Alkane hydroxylations

The INDO /1 semi‐empirical method is used to analyze the hydroxylation of alkanes by models for the putative active species in cytochromes P–450. In the insertion of oxygen into the CH bond by a M—O complex there are four atoms of particular interest, the metal and oxygen atoms of the oxidant plus the C and H atom of the CH bond to be activated. With four atoms being studied (3N‐6), 6 degrees of freedom need to be described and a nearly intractable computational problem results. Thus, two reaction coordinates ( CHI and OAT ) were developed in an attempt to describe alkane hydroxylation in a chemically meaningful way. CHI (CH insertion) is a measure of the extent to which the CH bond has been activated and transferred to the O atom. OAT (oxygen atom transfer) is a measure of the extent to which the CO bond in the alcohol product has formed. The models chosen for the present study are manganyl (MnO) and ferryl (FeO) porphyrin complexes. Experimental evidence for manganese prophyrin complexes shows that as the axial ligand is made a stronger donor the insertion of the O atom into the CH bond becomes less concerted. The INDO /1 method was used to calculate the points on the PES determined by the changes in OAT and CHI . The results point to the axial ligand modifying the pathway by changing the relative electro‐/nucleophilicity of the oxygen atom.