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Rate constants for the attachment of CH(CN)2^- to Fischer carbene complexes of the type (CO)5M&dbd;C(XR)C6H4Z with M = Cr, W and XR = OMe, OEt, SMe in 50% MeCN−50% water (v/v) at 25 °C are reported. The malononitrile anion shows a much higher reactivity toward these carbene complexes than OH^-, primary aliphatic amines (e.g. n-butylamine), or secondary alicyclic amines (e.g. piperidine) but is slightly less reactive than thiolate ions (e.g. HOCH2CH2S^-). The alkoxycarbene complexes react more quickly than the thiomethyl derivatives, consistent with previous findings for alkoxide ion, OH^-, amine, and thiolate ion nucleophiles. Hammett ρ values are 0.69 for the reaction with (CO)5Cr&dbd;C(SMe)C6H4Z and 2.88 for the reaction with (CO)5Cr&dbd;C(OMe)C6H4Z. The much larger ρ value for the methoxycarbene complexes fits a pattern observed previously with other nucleophiles and can be explained by the π-donor effect of the methoxy group, which is much stronger than that of the thiomethyl group.

A kinetic study of the aminolysis of (CO)5Cr&dbd;C(OEt)C6H5 (7-Cr) and (CO)5Cr&dbd;C(OEt)C6H5(Cr(CO)3) (8-Cr) by five primary aliphatic amines and of the hydrolysis of 8-Cr in 50% MeCN−50% water (v/v) at 25 °C is reported. The introduction of the Cr(CO)3 group has only a minor effect on the electrophilic reactivity of the carbene carbon; that is, the nucleophilic attack by OH^- and the amines on 7-Cr and 8-Cr occurs at similar rates. However, the Cr(CO)3 group exerts a strong influence on the mechanisms of both types of reactions, which results in changes of what steps are rate limiting. In the hydrolysis reaction these changes lead to a more complex rate versus pH profile for 8-Cr compared to the previously reported one for 7-Cr. In the aminolysis the effect is to change the mechanism of general base catalysis from rate-limiting leaving group departure for the reactions of 7-Cr to rate-limiting proton transfer for the reactions of 8-Cr. We provide a detailed analysis of the reasons for these changes.

A kinetic study of the reactions of (CO)5Cr&dbd;C(SMe)C6H4Z (Z = 4-CF3, 4-Cl, 4-F, 4-Me, 4-MeO, and 4-Me2N) with n-butylamine, 2-chloroethylamine, piperidine, and 1-(2-hydroxyethyl)piperazine in 50% MeCN−50% water (v/v) at 25 °C is reported. In all cases the nucleophilic attachment of the amine to the carbene complex is rate limiting, which contrasts with the previously reported aminolysis of the methoxy analogues ((CO)5Cr&dbd;C(OMe)C6H4Z), where leaving group departure is rate limiting in most cases. The substituent effect on the nucleophilic attachment step is much weaker (average ρ = 0.55) than for the methoxy analogue (ρ = 2.03). It is shown that the small ρ value for (CO)5Cr&dbd;C(SMe)C6H4Z is “normal” and reflects the near cancellation of the substituent effects on the partial positive and partial negative charges at the transition state. These findings provide evidence that the high ρ value for (CO)5Cr&dbd;C(OMe)C6H4Z is “abnormal” and the result of the strong π-donor effect of the methoxy group on the reactant carbene complex.

The kinetics of the reactions of (CO)5Cr&dbd;C(SMe)Ph (4-Cr) with five primary aliphatic and 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 the amine and by OH^- was observed while with aminoacetonitrile, OH^- catalysis but no amine catalysis was detected. These results are interpreted in terms of a three-step reaction mechanism (Scheme 1):  the first step is nucleophilic attachment of the amine to the carbene complex, to form a zwitterionic intermediate, %@mt;sys@%T%@sx@%A%@be@%%@ital@%±%@rsf@%%@sxx@%%@mx@% ; the second step is a fast equilibrium deprotonation of %@mt;sys@%T%@sx@%A%@be@%%@ital@%±%@rsf@%%@sxx@%%@mx@% to generate the anionic intermediate %@mt;sys@%T%@sx@%A%@be@%-%@sxx@%%@mx@% ; the third step is MeS^- expulsion by two concurrent pathways, one being spontaneous (solvent catalyzed) expulsion of MeS^-, the other being RR‘NH2⁺ catalyzed loss of MeS^- from %@mt;sys@%T%@sx@%A%@be@%-%@sxx@%%@mx@% . For the reactions that follow the second-order rate law, the first step is rate limiting, while for the reactions that show amine and/or OH^- catalysis, the third step is rate limiting. The factors that determine which steps are rate limiting are discussed in detail.

A kinetic study of the reversible cyclization of (((2-hydroxyethyl)thio)arylcarbene)pentacarbonyltungsten(0) complexes, 2-SR(Z), in 50% MeCN−50% water (v/v) at 25 °C is reported. The results are consistent with Scheme 1, which shows a pathway in basic solution that involves rapid deprotonation of the OH group followed by rate-limiting cyclization and a pathway in acidic solution that involves direct nucleophilic attack by the OH group. Rate and equilibrium constants for the various elementary steps were determined as a function of the phenyl substituent. They were found to correlate well with the Hammett equation. The ρ values provide insights into (a) the reasons that the equilibrium constants are highly sensitive to substituent effects despite the fact that the negative charge of the adducts is strongly dispersed, (b) the mechanistic details of the cyclization pathway in acidic solution, and (c) the extra stabilizing effect of soft−soft interactions in previously studied adducts between a soft thiolate ion and the extra-soft carbene complex (2-SR(Z)).

The pKa = 13.41 of the title compound (5), determined by a kinetic method, is about 1.3 units lower than the pKa of (2-oxacyclopentylidene)pentacarbonylchromium(0) (1) in 50% MeCN−50% water at 25 °C. The acidifying effect of the methyl group is attributed to its stabilizing effect on the C&dbd;C double-bond resonance structure of the anion (5^-). The rate constant for deprotonation of 5 by OH^- is about the same as for deprotonation of 1, despite the higher acidity of 5. This means that the intrinsic rate constant for proton transfer from 5 is significantly lower than that from 1. This reduction in the intrinsic rate constant is mainly the result of the imbalanced nature of the transition state which deprives the transition state from the stabilizing effect of the methyl group. Based on precedents, the most likely mechanism for the hydrolysis of 5 involves rate-limiting protonation of 5^- concerted with metal−carbon bond cleavage. Even though the relatively small kinetic solvent isotope effect cannot rigorously exclude an alternative nucleophilic substitution mechanism, a comparison of the reactivity of 5 and 1 allows one to rule out this latter mechanism.

A study of the acidity of various Fischer-type (alkoxyalkylcarbene)pentacarbonylmetal complexes and of their rates of deprotonation by OH^- and amines in 50% acetonitrile−50% water at 25 °C is reported. The pKa values of (CO)5M&dbd;C(OMe)CH3 for M = W (12.36) and M = Mo (12.81) are quite comparable to that for M = Cr (12.50) determined previously, and the pKa of (CO)5W&dbd;C(OMe)CH2Ph (10.18) is similar to that of the chromium analog (10.40) reported earlier. These results indicate that the stabilization of the negative charge of the anion by the (CO)5M moiety depends little on the metal. The pKa of (CO)5Cr&dbd;C(OEt)CH3 (12.98) is 0.48 units higher than that of the methoxy analog; this increase probably reflects an enhanced stabilization of the zwitterionic resonance structure of the carbene complex, (CO)5Cr^-C(&dbd;O⁺Et)CH3, by the stronger electron-donating effect of the ethyl group. Intrinsic rate constants for the deprotonation of the carbene complexes by OH^- (ko^OH) were estimated based on the Marcus equation, and intrinsic rate constants for the deprotonation of some of the carbene complexes by amines (ko^B) were determined from Brønsted plots. The dependence of ko^OH and ko^B on the metal is rather small and hardly significant, except possibly for the reaction of (CO)5W&dbd;C(OMe)CH3 with secondary amines, for which ko^B appears to be lower for the tungsten complex than for the chromium and molybdenum derivatives; ko^OH for (CO)5Cr&dbd;C(OEt)CH3 is the same as for the methoxy analog.