Physical Organic Chemistry of Transition Metal Carbene Complexes. 18.¹ Kinetics of Reversible Cyclization of the Fischer Carbene Complexes (CO)₅MC(SCH₂CH₂OH)Ph and (CO)₅MC(OCH₂CH₂OH)Ph (M = Cr, W) in Aqueous Acetonitrile

Rate and equilibrium constants for the conversion of (CO) 5 Cr C(OCH 2 CH 2 OH)Ph ( 8-CrOOH ), (CO) 5 W C(OCH 2 CH 2 OH)Ph ( 8-WOOH ), (CO) 5 Cr C(SCH 2 CH 2 OH)Ph ( 8-CrSOH ), and (CO) 5 W C(SCH 2 CH 2 OH)Ph ( 8-WSOH ) into the corresponding cyclic adducts ( 9-CrOO - for M = Cr, X = O; 9-WOO - for M = W, X = O; 9-CrSO - for M = Cr, X = S; 9-WSO - for M = W, X = S) are reported. Under basic conditions the reaction occurs via the respective oxyanions, (CO) 5 Cr C(OCH 2 CH 2 O - )Ph ( 8-CrOO - ), etc.; under acidic conditions the reaction may either be concerted with water acting as a general base catalyst or stepwise via a zwitterionic intermediate. The absence of buffer catalysis suggests but does not prove the operation of the stepwise mechanism. The equilibrium constants for cyclization of 8-CrSO - and 8-WSO - are about 1 order of magnitude higher than for 8-CrOO - and 8-WOO -, respectively. For the rate constants the situation is reversed, with 8-CrOO - and 8-WOO - being substantially more reactive than 8-CrSO - and 8-WSO -, respectively. These results reflect transition state imbalances with respect to various factors such as steric effects, π-donor effects, resonance effects in the cyclic adduct, and possibly anomeric effects. Kinetic and equilibrium effective molarities of the cyclization reactions were also deduced and are discussed in the context of intramolecular reactions in general.