Electrophilic addition orbital interactions

Alkyl substituents accelerate electrophilic addition reactions of alkenes and retard nucleophilic additions to carbonyl compounds. The bonding orbital of the alkyl groups interacts with the n bonding orbital, i.e., the HOMO of alkenes and raises the energy (Scheme 22). The reactivity increases toward electron acceptors. The orbital interacts with jt (LUMO) of carbonyl compounds and raises the energy (Scheme 23). The reactivity decreases toward electron donors. 

The rr bond of an alkene results from overlapping of p orbitals and provides regions of increased electron density above and below the plane of the molecule. These electrons are less tightly bound than those in the a bonds, so are more polarizable and can interact with a positively charged electrophilic reagent. This forms the first part of an electrophilic addition, in which the electrons are used to form a a bond with the electrophile and leave the other carbon of the double bond electron deficient, i.e. it becomes a carbocation. This carbocation is then rapidly captured by a nucleophile, which donates its... 

Stereoselectivity found for electrophilic additions to norbornene and l-methoxy-2-cyclohexene is believed to originate from secondary orbital interactions rather than from orbital distorsion at the reaction centre144. Rather different results have been obtained from an ab initio MO study of the norbornene hydroboration145. 

PMB calculations indicate that jr-facial diastereoselection in the reaction of 2,3 -endo-disubstituted 7-methylenenorbornanes 285 with electrophiles such as (AcO)2Hg or I+ is determined by electrostatic asymmetry, favouring the electron-richer awti-facial attack 285 -> 286 (with R = F or CC Me)415. In contrast, hydroboration is controlled by orbital interactions giving the syw-addition product 287415. This is an alternative interpretation of the experimental results reported previously30. 

An electrophilic addition generally proceeds in two steps.126 In the first, the most important interaction occurs between the LUMO of the electrophile and the HOMO of the alkene. Orbital overlap is largest when the electrophile attacks at the center of the n bond ... 

Addition reactions with Mode B are not popular, but are occasionally useful. Eq. 4.7 indicates the reaction of ethyl bromoacetate and sugar vinyl ether with Bu3SnH initiated by AIBN. The ethyl acetate radical is electrophilic and it reacts with electron-rich sugar vinyl ether through SOMO-HOMO orbital interaction to form a ribosyl anomeric radical, as shown below. Then, the formed ribosyl anomeric radical abstracts... 

The carbon-silicon bond has two important effects on the adjacent alkenc. The presence of a high-energy filled CT orbital of the correct symmetry to interact with the n system produces an alkene that is more reactive with electrophiles, due to the higher-energy HOMO, and the same ff orbital stabilizes the carbocation if attack occurs at the remote end of the alkene. This lowers the transition state for electrophilic addition and makes allyl silanes much more reactive than isolated alkenes. 

Figure 6.1. HOMO-LUMO orbital interactions in the electrophilic addition to an bond.

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