Additionen von Benzvalen an Nitriloxide. Eine Synthese für Benzvalen-3-carbonitril

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Darstellung und Röntgenstrukturanalyse des Diels-Alder-Addukts von 4-Phenyl-4H-1,2,4-triazol-3,5-dion an Octavalen

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Electron Density Distribution in a Bicyclo[l.l.0]butane

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Pyromellitic acid dianhydride: crystal structure and anisotropic proton magnetic shielding

The nuclear magnetic shielding tensors of the protons in pyromellitic acid dianhydride were measured in single crystals by means of high resolution solid state multiple pulse N.M.R. techniques. In order to correlate the N.M.R. results to the molecules, the molecular and crystal structure of the compound was determined by direct X-ray methods. The molecules have approximately D 2h symmetry. There are Z = 4 molecules in the unit cell, they are all crystallographically equivalent and the two protons in each molecule are magnetically equivalent. The assignment of the measured proton shielding tensors to the various proton sites is based on linewidth differences in multiple pulse spectra which can be traced back to differences in dipolar interactions, and on local symmetry arguments. The least shielded direction of the protons is close to the normal of the aromatic ring, but the other two principal shielding directions do not conform to the D 2h symmetry of the molecule. The deviations are ascribed to intermolecular shielding contributions. These are calculated on the basis of a magnetic dipole model. The application of this model for assessing inter-molecular shielding contributions is discussed. The difference between the measured shielding tensor and the calculated intermolecular contribution conforms well to the D 2h symmetry of the molecule. We believe, therefore, that this difference can be identified with good accuracy with the proton shielding of an isolated pyromellitic acid dianhydride molecule. The results imply that the direction perpendicular to the bond within the molecular plane is the most shielded direction of the proton, whereas the bond direction itself is the inter-mediate principal shielding direction. It is believed that these features are characteristic for the shielding of aromatic protons in general

SEVERAL POLYCYCLIC VALENCE ISOMERS OF DIMETHYL[14]ANNULENE-1,8-DICARBOXYLATE - REACTIVITY OF A NONCONJUGATED BIS(BICYCLO[1.1.0]BUTANE)

Diels-Alder reaction of dimethyl 1,2,4,5-tetrazine-3,6-dicarboxylate (5) with benzvalene (4), norbornene, and norbornadiene afforded the azo compounds 7 and 8. Theseare derivatives of 2,3-diazabicyclo[2.2.2]oct-2-ene as is azo compound 3, which had been obtained previously from 5 and 2 equiv of benzvalene (4). The photochemical extrusion of nitrogen from 3, 7, and 8 has been studied. Whereas 7 and 8 on direct irradiation in benzene gave rise exclusively to the bicyclo[2.2.0]hexane derivatives 9 and 10, respectively, from 3 in addition to the bicyclo[2.2.0]hexane 11, the diolefin 1l was formed. Diolefin 12 has cisdouble bonds in the nine-membered ring and is fixed in a boat conformation in a manner so that the two bicyclobutane systems approach each other very closely. This geometry suggests the unusual ring opening of the intermediate 1,4-cyclohexanediyl diradical from a boat conformation, which arises by inversion of the primarily generated boat conformation. Sensitized photolysis of 3 as weilasthat of ll produced the saturated isomer 13 of 11 and 12. The proximity of the bicyclobutane systems in 1l causes unprecedented reactions leading to cage compounds. When ll was heated at 90 °C, a rearrangement to the pentacyclic product 10 took place. Utilization of tetradeuteriated substrate ll-d4 supported a pathway with two diradical intermediates. Behaving in a convcntional manncr, bicyclobutane 9 and bis(bicyclobutane) 11 took up 1 and 2 equiv of thiophenol most probably in a radical-chain addition to give the thioethers 28 and 19, respectively. In contrast, bis(bicyclobutane) ll was converted by 1 equiv of thiophenol into cagc compound 30 in a process involving both the strained a systems. Heating at 80 °C subjected 30 to a reversible Copc rearrangement, resulting in a 6:1 mixture of 31 and 30. When it was treated with bromine, 11 was transformed to cage compound 38. This addition is believed to proceed via a cationic intermediate. The structure of cage compound 10 was established by a singlc-crystal X-ray analysis of dialcohol 11 prepared from 20 and methyllithium