SHORT COMMUNICATION

Synthesis and Characterization of a Novel Cyclobutadiene-Octatetrayne Polymer Markus Altmann, Volker Enkelmann, and Uwe H. F. Bunz*

Max-Planck-Institut fur Polymerforschung, Ackermannweg 10, D-55021 Mainz, FRG

Received October 9, 1995

Key Words: Organometallic polymers I Coupling reactions I Butadiynyl complexes I Cobalt compounds

The synthesis of 3a starting from compound 1 is described. Copper-catalyzed oxidative coupling of 3b under Hay condi-

tions gives the novel polymer 9 with octatetrayne-cyclobuta- diene units.

We are investigating the synthesis of ethynyl-substituted n com- plexes[']. Little was known about this type of compound before we started our work, but a few early examples include Vollhardts 1,2- and 1,3-diethynylated cyclobutadiene-Cp cobalt complexes[2], and the derivatives of 1,l '-diethynylferrocene, independently described by Rausch and S~hlogI[~]. Complexes with two ethynyl substituents on the 71 ligand are interesting building blocks for new organomet- allic oligomers and liquid crystalline polymers[4].

The concept of homologization has led us from exploring poly- mers which contain diethynyl cyclobutadiene units - variations in their properties and structures - to the use of species doubly-sub- stituted with butadiyne as expanded monomers. Nothing has been reported about these doubly-butadiyne-substituted complexes in the literature.

Therefore we considered it worthwhile to prepare the expanded monomer 3a, derived from 1, in order to synthesize an organomet- allic polymer with cyclobutadiene-octatetrayne units (9) through copper-catalyzed coupling. Here we report the synthesis of 3a and 3b, the model compounds 7 and 8, as well as the synthesis of 9, achieved through oxidative alkyne coupling under Hay con- ditions['].

To obtain 3a, 1 was stirred with a fivefold excess of cis-1,2- dichloroethene and 5 mol% Pd(PPh3)dCuI in piperidine for 16 hours (Heck-Cassar-Sonoshigara-Hagihara conditions according to Alami and Linstrumelle[6b]). The doubly-chlorovinyl- ated compound 2 was isolated in a yield of 43% after chromatogra- phy. This low yield was not unexpected since double chlorovinyl- ation of organic substrates under similar conditions also gives only moderate yields[7d1 (see below). Reacting 1 with the much cheaper cisltrans mixture of 1,Z-dichloroethene or pure trans-l,2-dichloro- ethene allows isolation of only traces of 2.

After 2 dissolved in DMSO, was treated with an excess of KOlBu, a yellow organic phase was obtained upon aqueous workup, presumably containing 3b. Solvent removal resulted in a black residue, which we assume is the remnants of decomposed 3b. In order to circumvent this decomposition, we performed the elimination with see-BuLi in THF at -78°C. It is knownc81 that 1,3-diynes can be stabilized with trimethylsilyl groups, therefore we added chlorotrimethylsilane to the metallated butadiyne and iso- lated 3a (Scheme l), stable under ambient conditions, in quantita- tive yield. Crystals of 3a were grown from chloroform and an X- ray structure analysis was performed to prove its structure. Figure 1 shows an ORTEP plot of the molecular structure of 3a. The unit cell contains two independent molecules that have slightly different

Scheme 1

1

Piperidine

YMe3

L

1. sec-BuLUTHF 2. ClSiMe3 I

SiMe3 I

bond lengths and angles. The cyclobutadiene ring in both mol- ecules is distorted from a square to a parallelogram with angles of 85-87" associated with the carbon atoms bearing the trimethylsilyl groups and angles of 93 -94" for the other carbon atoms. The bond lengths in the cyclobutadiene rFnge from 1.44 to 1.49 A, and those in the Cp ring are 1.36- 1.41 A, and thus unremarkable. However, the following is noteworthy: due to its length, the diyne ligand is bowed. Thus it points away from the CpCo unit at an angle of

Chem. Ber 196,129,269-273 0 VCH Verlagsgesellschaft mbH, D-6945 1 Weinheim, 1996 0009-2940/96/0303-0269 $10.00+ .25/0 269

M. Altmann, V Enkelmann, U. H. F. Bunz SHORT COMMUNICATION

1. KOHEthanol 2. CuCliTMEDN02

Acetone

6.3-6.8". This behavior is of an electronic nature and has been observed previously in cyclobutadiene complexes[7b].

Figure 1. ORTEP plot of the crystal structure of 3aLa]

C i A

c22

C ? C c19

Selected bond lengths [A] and angles ["I: Co-C(5) Co-C(6) 1.989(7), Co-C(7) 1.978(7), Co-C(8) C(8)-Si(2) 1.843(8), C(5)-C(6) 1.46(1), C(6)-C(7) C(4) - C( 5) 1.4 1 (l), C( 3) - C(4) 1.19( I), C(2) - C( 3) C(l)-C(2) 1.20( l), C( 1)-Si( I) 1.82( 1); C(7)-Co-C(5) C(8)-Co-C(6) 65.1(3), C(5)-C(4)-C(3) 176.5(8). C(2)-C(1) 179.2(1 l), C( 13)-Si( 1)-C(l) 106.8(5).

Scheme 2

SiMe3 1

4

Pd(PPh3)4/CuI Piperidine

1.959(7), 1.999(8), 1.48( l), 1.39( l), 61.8(3), C(3)-

5

+

SiMe3 ? M e 3

I

In search of a model for polymer 9 with good solubility proper- ties, we synthesized compound 8 (Schemes 2 and 3). We are able to extend 4, analogous to the extension of 1, with an ethynyl group. We isolated 5 in 46% yield and 7 in 28Y0 yield (Scheme 2). The low yields of 2 and 5 are due to homocoupling. Although it was tried to optimize the Pd-catalyzed coupling of 4 with cis-dichloroethene,

we could not increase the yield of 5. The smooth reaction of 5 with see-BuLi in THF leads, after workup with chlorotrimethylsilane, similar to the case of 2) in almost quantitative yield to 6a.

Compound 6a was dissolved in pentane and stirred for 2 hours with a 1% solution of KOH in ethanol. After aqueous workup, a solution of 6b in pentane was obtained. Similar to 3b, compound 6b spontaneously decomposes to a dark-colored substance upon solvent evaporation. Without further workup, 6b dissolved in pen- tane, was added to an equimolar mixture of Cu2C12/TMEDA in acetone and coupled for 2 hours at room temperature while oxygen was bubbled through the reaction mixture. After evaporation of the solvent, aqueous workup and chromatography, the tetrayne was isolated in 98% yield (as calculated from 6a).

Scheme 3

SiMe3

SiMe3 4- SiMe3

c p c o SiMe3

6a

SiMe3 I

SiMe3 I

8

Because of the good solubility of 8, we were in the position to characterize this compound with the usual spectroscopic methods The 'H-NMR spectrum shows three singlets at 6 = 0.10, 0.23 and 4.95 that were assigned to the trimethylsilyl groups and the cyclo- pentadienyl Iigand. In the "C-NMR spectrum, the trimethylsilyl group peaks were at 0.92 and 1.10 (9); the three singlets of the cyclobutadiene ring appeared at 6 = 67.6, 76.6 and 78.2; the dou- blet of the cyclopentadienyl ligand occurred at 6 = 81.0; the reso- nances of the octatetrayne bridge were observed at 6 = 66.29, 68.42, 80.71 and 81.53. The impressive temperature stability of 8 should be noted; it decomposes at 306°C without melting. In order to determine the struclure of 8, single crystals were obtained from chloroform for the X-ray crystal structure. Figure 2 displays the ORTEP plot of 8. The bond lengths in the four-membered nng are 1.50-1.51 and 1.42-1.45 A for the two sets of opposing bonds. The bond angles are, as in structure 3a, in opposing pwrs at 91 -93" and 87.5-88" and belong to a distorted parallelogr?. The bond distances in the Cp ring lay between 1.34 and 1.44 A and reveal a somewhat unsymmetrical cyclopentadiene. The octate- trayne unit is completely planar and its bond lengths are as found in the The cyclopentadienyl ligands in 8[7d1 are trans to one another, a consequence of the molecular symmetry, not steric hindrance.

In Figure 3 the UV/Vis spectra of the model compounds 7 and 8 are shown. The highest wavelength absorption of compound 8 shows a bathochromic shift of 15 nm in comparison to that of 7.

270 Chem. Ber. 1996, 129, 269-273

Selected bond lengths [A] and angles ["I: C(I)-C(2) 1.18(2), C(2)-C(3) 1.38(2), C(3)-C(4) l.l9(2), C(4)-C(5) 1.44(2), C(5)-C(6) 1,50(2), c(6)-c(7) 1.45(2), C(S)-C(5) 1.42(2), C(7)-C(8) 1.51(2), C(7)-Si(2) 1.86(2), Co-c(5) 1.95(2), Co-C(8) 1 92(2); c(3)-c(2)-c(1) 178.7(25), c(4)-C(3)-C(2) 177.3(26), C(5)-C(4)-C(3) 176.4(22), C(5)-C(S)-C(7) 88.0(13), C(8)-C(7)-C(6) 91.4(13), C(7)-C(6)-C(5) 87.5(14), C(S)-C(S)-C(6) 93.0(14).

There are five distinct bands recognizable at h = 268, 293, 398, 431, and 474, whereas 7 shows only one broad absorption feature (h = 396).

Figure 4. GPC of Polymer 9

*t' Figure 3. UVNis spectrum of compounds 7 and 8

Wavelength [nm]

Because of the good yields of 8 from the coupling of 6b, a poly- merization of 3b via a Hay reaction seemed feasible. The prep- aration of polymer 9 was analogous to the preparation of 8. First, 3a was deprotected and, after aqueous workup and isolation, oxi- datively coupled for 2 hours (see Scheme 4). After partial removal of solvent, 9 was precipitated in methanol and isolated by centri- fugation in 89% yield as a light orange powder.

This powder showed poor solubility in dichloromethane, chloro- form and benzene. THF was a moderately good solvent. Figure 4 shows the GPC trace of polymer 9. Not surprisingly, the heptamer was the longest oligomer that could be detected because of the poor solubility of 9. Extending the reaction time did not increase the degree of polymerization. We explain this as follows: either the coupling is prematurely interrupted due to the oligomers precipitat- ing out of the reaction mixture or the polymers that do form are not recovered in the workup due to insolubility. Coupling failure is improbable because it is known that the Hay reaction allows the formation of substances with high molecular weights and high de- grees of polymeri~ation[~J.

Id Molecular Weight 5 lo3

The 'H-NMR spectrum of 9 shows signals of trimethylsilyl groups at 6 = 0.25, cyclopentadienyl groups at 6 = 4.96 and an- other weak signal at F = 2.52 which we assign to the butadiyne protons. Trials to prove the structure of 9 with solid-state 13C- NMR spectroscopy were unsuccessful due to the bad signal-to- noise ratio in the region of the quaternary C atoms. Long accumu- lation time allowed us to obtain a 13C-NMR spectrum from 9 in solution. This shows broad signals which were interpreted by com- parison with the spectra of 8. The quartet of the trimethyl silyl group appears at 6 = 0.55. The doublet of the cyclopentadienyl ring at 81.60. Analogous to 8 four signals were found at 6 = 65.36, 68.14, 76.18, and 80.84 which were assigned to the alkync carbon. A1 6 = 62.63 and 78.13 the least intensive and broadest signals, from the cyclobutadiene ring, appear. Spin-echo experiments sup- ported this assignment.

Chew. Ber 1996, 129, 269-273 27 1

SHORT COMMUNICATION M. Altmann, V. Enkelmann, U. H. F. Bum

1. KOHEthanol 2. CuCvTMEDNO2

Acetone

Scheme 4

SiMe3 = = SiMe3

c p d o I SiMe3

3

9

Figure 5. UVNis spectrum of polymer 9

, I 600

0 ' 300 400 500

Wavelength [nm]

The UV/Vis spectrum of 9 (Figure 5 ) is nearly identical to that of 8. When one compares these results with that for polymer we draw the conclusion that the octatetrayne bndges prevent conju- gation along the polymer backbone. The effective conjugation length of the butadiyne-bridged 10 is 7 units. However, the spectra

dimer and polymer 9 are nearly identical. The mixture of higher gomers of 9 is astonishingly thermally stable. Thermogravimetric

investigations show that its strong exothermic decomposition, simi- lar to 8, occurs at 270°C. This emphasizes the kinetic stability of the polymer despite the tretrayne units.

In two steps from compound 1, the extended monomer 3a, sub- stituted with two butadiyne units, was reached. Compouud 3a was

pled under Hay conditions to give the poorly-soluble oligomeric structure of 9 was proven through the comparison of its scopic data with that of model compound 8. In contrast to r 10, the conjugation of the cyclobutadiene units in 9 is

prevented by the tetrayne bridges.

We thank Prof. Dr. K. Mullen for his generous support. This work was financed by the Deutschen Forschungsgemeinschaft, the Volkswagen-Stiftung and the Fonds der Chemischen Industrie.

Experimental The Pd-catalyzed coupling reactions were performed under ni-

trogen atmosphere. Chemicals were purchased from Aldrich and used without further purification. Solvents were dried by the usual methods[l0I and distilled before each reaction. 'H and 13C NMR: Bruker AC 300. IR: Nicolet Magna 550. MS: VG Trio 2000. - Elemental analysis: Mikroanalytisches Laboratorium des Institutes fur Organische Chemie der Universitat Mainz.

{[2,4-Bis(4'-chlorbut-3'-en-l '-yn-l'-yl)](l,3-bistrimethylsilyl)- cyclobutadiene)(cyclopentadienyl)cobalt (2): In a Schlenk tube 948 mg (2.57 mmol) 1, 2.49 g (25.7 mmol) cis-1,2-dichloroethene, 297 mg (0.257 mmol) Pd(PPh3)4 and 49.0 mg (0.257 mmol) CuI were combined. Piperidine (50 ml) was added. The yellow solution was stirred at 21 "C for 16 h; over this time it became cloudy due to the precipitation of ammonium salts. The piperidine was removed un- der vacuum. Aqueous workup and chromatography over silica gel (pentane) gave 541 mg (43%) 2 as yellow crystals, m.p. 68°C. - IR (KBr): 0 [cm-'1 = 3086, 3020, 2956,2897, 2187, 1260, 844, 813. - 'H NMR (CDC13): 6 = 0.32 (s, 18H), 4.96 (s, 5H), 5.86 (d, ' J = 7.4 Hz, 2H), 6.35 (d, 2J = 7.4 Hz, 2H). - 13C NMR (CDCI3): 6 = -0.46 (q, 6 C), 64.39, 76.42 (2 S, 4 C), 81.45 (d, 5 C), 84.70, 97.05 (2 s, 4 C), 112.73, 125.92 (2 d, 4 C). - C23H27C12C~Si2 (489.47). calcd. C 56.44, H 5.56; found C 56.42, H 5.54.

[4-(4'-Ch10rbut-3'-en-lr-yn-l'-yl) (l,2,3-tristrimethylsilyl)- cyclobutadiene] (cyclopentadienyl) cobalt (5); {[(I ,2,3-Tristri- methylsrlyl) cyclobutadien-4-yl] (ryclopen tadienyl) cobalt) (I ,$-buts- diynyl) (7): Analogous to the preparation of 2, 420 mg (1.01 mmol) 4, 489 mg (5.05 mmol) cis-l,2-dichloroethene, 58.4 mg (0.051 mmol) Pd(PPh3), and 9.7 mg (0.051 mmolf CuI were reacted and purified. Chromatography (silica gelfpentane) gave, as a first frac- tion, 222 mg (46%) 5. Further elution led to the isolation of 118 mg (28%) 7 as a yellow solid - 5: m.p. 61 "C. - IR (KBr): P

3017, 2956, 2899, 2180, 1601, 1414, 1246. - 'H

, 2H). - I3C NMR

125.10 (2 d, 2 C). -

: 6 = 0.14 (s, 9H), z, 2H), 6.35 (d, zJ

(CDC13): 6 = 0.82, 1.41 (2 q, 9 C),

Cz2H3&1CoSi3 (477.15): calcd. C 7.21. - 7: m.p. 235°C. - IR (KB 2185, 2122, 1417, 1247, 840, 809.

0.74 (9, 12 C), 1.16 (q, 6 C), 70 80, 76.01, 77.31 (3 s, 8 C), 80.63 (d, 10 C), 79.30, 82.72 (2 s, 8 C); UV/Vis (CH,Cl,): Lax [nm] = 396 ( E 11750). - C40H64C02Si6 (831.32): calcd. C 57.79, H 7.76; found C 57.99. H 7.93.

.21, 28 29 (3 s, 4 C), 80.26 (d, 5 C), 84.60, 98.24 (2 s, 4 C

(s, ISH), 0.24 (s , 36H), 4.96 (s, IOH). - 13C NMR (CD2C12): 6 =

[2,4-Bis(w-trimethylsilylhutadiynyl) (1,3-histrimethylsilyl) - cyclobutadiene] (cyclopentadienyl) cobalt (3a): In a Schlenk tube, 541 mg (1.11 mmol) 2 in 30 ml THF was cooled to -78°C. 3.6 ml see-BuLi (1.3 M hexane solution) were slowly injected. After being warmed to O'C, the solution was stirred for 1 h. After cooling again to -78"C, 481 mg (4.42 mmol) chlorotrimethylsilane was added. The solution was allowed to warm to 21 "C and subjected to aqueous workup (pentane). Purification with chromatography followed (silica gel/pentane). 604 mg (97%) of 3a were isolated, m.p. 149°C. - IR (KBr): 0 [cm-'1 = 3107, 2960,2899,2184,2089, 1250, 843, 813. - 'H NMR (CDZC12): 6 = 0.20 (s, 18H), 0.28 ( s , 18H), 4.99 (s, 5H). - ''C NMR (CD2C12): 6 = -0.46 (q, 6 C), -0.33 (q, 6 C), 63.26, 79.22 (2 b, 4 C), 81.92 (d, 5 C), 75.65, 76.46 (2 s, 4 C), 89 19, 90.39 (2 s, 4 C). - UVNis (CH,CI,): La, [nm] = 282 (E 27300), 302 (E 26850), 387 ( E 4720). Cz9H4,CoSk (560.92): calcd. C 62.10, H 7.37; found C 61.97, H 7.35.

272 Chem. Ber. 1996, 129, 269-273

Novel Cyclobutadiene-Octatetrayne Polymer SHORT COMMUNICATION (4- (w- Trimethylsilylbutadiynyl) (1,2,3-tristrimethylsilyl)-

cyclobutadiene] (cyclopentadienyl) cobalt (6a): 222 mg (0.465 mmol) of 5 was added to 30 ml THF and, analogous to 2, was reacted with 0.75 ml see-BuLi. Reaction with 101 mg (0 930 mmol) chloro- trimethylsilane gave 234 mg (98%) 6a, m.p. 150°C. - IR (KBr): C [cm-'] = 3108,2955,2899,2193,2100, 1247, 842,808. - 'H NMR (CD,Cl,). 6 = 0.12 (s, 9H), 0.22 (s, 9H), 0.25 (s, lSH), 4.97 (s, 5H). - "C NMR (CD2ClJ: 6 = -0.12 (9. 3 C), 0.85 (q, 6 C), 0.13 (4. 3 C), 68.86, 75.90, 76.53 (3 s, 4 C), 80.76 (d, 5 C), 78.21, 80.19, 89.82, 89.94 (4 s, 4 C). - C25H4iC~Si4 (512.87): calcd. C 58.55, H 8.06; found C 57.79, H 7.86.

w- ([ (2,3.4-tristrimethylsilyl) cyclobutadien-1 - y l ] [cyclopen- tudienyl~cobaltf(l.8-octatetraynyl~ (8): In a Schlenk tube, 82.0 mg (0.160 mmol) of 6a was dissolved in 25 ml pentane and reacted with 25 ml of a 1% solution of KOH in ethanol. After 2 h, no further evidence of reactant (6a) could be detected through TLC- monitonng. Aqueous workup gave a solution of 6b in pentane. Solvent removal caused decomposition of 6b to a dark-colored resi- due. For the dimerization, the pentane layer containing 6b (ca. 100 ml) was added to a 500-ml, 2-necked flask), which already con- tained 18 9 mg (0.160 mmol) TMEDA and 16.1 mg (0.160 mmol) Cu2C12 in 300 ml of acetone. Oxygen was bubbled through the solu- tion at room temperature (21 "C) for 2 hours, until TLC monitoring indicated that all the monomer (6b) had reacted The workup was as follows: The solution was filtered and the solvent removed under slightly reduced pressure. The residue was extracted with water and pentane. After chromatography using neutral aluminum oxide (pentane) 68.9 mg (98%) 8 (yellow crystals) was isolated, m.p. 306°C (dec.). - IR (KBr). P [cm-'1 = 3107, 2954, 2897, 2175, 2062, 1411, 1246, 838, 813. - 'H NMR (CDC13): 6 = 0.10 (s, lSH), 0 23 (s , 36H), 4.95 (s, 10H). - I3C NMR (CD2ClZ): F = 0.92 (q, 12 C), 1.10 (q, 6 C), 67.64, 76.65, 78.21 (3 s, 8 C), 81.00 (d, 10 C), 66.29, 68.42, 80.71, 81.53 (4 s, 8 C). - 1JV/Vis (CH2C12): h,,, [nm] = 268 ( E 51380), 293 ( E 47020), 398 (c 23420), 431 ( E

19890), 474 ( E 14860). - C4Hfi4C02Si6 (879.37): calcd. C 60.10, H 7.34; found C 59.56, H 7.52.

Poly ({para-[ (1,3-bistrimethylsrlyl) c)~clobutadien-2,4-~~lene/- (cyc1opentndienyl)cobult joctntetraynylene j (9). To cleave the tri- methylsilyl groups from the butadiyne units, 550 mg (0.981 mmol) of 3a in pentane was added to a 1% solution of KOH in ethanol and stirred at 21 "C for 3 h. After this time, 3a could no longer be detected (TLC). Aqueous workup gave a solution of 3b in pentane (ca. 150 ml). This was added to a mixture of 114 mg (0 981 mmol) TMEDA and 97.2 mg (0,981 mmol) copper(1) chloride in 200 ml of acetone. Oxygen was bubbled through the reaction mixture as the mixture was stirred for 2 h at 21 "C. Without purification, the reaction mixture was concentrated under reduced pressure and pre- cipitated in methanol (ca. 50 ml residuell L methanol). Polymer 9 was isolated upon centrifugation as an orange, poorly-soluble pow- der in 89% yield (363 mg). IR (KBr): P [ m - l ] = 3107, 2954, 2897, 2175, 2062, 1411, 1246, 838, 813. - 'H NMR (CDCI?): 6 = 0.25 (s, 18 H), 2.52 (s, 2H), 4.96 (s, 5 H). - I3C NMR (CDzC12): 6 = 0.55 (4, 6 C), 62 63, 78.13 (2 s, 4 C), 81.60 (d, 5 C), 65.36, 68 14, 76.18. 80.84 (4 s, 8 C). - UVNis (CHzC12): h,,, [nm] = 310, 388, 431, 478. - C23H23CoSi2 (414.54): calcd. C 66 64, H 5.59, found C 66.82, H 7.29.

Crystal Data und Structui e Refinement of 8["]: C44H64Si6C02, M = 879 37, yellow needles, crystal size 0.70 X 0.30 X 0.30 mm,

monoclinic, P2,/c, a = 13.950(2), b = 9.848(2), c = 18.435(1) A. !3 = 106.269(40)", I/= 2431(1) A3, D , = 1.201 g ~ m - ~ , Z = 2, 3782 reflections, observed 1921 (F > 3a(F)), R = 0.083, R, = 0.085, p = 75.62 cm-'. Data collection was performed on an Enraf-Non- ius CAD-4 Diffractometer at 297 K, Cu-K,, (Graphite mono- chromator) 20,,, := 64". The structure was solved by heavy atom methods (Patterson) using an empirical absorption correction. The position of the H-atoms were calculated utilizing the known bind- ing geometry and refined in the riding mode with fixed isotropic temperature factors. The programs Molen and CRYSTALS were used.

Crystal Dalu and Structure Refinement of 3a["]: C29H4LSi4C~, A4 = 560.92, yellow crystals, crystal size 0.50 X 0.20 X 0.30 mm, triclinic, space group P i , a = 13.677(5), b = 14.849(6), c = 18.681(4) A, a = 91.36(3)', p = 110.73(5)", y = 104.00(3)", V = 3417(2) A?, Z = 4. two independent molecules in the single cell; 9036 reflections, observed 5302 (F > 3o(F)), R = 0.039, R, =

0.044, D, = 1.089 g ~ r n - ~ , 1-1 = 55.93 cm-'. The data collection was performed on an Enraf-Nonius CAD-4 Diffractometer at 297 K, Cu-K,, (Graphite monochromator) 20,,,, = 64". The structure was solved by heavy atom methods (Patterson) using an empirical absorption correction. The position of the H-atoms were calculated utilizing the known binding geometry and refined in the riding mode with fixed isotropic temperature factors. The programs Molen and CRYSTALS were used.

['I U. H. F. Bunz, Angew. Chem. 1994, 106, 1127; Angew. Chem. Znt. Ed. Engl. 1994, 33, 1073. U. H. F. Bunz, V. Enkelmann, Anzew. Chem. 1993. 105. 1712; Annew. Chem. Int. Ed. E n d 1993, 32, 1653.

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[Io] W. Bunge, E. Muller, Houben Weyl-Methoden der Organischen Claemie7 Bd. I/2, Thieme, Stuttgart 1959.

[ I1] Further details of the crystal structure investigations of 3a and 8 are available from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Lcopoldshafen (Germany), on quoting the depository number CSD-59178, the names of the authors and the journal citation.

[95157]

Chem. Ber. 1996,129, 269-273 273