Methyltrioxorhenium Complexes of Polydimethylsiloxane

Apr 7, 2009 - ... and 1 (1%), the epoxide yield was many times higher than those observed when other typical base additives were employed in place of ...
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Organometallics 2009, 28, 2855–2863

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Methyltrioxorhenium Complexes of Polydimethylsiloxane-Functionalized Pyridine as Efficient Olefin Epoxidation Catalysts in Solventless and Low-Polar Solvent Conditions Matthew Herbert, Agustı´n Galindo,* and Francisco Montilla* Departamento de Quı´mica Inorga´nica, Facultad de Quı´mica, UniVersidad de SeVilla, Apartado 1203, 41071 SeVilla, Spain ReceiVed February 12, 2009

The methyltrioxorhenium (MTO) complex of polydimethylsiloxane (PDMS)-functionalized pyridine (1), [Re(O)3(CH3)(1)] (2), was synthesized, characterized, and investigated as an olefin epoxidation catalyst employing 30% H2O2 as oxidant. In the epoxidation of cis-cyclooctene, the influence of factors such as solvent conditions and temperature on the reaction was analyzed, and comparisons were made with analogous MTO-pyridine catalytic systems. Compound 2 is completely compatible with systems employing aqueous 30% H2O2 and catalyst stability is markedly superior than for MTO-pyridine systems, enabling higher yields. DFT calculations for model compounds of 2 suggest that interactions with the PDMS chains play an important role in this stability. The nonpolar PDMS function solubilizes 2 in the olefin substrates and homogenizes the catalyst-substrate system, consequently facilitating catalytic activity in the absence of organic solvents. Solventless epoxidations of several olefins were catalyzed by 2 with good epoxide yields and selectivity. In the solventless epoxidation of cis-cyclooctene catalyzed by MTO (0.005%) and 1 (1%), the epoxide yield was many times higher than those observed when other typical base additives were employed in place of 1. Introduction Epoxides are valuable precursors in the manufacture of a range of important fine chemicals,1 and one of the most important synthetic routes to this preparation is olefin epoxidation, which has been known for a century.2 The issues of sustainability, waste reduction, and the use of safer reagents3 in chemical processes are being increasingly prioritized as a result of environmental awareness. With respect to this, transition-metal-catalyzed synthetic oxidation processes employing environmentally benign oxidants (e.g., hydrogen peroxide4,5) are often superior to traditional stoichiometric reactions. Many catalytically active systems employing a variety of transition metals have been developed, allowing the epoxidation of a wide range of alkenes using hydrogen peroxide.6 Furthermore, immobilization of active homogeneous catalysts on solid supports has been described, which could potentially render them recyclable.7 * Corresponding author. Fax: +34 954 557153. E-mail: [email protected], [email protected]. (1) Sienel, G.; Rieth, R.; Rowbottom, K. T. In Ulmann’s Encyclopedia of Industrial Chemistry, 6th ed.; Verlag Chemie: Weinheim, 2003; p 269. (2) Prileschajew, N. Ber. Deutsch. Chem. Ges. 1909, 42, 4811. (3) (a) Sheldon, R. A. Chem. Commun. 2008, 3352. (b) Poliakoff, M.; Licence, P. Nature (London) 2007, 450, 810. (c) Lancaster, M. Green Chemistry: An Introductory Text; RSC Paperbacks Royal Society, 2002. (d) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: Oxford, 1998. (4) (a) Fioroni, G.; Fringuelli, F.; Pizzo, F.; Vaccaro, L. Green Chem. 2003, 5, 1. (b) Lane, B. S.; Burguess, K. Chem. ReV. 2003, 103, 2457. (c) Grigoropoulos, G.; Clark, J. H.; Elings, J. A. Green Chem. 2003, 5, 1. (5) For recent advances in asymmetric epoxidations see: Arends, I. W. C. E. Angew. Chem., Int. Ed. 2006, 45, 6250. (6) See for example: Jo¨rgensen, K. A. Chem. ReV. 1989, 89, 431. (7) Jain, K. R.; Ku¨hn, F. E. J. Organomet. Chem. 2007, 692, 5532.

Methyl trioxorhenium8 (MTO) is a good olefin epoxidation catalyst, able to efficiently activate hydrogen peroxide as an oxidant,9 although notably it is inert toward the activation of the often more active organoperoxides. Since MTO is among the most effective of the known transition metal epoxidation catalysts, much lower reaction times under mild conditions are observed when comparing, for example, with mechanistically analogous molybdenum oxodiperoxo-catalyzed systems.10 MTOcatalyzed epoxidations were initially investigated in a homogeneous system using tBuOH as solvent with H2O2.11 Systems of this type were initially of limited practical use due to low selectivity for the epoxide product. Catalytic ring-opening hydrolysis of the epoxide by the Lewis acidic rhenium center to give the 1,2-diol product was observed in the presence of water. Several experimental methods have been employed in order to overcome this problem. Using the dry oxidant urea-H2O2 (UHP) instead of aqueous H2O2 limits the concentration of water present in the system to only that occurring as a reaction byproduct, thus reducing the potential for hydrolytic decomposition to diols.12 This approach was taken by Owens (8) (a) Espenson, J. H. Chem. Commun. 1999, 479. (b) Herrmann, W.; Ku¨hn, F. E. Acc. Chem. Res. 1997, 30, 169. (9) (a) Ku¨hn, F. E.; Santos, A. M.; Herrmann, W. A. Dalton Trans. 2005, 2483. (b) Roma˜o, C. C.; Ku¨hn, F. E.; Herrmann, W. Chem. ReV. 1997, 97, 3197. (10) Ku¨hn, F. E.; Santos, A. M.; Abrantes, M. Chem. ReV. 2006, 106, 2455. (11) (a) Herrmann, W. A.; Fischer, R. W.; Marz, D. W. Angew. Chem., Int. Ed. 1991, 30, 1638. (b) Herrmann, W. A.; Fischer, R. W.; Rauch, M. U.; Scherer, W. J. Mol. Catal. A 1994, 86, 243. (12) (a) Carreiro, E. P.; Burke, A. J.; Curto, M. J. M.; Teixeira, A. J. R. J. Mol. Catal. A 2004, 217, 69. (b) Adam, W.; Mitchell, C. M. Angew. Chem., Int. Ed. 1996, 35, 533. (c) Boehlow, T. R.; Spilling, C. D. Tetrahedron Lett. 1996, 37, 2717. (d) Adam, W.; Mitchell, C. M.; SahaMo¨ller, C. R.; Weichold, O. J. Am. Chem. Soc. 1999, 121, 2097.

10.1021/om900111n CCC: $40.75  2009 American Chemical Society Publication on Web 04/07/2009

2856 Organometallics, Vol. 28, No. 9, 2009

and Abu-Omar in their studies of MTO-catalyzed epoxidations also in an ionic liquid13 and by us in our investigation of Mocatalyzed epoxidations in ionic liquids.14 Alternatively, many studies have investigated the addition of Lewis base ligands, which can increase the selectivity toward epoxides by inhibiting the unwanted Re-catalyzed ring-opening and additionally often enhance the rate of the epoxidation reaction. In this manner great improvements in the selectivities and rates of MTOcatalyzed epoxidations have been realized by the groups of Herrmann,15 Sharpless,16 and others.17 Typically in a biphasic (aqueous/organic) system, the addition of a significant excess of an aromatic monodentate Lewis base (particularly pyridine, pyrazole, and their substituted derivatives) over the catalyst not only suppresses unwanted ring-opening but also accelerates alkene epoxidation and catalyst lifetime relative to an analogous system employing only MTO. Low molecular weight siloxane-based polymers are liquids at room temperature, chemically very stable, inert, and nonvolatile and display relatively good solubility in nearly all organic solvents including even very nonpolar media such as supercritical carbon dioxide (scCO2).18,19 Despite this, their use as functional groups in the design of novel catalysts that make use of these properties has not been described until relatively recently,20 but over the past few years a number of reports detailing their application in catalytic systems have appeared.21 Systems where the catalyst (Jacobsen22 or Grubbs23) is incorporated in a polydimethylsiloxane (PDMS) membrane have been reported. The functionalization of ligands with PDMS, from which catalytic coordination compounds are then prepared, is (13) Owens, G. S.; Abu-Omar, M. M. Chem. Commun. 2000, 1165. (14) Herbert, M.; Galindo, A.; Montilla, F. Catal. Commun. 2007, 8, 987. (15) (a) Herrmann, W. A.; Kratzer, R. M.; Ding, H.; Thiel, W. R.; Glas, H. J. Organomet. Chem. 1998, 555, 293. (b) Herrmann, W. A.; Ding, H.; Kratzer, R. M.; Ku¨hn, F. E.; Haider, J. J.; Fischer, R. W. J. Organomet. Chem. 1997, 549, 319. (16) (a) Rudolph, J.; Reddy, K. L.; Chiang, J. P.; Sharpless, K. B. J. Am. Chem. Soc. 1997, 119, 6189. (b) Cope´ret, C.; Adolfsson, H.; Sharpless, K. B. Chem. Commun. 1997, 1565. (c) Adolfsson, H.; Converso, A.; Sharpless, K. B. Tetrahedron Lett. 1999, 40, 3991. (17) For example:(a) Yamazaki, S. Org. Biomol. Chem. 2007, 5, 2109. (b) Sabater, M. J.; Domine, M. E.; Corma, A. J. Catal. 2002, 210, 192. (c) Adolfsson, H.; Cope´ret, C.; Chiang, J. P.; Yudin, A. K. J. Org. Chem. 2000, 65, 8651. (d) Rudler, H.; Gregorio, J. R.; Denise, B.; Bre´geault, J.-M.; Deloffre, A. J. Mol. Catal. A 1998, 133, 255. (e) Villa, A. L.; Des Vos, D. E.; Montes, C.; Jacobs, P. A. Tetrahedron Lett. 1998, 39, 8521. (18) (a) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice; Butterworth-Heinemann Ltd.: Boston, 1994. (b) Chandrasekhar, V. Inorganic and Organometallic Polymers; Springer: Berlin, 2005; Chapter 6. (19) The influence of trimethylsilyl groups and carbosilane dendrons in the solubility has been previously addressed by us: (a) Sellin, M. F.; Bach, I.; Webster, J. M.; Montilla, F.; Rosa, V.; Avile´s, T.; Poliakoff, M.; ColeHamilton, D. J. J. Chem. Soc., Dalton Trans. 2002, 4569. (b) Montilla, F.; Rosa, V.; Prevett, C.; Avile´s, T.; Nunes Da Ponte, M.; Masi, D.; Mealli, C. Dalton Trans. 2003, 2170. (c) Montilla, F.; Galindo, A.; Rosa, V.; Avile´s, T. Dalton Trans. 2004, 2588. (d) Montilla, F.; Galindo, A.; Andre´s, R.; Co´rdoba, M.; de Jesu´s, E.; Bo, C. Organometallics 2006, 25, 4138. (20) Grunlan, M. A.; Regan, K. R.; Bergbreiter, D. E. Chem. Commun. 2006, 1715. (21) (a) Miller, A. L., II; Bowden, N. B. Chem. Commun. 2007, 2051. (b) Mwangi, M. T.; Runge, M. B.; Hoak, K. M.; Schulz, M. D.; Bowden, N. B. Chemistry 2008, 14, 6780. (22) (a) Parton, R. F.; Vankelecom, I. F. J.; Tas, D.; Janssen, K. B. M.; Knops-Gerrits, P.-P.; Jacobs, P. A. J. Mol. Catal. A 1996, 113, 283. (b) Janssen, K. B. M.; Laquiere, I.; Dehaen, W.; Parton, R. F.; Vankelecom, I. F. J.; Jacobs, P. A. Tetrahedron: Asymmetry 1997, 8, 3481. (c) Guedes, D. F. C.; MacLeod, T. C. O.; Gotardo, M. C. A. F.; Schiavon, M. A.; Yoshida, I. V. P.; Ciuffi, K. J.; Assis, M. D. Appl. Catal., A: Gen. 2005, 296, 120. (d) Pliego, J. R., Jr.; Schiavon, M. A. J. Phys. Chem. C 2008, 112, 14830. (23) (a) Mwangi, M. T.; Runge, M. B.; Bowden, N. B. J. Am. Chem. Soc. 2006, 128, 14434. (b) Runge, M. B.; Mwangi, M. T.; Bowden, N. B. J. Organomet. Chem. 2006, 691, 5278.

Herbert et al. Chart 1

Scheme 1

another technique to be recently described. Kerton, Rayner, and co-workers synthesized PDMS-tagged arylphosphines and employed them as scCO2 solubilizing ligands in homogeneous catalysis.24 Similarly, we synthesized 4-(polydimethylsiloxanylethyl)pyridine (1) (see Chart 1) and used the compound to prepare a scCO2-soluble copper(II) acetate complex.25 The solubility of MTO and its coordination compounds of most simple nitrogen bases in media ranging from water to apolar organic solvents is generally fairly good, although solubility can be limited in very nonpolar solvents such as hydrocarbons. We therefore carried out a study to investigate the possibility that the nonpolar media solubilizing properties of a PDMScontaining coordinative species such as 1 might enable MTOcatalyzed epoxidations in nonpolar media such as hexane and scCO2, and also in the absence of solvent, with the substrate itself comprising the reaction media. The work presented here describes the isolation and characterization of the short chain PDMS-functionalized methyltrioxorhenium complex of 1 and the advantages and drawbacks of its application in homogeneously catalyzed epoxidation processes.

Results and Discussion Synthesis of 4-(Polydimethylsiloxanylethyl)pyridine (1) and MTO Coordination Compound 2. Similar to a previously described preparation,25 4-(polydimethylsiloxanylethyl)pyridine (1) was synthesized by the reaction of hydride-terminated polydimethylsiloxane, H2[SiMe2(OSiMe2)n], with 4-vinylpyridine (a hydrosilylation) using Kardstedt’s catalyst.26 In situ IR spectra of the reaction mixture were used to monitor the progress of the reaction. As the hydride is consumed, the Si-H peak of the hydride-terminated PDMS at 2126 cm-1 gradually disappears. Its absence indicates that the reaction is complete. After appropriate workup, 1 was obtained as a clear oil. NMR analysis indicated that the product contained a mixture of two isomers resulting from the R and β (Markovnikoff and anti-Markovnikoff) addition of the Si-H to the vinyl group in a ratio of 1:2 (Scheme 1).27,28 Attempts at controlling the ratio of isomers contained by adjusting the reaction conditions were unsuccessful. (24) Saffarzadeh-Matin, S.; Chuck, C. J.; Kerton, F. M.; Rayner, C. M. Organometallics 2004, 23, 5176. (25) Herbert, M.; Montilla, F.; Galindo, A. Inorg. Chem. Commun. 2007, 10, 735. (26) Tronc, F.; Lestel, L.; Boileau, S. Polymer 2000, 41, 5039. (27) The formation of a mixture of two isomers resulting from the R and β addition of the Si-H to the vinyl group has been also observed in the reactions of H2[SiMe2(OSiMe2)n] with styrene and 4-chlorostyrene (ref 28). (28) Herbert, M. Ph.D. Thesis, Universidad de Sevilla, in preparation.

Methyltrioxorhenium Complexes of Pyridine

Chromatographic separation of the isomers was also attempted, but results were unsatisfactory. The resulting product mixture was characterized by analytic and spectroscopic methods. The number average molecular weight of H2[SiMe2(OSiMe2)n] was determined by 1H NMR integration of Si-CH3 versus Si-H.26 A Mn of ∼525 was calculated, corresponding to n ≈ 6.3. We synthesized several samples of 1 during the course of our work. For each product the ratio SiMe2: pyridine was determined using 1H NMR integrals, and it was thus possible to calculate the average Mn. The number average molecular weight of 1 varied between preparations, probably as a result of concurrent action of the platinum catalyst on the silicon chains of the polymer during the hydrosilylation reaction. The ratio SiMe2:pyridine was found to vary within a range of 5 to 10 (Mn ) 530-900), although the R to β isomer ratio remained constant at around 1:2. Spectroscopic data for a representative product are collected together in the Supporting Information. For this sample, 1H NMR integrals indicate a ratio of approximately 5.5 SiMe2 to every terminal pyridine (i.e., Mn ) ∼500). The 1H NMR spectrum displays a broad singlet at approximately 0 ppm resulting from the Si(CH3)2 protons and appropriate alkyl signals corresponding to the -CH(CH3)- and -CH2-CH2- protons in a 2:1 ratio, respectively. The pyridyl protons clearly differentiate into separate peaks corresponding to the two isomers. In the 29Si{1H} NMR spectrum several signals are observed in the 21.6-22.1 ppm range, corresponding to the nonterminal O-SiMe2-O silicon atoms of the PDMS, and the peak occurring at -12.9 ppm can be attributed to the silicon atoms adjacent to the -C2H4- moiety.29 The hydrosilylative addition of PDMS to coordinated 4-vinylpyridine has been reported in the preparation of some iridium complexes;30 however in a previous investigation25 we observed that the direct hydrosilylation of coordinated 4-vinylpyridine in a copper acetate complex was not possible, probably due to deactivation of the Pt catalyst. Here therefore we elected to prepare the coordination compound of MTO and compound 1 via the reaction of the latter with MTO in dichloromethane, as opposed to hydrosilylation of the 4-vinylpyridine complex of MTO. The PDMS-functionalized MTO compound 2 was thus obtained as a brown oil, which was appropriately characterized. Clearly apparent in the 1H and 13C{1H} NMR spectra are signals corresponding to Si(CH3)2, -CH2-CH2-, and one pyridine isomer, but signals from the R isomer present in the spectra of 1 are no longer apparent. We have also observed this isomeric purification in the preparation of Pd complexes of 1 via a similar experimental procedure.28 The most likely explanation for the elimination of the R isomer during workup seems to be the lower solubility of its complexes in nonpolar media, leading to their removal by filtration. It is possible to view compound 2 either as an MTO complex possessing a short chain PDMS function or as an MTO complex supported on a liquid PDMS polymer, similar to how one might view heterogeneous catalysts where the catalytic complex is bound to the surface of a solid support. In contrast to such materials, 2 is intended for use in homogeneous rather than heterogeneous catalytic systems, and the [catalyst]:mass ratio is thus the important measure for its quantification, rather than surface area-related measurements. Similarly though, in catalytic experiments the complex could be viewed as being supported in solution. (29) See for example: Cheng, H. N. In Modern Methods of Polymer Characterization; Barth, H. G., Mays, J. W., Eds.; Wiley: New York, 1991; p 461. (30) DeRosa, M. C.; Mosher, P. J.; Yap, G. P. A.; Focsaneanu, K.-S.; Crutchley, R. J.; Evans, C. E. B. Inorg. Chem. 2003, 42, 4864.

Organometallics, Vol. 28, No. 9, 2009 2857 Table 1. cis-Cyclooctene Epoxidations in Chloroform or Hexane, in the Presence of No Base, Pyridine, or Compound 1, at rt or 60 °Ca

entry

solventb

ligand

T

conversion (%)c

yield (%)c

1 2 3 4 5 6 7 8 9

chloroform chloroform chloroform hexane hexane hexane hexane hexane hexane

pyridine 1 pyridine 1 pyridine pyridined 1d

rt rt rt rt rt rt 60 °C 60 °C 60 °C

100 100 100 88 97 90 29 29 99

97 90 92 82 89 82 29 29 99

a MTO (0.025 mmol), 30% H2O2 (3.0 mmol), cis-cyclooctene (1.0 mmol), ligand (0.1 mmol), treaction ) 18 h. b V ) 10 mL. c Determined by GC analysis. d Coordination compound of MTO-ligand previously prepared.

MTO-Catalyzed Epoxidation Reactions. Several studies were carried out to investigate the relative effectiveness of MTO-1- and MTO-pyridine-catalyzed systems as well as any advantages that the PDMS functionality might confer upon the MTO-1-catalyzed systems. Investigations of relative epoxidative efficacy depending on experimental conditions used the relatively active cis-cyclooctene as the olefin substrate, as is common in these types of studies. Excess 30% aqueous H2O2 (3.0 equiv) was the oxidant, and a catalyst:substrate ratio of 1:40 was used. Initially catalytic performance depending on the solvent was examined before the focus shifted to solventless systems where the olefin itself comprises the organic phase. The success of the MTO-1 catalyst under these conditions led to the generalization of the study to investigate its effectiveness applied to a more diverse range of olefin substrates. Also particularly interesting were the results obtained in the solventless epoxidation of cis-cyclooctene when the quantity of MTO was very low (0.005%, all percentages are molar), where 1 was seen to enhance the TON considerably more than the other bases tested. (a) Influence of the Reaction Conditions in the Epoxidation and Comparison with the MTO-Pyridine System. The epoxidation of cis-cyclooctene in both chloroform and hexane, in the absence of base and in the presence of pyridine or 1 in a ratio of 4:1 to MTO, was investigated (Table 1). The effect of temperature was also investigated by performing reactions at room temperature and at 60 °C. The exact reaction conditions are given in the footnotes. Examination of the results shown in Table 1 does not reveal any particularly significant differences in conversion, which is always complete for the reactions in chloroform and nearly complete for those in hexane. Somewhat oddly in chloroform the selectivity was slightly higher when no base was employed (entry 1) than when it was (entries 2 and 3). This makes little sense, as while it could be argued that the bases might compete for coordinative sites on the metal and inhibit the rate of catalysis, thus lowering the conversion (though previous studies indicate that N-bases actually accelerate conversion, although formation of N-O oxides has been observed to lower the conversion rate), they should have a positive impact on selectivity due to their capacity to inhibit hydrolysis. The differences are sufficiently minor to be attributable to experimental error in any case. In hexane it was anticipated that the MTO catalysts might have poor solubility, leading to low conversions, when no base or pyridine were used, while 1 might

2858 Organometallics, Vol. 28, No. 9, 2009

Herbert et al.

Table 2. Solventless cis-Cyclooctene Epoxidations with Different Ligand Additivesa

entry

compound

ligand

t (h)

conversion (%)b

yield (%)b

1 2 3 4 5

MTO MTO 2 2 2

pyridine -

2 2 1 2 18

40 84 45 97 100

30 76 45 97 100

a

MTO (0.025 mmol), 30% H2O2 (3.0 equiv), cis-cyclooctene (0.5 mL, 3.65 mmol), ligand (0.1 mmol), rt. b Determined by GC analysis.

solubilize the catalyst and permit normal homogeneous catalysis, consequently giving higher conversions. However, conversions were approximately the same in all cases, suggesting there was no difference in the mechanism of catalysis in any of the experiments. Notably, the PDMS-containing catalyst MTO-1 is entirely compatible with the use of the aqueous H2O2, in contrast to some existing examples where the catalyst is occluded in a PDMS membrane, which acts as a barrier to H2O2, precluding the oxidation catalysis.22c The results had therefore established that 1 did not offer any notable advantage over unsubstituted pyridine in these systems, presumably because of complete homogeneity in all cases. With 1 offering little advantage by way of solubility, the possibility that the PDMS functionalization might lead to greater resistance to decomposition of the organometallic MTO was therefore examined. Under the basic oxidizing conditions used for the epoxidation, MTO is known to undergo decomposition via loss of its methyl group by which it converts to largely inactive oxide species.31 The experiments in hexane were therefore repeated at 60 °C to see if any differences would be observed. The results clearly show that the coordination complex of 1 allows a very significantly higher yield under these conditions than if unsubstituted pyridine is used. The retarded yields observed using pyridine (entries 7 and 8) compared with the analogous reaction employing 1 (entry 9) or pyridine at room temperature (entry 5) seem likely to result from decomposition of the MTO complex under the harsher conditions. Additional evidence for this comes from visual observations of the reaction mixtures after reaction. For entries 5 and 9 they were yellow due to the presence of the catalytically active rhenium peroxide complex, but for entries 7 and 8 the mixtures were colorless, indicating the rhenium species had likely decomposed. The identical conversions observed in entries 7 and 8 provide evidence that the complex had no problem forming in situ. It can be concluded that complex 2 (MTO-1) possesses superior resistance to decomposition under the oxidative conditions of the catalytic system compared to MTO-pyridine. (b) Solventless Epoxidation Reactions. The potential capacity of PDMS to solubilize MTO-1 in nonpolar media did not lead to any observable benefit in the experiments so far described. However, the study was extended to see if any advantages would be evident in solventless reactions, where the olefin substrate and reaction products alone comprise the organic phase. Table 2 shows the results obtained for a series of reactions investigating the solventless epoxidation of cis-cyclooctene. It was found that in the absence of base (entry 1) the conversion after 2 h was significant but far from complete, with (31) See for example:(a) Adam, W.; Saha-Moeller, C. R.; Weichold, O. J. Org. Chem. 2000, 65, 5001. (b) Abu-Omar, M. M.; Hansen, P. J.; Espenson, J. H. J. Am. Chem. Soc. 1996, 118, 4966.

Table 3. Solventless 1-Octene Epoxidations with and without Catalyst Pretreatment with H2O2a

entry

ligand

time stirred with H2O2 prior to commencing reaction (h)

yield (%)b

1 2 3 4

pyridine 1 pyridine 1

0 0 2 2

50 58 23 56

a MTO (0.025 mmol), 30% H2O2 (3.0 equiv), 1-octene (0.585 mL, 3.65 mmol), ligand (0.1 mmol), rt, treaction ) 18 h. b Determined by GC analysis.

some hydrolysis of the product to the dialcohol taking place. It is likely that in this reaction the MTO catalyst was dissolved in the aqueous phase, separated from the substrate, thus inhibiting the rate of the reaction and making the catalyst more vulnerable to decomposition, thus limiting the maximum obtainable yield. In the absence of base, hydrolysis of the product would also be less inhibited. The results observed support these hypotheses. When pyridine was employed (entry 2), the conversion was doubled and selectivity was improved, although with some hydrolysis still taking place. Better solubility of the catalyst in the organic substrate due to the coordinated pyridines and inhibition of the hydrolytic epoxide decomposition may explain these improvements. For both entries 1 and 2 the postreaction mixture was observed to be colorless after reaction, indicating an absence of rhenium peroxide species and that the catalyst had decomposed over the course of the reaction and was thus now inactive. When compound 2 was employed as catalyst (entries 3-5), faster conversion with complete selectivity to the epoxide product was observed. Practically complete conversion was observed after 2 h (entry 4), and the strong yellow color of the rhenium peroxide complex could be clearly seen in the organic phase, indicating that the catalyst remained active. The better results obtained in this system may be due to several factors. These include superior catalyst solubility in the organic substrate than the two analogues and insolubility in the aqueous phase, inducing more rapid conversion while preventing hydrolysis, which should be more prevalent in the aqueous phase. Reduced contact with the aqueous phase should also lead to greater resistance to decomposition, and it would thus be logical to predict that higher TONs would be achievable using this catalyst (see below). The reactions described up until this point all employed the cis-cyclooctene substrate, which is relatively activated to epoxidation for both steric and electronic reasons. However it was important that the scope of the study was extended to a greater range of olefin substrates to investigate how the different solvent environments, substrate activities to epoxidation, and presence of other functional groups would affect the activity of the catalyst. Terminal alkyl olefins are less electron rich and therefore less easily epoxidized than many other alkenes, as is regularly observed in catalytic epoxidation studies. The epoxidation of 1-octene would therefore be expected to proceed more slowly than that of cis-cyclooctene. A comparison between the activities of systems employing 2 and MTO-pyridine as catalysts in the epoxidation of 1-octene was therefore made. Additionally, to investigate any difference in the stabilities of the catalysts when exposed to aqueous H2O2, the catalytic tests were repeated after the catalysts had been stirred with the oxidant for 2 h prior to the reaction. The results are summarized in Table 3.

Methyltrioxorhenium Complexes of Pyridine Table 4. Solventless Olefin Epoxidations Employing MTO-1 as Catalysta

Organometallics, Vol. 28, No. 9, 2009 2859 Table 5. Solventless cis-Cyclooctene Epoxidations with a Very Low Catalyst Loadinga entry

ligand

yield (≈conversion)b

TON

1 2 3 4 5

pyridine 3-methylpyrazole 3-methylpyrazolec 4-tridecylpyridine 1

0.23% 0.30% 0.66% 0.06% 11.60%

45 61 133 12 2320

MTO (7.3 × 10-4 mmol, 0.00005 equiv, 11 mM in CH2Cl2), ligand (0.146 mmol, 0.01 equiv), 30% H2O2 (3 equiv), cis-cyclooctene (2 mL, 14.6 mmol), rt, treaction ) 17 h. b Determined by GC analysis. c 0.1 equiv of ligand. a

a MTO (0.025 mmol), 1 (0.1 mmol), 30% H2O2 (3.0 equiv), olefin (3.65 mmol), rt, treaction ) 18 h. b Determined by GC analysis. c Determined by 1H NMR integral analysis.

Comparison of entries 1 and 2 shows that the two catalysts had relatively similar activities, the yield obtained employing 2 only slightly greater than that obtained with MTO-pyridine. This would seem to indicate that a difference in the catalyst solubilities is not a factor affecting the conversion rates here. A more likely explanation is a slight difference in the activities of the catalysts or their stability to decomposition during the course of the reaction. Entries 3 and 4 provide evidence supporting the latter. A comparison of the yields observed in entries 2 and 4 reveals only a negligible difference, indicating that compound 2 underwent no significant decomposition during 2 h stirring with aqueous H2O2. In contrast, the yield was observed to decrease by over 50% between entries 1 and 3. Stirring the catalyst with hydrogen peroxide produced a change that was highly detrimental to the activity of the system, with decomposition of the catalyst the most likely explanation. Note that H2O2 decomposition by the MTO is unlikely to be responsible for the diminished yield; previous studies have shown that MTO compounds do not in fact decompose H2O2, so this should not be a factor.9a,32 Compound 2 therefore appears to possess a far greater stability to decomposition by aqueous hydrogen peroxide than its pyridine analogue, which indicates a potential for higher yields and TONs, as was subsequently investigated. Attention then shifted to the investigation of the range of olefin substrates to which the solventless 2/H2O2 epoxidation system might be applied. A selection of olefins were thus tested, and the results are summarized in Table 4. The cyclic olefins cis-cyclooctene and cyclohexene were found to convert completely in 18 h (entries 1 and 5, respectively), as expected for these activated substrates. Among the alkyl olefins, the inactive primary 1-octene converted slowly, giving only a 55% yield (entry 2), while the more active secondary trans-2- and trans-4-octene substrates converted almost completely (entries 3 and 4, respectively). In many epoxidation systems styrene has a tendency to over-oxidize, giving products such as benzoic acid and benzaldehyde, though (32) Al-Ajlouni, A. M.; Espenson, J. H. J. Org. Chem. 1996, 61, 3969.

there was no evidence of such products here and almost complete conversion to the epoxide was seen (entry 6). The oxidation of allylic alcohol (entry 7) was chemoselective, and a good conversion to the epoxide was observed, indicating that the close proximity of the alcohol group did not present a problem. However, only trace evidence of the epoxide was observed in the epoxidation of ethyl acrylate (entry 8). (c) Investigation of N-Donor Bases in Solventless Epoxidations in the Presence of Very Low (0.005%) MTO Catalyst Loadings. It was previously seen that in smallscale reactions employing ∼0.68% MTO and ∼2.7% base in the epoxidation of cis-cyclooctene, only minor differences in activity are observable between systems employing 1 and pyridine as base (Table 1). However, a further study was conducted using a much lower MTO:substrate ratio, raising the maximum achievable TON to 20 000. Under these conditions, differences in the maximum achievable yields, resulting from the catalyst lifetimes, for the systems tested should become apparent. A series of reactions employing only 0.005% MTO and 1% of selected N-donor base were thus run (see Table 5). A comparison of entries 1 and 5 now shows a clear difference between the conversions achieved using unsubstituted pyridine (entry 1) and 1 (entry 5), with a far higher yield and thus TON obtained in the MTO-1 system. A trial was also made using 4-tridecylpyridine (entry 4), but this system also performed poorly. The highest TON yet observed in an MTO-catalyzed olefin epoxidation was 20 000, employing 10% 3-methylpyrazole in the oxidation of cis-cyclooctene in chloroform.17a Under solventless conditions, this reaction was attempted using both 1% (entry 2) and 10% (entry 3) 3-methylpyrazole, but in neither case were the TONs observed even nearly of the magnitude of those seen using MTO-1. Thus, in the solventless epoxidation of cis-cyclooctene by MTO the addition of just 1% of 1 resulted in yields many times higher than those observed for simple bases typically employed in these reactions. Notably the yield was considerably higher than that observed even when 10% of 3-methylpyrazole was employed, established as an optimal system when carrying out this epoxidation in chloroform.17a The greater activity of the MTO-1 system seems to result from the catalyst’s high resistance to decomposition compared to other MTO-base systems. Theoretical Calculations. The experimental work demonstrated that the MTO-1 catalyst possessed a higher stability to decomposition under oxidative conditions compared to MTOpyridine. In order to gain further insight into the possible reasons for this, a theoretical investigation of the behavior of the PDMS chains of the compounds was therefore conducted. Preliminary results from the study of the catalyst precursor compound 2 are thus here presented. DFT calculations were made for several model compounds representing compounds 1 and 2, and some other related structures were also studied for comparative

2860 Organometallics, Vol. 28, No. 9, 2009

Herbert et al.

Figure 1. Models used in the DFT study.

purposes (Figure 1). In the case of compound 1 the model employed was a short PDMS chain terminating at one end with the appropriate pyridine function and at the other with a SiMe3. Other possibilities including a linear polymer terminating at both ends with pyridine or other terminal groups and branching structures were not considered. The models used to study compound 1, labeled 1a and 1b, correspond to the R and β isomers respectively, and possess the structural formula NC5H4C2H4-(SiMe2O)n-SiMe3 (n ) 2, 4). The two variants of each of 1a2, 1a4, 1b2, and 1b4 were numbered according to the value of n. Coordination of the terminal pyridine to MTO affords the model compounds of 2, labeled 2a2, 2a4, 2b2, and 2b4, as appropriate. Finally, two additional models were studied: the (4-heptylpyridine)methyltrioxorhenium complex and the adduct formed between MTO and a nonfunctionalized PDMS chain, Me3SiO-(SiMe2O)3-SiMe3. The optimized structures calculated for several selected models are shown in Figure 2; the others can be found in the Supporting Information. Examining first the energetic properties of the models, only a negligible energy difference between the R and β isomers of 1 is observed, regardless of the polymer length. Similarly, models 2a and 2b were calculated to possess almost the same electronic energy (the difference only approximately 1 kcal · mol-1). More interesting, however, is a comparison between the optimized geometries and structural parameters of 2a2, 2b2, and MTO-(4-heptylpyridine). Figure 2 allows comparison of the arrangement of PDMS chains in models 2a2 and 2b2 with that of the alkyl chain in MTO-(4heptylpyridine), which contains the same number of atoms. In both 2a2 and 2b2 the PDMS chains are observed to have moved into proximity with the oxo ligands of the rhenium, indicating C-H · · · O interactions. The importance of such interactions in MTO has been previously investigated,33 and the C-H · · · O lengths computed here of 2.749 and 3.581 Å (2a2) and 2.673 and 2.789 Å (2b2) are in the range of those previously computed33 and observed experimentally via diffraction methods34 for MTO. In order to quantify energetically the magnitude (33) Vaz, P. D.; Ribeiro-Claro, P. J. A. Eur. J. Inorg. Chem. 2005, 1836.

of the C-H · · · O interaction, we made a computational study of the adduct of MTO and nonfunctionalized PDMS, Me3SiO-(SiMe2O)3-SiMe3. The electronic energy calculated for the C-H · · · O interactions (bond lengths of 2.858, 2.989, 3.013, and 3.337 Å) is small but significant (2.2 kcal · mol-1). Examining the Re-N bond distances, it is found that in 2a2 and 2b2 these are shorter (2.538 and 2.542 Å, respectively) than the 2.565 Å found for MTO-(4-heptylpyridine), although all of these values are somewhat larger than Re-N bond lengths determined crystallographically for other MTO complexes of pyridine-type ligands (normally 2.41-2.50 Å).35 These factors are likely to have an important contribution to the explanation of the observed stabilization of the MTO-1 system that we have previously discussed.

Conclusions The work presented here covers the synthesis, characterization, epoxidative catalytic activity, and computational study of the polydimethylsiloxane-functionalized pyridine (1) complex of MTO, [Re(O)3(CH3)(1)] (2). In the catalytic studies, the activity of 2 was compared with that of MTO-pyridine, and in several cases the PDMS-supported catalyst was observed to possess significant advantages. For the epoxidation of ciscyclooctene with 30% H2O2 as oxidant, the influence of the solvent and temperature was investigated, concluding that while the PDMS did not confer any solubility advantages even in hexane, it did result in the catalyst being more resistant to decomposition at higher temperatures. It is also notable that, unlike PDMS membranes, which can act as a barrier to H2O2 and preclude oxidative catalysis, the PDMS-supported catalyst is compatible with the aqueous 30% H2O2 oxidant. In studies (34) Herrmann, W. A.; Scherer, W.; Fischer, R. W.; Blu¨mel, J.; Kleine, M.; Mertin, W.; Gruehn, R.; Mink, J.; Boysen, H.; Wilson, C. C.; Ibberson, R. M.; Bachmann, L.; Mattner, M. J. Am. Chem. Soc. 1995, 117, 3231. (35) (a) Ku¨hn, F. E.; Santos, A. M.; Roesky, P. W.; Herdtweck, E.; Scherer, W.; Gisdakis, P.; Yudanov, I. V.; di Valentin, C.; Rosch, N. Chem.-Eur.J. 1999, 5, 3603. (b) Santos, A. M.; Ku¨hn, F. E.; Xue, W.-M.; Herdtweck, E. J. Chem. Soc., Dalton Trans. 2000, 3570.

Methyltrioxorhenium Complexes of Pyridine

Organometallics, Vol. 28, No. 9, 2009 2861

Figure 2. Optimized structures of model compounds 1a4, 1b4, 2a2, 2b2, and MTO-(4-heptylpyridine).

of catalytic activity in the absence of solvent, compound 2 was found to offer faster, more selective conversion than the MTO-pyridine analogue, with the catalyst also much more resistant to decomposition. It was concluded that this is due, at least in part, to the capacity of the PDMS to solubilize the coordinated MTO in the olefin more effectively than unfunctionalized analogues. This solventless epoxidation system was successfully applied to a variety of olefins, achieving good epoxide yields in many cases. Additionally, the catalyst also performed well when the MTO-base:substrate ratio was lowered in order to investigate the maximum attainable yields in the epoxidation of cis-cyclooctene. Apparently as a result of superior catalyst stability, a far higher TON was achievable using the MTO-1-catalyzed system compared to other typical MTO-base combinations.

Experimental Section Reactions were performed under a dry, oxygen-free, nitrogen atmosphere using standard Schlenk-type techniques, unless otherwise stated. All solvents were dried and degassed appropriately before use. We have previously described the synthesis of compound 1,25 but a modified procedure was employed in this work, which is reported here. The preparation of 4-tridecylpyridine was based on a similar synthesis described in the literature.36 Hydride-terminated polydimethylsiloxane H2[SiMe2(OSiMe2)n] was obtained from Aldrich and used as supplied. The average molecular weight of H2[SiMe2(OSiMe2)n] was determined by the 1H NMR integrals of Si-CH3 versus Si-H (n ) 10). Karstedt’s catalyst (platinum(0)-1,3-divinyl1,1,3,3-tetramethyldisiloxane in xylenes) was supplied by Ald(36) Kiehne, U.; Bunzen, J.; Staats, H.; Lu¨tzen, A. Synthesis 2007, 7, 1061.

rich, while other chemicals were obtained from commercial sources and used as supplied. Infrared spectra were recorded on a Perkin-Elmer model 883 spectrophotometer (liquid film supported on NaCl discs). NMR spectra were obtained using a Bruker AMX-300 spectrometer. 13C{1H} and 1H shifts were referenced to the residual signals of deuterated solvents, while 29 Si{1H} shifts were referenced to external Me4Si. All data are reported in ppm downfield from Me4Si. Gas chromatography was carried out using a Varian CP-3800. Elemental analysis (C, H, N) was conducted by the Microanalytical Service of the Universidad de Sevilla (CITIUS). Preparation of 4-(Polydimethylsiloxanylethyl)pyridine Polymer (1). 4-Vinylpyridine (2.4 mL, 10.5 mmol) and a solution of Karstedt’s catalyst in xylenes (285 µL, 0.046 mmol, approximately 2% Pt) were mixed, and to this was added hydrideterminated polydimethylsiloxane H2[SiMe2(OSiMe2)n] (6.0 mL, 16.7 mmol) in dry dichloromethane (5 mL). A static vacuum was applied to prevent excessive pressure and the reaction heated at 60 °C with stirring for 36 h. After this time the solvent was evaporated by applying vacuum, and complete reaction was confirmed by checking for the absence in the IR spectrum of the Si-H peak at 2126 cm-1. The crude product was dissolved in hexane (15 mL), dried (MgSO4), and filtered through silica gel to remove the platinum catalyst. If necessary the solution was then centrifuged to remove any remaining sediments before the hexane was removed by applying vacuum at 60 °C for several hours. The product was obtained as a clear oil and was stored under nitrogen in a refrigerator. As discussed in the results section, the R and β addition products were always observed in a ratio of 1:2, while the ratio of SiMe2 to pyridine was found to vary between samples, generally within a range of 5 to 10. In a sample product, 1H NMR indicated a ratio of approximately 5.5 SiMe2 to every terminal pyridine (i.e., Mn ) ∼500). Yield: 5.592 g, 506.2 g mol-1, a 66% yield based on terminal pyridine groups. IR (cm-1, selected absorptions): ν(Si-O) 1090, 1025, ν(Si-CH3) 1260. 1H NMR (CDCl3): δ -0.1 to 0.1

2862 Organometallics, Vol. 28, No. 9, 2009 (br s, Si(CH3)2), R isomer: 1.29 (d, JHH ) 7.35 Hz, 3H, pyCH(CH3)-Si), 1.84 (q, JHH ) 8.04 Hz, 1H, py-CH(CH3)-Si), 6.92 (m, 2H, m-NC5H4), 8.30 (m, 2H, o-NC5H4); β isomer: 0.82 (t, JHH ) 8.7 Hz, 2H, py-CH2CH2-Si), 2.57 (t, JHH ) 8.6 Hz, 2H, pyCH2CH2-Si), 7.05 (m, 2H, m-NC5H4), 8.37 (m, 2H, o-NC5H4). 13 C{1H} NMR (CDCl3): δ 0-5 (several s, Si(CH3)2), 13.3 (s, pyCH(CH3), R isomer), 18.9 (s, py-CH2CH2-Si, β isomer), 28.8 (s, py-CH2CH2-Si, β isomer), 31.5 py-CH(CH3), R isomer), 122.8 (s, m-NC5H4, R isomer), 148.8 (s, o-NC5H4, R isomer), 153.0 (s, p-NC5H4, R isomer), 123.3 (s, m-NC5H4, β isomer), 149.4 (s, o-NC5H4, β isomer), 154.1 (s, p-NC5H4, β isomer). 29Si{1H} NMR (CDCl3): δ -21.6 to -22.1, -21.1, -20.8 (several s, O-Si(CH3)2O),-12.9 (br s, C2H4-Si(CH3)2-O). Anal. Calcd for a sample where n ) 10, C36H82N2O10Si11: C, 42.73; H, 8.17; N, 2.77. Found: C, 43.56; H, 8.36; N, 3.23. Synthesis of Compound 4-(Polydimethylsiloxanylethyl)pyridinyl Methyltrioxorhenium (2). To methyltrioxorhenium (17.9 mg, 0.07182 mmol) in a Schlenk vial equipped with a magnetic stirrer bar was added a solution of 1 (0.040 g, Mn ) ∼560, 1 equiv, 0.072 mmol) in dry dichloromethane (10 mL), and the mixture was stirred, resulting in the dissolution of the rhenium to give a pale yellow solution. A static vacuum was applied and the solution left stirring for 4 h at 40 °C. Afterward the solvent was evaporated by applying vacuum, leaving a brown oil and dark solids. Dry ethanol (10 mL) was added and the solution again stirred at 40 °C for 4 h. The solvent was removed by applying vacuum, leaving an oil, to which was added dry hexane (10 mL), and the solution was stirred at rt for 1 h. Afterward the resulting turbid solution was left to settle overnight, leaving a clear red-brown solution above insoluble precipitates, from which the solution was separated by decantation. The hexane was evaporated under vacuum to leave the product as a brown oil (0.0418 g, 0.0301 mmol). 1H NMR analysis indicated that the ratio of SiMe2:terminal pyridyl-MTO in the product was approximately 14 (1390 g · mol-1 Re, 42% yield). IR (cm-1): 3414, 6052, 2962, 2905, 1944, 1638, 1618, 1561, 1498, 1414, 1257, 1016, 912, 795, 703. 1H NMR (C6D6): δ 0.39 (s, 84H, Si(CH3)2), 0.79 (t, JHH ) 8.79 Hz, 2H, py-CH2CH2-Si), 1.65 (s, 2.2H, ReCH3), 2.46 (t, JHH ) 8.6 Hz, 2H, py-CH2CH2-Si), 6.76 (m, 2H, m-NC5H4), 8.45 (m, 2H, o-NC5H4). 13C NMR (C6D6): δ 0-2 (several s, Si(CH3)2), 18.6 (s, py-CH2CH2-Si), 23.9 (s, ReCH3), 28.7 (s, pyCH2CH2-Si), 123.7 (s, m-NC5H4), 148.0 (s, o-NC5H4), 155.1 (s, p-NC5H4). 29Si{1H} NMR (CDCl3): δ -21.3 to -22.1, -20.8 (several s, O-Si(CH3)2-O), -10.7 (br s, CH2CH2-Si(CH3)2-O). Anal. Calcd for C41H110NO19ReSi16.5: C, 31.35; H, 7.06; N, 0.89. Found: C, 36.54; H, 7.85; N, 1.45%. Preparation of 4-Tridecylpyridine. A solution of iPr2NH (1.05 equiv, 3 mL, 21 mmol) in dry THF (50 mL) was cooled to -78 °C, and to it was added 1.6 M nBuLi in hexane (1.05 equiv, 12.5 mL, 21 mmol), forming the LDA reagent, followed by 4-picoline (1940 µL, 20.0 mmol). The solution was allowed to warm to 0 °C in an ice bath and left stirring for 5 min. Next, 1-iodododecane (5.0 mL, 20 mmol) was added, and the solution was again stirred at 0 °C for 5 min, before allowing it to warm to room temperature and leaving it to stir for 36 h. The solution was then concentrated to approximately 10 mL, dissolved in dichloromethane (50 mL), and washed with water (3 × 25 mL). To prevent significant loss of product in the washes, the aqueous fractions were then themselves extracted with dichloromethane (3 × 25 mL), and all of the organic solutions were then combined. The resulting solution was dried (MgSO4), filtered, and evaporated on a rotovapor, leaving the crude product as an orange-brown oil. This was purified by column chromatography employing silica gel and 0.5% NEt3 in 1:1 hexane-ethyl acetate as eluent. The final product was a pale yellow oil. 1H NMR (CDCl3): δ 0.81 (t, 3H, CH3), 0.9-1.7 (m, 22H, CH2), 2.53 (t, 2H, CH2 adjacent to py), 7.03 (m, 2H, m-NC5H4), δ 8.41 (s, 2H, o-NC5H4). 13C{1H} NMR (CDCl3): δ 14.0 (s, terminal CH3), 20-32 (several s, intermediate CH2), 35.1

Herbert et al. (s, CH2 adjacent to py), 123.7 (s, m-NC5H4), 149.4 (s, o-NC5H4), 151.6 (s, p-NC5H4). Anal. Calcd for C18H31N: C, 82.69; H, 11.95; N, 5.36. Found: C, 80.66; H, 11.65; N, 5.33. General Considerations for MTO-Catalyzed Epoxidations. Catalytic reactions were conducted in sealable 50 mL Schlenk vials equipped with magnetic stirrer bars. Due to the nature of the reactions, an inert atmosphere was not necessary and so reagents could be charged under atmospheric conditions, but during reaction vials were sealed to prevent the entry or escape of gases. Where appropriate, heating was achieved by means of a thermostated oil bath. When reactions were complete, they were cooled for 5 min in an ice bath before opening. The majority of reactions were analyzed by GC, and these experiments were immediately prepared for analysis by addition of 25 mL of diethyl ether and a measured quantity of an appropriate internal standard. The solutions were dried with MgSO4, and a sample was then taken and analyzed. For the few reactions that were analyzed by NMR, deuterated chloroform was added to the mixture straight after reaction, dried with MgSO4, and then analyzed. For some reactions the coordinated catalyst was prepared prior to reaction rather than being left to form in situ. To do this, the required quantities of MTO and the coordinative species were weighed into a vial and left stirring at 40 °C in 10 mL of dry chloroform for 1 h. Afterward the solvent was removed by application of vacuum and the other reagents were then charged to the vial as appropriate for the desired reaction. Computational Details. The electronic structure and geometries of the model compounds were computed within the density functional theory at the B3LYP level.37,38 The Re atom was described with the LANL2DZ basis set,39,40 while the 6-31G* basis set was used for the Si, C, O, N, and H atoms. The optimized geometries of the model compounds 1a, 1b, 2a2, 2b2, and MTO-4heptylpyridin were characterized as energy minima either by a nonexistence of imaginary frequencies (NImag ) 0) or by very low vibrational frequencies (