Transition Metal Carbene Chemistry. 4. Nucleophilic Attachment of DABCO to Fischer Carbene Complexes in MeCN ² Mahammad Ali,* Abhijit Dan, Ambarish Ray, and Kaushik Ghosh Department of Chemistry, JadaVpur UniVersity, Kolkata 700 032, India Received March 7, 2005 Rate constants for the attachment of DABCO (1,4-diazabicyclo[2.2.2]octane) to Fischer carbene complexes of the type (CO) 5 CrdC(XR)C 6 H 4 Z (X ) O and S) in dry MeCN at 25 °C are reported. Hammett F values are 2.18 ± 0.13 and 0.89 ± 0.07 for DABCO reactions with (CO) 5 CrdC(OMe)C 6 H 4 Z (10-Cr-Z) and (CO) 5 CrdC(SMe)C 6 H 4 Z (11- Cr-Z), respectively. The F values for the reaction of 10-Cr-Z and 11-Cr-Z with CH(CN) 2 - in 50% MeCN-50% H 2 O (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) 3 P with (CO) 5 WdC(OMe)C 6 H 5 -Z (23) which also provided a F value 2.22. The much higher F 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) 5 Cr 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 k 1 (W)/k 1 (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. Introduction The extensive and ever increasing activity in the area of transition metal carbene complexes of the Fischer type has mainly been driven by their usefulness in organic synthesis. The key to the rich chemistry of this class of compounds is that they can undergo reactions at several sites: (i) modifica- tion of the carbene fragment; (ii) reactions on the sidearm; (iii) reactions on metal involving ligand exchange or change in oxidation state; (iv) insertion of unsaturated organic molecules into the metal-carbene bonds. 1 Of these, the most extensive studies have been made on the type i reactions, because most transition metal carbene complexes of the Fischer type are highly electrophilic at the carbene carbon. 1-3 Nucleophilic substitution of an alkoxy or akylthio group of carbene complexes such as 1 is one of the prototypical reactions of Fischer carbene complexes. It has generally been assumed to proceed via a tetrahedral intermediate 2 in analogy to the reaction of carboxylic esters with nucleophiles. Equation 1 shows the reaction scheme involving an anionic nucleophile, while reactions with neutral nucleophiles such as amines include a proton transfer as an additional step (eq 2). * Author to whom correspondence should be addressed. E-mail: mali@chemistry.jdvu.ac.in. ² Part 3: Ali, M. Indian J. Chem., Sect. A, in press. (1) (a) Do ¨tz, K. H.; Fischer, H.; Hofmann, P.; Kreissl, F. R.; Schubert, U.; Weiss, K. Transition Metal Carbene Complexes; Verlag Chemie: Deerfield Beach, FL, 1983. (b) Do ¨tz, K. H. Angew. Chem., Int. Ed. Engl. 1984, 23, 587. (2) Bernasconi, C. F. Chem. Soc. ReV. 1997, 26, 299. (3) Bernasconi, C. F. AdV. Phys. Org. Chem. 2003, 37, 137. Inorg. Chem. 2005, 44, 5866-5871 5866 Inorganic Chemistry, Vol. 44, No. 16, 2005 10.1021/ic050349t CCC: $30.25 © 2005 American Chemical Society Published on Web 07/14/2005