fand nur ein stereospezifischer Br/H-Austausch zum trans-2-Brom-I-methyl-cyclopropancarbonsiureethylester statt. Dies zeigt, daO sich ein auDergewohnlich stabiles Carbanion in syn-Stellung zur Esterfunktion bildet, eine Tatsache, die in Einklang mit der Stereochemie der beiden Cyclisierungsprodukte (7)
A suspension of tetraphenylphosphonium chloride (1 2.438, ~3 3 mmol) in THF (100 ml) is added dropwise with stirring to the yellow suspension at 220 K. After warming to 298 K the suspension is stirred for ca. 20 h at 298 K.Red, rod-shaped crystals (length = 0.4 mm) crystallize out from the filtrate within a week. Later, yellow amorphous "higher phosphanes" also separate out. However, the red crystals of (Ph4P)2P16 can easily be separated mechanically from the by-products. Compared to salts like Na3P7, Na3Pll and their derivatives, the hexadecaphosphide is remarkably stable. The crystals, which are practically insoluble in anhydrous solvents, hydrolyze and turn brown within two days on exposure to air. They decolorize within minutes in H20/ethanol mixtures, but their morphology is preserved. The phosphane odor typical for such reactions does not occur. Up to 570 K the mass spectrum (QMS) shows only the fragments of the cation and of P4. In the presence of Me3SiCl the fragment P16R: (m/z=642) is observed.The X-ray structure analysis[41 gave a surprising result: The compound does not contain the anions p;-, as would be expected in the synthesis, but the novel polycyclic hexadecaphosphorus dianion P: , (Fig. 1). Its skeletal structure, with mm2 symmetry, is made up of two P7 units which are coupled via a common P2 dumbbell; it can also be described in terms of the combination of a P,-unit with a P9-unit. The bond lengths show some peculiarities: The P-P distance a of 232 pm is very long. Here the strain caused by the 3m symmetry of the nortricyclic unit and the yr-tetrahedral configuration of the bridgehead atoms linked directly to the P2-dumbbell is effective. The shortest bond lengths (hand i) occur at the two divalent P'-atoms. The height 312 pm of the nortricyclene moieties lies directly between those in p;and those in the neutral P,R3I5'. D 2-Fig. I . The dianion in the (Ph4P),P16 structure. The bonds of the P,-fragment are shown in bold lines. Bond lengths P-P in pm ( 0 5 0 . 6 ) : a=232.0, b=219.7, c=220.6, d=219.6, e=222.3, f=223.6, p 2 2 1 . 4 , h=217.0, i=214.1, k=220.2, 1=220.4, m=219. 7.The slow crystallization of the sparingly soluble salt would indicate slow formation of the PI,-system. A number of disproportionation reactions are conceivable, which, e.g., start with P:-+Pi-+P4 and then lead from 2P:-and P4 directly or via Pi-as intermediate to P: , and P:-. The Pg-fragment of the dianion is isomeric with the framework of the prismane derivative P9R3 discussed earlierI2I, into which it can be converted via a simple route by a double 1,2-bond shift. pm, B=137.05(3)0, 2=4, pcAic= 1.435 g ~m -~; diffractometer data, 2443 reflections hkl, Mo,, radiation 2 0 5 4 0 " , largedropin intensity, 0=0.06 A: R=0.085.The thermally induced 1,5-homodienyl hydrogen shift (1)+(2) ( n = 1) belongs to the intramolecular ene reactions, which are of importance in the stereoselective synthesis of cyclic compounds [']. With n> 2, the equilibrium (l)*(2) lies on the side of the carbocycle, whereas with n = 1 ring strain effect...
The photosensitized singlet oxygenation of 7,7-dimethyl-(1 a), 7-(methoxycarbonyl)-7-methyl-(1 b), 7-methyl-7-phenyl-(1 c) and 7-[4-(methoxycarbonyl)phenyl]-7-methyl-(1 d) 1,3,5-~yclohepta-trienes has been investigated. While 1 b -I d give only the respective norcaradiene endoperoxides 2b-N-2d-N, the dimethyl derivative l a affords a 60: 40 mixture of tropilidene (2a-T) and norcaradiene (2a-N) endoperoxides. In addition to full spectral characterization, the norcaradiene endoperoxides 2 b-N -2d-N where thermally isomerized to their diepoxides 3 b -3d. X-ray analysis confirmed that for 3b the stereochemistry of the substituents is 7-endo-methyl and 7-exomethoxycarbonyl and for 3c it is 7-endo-phenyl and 7-exo-methyl, showing that the steric size of the substituents in the cycloheptatrienes is Ph < Me < C0,Me. While the tropilidene endoperoxide 2a-T was a stable compound and could be fully characterized, the norcaradiene derivative 2a-N was too labile and had to be reduced with diimide to the saturated endoperoxide 4a to enable complete characterization.It is shown that the cycloaddition of singlet oxygen is a useful method to assess qualitatively the substituent effects on the tropilidene-norcaradiene valence isomerization, especially for those cycloheptatrienes in which the N/T ratio is too low for measurement by dynamic NMR methods. Stabilization of the norcaradiene isomer by 7-methyl substitution is rationalized in terms of steric effects. Stereochemie und Produktverteilung bei der Cycloaddition vonSingulettsauerstoff an 7,7-disubstituierte 1,3,5-Cycloheptrariene Die photosensibilisierte Oxygenierung von 7,7-Dimethyl-(1 a), 7-(Methoxycarbonyl)-7-methyl-(1 b), 7-Methyl-7-phenyl-(1 c) und 7-[4-(Methoxycarbonyl)phenyl]-7-methyl-l,3-cycloheptatrien (Id) wurde durchgefiihrt. Wahrend 1 b -I d nur die jeweiligen Norcaradien-endoperoxide 2b-N -2d-N lieferten, fiihrte das Dimethylderivat l a zu einem 60: 40-Gemisch von Endoperoxiden 2a-T und 2a-N. Zusatzlich zu der spektroskopischen Charakterisierung wurden die Norcaradienendoperoxide 2b-N -2d-N thermisch in ihre Diepoxide 3b -3d isomerisiert. Rontgenstrukturanalyse bestatigte, dal3 die Substituenten in 3 b 7-endo-Methyl und 7-exo-Methoxycarbonyl und in 3c 7-endo-Phenyl und 7-exo-Methyl angeordnet sind. Die sterische GrbRe dieser 7-Substituenten 1-N'Electronic and steric effects of substituents on the tropilidene-norcaradiene equilibrium [Eq.(l)] have been investigated actively in recent years') with the purpose of understanding the mechanistic subtleties of such valence isomerizations. For monosubstitution at the 7-position, the general conclusion obtains that x-electron acceptors, e.g. CN, C02R, Ph, etc., displace the equilibrium towards the norcaradiene form 1-N, while electron donors, e.g. CH,, MeO, R2N, etc., displace the equilibrium towards the tropilidene form 1-T, relative to the unsubstituted cycloheptatriene2). In fact, spectroscopic evidence for the existence of the norcaradiene isomer 1-N (R = R = H) in such equilibria is still lacking even at very low tem...
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