Any process by which the normal alternating donor and acceptor reactivity pattern of a chain, which is due to the presence of O
heteroatoms, is interchanged. Reactivity umpolung is most often achieved by temporary exchange of heteroatoms (N
) by others, such as P
. The original meaning of the term has been extended to the reversal of any commonly accepted reactivity pattern. For example, reaction of R–C≡CX
= halide) as a synthon for 'R–C≡C+
(i.e. electrophilic acetylene) is an umpolung of the normal more common acetylide, R–C≡C−
(i.e. nucleophilic) reactivity.
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1174 (https://doi.org/10.1351/pac199466051077)