Anionic triazine systems†

Melanie Kingston, Shan-Jia Chen, Enno Lork and Rüdiger Mews*Institut für Anorganische und Physikalische Chemie der Universität Bremen, Postfach 330440,Leobener Straße, 28334 Bremen, Germany. E-mail: mews@chemie.uni-bremen.de

Received 18th February 2004, Accepted 22nd March 2004First published as an Advance Article on the web 5th April 2004

The synthesis of TAS�C3N3F4� (1) (TAS� = (Me2N)3S

�) and the reactions of 1 with Me3SiOSiMe3 andMe3SiCF3 to give TAS�C3N3F2O

� (2) and TAS�[(NCF)(NCCF3)(NC(CF3)2]� (4) are reported. An isomer of 4,

TAS�[(NCCF3)2(NCFCF3)]�, compound 6, was obtained by fluoride ion addition to (CF3CN)3. From the reactions

with Me3SiNMe2 neutral fluoroamino triazines C3N3Fn(NMe2)n�1 (n = 1, 2) were isolated. Possible reaction pathwaysare discussed, the X-ray structures of 1, 2, 4 and 6 were determined.

IntroductionThe search for selective anion receptors, especially for the fluor-ide ion is an active research topic, not only in inorganic but alsoin organic chemistry.1 We have concentrated our research tofinding host molecules which can be used to selectively complexa fluoride. Using TASF [(Me2N)3S

�Me3SiF2�] 2 as a fluoride

source, it is possible to transfer a fluoride onto the multi-functional heterocyclic systems phosphazenes (F2PN)n

3 andoxothiazines (F(O)SN)3

4 (Scheme 1). In the resulting cyclic

anions intramolecular interactions between the fluorine atomand the acceptor centres of the host molecule are observed. Asimilar result has been published by Corriu and co-workers,who prepared a heptafluorosilacyclohexane anion, for whichintramolecular interactions were also postulated.5

It has been determined that several carbon containingheterocyclic systems can also act as host molecules, yieldingcyclic anions. In the case of the phosphatriazine (NPF2)-(NCCl)2 the fluoride is transferred to the phosphorus atom.6

The anion is, however, unstable and decomposes quickly underthe formation of PF6

�. In the case of the fluorothiatriazines:(NCF)n(NSF)3�n (n = 1, 2), the fluoride is transferred to thecarbon atom.7 The ability of triazines (C3N3X3) to act as hostmolecules has not been greatly investigated. Chambers et al.reported the formation of a cyclic anion through the reactionof C3N3F3 with CsF (Scheme 2).8 The definite characterisationof this product was, however, not published. In the presentpaper the synthesis of TAS�C3N3F4

� (1), the reactions of 1 withMe3SiOSiMe3 and Me3SiCF3 to give TAS�C3N3F2O

� (2) andTAS�[(NCF)(NCCF3)(NC(CF3)2]

� (4) are reported. An isomerof 4, TAS�[(NCCF3)2(NCFCF3)]

� (6), was obtained by fluorideaddition to C3N3(CF3)3. From the reactions with Me3SiNMe2

neutral fluoroamino triazines C3N3Fn(NMe2)3�n (n = 1, 2) wereisolated. Possible reaction pathways are discussed, the X-raystructures of 1, 2, 4 and 6 were determined.

Scheme 1

† Dedicated to Professor Dr G.-V. Röschenthaler on the occasion of his60th birthday.

Results and discussion

Reaction of C3N3F3 with TASF

The anion C3N3F4� (1) was prepared using TASF as the fluor-

ide source via a simple fluoride addition to a carbon centre ofC3N3F3. After removal of the solvent and all volatile productsin vacuo, a colourless solid remained in quantitative yield(Scheme 2). The compound shows two signals in the 19F NMRspectrum, due the presence of two magnetically nonequivalentfluorine groups. This indicates the absence of fast intra-molecular fluorine exchange, which was found e.g. in cyclicfluorophosphazenates.3

Colourless crystals of 1, suitable for X-ray crystal structuredetermination, were grown from CH3CN and Et2O at �35 �C.The structure of the salt is shown in Fig. 1; selected bondlengths and angles for the anion are listed in Table 1.

Scheme 2

Fig. 1 The structure of TAS�C3N3F4� (1).D

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There are no apparent interactions between anion andcation; the C3N3 ring is, as expected, planar and C1 is in atetrahedral environment. According to the bond distancesobserved the anion is best described by the resonance structuresA and B (Scheme 2). The bond lengths C1–N1 (140.3(4) pm)and C1–N3 (140.0(4) pm) are in the region of single bonds;N1–C2 (127.7(4) pm) and C3–N3 (127.9(4) pm) are found to bedouble bonds. C2–N2 (132.9(4) pm) and N2–C3 (131.9(4) pm)lie between single and double bonds. Since the s character ofthe bonds at C1 is lower, the fluorine bonds are up to 6 pmlonger than those at the sp2 hybridised carbon centres C2 andC3. This highlights a possible synthesis method for selectivesubstituted anionic triazine systems. Since the C1–F bonds arerelatively weak they might selectively react with, for example,Me3Si-derivatives. The driving force of these reactions is theformation of the thermodynamically favoured Si–F bond ofMe3SiF.

Reaction of TAS�C3N3F4� with (Me3Si)2O

The oxotriazinate (2) was prepared using hexamethyldisiloxaneas an oxygen source (Scheme 3). The first step of the reaction isthe transfer of a fluoride to silicon, followed by cleavage of theSi–O bond and loss of two Me3SiF molecules.

Compound 2 is a colourless solid and shows only one signalin the 19F NMR spectrum for the CF groups. The 13C spectrumalso confirms the oxotriazinate structure, showing two multi-plets for the CF and CO carbon atoms. Since the chemical shiftsof the carbon atoms are similar, the assignments were madefrom the coupling constants (δCO 171.1, 3JCF = 18 Hz, δCF 171.7,1JCF = 270 Hz, 3JCF = 24 Hz). Single crystals suitable for X-raycrystal structure determination were grown from CH3CN andEt2O at �35 �C. The structure of the salt is shown in Fig. 2;selected bond lengths and angles for the anion are listed inTable 1.

The C3N3 ring is also planar and the C–N bonds follow thesame trend observed in the perfluorotriazinate 1 (Table 1).According to the bond lengths the bonding situation is best

Scheme 3

Table 1 Selected bond lengths (pm) and angles (�) for the anionsC3N3F4

� (1) and C3N3F2O� (2)

C3N3F4� (1) C3N3F2O

� (2)

C1–F1 140.0(4) –C1–F2 138.3(4) –C1–O1 – 122.5(4)C1–N1 140.3(4) 139.0(5)C1–N3 140.0(4) 139.1(5)N1–C2 127.7(4) 128.8(5)N3–C3 127.9(4) 128.1(5)C2–N2 132.9(4) 131.0(6)N2–C3 131.9(4) 132.2(6)C2–F 134.5(3) 135.0(5)C3–F 134.8(3) 134.2(5)

N1–C1–N3 120.4(3) 119.4(3)N1–C2–N2 132.0(3) 132.1(4)F1–C1–F2 101.1(2) – described by resonance structure C (Scheme 3). The bond

length of C1–O1 (122.5(4) pm) is in the region of a doublebond. Each carbon atom is sp2 hybridised and since 2 is iso-electronic with (NCF)3, it is of interest to compare the struc-tural data for the two compounds. The structure of (NCF)3 wasrecently determined in the gas phase by electron diffraction.9

Although comparison might be questionable because structuredeterminations in different phases are compared and becauseof the large alternation of the bond distances in the anion, thedata show that the structures of the planar heterocycles arerather similar. Due to the negative charge in the ring the sum ofthe C–N bond lengths is only slightly larger in the anionic com-pound (798.6 pm compared to 793.2 pm in (NCF)3), an increaseof the C–F bond distances (average C–F bond length in theanion 134.4 pm, in (NCF)3 131.1 pm) is also observed.

Reaction of TAS�C3N3F4� with Me3SiNMe2

The attempt to selectively introduce an amino group on to thetriazine ring did not yield an isolable anionic species. Character-isation of the products by mass spectroscopy showed the pres-ence of neutral amino triazines (Scheme 4). It is believed thatthe C3N3 rings with electron donating groups cannot accom-modate an additional fluoride. Experiments to introduce otherelectron donating groups should also result in neutral species.

Reaction of TAS�C3N3F4� with Me3SiCF3

Me3SiCF3 (Ruppert�s Reagent) 10 has recently become animportant reagent to introduce CF3 groups into organic mole-cules.11 From the reaction with the triazinate 1, a selective sub-stitution was not observed (Scheme 5); a mixture of productswas characterised via NMR spectroscopy.

Fig. 2 The structure of TAS�C3N3F2O� (2).

Scheme 4

Scheme 5

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The 19F NMR spectrum is shown in Fig. 3 and shows severalsinglets which are assigned to compounds 3–5, as summarisedin Table 2. The signals at δ �46.2 and �46.3 (intensity 2 : 1)were assigned as to CF for 3 and 4; those at �75.3 and �75.1(intensity 3 : 1) to CCF3 for 4 and 5; and those at �82.2, �81.9and �81.6 (intensity 6 : 6 : 1) to C(CF3)2 for 3, 4 and 5respectively.

The postulated structure of 4 was confirmed by X-ray crys-tallography (Fig. 4). Suitable crystals were grown from CH3CNand Et2O at �35 �C. In spite of repeated crystallisationattempts, it was not possible to obtain single crystals of 3 or 5.In 4 again the C3N3 ring is planar and, as observed in the pre-vious examples, the CN bonds follow the same trend of singleand double bonds followed by a bond of intermediate length(Table 3) The difference in length of the C1–C2 (153.6(5) pm)and C4–C5 (145.8(8) pm) lies in the hybridisation of C1 andC4. At the sp3 hybridised C1 atom the bonds are longer thanthose at the sp2 hybridised C4 atom. Within the crystal the CF3

group on the sp2 hybridised carbon atom is slightly disordered.Surprisingly it was observed that two CF3 groups were

located on one carbon atom. Recently several articles haveappeared highlighting the reaction of Me3SiCF3 with fluoridedonors, yielding labile anionic species.12 Since 1 can act as apotential fluoride donor, we suggest that the initial reactiontakes place between 1 and Me3SiCF3 forming a reactive silane, asource of the elusive CF3 anion, which can then attack theneutral triazine (Scheme 6). Through rearrangement, elimin-ation, and further addition reactions the observed products 3, 4and 5 are formed (Scheme 6). Attempts to isolate any of thepostulated intermediates from this reaction failed.

Fig. 3 19F NMR spectrum for the reaction of Me3SiCF3 with 1.

Fig. 4 The structure of TAS�C3N3F(CF3)3� (4).

Table 2 19F NMR data (ppm) for compounds 3–5

3 4 5

δ(CF) �46.3 �46.2 δ(CCF3) �75.3 �75.1δ(C(CF3)2) �82.2 �81.9 �81.6

Reaction of C3N3(CF3)3 with TASF

Compound 6, the isomer of 4 and a missing link in the pro-posed reaction Scheme 6 was prepared from TASF andC3N3(CF3)3 via a fluoride addition reaction (Scheme 7). Com-pound 6 is a colourless solid. The 19F NMR spectrum containstwo signals (�75.3 and �87.8 ppm) which were assigned toCCF3 and CF(CF3) respectively; a signal for CF(CF3) was notobserved. In the 13C NMR spectrum, however, a signal could beassigned to each carbon atom. The quartets at 117.7 and 119.6

Scheme 6

Scheme 7

Table 3 Selected bond lengths (pm) and angles (�) for the anions[(NCF)(NCCF3)(NC(CF3)2]

� (4) and [(NCCF3)2(NCFCF3)]� (6)

4 6

C1–N1 144.4(6) 145.6(4)N1–C3 126.3(7) 130.2(4)C3–N2 133.2(8) 133.4(4)C1–C2 153.6(5) 152.6(4)C4–C5 145.8(8) –C5–C6 – 151.4(4)

N1–C1–N3 118.3(4) 117.0(2)Σangles 720.3 716.7

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ppm showing a 1JCF coupling (∼280 Hz) were assigned toCF(CF3) and CCF3, respectively; another quartet at 158.1 ppmwas assigned to CCF3 due to the smaller 2JCF coupling (∼35 Hz)and the doublet at 122.7 ppm to CF(CF3) (

1J = 285 Hz).Suitable crystals for X-ray crystal diffraction were grown

from CH3CN and Et2O at �35 �C. The structure of 6 isshown in Fig. 5; selected bond lengths and angles of the anionare listed in Table 3. The fluorine atoms of the CCF3 groupsare disordered. The bonding situation within the C3N3 ring isanalogous to that of the previous examples.

A comparison of the structures shows that although 4contains a planar C3N3 ring, the C3N3 ring of 6 is not planar(Fig. 6). The atom C1 deviates 18� from the plane created byN1, C3, N2, C5 and N3. The non-planarity can also be seenthrough the sum of the intra-annular angles (Σ = 716.9�).

ConclusionsThe potential of using TAS�C3N3F4

� as a starting material fornovel cyclic triazine anions was highlighted via the crystal struc-ture determination, which showed that the C–F bonds at the sp3

hybridised carbon were substantially longer than those at thesp2 carbons. Anion formation activates the bonds in the CF2

group, substitution of the fluorine for the electron withdrawinggroups CF3 and O resulted in cyclic anionic species. The triazinering, however, cannot accommodate an electron donatinggroup and a negative charge, therefore the introduction of aNMe2 group resulted in neutral triazines. The novel cyclicanions were characterised spectroscopically and by X-raycrystallography.

Experimental

Materials and methods

The starting materials C3N3F3,13 C3N3(CF3)3,

14 TASF,2

(Me3Si)2O,15 and Me3SiNMe216 were prepared according to the

literature, Me3SiCF3 was a commercial product. The solventswere dried and distilled prior to use. NMR spectra wereobtained on a Bruker DPX200 spectrometer, the mass spectra

Fig. 5 The structure of TAS�C3N3F(CF3)3� (6).

Fig. 6 A side view of the anions [(NCF)(NCCF3)(NC(CF3)2]� (4) and

[(NCCF3)2(NCFCF3)]� (6).

(Fast Atom Bombardment, NBA Matrix; Electron SprayInduction) on a Finnigen MAT 8200 spectrometer, and the IRspectra on a Perkin Elmer Paragon 500 FT-IR spectrometer.The elemental analyses were performed by the Mikro-analytisches Labor Beller, Göttingen, Germany. All experi-ments were carried out in a dry nitrogen atmosphere usingSchlenk apparatus; volatile materials were transferred via aglass vacuum line.

Reaction of TASF with C3N3F3

A 5 ml portion of CH3CN and 0.45 g (3.33 mmol) C3N3F3 werecondensed onto 0.46 g (1.67 mmol) TASF at �196 �C. Thereaction mixture was warmed to �40 �C and stirred at thistemperature for 45 min. After removal of the volatile com-ponents, the remaining colourless solid was washed with Et2O.0.51 g of TAS�C3N3F4

� (1.6 mmol, quantitative yield) wasobtained, mp 48–50 �C (with decomposition); NMR (CD3CN,�50 �C): 19F: δ �51.2 (2F, CF), δ �3.77 (2F, CF2); IR (Nujol/Kel-F, KBr, cm�1) 2918m, 2815w, 1669s, 1640m, 1625msh,1590w, 1541s, 1521m, 1452w, 1418m, 1398s, 1373m, 1344m,1308w, 1273m, 1197vs, 1171s, 1153m, 1106vw, 1083w, 1062m,1027s, 968vs, 948vs, 910s, 807w, 769m, 719s, 684m, 672m,652vw, 585w, 567w, 517vw, 482vw, 438vw, 415w. Anal. Calc.(found): C 34.0 (34.1), N 26.4 (26.3), H 5.7 (5.81%).

Reaction of TAS�C3N3F4� with (Me3Si)2O

A 5 ml portion of CH3CN and 0.84 g (5.2 mmol) (Me3Si)2Owere condensed onto 1.0 g (3.1 mmol) of TAS�C3N3F4

� at�40 �C. The reaction mixture was warmed to �10 �C andstirred at this temperature for 30 min. After cooling to �50 �C a15 ml portion of Et2O was condensed onto the reaction mix-ture. On storage at �40 �C, colourless crystals were obtained;NMR (CDCl3, 20 �C): 19F: δ �44.6 (1F, CF); 13C: δ 38.8 (6C,TAS�), 171.1 (t, 1C, 3JCF = 18 Hz, CO), 171.7 (dd, 2C 1JCF = 220Hz, 3JCF = 24 Hz, CF), 1H: δ 2.9 (18H, TAS�); IR (Nujol, KBr):2924m, 1667m, 1637s, 1542m, 1458w, 1400w, 1364m, 1274w,1196s, 1157m, 1089w, 947s, 910m, 817s, 767m, 720s, 671m,648w, 618w, 565m; MS (FAB positive) m/z: 164 [(Me2N)3S

�,100%], 120 [(Me2N)2S

�, 81%], 76 [(Me2N)S�, 19%]; MS(FAB negative) m/z: 265 [(C3N3F2OH)C3N3F2O

�, 23%], 132[C3N3F2O

�, 100%]

Reaction of TAS�C3N3F4� with Me3SiCF3

A 5 ml portion of CH3CN and 1.29 g (9.1 mmol) Me3SiCF3

were condensed onto 0.6 g (1.9 mmol) TAS�C3N3F4� at

�196 �C. The reaction mixture was warmed to �40 �C andstirred at this temperature for 45 min. After removal of thevolatile components the remaining yellow solid was washedwith Et2O. Analysis of this solid showed a mixture of productswhich could not be separated. Spectroscopic analysis of theproduct mixture indicated the presence of three compounds:NMR (CDCl3, 20 �C): 19F: 3: δ �46.2 (2F, CF), �82.2 (6F,C(CF3)2); 4: δ �46.3 (1F, CF), �75.3 (3F, CCF3), �81.9 (6F,C(CF3)2); 5: δ �75.1 (6F, CCF3), �81.6 (6F, C(CF3)2),

1H:δ 2.85 (18H, TAS�); MS (FAB positive), m/z: 3: 582 [(TASC3-N3F2(CF3)2)TAS�, 21%]; 4: 632 [(TASC3N3F(CF3)3)TAS�,11%]; 5: 682 [(TASC3N3(CF3)4)TAS�, 3%]; additional signals:164 [(Me2N)3S

�, 100%], 120 [(Me2N)2S�, 20%], 76 [Me2NS�,

4%], 58 [Me2N2�, 9%], 44 [Me2N

�, 9%]; MS (FAB negative) m/z:3: 672 [(TASC3N3F2(CF3)2)C3N3F2(CF3)2

�, 9%], 254 [C3N3F2-(CF3)2

�, 43%]; 4: 772 [(TASC3N3F(CF3)3)C3N3F(CF3)3�, 27%],

722 [(772 � CF2)�, 23%], 304 [C3N3F(CF3)3

�, 100%]; Product 5:872 [(TASC3N3(CF3)4)C3N3(CF3)4

�, 4%], 822 [(872 � CF2)�,

13%], 354 [C3N3(CF3)4�, 44%]

Reaction of TASF with C3N3(CF3)3

A 5 ml portion of CH3CN and 0.59 g (2.07 mmol) C3N3(CF3)3

were condensed onto 0.43 g (1.56 mmol) TASF at �196 �C. The

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Table 4 Crystal data, data-collection and refinement parameters for compounds 1, 2, 4 and 6 a

Data 1 2 4 6

Formula C9H18F4N6S C9H18F4N6OS C12H18F10N6S C12H18F10N6SMr 318.4 296.35 468.38 468.38T /K 173 173 173 173Colour, habit Colourless Colourless Colourless ColourlessCrystal size/mm 0.3 × 0.2 × 0.1 0.6 × 0.3 × 0.2 0.6 × 0.4 × 0.3 0.6 × 0.4 × 0.3Crystal system Orthorhombic Monoclinic Monoclinic MonoclinicSpace group P212121 P21/n P21/m P21/na/pm 947.3(2) 1151.3(14) 911.8(1) 884.5(14)b/pm 1195.8(2) 914.4(8) 1138.8(2) 1166.0(1)c/pm 1306.6(3) 1352.7(3) 923.2(1) 1994.1(2)β/� 90 91.35(6) 95.11(1) 102.49(1)V/nm3 1.4801(5) 1.42(1) 0.9548(2) 2.0078(4)Z 4 4 2 4µ/mm�1 0.262 0.253 0.275 0.262No. of unique reflections measured 16774 4227 5178 3633Observed, |Fo| > 4σ(|Fo|) R1 (I > 2σ(I ), wR2 (all data) b 0.043, 0.120 0.071, 0.214 0.081, 0.250 0.043, 0.107

a Details in common: graphite monochromated Mo-Kα radiation, refinement based on F 2. b R1 = Σ|Fo| � |Fc||/Σ|Fo|; wR2 = {Σ[w(Fo2 � Fc

2)2]/Σ[w(Fo

2)2]}1/2.

reaction mixture was warmed to �45 �C and stirred at thistemperature for 45 min. After removal of the volatile com-ponents, the remaining colourless solid was washed with Et2O.The reaction is quantitative. NMR (CDCl3, 20 �C): 19F: δ �75.3(6F, CCF3), �87.9 (3F, CF(CF3));

13C δ 38.5 (6C, TAS�), 117.7(q, 1C, 1JCF = 278 Hz, CF(CF3)), 119.6 (q, 2C, 1JCF = 278 Hz,CCF3), 122.7 (d, 1C, 1JCF = 285 Hz, CF(CF3)), δ 158.2 (q, 2C,2JCF = 34 Hz, CCF3);

1H δ 2.9 (18H, TAS�); IR (Nujol, KBr,cm�1): 2920s, 2860s, 1724m, 1629m, 1450s, 1305w, 1190w,1123m, 1055w, 940m, 915w, 852w, 739m, 725m, 617m, 535w;MS (ESI positive) m/z: 363 [(Me2N)3SCl)(Me2N)3S

�, 2%], 164[(Me2N)3S

�, 100%], 120 [(Me2N)2S�, 54%]; MS (ESI negative):

304 [C3N3F(CF3)3�, 10%], 302 [C3N3(CF3)3OH�, 100%], 267

[C3N3(CF3)(CF2H)�, 5%], 232 [C3N3(CF3)2O�, 40%]

Crystallographic analysis

The data collections for TAS�[(NCF)(NCCF3)(NC(CF3)2]� (4,

monoclinic, P21/n), TAS�[(NCCF3)2(NCF(CF3)]� (6, mono-

clinic, P21/m) and TAS�C3N3F2O� (2, monoclinic, P21/n) were

carried out on a Siemens P4 diffractometer; the data forTAS�C3N3F4

� (1, orthorhombic, P212121) was collected on aStoe IPDS diffractometer. Mo-Kα (0.71073 Å) radiation wasused with a graphite monochromator on both diffractometers.The crystals were mounted by the oil drop technique usingKel-F oil and a thin glass fibre. Details of the data collectionand refinement are given in Table 4. The programs SHELX-97 17

and DIAMOND 18 were used and the structures were solved bydirect methods (SHELXS).17 Subsequent least squares refine-ment (SHELXL-97) 17 located the positions of the remainingatoms from the electron density maps. Non-hydrogen atomswere refined anisotropically. Hydrogen atoms were placedin calculated positions using a riding model and defined iso-tropically in blocks.

CCDC reference numbers: 1: 231366, 2: 231367, 4: 231368, 6:231369.

See http://www.rsc.org/suppdata/dt/b4/b402527j/ for crystal-lographic data in CIF or other electronic format.

Acknowledgements

The generous gift of CF3SiMe3 by Dr Marhold (Bayer AG,Leverkusen) is gratefully acknowledged.

Notes and references1 (a) F. P. Schmidtchen, Nachr. Chem-Tech. Lab., 1998, 36, 8;

(b) B. Dietrich, Pure Appl. Chem., 1993, 65, 1457; (c) D. E.Kaufmann and A. Otten, Angew. Chem., 1994, 106, 1917.

2 W. J. Middleton, Org. Synth., 1985, 64, 221.3 E. Lork, D. Böhler and R. Mews, Angew. Chem., Int. Ed. Engl.,

1995, 34, 2696.4 E. Lork and R. Mews, J. Chem. Soc. Chem. Commun., 1995,

1113.5 D. Brondani, F. H. Carré, R. J. P. Corriu, J. J. E. Moreau and

M. Wong Chi Man, Angew. Chem., 1996, 108, 349.6 S. J. Chen, PhD Thesis, University of Bremen, 1993.7 R. Maggiulli, PhD Thesis, University of Bremen, 1989.8 R. D. Chambers, P. D. Philpot and P. I. Russel, J. Chem. Soc., Perkin

Trans. 1, 1977, 4, 1605.9 S. J. Chen, E. Lork and H. Oberhammer, R. Mews, to be published.

10 I. Ruppert, K. Schlich and W. Volbach, Tetrahedron Lett., 1994, 25,2195.

11 (a) G. K. S. Prakash and A. K. Yudin, Chem. Rev., 1997, 97, 757;(b) R. P. Singh and J. M. Shreeve, Tetrahedron, 2000, 56, 7613.

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