Proton Transfer and Nucleophilic Addition to Fischer Carbene Complexes. Effect of the Alkyl p-donor Substituent on the Reactivity

MARTIN E. ZOLOFF MICHOFF; DIEGO M. ANDRADA; ALEJANDRO M. GRANADOS

Florianópolis - Brasil

Conferencia; 8th Latin American Conference on Physical Organic Chemistry; 2005

Fischer carbene complexes are organometallic compounds of transition metals such as chromium, molybdenum or tungsten. These metallic atoms have good p-acceptor ligands (CO and phosphines being the most common) and a carbene ligand in their coordination sphere. The carbene ligand is bound to at least one atom like O, N or S that can participate as a p-donor stabilizing the complex. When there is a proton at the a position of the carbene carbon the hydrolysis mechanism involves the proton transfer prior to ulterior decomposition steps. On the other hand, when the a position has non-ionizable protons the hydrolysis involves the nucleophilic attack to the carbene carbon. All these reaction have high intrinsic barriers and some of the various factors that affect the intrinsic barriers of these reactions have already been investigated. We will present some of the results on the reactions of the non-ionizable Fischer carbenes  with H2O, OH- and CH3O- as nucleophiles, where we have systematically changed the size of the R1 group in order to determine the effect of these changes on the intrinsic barrier of the reaction. We have found that with these substrates the reaction is mainly governed by the steric effect of the substituent and that the intrinsic barrier is not affected. We will also report results on the reactions of the acidic thiocarbenes  with different amines.