Towards the rehabilitation of the Leuckart reductive amination reaction using microwave technology

Loupy, André; Monteux, Daphne A.; Petit, Alain; Aizpurua, J. M.; Domı́nguez, Esther; Palomo, Claudio

doi:10.1016/0040-4039(96)01865-5

Cited by 55 publications

(28 citation statements)

References 12 publications

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“…This transformation was demonstrated by Loupy and co-workers to exhibit a dramatic nonthermal microwave effect when performed in a single-mode reactor at 202 8C (IR temperature measurement): while the conventionally heated experiment resulted in only 2 % conversion within 30 min, the microwave heated run provided 99 % conversion toward the desired amide product during the same time period (97 % isolated yield). [33] This striking difference in conversion was rationalized in terms of a nonthermal microwave effect, for which the dipolar transition state in the Leuckart reductive amination is stabilized by the electromagnetic field, thereby leading to a lowering of activation energy and thus an enhancement of reactivity. [33,34] Comparison studies between Pyrex and SiC vial singlemode microwave experiments using the identical reaction mixture composition but employing internal FO temperature monitoring quickly demonstrated that microwave effects are probably not involved in this chemistry.…”

Section: Wwwchemeurjorgmentioning

confidence: 98%

“…[33] This striking difference in conversion was rationalized in terms of a nonthermal microwave effect, for which the dipolar transition state in the Leuckart reductive amination is stabilized by the electromagnetic field, thereby leading to a lowering of activation energy and thus an enhancement of reactivity. [33,34] Comparison studies between Pyrex and SiC vial singlemode microwave experiments using the identical reaction mixture composition but employing internal FO temperature monitoring quickly demonstrated that microwave effects are probably not involved in this chemistry. For both types of experiments, high conversions could be realized within 30-40 min (Figure 4).…”

Section: Wwwchemeurjorgmentioning

confidence: 98%

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Sintered Silicon Carbide: A New Ceramic Vessel Material for Microwave Chemistry in Single‐Mode Reactors

Gutmann

Obermayer

Reichart

et al. 2010

Chemistry A European J

105

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Silicon carbide (SiC) is a strongly microwave absorbing chemically inert ceramic material that can be utilized at extremely high temperatures due to its high melting point and very low thermal expansion coefficient. Microwave irradiation induces a flow of electrons in the semiconducting ceramic that heats the material very efficiently through resistance heating mechanisms. The use of SiC carbide reaction vessels in combination with a single-mode microwave reactor provides an almost complete shielding of the contents inside from the electromagnetic field. Therefore, such experiments do not involve electromagnetic field effects on the chemistry, since the semiconducting ceramic vial effectively prevents microwave irradiation from penetrating the reaction mixture. The involvement of electromagnetic field effects (specific/nonthermal microwave effects) on 21 selected chemical transformations was evaluated by comparing the results obtained in microwave-transparent Pyrex vials with experiments performed in SiC vials at the same reaction temperature. For most of the 21 reactions, the outcome in terms of conversion/purity/product yields using the two different vial types was virtually identical, indicating that the electromagnetic field had no direct influence on the reaction pathway. Due to the high chemical resistance of SiC, reactions involving corrosive reagents can be performed without degradation of the vessel material. Examples include high-temperature fluorine-chlorine exchange reactions using triethylamine trihydrofluoride, and the hydrolysis of nitriles with aqueous potassium hydroxide. The unique combination of high microwave absorptivity, thermal conductivity, and effusivity on the one hand, and excellent temperature, pressure and corrosion resistance on the other hand, makes this material ideal for the fabrication of reaction vessels for use in microwave reactors.

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Section: Wwwchemeurjorgmentioning

confidence: 98%

Section: Wwwchemeurjorgmentioning

confidence: 98%

Sintered Silicon Carbide: A New Ceramic Vessel Material for Microwave Chemistry in Single‐Mode Reactors

Gutmann

Obermayer

Reichart

et al. 2010

Chemistry A European J

105

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show abstract

“…This reaction is well known but, unfortunately, using classical procedures is only possible under very harsh conditions (temperature 240 • C, sealed containers, and extended reaction times) and gives modest yields (≤ 30%) [219]. These difficulties are a good challenge to check the effectiveness of MW irradiation because the mechanism develops a dipolar TS (Scheme 4.26, Table 4.32) [220], and this should also favor involvement of a MW 210 12 This is a distinctive example of a pronounced MW effect for a reaction occurring with a very late dipolar TS. It could occur as a ''miraculous'' MW effect, which can be, finally, easily justified.…”

Section: Conclusion: Suitable Conditions For Observation Of Specific mentioning

confidence: 99%

Nonthermal Effects of Microwaves in Organic Synthesis

Perreux¹,

Loupy²,

Petit³

2012

Microwaves in Organic Synthesis

Self Cite

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“…Similarly, the same group [88] used the SiC vial concept to investigate more challenging chemical transformations for which specific or non-thermal MW effects had been documented in the literature, namely the Leuckart reductive amination of benzophenone with formamide [89] and the Co-catalyzed [2 + 2 + 2]-cyclotrimerization of 1,2-dipropargylbenzene with benzonitrile [90]. In both cases, thermal decomposition of formic acid and the Co catalyst, respectively, seemed to be the reasons for the unexpected results because of the higher surface area provided by the SiC ceramic (of higher porosity) compared with Pyrex vials.…”

Section: Use Of Silicon Carbide Vesselsmentioning

confidence: 99%

Elucidation of Microwave Effects: Methods, Theories, and Predictive Models

Hoz

Díaz‐Ortiz

Gómez

et al. 2012

Microwaves in Organic Synthesis

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IntroductionThe impact of microwave (MW) irradiation in organic synthesis has increased over the years and especially since the development of new dedicated and reliable MW instruments [1].MW-assisted organic synthesis has been characterized by the spectacular accelerations produced in many reactions as a consequence of a heating rate which, in most cases, cannot be reproduced by classical heating. This effect is particularly important in (i) the preparation of isotopically labeled drugs for positron emission tomography (PET) that have a short half-life ( 11 C, t1 / 2 = 20 min; 122 I, t1 / 2 = 3.6 min; 18 F, t1 / 2 = 100 min) [2], (ii) high-throughput chemistry (combinatorial and parallel chemistry) where a rapid synthesis may increase the efficiency [3], (iii) catalysis where the short reaction time protects the catalyst from decomposition and increases the catalyst efficiency [4], and (iv) the synthesis of unstable or sensitive compounds, for example, natural products, where the short reaction time prevent the decomposition that occurs with long reaction times [5].The occurrence of results that cannot be explained exclusively by rapid heating led several authors to postulate the existence of a so-called ''microwave effect.'' Hence acceleration or modifications of the selectivity and reactivity could be explained by an effect of the electromagnetic radiation and not merely by the heating effect. In this way, in combination with the rapid heating, specific thermal effects and non-thermal effect could be the responsible for the improvement of many chemical processes.The occurrence of MW effects has been the subject of much controversy. In this chapter, we describe the state-of-the-art of this subject and we present many examples in favor of and against the presence of MW effects with a critical appraisal.Microwaves in Organic Synthesis, Third Edition.

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Towards the rehabilitation of the Leuckart reductive amination reaction using microwave technology

Cited by 55 publications

References 12 publications

Sintered Silicon Carbide: A New Ceramic Vessel Material for Microwave Chemistry in Single‐Mode Reactors

Sintered Silicon Carbide: A New Ceramic Vessel Material for Microwave Chemistry in Single‐Mode Reactors

Nonthermal Effects of Microwaves in Organic Synthesis

Elucidation of Microwave Effects: Methods, Theories, and Predictive Models

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