State substructures

Compound 6 is a pivotal intermediate in Schreiber s synthesis. It was hoped that the conspicuous and strained bridgehead cyclobutene substructure in 6 would undergo a conrotatory electrocyclic ring opening upon thermolysis to give an isomeric 1,3-diene (8, Scheme 1). In the event, when a solution of cyclobutene 6 in toluene is confined to a sealed tube and heated to 180°C for 12 h, a stereoisomeric mixture of 1,3-dienes 7 and 8 is produced in an excellent yield of 95% (7 8 ca. 5 1). Finally, irradiation of the 5 1 mixture of cis-7 and trans-8, or of each independently, establishes a photostationary state in which the desired trans isomer 8 predominates (8 7 ca. 10 1). 

CD-ROM with state-of-the-art substructure and reaction-center seardi possibllties for rapid access... 

Fig. 18. Section of the solid-state structure of ft-gallium. The tubular substructure is marked, and a single layer is pointed out.

FIG. 19. Section of the solid-state structure of y -gallium. View along the crystallographic c-axis. The different substructures are marked. 

At this point, the picture which evolves from all our preparative, kinetic, and mechanistic work with the carbenoid fragments [(dtbpm)Pt(O)] and [(dcpm)Pt(O)] and with different organosilanes suggests that the platinum center of these extremely reactive and electronically most unusual (vide supra) intermediates interacts simultaneously with all three atoms (i.e. with both bonds or bonding pairs) of H-C-Si substructures of organosilane substrates near or at the transition state (Scheme 4 [24]). 

Based on the triglyme theozyme, additional catalysts were designed.1181 Of the potential catalysts examined, 6 was predicted to provide the best differential stabilization of the reactants and transition state. This molecule contains a polyether substructure as well as an additional hydrogen bond donor (the carbamate NH) which may further promote departure of the aryloxide leaving group. In addition, this catalyst is preorganized such that its polyether array is predisposed towards efficient... 

Thiophene and thienothiophene units have been frequently used in thienoquinoid or condensed forms in the design of new molecular architecture (16). Incorporation of the thiophene or thienothiophene unit as a rc-bridge linker could stabilize the quinoidal structure in the reduced forms (Figures 13 and 14). These two-types of new dications (222+ and 232+) stabilize the presumed two-electron reduction state by contribution of the thienoquinoid substructures (22 and 23) instead of the quinoidal form in the dication 212+ (77). 

The general structure with a cyanine unit at one terminus is represented in Figure 16. Two-electron transfer of the hybrid system produces another cyanine substructure via neutral radical state. In this case, a two step redox reaction is expected, because the neutral radical state is stabilized by the capto-dative substituents effect (19). Therefore, three colored sates will be achieved by the hybrid system. We call this system a cyanine-cyanine hybrid. 

Di(l-azulenyl)(6-azulenyl)methyl cation (24+) represented in Figure 17 exemplifies the cyanine-cyanine hybrid (20). Di(l-azulenyl)methylium unit in 24+ acts as a cyanine terminal group. The tropylium substructure stabilizes the cationic state (24+). Reduction of 24+ should afford the neutral radical 24, which is stabilized by capto-dative substitution effect, because 24 is substituted with azulenes in the donor and acceptor positions. The anionic state (24") is also stabilized by contribution of the cyclopentadienide substructure, which should exhibit the third color change in this system. 

Epr signals due to the triplet and quintet states were obtained when single crystals of diacetylene were photolysed (Huber and Schwoerer, 1980 Schwoerer et al., 1981 Sixl et ai, 1986). The quintet states are considered to arise from substructures such as [25]. 

As the analysis progresses, evidence is accumulated supporting the presence or absence of defined substructures. The evidence is combined by the Reasoner module to form a belief function, which describes the degree to which each substructure is currently believed. This information is stored in the chemical database, where it is available to the Expert modules and to the Controller as it decides the course of the analysis. As the belief function evolves, the current state is displayed graphically to the user, who may halt the analysis, query the current state, and redirect the course of the analysis by supplying evidence for or against a substructure. 

Bioisosteric replacement can be made from a position of knowledge, if the desirable properties of the substituent or substructure to be changed have been characterized. Such properties can include (with typical parameters) (a) size (volume, molar refractivity, surface area, Taft s) (b) shape (Verloop length and breadth, bond angles, interatom distances) (c) lipophilicity (log P, tt,/) (d) solubility (log S) (e) ionization state (pKg, a) ... 

If it is correct that the excited states of ethylene are twisted the torsional vibration must be active in the transitions. There is no direct evidence of this, though the possibility exists that the diffuseness of the spectrum owes less to predissociation than to an unresolved thicket of lines resulting from the torsional motion and other vibrational substructure. The torsional vibration does appear in association with the Rydberg transition at 1750 A (Wilkinson and Mulliken, 1955). 

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