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47.1.6.2.1.1 Variation 1: Catalytic Hydrogenation

DOI: 10.1055/sos-SD-047-00418

Aitken, R. A.; Aitken, K. M.Science of Synthesis, (2010471020.

As 1,2-dienes are much more reactive toward hydrogenation than the resulting alkenes the full range of normal hydrogenation catalysts can be used, since the reaction can be controlled by monitoring the uptake of hydrogen and stopping the process after exactly 1 equivalent of hydrogen has been absorbed. Addition usually favors the formation of the most highly substituted alkene or the one in which the remaining double bond is conjugated; the E/Z ratio of products favors those with a Z configuration. A representative selection of results for mono- and 1,1-disubstituted allenes is shown in Scheme 7.[‌15‌‌20‌] Trisubstituted allenes show a lower selectivity for the formation of the most highly substituted alkenes 11A and 11B. Furthermore, this selectivity can be overridden by the presence of a conjugating group in the R3 position, which favors the formation of the less hindered alkenes 12A and 12B. In the case of the cyclic allene cyclonona-1,2-diene, hydrogenation with palladium on charcoal provides convenient access to (Z)-cyclononene in 76% yield.[‌21‌]

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M8 M8 M8 Meeeeeee Meeeeee Meeeeeeeeeee (%) Mee
88M 88M 88M 88M
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Me/MeMM8
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MM8Me M M Me/M 88 8 [‌88‌]
Me Me Me Me/M 88e 88 8 [‌88‌]
Me Me MM8M Me/MeMM8 88e 88 88 [‌88‌]
Me Me MM8Me Me/MeMM8 88e 88 88 [‌88‌]

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Meeeeee (8M)-Meee-8-ee-8-eeeeeeeeeeeee (88):[‌88‌]

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