2023
DOI: 10.1002/chem.202300094
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No Transition Metals Required – Oxygen Promoted Synthesis of Imines from Primary Alcohols and Amines under Ambient Conditions

Abstract: The synthesis of imines denotes a cornerstone in organic chemistry. The use of alcohols as renewable substituents for carbonyl-functionality represents an attractive opportunity. Consequently, carbonyl moieties can be in situ generated from alcohols upon transition-metal catalysis under inert atmosphere. Alternatively, bases can be utilized under aerobic conditions. In this context, we report the synthesis of imines from benzyl alcohols and anilines, promoted by KO t Bu under aerobic conditions at room tempera… Show more

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Cited by 2 publications
(5 citation statements)
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“… [12a] Initially, we performed the β‐alkylation reaction with our optimized conditions but in the presence of 25 mol% 18‐crown‐6 and observed a yield of only 15 % suggesting that the potassium ion plays a large role in the mechanism in line with some of the previous literature reports (Scheme 2, a) [7b,12a] . On the other hand, when the N ‐alkylation reaction was performed with 25 mol% 18‐crown‐6, no significant effect on the yield was observed, which is in line with the report by Chaudhary and coworkers on NHC catalyzed N ‐alkylation (Scheme 2, b) and also with the recent report on imine synthesis by Kirchner [8b,24] . This difference between the two reactions possibly arises from the fact that in β‐alkylation, there is an intermediate aldol reaction where the K + metal ion is possibly acting as a Lewis acid whereas for N ‐alkylation, the intermediate reaction is imine formation between benzaldehyde and aniline which does not require the presence of the K + metal ion.…”
Section: Resultssupporting
confidence: 91%
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“… [12a] Initially, we performed the β‐alkylation reaction with our optimized conditions but in the presence of 25 mol% 18‐crown‐6 and observed a yield of only 15 % suggesting that the potassium ion plays a large role in the mechanism in line with some of the previous literature reports (Scheme 2, a) [7b,12a] . On the other hand, when the N ‐alkylation reaction was performed with 25 mol% 18‐crown‐6, no significant effect on the yield was observed, which is in line with the report by Chaudhary and coworkers on NHC catalyzed N ‐alkylation (Scheme 2, b) and also with the recent report on imine synthesis by Kirchner [8b,24] . This difference between the two reactions possibly arises from the fact that in β‐alkylation, there is an intermediate aldol reaction where the K + metal ion is possibly acting as a Lewis acid whereas for N ‐alkylation, the intermediate reaction is imine formation between benzaldehyde and aniline which does not require the presence of the K + metal ion.…”
Section: Resultssupporting
confidence: 91%
“…[10,28] The mechanism for oxidation of benzyl alcohol in air was recently investigated in some detail. [24] A similar mechanism has been reported by Xu and coworkers in the copper catalyzed β-alkylation of alcohols in the presence of air. [29] Given that only a catalytic amount of the oxidized alcohol is needed for reaction it cannot be ruled out that the role of the transition metal in many TM catalyzed N-alkylation and βalkylation reactions reported in the literature is to act as a stochiometric oxidant that dehydrogenates a small amounts of the alcohol to the respective aldehyde which can further catalyze the reaction as shown in Scheme 2.…”
Section: Entrysupporting
confidence: 64%
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“…Metal-free base catalyzed N-alkylation of amines have also been reported with alcohols. [62][63][64] However, the marked reversal in the catalytic activity (TH and AAD vs. ADC) of complexes containing the PPh 3 ligand (3a, 3b, and 3c) indicates the role played by these complexes during the imine formation. Analysis of the mass spectrogram of the catalytic mixture provided proof of a metal bound aldehyde, bound as a chelate after a Murai type C-H activation of the phenyl ring.…”
Section: Mechanism For Acceptorless Dehydrogenative Coupling Of Alcoh...mentioning
confidence: 99%
“…33,[45][46][47] Moreover, alternative synthesis of imines and N-alkylation of amines by alcohols have also been reported without transition metal catalysts. [62][63][64] The vast applications of these related reactions in synthetic chemistry and new energy pathways highlight the need for new catalysts that are selective towards one type of reaction. Although the effects of the reaction temperature or steric effects due to ligands or substituents on the substrates are common, the electronic effects of spectator ligands on selectivity control have not been documented.…”
Section: Introductionmentioning
confidence: 99%