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On-line version ISSN 0717-9707
J. Chil. Chem. Soc. vol.55 no.1 Concepción 2010
J. Chil. Chem. Soc, 55,on° 1 (2010), pág: 137-140
DIFUNCTIONAL SILARYLENE-CONTAINING ALIPHATIC COMPOUNDS. SYNTHESIS AND CHARACTERIZATION OF BIS(p (CARBOXYMETHYL)PHENYL)(R)PHENYLSILANE AND BIS(p-(2-AMINOETHYL)PHENYL)(R)PHENYLSILANE (R = Me, Ph)
CLAUDIO A. TERRAZA1*, LUIS H. TAGLE1 AND HANS LEMBACH1
1 Pontificia Universidad Católica de Chile, Faculty of Chemistry, Department of Organic Chemistry. P.O. Box 306, Santiago, Chile,
This work describes the synthesis of aliphatic diacids and diamines containing a silarylene moiety in the structure. These compounds can be employed as monomers for the synthesis of poly(amides) and poly(esters) or other condensation polymers. Thus, bis(p-methylphenylen)(R)phenylsilanes were obtained from p-bromotoluene and dicloro(R)phenylsilane (R= Me, Ph). Also, the dibenzyl nitrile compounds bis(p-(cyanomethyl)phenylen)(R)phenylsilanes were synthesized from the respective bis(p-(bromomethyl)phenylen) (R)phenylsilanes derivatives. The hydrolysis of the dibenzyl nitrile derivatives gives the respective diacids bis(p-(carboxymethyl)phenylen)(R)phenylsilanes with low yield. A direct Grignard reaction type was also studied using as precursors the dibenzyl bromide compounds. In a second stage, the synthesis of the phenethylamine bis(p-(2-aminoethyl)phenylen)(R)phenylsilane was studied by the reduction of the dibenzyl nitrile compounds. All compounds were characterized by IR-TF, 1H, 13C and 29Si NMR.
Keywords: Silicon, silarylene, dibenzyl bromide, dibenzyl nitrile, diphenylacetic acid, phenethylamines
The route for synthesizing diphenylacetic acids and phenethylamines containing silicon as central atom in their structure has been employed by some authors with tolyl derivatives as the initial material. In this sense, Frankel et al.1 prepared a series of silicon-containing phenethylamine derivatives: p-triethylsilylphenethylamine,p-trimethylsilylmethyl phenethylamine and D,L-l-p-trimethylsilylmethylphenyl-2-aminopropane from triethyl-p-tolylsilane. On the other hand, Migdal et al.2 synthesized bis(o-(carboxymethyl)phenylen) dimethylsilane in order to prepare thermoplastic poly(esters) of low molecular weight by interfacial polymerization. Thep-carboxymethylene isomer also was prepared2,3 and together with bis(p-(hydroxymethyl)phenylen)dimethylsilane4 was employed to prepare several poly(urethanes), poly(carbonates) and poly(esters). Other silicon-containing aliphatic difunctional compounds based on Ar-Si(CH3)2-Ar moiety were prepared by Rotman et al.3: bis(p-(formylphenylen)dimethylsilane, by hydrolysis of the respective dibenzyl bromide, and bis(p-(2-hidroxyethyl)phenylen)dimethylsilane, by reduction of the corresponding diphenylacetic acid. Later, a patent5 appeared where similar silicon-containing derivatives were synthesized, including bis(p-(dibromomethyl)phenylen)dimethylsilane, bis(p-(chlorocarbonymethyl) phenylen)dimethylsilane, and bis(p-(N-methylcarbamoylmethyl)phenylen) dimethylsilane.
Kim et al.6 worked on compounds containing the Ar-Si(Ph)2-Ar, Ar-Si(Me) (nHex)-Ar or Ar-Si(nBu)2-Ar unit. The authors synthesized the series of bis(p-tolyl), dibenzyl bromides and dibenzyl triphenylphosphonium salt derivatives from p-bromotoluene, in order to obtain poly(p-phenylenevinylene)-related polymers.
Silicon-containing aromatic compounds have been synthesized, characterized and employed as monomers of polycondensation materials. Thus, it is possible to find structures based on bis(p-hydroxyphenylene)7-13, bis(p-bromophenylene)14, bis(aminophenylene)meta and para isomers,10,14-16 and bis(p-carboxyphenylene)111718, bis(p-ch orocarbonylphenylene)1117-21, bis(p-carboxyalkylphenylene)20,22, bis(p-(3,4 dicarboxyphenylene)) dianhydride1623-26, bis(p-isocyanatephenylene)27, bis(p-carbohydrazidophenylene)22, bis(p acidazidephenylene)14 and bis(p-chloroformatephenylene)1328 moieties.
The several polymers obtained from these aromatic monomers show low chain-mobility with low to moderate glass transition temperatures (Tg). On the other hand, the solubility of these polymers in common organic solvents is strongly limited. Therefore, the incorporation of two methylene units in the new monomers and consequently in the repetitive unit of the polymers could be a positive factor in decreasing Tg while increasing the solubility of the materials without loss of the mechanical properties or great decrease of the thermal decomposition temperature (TDT). So, the next step of this work will be the synthesis of poly(esters), poly(amides), poly(imides) and/or polymer with combinations of these organic functions, containing monomers with and without methylene units as flexible elements.
N-Bromosuccinimide (NBS), benzoyl peroxide (BPO), sodium cyanide, lithium aluminium hydride (LiAlH4), diethylene glycol (DEG), tetrabutylammonium bromide (TBAB) and PhSi(R)Cl2 (R= Me, Ph) were obtained from Aldrich Chemical (Milwaukee, WI) and used without further purification. Tetrahydrofuran (THF) and diethyl ether (Aldrich Chemical, Milwaukee, WI) were dried over sodium previous to use. All the reagents and solvents were purchased commercially as analytical-grade and used without further purification.
The IR spectra (KBr pellets) were recorded on a Perkin-Elmer (Fremont CA) 1310 spectrophotometer over the range of 4000-450 cm-1. Melting points (uncorrected) were obtained on a SMP3 Stuart Scientific melting point apparatus. 1H, 13C and 29Si NMR spectra were carried out on a 400 MHz instrument (Bruker AC-200) using CDC13 as solvent and TMS as the internal standard.
Bis(p-methylphenylen)(R)phenylsilane (1a:R=Me, 1b:R=Ph) derivatives were obtained according to the following procedure:11,29,30 A solution of p-bromotoluene (0.163 mol) in Et2O anh. was slowly added over a mixture of Li (0.40 mol) and Et2O anh. The p-tolyllithium solution was refluxed for 2.5 h, cooled at 0 °C and then added the solution of the respective dichloro(R) phenylsilane (8x10-2 mol) in anh. Et2O. The system was refluxed for 4 h, cooled at room temperature and a HC1 aq. solution was added in order to destroy the main portion of the remainder Li. After filtration, the organic portion was washed with water, dried using anh. MgSO4 and the solvent removed. The white solid obtained was recrystallized from ethanol and characterized.
The dibenzyl bromide (2a:R=Me, 2b:R=Ph) derivatives were synthesized according to a procedure described previously.5,6 This reaction uses benzoyl peroxide (0.75 mol%) as radical initiating agent and NBS (0.016 mol) in 100 mL of CC14 under nitrogen atmosphere. The mixture was heated and a solution of the respective bis(p-methylphenylen)(R)phenylsilane (8xl02 mol) in CC14 was slowly added. The system was refluxed for 6 h and then stirred at room temperature during 24 h. After cooling, the remainder NBS was removed by filtration and the solution was washed with water. The organic portion was treated with anh. CaCl2, filtered and the solvent was removed by distillation. The white solid obtained was used without further purification due to its high spectroscopic purity.
Bis(p-(bromomethyl)phenyenl)methylphenylsilane (2a): Yield 32%, mp 120-122 °C. IR (KBr) (n) (cm-1): 3067 (C-H arom.), 2957, 2928 (C-H aliph.), 1598,1503 (C=C arom.), 1426,1109,726 (silyl-Ph), 1228,787 (silyl-CH3), 832, 700 (arom. p-subst), 745 (arom. mono-subst), 605 (C-Br). 1H NMR (CDC13) (d) (ppm): 0,86 (s,3H,Si-CH3), 4,52 (s,4H,CH2), 7,24-7,68 (m,13H,Ph-H). 13C NMR (CDC13) (d) (ppm): -3.39 (Si-CH3), 33.3 (CH2), 120.0 (C7), 129.7 (C3), 129.7 (C8), 134.3 (C6), 135.6 (C2), 136.4 (C5), 137.0 (Cl), 139.1 (C4). 29Si NMR (CDC13) (d) (ppm): -10.8.
Bis(p-(bromomethyl)phenylen)diphenylsilane (2b): Yield 38%, mp 211 - 214 °C. IR (KBr) (n) (cm-1): 3066 (C-H arom.), 2957, 2916 (C-H aliph.), 1484, 1427 (C=C arom.), 1395, 1108, 726 (silyl-Ph), 785, 700 (arom. p-subst), 737 (arom. mono-subst.), 606 (C-Br). 1HNMR(CDC13) (d) (ppm): 4.52 (s,4H,CH2), 7.21-7.72 (m,18H,Ph-H). 13C NMR (CDC13) (d) (ppm): 33.3 (CH2), 125.9 (C7), 128.0 (C3), 125.8 (C8), 129.9 (C6), 130.2 (C2), 134.5 (C5), 136.3 (Cl), 139.2 (C4). 29Si NMR (CDC13) (d) (ppm): -14.3.
These syntheses were developed following classic references for phase-transfer reactions31. Sodium cyanide (0.10 mol) and TBAB (1x10-2 mol) were dissolved in 40 mL of water. The solution was stirred for 15 min. at a temperature close to 50 °C. Later, the mixture was cooled at room temperature and 5x10-2 mol of the respective dibenzyl bromide derivative dissolved in 20 mL of CHC13 was added. The systems was refluxed for 2 h and then cooled. Three portions of water were used for removing salts and the organic portion was treated with anh. CaCl2. After filtration the solvent was removed and the crude product was dried at 50 °C during 12 h.
3b (R = Ph) was successively recrystallized from ethanol in order to obtain a white solid, while 3a (R= Me) was dissolved in n-hexane and purified by chromatographic techniques using silica gel (Merck, 70-230 mesh) and a mixture of n-hexane/ethyl acetate (90/10 vol/vol) as eluent.
Bis(p-(cianomethyl)phenylen)methylphenylsilane (3a): Yield 38%, mp 35 - 38 °C. IR (KBr) (n) (cm-1): 3068 (C-H arom.), 2959 (C-H aliph.), 2250 (CN), 1599, 1427 (C=C arom.), 1395, 725 (silyl-Ph), 1257, 784 (silyl-CH3), 806, 699 (arom. p-subst), 725 (arom. mono-subst.). 1H NMR (CDC13) (d) (ppm): 0.83 (s,3H,Si-CH3), 3.69 (s,4H,CH2), 7.28-7.58 (m,13H,Ph-H). 13C NMR (CDCLJ (d) (ppm): -3.38 (Si-CH3), 23.6 (CH2), 117.8 (CN), 127.6 (C7), 128.2 (C8), 128.8 (C4), 129.9 (C3), 131.5 (Cl), 135.2 (C2), 135.9 (C5), 136.1 (C6). 29Si NMR (CDC13) (d) (ppm): -10.6.
Bis(p-(cianomethyl)phenylen)diphenylsilane (3b): Yield 47%, mp 137 - 139 °C. IR (KBr) (n) (cm-1): 3067 (C-H arom.), 3025, 2915 (C-H aliph.), 2250 (CN), 1598, 1427 (C=C arom.), 1415, 1109, 726 (silyl-Ph), 799, 702 (arom. p-subst), 737 (arom. mono-subst.). :H NMR (CDC13) (d) (ppm): 3.76 (s,4H,CH2), 7.24-7.57 (m,18H,Ph-H). 13C NMR (CDC13) (d) (ppm): 23.6 (CH2), 117.6 CN), 127.5 (C7), 128.1 (C3), 129.9 (C8), 131.5 (C4), 133.2 (C5), 134.1 (Cl), 136.5 (C6), 137.1 (C2). 29Si NMR (CDC13) (d) (ppm): -14.4.
The preparation of diphenylacetic acid (4a:R=Me, 4b:R=Ph) derivatives was realized following techniques already reported by Rotman et al. for dimethylsilane compounds3. Thus, 3xl-3 mol of the dibenzyl nitrile derivative was dissolved in 15 mL of DEG and the solution was heated at 40 °C during 30 min. Then 7 mL of aq. NaOH 50% solution was added and then refluxed at 90 DC for 48 h. After this, the system was cooled and aq. HC1 (5%) solution was added until pH 1 was obtained. The organic portion was separated and washed with water and then treated with aq. NaOH 4% solution. The alkaline portion was acidified with aq. HC1 and the crude product obtained was re-dissolved employing NaOH solution. A semi-solid of adequate spectroscopic purity was obtained after extracting with Et2O, wash with water, dry with anh. MgSO4 and distil the solvent.
Bis(p-(carboxymethyl)phenylen)methylphenylsilane (4a): Yield 37%. IR (KBr) (n) (cm-1): 3396 (COO-H), 3069, 3045 (C-H arom.), 2958, 2922 (C-H aliph.), 1708 (C=0), 1599, 1427 (C=C arom.), 1397, 1109, 736 (silyl-Ph), 1402, 1251 (silyl-CH3), 785, 700 (arom. p-subst), 723 (arom. mono-subst.). [H NMR (CDC13) (d) (ppm): 0.82 (s,3H,Si-CH3), 3.66 (s,4H,CH2), 7.27-7.79 (m,13H,Ph-H), 10.0 (s,2H,COOH). 13C NMR (CDCLJ (d) (ppm): -3.37 (Si-CH3), 41.1 (CH2), 127.9 (C7), 128.8 (C3), 128.9 (C8), 134.5 (Cl), 134.9 (C5), 135.8 (C4), 136.4 (C2), 136.7 (C6), 177.5 (COOH). 29Si NMR (CDC13) (d) (ppm): -10.8.
Bis(p-(carboxymethyl)phenylen)diphenylsilane (4b): Yield 41%. IR (KBr) (n) (cm-1): 3404 (COO-H), 2995, 2980 (C-H arom.), 2941, 2875 (C-H aliph.), 1757 (C=0), 1523, 1400 (C=C arom.), 1377, 1158, 726 (silyl-Ph), 869, 717 (arom. p-subst), 745 (arom. mono-subst.). :H NMR (CDCLJ (d) (ppm): 3.55 (s,4H,CH2), 7.18-7.61 (m,18H,Ph-H), 8.78 (s,2H,COOH). 13C NMR (CDCLJ (d) (ppm): 41.3 (CH2), 128.0 (C7), 128.1 (Cl), 129.1 (C3), 129.8 (C8), 134.1 (C5), 135.3 (C4), 136.5 (C6), 136.8 (C2), 177.0 (COOH). 29Si NMR (CDCLJ (d) (ppm):-14.5.
The phenethylamine (5a:R=Me, 5b:R=Ph) derivatives were synthesized by reduction of the dibenzyl nitrile compounds according to described procedures1,3. Thus, 5x10-2 mol of the respective bis(p-(cianomethyl)phenylen) (R) phenylsilane was dissolved in 70 mL of THF and placed in a water-ice bath. Then a solution of 3x10-2 mol of LiAlH4 in 100 mL of THF was slowly added. The system was refluxed for 4 h and then stirred at room temperature during 12 h. After this, the system was cooled at 0 °C and 5 mL of water were added with stirring for 1 h. After filtration, the solution was treated with anh. CaCl2 and the solvent removed by distillation.
Bis(p-(aminomethyl)phenylen)methylphenylsilane (5a): Yield 58%. IR (KBr) (n) (cm-1): 3373 (N-H), 3066, 3011 (C-H arom.), 2954, 2851 (C-H aliph.), 1599, 1427 (C=C arom.), 1394, 1109, 726 (silyl-Ph), 1394, 1252 (silyl-CH3), 784, 699 (arom. p-subst), 723 (arom. mono-subst.). :H NMR (CDC13) (d) (ppm): 0.93 (s,3H,Si-CH3), 1.97 (s,4H,NH2), 2.68-2.80 (t,4H,Ph-CH2), 2.82-3.00 (t,4H,CH2-N), 7.14-7.78 (m,13H,Ph-H). 13C NMR (CDC13) (d) (ppm): -3.00 (Si-CH3), 40.1 (Ph-CH2), 43.4 (CH2-N), 127.6 (C3), 128.6 (C7), 129.5 (C8), 132.8 (Cl), 135.4 (C2), 136.2 (C6), 139.7 (C5), 141.2 (C4). 29Si NMR (CDC13) (d) (ppm): -10.6.
Bis(p-(aminomethyl)phenylen)diphenylsilane (5b): Yield 55%. IR (KBr) (n) (cm-1): 3359 (N-H), 3067, 3011 (C-H arom.), 2919,2851 (C-H aliph.), 1599, 1428 (C=C arom.), 1393, 1108, 726 (silyl-Ph), 803, 701 (arom.p-subst), 745 (arom. mono-subst). :H NMR (CDCLJ (d) (ppm): 2.12 (s,4H,NH2), 2.72-2.79 (t,4H,Ph-CH2), 2.80-3.01 (t,4H,CH2-N), 7.12-7.76 (m,18H,Ph-H). 13C NMR (CDC13) (d) (ppm): 39.9 (Ph-CH2), 43.2 (CH2-N), 127.8 (C3), 128.5 (C7), 128.8 (C8), 134.4 (Cl), 136.4 (C2), 136.8 (C6), 139.8 (C5), 141.2 (C4). 29Si NMR (CDC13) (d) (ppm): -14.4.
RESULTS AND DISCUSSION
Difunctional silarylene-containing compounds were synthesized from p-bromotoluene and Cl2SiCH3Ph or Cl2Si(Ph)2 as starting materials. Thus, the bis(p-tolyl) derivatives were converted into the respective dibenzyl bromides by reaction with NBS and BPO. Later, the dibenzyl nitriles were obtained by reaction of the bromide compounds with NaCN and then hydrolyzed or reduced in orderto prepare the diphenylacetic acids orthe respective phenethylamines.
The synthesis and spectroscopic characteristics of bis(p-methylphenylene) derivatives (1) were already described11,29,30. In these procedures we obtained high final yields using a20%-mol of lithium excess inthep-methylphenyllithium preparation: 77% (la) and 82% (1b). After recrystallization from ethanol the white solids showed a narrow melting point interval (82-83 °C and 118-119 °C for R= Me and Ph, respectively).
The synthesis of the dibenzyl bromide compounds (2) followed the Wohl-Ziegler methodology5,6, which uses radical reactions promoted by PBO and NBS. The general route starting from bis(p-methylphenylen)methylphenylsilane and bis(p-methylphenylen)diphenylsilane is shown in figure 1.
In this reaction, the mixture time between BPO and NBS before adding the respective bis(p-tolyl) derivative (1) should ensure the formation of bromine radical and minimize the formation of molecular bromine. According with our observations, the optimal condition is accompanied by a change in the color of the system from yellow to light-orange. In both reactions the yields are below 40%; however, when we increase the mixture time, lower yields are obtained. These results agree with the general conclusión: the yield of reaction is strongly dependent on this initial mixture time.
The isolation procedure allows obtaining crude yellow-solids, which were purified by repeated treatment with Et2O. Thus, white solids were characterized by their melting points: near 120 °C for the methyl derivative (2a) and 212 °C for the phenyl compound (2b). On the other hand, the IR-FT spectroscopic results showed common bands for bis(p-methylphenylene) derivatives, standing out the C-Br stretching band at 605 cm-1 (Table 1). IR and 1H NMR data for bis(p-(bromomethyl)phenylen)diphenylsilane (2b) were already reported by Kim et al.5. Our characterization for this compound includes 13C and 29Si NMR data (Experimental Part).
The disappearance of the signal at 2.3 ppm related with the methyl group from p-tolyl derivatives and the appearance of a new signal at 4.5 ppm in the 1H NMR spectrum is indicative of the formation of the -CH2Br group. A similar conclusion is obtained from 13C NMR where the signal at 21.7 ppm is replaced by a new one at 33.3 ppm. Itis clear that the silicon atom shields near the nuclei, so, the protons from the methyl group that is directly bonded to Si show a chemical shift of 0.86 ppm, while the carbon atom from same group appears at -3.39 ppm. This effect was possible to see in all spectra of the several compounds synthesized.
For the synthesis of dibenzyl nitriles (3a,b) two general reaction conditions were developed using the dibenzyl bromide derivatives as starting materials. The direct cyanidation reaction agrees with reports5 using an ethanol-water mixture as solvent under different substrate-NaCN molar relation (1:3 and 1:6), temperature (40 and 90 °C) and time (2-96 h.), in the sense that do not promote obtaining of product. Due to this, phase-transfer conditions were developed using an organic solvent-water mixture and a phase transfer catalyst31. The solvents were CC14, CH2C12, Et2O, nhexane, toluene and CHC13 while tetrabutylammonium bromide (TBAB) and hexadodecyltributylphosphonium bromide were used as transfer agent. Also, the time and molar relation between the substrate and NaCN were studied. The reaction optimal conditions were found for CHC13, TBAB and a molar relation near 1:20 for dibenzyl bromide:NaCN. The yields obtained, between 38-47%, agree with the specific nature of the substrate. If this relation between reagents is also used in the direct cyanidation reactions, the respective dibenzyl nitrile compounds are obtained. The main factor in the synthesis of these derivatives is the relation between substrate and NaCN.
The semi-solid crudes of brown color obtained from phase-transfer reactions can be purified by two methodologies: one is recrystallization from ethanol with great loss of mass because at least four processes are necessary to obtain white solids of high purity. The other method is to use a chromatographic technique. According to the different retention factors shown by dibenzyl bromides (Rf= 0.77) and dibenzyl nitriles (Rf= 0.36) in the SiO2-n-hexane/ethyl acetate (70/30 vol/vol) system, it was possible to isolate the producís in good yields. The specific signnals in the IR and NMR spectra are shown in Table 1.
The mtrile stretching band is the most important signal in the IR spectra and it appears at 2250 cm-1. The NMR data show the displacement of the signal of the methylene group at high field due to the substitution of the bromine atom by the cyano group. On the other hand, for 13C NMR the nitrile carbon appears clearly at 117 ppm.
The synthesis of diphenylacetic acids (4a,b) was developed following two routes: The first included the use Grignard's techniques from the respective dibenzyl bromide (2). The second option was the direct basic hydrolysis of dibenzyl nitriles (3).
The Grignard reaction was employed by Kitamura et al.32to obtain silicon-containing polymers. These authors reacted 2,7-dibromo-9,9-R1R2fluorenyl magnesium with several silicon-containing substrates. In our case, we tried to prepare the Grignard reagent from a silarylene compound and then add over C02. In spite of the experimental modifications carried outthe results were not good. By using different anhydrous organic solvents, such as Et2O and THF, several times and the reaction temperature employed in step 1 (0.5-10 h. and 20 °C-boiling point, respectively) or the use of iodine-crystal, do not promote the formation of the organo-magnesium intermediate. Due to these results, the basic hydrolysis of dibenzyl nitrile compounds was used to obtain the disalt first and then by acid treatment the respective diphenylacetic acid. For this purpose, we dissolved dibenzyl nitrile in ethanol and then added the aq. KOH solution33. After the reaction and the isolation process, the yield was under 10%. When we replaced ethanol by diethyleneglicol, according to the Rotman et al. technique5, the yield obtained was better, but still very low (near 40%).
Although the product obtained after the re-precipitation in basic and then acid medium gives a semi-solid, this compound presents an appropriate spectroscopic purity. In Table 1 are summarized the principal bands found in the IR spectra, standing out the C=0 and O-H stretching bands of carboxylic acid group near 1600 and 3400 cm-1, respectively. The 1H NMR spectram of 4a shows similar values for the methylene group while the 4b compound shows a slight but clear displacement at low field. In both 13C NMR spectra no signnals associated to the CN group was found and a new signal at 177 ppm appears (-COOH).
Finally, the reduction of dibenzyl nitrile compounds to obtain the respective phenethylamine was developed using LiAlH4, in agreement with previous reports1,3. The crude yields were moderate, with values near 60%. The phenethylamines (5a,b) were characterized without further purification because they decomposed on prolonged heating. The spectroscopic data show the characteristic signnals for a primary amine: In the IR spectra it is possible to see the stretching bands associated to the N-H bond (Table 1). On the other hand, the 1H NMR spectra show the -CH2CH2- unit with adequate chemical shifts, multiplicity and integration values. Likewise, the 13C NMR analyses show two methylene groups with displacements according with their electronic environments; CH2-NH2 appears to a lower field than the CH2-Ph unit. The nature of methylene groups in all synthesized compounds was ratified by DEPT-135° analyses, whose results were not included in this work.
For all synthesized compounds: bis(p-tolyl), dibenzyl bromides, dibenzyl nitriles, diphenylacetic acids, and phenethylamines, the 29Si NMR spectra show two environments for the silicon atom. When the silicon atom is bonded to phenyl groups only, their chemical shift is approximately -14.3 ppm. If a phenyl group is replaced by an aliphatic group such as the methyl group, the chemical shift appears at a lower field (approximately - 10.7 ppm). This effect was already reported by our group,13 whereby an electronic retrodonation could be taking place between the p-aromatic system and the silicon atom d orbital.
A series of difunctional aliphatic compounds containing a silarylene unit in their structure were synthesized and characterized: bis(p-methyl-phenylen)methylphenylsilane, bis(p-methylphenylen)diphenylsilane, bis(p-(bromomethyl)phenylen)methylphenylsilane, bis(p-(bromomethyl)phenylen) diphenylsilane, bis(p-(cyanomethyl)phenylen)methylphenylsilane, bis(p-(cyanomethyl)phenylen)diphenylsilane, bis(p-(carboxymethyl)phenylen) methylphenylsilane, and bis(p-(carboxymethyl)phenylen)diphenylsilane.
The cyanidation reaction of the dibenzyl bromide derivatives is especially sensitive to the concentration of NaCN. It is necessary to use the system near saturation, maintaining the molar proportion 1:20 for substrate:cyanide ion. On the other hand, the reduction reaction of the dibenzyl nitrile derivatives, allows to obtain the phenethylamines, bis(p-(2-aminoethyl)phenylen) methylphenylsilane and bis(p-(2-aminoethyl)phenylen)diphenylsilane, with yields of 58% and 55%, respectively. In all cases, the spectroscopic analyses of IR-FT, 1H- and 13C-NMR clearly allowed to identify the structures.
For all the precursors and monomers 29Si-NMR analyses were carried out. The chemical displacements obtained evidence the different environments of the silicon atoms, whose magnitudes agree with the nature of the groups bonded to the heteroatom. Thus, for aliphatic groups the chemical displacement appears at lower field that that when the silicon atom is bonded to phenyl groups. For these latter substituents, it is possible to postulate an electronic retrodonation between the p-aromatic system and the silicon atom d orbital, which would shield the analyzed nucleus.
The authors acknowledge Fondo Nacional de Investigaclosn Científica y Tecnológica, FONDECYT, through Project 1070778.
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(Received: August 27, 2009 - Accepted: December 2, 2009)