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The synthesis and characterization of [Cp(NO)(PPh3)Re(2,5-dimethyl-3-thienyl)carbene]+ (3H+(S)), a novel Fischer carbene complex, is described. 3H+(S) undergoes reversible deprotonation with a pKa of 2.64 in 50% MeCN−50% water (v/v). Rate constants for the deprotonation of 3H+(S) by carboxylate ions and by primary aliphatic and secondary alicyclic amines were determined. A major focus of this paper is a comparison of the kinetic and thermodynamic acidities of 3H+(S) with those of [Cp(NO)(PPh3)Re(2-thienyl)carbene]+, 1H+(S). The pKa of 1H+(S) (2.51) is virtually the same as that of 3H+(S). This appears to be the result of a fortuitous cancellation of acidity-reducing and acidity-enhancing factors, a conclusion supported by a computational study of gas phase acidities. An analysis of Bronsted plots indicates that the intrinsic rate constants for proton transfer are about 2 orders of magnitude lower for 3H+(S) than for 1H+(S). This reduction is the result of steric effects and of electronic effects connected...

Rate constants for the attachment of CH(CN)2- to Fischer carbene complexes of the type (CO)5MC(XR)C6H4Z with M = Cr, W and XR = OMe, OEt, SMe in 50% MeCN−50% water (v/v) at 25 °C are reported. The ...

A kinetic study of the aminolysis of (CO)5CrC(OEt)C6H5 (7-Cr) and (CO)5CrC(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)5CrC(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)5CrC(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)5CrC(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)5CrC(OMe)C6H4Z is “abnormal” and the result of the strong π-donor effect of the methoxy group on the reactant carben...

The kinetics of the reactions of (CO)5CrC(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, ; the second step is a fast equilibrium deprotonation of to generate the anionic intermediate ; 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 ...

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 co...

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 CC 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 e...

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)5MC(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)5WC(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)5CrC(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(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- (koOH) were estimated based on the Marcus equation, and intrins...

Evidence is presented that the reaction of (methoxymethylcarbene) pentacarbonylchromium (0) with OH − and piperidine leads to reversible deprotonation of the methyl group. The rate constants are found to be remarkably low, even for a proton transfer from carbon that leads to the formation of a resonance-stabilized carbanion