Ligogenic Processes

A ligogenic process is one in which distinct a or a/n (but not just n) bonding takes place between two unbonded atoms. For canonical structures, one uses localized bonding models. In a ligogenic process, the bond order between two reactive atoms starts at 0 and increases to integral... 

A complex process incorporates ligogenic as well as ligolytic components. For example, in transformation (l,l)f (l,2) , there is a (l,l)-ligolytic process with a loss of a obond, and a (1,2)-ligogenic process in which two o bonds are formed. The reverse process is designated as (1,1) g ... 

In determining selectivity in ligogenic processes (Chapter 9), the center of attention is usually on either whole molecules Sj vs. S2, or, molecular sites tj vs. t2. In the former instance, one deals with morphoselectivity - selectivity based on morphic relationships between molecules in contrast, in the latter instance, one invokes situselectivity - selectivity based on topic relationships between molecular sites. In this chapter we deal with morphoselectivity situselectivity is treated in the next chapter. The concept of morphoselectivity is synonymous with substrate selectivitystructural selectivity," intermolecular selectivity," shape selectivityand, enzyme substrate selectivity it is also implicit in intermolecular chemoselectivity. This chapter deals with the concept of morphoselectivity, and establishes the commonality of all the above literature terms. 

In specifying selectivity of ligogenic processes, the focus of attention is on either whole molecules, or, molecular sites. Selectivity for whole molecules is based cn their morphic relationships and is termed morphoselectivity. In contrast, if the emphasis is on selective transformations at molecular sites, then one is dealing with situselectivityP The latter term is synonymous with site selectivity, site specificity, positional selectivity, locoselectivity, atroposelectivity, regioselectivity (intramolecular), regiospecificity, chemoselectivity (intramolecular), intramolecular selectivity, fimctional selectivity, and sequence selectivity. This chapter deals with the concept of situselectivity, and establishes the commonality of all the previous terms. 

We have systematically analyzed the outcome of a wide variety of (l,2)-atom-face and (2,2)-bond-face ligogenic reactions of prototype alkenes, belonging to the eleven fundamental subclasses, with monojimctive/bijunctive C2-symmetric/non-C2-symmetric, achiral/chiral reagents of general types Cab, C=Cxy, C=C=Cxy, and x-y, (x=y and x ty x,y= achiral or chiral ligands). We have also examined (2,2)-face-face ligogenic processes based on [2+2] and [2+4] cycloadditions - those involving reactants of types xyC=Cwz and xyC=Ca-Cb=Cwz. 

Table 12.2. Quartets of Conjunctive States (ql-q45). Quartet Component Designations, and Quartet Compositions in (1,2)- and (2,2)-Ligogenic Processes...

We have shown previously how a wide variety of chemical transformations may be represented as junctive/ligogenic processes (cf. Volume 2, Chapters 8 and 9). Figure 13.1 depicts the most common types (i-vii) of such processes in organic chemistry. The first three of these represent (1,1)-, (1,2)-, and (2,2)-junctive processes, and involve interactions between two junctive simplexes the... 

These designations are independent of the nature of the ligogenic process hence, the vectorial permutations shown apply equally well for (1,1)-, (1,2)-, and (2,2)- ligogenic processes. However, the two vectors here belong to a common plane (considered to be the reference plane). Molecular examples of the above-mentioned interacting bijunctive vectoplexes are shown in Figure 13.10. 

In the chapter on facioselectivity (Chapter 11) we examined (2,2)-face-face ligogenic processes involving four face-face combinations -1-3, 2-3,1-4, and 2-4. When vectoselectivity is also taken into accovmt, such a process would lead to a maximum of eight conjunctive states (i)-(viii), and is portrayed as an octet (Figure 13.26, p. 144). 

In ligogenic processes, the formation of o-bonds involves hybridized (sp, sp, sp) or simple atomic (s,p) orbitals (Figure 14.1). 

Angulospecific states refer to partially bonded enhties as in transition states they are not synonymous with conformational states, be it conformations or conformers. Where more than one a bond is formed, the angulospecific state must define the alignment of the elements with respect to each and every a bond that is being formed. We will next discuss the concept of anguloselectivity for (1,1)-, (1,2)- and (2,2)-ligogenic processes. 

The various permutations for couplings in (l,l)-ligogenic processes using prototypes 1-13 (Figure... 

Conformational (l,l)-ligogenic processes are exemplified by the coupling of free radicals to form a single, o-bonded, conformationally-labile, coupled product (Figure 14.8). 

Figure 14.8. Vectoselectivity and Anguloselectivity in Conformational (1,1 )-Ligogenic Processes...

Ligogenic processes are exemplified by the photochemical [2+2] cycloaddition to form cyclobutanes (Figure 14.10). 

Note in the case (2,2)-ligogenic processes, unlike the (1,1)- and (l,2)-cases discussed above, one has cases of vectoastereoselectivity and vectononequiselectivity, in addition to vectoaselectivity, vectononselectivity and vectostereoselectivity. 

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