Suprafacial processes

Dihydrothiophene-1,1-dioxides (42) and 2,17-dihydrothiepin-1,1-dioxides (43) undergo analogous 1,4 and 1,6 eliminations, respectively (see also 17-38). These are concerted reactions and, as predicted by the orbital-symmetry rules (p. 1067), the former is a suprafacial process and the latter an antarafacial process. The rules also predict that elimination of SO2 from episulfones cannot take place by a concerted mechanism (except antarafacially, which is unlikely for such a small ring), and the evidence shows that this reaction occurs by a non-concerted pathway.The eliminations of SO2 from 42 and 43 are examples of cheletropic reactions, which are defined as reactions in which two a bonds that terminate at a single atom (in this case the sulfur atom) are made or broken in concert. 

We have been discussing a cycloaddition where bonds are made or broken on the same face (suprafacial process). The alternative process is one where the bonds are made or broken on opposite faces of the reacting system (antarafacial) ... 

So in a [1, 5] suprafacial thermal rearrangement, there is retention of configuration but in a [1, 3] suprafacial process, there is inversion of configuration. 

Examples of shifts of higher order than [1, 5] are relatively rare. But an example of a thermal suprafacial process involving [1, T shift and inversion of the migrating group is afforded by bicyclononatrienes. 

This system covers concerted reactions of the n electron systems on two reactants to form new a bonds yielding carbocyclic rings with a single unsaturation. If the reaction follows the rule of maximum orbital overlap, then it is a suprafacial, suprafacial process and is termed a [,r4 + r t] reaction. By the Woodward-Hoffmann rules this is a symmetry-allowed thermal reaction [13]. 

In conclusion, the ene reaction of singlet oxygen with simple alkenes is a highly stereospecific suprafacial process. It proceeds via an irreversible formation... 

This reaction is a reverse [4 + 2] cycloaddition. The reacting orbitals have the correct symmetry for the reaction to take place by a favorable suprafacial process. 

This thermal sigmatropic rearrangement is a suprafacial process since five electron pairs are involved in the reaction. 

A suprafacial process is one in which the bonds made or broken lie on the same face of the system undergoing reaction. When the newly formed or broken bonds lie on the opposite faces of the reacting systems, it is known as an antarafacial process. 

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. 

Because in the ground state the HOMO is 4/3, the hydrogen shift is controlled by the symmetry of the vj 3 of the pentadienyl radical. The v) 3 has similar signs on the terminal lobes i.e. it is symmetrical. Thus, the [ 1,5] -hydrogen shift is thermally allowed and occurs in a suprafacial process. This involves a transition state in which the C-1 and C-5 orbitals overlap with Is hydrogen orbital. This shift is both symmetry allowed and geometrically favourable, as shown in Fig. 8.55. 

Further biosynthetic studies by Cane et have shown that the conversion of farnesyl pyrophosphate (15) into nerolidyl pyrophosphate (16) proceeds by a net syn (suprafacial) process and that the subsequent cyclization to cyclonerodiol (17) occurs in a trans manner (Scheme 4). This careful piece of work was achieved by incorporation studies with doubly labelled nerolidol and mevalonate precursors and then by ascertaining the chirality of the acetate derived by Kuhn-Roth oxidation through enzymatic conversion (malate synthase/fumarase incubation) into labelled malate. In a subsequent series of experiments with labelled precursors, Cane et have confirmed that (i) only the C-7 hydroxy-group in cyclonerodiol is derived from water whereas the C-3 hydroxy-group is derived... 

The usually high stereoselectivity on formation of a new double bond or a stereogenic center is due to the stereodifferentiating ability of envelope-like transition states of this concerted suprafacial process. The envelope conformation of cyclopentane serves as a model for the transition state of the [2,3] sigmatropic rearrangement6,7. 

Diels-Alder reactions of Ceo are generally believed to proceed via a thermally allowed concerted (suprafacial) process or a photochemical concerted (antarafacial) process [283-286]. However, an alternative stepwise (open-shell) mechanism for the Diels-Alder reaction has recently merited increasing attention [287-294], Along this line several reports describe an electron transfer with the formation of radical ion pairs as primary step of the Diels-Alder reactions, followed by a stepwise bond formation [295-301], The photochemical Diels Alder reaction of Ceo with an-... 

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. 

In the [1,5] shift, brr electrons are involved. In this case, bonding can be maintained throughout a suprafacial process, and this type of reaction occurs with ease. 

Under thermal conditions, the [2+2]-cycloaddition of olefins is symmetrically forbidden, according to the Woodward-Hoffmann rules. However, under photochemical conditions, [2+2]-cycloadditions become a suprafacial process for both components The orbital geometry of the interacting orbitals is equal and therefore the entire reaction is symmetrically allowed. 

Sigmatropic rearrangement is termed the 1,3-shift of an allylic atom pair ——Y with a concomitant 1,2-shift of an electron pair from Y to X (Scheme 1). It is symmetry allowed as a concerted suprafacial process. 

The molecular orbitals of the 7r-allyl fragment left by the migrating hydrogen atom are given in Pig. 10. The highest occupied molecular orbital ( 2) in the suprafacial process is antisymmetric and nonbondins at the transition state (XXXVII). 

HOMO of ethylene. The dashed lines represent a suprafacial process on this component. 

All the reactions we have looked at so far involve only the geometrically easy, and hence commonly observed, suprafacial processes. If we restrict ourselves to all-suprafacial processes, we may note that the allowed reactions involve an aromatic (4n + 2) number of electrons, and the forbidden reactions an antiaromatic (4n) number of electrons. (The number of electrons involved is easily counted it is twice the number of curly arrows.) This is the simplest version of the Woodward-Hoffmann rules it was first pointed out by Evans126 in 1939 and recalled many years later by Dewar.127... 

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