Nucleophilic attack orbital interactions controlling

To make the task more manageable this chapter will focus specifically on the interaction between the nucleophile and a double bond and not consider in any depth subsequent steps. We will also only briefly consider reactions in which there is a preassociation or complexation of the double bond with a Lewis acid prior to nucleophilic attack. Finally we shall concentrate on conventional nucleophilic attack and not discuss mechanisms involving single electron processes. In Section II we shall examine the types of double bonds that undergo nucleophilic attack, in particular examining relative reactivity, where available, and models for explaining this order. In Section III we shall review the orbital interactions that control the approach of a nucleophile to the double bond and the associated geometrical constraints. Then in Section IV we shall consider the implications of these constraints on selective reactions. 

It is convenient to separate the total electron density at each atom into a- and 71-components. It is likely to be the 7t-density that will be important in reactions with nucleophiles, since in an orbitally controlled reaction (Chapter 1) the donor orbital of the incoming nucleophile will initially interact with the lowest vacant 7i -orbital. The overall pattern of charge alternation is repeated in both the 7t- and the a-electron densities, and nucleophiles are expected to attack at the 2- or 4-positions. This is exactly the pattern that is seen in... 

As both HOMO-LUMO attraction and HOMO-HOMO repulsion promote approach from an obtuse angle, this explains why this trajectory is so highly favored. According to calculations,47 an additional 5 kcal mol-1 are required to provoke a nucleophilic attack from an acute angle, whereas only 1 kcal mol-1 suffices to switch an electrophilic attack from acute to obtuse (the electrophilic case is controlled by HOMO-LUMO interactions alone). It is the repulsions between the occupied orbitals which displace the nucleophile from the n co plane during an addition to an aldehyde (p. 176). 

The proximity of the diffusion limit also inhibits a detailed discussion of the data in Table 7, but a significant difference to the substituent effects discussed in Section III.D.4 is obvious. Whereas the reactivities of terminal alkenes, dienes, and styrenes toward AnPhCH correlate with the stabilities of the new carbenium ions and not with the ionization potentials of the 7r-nucleophiles [69], the situation is different for the reactions of enol ethers with (p-ClC6H4)2CH+ [136]. In this reaction series, methyl groups at the position of electrophilic attack activate the enol ether double bonds more than methyl groups at the new carbocationic center, i.e., the relative activation free enthalpies are not controlled any longer by the stabilities of the intermediate carbocations but by the ionization potentials of the enol ethers (Fig. 20). An interpretation of the correlation in Fig. 20 has not yet been given, but one can alternatively discuss early transition states which are controlled by frontier orbital interactions or the involvement of outer sphere electron transfer processes [220]. 

In the reaction of the bromoacyl chloride with methanol, attack occurs at the carbonyl group with an alcohol because oxygen nucleophiles are hard nucleophiles (controlled by charge interactions). If we want to displace the a-bromo group we can use any soft (orbital-dominated) nucleophile. Triphenylphosphine PI13P is particularly important—the product is a phosphonium salt, employed in Wittig reactions and discussed in Chapter 31. 

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