7i-complexes

This scheme requires a rate-determining (second) proton-transfer, against which there is considerable experimental evidence in the form of specific-acid catalysis, the solvent isotope effect and the hg dependence discussed earlier. Further, application of the steady-state principle to the 7i-complex mechanism results in a rate equation of the form... 

Tetracyanoethylene yields a colored 7i-complex with aromatic compounds in the case of aromatic amines, phenols and indoles these then react to yield the corresponding tricyanovinyl derivatives [3, 4]. 

Polymeric pseudocrown ether networks have been generated in situ by the photopolymerization of poly(ethylene glycol) diacrylate transition metal complexes <00CM633>, and the effect of metal ion templation was evaluated. The 1,6,13,18-tetraoxa[6.6]paracyclophane-3,15-diyne (termed pyxophanes) was prepared from hydroquinone and l,4-dichlorobut-2-yne it forms size-selective 7i-complexes with alkali metal cations <00CC2377>. Dibenzo[ ]crown-m have been used in numerous elegant studies in which they were the needles that were threaded by diverse reagents the resultant... 

The formation and decay of these product-catalyst-7i-complexes are expected to occur according to the sequence of reactions as outlined in Scheme 12.4. The kinetic constants associated with the occurrence of kHYD and the decay of k0FF> respectively, can both be determined by PHIP-NMR using a process termed dynamic PASADENA (DYPAS) spectroscopy, as has been outlined previously [37]. For this purpose the addition of parahydrogen to the reaction is synchronized with the pulse sequences of the NMR spectrometer, whereby the time for acquiring the NMR spectra is delayed by variable amounts. The results thereof are listed in Table 12.3. A variety of kinetic constants can be determined, and the method is reasonably accurate the margins of error are also indicated in Table 12.3 [37]. 

The affinity of [2.2]paracyclophane for nitrosonium cation is much greater than that of para-xylene, presumably owing to stacking interaction between the aromatic rings in the 7i-complex. Low isotope effect on the aromatic carbon... 

Following olefin coordination, the Chalk-Harrod mechanism proceeds by olefin insertion into the M-H bond, whereas with the modified Chalk-Harrod mechanism, olefin coordination is followed by insertion into the M-Si bond. This step distinguishes the two mechanisms. Thus, the coordination of styrene to the hydridosilyl complex to form an olefin 7t-complex may be the first step of the catalytic cycle that discriminates between the two mechanisms. We have examined this coordination process as well as the relative energies of the many isomers of the 7i-complex that are possible. 

In the calculations performed with model B the to-7i-complex is destabilized by 1.6 kcal/mol with respect to the corresponding endo isomer. If we were to follow the more stable endo rc-complex isomer through the Chalk-Harrod mechanism, this would lead to the R form of the product as shown on the left-hand-side of Figure 15. Since it is actually the S form of the product that dominates when styrene is the substrate, the formation of the tt-complex cannot be the stereodetermining step of the catalytic cycle. 

The transfer of the silyl ligand onto the a-carbon of the substrate is followed by the formation of an intermediate in which the (1-phenylethyl)trichlorosilane product still weakly coordinated in a rj2-fashion. The corresponding endo and exo intermediates (11a and lib, respectively) are 3.5 and 4.2 kcal/mol, respectively, more stable than the endo 7i-complex, 8a Therefore the exo-stereoisomer is again thermodynamically more stable than the endo form. Finally, the oxidative addition of a molecule of trichlorosilane occurs with concomitant liberation of the products. The formation of both R and S products is exothermic by -20 kcal/mol. 

Similar to alkynes, cyclopropenes also readily form transition metal 7i-complexes which can isomerize to carbene complexes thermally, photochemically, or che-... 

Complexation of HC C-EHs (E is C, Si, or Ge) to Ni and Cu differs from the alkene complexation in that conventional 7i-complexes are the global minima for all the elements However, when E = Si or Ge, the agostic complexes... 

With a tungsten pentacarbonyl catalyst, the calculated mechanisms are summarized in Scheme 4.15 [26]. Coordination of the 4-pentyn-l-ol substrate to the pentacarbonyl tungsten leads to the formation of the 7i-alkyne-W(CO)5 adduct Wl. This coordination process was calculated to be exothermic by 24.3 kcal mol. The cydoisomerization leading to a five-membered-ring exo product starts with the 7i-complex Wl via a one-step process with a barrier of 46.5 kcal mol (path a of Scheme 4.15). The barrier calculated here is comparable with that calculated for the catalyst-free process. From Wl to W3, the tungsten metal center does not play a significant role in the isomerization process. 

Furthermore, when o-ethynylphenyl methyl ketone 104 (R = Me) was treated with W(CO)5(thf) in the presence of 5 equiv of water, o-acetylacetophenone 123 (R = Me) was obtained in about 50% yield. Direct observation of the reaction mixture inTHF-dg clearly showed the formation of a methyleneisobenzofuran derivative 122 (R = H) at rt. This compound is produced by the exo-mode of attack of the carbonyl oxygen on the W(CO)5-7i-complexed alkyne to give 121 (R = Me), followed by protonation of the C—W bond and deprotonation from the methyl group (Scheme 5.36). 

In a series of reports published over the last 10-15 years, Mayr and co-workers obtained second-order rate constants for reactions of carbocations and other electrophiles such as metal-7i complexes with a series of nucleophiles, especially 7t-nucleophiles where a C C bond is formed. An impressive body of reactivity data has been accumulated, and, including data from other groups, correlated by the following equation. 

However, ab initio calculations [QCISD-(T)/6-31G //UMP2/6-31G ] on ethylene and its radical cation support an anti -7i-complex, in which the two components are joined by one long bond (190 pm), rather than the sandwich -type 71 complex. The complex is connected to two transition states leading to a (rhombic) cyclobutane radical cation (see above) or, by 1,3-H-shift, to 1-butene radical... 

These results lead to a general mechanism for the reaction nitrenium ions with aromatic compounds (Fig. 13.71). Initial encounter leads to a 7i-complex (141). The latter is converted into isomeric a complexes (142-144) which, in turn, either tau-tomerize to give stable adducts (145-147) or else dissociate to give radicals. The relative rates of these processes depend on the reactivities of the nitrenium ion and the arene. With less reactive nitrenium ions the 7i-complex is relatively long lived. With more reactive nitrenium ions the n complex forms in a low steady-state... 

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