Curly arrows cycloadditions

Crudely, but adequately for now, we may state rule governing which cycloadditions can take place and which not. A thermal pericyclic cycloaddition is allowed if the total number of electrons involved can be expressed in the form (4n+2), where n is an integer. If the total number of electrons can be expressed in the form 4n it is forbidden. Another way of saying the same thing is that reactions with an odd number of curly arrows are allowed and those with an even number are forbidden. This rule needs to be qualified, as we shall see shortly, and in due course in Chapter 3 made more precise, along with the rules for all the other kinds of pericyclic reaction, in one all-encompassing rule. For now, we need to introduce the rule for photochemical pericyclic cycloadditions. 

This ring opening is clearly pericyclic—the electrons go round in a ring, and the curly arrows could t>e drawn either way—but it is neither a cycloaddition (only one 7t system is involved) nor a sigmatropic rearrangement (a a bond is broken rather than moved). It is, in fact, a member of the third and last kind of pericyclic reaction, an electrocyclic reaction. 

Let us go through the same steps for a symmetry-forbidden reaction, the ln s+A] cycloaddition 6.141. We first draw the reaction and put in the curly arrows—the orbitals are evidently the n and n of each of the n bonds. There are two symmetry elements maintained this time—a plane like that in the Diels-Alder reaction, bisecting the n bonds, but also another between the two reagents, which reflect each other through that plane. 

In order to answer the question first posed in Chapter 1 and repeated above, we begin by ignoring the substituents and counting only those parts of the conjugated system directly involved in the reaction. (We shall return to the crucial role of the substituents later in the chapter.) Thus the Diels-Alder reaction is simplified to that of butadiene reacting with ethylene the former component has four rc-electrons and the latter two, and these are the only electrons directly involved, as we can see from the curly arrows. Such a reaction is called a [4 + 2] cycloaddition. We now examine the signs of the coefficients of the frontier orbitals on the atoms which are to become bonded (Fig. 4-1). We are not yet concerned with the magnitude of the coefficients of the frontier orbitals, and therefore in this section all orbitals are drawn the same size, so as not to... 

Palladium-catalysed processes typically utilise only 1-5 mol% of the catalyst and proceed through small concentrations of transient palladium species there is a sequence of steps, each with an organopalladium intermediate, and it is important to become familiar with these basic organopalladium processes in order to rationalise the overall conversion. Concerted, rather than ionic, mechanisms are the rule, so it is misleading to compare them too closely with apparently similar classical organic mechanisms, however curly arrows can be used as a memory aid (in the same way as one may use them for cycloaddition reactions), and this is the way in which palladium-catalysed reactions are explained in the following discussion. (For convenience, an organometallic component can be referred to as the nucleophilic partner and the halide as the electrophilic partner, but this should not necessarily be taken to imply reactivity as defined in classical chemistry. Also, references to the halide should be understood to include all related substrates, such as triflates.)... 

We might reasonably ask why these 4 + 2-cycloadditions are favorable, when 2 + 2-cycloadditions do not proceed under thermal conditions. We could write apparently sensible curly arrows for 2 -I- 2-cydoadditions, but they don t readily occur on heating (Figure 11.79). 

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