Science of Synthesis

Yan, L.; Lin, S.; Liu, P., Science of Synthesis, (2007) 20, 738.

Ruthenium complexes [e.g., RuH2(PPh3)4[‌39‌] or Ru3(CO)12[‌40‌]] or rhodium complexes [e.g., RhH(CO)(PPh3)3[‌41‌]] catalyze the oxidative condensation of aldehydes and alcohols to give esters ‌43‌. A possible cyclic mechanism involving the successive formation of a hemiacetal complex ‌40‌, an alkoxyruthenium complex ‌41‌ and a dihydridoruthenium complex ‌42‌ is illustrated in ‌Scheme 15‌. In a specific example of the procedure dihydridotetrakis(triphenylphosphine)ruthenium(II) is known to catalyze the formation of butyl butanoate (‌44‌) from butanal and butan-1-ol (‌Scheme 15‌). Note, however, that in this illustration both the aldehyde and the alcohol have the same hydrocarbon stem; when this is not the case, then cross condensation may give a mixture of esters. This problem arises mainly because of hydrogen transfer from the appropriate dihydridoruthenium complex ‌42‌ to the starting aldehyde leading to the generation of an alternative alcohol. Fortunately, hydrogen transfer is inhibited by adding a hydrogen acceptor, such as mesityl oxide. In this case, for example, methyl octanoate (‌45‌) can be obtained in 66% yield from octanal and methanol (‌Scheme 15‌).[‌39‌]

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