Divalent reaction centers

P, Si, some metals) can be prepared in chiral form and do invite the use of standard techniques.( ) However, many important displacement reactions take place at mono- or divalent reaction centers, where there is no possibility of using a chiral probe of mechanism. In this paper, we report the results of experiments designed to probe the preferred geometry and mechanism of radical displacements at divalent sulfur, a reaction center characterized by a plane of symmetry. Further, the technique is general and can presumably be used to study the geometries of other displacement reactions at centers which are inherently achiral or cannot be easily prepared in chiral form.(2)... 

Gordon PM, Pong R, Piccirilli lA. A second divalent metal ion in the group II intron reaction center. Chem. Biol. 2007 14 607-612. 

This type of reaction could not be effected using alkyl-lithium or alkyl-Grignard reagents. Also, the products represented by 2.225 and 2.227 with presumed divalent cobalt centers as offered by Johnson and coworkers should be considered as being tentative only no conclusive data were actually given in support of the proposed formulations. There is also a discrepancy between the main textual body and the experimental section of the 1973 paper by Johnson and coworkers as to which Co(III) derivative was actually used in these reactions. In the main body, it is stated that the square planar (implying pyridine-free) Co(III) corrole is used, whereas in the experimental section, it is stated that the pyridine derivatives were used. 

Until recently, there were only few ways to introduce specific modifications into the reaction center structure which were suited for a parametric study of charge separation processes. Selective modifications were limited to the two quinones, and Qg (Warncke Dutton), the presence and nature of the divalent metal ion located between and Qg [3], and the removal of the H-protein subunit [4]. The only possibility to interfere with one of the four bacteriochlorophylls, the one at the site Bg in reaction centers of Rhodobacler (Rb.) sphaeroides, consisted in the borohydride treatment [5] and the chemistry of this procedure is still under debate 16]. 

Carbenes are neutral divalent derivatives of carbon. Carbenes can be included with carbanions, carbocations, and carbon-centered radicals as among the fundamental intermediates in the reactions of carbon compounds. Depending on whether the nonbonding electrons are of the same of opposite spin, they can be triplet or singlet species. 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

Some of the silylene-like reactions which take place at the silylene centers are displayed in Scheme 12. The divalent silicon inserts into the H—H bond of dihydrogen, the S—H and O-H bonds of H2S and H20, and the C-S bond of Me2S348. 

The remarkable progress which has been made in the study of silylene centers on silica reflects the recent advances in surface spectroscopy. The silylene centers, (=Si—0)2Si , represent a strongly stabilized form of divalent silicon. The reactions which they undergo may have important implications for the reactivity of silylenes generally. 

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