Dimethyl methylene-bridged

Camphor has two dissimilar chiral centres (C-l and C-U) and therefore four enantiomorphs might be expected. But only one pair of enantiomers is known. This is due to the fact that only the ois-form is possible trans fusion of the gem-dimethyl-methylene bridge to cyclohexane ring is impossible. Thus only the enantiomers of cis-isomer are known. 

Freedom of rotation about the double methylene bridge in the compound (7) (dimethyl 4,4 -(l,2-ethanediyl)bisben2oate [797-21-7]) destroys the rod shape of the molecule and prevents Hquid crystal formation. The stilbene derivative (8) (dimethyl 4,4 -(l,2-ethenediyl)bisben2oate [10374-80-8]) however, is essentially linear and more favorable for Hquid crystal formation. 

Concerning steric factors, 43 is attacked in the most hindered position ( inverse effect of substitution ) likewise, 39 is attacked at the most hindered carbon. Obviously, the transition states for the formation of 44 or 50 show limited sensitivity to the degree of substitution, and the relief of ring strain is a more significant factor than the steric hindrance in the transition state. On the other hand, steric factors are important in systems such as P-phellandrene radical cation 40 which is attacked at the xo-methylene carbon (most easily accessible), or the tricyclane radical cation 56 which is attacked at the less hindered 3° carbon further removed from the dimethyl-substituted bridge (approach a). Both reactions also benefit Irom the formation of the most highly substituted, hyperconjugatively stabilized free radicals. 

Asymmetric transfer hydrogenation.1 Bis(oxazolines) lacking a methylene bridge such as 3 are prepared by reaction of dimethyl oxalate with a-amino alcohols,... 

Methylated arsenic is also found in compounds in which arsenic replaces phosphorus of natural biomolecules. Several arsenic-containing ribofuranosides have been identified in brown kelp . The dimethylated arsenic is linked to the C-4 of ribose by a methylene bridge. 

Both resole and novolac types of phenolic resin are used in aerospace applications. The resoles are generally made by using alkaline catalysts and an excess of formaldehyde. The resole structure is highly complex and involves both methylene and dimethyl ether bridges between the phenolic moieties. During cure both water and formaldehyde are evolved. 

With ketones that are locked due to a bridged ring system, then it is normally straightforward to predict which is the less-hindered side of the carbonyl group. Reduction of camphor with lithium aluminium hydride, for example, leads mainly to the exo alcohol (isobomeol), whereas norcamphor (which lacks the gem-dimethyl group on the bridging carbon atom), in which approach of the hydride anion is now easier from the side of the methylene bridge, leads mainly to the endo alcohol. 

Table 93.3 compiles the experimental a data for the photolysis of azoalkanes 1,16, 5b, and 17. As expected, the lowest a value (0.050 0.003) is observed for the unsubstituted (parent) DBH 1. As is evident from Table 93.3, dimethyl substitution in the methylene bridge (azoalkane 16) increases the a value ca. threefold (0.146 0.005) compared to the parent 1. This is also expected because in a more viscous solvent, the inversion process along the Sjj2 trajectory should be slowed down mainly due to frictional impediment on the inversion motion (flap mode) of the methylene bridge. Substitution in the methylene bridge should enhance the frictional imposition by the liquid medium on the flap mode of the skeletal inversion. 

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