Suprafacial pathway

In contrast with the thermal process, photochemical [2 + 2] cycloadditions me observed. Irradiation of an alkene with UV light excites an electron from i /, the ground-slate HOMO, to which becomes the excited-slate HOMO. Interaction between the excited-state HOMO of one alkene and the LUMO of the second alkene allows a photochemical [2 + 2j cycloaddition reaction to occur by a suprafacial pathway (Figure 30.10b). 

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. 

Both Cope and Claisen rearrangements involve reorganization of an odd number of electron pairs (two tt bonds and one a bond), and both react by suprafacial pathways (Figure 30.13). 

The occurrence of a 1,7-photochemical shift of H in this compound does not, of itself, establish directly that this shift proceeds via a suprafacial pathway. The relatively rigid cyclic structure of (47) must, however, rule out the possibility of the shift having proceeded via the antarafacial route. 

So we find that both Cope and Claisen rearrangements are [3, 3] sigmatropic rearrangements and proceed by suprafacial-suprafacial pathway. 

Alkyl shift is evident in the Cope rearrangement. A Cope rearrangement is a [3,3] sigmatropic rearrangement of a 1,5-diene. This reaction leads to the formation of a six-membered ring transition state. As [3,3] sigmatropic rearrangements involve three pairs of electrons, they take place by a suprafacial pathway under thermal conditions. 

The stereochemistry of the ene reaction 6.90 + 6.91 —> 6.93 is such that the hydrogen atom delivered to the enophile 6.91 leaves from the same surface of the ene 6.90 as the surface to which the C—C bond is forming, and the hydrogen atom is delivered to the same surface of the enophile as the forming C—C bond 6.92, so that both components are reacting suprafacially. The full stereochemistry is not proved in this example, because neither the methyl group, C-l, nor the (3 carbon has any stereochemical label, but the all-suprafacial pathway provides a reasonable explanation for the relative stereochemistry set up between the a carbon and C-3. 

Several reactions in organometaUic chemistry also appear to contravene the rule, but which can be explained in a somewhat similar way. Hydrometallation [5.45, see (Section 5.1.3.4) page 162], carbometallation, metallo-metallation, and olefin metathesis reactions are all stereospecifically suprafacial [2 + 2] additions to an alkene or alkyne, for which the all-suprafacial pathway is forbidden. Hydroboration, for example, begins with electrophilic attack by the boron atom, but it is not fully stepwise, because electron-donating substituents on the alkene do not speed up the reaction as much as they do when alkenes are attacked by electrophiles. Nevertheless, the reaction is stereospecifically syn—there must be some hydride delivery more or less concerted with the electrophilic attack. The empty p orbital on the boron is the electrophilic site and the s orbital of the hydrogen atom is the nucleophilic site. These orbitals are orthogonal, and so the addition 6.126 is not pericyclic. 

The 1,3-dipolar cycloaddition reactions proceed via a concerted suprafacial pathway, which ensures the complete transfer of stereochemical information from the substrates to the cycloadducts. Thus, the stereochemistry of the alkene is retained in the product [40] (Scheme 10.15). With these reactions, up to four stereogenic centers can be formed in one step. Stereodifferentiating groups can be introduced either in the dipole or in the dipolarophile. 

Hydrogen shifts can take place if the reaction is carried out under photochemical conditions because the HOMO is symmetric under photochemical conditions, which means that hydrogen can migrate by a suprafacial pathway (Table 29.4). 

Sigmatropic migrations of hydrogen are well known. They involve three pairs of electrons, so they take place by a suprafacial pathway under thermal conditions. 

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