| Abstract
| - The kinetics of the reactions of (CO)5CrC(SMe)Ph (4-Cr) with five primary aliphaticand four secondary alicyclic amines have been investigated in 50% MeCN−50% water (v/v)at 25 °C. With n-butylamine, 2-methoxyethylamine, 2-chloroethylamine, glycinamide,piperidine, piperazine, and 1-(2-hydroxyethyl)piperazine, the reaction is strictly second order,i.e., first order in 4-Cr and first order in the amine. With morpholine, base catalysis by theamine and by OH- was observed while with aminoacetonitrile, OH- catalysis but no aminecatalysis was detected. These results are interpreted in terms of a three-step reactionmechanism (Scheme ): the first step is nucleophilic attachment of the amine to the carbenecomplex, to form a zwitterionic intermediate,; the second step is a fast equilibriumdeprotonation ofto generate the anionic intermediate; the third step is MeS-expulsion by two concurrent pathways, one being spontaneous (solvent catalyzed) expulsionof MeS-, the other being RR‘NH2+ catalyzed loss of MeS- from. For the reactions thatfollow the second-order rate law, the first step is rate limiting, while for the reactions thatshow amine and/or OH- catalysis, the third step is rate limiting. The factors that determinewhich steps are rate limiting are discussed in detail.
- The nucleophilic substitution of (CO)5CrC(XMe)Ph (X = O or S) with amines proceeds by the three-step mechanism shown. For X = O, the/k-1and/k-1Kw ratios are small, which means that leaving group departure is rate limiting and base catalysis by the amine and OH- is observed for all amines studied. For X = S, these ratios are substantially larger so that the k1 step becomes rate limiting and no base catalysis can be observed except for the reactions with weakly basic amines.
|
