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Exploring Flow Procedures for Diazonium Formation molecules Article Exploring Flow Procedures for Diazonium Formation Te Hu, Ian R. Baxendale * and Marcus Baumann Department of Chemistry, University of Durham, South Road, Durham DH1 3LE, UK; [email protected] (T.H.); [email protected] (M.B.) * Correspondence: [email protected]; Tel.: +44-191-3342185 Academic Editor: Alessandro Palmieri Received: 12 June 2016; Accepted: 5 July 2016; Published: 14 July 2016 Abstract: The synthesis of diazonium salts is historically an important transformation extensively utilized in dye manufacture. However the highly reactive nature of the diazonium functionality has additionally led to the development of many new reactions including several carbon-carbon bond forming processes. It is therefore highly desirable to determine optimum conditions for the formation of diazonium compounds utilizing the latest processing tools such as flow chemistry to take advantage of the increased safety and continuous manufacturing capabilities. Herein we report a series of flow-based procedures to prepare diazonium salts for subsequent in-situ consumption. Keywords: diazonium salts; flow chemistry; meso reactor; processing; supported reagent 1. Introduction The formation and continuous processing of highly reactive or potentially unstable intermediates has proven to be a strong driver for the adoption of flow based chemical synthesis [1–3]. Indeed, the ability to continuously prepare transient species using small volume reactor technology and directly couple their formation into a subsequent consuming reaction step has significantly enhanced the safety profile of many chemical sequences [4–7]. For this reason we are currently experiencing a resurgence of interest in many classical transformations that have historically been relegated to almost obscurity because of inherent batch based safety concerns and an inability to scale the transformation [8–16]. The improved mixing efficiencies and greater temperature control imparted through the application of flow based reactor technologies is thus enabling their reinvestigation. From our own repertoire of studies the diazonium functionality has shown particular versatility as a reactive intermediate [17–20] that benefits from being prepared and directly reacted in a continuous flowing process [21–37]. Within this manuscript we describe a number of methods that can be used to conveniently prepare these species at differing scale and with contrasting processing advantages. 2. Results and Discussion We initially started our investigations by evaluating the various methods of forming aryl diazonium salts under flow conditions. These can be broadly classified into three general methods based upon the phases used, namely: aqueous (Section 2.1); organic (Section 2.2) and solid phase (Section 2.3). 2.1. Formation of Aryl Diazonium Species under Aqueous Conditions For the development of the aqueous conditions for preparing diazonium salts in flow we evaluated the classical combination of sodium nitrite and hydrochloric acid. To aid in the rapid optimization of the transformation we incorporated flow ReactIR analysis into the process [38,39]. The reactor setup consisted of six paired HPLC pumps used to deliver three variable concentrations of the different reaction inputs (Figure1). Stream one contained the sodium nitrite solution, stream two an aqueous solution of hydrochloric acid and stream three the aniline component as its mono-hydrochloric acid Molecules 2016, 21, 918; doi:10.3390/molecules21070918 www.mdpi.com/journal/molecules MoleculesMolecules2016 2016, 21, 21, 918, 918 2 of2 of23 23 mono-hydrochloric acid salt [40], also dissolved in water. The final configuration of T-mixing pieces saltas [shown40], also in dissolvedFigure 1 was in water.determined The finalthrough configuration experimental of T-mixingtesting. It pieceswas found as shown that mixing in Figure the 1 washydrochloric determined acid through and nitrite experimental stream prior testing. to introduction It was found of thatthe aniline mixing gave the hydrochloricconsistently superior acid and nitriteresults stream [41]. It prior was tofurther introduction observed of that the altering aniline gavethe delay consistently time between superior thisresults initial mixing [41]. It event was further and observedsubsequent that introduction altering the delayof the timeaniline between streamthis had initial no detectable mixing eventeffect andon the subsequent tested reactions introduction (delay of therange aniline 1.2–40 stream s). In had this no regard detectable the aqueous effect onacid the cata testedlyzed reactions decomposition (delay of range inorganic 1.2–40 nitrite s). In to this nitrous regard theacid aqueous and further acid catalyzed to nitric decompositionoxide is known ofto inorganic be a very nitrite rapidly to es nitroustablished acid equilibrium and further process to nitric [42]. oxide isThe known presence to be of a veryvery rapidlylow levels established of a NO triple equilibrium bonded processspecies as [42 dete]. Thermined presence by ReactIR of very at low 2190–2215 levels of a NOcm−1 triplewas noted, bonded but speciesit was not as deemed determined useful by to ReactIRquantify atits 2190–2215formation (generation cm´1 was of noted, calibration but it curves) was not deemedunder these useful equilibrium to quantify conditions its formation. Instead (generation monitoring of of calibration the subsequent curves) diazonium under these formation equilibrium step conditions.was examined Instead instead. monitoring of the subsequent diazonium formation step was examined instead. FigureFigure 1. 1.Configuration Configurationof of flowflow reactorsreactors for diazoniumdiazonium formation formation using using aqueous aqueous conditions. conditions. ToTo test test the the range range of viable reactant reactant concentrations, concentrations, 1.0 1.0M stock M stock solutions solutions of the ofthree the reagents three reagents were wereprepared; prepared; water water was wasused used as the as diluent the diluent for the for thre thee threemakeup makeup pumps. pumps. Serial dilution Serial dilution profiles profiles for each for reagent stream were systematically produced whilst always ensuring the required theoretical minimum each reagent stream were systematically produced whilst always ensuring the required theoretical 1:1:1 reagent stoichiometry. It was quickly found that a viable concentration window of 0.2 + 0.25 M minimum 1:1:1 reagent stoichiometry. It was quickly found that a viable concentration window aniline, 1.8 equivalents of HCl and 1.3 equivalents of sodium nitrite worked well across a representative of 0.2 + 0.25 M aniline, 1.8 equivalents of HCl and 1.3 equivalents of sodium nitrite worked well subset of anilines (Ar = 4-Me, 2-OMe, 2-F, 4-OMe, 4-Br, 3-NO2, 4-NO2 and 3-CF3). In general higher across a representative subset of anilines (Ar = 4-Me, 2-OMe, 2-F, 4-OMe, 4-Br, 3-NO , 4-NO and aniline concentration led to precipitation issues and significant by-product formation2 (triazine2 3-CF ). In general higher aniline concentration led to precipitation issues and significant by-product formation3 with low acid concentration). The use of 1.8 equivalents of hydrochloric acid gave excellent formationconversions (triazine to the formation diazonium with salt lowin every acid concentration).case (>93% purity), The however, use of 1.8 using equivalents a higher of ratio hydrochloric for the acidmore gave electron excellent rich conversions anilines (Ar to = the4-OMe diazonium {2.2 M} saltand in 2-OMe every case{2.0 M}) (>93% was purity), found however,to completely using asuppress higher ratio the forformation the more of small electron quantities rich anilines of diazo (Ar coupled = 4-OMe material {2.2 M}and and other 2-OMe unidentified {2.0 M}) side was foundproducts to completely [43–49]. Importantly, suppress the it formationshould be ofnoted small th quantitiesat in each ofinstance diazo coupledthe first equivalent material and of otherthe unidentifiedhydrochloric side acid products was always [43– present49]. Importantly, as a part of it the should aniline be stock noted solution that in (HCl each salt), instance facilitating the first equivalentsolubility of of the the hydrochloric aniline substrate acid was in alwaysthe aque presentous media. as a part Therefore of the aniline only the stock further solution excess (HCl (>1 salt), facilitatingequivalent) solubility is supplied of the as a aniline separate substrate stream inand the us aqueoused in the media. generation Therefore of the onlyreactive the nitrosonium further excess (>1cation equivalent) prior to is its supplied combination as a separate with the stream aniline and flow used stream in the (Figure generation 1). This of is the important reactive because nitrosonium as cationidentified prior earlier to its combinationit was found that with pre-generation the aniline flow of the stream intermediate (Figure1 ).NO+ This containing is important solution because prior as identifiedto its unification earlier it waswith found the aniline that pre-generation yielded improved of the results. intermediate This was NO+ additionally containing solutionconfirmed prior by to itsaltering unification the reaction with the setup aniline to yieldedeliminate improved the separa results.te acid This stream was and additionally alternatively confirmed supply bythe alteringsame thequantity reaction of setup hydrochloric to eliminate acid theas a separate homogeneous acid stream mixture and with alternatively the aniline supply starting
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