Hydrogen geminal

In the case of t-butyl acetate, shown in reaction 41, three successive geminal hydrogen abstractions and insertions of 303 take place a product of reductive dimerization (306) is also formed in small yield626. 

Pople,J. A., Bothner-By, A.A. Nuclear Spin Coupling between Geminal Hydrogen Atoms. J. Chem. Phys. 42, 1339 (1965). 

The H-NMR spectra of pavines can provide appreciable assistance in structural elucidation. The oxygenation pattern of a pavine may be deduced from a careful examination of the methine (H j) and methylene (H. f) proton absorptions (Ic). In the case of 2,3,8,9 substitution, the abc and def protons furnish two superimposable ABX patterns. A doublet integrating for two protons at the lower field end of the system at approximately 8 4.0 represents the bridgehead protons, Ha and Hj. At 60 MHz, it appears as if these protons are coupled to only one of the neighboring protons (J = 6 Hz) (18,20,25). Furthermore, the geminal hydrogens couple to each other with a coupling constant of 17 Hz (18,20). On... 

Highly regioselective ene reactions of singlet oxygen with alkyl-substituted alkenes can also be used for synthetic purposes. For example, in the photooxygenation ofl-t-butylcycloheptene only one allylic hydroperoxide, by the exclusive geminal hydrogen... 

Geminal hydrogens (Section 14.4) Hydrogens bonded to the same carbon. 

The presence of a center of prochirality is not an obligatory condition for a molecule to be prochiral. Indeed, other elements of prochirality exist, namely axes and planes of prochirality. Thus, the prochiral allene derivative LX displays two geminal hydrogen atoms which are enantiotopic. 

While the presence of enantiotopic groups in a molecule necessarily implies the presence of an element of prochirality, diastereotopic groups imply prostereoisomerism as an element of prochirality, or of proachirality. For example, chloroethylene (LXII) contains two geminal hydrogen atoms which are diastereotopic. But no... 

Fig. 18. The expected percentages of various labelled products of the dioldehydratase reaction using 25 as substrate. The calculation was based on the following facts and assumptions (1) The enzyme does not differentiate between the enantiotopic hydrogen positions (conclusion from experiments with species 17 and 18 shown in Fig. 14) (2) in the competition between vicinal hydrogen atoms there is an intramolecular kinetic deuterium isotope effect of 2.6 (Fig. 15) (3) this effect is 10 for geminal hydrogen atoms (4) the migrating hydroxyl group substitutes one of the hydrogen atoms in the vicinal position stereospecifi-cally (i.e., with inversion).

Organolithium compounds RLi (R = Me, Et, //-Bu, Ph) are excellent dehydrohalo-genating agents of vinyl halides in ether or THE solution under very mild conditions at temperatures below 0 °C. In a first step the acidic geminal hydrogen is replaced by lithium in a slow step to give an alkenyllithium which can be isolated at low temperature. In a second fast step lithium halide is eliminated to furnish an acetylene (equation 61). Two equivalents of alkyllithium are needed according to this equation. 

Similarly, for the methyloctalins, an axial methyl has a larger effect than an equatorial methyl on the orientation of readsorption. Each of the methyldecalin stereoisomers that was formed in the low yield of 0.5 to 1.0% had as its precursor an octalin whose methyl group not only was axial but was on the side of the molecule facing the catalyst. Most stereoisomers that occurred in yields of 4 to 6% had octalin precursors that could be adsorbed with minimum steric interference. Thus, each not only had an equatorial methyl group but was adsorbed in such a way that the axial geminal hydrogen was directed away from the... 

Molecules having some hydrogen atoms trans can readily be considered in this system by using the assumption that the angle between geminal hydrogen atoms is 120°. 

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