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The development of modified Pd catalysts for chemoselective hydrogenation has been a long-standing goal in synthetic chemistry. Several applications of catalyst poisons for this purpose have been studied. But these methods usually lack rules of generality except for a few examples such as Lindlar catalyst and Rosenmund's reaction. Recently, we found that the addition of a nitrogen containing base such as ammonia, triethylamine, pyridine, ammonium acetate, to a Pd/C-catalyzed reduction system selectively inhibited the hydrogenolysis of an aliphatic benzyl ether with smooth hydrogenation of other reducible functions such as olefin, N-Cbz, benzyl ester and azido. However, the selective suppression of hydrogenolysis was not applicable to the benzyl protective group with phenolic hydroxyl functions. The problem has been temporarily, solved by the employment of a 4-methoxybenzyl (MPM) protective group instead of the more reducible benzyl group with phenolic hydroxyl functions. During the course of our further study on the Pd/C-catalyzed chemoselective hydrogenolysis, we further found large differences in the suppressive effect on the hydrogenolysis of O-benzyl protective groups depending upon the nitrogen-containing bases employed as additives. By the use of Pd/C-2,2'-dipyridyl combination as a catalyst for the hydrogenation, aliphatic and phenolic O-benzyl protective groups can be retained without any hydrogenolysis. Further-more, we found that the Pd/C catalyst formed an isolable complex with ethylenediamine employed as a catalytic poison, selectively catalyzing the hydrogenation of various functional groups without hydrogenolysis of O-benzyl protective groups even in phenolic benzyl ethers.

Development of a Novel Type of Pd/C-catalyzed Chemoselective Hydrogenation Using a Nitrogen Catalyst Poison