The synthesis and stereoelectronic structure of 1-and 2-(trimethoxysilylmethyl)- and 1-and 2-(silatranylmethyl)benzotriazole

Only 2-benzotriazolyl sodium is formed in the reaction of benzotriazole (BtH) with sodium methoxide. Its reaction with trimethyl-or trimethoxy-(chloromethyl)-silane affords a mixture of 1-and 2-(trimethylsilylmethyl)- 1 and 2 , and 1-and 2-(trimethoxysilylmethyl)benzotriazoles, 3 and 4 , which were separated into the individual isomers. trans-Etherification of the latter with tris-(2-hydroxyethyl)amine leads to 1-and 2-(silatranylmethyl)benzotriazoles 5 and 6 . The electronic structures of the compounds prepared were investigated using IR, UV, multinuclear NMR spectroscopy, mass spectrometry, and X-ray diffraction. The organosilicon benzotriazole derivatives 1 , 3 and 5 show a benzenoid structure, whereas 2 , 4 and 6 have ortho-quinonoid structures.


A B
Unfortunately, the available experimental and theoretical results are often contradictory.Thus, for example, it has been reported that according to UV spectra the 2H-tautomer in the gas phase is approximately 4 kcal/mol more stable than the 1H-tautomer. 16][7][8] The microwave spectrum at 90°С has established only 1H-BtH. 7All the peculiarities of the fluorescence excitation spectrum of BtH at 70°С are explained by the presence of the 2Нtautomer. 1,6][11][12] According to the UV spectra of BtH, and 1-Me-and 2-Me-BtH at 90°C, there is a mixture of both BtH tautomers in an approximately 80/20 ratio. 1 The probable reason why ≈20% of the 2Htautomer has not been detected is easy to understand, considering the corresponding dipole moments (4.2 and 0.4 D, respectively).Since the microwave signals are proportional to the square of the dipole moments, the signals of the 1H-tautomer should be almost 500 times more intense than those of the 2H-tautomer.
The proportion of the 2H-tautomers is about 45, 35 and 25% at 30, 50 and 80 0 C, respectively, and shows 2 that, contrary to all available calculations, the 2H-tautomer is enthalpically more stable than the 1H-tautomer by about 3.8 kcal mol -1 .
The intrinsically most stable tautomer of benzotriazole is the 2H-, and the origin of the difference in stability with regard to the 1H-tautomer results in a balance between the lone-pair repulsion (6.5 kcal mol -1 ) [16][17][18] which favors the 2H tautomer and the aromaticity which favors the 1H tautomer. 1he goal of our research is to synthesize the individual organosilicon BtH derivatives containing at the nitrogen atoms organosilicon substituents such as -CH 2 SiMe 3 (1 and 2), -CH 2 Si(OMe) 3 (3 and 4), and -CH 2 Si(OCH 2 CH 2 ) 3 N (5 and 6) 19 in position 1 or 2, and to study their electronic structure using IR, UV, multinuclear NMR spectroscopy, mass spectrometry and X-ray diffraction.The interest in the BtH derivatives containing at the nitrogen atoms such bulky substituents is explained by the stability of their 1-and 2-isomers to interconversion, as well as the possibility of establishing the influence of electron-donor inductive effect of -CH 2 SiMe 3 , -CH 2 Si(OMe) 3 and -CH 2 Si(OCH 2 CH 2 ) 3 N groups (σ I = -0.04,σ* = -0.25 for 1 and 2; σ I = -0.10,σ* = -0.62 for 3 and 4 and σ I = -0.36,σ* = -2.241][22] The clearly defined crystalline structure of N-silatranylmethyl derivative 6 is favorable for structural investigation using X-ray diffraction.
We have established that the reaction of 1-BtH with sodium methoxide in methanol in the temperature range from -5 to 60°С gives only (2-benzotriazolyl)sodium 7 in all cases.
Compounds 5 and 6 are colorless crystals, soluble in DMF and DMSO.Their physicochemical characteristics and elemental analysis data are also presented in Table 1.Upon keeping, even in a sealed ampoule, the crystals of 5 become turbid and this did not permit structural determination by X-ray diffraction.See Table 1

Spectroscopic studies UV and IR spectroscopy
The UV spectrum of (2-benzotriazolyl)sodium 7 in polar solvents (МеОН, DMF) shows only one intense maximum at 283 nm (Table 2) similar to the spectra of 2-Ме-Bt 1,2 which has an ortho-quinonoid structure.Its IR spectrum also indicates the sodium atom to be in the benzotriazole ring position 2 (Table 3).
See Table 2 on page 235.
The UV spectra of the organosilicon derivatives 1, 3, 5, and 2, 4, 6, respectively, which show benzenoid and ortho-quinonoid structures, differ in both the number and intensity of electronic transitions.Thus, in the spectra of solutions of the 1-substituted BtH 1, 3 and 5 there are two absorption maxima in the 270-290 and 250-270 nm regions, which are analogous to those of 1Н-and 1Ме-Bt. 1,2,5The UV spectra of the 2-isomers 2, 4 and 6 display only one maximum in the 270-285 nm region, as in the spectra of 2Ме-and 2Na-Bt (Figure 1, Table 2).From what is said above, it follows that UV spectroscopy provides an adequate instrument for establishing of the N-substituent position in the benzotriazole ring (1-or 2-).A slight hypsochromic shift of bands in the UV spectra of the (trimethoxylsilylmethyl)benzotriazoles 3 and 4 compared to the spectra of the corresponding trimethylsilylmethyl-( 1 and 2) and silatranylmethyl-(5 and 6) BtH derivatives is likely to be brought about by the effect of n,σ*hyperconjugation in the N-CH 2 Si fragment, 24,25 absent in molecules 1, 2, 5 and 6.The benzenoid 1, 3 and 5, and ortho-quinonoid structures 2, 4, 6 and 7, are clearly expressed in their IR spectra as well (Table 3). 19See Table 3 on page 236.

Nuclear magnetic resonance
The 1 H-, 13 C NMR chemical shifts of the BtH fragment of 1-and 2-isomers of compounds 3-6 and the corresponding N-methylbenzotriazoles 26 (Table 4) are practically identical.The same is also true for the proton chemical shifts in the spectra of compounds 1, 3-6, with an identical position of the N-substituent in the exocyclic fragment -N-CH 2 Si.The 13 C-chemical shifts of the 1-substituted BtH, 3 and 5, in the -N-CH 2 Si fragment are displaced upfield by ~ 10 ppm compared to those of the 2-substituted BtH.The 29 Si-resonance in the spectra of the isomeric silatrane derivatives 5 and 6 is displaced by 25-ppm upfield compared to that of the corresponding trimethoxysilylmethyl derivatives 3 and 4.This is due to the fact that in compounds 5 and 6 the silicon atom is pentacoordinate, whereas in 3 and 4 it is in the tetracoordinate state.At the same time, the difference of the organosilicon substituent's position (1-or 2-) in compounds 3-6 does not influence much the 29 Si chemical shift.In this case, the  4).Differences in the chemical shifts of the 1-and 2-isomers are over 100 ppm.
In the spectra of the 1-and 2-(trimethoxysilylmethyl)benzotriazoles, 3 and 4, and the 1-and 2-methylbenzotriazoles, 27 the 15 N-chemical shifts differ insignificantly (1-5 ppm).Compared to 1-(trimethoxysilylmethyl)benzotriazole 3, in the spectrum of 1-(silatranylmethyl)benzotriazole 5 the 15 N chemical shift of the N-1 atom is displaced downfield by 12 ppm, whereas the atom N-3 shows a 5-ppm displacement upfield.The chemical shifts of the N-2 atom in the spectra of 3 and 5 are nearly identical, and displaced slightly upfield (Table 4).The 15 N-resonance signal of the nitrogen atom in the silatrane skeleton of 1-and 2-(silatranylmethyl)benzotriazoles, 5 and 6, is practically the same.
Thus, the 15 N NMR spectra of BtH organosilicon derivatives 3-6 provide evidence that the nature of the CH 2 SiХ 3 substituent does not much influence the electron density of all the nitrogen atoms, but is sufficiently dependent on the nitrogen position (1-or 2-).
See Table 4 on page 237.

Mass spectrometry
Electron-impact induced disintegration of 1-and 2-(trimethylsilylmethyl)-1 and 2, and 1-and 2-(trimethoxysilylmethyl)benzotriazoles, 3 and 4, depends on the position of the CH 2 SiX 3 substituted in the benzotriazole ring.None of the spectra of the 1-isomers, 1 and 3, contains molecular ions; the heaviest are generated by the elimination of molecular N 2 .The initial elimination of N 2 is a process typical of 1-BtH and its 1-methyl-, 1-vinyl-, and 1-methoxy derivatives. 28he disintegration of compound 1 involves elimination of the hydrogen atom from the СН 2 group in fragment NCH 2 Si atoms, and formation of an ion m/z 176 (82%) (Scheme 1).

Scheme 1
The significant intensity of this ion, and subsequent disintegration by elimination of the HCN molecule to form an ion with m/z 149 (82%), suggests its possible bicyclic structure.
Despite the above-mentioned similarity of the mass spectra of 1-and 2-(trimethylsilylmethyl)and 1-and 2-(trimethoxysilylmethyl)-benzotriazoles, 3 and 4, they are sufficiently different.In the spectra of compounds 3 and 4 there is no peak for a [М-Х] + ion with m/z 190 (6 and 28 %, respectively), characteristic of compounds 1 and 2, but there are ion peaks arising as a result of benzotriazole cycle fragmentation, with m/z 105 (6 and 0 %), m/z 91 (70 and 77 %), m/z 77 (15 and 4 %, respectively).The high intensity of the ion with m/z 91 seems to be due to overlapping of the isobaric [Si(OMe) 3 -OCH 2 ] + ion with m/z 91.
The mass spectra of 1-and 2-(silantranylmethyl)benzotriazoles, 5 and 6, are rather meager.They contain a maximum peak of the silatrane skeleton ion with m/z 174 (100%), and lowintensity peaks of ions brought about exclusively by its disintegration (ions m/z 144 (7%), 130 (1%)).Minor disintegration of these ions is conditioned by C-Si bond breakage in the molecular ion with the positive charge localization on the silatrane skeleton Si(OCH 2 CH 2 ) 3 N + .The intensity of molecular ion peak of silatrane derivatives 5 and 6 is about 2%.
The bond lengths and valence angles in five-membered SiOCCN cycles of the silatranyl group in the molecule 6 are close (within the limits of error) to analogous parameters of molecules of (1silatranylmethyl)-1,2,4-triazole, -pyrrole, -indole and -carbazole.Their conformation is a poorly defined envelope with the С α atoms displaced from its plane.

Experimental Section
General Procedures.UV absorption spectra of solutions of compounds 1-7 were registered on a SPECORD UV-Vis spectrophotometer.IR absorption spectra were recorded on a SPECORD 75 IR spectrometer (thin films or KBr pellets). 1H-, 13 C-, 15 N-, and 29 Si NMR spectra were run on a Bruker DPX-400 spectrometer (frequency 400.13 MHz, 100.61MHz, 40.56 МHz, 79.49МHz).Those of compounds 1-7 were recorded as solutions in CDCl 3 (TMS internal standard). 29Si NMR spectra were obtained by using the INEPT pulse sequence. 15N-NMR chemical shifts were measured from inverse two-dimensional 1 Н-15 N spectra using the HMBCGP technique.The precision of measurements of the 1 Н-and 13 С-chemical shifts was 0.01 and 0.02 ppm, respectively, and 0.1 ppm for 15 N-and 29 Si-nuclei.Mass Spectra were recorded on an LKB-2091 mass spectrometer, ionizing voltage 60 eV, using direct introduction of the sample into the ion source (T source 250 о С).The intensities of 3608 independent reflections were measured on a Syntex P21 automatic diffractometer (МоKa-radiation, graphite monochromator, w-scanning).

2-Benzotriazolyl sodium (7).
A solution of freshly prepared MeONa (0.1 g of sodium in 10 ml of dry methanol) was added dropwise to a solution of 0.5 g of 1Н-Bt in 5 ml of dry methanol.The reaction mixture was stirred for about 20 min.Methanol was distilled off, using a rotary evaporator.The product 1 thus obtained was used in further reactions.

Figure 1 .
Figure 1.UV absorption spectra of the compounds 3 (a) and 4 (b) in MeCN; reaction mixture from 3 and 4 in DMF, dissolved in MeCN (c).

13 C
-signals of the N-CH 2 Si fragment in the spectra of the tetracoordinate silicon derivatives 3 and 4 are displaced upfield by more than 7 ppm compared to the 13 C-signals in the spectra of the corresponding silatranyl BtH derivatives, 5 and 6.The 15 N-NMR chemical shifts in the spectra of 1-and 2-(trimethoxysilylmethyl)-3 and 4, and the 1-and 2-(silatranylmethyl)benzotriazoles, 5 and 6, are strongly dependent on the organosilicon substituent's position in the triazole ring (1-or 2-) (Table