Strain-induced complexation

Fig. 7. Conjectured cross-sectional representation of intermediate cycloaddition complexes substrates + cucurbituril and product -t- cucurbituril (R = H or t-Bu). Outlines are drawn as for Fig. 4. Shaded region corresponds to strain-induced compression of substrates, promoting reaction (cf. Sect. 3.2) from [15] with permission...

In the author s own laboratory the Cu(II)-catalyzed hydrolysis of the phosphate ester derived from 2-[4(5)-imidazolyl] phenol recently has been investigated146. The pertinent results are (a) the pre-equilibrium formation of a hydrolytically labile Cu(II)-substrate complex (1 1), (b) the occurrence of catalysis with the free-base form of the imidazolyl and phosphate moieties and (c) the extraordinary rate acceleration at pH 6 (104) relative to the uncatalyzed hydrolysis146. The latter recalls the unusual rate enhancement encountered above with five-membered cyclic phosphates and suggests a mechanism in which the metal ion, at the center of a square planar complex or a distorted tetrahedral complex, might induce strain in the P-O ester bonds (60). viz. 

We [47] further studied quantitatively the strain-induced complex formation in PVA films in dilute iodine solutions whose iodine concentration is lower than the threshold required for the complex formation. We were interested in the effects of degree of hydration D.H. of PVA films and the iodine concentration of the soaking solutions on the strain-induced complex formation. PVA films were stretched in iodine-KI soaking solutions whose iodine concentration was in the range of 2 x 10 9 x 10 mol/1. No boric acid was added to the solution. No complex forms in these solutions, and therefore films remain brown in color before extension. However, when stretched in the solution the color turns to blue at the points indicated by arrows on the stress-strain curves shown in Fig. 11, which shows the beginning of the formation of the complex. The strain-induced complex formation is also shown by the visible ray absorption spectra measured before and after extension in a solution of 3x 10 mol/1 iodine concentration at 30 °C as shown in Fig. 12. A broad peak of the complex with... 

Figure 13 shows the absorbance at km of a specimen of D.H. = 0.84, soaked in 3 x 10 4 mol/1 iodine concentration, as a function of the extension at 30 °C. This absorbance is proportional to the amount of complex formed. The amount of the complex increases successively with extension after passing the point indicated by an arrow on the stress-strain curve. All the swollen PVA films used are so highly elastic that the strain is released when the extension is not more than about 100%. The complex disappears after removal of the extension force when strain is released. This reversible strain-induced complex formation is explained by the increased free energy of PVA chains due to the extension. 

The highly strained molecule of 3,4-diphenylcycloocta-l,5-diyne (165), stabilized by the coordination of Co2(CO)6, was prepared from the bis-propargylic alcohol 163. Cyclization of the propargylic dication complex 164, induced by reduction with Zn,... 

The study of the strain-induced polymorphic transitions by microhardness measurement offers the opportunity to gain additional information on the deformation behaviour of more complex polymer systems such as polymer blends. Since polymer blends are usually multicomponent and multiphase systems the question arises of how the independent components and phases react under the external load. The polymorphic transition will reflect the behaviour of the crystalline phases provided strain-induced polymorphic transition is possible. 

In summarizing the results from the last three sections, one can conclude that the systematic variation of microhardness under strain performed on (a) homo-PBT (Section 6.2.1), (b) its multiblock copolymer PEE (Section 6.2.2) and (c) on blends of both of these (this section) is characterized by the ability of these systems to undergo a strain-induced polymorphic transition. The ability to accurately follow the strain-induced polymorphic transition even in complex systems such as polymer blends allows one also to draw conclusions about such basic phenomena as cocrystallization. In the present study of a PBT/PEE blend two distinct well separated (with respect to the deformation range) strain-induced polymorphic transitions arising from the two species of PBT crystallites are observed. From this observation it is concluded that (i) homo-PBT and the PBT segments from the PEE copolymer crystallize separately, i.e. no cocrystallization takes place, and (ii) the two types of crystallites are not subjected to the external load simultaneously but in a sequential manner. 

Cholesta-3,5-diene and similar transoid dienes undergo a variety of complex photochemically induced reactions, which differ for singlet- and triplet-excited states. Singlet products are derived in part from the very strained 3,5 4,6-bis-cyclo ( bicyclobutane ) intermediates of the type (315), which open by solvent attack to give such products as (316). The singlet-excited diene may alternatively react via... 

Protonation of (norbornadiene)cobalt complex 43 induces C-C bond cleavage of the norbornenyl ring to form cationic complex 44 [63,64]. Re-protonation of the reduced complex 45 induces a second cleavage of a non-strained cyclopen-tene ring to give an open r)5-pentadienyl complex 46. It is postulated that a three-center interaction of the highly electrophilic metal center with the a-electrons of the adjacent C-C bond is involved. 

A SCSC [2+2] photodimerziation involving 7-hydroxy-4-methylcoumarin has been shown to occur in the cavity of (3-cyclodextrin, as reported by Stezowski and co-workers (Fig. 2.3.6) [53]. The SCSC reaction was facilitated by the inclusion of both reactants within a single cyclodextrin cavity. In that way, the photodimerization produced very small changes in the overall shape of the inclusion complex. In addition, water molecules of crystallization were shown to afford an environment favorable for the SCSC transformation by providing a way to relieve the strain induced by the reaction. 

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