Suprafacial reactions

Because a [1,5] sigmatropic rearrangement involves three electron pairs (two ir bonds and one cr bond), the orbital-symmetry rules in Table 30.3 predict a suprafacial reaction. In fact, the 1,5] suprafacial shift of a hydrogen atom across... 

Suprafacial (Section 30.6) A word used to describe the geometry of pericyclic reactions. Suprafacial reactions take place on the same side of the two ends of a -n electron system. 

Pericyclic Diels-Alder reactions are suprafacial reactions and this manner of bond formation preserves in the cycloadduct the relative stereochemistry of the substituents at Ci and C4 and at Ci and C2 of the parents diene and dienophile, respectively (Scheme 1.7). The relative stereochemistry of the substituents in the... 

If j is of the form 4n 1, suprafacial migrations are photochemically allowed. For antarafacial migrations, the restrictions are reversed. See also Antarafacial Suprafacial Reactions... 

All the other cycloadditions, such as the [4+2] cycloadditions of allyl cations and anions, and the [8+2] and [6+4] cycloadditions of longer conjugated systems, have also been found to be suprafacial on both components, wherever it has been possible to test them. Thus the trans phenyl groups on the cyclopentene 2.65 show that the two new bonds were formed suprafacially on the rrans-stilbene. The tricyclic adducts 2.61, 2.77, 2.79, and 2.83, and the tetracyclic adduct 2.82, show that both components in each case have reacted suprafacially, although only suprafacial reactions are possible in cases like these, since the products from antarafacial attack on either component would have been prohibitively strained. Nevertheless, the fact that they have undergone cycloaddition is important, for it is the failure of thermal [2+2], [4+4] and [6+6], and photochemical [4+2], [8+2] and [6+4] pericyclic cycloadditions to take place, even when all-suprafacial options are open to them, that is significant. 

A retro-Diels-Alder reaction 3.20 will allow us to look at an example involving single bonds. We have three components two a-bonds and one n-bond. Let us just do two, 3.21 and 3.22, of the several possible equivalent ways of drawing this reaction. Notice that the dashed lines correspond to the stereochemistry we must conform to—the experimentally observed counterpart to a suprafacial reaction on both components in the normal direction. 

It is generally a good idea to try to draw the dashed lines to make as many suprafacial components as you can, for it is possible to simplify the rule further when you do. If the total number of electrons involved is a (4n+2) number, the all-suprafacial reaction will be allowed, as in 3.21. This can be made to apply to a high proportion of the pericyclic reactions you will ever come across, and especially to cycloadditions, as we saw with a preliminary... 

In a [1,7] hydrogen shift, the allowed pathway is an antarafacial shift, in which the hydrogen atom leaves the upper surface at C-l, and arrives on the lower surface at C-7. This can be drawn 5.3 as a [a2s+n6a] process or 5.4 as a [a2a+K6s] process. This time it is structurally an antarafacial shift, but the developing overlap that happens to be illustrated can be described with one suprafacial and one antarafacial component either way round. It is helpful to draw as many suprafacial components as possible, i.e. preferring 5.1 to 5.2, since the structurally suprafacial reaction is then also described with suprafacial overlap developing. Similarly it is helpful to draw 5.3 rather than 5.4, since that makes the antarafacial component the triene system, from one side of which to the other the antarafacial shift of the hydrogen is taking place. 

The reaction is symmetry-allowed when it is suprafacial on all three components, but there are two reasonably accessible transition structures for an all-suprafacial reaction, chair-like 5.52 and boat like 5.53, both of which are [ g+ g+ J. In the reaction 5.48 - 5.49, it must be boat-like to give two cis double bonds in the product, but this is probably constrained by the... 

In the event that two substituents at different carbon atoms in the aziridine cycle are hydrogen atoms (for example, A and C in Scheme 1.33), the ylides being formed are called S-shaped, W-shaped, or U-shaped [115] (Scheme 1.34). The reaction of dipolar cycloaddition of S-shaped ylides forms adducts with trans orientation of the protons, while W- and U-shaped ylides lead to cis adducts (for suprafacial reactions). 

The C-H bond is parallel with the p orbitals of the ene so that the orbitals that overlap to form the new k bond are already parallel. The two molecules approach one another in parallel planes so that the orbitals that overlap to form the new o bonds are already pointing towards each other. Because the electrons are of two types, n and O, we must divide the ene into two components, one K2 and one a2. We can then have an all-suprafacial reaction with three components. 

Now it works In fact, extension of this reasoning to other electrocyclic reactions tells you that they are all allowed—provided you choose to make the conjugated system react with itself supra-facially for (An + 2) K systems and antarafaciaWy for (4n) n systems. This may not seem particularly informative, since how you draw the dotted line has no effect on the reaction product in these cases. But it can make a difference. Here is the electrocyclic ring closure of an octatriene, showing the product from (a) suprafacial reaction and ( ) antarafacial reaction. 

We have drawn little green arrows on the two diagrams to show how the methyl groups move as the new o bonds form. For the allowed suprafacial reaction of the 6k electron system they rotate in... 

If the dashed, bold or coloured lines are not those for an all-suprafacial reaction, as on the right-hand side of Fig. 6.7, for example, all is not lost—simply do the sum to find out whether the drawing corresponds to an allowed reaction or not. The all-suprafacial drawing is no better than the other representations, but it is a quick way to arrive at a drawing showing that a (An + 2) reaction is reasonable and allowed, and it works for a very high proportion of pericyclic reactions. 

However, the [1,5]-sigmatropic rearrangement is a concerted suprafacial reaction because the HOMO and LUMO (of migrating group, which is H, and of polyene component, pentadienyl) can interact in a suprafacial process (Fig. 8.53). The six electrons involved are considered to occupy the hydrogen HOMO (two) and the (two) and F2 (two) of pentadienyl. The LUMO of pentadienyl is Fs. 

The ene reaction of an azodicarboxylate ester was first observed in 196218-19 as a process competing with cycloaddition to dienes, but little additional work has been done on the ene reaction with simple alkenes. An elegant study in 1976 provided evidence for a concerted, suprafacial reaction between dimethyl azodicarboxylate and (S)-(Z)-l-deutero-4-methyl-l-phenyl-2-pentene (15) based on the direction and high level of chirality transfer observed20. 

Obviously, [2,3]-sigmatropic rearrangements of sulfur-containing systems comprise a class of reactions with a considerable constitutional scope. But the configurational consequences of the concerted suprafacial reaction mode are even more exciting, both from a stereochemical point of view and for synthetic reasons. 

Essentially, a suprafacial-suprafacial or an antarafacial-antarafacial cycloaddition is equivalent to a concerted syn addition. A suprafacial-antarafacial or an antarafacial-suprafacial process is equivalent to a concerted anti addition. The Diels-Alder reaction is suprafacial for both components, so that the stereochemical relationships among the substituents are maintained in the product. In Example 6.6, suprafacial addition to the dienophile component means that the two carbomethoxy groups that are cis in the starting material also are cis in the product. Suprafacial reaction at the diene component leads to a cis orientation of the two methyl groups in the product. 

There are four possible products two arise from suprafacial reaction and two more from antarafacial reaction. 

In agreement with the theoretical prediction of suprafacial reaction, compounds 6-12 and 6-13 were produced rather than 6-14 and 6-15. Because of the favorable geometry for suprafacial migration, there are many examples of thermal [1,5] sigmatropic shifts, which occur with ease. 

The [1,5] shift would be a suprafacial reaction. The [1,7] shift must be antarafacial in the v component to be thermally allowed. The proton, located at carbon A in 6-20, moves from the bottom side of the six-mem-bered ring to the top side of the w system at the other end. In general, the antarafacial process, necessary for a thermal [1,7] sigmatropic shift, occurs with ease, especially when compared to the antarafacial process that would be required for a thermal [1,3] sigmatropic shift. 

There is a special kind of site-selectivity which has been called periselectivity. When a conjugated system enters into a reaction, a cycloaddition for example, the whole of the conjugated array of electrons may be mobilized, or a large part of them, or only a small part of them. The Woodward-Hoffmann rules limit the total number of electrons (to 6, 10, 14 etc. in all-suprafacial reactions, for example), but they do not tell us which of 6 or 10 electrons would be preferred if both were feasible. Thus in the reaction of cyclopentadiene (355) and tropone (356), mentioned at the beginning of this book, there is a possibility of a Diels-Alder reaction, leading to 354, but, in fact, an equally allowed, ten-electron reaction is actually observed,121 namely the one leading to the adduct (357). The product is probably not thermodynamically much preferred to the... 

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