The study of mass-selected complexes of bare Fe+ with unsaturated nitriles, CH3(CH2)nCH=CH(CH2)mCN, in the gas phase permits a detailed understanding of the chain-length-dependent interaction of the two functional groups with the transition metal ion and its implications for the direction of allylic carbon-carbon bond cleavage processes. This fundamental reaction, which proceeds via a metal-containing cyclic intermediate, is operative only for systems with m > 2, and it involves, without exception in all cases studied, the "exocyclic" allylic CC bond (i.e., loss of alkenes from the CH3(CH2), part of the nitrile). Double-bond migration constitutes a side reaction. In those cases in which this reaction is observed, the double bond preferentially migrates away from the CN group. The actual nature of bidentate complexation (and, in particular, the question of "side-on" versus "end-on" complexation of the nitrile group) is dependent upon the chain length (CH2)m. For m = 2, 3 the data suggest a bidentate complex in which both the CC double bond and the CN triple bond act as π donors (21); however, for larger chain lengths (m ≥ 4) the bidentate complex may be described by an "end-on" complexation of the CN group to the metal ion which still interacts with the CC double bond (23). For systems in which the two functional groups are separated by one or no CH2 unit, the Fe+ does not "feel" the presence of the double bond. The gas-phase chemistry of these systems (m = 0, 1) is practically identical with that of the previously described saturated alkyl nitriles for which "end-on" complexation (22) induces functionalization of remote CH bonds. The experimental findings are supported by the study of labeled isotopomers.
|Original language||English (US)|
|Number of pages||5|
|Journal||Journal of the American Chemical Society|
|State||Published - 1989|
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