Deuterium oxide, pyrolysis

Early stereochemical evidence supported the syn elimination mechanism outlined in equation (3). Pyrolysis of the co /iro-2-amino-3-phenylbutane (1) gave mainly the (Z)-alkene (2 equation 4), whereas the threo isomer (4) gave more of the ( )-alkene (3 equation This stereochemistry has since been confirmed by deuterium labeling, the rrarw-labeled cyclooctyldimethylamine oxide (5) giving cij-cy-clooctene with no loss of label, and the c -labeled isomer (6) giving partially labeled product (78% d, 22% do) consistent with an isotope effect of about 3. ... 

This type of reaction is termed an a -(3 elimination and it is isoelec-tronically similar to the pyrolysis of amine oxides 30>. However, the occurrence of an a -(3 elimination was subsequently disproven in this particular reaction because the trimethylamine which forms in the decomposition of s-2-phenyl-2-deuterocyclohexyltrimethylammonium hydroxide contains no deuterium T>. Therefore, a cis E-2 mechanism was proposed to account for the experimental observations 36>. 

That the elimination is syn has been established by use of deuterium labels. Deuterium was introduced stereospedfically by LiAlD4 reduction of cis- and frans-stilbene oxide. The product of the subsequent ester pyrolysis is frans-stilbene because of eclipsing effects in the transition state. Tlie syn elimination is demonstrated by retention of deuterium in the olefin from frans-stilbene oxide and its absence in the olefin from c -stilbene oxide. " ... 

Ester pyrolysis has been shown to be a syn elimination by use of deuterium labels. In the case of stilbene formation, deuterium was introduced by stereospecific reduction of cis- and trans-stilhene oxide by LiAlD4. The syn elimination is demonstrated by retention of deuterium in the product from the trans epoxide and its elimination in the product from the cis epoxide. 

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