Orientation of reactions

Substituents can be classified as (i) ortho-lpara-directing activators (ii) ortto/para-directing deactivators or (iii) me/a-directing deactivators. 

The relative ease of attack (by an electrophile) at the ortho-lmeta-lpara-positions can be determined experimentally from partial rate factors. These compare the rate of attack at one position in the monosubstituted benzene with the rate of attack at one position in benzene. The higher the partial rate factor at a given position, the faster the rate of electrophilic substitution. 

Electron-donating +1 and/or +M groups (EDGs), which make the ring more nucleophilic than benzene, will stabilise the intermediate carbocation most effectively when new substituents are introduced at the ortho- or para- position. For the meta- isomer, the positive charge in the carbocation intermediate does not reside adjacent to the EDG in any of the resonance forms. 

The EDG can stabilise both of these cations more effectively than the cation derived from meta- attack 

An additional resonance structure can be drawn for intermediate car-bocations bearing the +M but not +1 groups (at the 2- or 4-position). 

The slowest step is the rale-determining step (Section 4.9.2.1) 

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As stated above, simple catenanes were first obtained by a statistical approach. However, the successful syntheses of more complicated topological molecules would not have been possible without an enforcement of spatial orientation of reaction substrates called preorganization, introduced in Section 2.3. The latter effect can be achieved by... 

Orientation of reactions of amino-pyridines and -azines with electrophiles... 

Scheme 3.10 Closed spaces inside a capsule impose restrictions in the orientation of reaction partners in a 1,3-dipolar...

This sequence applies (a) to the various hydrogens within a single alkane and hence governs orientation of reaction, and (b) to the hydrogens of different alkanes and hence governs relative reactivities. 

Figure 6.4. Molecular structure and orientation of reaction. Stability of transition state parallels stability of carbonium ion more stable carbon-ium ion formed faster.

The orientation of reaction of aminopyridazines with electrophiles, and further reaction of the intermediates is illustrated by a study with 4-amino-5-aroylpyridazines. Acylation and sulfonylation occurs on the amino nitrogen, while alkylation occurs, as expected, at N-1 the acylamino derivatives are relatively unstable, but the toluenesulfonamide is quite stable to acid and to base. Methylation of the toluenesulfonamide gives a mixture of ring and amino alkylated products with the latter predominating. Hydrolysis of the ring A-alkylated compounds gives 4(l//)-pyridazinones (Scheme 57). Selective amino-alkylation of the 4-amino-5-aroylpyridazines is achieved via the imidate by reduction and subsequent oxidation of the alcohol <85H(23)265l>. 

Orientation of reaction is determined by more activating (or less deactivating) substituent (marked in each case below). Again, the para product can be expected to predominate where a choice of ortho or para exists. 

Predict the products of additions, oxidations, reductions, and cleavages of alkynes, including orientation of reaction (regiochemistry) and stereochemistry. 

Paillotin, G., Vermeglio, A., Breton, J. Orientation of reaction center and antenna chromophores in the photosynthetic membrane of Rhodopseudomonas viridis. Biochim. Biophys. Acta 545, 249-264 (1979)... 

Rafferty, C.N., Qayton, R.K. The orientations of reaction center transition moments in the chromatophore mcanbrane of Rhodopseudomonas sphaeroides, based (m new linear dichro-ism and photoselection measurements. Biochim. Biophys. Acta 546, 189-206 (1979)... 

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