Vectoselectivity

The concept of the junctivity lays the framework for discussing the concepts of ligogenicity (Chapter 9), vectoselectivity (Chapter 13), and regioselectivity (Chapter 13). 

With the definitions of junctivity and ligogenicity given in this and the previous chapter, the stage is set for a discussion of vectoselectivity and regioselectivity (Chapter 13). 

The preceding discussion was intended for two-component systems. A wide variety of chemical transformations do yield 3multicomponent systems, we will first extend the concept of selectivity to encompass situselectivity (Chapter 11), facioselectivity (Chapter 12), and vectoselectivity (Chapter 13). The specification of the various compositions of 2-, 3-, 4-component mixtures was given previously in Addendum A, Volume 1 (p. 143). 

In organic, organometallic and biochemical reactions involving paired molecular (stereotopic) faces, one is confronted with two fundamental types of selectivity - facioselectivity and vectoselectivity. Facioselectivity characterizes the preferential reaction at molecular faces, and will be discussed in this chapter. Vectoselectivity refers to the relative alignment of reactants, and will be covered in the following chapter. 

Reactions of enantiotopic faces "e" are either afacioselective (ql) or nonfacioselective with achiral reagents (ql0,qll,ql2,ql5,ql6,ql7,ql8), but stereofacioselective (enantiofacioselective) with chiral reagents (q23,q24,q27,q37,q38,q41). With only enantio-facioselectivity at work, and no role for vectoselectivity - e.g. with C2-symmetric reagents - one would expect two chiral diastereomers (q23). With non-C2-symmetric reagents, vectoselectivity would come into play, and more complex mixtures may result (vide infra). Here, enantiofacioselectivity refers to the face type in the reactant substrate. 

Vectoselectivity between Ma and Mg - moving left to right - is defined by ... 

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. 

Where zero (154a/b) or one vectoplex (155a/b) is involved, every point on one pathway has a homometric counterpart on the other hence, the pathways are exactly superimposable and no selectivity is possible these are cases of vectoaselectivity. Qn the other hand, for per/aper alignments such as those in 156a/b and 157a/b, every point on one pathway has an enantiometric coimterpart on the other in the absence of chiral influences, the pathways are isoenergetic, though obviously not superimposable, and while the resultant transition states/intermediates/products are enantiomorphic, no vectoselectivity is expected - these are cases of vectononselectivity. 

In order to establish the relationship between vectoselectivity and regioselectivity, we consider the vectorial reversal of monojunctive and bijimctive elements. 

Figure 13.16. Examples of Vectoaselective, Vectononselective and Vectoselective Processes...

We conclude that vectoselectivity is a universal concept that encompasses regioselectivity. Vectoselectivity is applicable to not just two-, but also to three- and more interacting junctive elements, such as those required in conjunctive states - be it transition states or products. 

To emphasize the importance of distinguishing between vectoselectivity/regioselectivity and situselectivity, we consider examples (Figure 13.18, p. 128), in each of which we illustrate the need for both terms - situselectivity and regioselectivity. 

Vectoselectivity is classified into vectostereoselectivity and vectonontereoselectivity. Each of these selectivities may reduce into aselective or nonselective variants. Further, vectostereoselectivity is subclassified, in principle, into "vectoenantioselectivity" (vide infra) and vectodiastereoselectivity, while vectononsteroeselectivity is subclassified into vectoastereoselectivity and vectononequiselectivity. Figure 13.19 depicts the overall classification of vectoselectivity. 

For a vectoselective process leading to two structurally- distinct products Pj and P2, vectoselectivity is defined by Equation 13.1 ... 

Vectoselectivity may be classified into vectostereoselectivity or vectononstereoselectivity depending on whether the ensuing transition states/intermediates/products are stereomorphic or nonstereomorphic, respectively ... 

In order for vectoselectivity to be possible for three-component systems, at least two of the three elements must be vectogenic (vide infra). 

In a vectoselective process leading to n products Pi/ P2/ P3/ Pn/ through n competing vectoselective pathways, vectoselectivity naay be defined for a specific pair of products, say PN-i and Pfj-... 

The usefulness of these equations lies in the ability to focus on a given pair of products and thus define the appropriate vectoselectivity arising from a single vectorial reversal. In Addendum A (Vol. 1, p. 143), we have discussed the quantitative treatment of 2-, 3-, and 4-component systems, and detailed how these individual pairwise differences can be used to described a given 3-, or 4-component system. 

Vectoselectivity may also be given as a ratio (see Volume 1, Addendum B, p. 149) ... 

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