MECHANISM OF C-N BOND SCISSION - HYDRODENITROGENATION OF 1,2,3,4-TETRAHYDROQUINOLINE AND OF 1,2,3,4-TETRAHYDROISOQUINOLINE

The transformations of 1,2,3,4-tetrahydroisoquinoline (1234THIQ) and of 1,2,3,4-tetrahydroquinoline (1234THQ) were compared on a sulfided commercial NiMo-Al2O3 catalyst in a flow microreactor at 623 K (total pressure: 7 MPa). In the case of compounds such as 1234THQ hydrogenation of the benzenic ring is necessary to eliminate nitrogen as NH3 and so one cannot draw conclusions concerning the mechanism of C-N bond scission. 1234THIQ leads essentially to ethyltoluene as a denitrogenation product which indicates that in this case the principal mechanism of C-N bond scission is not a Hofmann type elimination but rather a nucleophilic displacement by SH-, followed by hydrogenolysis of the intermediate thiol. It is also shown that the degree of substitution of the carbon atom-alpha with respect to the nitrogen can modify the product distribution, suggesting that the mechanism probably changes to some extent from S(N)2 to S(N)1 or to a Hofmann elimination.