Geminal bond participation

Keywords Chemical orbital theory, Cw-stability, Cyclic conjugation. Disposition isomers. Diradicals, Donor-acceptor, Electron delocalization, Geminal bond participation, Inorganic heterocycles. Ring strain. Orbital phase. Orbital phase continuity. Polarization, Preferential branching. Reactivity, Selectivity, Stability, Tautomerism, Z-selectivity... 

Interactions of vicinal bonds have been extensively studied and are well known as hyperconjugation, resonance, and others [89-91], The a bonds vicinal to a reacting 71 bond have been proposed to participate in organic reactions and to control the selectivity [92, 93], Recently, we noticed the importance of the participation of the a bonds geminal to a reacting % bond (Scheme 35) [94] and have made extensive applications [95-102], Here, we present an orbital phase theory for the geminal bond participation and make a brief review. 

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

H3SiO SiH3 Scheme 38 Geminal bond participation and reactivity... 

In 1987, more than a decade before our proposal of the geminal bond participation [94], ene reactions of allylsilanes with singlet oxygen was reported to afford the Z-isomers more than the -isomers of ally lie hydroperoxides (Scheme 40) [106],... 

The Z-selectivity is understood in terms of the geminal bond participation. The C-Si bond, which is more electron-donating than the C-H bond, occupies the Z-position. 

The unsaturated substituents with low-lying vacant orbitals were found [99] to favor the inward rotation more than expected from the geminal bond participation. 

Geminal bond participation has been reported to control Z-selectivity in Carroll rearrangements (Scheme 90). " ... 

Orbital phase continuity for the participation of an electron-donating geminal bond... 

Such examples are rare because GcCI3 counterion, unlike the counterions of classical superacids, usually recombines with the carbenium ions with the formation of Ge—C bonds. In the present case, the ion pair 58 does not yield a stable recombination product and instead it participates in an intermolecular reaction. This is probably due to the fact that, for steric reasons, the recombination product with two geminal GeCl3 groups is unstable at an elevated temperature. 

The photodissociation of I2 dissolved in CCI4 was studied (at a concentration of 29 x 10 3 mol/L). Although the process might look quite simple, it is actually quite complex. Several excited electronic states of I2 participates in the process and after the dissociation, the solvent cage may trap the atoms leading to geminate recombination or the atoms may escape from the solvent cage and recombine with other partners. With the time resolution available at a pulsed synchrotron source ( 100 ps), it is not possible to directly follow atomic motion, i.e., the dynamics of the chemical bond. Furthermore, it is expected that incoherent motion will set in... 

Hair and Hertl [109] showed that isolated hydroxyls, not hydrogen bonded hydroxyls, participate in Reactions 24-26. Reaction 24 obeys first-order kinetics [108], whereas Reactions 25 and 26 are expected to obey first-and second-order kinetics, respectively. A mixed, 1.6-order reaction is observed [108] suggesting that Reactions 25 and 26 both occur and that 60% of the isolated silanols must be sufficiently close to each other (neighboring, isolated silanols or geminal silanols) that they can react in a bifunctional manner. 

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