Functional Modeling of Cobalamine-Independent Methionine Synthase with Pyrazolylborate−Zinc−Thiolate Complexes

A series of new pyrazolylborate−zinc−thiolate complexes TpPh,MeZn−SR and TpMe,MeZn−SR, including two
homocysteine derivatives, were prepared and structurally characterized. Their reactions with methyl iodide in
nonpolar media resulted in the formation of the thioethers MeSR, including two methionine derivatives, and
TpR‘,MeZn−I in all cases. Methylation of the thiolates could also be achieved with dimethyl sulfate and
trimethylsulfonium iodide but not with trimethyl phosphate or N-methylpyridinium salts. The accumulated evidence
indicates that the methylation occurs intramolecularly, i.e., at the zinc-bound thiolates:  (i) The reactions occur
readily in nonpolar media. (ii) Thiolate exchange at TpPh,MeZn−SR with [PPN]SR‘ is slower than thiolate alkylation.
(iii) The methylation of TpPh,MeZn−SBn with MeI is a clean second-order reaction with k‘ ‘ = 1.75 × 10-2 M-1
s-1 at 300 K.