Co-induced transformations

Fig. 6. Schematic energy diagram for the CO-induced transformation of the surface structure of Pt(IOO). (From Ref. 46.)...

To date, only one example of a combination of a photochemically induced transformation with a transition metal-catalyzed reaction has been found in the literature. This hv/Pd°-promoted process allows the synthesis of five-membered cyclic y-keto esters 5-119 from 5-iodoalkenes 5-117 in the presence of CO and an alcohol 5-118 as a nucleophile (Scheme 5.24) [41]. The yields are high, and differently substituted iodoalkenes can be employed. 

The Pauson-Khand reaction is the Co-induced formation of cyclopentenones from ene-ynes and CO. One impressive example of a domino Pauson-Khand process is the synthesis of fenestrane 6/4-15, as reported by Keese and colleagues [278]. The transformation is initiated by a double Grignard reaction of 4-pentynoic acid 6/4-12, followed by protection of the formed tertiary hydroxyl group to give 6/4-13. The Co-induced polycyclization of 6/4-13 led directly to the fenestrane 6/4-15... 

If PAH-degrading microorganisms use broad-specificity enzymes or common pathways to transform multiple PAHs, then inducers for the metabolism of one PAH substrate might co-induce the transformation of a range of PAHs. Preliminary evidence indicated that the transformation of naphthalene, phenanthrene, fluoranthene, and pyrene by Pseudomonas saccharophila P15 was stimulated by salicylate [132], a known inducer of naphthalene metabolism in pseudomonads [43]. However, Chen and Aitken [181] reported in more detail the inducing effects of salicylate on the transformation of various HMW PAHs by Pseudomonas saccharophila P15 isolated from contaminated soil, including... 

The clean Pt(210) surface is nonreconstructed and there is also no indication for a CO-induced alteration of the atomic positions within the unit cell. In this case, however, under reaction conditions, continuous faceting, again with creation of other crystal planes, preferably (110), takes place. The latter may then in turn be subject to periodic structural transformations, which then accounts for the observed integral oscillatory behavior. 

The atomic density of the hex phase is about 20% higher than that of the 1 x 1 phase. As could be demonstrated by scanning tunneling microscopy (STM) (49), during the CO-induced hex — 1 x 1 transformation, these additional atoms are squeezed out from the surface layer, on top of which they are aggregating to new small I x 1 patches, a result which could also be successfully modeled by computer simulations (50). 

Fig. 7. Scanning tunneling microscopy image from a Pt(110) surface showing nucleation of the CO-induced 1 x 2— 1 x I transformation (a) and corresponding ball model (b). (From Ref. 57.)...

More recently, a kinetic model for describing the occurrence of temporal oscillations on Pt( 110) was developed, which is more simplified than that originally derived for Pt(100), but nevertheless reproduces the qualitative features well and enables, in addition, analysis of further effects, such as forced oscillations, etc. (69). It starts from Eqs. (4) and (5) outlined previously, which were the basis for successful description of the overall kinetics. The system was then extended in order to include the CO-induced structural transformation of the surface as well as the different oxygen sticking coefficients on the two modifications. 

From the microscopic point of view, again the CO-induced 1 x 2- 1 x 1 structural transformation of the Pt(l 10) surface (as also underlying the mechanism of temporal oscillations) is of crucial importance for the development of facets, as becomes evident from the fact that this effect is restricted to conditions of high stationary CO coverages. Simply speaking, CO adsorption lifts the 1 x 2 reconstruction and simultaneously creates... 

The mechanism of the kinetic oscillations occurring with the CO + 07 reaction on clean Pt( 100) and Pt( 110) surfaces was based on the reversible transformation of the surface structure by the presence of adsorbed CO and by an associated variation of the oxygen sticking coefficient that increased upon CO-induced lifting of the reconstruction of the clean surface. The most densely packed Pt(lll) surface is not reconstructed and its structure is also not affected by CO adsorption. Accordingly, kinetic oscillations with a clean Pt(lll) surface (i.e., for partial pressure <10 3 torr) could never be observed (13, 26, 27, 38). Again no reconstruction... 

The bis(methylthio)-substituted quarterpyridine, 4, 4"-bis(methylthio)-2,2 6, 2" 6, 2" -quaterpyridine (44), yields a binuclear copper(I) complex, but a mononuclear species with copper(II). ° A spectroelectrochemical study indicated that a redox-induced transformation occurs between these two complex species. The X-ray structure of the 2 2 [copper(I) ligand] helical complex confirmed that each copper is tetrahedrally co-ordinated to four pyridyl nitrogens from two ligand strands, with a short Cu-Cu distance of 3.32 A being present. 

Z pp and co-workers (1976) found that photolysis of methoxychlor, although much more rapid than that of DDT, is still a very slow environmental process. The major product of light-induced transformation is l,l-bis(p-methoxyphenyl)-2,2-dichloroethylene (DMDE, 24). This product is subject to rapid photodegradation in both aqueous and hydrocarboi) media. 

Kamachi and co-workers reported a base-induced transformation of trifluoroacetaldehyde azine 86 into 1,4-bis(2,2,2-trifluoroethyl)-3,6-bis(trifluoromethyl)-l,4-dihydro-l,2,4,5-tetrazine 92. The proposed reaction mechanism involves addition of triethylamine to the C=N bond followed by addition of the adduct 87 to another molecule of 86 to give the zwitterion 88. Intermediate 88 undergoes hydride shift and elimination to give the intermediate 90. Repetition of the latter part of the reaction sequence then results in cyclization to give dihydrotetrazine 92 (Scheme 22) < 1995BCJ2025>. 

FIGURE 2.21. Hysteresis effects in the CO-induced hex 1x1 structural transformation of Pt(l 0 0) [25] (a) structural parameter reflected by the Rutherford backscattering yield (b) CO coverage. 

The CO-induced structural transformations of both the Pt(l 0 0) and Pt(l 10) surfaces will be of decisive importance for the occurrence of kinetic oscillations in CO oxidation, as will be discussed in detail in Chapter 7. 

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