43184-82-3Relevant academic research and scientific papers
Transition metal carbene chemistry. 4. Nucleophilic attachment of DABCO to fischer carbene complexes in MeCN
Ali, Mahammad,Dan, Abhijit,Ray, Ambarish,Ghosh, Kaushik
, p. 5866 - 5871 (2005)
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.
