Homogeneous Hydrogenation

Abstract

While electron transfer appears to be the biological function of smaller proteins having sites 1-3, the possibility that these sites or variants thereof may be capable of catalytic transformations should be borne in mind. Hydrogenase is the best current example of an iron-sulfur enzyme containing no other metals or prosthetic groups. Given the numerous precedents in metal complex chemistry,63 it is entirely probable that dihydrogen activation and heterolytic cleavage, evi-denced by H/D exchange in the absence of an electron carrier,64 occurs at a 4-Fe site. The terminal mono-oxygenase of the 4-methoxybenzoate O-demethylase system from Pseudomonas putida has been described as a nonheme protein whose catalytic site may be similar to the 2-Fe sites of Fd proteins.65 The catalytic site(s) of the Fe-Mo protein of nitrogenase remains undefined despite massive research attention to this enzyme, rendering it perhaps the next most exhaustively examined metallobiomolecule complex after hemoglobin and cytochrome oxidase. Numerous model studies, which have afforded catalytic systems for the N2-· NH3 conversion66 involving unidentified inter-mediates and direct ammonia formation from defined dinitrogen complexes,67 have focussed heavily on mo-lybdenum as the biocatalytic center. The large complement of Fe-S sites58,60,62 demands consideration of 2 or 3 or a yet-unrecognized Fe58,62 or Fe-Mo68 coordination unit as a binding and activation center. Consequently, the analogues of Table II may be, or may serve as precursors to, complexes useful as test vehicles for stoichiometric or catalytic conversions of biological substrates.