Syn Configuration Research Articles

Inner Coordination Sphere Control of Metal-Metal Superexchange in Ruthenium Dimers.

The dinuclear Ru(III) complexes trans-[{(NH(3))(4)Ru(py)}(2)(&mgr;-L)][PF(6)](4), where py represents pyridine and L represents 1,4-dicyanamidobenzene dianion (dicyd(2)(-)) derivatives dicyd(2)(-) (1), Me(2)dicyd(2)(-) (2), Cl(2)dicyd(2)(-) (3), and Cl(4)dicyd(2)(-) (4), have been prepared and characterized by electronic absorption spectroscopy and cyclic voltammetry. A crystal structure of the complex trans-[{(NH(3))(4)Ru(py)}(2)(&mgr;-dicyd)][PF(6)](4).(1)/(2)H(2)O showed the dicyd(2)(-) ligand to be approximately planar with the cyanamido groups in a syn configuration. Crystal structure data are space group P2(1), with a, b, and c = 7.826(3), 20.455(7), and 14.428(5) Å, respectively, beta = 95.76 (3) degrees, V = 2296.7(14) Å(3), and Z = 2. The structure was refined by using 3292 reflections with I > 2.5sigma(I) to an R factor of 0.069. Solid state magnetic susceptibility measurements of the Ru(III)-Ru(III) dimers showed diamagnetic behavior at room temperature, and this is suggested to be due to strong antiferromagnetic superexchange via the HOMO of the dicyd(2)(-) ligand. The bridging ligand dependence of metal-metal coupling in the Ru(III)-Ru(II) complexes of 1, 2, 3, and 4 in acetonitrile solution was demonstrated by the trend in comproportionation constants, 1.5 x 10(6), 5.7 x 10(6), 1.4 x 10(4), and 1.1 x 10(3), respectively. In addition, comparison to the analogous pentaammineruthenium dimers showed that the magnitude of metal-metal superexchange could be controlled by the nature of the spectator ligand. Spectroelectrochemical methods were used to acquire the absorption spectra of the mixed-valence complexes, and the intervalence band properties were modeled with PKS theory. Metal-metal coupling in the Ru(III)-Ru(II) complexes of 1, 2, 3, and 4 was analyzed by using Hush and CNS theories.

Epimer B of Tetraphenylarsonium (Mercaptoacetylalanylglycylglycinato-N,N',N'',S)oxotechnetate(V) (99Tc) Chloroform Solvate

In the title compound, (C24H20As)TcO(C9H11N3O5S).CHCl3, technetium is coordinated to three amide N atoms and one S atom which form a square pyramid, with the oxo group occupying the apical position such that the Tc atom lies 0.771 (5) Å above the least-squares basal plane. The methyl group adopts a syn configuration with respect to the oxo-metal group. The chloroform solvate molecule exhibits site occupation disorder with three positions for the Cl atoms.

Long‐distance control in stereoselective reduction of 3‐[3‐(4′‐bromo[1,1′‐biphenyl]‐4‐yl)‐3‐keto‐1‐phenylpropyl]‐4‐hydroxy‐2H‐1‐benzopyran‐2‐one: Relative configuration of prevailing diastereomer and absolute configuration of its enantiomers

Depending on the reducing agent and reaction conditions, diastereoselective reduction of 3-[3-(4′-bromo[1,1′-biphenyl]-4-yl)-3-keto-1-phenylpropyl]-4-hydroxy-2H-1-benzopyran-2-one (2) proceeds with different stereoselectivity; a surprisingly high, approximately 90% d.e. of 4A is achieved with NaBH4 in MeOH at low temperature. Resulting diastereomeric racemates 4A and 4B are separated and their respective syn and anti configurations are assigned on the bases of mechanic considerations, supported by the 1H-NMR spectra and conformational analysis based on MM2 calculations. The syn diastereomer 7A, 4-OMe derivative of 4A, was partially resolved by acylation at C(3)-OH with S-(−)-camphanic acid to camphanyl ester 12 of (−)-7A, leaving (+)-enantiomer 7A. The assignment of absolute 1R,3S-configuration to (−)-7A is based on comparison of its CD spectrum with those of the model compounds S-14 and R-15, which represent partial chromophores 4-hydroxy-2H-1-benzopyran-2-one (4-hydroxycoumarin) A, and 4′-bromo-1,1′-biphenyl B; their exciton coupling in (−)-7A is suggested. Chirality 9:512–522, 1997. © 1997 Wiley-Liss, Inc.

Long-distance control in stereoselective reduction of 3-[3-(4?-bromo[1,1?-biphenyl]-4-yl)-3-keto-1-phenylpropyl]-4-hydroxy-2H-1-benzopyran-2-one: Relative configuration of prevailing diastereomer and absolute configuration of its enantiomers

Depending on the reducing agent and reaction conditions, diastereoselective reduction of 3-[3-(4′-bromo[1,1′-biphenyl]-4-yl)-3-keto-1-phenylpropyl]-4-hydroxy-2H-1-benzopyran-2-one (2) proceeds with different stereoselectivity; a surprisingly high, approximately 90% d.e. of 4A is achieved with NaBH4 in MeOH at low temperature. Resulting diastereomeric racemates 4A and 4B are separated and their respective syn and anti configurations are assigned on the bases of mechanic considerations, supported by the 1H-NMR spectra and conformational analysis based on MM2 calculations. The syn diastereomer 7A, 4-OMe derivative of 4A, was partially resolved by acylation at C(3)-OH with S-(−)-camphanic acid to camphanyl ester 12 of (−)-7A, leaving (+)-enantiomer 7A. The assignment of absolute 1R,3S-configuration to (−)-7A is based on comparison of its CD spectrum with those of the model compounds S-14 and R-15, which represent partial chromophores 4-hydroxy-2H-1-benzopyran-2-one (4-hydroxycoumarin) A, and 4′-bromo-1,1′-biphenyl B; their exciton coupling in (−)-7A is suggested. Chirality 9:512–522, 1997. © 1997 Wiley-Liss, Inc.

Crystal and Molecular Structures of Two Isomers of Phenylglyoxal Bis(amidinohydrazone) [‘Phenylglyoxal Bis(guanylhydrazone)’] Sulphate

The first crystallographic study on an aromatic analogue of the antileukemic agent methylglyoxal bis(amidinohydrazone) (MGBG) is reported. Thus, the crystal and molecular structures of two different geometrical isomers of phenylglyoxal bis(amidinohydrazone) (PhGBG) sulphate were determined by single-crystal X-ray diffraction. Crystals were prepared by recrystallizing PhGBG sulphate using either water or aqueous ethanol (volume ratio ethanol:water 1:4) as the solvent. Depending on the solvent, different types of crystals were obtained although the PhGBG sulphate employed was in the both cases from the same synthesis batch that had been prepared according to classical methods from the corresponding glyoxal. When a crystal obtained from water was studied, PhGBG was found to exist solely in the form of the anti-anti isomer. e. the same isomer that has been observed in the cases of all mono- and dialkylglyoxal bis(amidinohydrazones) so far studied. However, when PhGBG sulphate was recrystallized from 20 % aqueous ethanol, the crystals obtained consisted of a different geometrical isomer. In this anti-syn isomer the carbon-nitrogen double bond closest to the phenyl group had the syn configuration. In the anti-syn isomer, there is an internal hydrogen bond between the two amidinohydrazone moieties, which may markedly contribute to the stabilization of the isomer. The anti-syn isomer of PhGBG is analogous to the only isomer of trifluoromethylglyoxal bis(amidinohydrazone) so far observed. PhGBG sulphate constitutes the first case in which two different geometrical isomers of a bis(amidinohydrazone) have been observed. In the case of the much studied aliphatic mono- and dialkylglyoxal analogues, isomerization of the bis(amidinohydrazone) backbone has never been observed. The structural flexibility of the bis(amidinohydrazone) chain of PhGBG is obviously attributable to the electron-withdrawing resonance effect and perhaps also to the inductive and hyperconjugative effects of the phenyl group. The obviously facile isomerization of PhGBG may markedly influence the biochemical properties of the compound.

2-(2-Pyridyl)pyridinium Triiodide

The title structure, C10H9N2+.I3−, consists of 2-(2- pyridyl)pyridinium cations and near-symmetric essentially linear triiodide anions [I—I = 2.902 (1) and 2.920 (1) Å, and I—I—I = 177.88 (5)°]. The cation has a syn configuration, with an intramolecular N—H⋯N hydrogen bond: H—N2 and H⋯N1 = 1.13 (11) and 1.83 (10) Å, respectively, and N2—H⋯N1 = 124 (8)°. The two heterocyclic rings are not coplanar, the angle between the rings being 9.6 (8)°.

Asymmetric aldol condensation as a route to polypropionate derived pheromones

The synthesis of the polypropionate-derived pheromones sitophilate ( 1) and sitophilure ( 2) are described using an asymmetric aldol condensation as the key step to adduct 6; compound 6 was smoothly converted to the antipodes of each pheromone. This procedure can be expanded to more complicated structures with the same type of syn configuration such as stegobinone ( 3) and serricornin ( 4).

Synthesis and Characterization of Dibromo-Containing Ruthenium(IV) η3-Allyl and Ruthenium(IV) η4-Diene Complexes. Formation of [Ru(η5-C5Me5)Br3]- and [Ru(η5-C5Me5)Br3]2

The reaction of Br2 with Ru(η5-C5H5)(η4-diene)Br (diene = 1,3-butadiene (1a), 2-methyl-1,3-butadiene (1b), 1,3-hexadiene (1c)), Ru(η5-C5Me5)(η4-diene)Br (diene = 1,3-butadiene (2a), 2-methyl-1,3-butadiene (2b), 3-methyl-1,3-pentadiene (2c), 1,3-hexadiene (2d), 1-methoxy-1,3-butadiene (2e), 2,4-hexadiene (2f), phenyl-1,3-pentadiene (2g), diphenyl-1,3-butadiene (2h), 1,3-cyclohexadiene (2i), 2,3-dimethoxy-1,3-butadiene (2j), 2,3-dimethyl-1,3-butadiene (2k), 1,2-dimethylenecyclohexane (2l)), and Ru(η5-C5Me4Et)(η4-diene)Br (diene = 2,4-hexadiene (3)) has been studied. 1,3-Butadiene and mono-, and 1,2-disubstituted-1,3-butadiene complexes afford bromo-substituted Ru(IV) anti η3-allyl complexes in high yields. This process involves addition of bromine on the exo face of the diene ligand taking place regioselectively at the terminal carbon bearing no substituent. Unexpectedly, bromination of 2d yields the dibromoruthenium(IV) η-(1−3)-hexa-1,4-dien-3-yl complex (6). In the course of this process HBr is liberated involving the intermediacy of bromo-substituted Ru(IV) η3-allyl complexes. The molecular structure of 6 has been determined. 1,4-Disubstituted-1,3-butadiene complexes 2f−h and 3 react with Br2 to form bromo-substituted Ru(IV) η3-allyl complexes 10a−c and 11 adopting exclusively the syn configuration. These compounds are not stable in solution and decompose to give either a dibromoruthenium(IV) η-(1−3)-hexa-1,4-dien-3-yl complex, as a result of HBr elimination, or the dimeric Ru(IV) complexes [Ru(η5-C5Me5)Br3]2 (13) and [Ru(η5-C5Me4Et)Br3]2 (14), respectively. In order to explain the observed stereochemistry and reactivity of complexes 10a−c and 11 a weak three-center 4e- C−Br···Ru interaction is proposed. In case of 2i, bromination leads to the formation of the complex salt [Ru(η5-C5Me5)(η6-C6H6)][Ru(η5-C5Me5)Br3] (15) and of the dimeric Ru(III) complex [Ru(η5-C5Me5)Br2]2. 15 features a novel monomeric 17e- half-sandwich Ru(III) complex as counteranion. The molecular structure of 15 has been determined. By contrast, bromination of 2,3-disubstituted-1,3-butadiene complexes 2j,k affords the novel cationic Ru(IV) η4-diene complexes [Ru(η5-C5Me5)(η4-diene)Br2]Br (diene = 2,3-dimethoxy-1,3-butadiene (16a), 2,3-dimethyl-1,3-butadiene (16b)). Complexes 16a,b are not stable in solution in the presence of Br-. On replacement of the bromide counterion by CF3SO3- the stable complexes 17a,b are obtained. The molecular structures of both 16a and 17a have been determined. Complexes 16 and 17 appear to be the first late transition-metal complexes approaching a σ2,π-metallacyclopentene structure.

Stereoselective Reduction of Ethyl 2-Methyl-3-oxobutanoate by Bacteria

More than 450 bacterial strains were examined for their ability to reduce ethyl 2-methyl-3-oxobutanoate (1) to ethyl 3-hydroxy-2-methylbutanoate. Nine strains of enteric bacteria were found to predominantly yield the product with a syn configuration. Of these strains, five gave ethyl (2R,3S) 3-hydroxy-2-methylbutanoate (2a) with more than 98% diastereoisomeric excess (d.e.) and over 99%. enantiomeric excess (e.e.) Klebsiella pneumoniae IFO 3319 was found to give the highest d.e., e.e., and chemical yield at a high concentration of 1. Reduction of 1 was then done with the bacterium in a 200-liter fermentor. Two kilograms of 1 was converted to 2a with 99 %d.e., >99% e.e., and 99% chemical yield.

Synthesis, Characterization, and Theoretical Study of Sulfur-Containing Donor-Acceptor DCNQI Derivatives with Photoinduced Intramolecular Electron Transfer.

The donor-acceptor compounds N,N'-dicyanobenzo[b]naphtho[2,3-e][1,4]dithiin-6,11-quinonediimine (9a) and N,N'-dicyanobenzo[b]naphtho[2,3-e][1,4]oxathiin-6,11-quinonediimine (10a) and their methyl-substituted derivatives (9b and 10b-d, respectively) have been prepared, and their structural and electronic properties have been characterized by both experimental techniques and quantum-chemical calculations. The (1)H-NMR spectra evidence the existence of a syn/anti isomerism in solution. Both experimental and theoretical data suggest that the preferred configuration of the N-CN groups corresponds to a syn isomer for 9 and to an anti isomer for 10. The X-ray analysis performed for 9b reveals that molecules are not planar and pack in vertical stacks showing an overlap between donor and acceptor moieties of adjacent molecules. In agreement with X-ray data, theoretical calculations predict that both for 9 and 10 the acceptor DCNQI moiety is folded and adopts a butterfly-type structure and the donor moiety is bent along the line passing through the heteroatoms. The energy difference between planar and butterfly structures is calculated to be < 3 kcal/mol at the ab initio 6-31G level. The UV-vis spectra present a broad absorption in the visible which corresponds to a photoinduced intramolecular electron transfer from the high-energy HOMO furnished by the donor moiety to the low-energy LUMO located on the DCNQI fragment. Cyclic voltammetry displays one oxidation peak to the cation and two one-electron reduction waves to the anion and dianion. Theoretical calculations show the planarization of the acceptor/donor moiety induced by reduction/oxidation. The formation of stable radical anions is corroborated by the intense EPR signals recorded for reduced 9. The assignment of the hyperfine coupling constants of the EPR spectra is consistent with the existence of a preferred syn configuration.

Synthesis and structure of a 5,15-bis(4-pyridyl)purpurin

Base catalyzed cyclization of a 5,15-di(4-pyridyl)octaalkylporphyrin-5-acrylate 6 yields smoothly the corresponding purpurin 2. Quaternization with methyl iodide yielded the corresponding cationic N,N′-dimethylpyridiniumpurpurin 3; X-ray crystallography of purpurin 2, the first purpurin to be studied crystallographically, shows the two β-alkyl groups on the saturated ring of the purpurin have a syn configuration, and the macrocycle to have a slight saddle shape with a mean deviation of 0.230 Å from the least-squares plane calculated for the core atoms.

DNA determinants and substrate specificities of Escherichia coli MutY.

Potential DNA contacts involved in the specific interaction between the Escherichia coli MutY protein and a 40-mer oligonucleotide containing an A/G mismatch have been examined by alkylation interference techniques. Ethylation interference patterns suggest that more than five phosphates are involved in electrostatic interactions between MutY and DNA. Interestingly, MutY has more contacts on the G-strand than on the A-strand. Methylation at both the N-7 position of the mismatched G and the N-3 position of the mispaired A interfere with MutY binding. In addition to these mismatched bases, MutY also contacts purines on both sides of the mismatch. Binding and endonuclease activities of MutY were assayed with 20-mer oligonucleotides containing A/G, A/C, A/7,8-dihydro-8-oxo-guanine (A/GO), A/inosine (A/I), A/2-aminopurine (A/2AP), nebularine/G (N/G), inosine/G (I/G), 2AP/G, and 7-deaza-adenosine/G (Z/G) mispairs. The C-8 keto group of GO in A/GO contributes to a much tighter binding but weaker endonuclease activity than is seen with A/G. Because A/I is not specifically well recognized by MutY, the 2-amino group of G in A/G is essential for recognition. The C-6 keto group present in A/G but absent in A/2AP is also important for recognition. The 6-amino group of adenine appears not to be required for either binding or endonuclease activity because N/G is as good a substrate as A/G. The 2AP/G mispair is bound and cleaved weaker than is the A/G mispair. Binding and endonuclease activities are abolished when the N-7 group of A is replaced by C-7 as in the Z/G mispair. When a C-6 keto group is present as in the I/G pair, its binding by MutY is as good as for A/G, but no endonuclease activity is observed. Taken together, our data suggest that DNA sequences proximal to and specific functional groups of mismatched bases are necessary for recognition and catalysis by MutY protein.

Spectroscopic study of the isomerization of Z- to E-pyridine-2-formyl thiosemicarbazone

Pyridine-2-formyl thiosemicarbazone (PATS2) was synthesized in the E (anti) and Z (syn) configurations. Raman spectra were obtained in the solid state, and in DMSO and aqueous solutions. The SER spectra of PATS2 adsorbed on a silver electrode in DMSO and in aqueous solutions showing striking differences. The comparison of Raman data revealed that PATS2 is present on the silver electrode surface in the same configuration as the predominant isomer of the corresponding parent solutions. The prevalence of the E (anti) isomer in DMSO solutions was confirmed by Raman and 1 H NMR spectroscopies, and by means of the latter technique, the anti configuration is also proved to be predominant in aqueous solutions of different pHs, even starting with Z -PATS2 solutions. A strong interaction of E -PATS2 with water molecules by hydrogen bonding accounts for marked differences in Raman spectra of this isomer in DMSO and water solutions, and for the corresponding E -PATS2 species adsorbed on the silver electrode surface.

Molecular and electronic structures of the purple chromophore RC(NH2)(NSePh) (R = H, 4-CH3C6H4)

The reaction of Li[4-CH3C6H4C(NSiMe3)2] with PhECl (E = S, Se) in a 1:1 molar ratio produces the monosubstituted derivatives 4-CH3C6H4C(NEPh)[N(SiMe3)2] (3b, E = Se; 3d, E = S). The thermolysis of 3b at 130 °C produces PhSeN(SiMe3)2 and 4-CH3C6H4CN. Crystals of 3b slowly acquire a purple stain upon prolonged standing at 23 °C owing to the formation of 4-CH3C6H4C(NH2)(NSePh) (4c, λmax = 551 nm). The formamidine derivative HC(NH2)(NSePh) (4a, λmax = 472 nm) was isolated from the reaction of HCN2(SiMe3)3 with PhSeCl (1:3 molar ratio) in dichloromethane. The structure of 4a was shown by X-ray crystallography to involve a syn configuration at the [Formula: see text] bond and hydrogen bonding between amino nitrogen atoms. The crystals of 4a are monoclinic, space group P21 (No. 4), with a = 7.514(1), b = 5.575(1), c = 8.925(1) Å, β = 100.71(1)°, V = 367.40(6) Å3, and Z = 2. The final R and Rw values were 0.055 and 0.060, respectively. The 1H and 77Se NMR spectra indicate that both 4a and 4c exist as a mixture of tautomers in THF solution. DFT calculations for the model tautomers HC(NH2)(NER) and HC(NH)(NHER) (E = S, Se; R = H, Ph) show that the former is more stable in all cases, but the difference in total bonding energy is smaller for selenium than for sulfur (17.5 vs. 29 kJ mol−1). The visible absorption band for 4a is assigned to the HOMO–LUMO transition (ΔE = 2.90 eV, λmax = 427 nm) of the minor tautomer. Keywords: formamidine, tautomer, selenium, purple chromophore.

Synthesis of biliverdins with stable extended conformations. Part II

The synthesis of two hexacyclic and one heptacyclic biliverdin with extended conformations was achieved using base catalyzed intramolecular substitution reactions of 2-chloroethyl biliverdins. The 2-chloroethyl residues were located at selected β-pyrrole positions as to enable them to react with proximal basic nitrogens at the adjacent pyrrole rings. Seven membered rings were thus formed which distorted either two or the three exocyclic double bonds at the biliverdin meso-bridges away from their usual Z- syn configuration. The hexacyclic biliverdin 9 is isomorphous with the chromophores of C-phycocyanin, biliverdin 19 is an isomer of isophorcabilin, and the heptacyclic biliverdin 34 has the fullest extended conformation that the biliverdin backbone can achieve.

Electrostatic potential surface analysis of the transition state for AMP nucleosidase and for formycin 5'-phosphate, a transition-state inhibitor.

AMP nucleosidase hydrolyzes the N-glycosidic bond of AMP to yield adenine and ribose 5-phosphate. Kinetic isotope effects have been used to establish an experimentally based transition-state structure for the native enzyme and a Vmax mutant [Mentch, F., Parkin, D. W., & Schramm, V. L. (1987) Biochemistry 26, 921-930; Parkin, D. W., Mentch, F., Banks, G. A., Horenstein, B. A., & Schramm, V. L. (1991) Biochemistry 30, 4586-4594]. The transition states are characterized by weak reaction coordinate bonds to C1' and substantial carbocation character in the ribose ring. The N9-C1' bond to the leaving group is nearly broken and the adenine ring is protonated at the transition state. Formycin 5'-phosphate and other purine nucleoside 5'-phosphate analogues with syn-glycosyl torsion angles bind better than substrate, supporting a syn configuration in the enzyme-substrate complex and presumably in the transition state [Giranda, V. L., Berman, H. M., & Schramm, V. L. (1988) Biochemistry 27, 5813-5818]. Access to a geometric model of the transition state permits the analysis of its molecular electrostatic potential surface as enforced by the enzyme. Comparison of the molecular electrostatic potential surfaces for AMP, formycin 5'-phosphate, and the transition state reveals a striking similarity in the surface charges of formycin 5'-phosphate and the transition state. The enzyme-stabilized transition state for AMP hydrolysis is characterized by new positive electrostatic potential in the adenine ring as a result of protonation by the enzyme. This is closely matched by the protonated pyrazole ring of formycin 5'-phosphate. The molecular electrostatic potential surfaces of formycin 5'-phosphate and the transition state for AMP are similar and are likely to be a factor in the Km/Ki value of > 10(3) for formycin 5'-phosphate as a transition-state inhibitor of AMP nucleosidase.

N,N'-Dicyano-4,7-indanquinone diimine

The title compound, C 11 H 8 N 4 , has a syn configuration in the solid state in contrast to syn/anti isomerization in solution. The structural details help to explain the influence of the anellated five-membered ring on the properties of the quinoid π system

NMR study of the configuration and protonation equilibria of a pyrazolotriazole azomethine dye

1 H NMR and NOE spectroscopy have been used to investigate the ground state configuration and proton equilibria of a 7H-pyrazolo [5,1-c][1,2,4]triazole phenylamino azomethine dye. NOE experiments show that the dye exists in the syn configuration at room temperature. Two protonation equilibria can be identified in CDCl3–CF3CO2H at 1.6 ± 0.2 and 0.0 ± 0.2 –log [acid] units. The first equilibrium involves protonation of the azomethine nitrogen and leads to changes in the NMR spectrum which are interpreted as arising from increasing deviation from planarity about the azomethine bond because of steric interactions. The second equilibrium involves protonation of the phenylamino nitrogen.

Asymmetric synthesis of the taxol and taxotère C-13 side chains

A practical and efficient asymmetric synthesis of the 3-benzoylamino-2-hydroxy-3-phenylpropionic acid derived side chain of the important anticancer agent taxol is described. The pivotal synthetic transformation relies upon the highly diastereoselective tandem lithium amide conjugate addition–electrophilic hydroxylation of tert-butyl cinnamate 4. The resultant antiβ-amino-α-hydroxy acid derivative is readily converted to the anti diastereoisomer of the taxol side chain methyl ester, from which the naturally occurring syn configuration is secured by a simple Mitsunobu inversion sequence via a dihydrooxazole intermediate. Under optimal conditions, this straightforward approach provides the taxol side chain methyl ester (–)-15(natural enantiomer) in four steps and 60% yield from tert-butyl cinnamate 4. The protocol is applied to the preparation of all four taxol side chain stereoisomers and is extended to allow for the synthesis of the side chain of taxotére, a potent taxol analogue.

Komplexkatalyse: XLI. Die Kristallstruktur des η 3,η 2-Octa-2( E),7-dien-1-yl-(triphenylphosphin) nickel(II)-tetrafluoroborats, ein Beitrag zur Klärung der Koordinationsverhältnisse in der allylnickelkomplexkatalysierten Butadienpolymerisation

Single crystals of the title complex suitable for X-ray structure analysis could be obtained from the solution in dichloromethane by crystallization after addition of ether. The nickel is quasi-planar coordinated by the η3-bonded allyl group existing in the syn configuration, by the double bond at the other end of the C 8-chain, and by the phosphine ligand. There is no coordinative interaction with the tetrafluoroborate anion. Thus, the existence and the structure of stable planar allyl mono(ligand) nickel(II) cations with one chelate coordinated olefinic double bond is proved, and for the mechanism of trans polymerization a sterically plausible explanation for CC bond formation can be formulated.