Neopentyl compounds

Se2 reactions generally occur with retention of configuration. In some cases, this stereochemistry has been directly proved.142 It can also be deduced from observations such as the very easy substitutions which occur in neopentyl compounds (whose rear face is sterically blocked),143 and at bridging atoms.144 According to March,145 inversion of configuration only occurs when the leaving group side is severely hindered. 

FIGURE 7.25 Even neopentyl compounds, in which a r/-butyl group shields the rear of the C—L bond, are hindered enough so that the rate of the Sn2 reaction is extremely slow. 

Efforts to find conditions where the formyl group will be attached to a branched carbon atom have met with only limited success. With isobutylene at higher temperature (220 °C) and pressure (425 atm) required for catalyst stability, an 8% yield of neopentyl compounds is obtained, together with 61.6% yield of isopentyl compounds. At the same time, 25% of the starting material was hydrogenated [237]. Good yields of the neopentyl structures have been obtained only at room temperature with stoichiometric quantities of hydrocarbonyl [35] (see also p. 9). 

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