Acyclic cationic systems

Acyclic cationic systems, chalcogen-halogen compounds, 35 299-301 Acylarsonic acids, 44 217 Acyldiazenido complexes, mechanisms of formation, 27 223-225 Acylium cation, 9 231, 241-243 Acylphosphonic acids, 44 217 Acylsilane, hydrolysis, 42 173 Adamantane... 

Chalcogen dihalides, substituted, chalcogen(II) compounds, 35 278-279 Chalcogen-halogen compounds, 35 235-237 acyclic cationic systems, 35 299-301 cations containing homoatomic chalcogen rings, 35 295-298... 

As in the photolysis of protonated eucarvone, an acyclic intermediate is proposed in the mechanistic pathway. The protonated dienones 73 and 74 should be thermally stable, since a symmetry-allowed ring closure in the conrotatory mode is precluded in the cyclic system (Woodward and Hoffmann, 1970). Upon irradiation it can undergo a conrotatory ring opening however, to produce the acyclic cations 79 and 80 which in... 

The analogous cycloheptadienyl complex (5 equation 1) was similarly prepared by Dauben and Ber-telli,4 but the acyclic pentadienyl systems were a little more difficult to obtain. The triphenylmethyl cation does not remove hydride from tricarbonyl(frans-pentadiene)iron (6 equation 2). The corresponding c/s-pentadiene complex (7 equation 3) cannot be prepared directly from the diene and an iron carbonyl,... 

The size of the K-system chosen has important implication on the structural and functional aspects of metal binding. To explore the size effect calculations were performed on the cation-ir complexes of Li+ and Mg + with the Jt-face of linear and cyclic unsaturated hydrocarbons [45]. In the case of the acyclic Jt-systems, we started with the simplest system, e.g. ethylene followed by buta-1,3-diene, hexa-l,3,5-triene, and octa-1,3,5, 7-tetraene with 2, 3 and 4, conjugated jt units, respectively. These linear systems with two and more number of jt units can have various conformations wherein the jt units can have cis, trans or a combination of both cis and trans orientations. Similarly for cyclic systems cyclobutadiene, benzene, cyclooctateraene, naphthalene, anthracene, phenanthrene and naphthacene have been included. Thus a wide range of sizes for aromatic systems have been covered. 

In the pentadienyl cationic complexes described so far the five sp hybridized carbon atoms are necessarily held in a cis configuration. This restriction of configuration does not hold in the case of acyclic pentadienyl systems. Nevertheless in each of the acyclic pentadienyl-Fe(CO)3 cations so far prepared the arrangement of carbon atoms is cis. Reactions which might be expected to yield the trans configuration either fail to produce a stable cationic system or proceed to yield the cis isomer. 

Acyclic Se-N chlorides that are potential building blocks (as a source of the SeNSe unit) for cyclic Se-N systems are generated by the reactions of [SeChJiAsEg] or SeCU with (Mc3Si)3N as illustrated by the sequence of transformations that produce the five-membered [Se3N2Cl] cation (Scheme 2.4 and Section 8.4). 

Interconversion between two tautomeric structures can occur via discrete cationic or anionic intermediates (scheme 24, where T is an anion capable of reacting with a proton at a minimum of two distinct sites). Alternatively, interconversion can occur by simultaneous loss and gain of different protons (scheme 25, w here T has the same definition as in scheme 24). These mechanisms are well established for acyclic compounds, but they have been much less thoroughly investigated for heteroaromatic systems. The rate of interconversion of two tautomers is greatest when both of the alternative atoms to which the mobile proton can be attached arc hetero atoms, and isolation of the separate isomers is usually impossible in this case. If one of the alternative atoms involved in the tautomerization is carbon, the rate of interconversion is somewhat slower, but still fast. When both of the atoms in question are carbon, however, interconversion is... 

Closely related to the polyepoxide cascade procedure for the synthesis of polycyclic systems is Corey s biomimetic-type, nonenzymatic, oxirane-initiated (Lewis acid-promoted) cation-olefin polyannulation. By this strategy, compound 96, containing the tetracyclic core of scalarenedial, was constructed by exposure of the acyclic epoxy triene precursor 95 to MeAlCl2-promoted cyclization reaction conditions (Scheme 8.25) [45]. 

Experimentally, there are two approaches to the elucidation of the structure of vinyl cations first, preparation and solvolysis of systems where because of geometric restrictions the intermediate vinyl cation by necessity is bent and second, by a careful examination of the stereochemistry of solvolysis of appropriate acyclic substrates. 

The suprafacial shift along the carbon framework is not restricted to cyclic systems but may also prevail in acyclic cases. In the example given in Scheme 11, minimization of dipolar repulsion between the two C-0 bonds mandates a preferred conformation of the initial radical, leading to a stereo-chemically defined alkene radical cation and, ultimately, to a single diastere-omer of the product [119]. 

Electrochemical recognition, 39 80 anions, macrocyclic and acyclic polycobalti-cinium ligand systems, 39 134-140 cations, 39 81-133 ammonium cation, 39 128-133 crown ether and bis crow ether ligands containing bipyridyl transition metal recognition sites, 39 111 crown ether dithiocarbamate and dithi-olene complexes, 39 123-124 di-h-cyclopentadienyl-molybdenum(IV) and -tungsten(IV) crown ether complexes, 39 107-108... 

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