Suprafacial cycloaddition

In general, stereochemical predictions based on the Alder rule can be made by aligning the diene and dienophile in such a way that the unsaturated substituent on the dienophile overlaps the diene n system. The stereoselectivity predicted by the Alder rule is independent of the requirement for suprafacial-suprafacial cycloaddition, since both the endo and exo transition states meet this requirement. 

The selection rules for cycloaddition reactions can also be derived from consideration of the aromaticity of the transition state. The transition states for [2tc -f 2tc] and [4tc -1- 2tc] cycloadditions are depicted in Fig. 11.11. For the [4tc-1-2tc] suprafacial-suprafacial cycloaddition, the transition state is aromatic. For [2tc -F 2tc] cycloaddition, the suprafacial-suprafacial mode is antiaromatic, but the suprafacial-antarafacial mode is aromatic. In order to specify the topology of cycloaddition reactions, subscripts are added to the numerical classification. Thus, a Diels-Alder reaction is a [4tc -f 2 ] cycloaddition. The... 

Note that both suprafacial and antarafacial cycloadditions are symmetry-allowed. Geometric constraints often make antarafacial reactions difficult, however, because there must be a twisting of the it orbital system in one of the reactants. Thus, suprafacial cycloadditions are the most common for small tt systems. 

Cycloaddition reactions are those in which two unsaturated molecules add together to yield a cyclic product. For example, Diels-AJder reaction between a diene (four tt electrons) and a dienophile (two tt electrons) yields a cyclohexene. Cycloadditions can take place either by suprafacial or antarafacial pathways. Suprafacial cycloaddition involves interaction between lobes on the same face of one component and on the same face of the second component. Antarafacial cycloaddition involves interaction between lobes on the same face of one component ancl on opposite faces of the other component. The reaction course in a specific case can be found by looking at the symmetry of the HOMO of one component and the lowest unoccupied molecular orbital (LUMO) of the other component. 

Several oxidizing reagents react with alkenes under mild conditions to give, as the overall result, addition of hydrogen peroxide as HO—OH. Of particular importance are alkaline permanganate (MnO40) and osmium tetroxide (0s04), both of which react in an initial step by a suprafacial cycloaddition mechanism like that postulated for ozone. 

An anomalous pair of cheletropic reactions looking like forbidden [2+2] suprafacial cycloadditions ... 

Draw the frontier orbital interactions for the all-suprafacial cycloaddition of an allyl anion to an alkene and for an allyl cation to a diene showing that they match, and show that the alternatives, allyl cation with alkene and allyl anion with diene are symmetry-forbidden. 

The preservation ofalkene geometry in these concerted cycloadditions implies a mode of reaction in which, for each alkene unit, the two new bonds are formed to the same face of the Jt-bond. This mode is called suprafacial-suprafacial cycloaddition, and is one of... 

Fbr a successful cycloaddition to take place, the terminal a lobe of the two reactants must have the correct symmetry for bunding to occur This can happen in either of two ways, demited suprafociat and antarafaeiat. Suprafacial cycloadditions take place when a bonding interaction occurs between lobes on the same face of one reactant nd lobes on the Mime face of the other reactant (Figure 30.8>. 

Similarly, it is clear from the analysis of FMO for [6s-f2s] suprafacial cycloaddition that supra-supra cycloaddition is symmetry forbidden under thermal conditions as shown in Fig. 8.31. 

The discovery of the [47t -i- 2tu] cycloaddition reaction by Otto Diels (Nobel Prize, 1950) and Kurt Alder (Nobel Prize, 1950), a landmark in synthetic organic chemistry, permits the regio- and stereoselective preparation of both carbocyclic and heterocyclic ring systems. Its application can result simultaneously in an increase of (1) the number of rings, (2) the number of asymmetric centers, and (3) the number of functional groups. The reaction controls the relative stereochemistry at four contiguous centers. The Diels-Alder reaction can be depicted as a concerted -1- (suprafacial) cycloaddition. While depicted as a concerted process, the reaction has been proposed to proceed in a nonsynchronous manner via an unsymmetrical transition state. °... 

Figure 30.8 (a) Suprafacial cycloaddition occurs when there is bonding between lobes on the same face of one reactant and lobes on the same face of the other reactant, (b) Antarafacial cycloaddition occurs when there is bonding between lobes on the same face of one reactant and lobes on opposite faces of the other, which requires a twist in one 77- system. 

We have established earlier in the chapter that there will be favourable Frontier Orbital HOMO-LUMO interactions when two molecules approach for a cycloaddition reaction if there are 4n + 2 electrons involved in a fully suprafacial reaction, or 4n electrons if there is an antarafacial component. For delocalization of electrons in the transition state, the fully suprafacial cycloaddition reaction will result in a continuous cyclic overlap of atomic orbitals in the transition state without a phase change, for which 4n + 2 electrons will give aromatic stabilization. For a cycloaddition with one antarafacial component, the cyclic overlap of orbitals will give a Mobius system for which 4n electrons will provide stabilization. Thus the two approaches, Frontier Orbitals and the Aromatic Transition State will always be in agreement favourable... 

Here are several examples of photochemical reactions, for which the thermal equivalents were forbidden. In most cases the corresponding thermal reactions simply did not occur, like the [2 +2] suprafacial cycloadditions 8.24 -+ 8.25,1097 the [4 +4] suprafacial cycloadditions 8.26 — 8.271099 and 8.28 — 8.29,1100 and the 1,3-suprafacial sigmatropic rearrangement 8.32 —> 8.33.1101 In others, the thermal reactions have a different stereochemistry, as in the antarafacial cheletropic extrusions of sulfur dioxide 8.30 > 8.31,1102 or a different regiochemistry, as in the 1,7-suprafacial sigmatropic rearrangement 8.34 > 8.35.1103 1104... 

The selection rules for [tt4 + tt2 ] and other cycloaddition reactions can also be derived from consideration of the aromaticity of the TS3 In this approach, the basis set p orbitals are aligned to correspond with the orbital overlaps that occur in the TS. The number of nodes in the array of orbitals is counted. If the number is zero or even, the system is classified as a Htickel system. If the number is odd, it is a Mobius system. Just as was the case for ground state molecules (see p. 716), Htickel systems are stabilized with 4 + 2 electrons, whereas Mobius systems are stabilized with 4n electrons. For the [tt4 + tt2] suprafacial-suprafacial cycloaddition the transition state is aromatic. 

Pericyclic reactions in which p-orbitals at the ends of the rt-component of each system overlap and form the new a-bonds on the same surface are called suprafacial cycloaddition. Almost aU pericyclic cycloaddition reactions are suprafacial on both systems and thus the stereochemistry is maintained due to their concerted nature. This specification is usually made by placing a suitable subscript (s or a) after the number referring to the Tr-component. For example,... 

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