Methyl groups geminal

For example, molecular mechanics calculations (MM2) showed that the methyl group geminal to the neopentyl group in 2,3,5,5-tetramethyl-2-hexene (51, Scheme 18) has the lowest rotational barrier and is the most reactive [79], Furthermore, the ethyl group in 2,3-dimethyl-2-pentene (52) has a much higher rotational barrier (5.76 kcal/mol) than the methyl groups and is totally inactive to 102. Similar trends hold with 2-methyl-2-butene (4). However, the barrier to rotation does not always predict the regioselectivity of the ene reaction of 102 with alkenes. As it was shown later [62], it is the nonbonded interactions in the isomeric... 

If the CXOH (X = O, H2) substituent has no geminal group (71,73), the rotations can be predicted correctly. With a methyl group geminal to CXOH the TTOR approach fails. In 72 and 74 the CXOH group is strongly shielded. 

The stereochemistry of the reduction is interesting. The hydride can approach the camphor molecule more easily from the bottom side (endo approach) than from the top side (exo approach). If attack occurs at the top, a large steric repulsion is created by one of the two geminal methyl groups. Geminal methyl groups are groups that are attached to the same carbon. Attack at the bottom avoids this steric interaction. 

An unactivated methyl group can be functionalized by the cyclopalladation of oximes. The equatorial methyl of geminal methyls in steroids or hexapyr-anosides is selectively aceto.xylated by the reaction of the palladation complex 523 of the 3-oxime with lead tetraacetate[467,468]. 

The stepwise replacement of fluorine atoms in fluorocyclophosphazenes, (NPF2) n = 3—5), by methyl groups has been followed.and n.m.r. spectroscopy showed that methyl-lithium in diethyl ether generally effects a geminal replacement pattern. With N3P3F6, only mono- and di-methyl derivatives were obtained, but with N4P4F8 the dimethyl (52), trimethyl (53), and octamethyl derivatives were noted. The formation of... 

Polar functional groups such as carbonyl, cyano, and sulfoxide, as well as silyl and stannyl groups, exert a strong directing effect, favoring proton removal from the geminal methyl group.171... 

At 100 °C, the ROESY spectrum has shown chemical exchange of NH-protons and also pairwise chemical exchange of the methyl groups on the dialdehyde units, methine protons as well as the geminal CH2 protons. This behaviour was explained as inversion of the whole molecule in an... 

Methyl deoxypodocarpate 127 (Scheme 1) 129) represents a simple problem since the ketone 132 is well-known and readily available from Hagemann s ester in three steps. The problem of geminal alkylation of this ketone stems from its existence as an EjZ mixture of ring fusion isomers. Recognizing that decarbonylation of aldehydes occurs readily with Wilkinson s catalyst creates a structural equivalence of an acetaldehyde chain and a methyl group as in 128. This simple relationship immediately establishes several options, a simple one uses a thioacetal such as 129 as a synthon for the aldehyde. The presence of a carbonyl group three carbons away... 

In sharp contrast to 20a, 4,5,7,8,12,13,15,16-octamethyl[2.2]para-cyclophane 22 is extraordinarily unstable 61>. This substance polymerizes at room temperature, both in solution and in the solid state, even in an inert atmosphere. The reason is that the accumulation of pseudo-geminal methyl groups leads to steric overcrowding which cannot be circumvented because of the rigidity of the [2.2]paracyclophane system. Accordingly,... 

---