Dinuclear Silicon Complexes of Uracil, Barbituric Acid, and 5,5

Organometallics 2010, 29, 5435–5445 DOI: 10.1021/om100461b

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Dinuclear Silicon Complexes of Uracil, Barbituric Acid, and 5,5-Dimethylbarbituric Acid: Hydrolytic Formation of Cyclic Oligomers1,† Evgenia Kertsnus-Banchik,§ Boris Gostevskii,§ Mark Botoshansky,‡ Inna Kalikhman,*,§ and Daniel Kost*,§ §

Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel, and ‡Department of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel Received May 13, 2010

The synthesis and properties of dinuclear silicon complexes derived from uracil, barbituric acid, and 5,5-dimethylbarbituric acid, through the reaction with (chloromethyl)dimethylchlorosilane and (chloromethyl)dichloro(methyl)silane are described. The oxygen atoms are weakly coordinated to silicon in all three groups of compounds. Mild hydrolysis of the dinuclear compounds leads to formation of cyclic oligomers, with 2-9 monomer units in each, in which the individual units are connected through siloxane groups. Lithium chloride is shown to act as an effective template, restricting oligomerization to dimers and preventing higher-mass product formation.

Introduction The chemistry of organic hypercoordinate silicon compounds has received considerable attention in recent years.2 In most of the studies, one or two chelating agents are attached to one silicon atom, generating a mononuclear penta- or hexacoordinate complex. In the present study we

aim at the reverse situation: preparation of dinuclear complexes in which two hypercoordinate silicon centers are attached to one bidentate ligand moiety.3 Suitable ligands for this purpose should be bidentate, able to bind to two silicon atoms. However, most of the bidentate ligands generally attach to the same silicon atom, forming a chelate ring.2g In order to avoid the formation of chelates, the bidentate ligand should have its binding sites well separated from each other, so that attachment of two different silicon atoms is preferred over chelate formation with one silicon center (Scheme 1). Dinuclear silicon compounds formed in this manner are expected to be primarily tetracoordinate. In order to obtain dinuclear compounds of hypercoordinate silicon moieties, the synthetic method must have a way to convert the tetra- to pentacoordinate (or higher) silicon compounds. This goal is achieved through the introduction of a well-known class of reagents, chloromethylsilanes (Scheme 2). These reagents have been shown to react with silyl-amides,4 silylhydrazides,5 and related compounds in two steps: in the first, transsilylation takes place, followed by a molecular rearrangement in which chloride is displaced by an adjacent nitrogen

† Part of the Dietmar Seyferth Festschrift. Dedicated to Professor Dietmar Seyferth, Editor in Chief, on his retirement from Organometallics. *To whom correspondence should be addressed. E-mail: kostd@bgu. ac.il. (1) For a preliminary account: Bannikova, O. B.; Gostevskii, B. A.; Kalikhman, I. D. Abstract VII Conference for Chemistry, Technology and Utility of Organosilicon Compounds; Irkutsk: SSSR, 1990; p 59. (2) Selected reviews on hypercoordinate silicon compounds: (a) Brook, M. A. In Silicon in Organic, Organometallic and Polymer Chemistry; Wiley: New York, 2000; pp 97-115. (b) Bassindale, A. R.; Glynn, S. J.; Taylor, P. G. In The Chemistry of Organic Silicon Compounds; Rappoport, Z.; Apeloig, Y., Eds.; Wiley: Chichester, U.K, 1998; Vol. 2, Part 1, pp 495-511. (c) Chuit, C.; Corriu, R. J. P.; Reye, C. In The Chemistry of Hypervalent Compounds; Akiba, K., Ed.; Wiley-VCH: Weinheim, Germany, 1999; pp 81-146. (d) Kira, M.; Zhang, L.-C. In The Chemistry of Hypervalent Compounds; Akiba, K., Ed.; Wiley-VCH: Weinheim, Germany, 1999; pp 147-169. (e) Tacke, R.; P€ulm, M.; Wagner, B. Adv. Organomet. Chem. 1999, 44, 221–273. (f) Kost, D.; Kalikhman, I. Adv. Organomet. Chem. 2004, 50, 1–106. (g) Kost, D.; Kalikhman, I. In The Chemistry of Organic Silicon Compounds; Rappoport, Z.; Apeloig, Y., Eds.; Wiley: Chichester, U.K, 1998; Vol. 2, Part 2, pp 1339-1445. (h) Pestunovich, V.; Kirpichenko, S.; Voronkov, M. In The Chemistry of Organic Silicon Compounds; Rappoport, Z.; Apeloig, Y., Eds.; Wiley: Chichester, U. K, 1998; Vol. 2, Part 2, pp 1447-1537. (i) Negrebetsky, V. V.; Tandura, S. N.; Baukov, Yu. I. Usp. Khimii. 2009, 78, 24-55. English translation: Russ. Chem. Rev. 2009, 78, 21-51. (j) Kost, D.; Kalikhman, I. Acc. Chem. Res. 2009, 42, 303–314. (3) For other dinuclear hypercoordinate silicon compounds see: (a) Loy, A. D.; Small, J. H.; Shea, K. J. Organometallics 1993, 12, 1484. (b) Cerveau, G.; Chuit, C.; Colomer, E.; Corriu, R. J. P.; Reye, C. Organometallics 1990, 9, 2415. (c) Laine, R. M.; Blohowiak, K. Y.; Robinson, T. R.; Hoppe, M. L.; Nardi, P.; Kampf, J.; Uhm, J. Nature 1991, 353, 642. (d) Tacke, R.; M€ uhleisen, M.; Jones, P. G. Angew. Chem., Int. Ed. 1994, 33, 1186. (e) Arya, P.; Boyer, J.; Corriu, R. J. P.; Lanneau, G. F.; Perrot, M. J. Organomet. Chem. 1988, 346, C11. (f) Kalikhman, I. D.; Gostevskii, B. A.; Bannikova, O. B.; Voronkov, M. G.; Pestunovich, V. A. J. Organomet. Chem. 1989, 376, 249.

(4) (a) Onan, K. D.; McPhail, A. T.; Yoder, C. H.; Hillyard, R. W. J. Chem. Soc., Chem Commun. 1978, 209–210. (b) Hillyard, R. W.; Ryan, C. M; Yoder, C. H. J. Organomet. Chem. 1978, 153, 369–377. (c) Yoder, C. H.; Ryan, C. M; Martin, G. F.; Ho, P. S. J. Organomet. Chem. 1980, 190, 1–7. (d) Kalikhman, I. D.; Albanov, A. I.; Bannikova, O. B.; Belousova, L. I.; Voronkov, M. G.; Pestunovich, V. A.; Shipov, A. G.; Kramarova, E. P.; Baukov, Yu. I. J. Organomet. Chem. 1989, 361, 147–155. (5) (a) Kalikhman, I. D.; Pestunovich, V. A.; Gostevskii, B. A.; Bannikova, O. B.; Voronkov, M. G. J. Organomet. Chem. 1988, 338, 169–180. (b) Macharashvili, A. A.; Shklover, V. E.; Struchkov, Yu. T.; Gostevskii, B. A.; Kalikhman, I. D.; Bannikova, O. B.; Voronkov, M. G.; Pestunovich, V. A. J. Organomet. Chem. 1988, 356, 23–30. (c) Macharashvili, A. A.; Shklover, V. E.; Struchkov, Yu. T.; Voronkov, M. G.; Gostevskii, B. A.; Kalikhman, I. D.; Bannikova, O. B.; Pestunovich, V. A. J. Organomet. Chem. 1988, 340, 23–29. (d) Kalikhman, I. D.; Bannikova, O. B.; Pestunovich, V. A.; Voronkov, M. G. Dokl. Akad. Nauk SSSR 1986, 287, 870–873. (e) Yakubovich, S.; Gostevskii, B.; Kalikhman, I.; Kost, D. Organometallics 2009, 28, 4126–4132.

r 2010 American Chemical Society

Published on Web 07/22/2010

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Organometallics, Vol. 29, No. 21, 2010

Kertsnus-Banchik et al.

Scheme 1. Schematic Description of Dinuclear Complex vs Intramolecular Chelate Formation Resulting from Relative Locations of Binding Sites (bs) on Ligand Molecule

Scheme 2. Two-Step Reaction Sequence Leading to Chelate Ring Formation and Increase of Silicon Coordination Number by One

or oxygen nucleophile, with formation of a chelate ring and an increase of the silicon coordination number by one. Chlorosilanes are readily hydrolyzed to form siloxane oligomers and polymers.6 The dinuclear chlorosilanes described in the present paper are shown to form cyclic oligomers of various sizes upon partial hydrolysis. The size of the oligomers can be restricted to dimers by using LiCl as template.

Table 1. 29Si NMR Chemical Shifts of Dinuclear Complexes at Various Temperatures

Results and Discussion

reactant ClCH2SiMe2Cl (2a)

1. Dinuclear Complexes. The bidentate ligand forming reagents were chosen and prepared from the readily available di- and trihydroxy heterocycles: uracil, barbituric acid, and 5,5-dimethylbarbituric acid, in which the hydroxy groups are sufficiently separated to prevent binding to the same silicon atom. After silylation with trimethylchlorosilane (1, 4, and 6) they were allowed to react with chloro(chloromethyl)dimethylsilane (2a) and (chloromethyl)dichloro(methyl)silane (2b), as shown in eqs 1-3.

product 3a 5a 7a

ClCH2SiMeCl2 (2b)

3b 5b 7b, 70 b

The products of the reactions have been characterized by their 1H, 13C, and 29Si NMR spectra and elemental analysis. Some of the products were further subjected to X-ray crystallographic analysis and to solid-state NMR spectroscopy. Uracil Complexes. The 29Si NMR chemical shifts of 3a-7b are listed in Table 1. It is evident from the table that the two silicon atoms in 3a are different, as expected. The chemical shift difference is quite substantial, with one resonance at -5.7 and the other at 13.7 ppm at 295 K, suggesting that (6) (a) Noll, W. Chemistry and Technology of Silicones; Academic Press: New York, 1968. (b) Eaborn, C. Organosilicon Compounds; Butterworths, 1960; pp 229-231.

T (K) 265 295 330 270 295 330 260 270 295 330 295 320 260 270 295 255 270 310 330

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Si chemical shift (ppm) -13.2, 12.6 -5.7, 13.7 -0.7, 14.9 1.1 2.3 3.8 16.7 17.3 18.6 19.9 -20.8, 13.9 -15.3, 14.4 -21.0 -19.9 -18.3 13.7, 11.8, 21.9 15.8, 14.6, 21.8 17.7, 16.9, 21.8 18.3, 17.7, 21.7

perhaps only one of the silicon functions is coordinated to the carbonyl oxygen (the one resonating at higher field), while the other one remains tetracoordinate and does not bind to oxygen to any significant extent. This is supported by the temperature dependence of the 29Si resonance of the highfield signal and the essential temperature independence of the low-field signal, in agreement with expectations for a pentacoordinate vs a tetracoordinate compound, respectively.7 Clearly the high-field silicon resonance corresponds to a pentacoordinate silicon, coordinated to its neighboring carbonyl oxygen. Evidence discussed below shows that the stronger oxygen donor toward coordination to silicon is the amide oxygen, leaving the urea-type oxygen uncoordinated and the adjacent silicon group tetracoordinate. Surprisingly, even in the tetrachloro compound 3b, in which the additional chlorine atoms may be expected to render silicon more electrophilic relative to 3a, it is still found that one of the silicon atoms is pentacoordinate and the other one tetracoordinate. This is deduced from the substantial difference in chemical shifts, and the significant temperature dependence of the high-field signal, relative to the low-field signal (see Table 1), in analogy with 3a. Barbituric Acid Complex. The barbituric acid derived complexes 5a and 5b were prepared as shown in eq 2. A single crystal of 5a was grown from acetonitrile solution and (7) (a) Tandura, S. N.; Voronkov, M. G.; Alekseev, N. V. Top. Curr. Chem. 1986, 131, 99. (b) Klebe, G. J. Organomet. Chem. 1987, 332, 35. (c) Kummer, D.; Chaudhry, S. C.; Seifert, J.; Deppisch, B.; Mattern, G. J. Organomet. Chem. 1990, 382, 345. (d) Kummer, D.; Abdel Halim, S. H.; Kuhs, W.; Mattern, G. J. Organomet. Chem. 1993, 446, 51.

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Figure 1. Molecular structure of 5a in the crystal, depicted at the 50% probability level with omission of hydrogen atoms. Table 2. Selected Crystallographic Bond Lengths and Angles for 5a and 9 5a

9 Bond Lengths, A˚

Si1-O1 Si2-O2 Si2-C8 Si1-C1 O1-C4 O2-C6 Si2-Cl

1.722(2) 1.954(2) 1.886(3) 1.875(3) 1.317(3) 1.277(3) 2.3350(13)

Si1-O1 Si2-O6 Si1-O2 Si2-O7 Si2-Cl1 Si1-C1 O1-C5 O6-C9

1.992(4) 2.036(4) 1.939(4) 1.874(4) 1.879(5) 1.884(5) 1.247(6) 1.238(6)

Figure 2. Crystal packing of 5a showing the chloro ligand alignment of one molecule along the apical axis of a neighboring molecule’s silicon atom. Scheme 3. Chloro Ligand Migration from One Silicon to the Other in 5aa

Bond Angles, deg C(1)-Si(1)-O(1) C(2)-Si(1)-O(1) C(3)-Si(1)-O(1) C(2)-Si(1)-C(3) C(2)-Si(1)-C(1) C(3)-Si(1)-C(1) C(10)-Si(2)-C(9) C(10)-Si(2)-C(8) C(9)-Si(2)-C(8) C(8)-Si(2)-O(2) O(2)-Si(2)-Cl Si(1)-C(1)-N(2) Si(2)-C(8)-N(1)

91.23(11) 108.00(13) 106.21(14) 113.59(18) 116.89(15) 117.32(16) 119.76(16) 122.15(16) 117.96(16) 83.39(11) 170.91(8) 102.99(17) 110.35(19)

C(1)-Si(1)-O(1) C(2)-Si(1)-O(1) C(3)-Si(1)-O(1) C(2)-Si(1)-C(3) C(2)-Si(1)-C(1) C(3)-Si(1)-C(1) C(12)-Si(2)-C(13) C(12)-Si(2)-C(11) C(13)-Si(2)-C(11) O(2)-Si(1)-O(1) O(7)-Si(2)-O(6) Si(1)-C(1)-N(1) Si(2)-C(11)-N(2)

82.84(18) 90.2(2) 90.9(2) 118.1(3) 121.2(3) 120.3(3) 119.3(3) 121.3(3) 118.6(3) 172.12(16) 167.94(18) 109.7(3) 110.1(3)

its crystal structure was determined. The molecular structure in the solid state is depicted in Figure 1, and selected bond lengths and angles are listed in Table 2. It is evident that one of the silicon atoms is tetracoordinate and essentially tetrahedral, while the second silicon atom is clearly pentacoordinate and located within a near trigonal-bipyramidal (TBP) geometry. This difference is dictated by the conjugation between the different oxygen atoms through the central ring, one being a carbonyl oxygen and the other an enol oxygen. The different roles of these two oxygen atoms are also manifest in the substantially different O-Si bond lengths: 1.722 (covalent) vs 1.954 A˚ (dative). In contrast with the lack of symmetry between the silicon moieties in the solid state, the solution NMR spectra of 5a showed the two silicon atoms to be equivalent on timeaverage, down to 270 K.8 It follows that migration of the chloro ligand between the two silicon centers must be rapid relative to the NMR time scale, exchanging tetracoordinate and pentacoordinate silicon environments. Due to the distance between the two silicon atoms, intramolecular chloride migration seems unlikely. Instead, intermolecular chloride (8) 5a was found sufficiently soluble only in CD3CN, the solution of which cannot be cooled much further.

a

Above: intramolecular. Below: intermolecular.

migration is proposed, as shown in Scheme 3. Some support for a possible intermolecular exchange comes from observation of the crystal packing (Figure 2): the chloro ligand of one molecule is positioned along the apical axis of a neighboring molecule’s tetracoorinate silicon, at a distance of 3.573 A˚, perfectly oriented for the exchange. A second molecular exchange may be expected as a result of a “flip flop” motion of the coordinated silicon from the amido to the ureido-type oxygen (eq 4). However, no trace of this exchange can be observed, either because it is too rapid relative to the NMR time scale or because coordination of the silicon to the urea-like oxygen is too weak to support such exchange.

The situation is very similar also in 5b, where two additional chlorine atoms are present. Despite the expected increase in silicon electrophilicity, the stability of the pentacoordinate silicon moiety is insufficient to slow the chloride migration, and as a result, the molecule appears symmetric

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Figure 3. Comparison of the solid state (A) and CD3CN solution (B) sidebands.

13

Figure 4. Comparison of the solid state (A) and CD3CN solution (B) sidebands.

29

on time-average in all three solution spectra: 1H, 13C, and 29 Si NMR. Only in the VACP-MAS solid-state NMR spectra (like in the crystal structure) are the two silicon atoms in the molecule observed as nonequivalent groups: the solution (CD3CN) and solid-state 13C and 29Si NMR spectra of 5a are compared in Figures 3 and 4, respectively. It is immediately evident that the dynamics that are observed in solution can no longer be found in the solid state. Not only are the two silicon resonances very different in the solid state (44.4 and -35.2 ppm, averaging 4.6 ppm, in excellent agreement with the time-averaged solution resonance at 2.3 ppm), but also the various skeletal carbon atom resonances are doubled, reflecting the loss of average molecular symmetry in the solid. It is interesting to note that the four siliconmethyl groups give rise to four different singlets in the solid, despite the apparent Cs molecular symmetry. The reason for this is found in the crystal lattice, in which this symmetry

C NMR spectra of 5a. Signals labeled with * are due to

Si NMR spectra of 5a. Signals labeled with * are due to

is lost (different environments in the vicinity of each of the geminal methyl groups). 5,5-Dimethylbarbituric Acid Complex. The synthesis shown in eq 3 suggests that the reactant 6 has two stable isomeric forms, the C2v symmetric 6 and the Cs symmetric 60 . The 29Si NMR spectrum of 6 at 295 K shows the two isomers in ca. 6:1 molar ratio, respectively. However, once the isomer mixture has reacted further to form 7a, this isomer distinction is lost because the products formed from each isomer (7a and 70 a) can rapidly exchange and are indistinguishable in their solution NMR spectra. The low activation barrier for exchange of 7a and 70 a indicates weak coordination of the oxygen with silicon. This is also reflected in the relatively low-field 29Si NMR chemical shift (Table 1), characteristic of weak coordination and well into the tetracoordinate silicon region. One may wonder why OfSi coordination in 7a is so much weaker than in 5a. This is probably the result of the

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Figure 5. Molecular structure of 9 in the crystal, depicted at the 50% probability level with omission of hydrogen atoms.

conjugated system in 5a and its absence in 7a: in both compounds the amide-oxygen coordinates with silicon by electron donation. However, the donor ability of the oxygen, and the resulting OfSi bond strength in 5a, may be enhanced by electron donation from the second oxygen atom through the double bond (resonance contribution of canonic structure 8, eq 5), an interaction that is not available in 7a.

Substitution of the chloro by triflato ligands (eq 6) resulted in a more strongly coordinated dinuclear complex (9), as evident from its significantly higher-field 29Si resonance (-16.5, as opposed to 18.6 ppm in 7a). A crystal structure analysis of 9 (Figure 5, Table 2) shows two well-defined pentacoordinate silicon moieties, occupying the central position in slightly distorted TBP geometries.

7b was prepared in analogy with 7a and has two chloro ligands attached to each of the silicon atoms. As a result, its oxygen to silicon coordination is stronger than in 7a, and for the first time in this series of dinuclear complexes the coordination isomers can be observed by 29Si NMR spectroscopy; that is, the barrier for cleavage of the dative bond is sufficiently high to allow individual site observation on the NMR time scale. The predominant isomer is the C2v symmetric 7b, possessing a single silicon resonance at 15.8 ppm, while the Cs isomer 70 b gives rise to two signals (14.6 and 21.8 ppm) at 270 K, corresponding to the two different silicon (9) We found no conclusive evidence to determine which of the two carbonyl oxygen atoms in uracil is a stronger base or nucleophile; one computational study [Kallies, B.; Mitzner, R. J. Mol. Struct. (THEOCHEM) 1998, 428, 267-282] reported that the urea-type oxygen is more basic in the gas phase toward protonation relative to an amide oxygen. However, the present study suggests that the oxygen affinity toward silicon follows the opposite trend.

atoms, in a 10:1 isomer ratio, respectively. This observation provides independent evidence that the electron-donor strength of the amide-oxygen is greater than the donor ability of the urea-type oxygen in this heterocyclic compound series.9 2. Oligomerization through Partial Hydrolysis. Compounds 3a, 5a, and 7a are highly moisture sensitive and upon exposure to the laboratory environment rapidly undergo partial hydrolysis, to form cyclic oligomers in which the monomer units are connected through siloxane groups. Uracil-Derived Cyclic Oligomers. The hydrolysis of 3a is shown in eq 7. The results were analyzed by APCI and MALDI-TOF mass spectra, which are depicted in Figures 6 and 7. Figure 6 shows the formation of cyclo-oligomers composed of n = 1-5; higher mass oligomers, up to n = 8 (and in some cases 9 and 10) could be observed in the MALDI spectrum in Figure 7. The molecular masses obtained in the MS measurements correspond only to the masses of cyclic compounds, and no significant signals corresponding to linear oligomers have been observed. The strong signal at m/z = 270 indicates intramolecular cyclization upon hydrolysis, to form the eight-membered heterocyclic 10.

The oligomers were further analyzed by NMR spectroscopy: the oligomer mixtures give rise to remarkably sharp spectra. Comparison of the 13C NMR spectra (Figure 8) reveals very minor spectral changes between 3a and its hydrolysis product, 3a-hyd. The major change is found in the chemical shifts of the silicon-methyl groups, which in 3a are adjacent to a chloro ligand and in the oligomer are next to oxygen. A distinct change has also been found in the 29Si NMR spectrum, where the two signals for 3a, corresponding to tetra- and pentacoordinate silicon atoms, have essentially collapsed into one narrowly distributed group of signals centered near 5 ppm, indicating loss of coordination in the cyclic oligomer.

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Figure 6. APCI-MS spectrum of 3a-hyd in the positive ion mode. Signals corresponding to the cyclic isomers with n = 1-5 are found.

Figure 7. MALDI-TOF spectrum of 3a-hyd. Signals corresponding to cyclic oligomers with n = 2-8 are observed.

In principle, the cyclic oligomers could each have several geometrical isomers, differing in the relative orientations of the uracil rings. This was tested and confirmed only for the simplest case, the dimer. Indeed, two isomers were detected by the UV (230 nm) detector of a HPLC-MS instrument at m/z = 541, with retention times of 20 and 25 min, which were assigned to the C2v and C2h diastereomers shown in Scheme 4. Barbituric Acid-Derived Cyclic Oligomers. The hydrolysis of 5a in chloroform solution (eq 8) led to formation of up to nine-mers, as evident from the MALDI-TOF spectrum (Figure 9). The conversion is essentially quantitative, leading to a higher symmetry cyclic oligomer-mixture product (local C2v) and, hence, resulting in sharp and simple NMR spectra (Figure 10). The spectra in Figure 10 provide the necessary support for the structural assignment: the two significant spectral changes are in the silicon-methyl groups, which lost the neighboring chlorine atom and the pentacoordination and moved upfield, and in the ring proton, which became a saturated CH2 group and shifted from 5.46 to 3.65 ppm. The

absence of any resonances for different end groups supports the cyclic structure. The smallest cyclic product is the dimer (n = 2) at molecular mass 594.2 (2M þ Na), indicating that in the present case intramolecular cyclization is not possible, or is exceedingly slow.

5,5-Dimethylbarbituric Acid-Derived Cyclic Oligomers. The results of partial hydrolysis of 7a are very similar to those of 5a. Oligomerization up to n = 7 could be observed by MALDI analysis. The 1H and 13C NMR spectra are barely modified relative to the starting monomer (see Experimental

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Figure 8.


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C NMR spectra of 3a (A) and 3a-hyd (B) in CDCl3 solution.

Figure 9. MALDI-TOF spectrum of the hydrolysis products of 5a. Signals corresponding to cyclic oligomers with n = 2-9 are observed, each with an additional Naþ ion. Scheme 4. Diastereomeric Cyclic Dimers (n = 2) Derived from 3a by Hydrolysis

Section). However, the 29Si NMR spectra show a substantial change, from 18.6 ppm in 7a to 4.7 ppm in the cyclic oligomer. Template Oligomerization. Examination of the molecular formulas of a cyclic dimer or trimer of the compounds

discussed above shows that a number of oxygen and nitrogen atoms form a part of the macrocycle. It occurred to us that these macrocycles might be capable of acting as crown ethers; that is, the hydrophilic atoms in the ring could bind a metal ion and be effective in transporting it between liquid phases. To test this hypothesis, some of the hydrolysis reactions were carried out in the presence of alkali metal salts acting as templates to support preferred cyclization, and preliminary results are described below. 3a, 5a, and 7a were each exposed to atmospheric hydrolysis in the presence of methanol solutions of LiCl, KCl, and CsCl. The resultant hydrolysis products were then analyzed using MALDI-TOF mass spectrometry. No significant selectivity could be observed with potassium and cesium ions. However, the hydrolysis in the presence of the lithium

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Figure 10. 1H NMR spectra of 5a (A) and 5a-hyd (B) in CDCl3 solution.

Figure 11. MALDI-TOF spectrum of the hydrolysis products of 7a. Signals corresponding to cyclic oligomers with n = 2-7 are observed, each with an additional Naþ ion.

salt resulted in exclusive formation of hydrolytic dimers, in sharp contrast with the wide molecular-weight distribution of the unassisted hydrolysis products. The MALDI-TOF spectra of the hydrolysis products of 3a, 5a, and 7a in the presence of LiCl are displayed in Figures 12-14. The mass spectrum shown in Figure 12 displays a single mass, 547.3, corresponding precisely to the cyclic dimer of 3a attached to a Li cation. Likewise, Figure 13 shows only two signals, with m/z values corresponding to the cyclic and linear dimers of 5a, without any other significant mass distribution. It is thus demonstrated that lithium cation is effective as a template, causing preferential cyclic dimerization over formation of higher-mass oligomers. The hydrolysis in 5a acts selectively (eq 9), to first hydrolyze the more labile Si-Cl bonds, forming a linear dimer, which only subsequently partially cyclizes to the cyclic dimer by a net addition of one water molecule (two H2O molecules are

consumed to open the five-membered chelate rings, and condensation releases one H2O molecule).

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Figure 12. MALDI-TOF spectrum of the hydrolysis product of 3a in the presence of excess LiCl. Only the cyclic dimer þ Liþ is found.

Figure 13. MALDI-TOF spectrum of the hydrolysis product of 5a in the presence of excess LiCl. Signals corresponding only to the cyclic and acyclic dimer þ Liþ are observed.

The partial hydrolysis of 7a (Figure 14) also leads only to the cyclic dimer, which is presumably formed by coordination with the Liþ template ion, and to an additional signal due to the same dimer to which a water molecule has been added (m/z 653.4). The additional water molecule may have opened the cyclic dimer to a linear dimer by cleavage of one of the siloxane groups or, alternatively, may just be attached to the lithium ion as a solvating agent.

Experimental Section The reactions were carried out under dry argon using Schlenk techniques. Solvents were dried and purified by standard methods. NMR spectra were recorded on a Bruker Avance DMX500 spectrometer operating at 500.13, 125.76, and 99.36 MHz, respectively, for 1H, 13C, and 29Si spectra. Spectra are reported in δ (ppm) relative to TMS, as determined from standard residual solvent proton (or carbon) signals for 1H and 13C and directly from TMS for 29Si. Silicon-containing starting materials were obtained from Gelest Inc., Morrisville, PA. Melting points

were measured in sealed capillaries using a Buchi melting point instrument and are uncorrected. Elemental analyses were performed by Mikroanalytisches Laboratorium Beller, G€ ottingen, Germany. Single-crystal X-ray diffraction measurements were performed on a Bruker Smart Apex on a D8-goniometer. Crystallographic details are listed in Table 3. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (CCDC). The CCDC numbers are listed in Table 3. Mass spectra were measured by a Bruker Daltonics Reflex IV matrix-assisted laser desorption ionization time of flight (MALDI-TOF) instrument, using 2,5-dihydroxybenzoic acid as a matrix. 1,3-Bis(chlorodimethylsilylmethyloxy)pyrimidine (3a). A mixture of 0.63 g (2.5 mmol) of 110 and 0.79 g (5.5 mmol) of 2a in 5 mL of chloroform was left standing at room temperature for 2 days. The volatiles were removed under reduced pressure (0.1 mmHg) to leave a white solid residue. The solid was washed twice with n-hexane to yield 0.7 g (96%) of 3a. Mp: 84-86 °C. (10) Takuzo, N.; Bunji, S.; Issei, I. Chem. Pharm. Bull. 1963, 11, 1470– 1477.

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Figure 14. MALDI-TOF spectrum of the hydrolysis product of 7a in the presence of excess LiCl. Signals corresponding only to the cyclic and acyclic dimer þ Liþ are observed. Table 3. Crystallographic Data and Experimental Parameters for the Structure Analyses of 5a and 9

CCDC no. empirical formula form mass, g mol-1 collection T, K cryst. syst. space group a, A˚ b, A˚ c, A˚ R, deg β, deg γ, deg V, A˚3 Z Fcalcd, Mg/m3 F(000) θ range, deg no. of coll reflns no. of indep reflns Rint no. of reflns used no. of params Goof R1, wR2 [I > 2σI)] R1, wR2 (all data) max./min. res electron dens (e A˚-3)

5a

9

780245 C10H17ClN2O3Si2 304.89 240(1) orthorhombic Pbca 11.190(2) 13.114(3) 19.667(4) 90 90 90 2886.0 (10) 8 1.403 1280 2.07-26.01 15 607 2700 0.0910 2652 164 0.932 0.0450, 0.0941 0.1169, 0.1113 0.256/-0.259

780246 C14H22F6N2O9S2Si2 596.64 240(1) monoclinic P21/c 15.212(3) 12.222(2) 15.921(3) 90 117.750(3) 90 2619.6(8) 4 1.513 1224 2.21-25.11 15 898 4446 0.075 4446 317 1.110 0.0719, 0.2021 0.1248, 0.2289 1.075/-0.645

1 H NMR (CDCl3, 295 K): δ 0.65, 0.66 (2s, 12H, SiMe2), 3.28, 3.55 (2s, 4H, CH2), 5.96, 7.36 (s, 2H, HCdC). 13C NMR (CDCl3, 295 K): δ 3.1, 5.6 (SiMe2), 35.0, 43.0 (CH2), 98.8 (CHCdO), 145.4 (NCH), 151.8 (N2CO), 164.6 (CCdO). 29Si NMR (CDCl3, 295 K): δ 13.7, -5.7. Anal. Calcd for C10H18Cl2N2O2Si2: C, 36.92; H, 5.58; N, 8.61. Found: C, 37.55; H, 5.80; N, 8.90. 1,3-Bis[dichloro(methyl)silylmethyloxy]pyrimidine (3b). 3b was prepared like 3a, from 0.61 g (2.4 mmol) of 110 and 0.85 g

(5.2 mmol) of 2b, to give 0.85 g (96%) of 3b. Mp: 102-106 °C. H NMR (CDCl3, 295 K): δ 0.99, 1.03 (2s, 6H, SiMe), 3.45, 3.71 (2s, 4H, CH2), 6.00, 7.48 (2d, J = 10 Hz, 2H, HCdC). 13C NMR (CDCl3, 295 K): δ 7.1, 10.3 (SiMe), 36.2, 44.5 (CH2), 98.5 (CCdO), 145.9 (CdCN), 150.5 (N2CO), 164.7 (CCdO). 29Si NMR (CDCl3, 295 K): δ 13.9, -20.8. Anal. Calcd for C8H12Cl4N2O2Si2: C, 26.24; H, 3.30; N, 7.65. Found: C, 26.30; H, 3.35; N, 7.69. 6-[(Chlorodimethylsilyl)methyl]-2,2-dimethyl-2H-[1,4,2]oxazasilolo[4,5-c]pyrimidine-5,7(3H,6H)-dione (5a). 5a was prepared like 3a, from 0.98 g (2.6 mmol) of 411 and 0.82 g (5.3 mmol) of 2a in CH3CN. The solution was heated in a 100 °C oil bath for 3 h. 5a (0.8 g, 95%) was obtained upon removal of solvent. A single crystal for X-ray analysis was grown from CH3CN. Mp: 235-238 °C. 1H NMR (CD3CN, 295 K): δ 0.53 (s, 12H, SiMe2), 3.07 (s, 4H, CH2), 5.26 (s, HC). 13C NMR (CD3CN, 295K): δ 3.4 (SiMe2), 34.8 (CH2), 76.6 (CH), 149.9 (N2CO), 167.8 (CCdO). 29 Si NMR (CD3CN, 295 K): δ 2.3. VACP-MAS solid state: 13C NMR: δ -0.5, 1.4, 8.0, 9.3 (SiMe2), 34.8, 37.0 (CH2), 77.3 (CH), 148.4 (N2CO), 166.2, 168.5 (CCO). 29Si NMR: δ -35.2, 44.4. Anal. Calcd for C10H17ClN2O3Si2: C, 39.40; H, 5.62; N, 9.19. Found: C, 39.40; H, 5.55; N, 9.11. 6-[(Dichloro(methyl)silyl)methyl]-2-chloro-2-methyl-2H-[1,4,2]oxazasilolo[4,5-c]pyrimidine-5,7(3H,6H)-dione (5b). 5b was prepared like 3a, from 0.89 g (2.6 mmol) of 411 and 0.85 g (4.7 mmol) of 2b in CH3CN. The solution was heated for 1 h. 5b (0.84 g, 94%) was obtained. Mp: 174-178 °C. 1H NMR (CD3CN, 295 K): δ 0.88 (s, 6H, SiMe), 3.27 (s, 4H, CH2), 5.46 (s, HC). 13C NMR (CD3CN, 295 K): δ 7.3 (SiMe), 36.0 (CH2), 77.3 (CH), 149.5 (N2CO), 166.7 (CCdO). 29Si NMR (CD3CN, 295 K): δ -18.3. Anal. Calcd for C8H11Cl3N2O3Si2: C, 27.79; H, 3.21; N, 8.10. Found: C, 27.91; H, 3.25; N, 7.94. 5,5-Dimethyl-4,6-bis(trimethylsilyloxy)5H-pyrimidin-2-one (6). To 5.0 g (32 mmol) of 5,5-dimethylbarbituric acid dissolved in 50 mL of diethyl ether were added 7.3 g (72 mmol) of Et3N and 1

(11) Studentsov, E. P.; Kokhanovskii, A. N.; Ganina, M. B.; Nikolaeva, N. I.; Fedorova, E. V.; Moskvin, A. V.; Ivin, B. A. Zh. Obsh. Khim. 2004, 74, 293-298. English translation: Russ. J. Gen. Chem. 2004, 74, 261-265.

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7.2 g (66 mmol) of Me3SiCl dropwise over 1 h, followed by 2 h of reflux. After cooling the Et3NHCl was filtered under argon and the solid washed twice with diethyl ether (2 20 mL). The solvent was removed from the combined solution under reduced pressure, followed by distillation. The fraction boiling between 67 and 73 °C at 0.1 mmHg, 8 g (85%), was collected as 6. 1H NMR (toluene-d8, 295 K): (the solvent was chosen to allow observation of two isomers in 1H NMR spectrum) major: δ 0.38 (s, 18H, SiMe3), 1.25 (s, 6H, CMe); minor: δ 0.23, 0.43 (SiMe3), 1.19 (Me2). 13C NMR (CDCl3, 295 K): major: δ 0.5 (SiMe3), 23.3 (CCH3), 47.3 (CMe2), 155.4 (N2CO), 177.0 (CCdO); minor: δ -0.8 (SiMe3), 23.3 (CCH3), 42.8 (CMe2), 158.6 (N2CO), 177.9, 180.7 (CCdO). 29Si NMR (CDCl3, 295 K): major: δ 17.8; minor: δ 16.3, 29.3. Anal. Calcd for C12H24N2O3Si2: C, 47.96; H, 8.05; N, 9.32. Found: C, 47.91; H, 8.10; N, 9.26. 1,3-Bis[(chlorodimethylsilyl)methyl]-5,5-dimethylpyrimidine2,4,6-trione (7a). 7a was prepared like 3a, from 0.50 g (1.7 mmol) of 6 and 0.49 g (3.4 mmol) of 2a in chloroform solution. The solution was heated for 7 h. The solid was washed twice with diethyl ether, yielding 0.61 g (98%) of 7a. Mp: 70-71 °C. 1H NMR (CDCl3, 295 K): δ 0.52 (s, 12H, SiMe2), 1.52 (s, 6H, CMe), 3.44 (s, 4H, CH2). 13C NMR (CDCl3, 295 K): δ 3.0 (SiMe2), 25.1 (CCH3), 34.7 (CH2), 46.6 (CMe2), 151.4 (N2CO), 172.7 (CCdO). 29Si NMR (CDCl3, 295 K): δ 18.6. Anal. Calcd for C12H22Cl2N2O3Si2: C, 39.02; H, 6.00; N, 7.58. Found: C, 39.11; H, 5.92; N, 7.47. 1,3-Bis[(dichloro(methyl)silyl)methyl]-5,5-dimethylpyrimidine2,4,6-trione (7b). 7b was prepared like 3a, from 0.56 g (1.8 mmol) of 6 and 0.62 g (3.8 mmol) of 2b in chloroform solution. The solution was refluxed for 16 h. The solid was washed twice with diethyl ether, to leave 0.7 g (90%) of 7b. Mp: 57-60 °C. 1H NMR (CDCl3, 295 K): two isomers were observed; major: δ 0.94 (s, 6H, SiMe), 1.52 (s, 6H, CMe), 3.65 (s, 4H, CH2); minor: δ 0.85, 0.94 (SiMe), 1.50 (CMe), 3.59, 3.65 (CH2). 13C NMR (CDCl3, 257 K): major: δ 7.6 (SiMe), 24.7 (CCH3), 35.7 (CH2), 46.3 (CMe2), 150.4, 172.3 (CdO); minor: δ 2.74, 7.77 (SiMe), 24.3 (CCH3), 34.8, 35.7 (CH2), 149.8, 172.2, 173.6 (CdO). 29Si NMR (CDCl3, 270 K): major: δ 15.8; minor: δ 14.6, 21.8. Anal. Calcd for C10H16Cl4N2O3Si2: C, 29.28; H, 3.93; N, 6.83. Found: C, 29.53; H, 4.03; N, 6.88. 1,3-Bis[((trifluoromethanesulfonato)dimethylsilyl)methyl]-5,5dimethylpyrimidine-2,4,6-trione (9). To a solution of 0.8 g (2.2 mmol) of 7a in acetonitrile was added 0.85 g (4.3 mmol)

of CF3SO3SiMe3.12 The reaction was boiled to reflux for 8 h and then cooled to room temperature. The precipitate was separated by decantation and dried, to yield 1.40 g (73%) of 9. A single crystal for X-ray analysis was grown from CH3CN. Mp: 211 °C (dec). 1H NMR (CD3CN, 295 K): δ 0.59 (s, 12H, SiMe2), 1.57 (s, 6H, CMe), 3.28 (s, 4H, CH2). 13C NMR (CD3CN, 295 K): δ 2.6 (SiMe2), 23.8 (CCH3), 33.3 (CH2), 44.6 (CMe2), ∼110 (CF3, under solvent peak), 149.7, 178.6 (CdO). 29Si NMR (CDCl3, 295 K): δ -16.5. Anal. Calcd for C14H22F6N2O9S2Si2: C, 28.18; H, 3.72; N, 4.70. Found: C, 28.37; H, 3.78; N, 4.53. Hydrolysis Products. The dinuclear silicon complexes were hydrolyzed from their CHCl3 solutions by opening the reaction flask and exposure to the atmosphere for 48 h. 3a-hyd. During hydrolysis the color changed to yellow. An oily product was obtained. 1H NMR (CDCl3, 295 K): δ 0.12 (m, 12H, SiMe2), 3.31, 3.44 (2s, 4H, CH2), 5.69, 7.27 (2s, 2H, HCdC). 13C NMR (CDCl3, 295 K): δ 0.0, 0.5 (SiMe2), 33.3, 41.4, 42.0 (CH2), 100.3 (CHCdO), 142.9 (NCH), 151.4, 162.9 (CdO). 29Si NMR (CDCl3, 295 K): δ 4.7, 5.3, 6.2. MALDITOF, see Figure 7; in the presence of LiCl, m/z 547.3 (dimer þ Liþ), calcd 547.2. 5a-hyd. Solid, mp 165 °C (dec). 1H NMR (CDCl3, 295 K): δ 0.11 (s, 12H, SiMe2), 3.35 (s, 4H, CH2), 3.65 (s, 2H, CH2). 13C NMR (CDCl3, 295 K): δ 0.5 (SiMe2), 33.9, 39.3 (CH2), 151.8, 164.6 (CdO). 29Si NMR (CDCl3, 295 K): δ 4.4. MALDI-TOF, see Figure 9; in the presence of LiCl, m/z 579.0 (cyclic dimer þ Li) calcd 579.1; 561.0 (linear dimer þ Li), calcd 561.2. 7a-hyd. Oily product. 1H NMR (CDCl3, 295 K): δ 0.15 (s, 12H, SiMe2), 1.53 (s, 6H, CCH3), 3.42 (s, 4H, CH2). 13C NMR (CDCl3, 295 K): δ 0.3 (SiMe2), 25.3 (CCH3), 34.2 (CH2), 46.9 (CMe2), 151.0, 172.3 (CdO). 29Si NMR (CDCl3, 295 K): δ 4.7. MALDI-TOF, see Figure 11; in the presence of LiCl, m/z 635.4 (cyclic dimer þ Li), calcd 635.2; 653.4 (linear dimer þ Li), calcd 653.3.

(12) (a) Marsmann, H. C.; Horn, H. G. Z. Naturforsch., B 1972, 27, 1448–1451. (b) Kingston, V.; Gostevskii, B.; Kalikhman, I.; Kost, D. Chem. Commun. 2001, 1272–1273.

Supporting Information Available: Crystallographic data for 5a and 9 in the form of CIF files are available free of charge via the Internet at http://pubs.acs.org.

Acknowledgment. We thank Dr. R€ udiger Berterman and Prof. Dr. Reinhold Tacke for the solid-state NMR spectra. Financial support from the Israel Science Foundation, grant no. ISF-242/09, is gratefully acknowledged.

Dinuclear Silicon Complexes of Uracil, Barbituric Acid, and 5,5

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