Decomplexation thermal

Examination of Dreiding and Corey-Pauling-Koltung molecular models reveals that the trans-ligand (b) cannot chelate a metal ion without severe steric interactions while the cis-(a) can do so easily. The fact that the nickel (II) complexation-cyanide decomplexation sequence leads to enrichment of the major isomers of VI and VII leads us to postulate that the major isomers are the cis-(a) species. We have equilibrated Via and VIb thermally to a ratio of 58 42 thus the macrocyclization gives a non-equilibrium mixture favoring the cis isomer, analogous to PPh and AsMe macrocyclizations that we have described recently (1, 7 ) ... 

The above mentioned conversion of 3,3-dimethylcyclopropene proceeded via 5-(2,2 -bipyridyl)-3,3,7,7-tetramethyl-tra 5-5-nickelatricyclo[4.1.0.0 ]heptane (9) when 2,2 -bipy-ridyl(cycloocta-l,5-diene)nickel was used. Upon thermal or oxidative decomplexation, the metallacyclic system 9 gave 3,3,6,6-tetramethyl-tra j-tricyclo[3.1.0.0 ]hexane. ... 

Similar to the nickel-catalyzed reactions, metallacycles of palladium are assumed to be intermediates of these conversions. They were isolable in some cases. Thus, 3,3-dimethylcyclopropene with -allyl( ) -cyclopentadienyl)palladium, on cyclizing oxidative addition, formed metallacyclopentanes 20 and metallacyclononanes 21, depending on the reaction conditions. Thermal decomplexation via reductive elimination gave cyclopropane systems and products thereof. ... 

The (cyclopropylamino)carbene complex 15 reacted with diphenylalkyne to give a nitrogen ylide, which was converted via thermal A -cyclopropyl to C-cyclopropyl rearrangement and decomplexation to a cyclopropyl-substituted lactam. 

Decomplexation of cyclopropanes from metallacyclobutanes is facilitated thermally or photo-chemically as well as by numerous reagents such as phosphanes, alkenes, cyanides, or oxidants [Ij, Oj, Ce(IV), etc.]. Depending on the method used, striking differences in product selectivities are observed, even with respect to stereochemical results. Thus, decomplexation of trans-1,2-... 

Treatment of tricyclo[3.2.0.0 ]hept-3-ene (13) with iron carbonyls leads to a primary complex 14, which undergoes thermal rearrangement and carbonylation to 4-(tricarbonyl)ferrate-tracyclo[4.2.1.0 .0 ]nonan-5-one (15). Photochemical conversion of the primary complex 14 gives 8-(tricarbonyl)ferra-2,3-f/-tricyclo[4.2.0.0 ]oct-2-ene (17) (in equilibrium with its acyl complex 16). This system after carbonylation undergoes rearrangement to 2-(tricarbonyl)ferra-6,7-f/-tricyclo[3.2.2.0 ]non-6-en-4-one (18), which upon decomplexation gives tricyclo-[3.2.1.0 ]oct-3-en-6-one (19). Several other products (not containing cyclopropane subunits) are obtained under modified conditions. ... 

Fig. 8. A light-driven movement of an pendant arm which bears an ammonium group and is covalently linked to a crown ether. UV irradiation induces the trans-to-cis rearrangement of the azobenzene fragment. The self-complexing process is favored by the establishing of hydrogen bonding interactions between the ammonium group and the oxygen atoms of the crown. Decomplexation (cis-to-trans re-isomerization) can take place either via irradiation with visible light or thermally...

TGA analysis showed that polymers 25 were thermally stable with the first weight loss starting between 225 and 241 °C as a result of the decomplexation of the cyclopentadienyliron moieties as well as partial decomposition of the polymer side chains. The second weight-loss beginning between 400 and 450 °C was attributed to the degradation of the polymer backbone. DSC analysis showed that the organoiron... 

If we consider that the stability constant for the complex is K< =k,. .pi.,/kA., pi. the requirement for a carrier is that the same molecule have a large and small decomplexation rate at the same time. Since this is impossible, compounds are chosen that achieve transport by a compromise of the complexation and decomplexation rate constants. One means of circumventing this problem is to use a ligand (binder) that can be made to form a more or less stable complex by a rapid chemical modification. This is usually referred to as "switching." Mechanisms to switch a ligand s binding ability based on ionization, thermal, photochemical, and redox properties have been described in the literature. 

An important feature of these tandem processes is the complementary electronic nature of the [4 + 2] and the [3 + 2] cycloadditions. The former operates under inverse-electron-demand and as such requires an electron-rich dienophile. The latter operates under normal-electron-demand and is more facile with an electron-deficient dipolarophile. As a consequence, both the dienophile and the dipolarophile may be simultaneously present in the reaction mixture or linked to each other and will not lead to cross-reactivity. Tandem cycloadditions that are initiated by the thermal or pressure activated [4 + 2] cycloadditions usually require an excess of the dienophile. Even though this 27t component is electron-rich, it can react further with the nitronate intermediate unless a more reactive, electron-deficient dipolarophile is also present. As a consequence, such tandem cycloadditions may be conducted as cascade tandem processes if all three components are present firom the beginning and react under the same conditions. On the other hand, tandem processes initiated with a Lewis acid activated [4 + 2] cycloaddition often require a work up to decomplex the Lewis acid li om the nitronate, which otherwise would inhibit the next [3 + 2] step. Because this work up constitutes a change of reaction conditions, such processes are usually run as consecutive or even sequential tandem processes, if the dipolarophile is added later. 

---