Electropositive substituents

The equilibrium situation for simple substituted 2-ulkenyl alkali metal derivatives can be estimated by a rule of thumb electron-accepting and electropositive substituents ( ) prefer the exo position, but electron-donating and electronegative substituents ( ), including alkyl groups, tend to occupy the endo position. With increasing steric demand of the substituent, the exoisomer becomes more favored. 

Here the nucleophilicity of the dienes was evaluated from the electrostatic potential the energy accounts only for Coulombic interactions, between a test electrophile (a proton) and the dienes. The electrophilicity of the dienophiles was evaluated from electrostatic potential between a test nucleophile (a hydride) and the dienophiles. They stated that, in the case of the reaction of 5-substituted cyclopentadienes, the approach of dienophile (an electrophile) will occur at the syn side of the substituents having lone pairs, and at the anti side of electropositive substituents. There are some... 

It has been reported that the degree of dissociation of unsymmetri-cal hexaarylethanes is usually somewhat less than the average of the related symmetrical ethanes.21-22 Unfortunately, the available ethanes do not include any with extremely electronegative and electropositive substituents in opposite parts of the ethane molecule. A marked difference in electronegativity of the two radicals might increase the strength of the central bond by increasing its ionic character. 

Atomic s character concentrates in orbitals directed toward electropositive substituents... 

In most of the cases these are ethylene derivatives which have electropositive substituents. The order of reactivity of vinyl monomers is as under ... 

The use of organic-solvent-free systems can be applied to the cyanohydrin synthesis of a wide range of acetophenone derivatives (Table 8.2) electronegative substituents (e.g. fluorine) facilitate high conversions and enantiomeric excess of the product, whereas electropositive substituents (e.g. methoxy-) result in low to no conversion into the corresponding cyanohydrins. 

Bisketenes (11) can decarboxylate and then ring close to give cyclopropenones (12) subsequent further decarboxylation yields alkynes (13). A theoretical study shows that the first reaction is favoured by electronegative substituents, whereas electropositive substituents favour the second. The calculations do not indicate conclusively whether cyclopropenone formation is concerted, or proceeds via a syn-ketenylcarbene (14). 

First of all, the introduction of an electropositive substituent onto the carbene center increases its nucleophilicity. In particular, the effect of a strong 7t-donating and electropositive ylide substituent is efficient as can be seen in the case of amino (ylide) carbenes and of cyclic vinylidenephosphorane type carbenes. Acyclic structures also increase the HOMO level but they also significantly diminish the S/T energy gap. 

Electropositive substituents stabilize the three-membered ring of borirene, whereas ir-donors (NH2, OH) coplanar with the ring lead to destabilization (85MI4). 

The introduction of an electropositive substituent on the same carbon atom as the geminally coupled hydrogens. 

Hie phosphorus atom is in an approximately tetrahedral environment using four er bonds of approximately sp3 character. We should expect the more electronegative oxygen atoms to bond to s-poor orbitals on the phosphorus and the two oxygen atoms in the ring do attract hybridizations of about 20% s.40 The most electropositive substituent on the phosphorus is the bromine atom and Bent s rule would predict an s-rich... 

If the cyclopropyl group is compared with a 2-propyl group, then electropositive substituents (er-donor substituents such as Li) stabilize the ring while strongly electronegative groups such as F, OH, etc. destabilize the ring. 

Referring to the effect of electron releasing groups, they suggest that these depress the rate of solvolysis merely because in this SN2 reaction bond-forming is more important than bond-breaking, and electropositive substituents would be expected adversely to affect the bondforming requirements. 

The geometry of exceptionally long or short Si—P bond lengths is given in Table 14 for compounds 143-145 and 146-148, respectively. The lengthening may be attributed to strain and steric congestion in compounds 144 and 145 and to the reduction of polarity in 143. An electropositive substituent on the phosphorus atom will reduce the Si—P bond length as found in 147 and 148. 

The equilibrium between the enol (5a) and the keto (5b) forms depends upon the nature of R, electropositive substituents destabilizing the keto form 5b. This destabilization is due to the adjacent carbon having some carbocationic character, as can be seen in the resonance form 5c. 

Inductive Effects Electronegative substituents increase the n(Si) - 3p(Si) transition energy and thus induce a blue-shift. The opposite is true for electropositive substituents. 

Tetrasubsubstituted derivatives 6 are easily obtained from N P Meg, and n-electron calculations show that antipodal disubstitution is to be expected for electropositive substituents, as found for R = Me3Si, Me (7). Monocarbanions are not obtained, principally because the intra-ring exothermic electrostatic interactions are reduced less by charge transfer from the second ylidic group than the first if the added base is strong enough to remove the first proton, it will also remove the second. 

The s-Characler Ride. The rule that the s-character of an atom tends to concentrate in orbitals the atom uses in bonds toward electropositive substituents 64>65> follows qualitatively from simple electrostatic considerations, Fig. 11. 

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