Diazo compounds are very versatile reagents in organic chemistry and meet the challenge of selective assembly of structurally complex molecules. Their leaving group is dinitrogen; therefore, they are very clean and atom-efficient reagents. However, diazo compounds are potentially explosive and extremely difficult to handle on an industrial scale. In this review, it is discussed how continuous flow technology can help to make these powerful reagents accessible on large scale. Microstructured devices can improve heat transfer greatly and help with the handling of dangerous reagents safely. The in situ formation and subsequent consumption of diazo compounds are discussed along with advances in handling diazomethane and ethyl diazoacetate. The potential large-scale applications of a given methodology is emphasized.
A powerful new continuous process for the formation and use of donor/acceptor-substituted carbenes is described. The safety profile of diazo group transfer on methyl phenylacetate was determined including kinetic studies in batch and in flow using in-line IR analysis. Batch work-up and liquid chromatography were circumvented by developing an optimized liquid/liquid flow separation method providing aryl diazoacetates in high purity. Fast screening of reaction conditions in flow with in-line IR analysis allowed rapid reaction optimization. Finally, a multistep process of diazo group transfer, extraction, separation and subsequent diazo decomposition combined with multiple X-H insertion reactions was established.
Four new metal complexes [Cu(ISO)2], [Cu(BUT)2] and [Zn(ISO)2], [Zn(BUT)2] of the polyhydroxychalcones (isoliquiritigenin and butein) are synthesized, structurally characterized and their antioxidant activity is investigated. The formation of the complexes [Cu(ISO)2] and [Zn(ISO)2] is followed by Job’s plot using NMR titration. The resulting compounds are characterized by mass spectrometry, IR spectroscopy, and elemental analysis. Studies on the radical scavenging activity are performed using DPPH as substrate. The results showed that the antioxidant activities of isoliquiritigenin and butein are enhanced after binding to copper or zinc.Graphical abstract
Electronic supplementary materialThe online version of this article (doi:10.1007/s00706-016-1822-7) contains supplementary material, which is available to authorized users.
A stereoselective synthesis of 2,3-dihydro-1H-indoles with a Rh(II) catalyzed C-H insertion is reported. The α-diazo carbonyl intermediates are obtained via a diazo-transfer reaction on 2-aminophenylacetic acids. Optimizations and kinetic studies were performed leading to increased yields of the diazo-transfer after mechanistic evaluation of the side product formation. trans-2,3-Dihydro-1H-indoles were obtained in high diasteromeric excesses (up to 94% de) and enantioselectivites (up to 94% ee).
Ethyl diazoacetate (EDA) is one of the most prominent diazo reagents. It is frequently used in metal-carbene-type reactions. However, EDA can also be used as a nucleophile under base catalysis. Whilst the addition of EDA to aldehydes can be performed using organic bases, the addition of EDA to other carbonyl electrophiles requires the use of organometallics such as lithium diisopropylamide (LDA). The generated ethyl lithiodiazoacetate is highly reactive and decomposes rapidly, even at low temperatures. Herein, we report a continuous flow protocol that overcomes the problems associated with the instantaneous decomposition of ethyl lithiodiazoacetate. The addition of ethyl lithiodiazoacetate to ketones provides direct access to tertiary diazoalcohols in good yields.
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