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Photo-Claisen Rearrangement of Allyl Phenyl Ether in Microflow: Influence of Phenyl Core Substituents and Vision on Orthogonality

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Photo-Claisen Rearrangement of Allyl Phenyl Ether in Microflow: Influence of Phenyl Core Substituents and Vision on Orthogonality Full Paper Photo-Claisen Rearrangement of Allyl Phenyl Ether in Microflow: Influence of Phenyl Core Substituents and Vision on Orthogonality Elnaz Shahbazali1, Timothy Noël1 and Volker Hessel1 1Department of Chemical Engineering and Chemistry, Micro Flow Chemistry and Process Technology, Eindhoven University of Technology, Eindhoven 5600 MB, The Netherlands Received: 29 June 2016; accepted: 22 August 2016 We converted diverse commercial meta-substituted phenols to the allyl-substituted precursors via nucleophilic substitution using batch technology to allow processing these in microflow by means of the photo-Claisen rearrangement. The latter process is researched on its own, as detailed below, and also prepares the ground for a fully continuous two-step microflow synthesis, as outlined above. It is known that batch processing of electronically deactivated phenols (e.g., bearing a cyano or nitro group) has several orders of magnitude lower reactivity than their parental counterparts [1]. Thus, we here explore if the high quantum yield of microflow, yet at very short residence time, is sufficient to activate the deactivated molecules. In addition, the realization of a true orthogonal two-step flow synthesis can open the door to a large synthetic scope of our approach and possibly overcome limitations due to missing orthogonality of our previously reported thermal approach of combined nucleophilic substitution-Claisen rearrangement in microflow. Consequently, we make for our photo microflow approach an orthogonality check, as previously reported for the thermal approach, and compare both. To get a broader picture, we have investigated some major parametric sensitivities such as the irradiation intensity, the choice of solvent, the reactant concentration, and, most notably, the influence of the substitution pattern. The irradiation intensity was varied by increasing distance between a lamp and the microflow capillary. In addition, the normal photo- Claisen microflow process (at room temperature) is compared to a high-temperature photo-Claisen microflow process, to check the potential of such novel process window [2]. This is difficult to realize in batch, as the combination of strong ultraviolet (UV) irradiation and high temperature causes a high hazard potential. Yet, under microflow, this can be safely handled. Keywords: flow chemistry, microreactors, photo-Claisen rearrangement, microreactor networks, flow orthogonality 1. Introduction acid or base removal techniques based on extraction and adsorp- tion. We discussed all the advantages and disadvantages. Finally, The Claisen rearrangement, which is the [3,3]-sigmatropic the preference was given to using Amberlyst A21 and A26 ion rearrangement of allyl phenyl ethers, makes a valuable synthetic exchanger in a packed bed configuration. Yet, we also noted a transformation for organic chemistry [3–5]. The thermal Claisen mismatch in the capacities of the synthesis and the purification, rearrangement requires high temperatures in order to result in which led to an unacceptable mismatch in the dimensions (inner sufficient conversion. diameter) of the reaction capillary and the packed-bed tube. The In the last decade, microreactor technology has revealed prom- latter was much too low. Therefore, we proposed a much faster ising potential for many synthetic applications and has even been exchange of packed-bed columns than common with these mate- applied on an industrial scale [6, 7]. Microreactor technology has rials to provide fresh ion exchanger material with shorter cycle. In also been proven beneficial for the Claisen rearrangement [8–11]. this way, the dimensional fit between reaction capillary and Recently, we reported on the combination of a nucleophilic adsorption packed was smaller, although still far from being ideal. substitution to the thermal-Claisen rearrangement in microflow, The latter would finally ask for a better ion exchange material i.e., a continuous microflow two-step synthesis [12]. In the first with, e.g., much higher packing density. step thereof, phenol and allyl bromide yield phenyl allyl ether Microreactors have also received a lot of attention in photo- which is then in a second step and directly converted by the chemical applications since they are capable of improving some thermal Claisen rearrangement. Targeting synthetic scope, the issues related to the batch photochemistry [15, 16]. In essence, motivation was to show that, in this way, many substituted the narrowness of the microreactor channels enables a better phenyl allyl ethers may derive from respectively substituted exposure of the reaction mixture to the irradiated light and phenols. Yet, as reported, just combining the individual micro- modern light-emitting diode (LED) technology avoids the flow syntheses was by no means sufficient, which is surprising. unwanted warming up of the reaction solutions. A greater Rather, both syntheses lacked considerably in orthogonality. amount of reactant molecules is excited under more defined This was partly due to the remaining base 1,8-diazabicyclo experimental protocol, and thus, the reaction is expedited. [5.4.0]undec-7-en (DBU), which acts as catalyst for the allyl As mentioned above, for the thermal-Claisen rearrangement, group substitution, and caused cleavage of the Claisen products the photo-Claisen rearrangement can be connected similarly to a to the phenol starting material. Unexpected was that the allyl nucleophilic substitution, which introduces the allyl group into bromide had similar strong effect. Under the high temperatures diverse phenols. Here, the main advantage is the possibility to used in the thermal Claisen rearrangement, which are typically generate para-substituted products besides the ortho-substitu- set in microflow even higher [13, 14], the allyl bromide decom- tion pattern (for details on mechanism of photo-Claisen rear- poses to give propene and HBr. The latter acid can lead to the rangement, one is referred to refs. [17–20]). Yet, it should be same decomposition rate of the Claisen product as the acid noted as well that this generates an extra separation issue. In does. As a consequence, we have discussed several continuous addition, it might be anticipated that the orthogonal fit between the two reaction steps might be better, as the photo-Claisen * Authors for correspondence: [email protected] and and DOI: 10.1556/1846.2016.00029 J. Flow Chem. 2016, 6(3), 252–259 © 2016 Akadémiai Kiadó E. Shahbazali et al. reaction is done at normal temperature so that (at least) the allyl The photo microflow synthesis avoids the formation of bromide decomposition should be much reduced or even migration of the double bound of the allyl group to give E/Z absent. 2-(prop-1-enyl)phenols which were reported for the corre- With such motivation on enlarged synthetic scope and sponding thermal (high-T) microflow synthesis of allyl phenyl improved orthogonality, we aim to develop a microflow proto- ether, especially at temperatures above 300 °C [14]. These col of the photo-Claisen rearrangement. substances can undergo ring closure to give 3-dihydro-2-meth- ylbenzo[b]furan and 2-methylbenzo[b]furan as further byprod- ucts of the thermal process which we did not observe. The latter 2. Results and Discussion effect was remarkably dependent on the solvent in the thermal 2.1. Parametric Sensitivities of the Photo-Claisen Rear- process, with alcoholic solvents giving the cleanest process. rangement of Allyl Phenyl Ether in Microflow. It is known 2.1.2. Solvent Effect. One of the key parameters both for the from the literature that the photo-Claisen rearrangement of allyl thermal and photo-Claisen rearrangement is the selection of an phenyl ether works well under given conditions in batch. Thus, appropriate solvent. Basic needs are to ensure that all reactants we started our parametric investigation with the unsubstituted are soluble (which is particularly relevant for microflow syn- precursor (also because this molecule is commercially available, thesis), the solvent is transparent to the desired wavelength, and whereas we needed to synthesize the substituted molecules it should not suppress the photochemical process. We have before the photo experiments). already shown that, for the thermal Claisen microflow process 2.1.1. Flow Process Development — Isomer Distribution/ variant, the performance varies largely with the solvent chosen. Phenol Content and Conversion/Yield. In a typical batch Even with the class of alcohols, considerable differences were experiment documented in the literature, the relative product found for diverse alkyl chain length, yet also even when com- distribution after the photoreaction of allyl phenyl ether is paring 1- and 2-alcohol isomers. reported as 21% (ortho, 2-), 31% (para, 4-), and 48% (phenol) Since Hughes et al. [22] state that increasing polarity causes in hexane and 42% (ortho, 2-), 42% (para, 4-), and 16% an increase in reaction rate for the photo-Claisen process, we (phenol) in MeOH (irradiation time = 4 h) [17]. Irradiation would like to continue this investigation with alcohols as sol- was conducted to about 30% conversion [17, 18]. In an own vents of the choice. The explanation for the acceleration is that batch experiment, after 5 h of irradiation in 1-butanol (10 mL the transition state is more polar than the initial reagent so that it solution), no product was detected, while in photo microflow, has more stabilization by interaction with the solvents. Besides we achieved 30% (ortho), 30% (para), and 21%
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