Bonding schemes, alternate

An alternative bonding scheme is a 7r-delocalized radical with a linear 3c, 4e bond involving O-S-O. Such 7r-radicals have been suggested for radicals 56a,b based on analysis of their spectra obtained using EPR and ESE techniques [152]. 

We could use several spatial configurations while retaining the same spin function. This corresponds to several alternative bonding schemes in a given model. For example... 

Ql) An alternative bonding scheme consistent with a geometry similar to that postulated in would have the hypervalent colinear three-center bond occuj d by three rather than four... 

The synthesis of aziridines through reactions between nitrenes or nitrenoids and alkenes involves the simultaneous (though often asynchronous vide supra) formation of two new C-N bonds. The most obvious other alternative synthetic analysis would be simultaneous formation of one C-N bond and one C-C bond (Scheme 4.26). Thus, reactions between carbenes or carbene equivalents and imines comprise an increasingly useful method for aziridination. In addition to carbenes and carbenoids, ylides have also been used to effect aziridinations of imines in all classes of this reaction type the mechanism frequently involves a stepwise, addition-elimination process, rather than a synchronous bond-forming event. 

An alternative pathway to impart diversity at the C-3 position of the pyrazinone scaffold is possible via the hnkage of the triazole moiety to the core using a C - C bond (Scheme 27). The C-3 acetylenic pyrazinone... 

However, an alternative mechanism has been suggested to us by Prof. H. Saku-rai. It involves the homolytic cleavage of the iron-silicon bond Scheme 3). 

For olefins, cyclic, or better hi- or tricyclic ring structures with large ring strain (norborn-2-enes or norbornadienes for instance) are required. Alternatively, 1-alkynes can be used. In this case, the term 1-alkyne polymerization applies. This reaction proceeds via a- or j6-insertion of the alkyne into the metal-carbon double bond (Scheme 1). Both insertion mechanisms lead to a conjugated polymer. With a few exceptions [1-3], polymerizations based on a-insertion are the preferred ones, since they offer better control over molecular weights due to favorable values of kj/kp (ki, kp = rate constants of initiation and propagation, respectively). 

The second category consists of starting from an appropriately mono- or di-substituted heterocycle and introducing the fifth atom, with the formation of two bonds (Scheme 49). An alternate method of construction of the second ring by forming two bonds is also shown in Scheme 49. It should be emphasized, however, that the classification involving the formation of two bonds is very arbitrary and is based on the overall synthesis. 

Asymmetric hydrometallation of ketones and imines with H-M (M = Si, B, Al) catalyzed by chiral transition-metal complexes followed by hydrolysis provides an effective route to optically active alcohols and amines, respectively. Asymmetric addition of metal hydrides to olefins provides an alternative and attractive route to optically active alcohols or halides via subsequent oxidation of the resulting metal-carbon bonds (Scheme 2.1). 

Some of the bonding schemes set out in Table 6.9 should not be taken too literally. In scheme (11) for Xe04, the promotion of four electrons to 5d orbitals must require a colossal amount of energy. The alternative semi-polar structure Xe4+(0 )4, with coordinate/dative bonds between Xe and O, is equally far-fetched. A complete VB description of the bonding in Xe04 would require resonance among a large number of structures such as ... 

Upon irradiation of a,a,a-trifluorotoluene and 1,3-dioxole in dioxane, two ortho cycloadducts and two meta cycloadducts are formed, next to small amounts of substitution products and some unidentified products. Mattay et al. [15] have proposed a mechanism in which an exciplex may be the common precursor of both types of adducts (Scheme 41). Formation of one bond within the exciplex gives rise to zwitterions in which the negative charge is optimally stabilized by the CF3 group. The formation of the second C—C bond then leads to the 1-substituted endo-and exo-ortho adducts. The meta adducts, which both have the exo configuration, may be formed either by simultaneous formation of two bonds in the exciplex or by an alternative bond formation in the zwitterion that also leads to the endo-ortho adduct. 

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