Stereospecific Hydrogenolysis of Benzylic Alcohols over Pd/C

Abstract

Hydrogenolysis of tertiary benzylic alcohols on palladium on carbon (Pd/C) generally proceeds with inversion of configuration. However, little is known about the hydrogenolysis mechanism of primary and secondary benzylic alcohols. Literature precedents suggest that these substrates may interact differently with the catalyst. To study the mechanism, we synthesized a pair of deuterated diastereoisomers with a chiral center at the benzylic position. Chemical derivatization of the hydrogenolysis products showed that the reaction proceeds with inversion of configuration for these substrates.

Is there a photochemical Hammond postulate?

“…The second observation—that the percentage is the opposite to what was expected—indicates that our predicted chair-like transition state was incorrect (in a previous publication, we have proven that the anti and syn configurations of the substrates are correctly assigned). 19 We found two likely explanations as to why the structure may be different in the ESTS.…”

“…The starting esters have been prepared previously. 19 Aryl cerium reagents allowed for successful addition of the aryl group to the ester without epimerization of the α-carbon carrying the OTBS group, and the substrates were obtained in good yields.…”

“…Starting from known benzylic ketones, 19 the ketone could be hydrogenolyzed over Pd/C, and the benzylic position was fully deuterated using D 2 gas over Pd/C ( Scheme 5 ). Then, addition of aryl lithium onto the ester gave substrates 19–36 .…”

Mechanistic investigation of a Ni-catalyzed electrochemical reductive cleavage of the α-O-4 bond in the lignin model compound benzyl phenyl ether

“…[46][47][48] Hydrogenolysis reactions achieved by these two catalysts have been shown to follow two different reaction mechanisms: Raney® nickel creates strong interactions between the benzylic ring and the neighboring oxygen on the catalyst surface (Scheme 3a), while the surface of Pd/C catalysts lacks the latter and only exhibits strong interactions with the benzylic ring of the substrate (Scheme 3b). 49,50 To further probe whether our reaction shares similarities with these mechanisms, we investigated how well the Ni/CP system performed with BPE derivatives that were sterically hindered on the benzylic side (Scheme 4, compounds b-f ). There is a clear trend between decreasing product yield with increasing degree of steric hindrance.…”

Automated Recognition of Nanoparticles in Electron Microscopy Images of Nanoscale Palladium Catalysts

“…One of the most critical commercial catalysts is palladium on carbon (Pd/C). As a few examples, it can catalyze various reactions, including [ 2 ] but not limited to hydrogenations [ 3 ], hydrogenolysis [ 4 ], cross-coupling [ 5 , 6 ], and others [ 7 ]. The highest catalyst performance with ~10 9 turnover numbers was recently achieved with Pd/C catalysts [ 8 ].…”