Miscellaneous

The control elements that did not find mention in the earlier chapters are dealt with here. The prominent among these elements are spiroconjugation, periselectivity in pericyclic reactions, torquoselectivity in conrotatory-ring openings, ambident nucleoph

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Miscellaneous

Abstract The control elements that did not find mention in the earlier chapters are dealt with here. The prominent among these elements are spiroconjugation, periselectivity in pericyclic reactions, torquoselectivity in conrotatory-ring openings, ambident nucleophiles and electrophiles, a-effect in nucleophilicity, carbene addition to 1,3-dienes, Hammett’s substituent constants, Hammond postulate, Curtin– Hammett principle, and diastereotopic, homotopic, and enantiotopic substituents.









Keywords Spiroconjugation Periselectivity Carbenes Ketenes Torquoselectivity Ambident nucleophiles and electrophiles a-effect Hammett’s substituent constants Hammond postulate Curtin–Hammett principle Diastereotopic Homotopic Enantiotopic substituents

















1 Spiroconjugation When one conjugated system is held at a right angle to another conjugated system, as in the case of a spirostructure, the p orbitals of one conjugated system can overlap with those of the other conjugated system, as indicated by the red curved lines on the front lobes and green curved lines on the rear lobes in structure 1, with a small overlap integral. In the situation when the symmetries match, the interaction leads to two new orbitals, one raised and the other lowered in energy, in the usual fashion that we have previously learnt elsewhere. However, when the symmetry elements do not match, the overlap is considered to have no effect.

© Springer Science+Business Media Singapore 2016 V.K. Yadav, Steric and Stereoelectronic Effects in Organic Chemistry, DOI 10.1007/978-981-10-1139-9_7

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7 Miscellaneous

1

2

3

Let us consider spiroheptatriene 2 with the unperturbed orbitals of the cyclopentadiene component shown on the left and that of the cyclopropene component shown on the right in Fig. 1. It is easy to see that the only orbitals that can interact are w2 on the left and p* on the right and that all the other orbitals possess wrong symmetry. For example, the top lobes of w1 and the upper p orbital of p (one lobe in the front and the other in the back) have one interaction in phase and the other out of phase, exactly cancelling each other. A similar situation exists between the lower lobes of w1 and the lobes of the lower p orbitals of p on the right. The interaction w2 ! p* creates two new orbitals, one raised and the other lowered in energy. Since there are only two electrons (originating from w2 ) to go into these orbitals and, also, since these two electrons will occupy the lowered orbital, the overall energy of the system is lowered. This lowering in energy, ΔE, is small because of poor overlap which is necessarily on account of having the two interacting orbitals significantly apart in energy. Nevertheless, it is generally

ψ4 π*

ψ3

ψ2

ΔE π

ψ1

Fig. 1 Molecular orbitals of spiroheptatriene

1 Spiroconjugation

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ψ4

ψ4

ψ3

ψ3

ψ2

ΔE *

ψ2

ΔE

ψ1

ψ1

3

Fig. 2 Molecular orbitals of spirononatetraene

concluded that if the total number of p electrons is 4n + 2 (n 6¼ 0), the spirosystem is