Junctive processes

The formation of a directed bond between two (or more) reactive atomic sites (belonging to one, two or more reactants/reactant moieties) en route to fransifion state, intermediate or product(s) (Figure 8.2) is, ipso facto, a junctive process. Such a process manifests itself in the formation of (a) incipient a bonds (l+2- [3] , 5+6- [7]), (b) full-fledged a bonds (l+2- 4, 5+6- 8, 9+l0- ll, 12+13- 14), (c) partial bonds due to dipole-dipole, dipole-ion (in forming host-guest complexes ... 

A junctive site is a reacting atom at which a change in directed bonding occurs. In Figure 8.2 (p. 3), all junctive sites are marked with arrows. A junctive simplex is the smallest portion of a molecule/molecular species that encompasses the reacting atomic sites involved in a junctive process. Jimctive simplexes are of two types - fundamental and topological. Processes are derived from fimdamental simplexes, or topological ones, or both (vide infra). 

In carbogenic systems, a hmdamental junctive simplex is, generally, an atom (type 26,27), a a-bond (type 28,29), or, a Jt-system (type 30,31), as represented in Figure 8.3 (p. 4). The atomicity of a simplex (ag) is the number of junctive atoms of the simplex taking part in a given junctive process. 

Fundamental junctive/disjunctive processes involving interactions between two or three fundamental simplexes vide infra) are described as binary and ternary processes, respectively. A given process - be it binary or ternary- is either simple or complex. The junctive process is simple, if only junctive or disjunctive components are present it is complex, if jimctive and disjunctive components are both present vide infra). 

Simple binary and ternary junctive processes are denoted as (m,n)j, (m,n,p)j, respectively (m, n, p are the atomicities as of the participating simplexes i.e. rtvi,p=l,2. ). The corresponding reverse processes are denoted as (mnld, (nvi,p)d and they constitute simple disjunctive processes (subscripted suffixes j and d, mean junctive and disjunctive, respectively). Figure 8.4 depicts cartoon representations and notational designations of simple binary and ternary junctive processes (the square bracketed entity, in each case, represents a conjunctive slate). ... 

The simplest binary junctive process (l,l)j results from atom-atom combination e.g. 32- [33], 34- [35], 36- [37]. The (l,2)j atom-bond, atom-face junctive processes are exemplified by transformations 38- [39] and 40- [41], respectively. The (2,2)j junctive processes resulting from bond-bond, bond-face, and face-face associations are represented by 42- [43], 44- [45] and 46- [47], respectively. 

The ternary (1,1,l)j junctive process resulting from atom-atom-atom associations is given by 48- [49]. Similarly, (l,l,2)j processes arise from atom-atom-bond, atom-atom-face interactions (e.g. 50- [51], 52- [53]), whereas (l,2,2)j processes stem from atom-bond-bond, atom-bond-face and atom-face-face interactions (e.g. 54- [55], 56- [57], 58- [59]. Finally, the ternary bond-bond-bond, bond-bond-face, bond-face-face, and face-face-face associations are designated (2,2,2)j as in 60- [61], 62- [63], 64- [65], and 66- [67], respectively. 

The notation for a reverse (disjunctive) process is identical with that for the corresponding junctive processes except for the subscript j (for junctive) the latter is modified to u (for disjunctive). For example, the reverse of the (l,2)j process 38- [39] is (1,2) process [39]- 38. Figure 8.4 includes the notational designations of all the pure reverse/disjunctive processes. 

Figure 8.7. Hypothetical Examples of Intermolecular and Intramolecular Examples of Topologically Junctive Processes...

The notation for junctive processes is applicable to mechanisms of organic reactions as well. In the two alternative mechanisms for the free-radical chlorination of cyclopropane, the reaction pathway proceeds towards the transition state through either (201+202- [203] ) or (l,2)j - (201+202- [206] ), whereas the transition state transforms into products through either (l,l)d- ([203] -204+205) or (l,2)d- ([206] -204+205). 

Figure 8.13. Examples of Unimolecular and Bimolecular Junctive Processes ...

Figure 8.15. Examples of Composite Trimolecular, Tetramolecular and Pentamolecular Junctive Processes...

The (m,n)j notation given here bears a certain similarity to Woodward-Hofmann s [nvn] notation for cycloaddition reactions. Nevertheless, the two are distinct. Thus the cycloaddition 12+13- 14 (Figure 8.2, p. 3) is a (2,2)j junctive process the numbers are coincidentally identical. Cycloaddihon reactions of the type ["2+"4], ["4+ 4], [ 2+ ] etc. are also (2,2)j junctive processes. The (2,2)j notation also applies to the transition states, as well as the products, in all these cycloadditions. 

Figure 12.5. Quartets ql-q45 for all Permutations of Conjunctive States in Junctive Processes...

We defined situselectivity in Chapter 10 we now delve into regioselectivity, and demonstrate that not only is it distinct from situselectivity, but that it is also a special case of a much broader concept, which we term vectoselectivity. The latter concept encompasses a comprehensive and wider range of orientational possibilities, and is one that is applicable to reactions involving three or more (rather than two) reactants, and is applied to (1,1)-, (1,2)-, (2,2)-, (1,1,2)-, (1,2,2)-, and (2,2,2)-junctive processes. 

Figure 143. Angular Joins in (l,l)-Junctive Processes Between Atomic Orbital Prototypes 1-13...

An associative process refers to the coming together of molecular entities, whereas a junctive process refers to the bonding aspect accompanying the association for examples see Cotton, F. A. Wilkinson, G., Advanced Inorganic Chemistry, 5 ed. Wiley New York, 1988 p. 37. 

In [m,n,p]j, [m,n]j and [m]j, m is the number of junctive atoms in molecule 1, n is the number of junctive atoms in molecule 2, and p is the number of jimctive atoms in molecule 3 the total number of numerals between the square brackets reflects the number of molecules/molecular entities taking in the junctive process. Thus, [nvn,p]j involves three molecules, [nwi]j involves two, and [m]j involves only one. The numerical value of each numeral indicates the number of junctive atoms in a molecule taking part in the junctive process. For example, in junctive process... 

In transformations in which superimposed a/n bonds are broken, the process is termed disjimctive, and the reverse of the said transformation would be a junctive process. 

In a given chemical transformation, reactive atoms may undergo changes in hybridization and valency. For example, a tetravalent sp atom in a reactant molecule may change to a (a) dsp pentavalent, (b) sp trivalent or (c) different sp-" tetravalent atom in the conjunctive state or product. The first of these is a junctive process (Figure 16.1, l->2). The second one is a disjunctive process (1 3). In the third case, the process is substitutive (1 4, as in an 5 2 transformation) -it is simultaneously "lytic" and "genic". 

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