Desulfurization of Isothiocyanato Silatrane - First Report on the Formation of Isocyano Silatrane Meenu1*, Manish Kumar2, Ravi Shankar3 and Suraj P. Narula4 ' University Institute of Engineering & Technology, Panjab University, Chandigarh-160014, India 2Panacea Biotec, E-4, Industrial Area, Phase II, Mohali-160055, India department of Chemistry, Indian Institute of Technology, New Delhi, India 4Department of Chemistry, Panjab University, Chandigarh-160014, India. ABSTRACT Photolytic cleavage of NCS group attached to silatrane results in the formation of isocyano silatrane with the elimination of elemental sulfur. Spectral data confirming of the new isocyanide silatrane derivative are presented. The method offers an opportunity to achieve silyl isocyanides from corresponding isothiocyanates. INTRODUCTION Desulfurization of organic isothiocyanates has been achieved by a variety of methods leading to cyanamides or various cyclized products /1, 21. Removal of the sulfur atom from isothiocyanate group, leaving behind isocyanide, has also been studied previously /3,4/. This reaction offers an interesting opportunity to obtain isocyanide derivatives from isothiocyanates attached to a variety of rigid frameworks. The N=C=S chromophore of organic isothiocyanates absorbs at 250 nm due to π-π * transition. This excitation may be employed for the photochemical cleavage of C=S of N=C=S moiety 15, 6/. 1-Isothiocyanatosilatrane, SCNSi(OCH2CH2)3N, has been synthesized and extensively studied by our group 111. The Si-NCS in this compound has been established as a fairly stable silicon-nitrogen bond. The coordinate bond, Ν-» Si, of the atrane cage, Si(OCH2CH2)3N, is the shortest of the known silatranes. Ab initio 111 and DFT calculations /8/ on this molecule have suggested that the electron density donated by coordinating nitrogen may be dissipated on the oxygen atoms and sulfur of NCS group. Hence there is a high electron density on the sulfur atom. The chemical reactivity of this compound towards transition metal compounds has suggested linkage of this sulfur atom to the metal centre. In an effort to achieve desulfurization of the isothiocyanate group in this pentacoordinate * Author for correspondence. E-mail: [email protected]. 201 Vol. 33, Nos. 4-5, 2010 Desulfurization of Isothiocyanato Silatrane First Report silicon compound, it was thought to subject the silatrane to photochemical irradiation at an appropriate wavelength, so that cleavage of C=S may be achieved. The results are presented herein. EXPERIMENTAL Materials and methods Silicon tetrachloride (Aldrich) was used without further purification. Triethanol amine, potassium thiocyanate, acetonitrile, diethyl ether and ethanol were purified and dried by standard procedures before use. 1-Isothiocyanato silatrane was prepared by reported procedure 111. All operations were performed under dry nitrogen atmosphere. Spectral studies IR spectra were recorded as neat liquids, as nujol mulls or HCB mulls on KBr optics on Perkin Elmer (model 1430) ratio-recording spectrophotometer. 'H, 13C and 29Si NMR spectra were obtained using Briiker 300 MHz spectrometer. Me4Si was used as internal standard. C, Η, Ν analyses were carried out on a Perkin Elmer model 2400 elemental analyzer. Irradiation experiments were carried out in a Raynot photochemical reactor (Applied Biophysics Ltd.) equipped with twelve 253.7 nm 10 watt lamps (Eye, G15T8). Photochemical reaction ofl-Isothiocyanato silatrane 1-Isothiocyanato silatrane (0.05 mmol, 0.12 g) was dissolved in CH3CN (15 ml). The solution was degassed and filled into a quartz glass tube. This tube was suspended in the photochemical reactor. The UV light was allowed to fall on the sample solution for 48 hours. Then the lamp was switched off. The solid precipitate out of the solution was filtered under nitrogen and was analysed by mass spectrum as elemental + + + + sulfur [MS: 258 (S8) , 194 (S6)\ 130 (S4) , 64 (S2) , 32 (S) ], The clear solution was again filled into the quartz cell and irradiated as previously for another 48 h. The solid precipitated out was again filtered off. Solvent was evaporated from the filtrate under reduced pressure and a solid was precipitated out using diethyl ether. The solid thus obtained was filtered and dried under vacuum. Anal. Found (Calcd. for C7H12N2O3SS1 + C7HI2N203Si in 3:7 ratio): C, 39.69 (39.77); H, 5.68 (5.67); N, 13.25 (13.26); S, 5.21 (5.30); Si, 13.25 (13.26). 1 IR (Nujol Mulls) cm" : 2110 i)asNCS/uNC, 2935 u(CH2 aliphatic), 1410 & 1375 u(CH2), 1080 uSiO. δ'Η 13 NMR (CD3CN): 4.02 (t, OCH2), 3.71 (t, OCH2), 3.11 (t, NCH2), 2.98 (t, NCH2) ppm. 6 CNMR (CD3CN): 13 i4 57.61 (OCH2), 50.32 (NCH2), 133.82 (t, NCS, J{ C- N} = 26.1 Hz) and 56.75 (OCH2), 49.90 (NCH2) and 29 29 l4 167.97 (Si-NC). 5 Si NMR (CD3CN): - 103.03 (t, J{ Si - N}= 28.5 Hz), - 105.85 ppm. 202 Meenu et al. Main Group Metal Chemistry RESULTS AND DISCUSSION Isothiocyanate group of the silatrane reveals a strong absorption at 235 nm. When a solution of this compound in CH3CN was irradiated with UV light, using a low pressure mercury lamp with λπ,3Χ 253.7 nm, for 48 h, a brownish solid precipitated out from the reaction mixture. This solid was separated. The mass spectrum of the solid suggested it to be elemental sulfur. The residual solution was further subjected to irradiation for another 48 h and sulfur was separated from the solution again. The usual workup of residual solution yielded a mixture of desulfurized and parent silatrane in the ratio 70:30 [Analytical data: Experimental Section]. CH3CN, hv SCNSi(OCH2CH2)3N • SCNSi(OCH2CH2)3N + CNSi(OCH2CH2)3N + S j Spectral Studies Infrared spectrum of the product revealed a broad band at 2110 cm"' which is assigned to i)asNCS and uNCof Si-NCS and Si-NC. Absorption due to -CN stretching in silyl cyanides is reported in the region 2180- 2200 cm"1. IR absorption frequencies of organic isocyanides are reported 19/ to be around 2120 cm"1. The other absorption bands characteristic of silatranyl cage are retained at their respective positions. 'H NMR spectrum of the sample in CD3CN reveals triplets at 4.02, 3.71, 3.11 and 2.98 ppm. The signals at 4.02 and 3.11 ppm are assigned to OCH2 and NCH2 of Si(OCH2CH2)3N skeleton in 1- isothiocyanato silatrane (parent compound) 111. Those at 3.71 and 2.98 ppm may be due to the silatranyl cage of the desulfurized product. The relative ratio of the two sets of signals suggest that desulfurized and parent silatrane may be present in the ratio 7:3. I3C NMR spectrum shows signals at 56.75, 49.90 and 167.97 ppm in addition to those reported for the parent silatarane [57.61 (OCH2); 50.32 (NCH2); 133.82 (t, NCS)] 111. The signals at 56.75 and 49.90 ppm may be attributed to OCH2 and NCH2 of the desulfurized silatrane. The signal at 167.97 ppm is assigned to Si-NC (isocyanate group). The cyanate group appears at 120-145 ppm, whereas isocyanate group resonates at 160-180 ppm. Therefore the desufurized product is characterized as 1-isocyanosilatrane. 29Si NMR exhibits a triplet centered at - 103.03 ppm and a singlet at - 105.85 ppm. The triplet is characteristic of 1- isothiocyanato silatrane due to 29Si-l4N coupling 111. The singlet at - 105.85 is assigned to isocyanosilatrane. The chemical shift value supports pentacoordination around silicon. 29Si chemical shifts are reported to experience more shielding on increasing the electronegativity of functionality at the apical Si atom /10/. The electronegativity values /ll/ of NCS (3.22) and NC (3.26) are comparable, whereas that of CN (2.69) is relatively less. 29Si NMR chemical shift values observed above are also comparable and favour isocyanide functionality on silicon. 203 Vol. 33, Nos. 4-5, 2010 Desulfurization of Isothiocyanato Silatrane First Report The spectral data presented above support the formation of isocyanato silatrane from a single step photolytic cleavage of isothiocyanato silatrane without disturbing the silatranyl cage. The future work will relate to the isolation of isocyanosilatrane in pure form. CONCLUSIONS Silyl isocyanides have resisted isolation so far. This isomeric form has been detected only by molecular spectroscopy /12/. Otherwise, only a small amount of isocyanide is found to be present in equilibrium with predominant silyl cyanide. The present study has offered a route for formation of silyl isocyanide. Silatrane, which is a fairly stable pentacoordinate silane, has been a successful model for such reactivity studies. ACKNOWLEDGEMENTS Authors are thankful to CS1R, New Delhi for financial assistance and respective affiliated institutes for supporting the work. REFERENCES 1.C-Y. Chen, F. F. Wong, J-J. Huang, S-K. Lin, M-Y. Yeh, Tet. Lett. 49 (2008), p. 6505-07. 2. (a) I. Shibuya, M. Goto, M. Shimizu, M. Yanagisawa and Y. Gama, Heterocycles, 51 (1999), 2667. (b) H. Ghosh, R. Yella, J. Nath and Β. K. Patel, European J. Org. Chem., (2008), 6189. 3. F. C. Bowden, R. Giles, R. N. Haszeldine, J. Chem. Soc. Chem. Comm., 1974, 578. 4. R. O. Harris, J. Powell, A. Walker and P. 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