Manganese(III) Porphyrin‐Catalyzed Dehydrogenation of Alcohols to form Imines, Tertiary Amines and Quinolines

Abstract

Manganese(III) porphyrin chloride complexes have been developed for the first time as catalysts for the acceptorless dehydrogenative coupling of alcohols and amines. The reaction has been applied to the direct synthesis of imines, tertiary amines and quinolines where only hydrogen gas and/or water are formed as the by‐product(s). The mechanism is believed to involve the formation of a manganese(III) alkoxide complex which degrades into the aldehyde and a manganese(III) hydride species. The latter reacts with the alcohol to form hydrogen gas and thereby regenerates the alkoxide complex.

Manganese Catalyzed Acceptorless Dehydrogenative Coupling Reactions

“…Interestingly, the phosphine‐free manganese catalyst Mn‐27 displayed similar catalytic activity and selectivity to that of the PNP‐manganese catalyst Mn‐2 . Madsen and co‐workers have also applied their porphyrin complex Mn‐28 for the synthesis of quinolines (Scheme c) …”

“…[18] Madsen and co-workers have developed high valent salen manganese(III)-complex Mn-5 and porphyrin Mn(III)-complex Mn-6 for the synthesis of imines via the ADC of alcohols with amines (Scheme 4e,f). [19]…”

Manganese‐Catalysed Dehydrogenative Coupling – An Overview

“…Recently Madsen and co‐workers reported the first Mn‐porphyrin‐catalysed synthesis of imines and tertiary amines by the acceptor‐less dehydrogenative coupling of alcohols with amines . For imine synthesis they have used catalyst 118 as the optimised one at a 2 mol% loading without any base while in the case of tertiary amine synthesis a different porphyrin was used (catalytic complex 119 at 3 mol% loading) along with 20 mol% of K 2 CO 3 (Scheme ).…”

“… Proposed catalytic cycle for Mn‐porphyrin complex‐catalysed C−N bond forming reactions (reproduced with permission from Ref …”

“…The strategy previously mentioned by Madsen and co‐workers on Mn‐porphyrin‐catalysed C−N bond forming reaction was prudently employed in the synthesis of various quinolines using secondary alcohols with 2‐aminobenzyl alcohols (Scheme ) . Mn complex 119 (5 mol%) and 20 mol% of pyridine along with 2 equivalents of KOH and t‐ BuOK in mesitylene in the presence of 4 Å molecular sieves at 164 °C were needed and the mixture was stirred for 60 h. Both electron‐rich secondary alcohols and 2‐aminobenzyl alcohols were found to be favourable for this reaction.…”