Antarafacial electrons

To apply the rule we first draw the orbital picture of the reactants and show a geometrically feasible way to achieve overlap. Then the (4q + 2) suprafacial electrons and 4r antarafacial electrons of the components is counted. If the total is an odd number, the reaction is thermally allowed. Let us take the hypothetical cycloaddition of ethene to give cyclobutane. 

The direct connection of rings A and D at C l cannot be achieved by enamine or sul> fide couplings. This reaction has been carried out in almost quantitative yield by electrocyclic reactions of A/D Secocorrinoid metal complexes and constitutes a magnificent application of the Woodward-Hoffmann rules. First an antarafacial hydrogen shift from C-19 to C-1 is induced by light (sigmatropic 18-electron rearrangement), and second, a conrotatory thermally allowed cyclization of the mesoionic 16 rc-electron intermediate occurs. Only the A -trans-isomer is formed (A. Eschenmoser, 1974 A. Pfaltz, 1977). 

How can we predict whether a given cycloaddition reaction will occur with suprafacial or with antarafacial geometry According to frontier orbital theory, a cycloaddition reaction takes place when a bonding interaction occurs between the HOMO of one reactant and the LUMO of the other. An intuitive explanation of this rule is to imagine that one reactant donates electrons to the other. As with elec-trocyclic reactions, it s the electrons in the HOMO of the first reactant that are least tightly held and most likely to be donated. But when the second reactant accepts those electrons, they must go into a vacant, unoccupied orbital—the LUMO. 

Thermal and photochemical cycloaddition reactions always take place with opposite stereochemistry. As with electrocyclic reactions, we can categorize cycloadditions according to the total number of electron pairs (double bonds) involved in the rearrangement. Thus, a thermal Diels-Alder [4 + 2] reaction between a diene and a dienophile involves an odd number (three) of electron pairs and takes place by a suprafacial pathway. A thermal [2 + 2] reaction between two alkenes involves an even number (two) of electron pairs and must take place by an antarafacial pathway. For photochemical cyclizations, these selectivities are reversed. The general rules are given in Table 30.2. 

Thermal reactions with an ven number of electron pairs are Conrotatory or Antarafacial. 

A change either from thermal to photochemical or from an even to an odd number of electron pairs changes the outcome from conrotatory/antarafacial to dis-rotatory/suprafacial. A change from both thermal and even to photochemical and odd causes no change because two negatives make a positive. 

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. 

The stereochemistry of any pericyclic reaction can be predicted by counting the total number of electron pairs (bonds) involved in bond reorganization and then applying the mnemonic "The Electrons Circle Around. " That is, thermal (ground-slate) reactions involving an even number of electron pairs occur with either conrotatory or antarafacial stereochemistry. Exactly the opposite rules apply to photochemical (excited-state) reactions. 

Antarafacial (Section 30.6) A pericyclic reaction that takes place on opposite faces of the two ends of a tt electron system. 

As expected, the Mobius-Hiickel method leads to the same predictions. Here we look at the basis set of orbitals shown in G and H for [1,3] and [1,5] rearrangements, respectively, A [1,3] shift involves four electrons, so an allowed thermal pericyclic reaction must be a Mobius system (p. 1070) with one or an odd number of sign inversions. As can be seen in G, only an antarafacial migration can achieve this. A [1,5] shift, with six electrons, is allowed thermally only when it is a Hiickel system with zero or an even number of sign inversions hence it requires a suprafacial migration. 

The reaction via a planar transition state is n2s + n2s. Here only one of the two new C—C bonds can be formed. This will raise its activation energy impossible to be reached. So there are two (4q + 2) electron suprafacial components and no antarafacial component. Since the total number of counting components is two, an even number, the reaction is thermally disallowed. 

The symbols ji, o and (0 are given respectively to the n systems, o bonds and lone p orbitals which participate in the transition state and the symbols (s) and (a) are indicated for their suprafacial and antarafacial use. The notation is completed by the number of electrons supplied by each component. Thus n2s denotes a two electron n system used in a suprafacial way. woa indicates a vacant p orbital used in an antarafacial way and so on. 

For transitions involving An + 2 and An electron systems, the pictures will be as fallows. The First leads to Suprafacial overlap while the second to antarafacial overlap. 

The linear cheletropic reactions in which the polyene is a suprafacial component (i.e., involving disrotatory motion of the termini) will be allowed if it has a total of (4n + 2) electrons. But linear cheletropic reactions in which the polyene is an antarafacial component (i.e., involving conrotatory movement of the termini) are allowed if it has a system of 4n electrons. 

The outcome of all this for photochemical sigmatropic shifts is that those most commonly observed are of order (1.3) or (1.7) these involve 4 or 8 electrons, respectively, and occur in a suprafacial manner. Examples of photochemical 1.3-shifts of hydrogen are found for monoalkenes (2.25) and for conjugated dienes 2.26). In the case of dienes a 1,3-shift is favoured over a 1,5-shift, because for the latter to occur photochemically it would have to take place in an antarafacial manner. Note that in both examples the direction of... 

Anomeric effect, 82, 310-311, 305 Antarafacial, 163 examples, 164 sigma bonds, 167 Anti-Bredt olefin, 102 Approximations of MO theory Born-Oppenheimer, 22 Hartree-Fock, 222 Huckel, 35, 86 independent electron, 35 LCAO, 229 nonrelativistic, 22 SHMO, 87... 

Loss of sulfur from these species would give the carbodiimide, whereas addition of alkenes would give the cycloadducts. The authors infer that this intermediate cannot add conceitedly to alkenes in the supra-supra fashion because it would involve a four-electron transition state, but that a stepwise addition would hardly rationalize the stereospecificity observed this suggests that the thiaziridine (or the 1,3-dipole) participates in an antara facial reaction.63 However, according to recent discussions antarafacial addition is seldom observed68 and stereospecificity need not be lost in a stepwise process.69... 

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