio as shown in Scheme 1. Compounds 1-3 are soluble in common organic solvents.They are stable both in the solid state as well as in solution without any decomposition under an inert gas atmosphere.The molecular structures of 1-3 were unequivocally established by single crystal X-ray structural analyses.Furthermore they were fully characterized by NMR spectroscopy, EI-MS and elemental analysis.
The 29 Si NMR spectrum of 1 shows a single resonance at δ −99.6 ppm, which is upfield shifted when compared with that of LSitBu (δ 61.5 ppm). 20The tBu protons of compound 1 in the 1 H NMR spectrum exhibit a double resonance of equal intensity for the tBu groups that reside on nitrogen atoms (δ 0.98 and 1.23 ppm).The tBu protons attached to the silicon atom resonate at 1.31 ppm.In addition 1 shows its molecular ion in the mass spectrum at m/z 498.
The siladiaziridine product 1 crystallizes in the triclinic space group P1 ˉand the molecular structure is shown in Fig. 1.We propose that the reaction of LSitBu with PhNvNPh followed the [1 + 2]-cycloaddition reaction.The silicon atom is five coordinate and the coordination environment is made up from four nitrogen atoms and a carbon atom.The structural index τ which defines the extent of deviation from trigonal bipyramidal to square pyramidal geometry (τ = 1 for perfect trigonal bipyramidal; τ = 0 for perfect square based pyramidal 23 ) is 0.28, indicating strong deviation from the regular trigonal bipyramidal geometry and closer to square pyramidal geometry.The N1-Si1-N2 bite angle between the silicon atom with the backbone ligand is 69.83( 6)°.The distance between the Si and the tBu carbon atom is 1.8933( 17 The silaaziridine 2 exhibits a single resonance at δ −104.6 ppm in its 29 Si NMR spectrum.Similar to compound 1, the tBu protons attached to the nitrogen atoms of compound 2 show a double resonance of equal intensity in the 1 H NMR spectrum (δ 0.76, 1.24 ppm).The tBu protons that reside at the silicon atom resonate at 1.35 ppm.The imino proton resonates at δ 3.81 ppm.Further compound 2 displays its molecular ion in the mass spectrum at m/z 497.

Conclusions
In summary, three-membered silicon heterocycles can be accessed by the reaction of base stabilized monoalkylsilylene LSitBu with PhNvNPh, PhNvCHPh and PhCuCPh.Due to the presence of the bulky tBu groups at the silicon(II) centers, the reactions of LSitBu with PhNvNPh and PhCuCPh show a different reactivity pattern compared to that of LSiCl.Compound 1 is the first structurally isolated example of a three-membered siladiaziridine compound.