Geminal position

Interaction of the donbly occnpied bonding orbital of a o bond with the vacant antibonding o -orbital of another bond at the geminal position was theoretically shown to be bonding (Scheme 4a) and antibonding (Scheme 4b) when the s-character... 

Molecules with donor and acceptor in trans- and a s-configurations give much higher nonlinearities, due to favorable linear donor-acceptor conjugation, than those with substituents at the geminal position, where only the weaker cross-conjugation is effective (see 89 vs 86 and 92, Fig. 8). 

FIGURE 8.3 Geometric and energetic relationship for electron thermalization by random walk in liquid hexane in the presence of the geminate positive ion. Here = fid). Reproduced from Mozumder and Magee (1967), with the permission of Am. Inst. Phys. . 

Geminate Recombination. Coulomhic forces attract the Ionisation electron back to Its geminate positive partner (reaction (1) + (2)). In water and other polar media, these forces are modified by the high dielectric constant and the possibility of solvation, giving a high probability of escape. This results In a high yield... 

The resonance structures 65 suggest that the pseudo-meta and pseudo-geminal positions are the preferred sites for substitution however, it has been observed (e.g. in the bromination and acetylation of 4-bromo[2.2]paracyclophane (67)) that there is predominant formation of pseudo-ortho and pseudo-para products 8 3b>8 5> (see Table 3) ... 

In the nitro- and carbonyl-substituted [2.2]paracyclophanes the most basic position is at the oxygen of the substituent, and substitution occurs in the pseudo-geminal position. 

The decisive step in this reaction is the transannular migration of the N-oxide oxygen atom (86+87). Such an intramolecular attack seems quite plausible, since model studies demonstrate that the oxygen atom of the N—0 group in 86 can approach within 2.6 A of the pseudo-geminal position in the other benzene ring. Analogous intermolecular reactions, such as oxidation of phenols to quinones by N-oxides, are known 87>. 

Haloalkene oxides form a particular group of xenobiotics generally known for being reactive and toxic. Here, we discuss two classes of compounds, separated according to the vicinal or geminal position of their halogen atom(s) relative to the oxirane ring. 

Limitations of this rule are apparent when bulky substituents interfere sterically, especially in the cis and geminal positions. 

Imine formation is also possible when a fluorine atom is in a geminal position to a suitable leaving group (e.g., halogen38 or O-substituents). Reaction of perfluorinated epoxides (e.g.. 

It had been established several decades ago that the reaction of 1-chloro-l-ni-troethane with sodium nitrite in aqueous-alcohol medium is second order overall and first order in each reactant (Hawthorne 1956). 1-Deutero-l-chloro-l-nitroethane reacts more slowly than its lighter isotopomer. This means that the kinetic isotopic effect is observed. The reaction proceeds only in moderately alkaline media in strongly alkaline media it does not take place. Only those geminal halo nitro compounds, which carry hydrogen in the geminal position, can undergo the conversion. Based on these facts, Hawthorne (1956) suggested the Sn2 substitution preceded by the isomerization of the initial substrate into the aci-nitro form ... 

The direct and sensitized photolyses of 26 possessing methoxycarbonyl groups at the geminal position give different regioisomeric vinylcydopropanes (27 and 28) via the regioisomeric DPM rearrangement under multiplicity control (Scheme 4.14)... 

This has been explained on the basis of the greater stability of the rotamer (A) than that of the rotamer (B) due to the gear effect between the two isopropyls (or cyclopropyls) at geminal positions, which generates distinct energy minima with respect to rotation of the groups (Sch. 69). 

HNO elimination with the H coming from the geminal position has also been proposed . 

Broader variations on the scaffold model have been reported in the patent literature. A prodrug of the rofecoxib furanone template has been reported (366). The cis-stilbene-like structure (68) can undergo metabolic oxidation to produce the selective 2,3-diary furanone. A variation on the 1,2 vicinal substitution is the reported analog (69), with geminal positioning of the two aryl rings off the exocy-clic olefin of the butyrolactone template (367, 368). 

Furthermore, the relative permittivity (or dielectric constant) ofthe liquid is an important parameter. The probability P of an electron (ef) which is thermalised at a distance r metres from its geminate positive ion (M +) escaping recombination with it is exp(-r/r) where is the distance at which the Coulomb potential between e and M is equal to thermal energy kT and is given by the Onsager expression where e is the elementary... 

The rates of reaction of amines with N3P3CI5R (R = primary or secondary amino) are hardly influenced by the size of R. If R is a primary amino ligand, the incoming amino group can also take a geminal position, due to initial... 

What factors, then, determine the conformation in these types of molecules It is rather obvious that van der Waals forces dominate, and molecular-mechanics computations on model di(primary-alkyl)ethylenes help to quantify the contributions from different interactions. It turns out that two alkyl groups on the same side of the olefin plane syn) repel each other if they are in c -vicinal or geminal positions relative to each other, but attract each other if they are trans substituted, regardless of the kind... 

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