51994-85-5

Upstream product

• 280-57-9 1,4-diaza-bicyclo[2.2.2]octane 
• 37823-96-4 pentacarbonyl(methoxyphenylmethylene)tungsten⁽⁰⁾ 

Relevant academic research and scientific papers

Transition metal carbene chemistry. 4. Nucleophilic attachment of DABCO to fischer carbene complexes in MeCN

Rate constants for the attachment of DABCO (1,4-diazabicyclo[2.2.2]octane) to Fischer carbene complexes of the type (CO)5Cr=C(XR)C 6H4Z (X = O and S) in dry MeCN at 25°C are reported. Hammett ρ values are 2.18 ± 0.13 and 0.89 ± 0.07 for DABCO reactions with (CO)5Cr=C(OMe)C6H4Z (10-Cr-Z) and (CO)5Cr=C(SMe)C6H4Z (11-Cr-Z), respectively. The ρ values for the reaction of 10-Cr-Z and 11-Cr-Z with CH(CN)2- in 50% MeCN-50% H2O (v/v) are comparable to the present reactions. The reaction of DABCO with 10-Cr-Z is more closely related to the reaction of (n-Bu)3P with (CO) 5W=C(OMe)C6H5-Z (23) which also provided a ρ value 2.22. The much higher ρ values and hence much higher reactivity of methoxy carbene complexes over the corresponding thiomethyl derivatives fit a pattern observed previously for alkoxide ion, OH-, amine, and thiolate ion nucleophiles, and a rational explanation comes from the consideration of the substituent effects not only on the transition state but also on the reactant. A major difference between 10-Cr-Z and 11-Cr-Z is that the π-donor effect of the methoxy group is much stronger than that of thiomethyl group. This leads to a substantial contribution of the zwitterionic form to the structure of 10-Cr-Z with much more localized positive charge on the methoxy group than the negative charge on the (CO)5Cr moiety. This leads to overall destabilization by an electron-withdrawing phenyl substituents resulting an increase in reactivity. The ethoxycarbene complexes are somewhat less reactive than their methoxy counterparts due to the somewhat more ground state stabilization through its stronger π donor effect and partly due to steric crowding exerted by the slightly larger ethoxy group in the transition state. Higher k1(W)/k1(Cr) ratios for (thiomethyl)carbene complexes than methoxy or ethoxycarbene complexes are related to the intrinsic rate constant which is higher for ((thiomethyl)carbene)tungsten complexes than the corresponding Cr ones resulting in an enhanced ratio.

Physical organic chemistry of transition metal carbene complexes. 4. Kinetics and equilibria of methoxide ion addition to [methoxy(phenyl)carbene]pentacarbonylchromium(0) and [methoxy(phenyl)carbene]pentacarbonyltungsten(0)

A stopped-flow kinetic study of the reversible addition of methoxide ion to [methoxy(phenyl)-carbene]pentacarbonylchromium(0) (2-Cr) and [methoxy(phenyl)carbene]pentacarbonyltungsten-(0) (2-W) in methanol at 25°C is reported. The reaction was investigated by approaching the equilibrium (eq 1) from either side; due to the rapid decomposition of the adduct, the reaction in the reverse direction had to be measured in a sequential stopped-flow apparatus a few milliseconds after the adduct had been generated. Evidence that the observed reaction refers to the formation of (CO)5M--C(Ph)(OCH3)2 (M = Cr, W) includes a comparison of the absorption spectrum of (CO)5W--C(Ph)(OCH3)2 with that of the ylide derived from 2-W and DABCO, low-temperature IR of the adducts, and demonstration of CH3O/CD3O exchange on the same time scale as the kinetic experiments. The following rate and equilibrium constants were obtained: k1 = 77.1 M-1 s-1, k-1 = 1.10 s-1, and K1 = 70.1 M-1 for 2-Cr and k1 = 186 M-1 s-1, k-1 = 1.68 s-1, and K1 = 111 M-1 for 2-W. Approximate intrinsic rate constants (k0 = k1 = k-1 when K1 = 1) were calculated from the experimental rate constants. The k0 values are substantially lower than those estimated for the addition of methoxide ion to methyl benzoate. This is consistent with significant charge delocalization in the methoxide ion adducts of the metal carbene complexes. Rate constants, k-1BH, for general acid catalyzed methoxide ion loss from the adducts by phenol and 3,5-dichlorophenol were also determined. They allow an estimate of a Br?nsted α value on the order of 0.9, suggesting a transition state in which proton transfer from the catalyst to the incipient methoxide ion is well advanced. The near-independence of K1 on the metal contrasts with the strong metal dependence (Cr vs W) of the pKa values of (η5-C5H5)M(CO)3H-type hydrido complexes and supports Norton's view that the acidity of these hydrido complexes is not governed by the stabilization of the negative charge in the respective anions.