We report the construction
and use of a vortex reactor which uses
a rapidly rotating cylinder to generate Taylor vortices for continuous
flow thermal and photochemical reactions. The reactor is designed
to operate under conditions required for vortex generation. The flow
pattern of the vortices has been represented using computational fluid
dynamics, and the presence of the vortices can be easily visualized
by observing streams of bubbles within the reactor. This approach
presents certain advantages for reactions with added gases. For reactions
with oxygen, the reactor offers an alternative to traditional setups
as it efficiently draws in air from the lab without the need specifically
to pressurize with oxygen. The rapid mixing generated by the vortices
enables rapid mass transfer between the gas and the liquid phases
allowing for a high efficiency dissolution of gases. The reactor has
been applied to several photochemical reactions involving singlet
oxygen (1O2) including the photo-oxidations
of α-terpinene and furfuryl alcohol and the photodeborylation
of phenyl boronic acid. The rotation speed of the cylinder proved
to be key for reaction efficiency, and in the operation we found that
the uptake of air was highest at 4000 rpm. The reactor has also been
successfully applied to the synthesis of artemisinin, a potent antimalarial
compound; and this three-step synthesis involving a Schenk-ene reaction
with 1O2, Hock cleavage with H+,
and an oxidative cyclization cascade with triplet oxygen (3O2), from dihydroartemisinic acid was carried out as a
single process in the vortex reactor.
We report the development of a scalable continuous Taylor vortex reactor for both UV and visible photochemistry. This builds on our recent report (Org. Process Res. Dev. 2017, 21, 1042) detailing a new approach to continuous visible photochemistry. Here we expand this by showing that our approach can also be applied to UV photochemistry and that either UV or visible photochemistry can be scaled-up using our design. We have achieved scale-up in productivity of over 300× with a visible light photo-oxidation that requires oxygen gas and 10× with a UV induced [2+2] cycloaddition obtaining scales of up to 7.45 kg day-1 for the latter. Furthermore, we demonstrate that oxygen is efficiently taken up in to the reactions of singlet O2 and, for the examples examined, that near-stoichiometric quantities of oxygen can be used with little loss of reactor productivity. Furthermore, our design should scalable to substantially larger size as well as having the potential for scaling-out with reactors in parallel.
Safety warning: Any experiment involving flammable organic solvents and air or pure oxygen is potentially hazardous, especially when partially contained, as is the case of the flask of our reactor. We took the following precautions and encountered no problems but we stress the need for readers to make safety assessments for their own experiments as peripheral circumstances may be different from ours. All experiments were carried out in a fume hood or ventilated enclosure with adequate ventilation and the front lowered. Any obvious sources of ignition were removed. Oxygen was fed from a cylinder fitted with a compliant regulator and was delivered at a maintained pressure of 1 bar using a mass flow controller compatible with oxygen. The equipment was maintained and cleaned free of grease at all times to prevent any incompatibilities with oxygen. Temperatures were kept at ambient. When working above the solvent flash point and LOC, care must be taken to ensure that all possible risks have been considered. Appropriate safeguards and suitable safety measures must be implemented.
Interaction of p‐tert‐butylcalix[8]areneH8 (L8H8) with [NaVO(OtBu)4] (formed in situ from VOCl3) afforded the complex [Na(NCMe)5][(VO)2L8H]⋅4 MeCN (1⋅4 MeCN). Increasing [NaVO(OtBu)4] to 4 equiv led to [Na(NCMe)6]2[(Na(VO)4L8)(Na(NCMe))3]2⋅10 MeCN (2⋅10 MeCN). With adventitious oxygen, reaction of 4 equiv of [VO(OtBu)3] with L8H8 afforded the alkali‐metal‐free complex [(VO)4L8(μ3‐O)2] (3); solvates 3⋅3 MeCN and 3⋅3 CH2Cl2 were isolated. For the lithium analogue, the order of addition had to be reversed such that lithium tert‐butoxide was added to L8H8 and then treated with 2 equiv of VOCl3; crystallisation afforded [(VO2)2Li6[L8](thf)2(OtBu)2(Et2O)2]⋅Et2O (4⋅Et2O). Upon extraction into acetonitrile, [Li(NCMe)4][(VO)2L8H]⋅8 MeCN (5⋅8 MeCN) was formed. Use of the imido precursors [V(NtBu)(OtBu)3] and [V(Np‐tolyl)(OtBu)3] and L8H8, afforded [tBuNH3][{V(p‐tolylN)}2L8H]⋅3 1/2 MeCN (6⋅3 1/2 MeCN). The molecular structures of 1 to 6 are reported. Complexes 1, 3, and 4 were screened as precatalysts for the polymerisation of ethylene in the presence of cocatalysts at various temperatures and for the copolymerisation of ethylene with propylene. Activities as high as 136 000 g (mmol(V) h)−1 were sometimes achieved; higher molecular weight polymers could be obtained versus the benchmark [VO(OEt)Cl2]. For copolymerisation, incorporation of propylene was 7.1–10.9 mol % (compare 10 mol % for [VO(OEt)Cl2]), although catalytic activities were lower than [VO(OEt)Cl2].
We
report the development of a small footprint continuous electrochemical
Taylor vortex reactor capable of processing kilogram quantities of
material per day. This report builds upon our previous development
of a scalable photochemical Taylor vortex reactor (Org. Process
Res. Dev.
2017, 21, 1042; 2020, 24, 201–206). It describes a
static and rotating electrode system that allows for enhanced mixing
within the annular gap between the electrodes. We demonstrate that
the size of the annular gap and the rotation speed of the electrode
are important for both conversion of the substrate and selectivity
of the product exemplified using the methoxylation of N-formylpyrrolidine. The employment of a cooling jacket was necessary
for scaling the reaction in order to manage the heat generated by
electrodes at higher currents (up to 30 A, >270 mA cm–2) allowing multimole productivity per day of methoxylation product
to be achieved. The electrochemical oxidation of thioanisole was also
studied, and it was demonstrated that the reactor has the performance
to produce up to 400 g day–1 of either of the corresponding
sulfoxide or sulfone while maintaining a very high reaction selectivity
(>97%) to the desired product. This development completes a suite
of vortex reactor designs that can be used for photo-, thermal-, or
electrochemistry, all of which decouple residence time from mixing.
This opens up the possibility of performing continuous multistep reactions
at scale with flexibility in optimizing processes.
We
report the use of a simple rotary evaporator as a semi-continuous
UV photochemical reactor. By generation of a thin film from the rotation
of a flask, better light penetration is achieved, and in this work
we used high-power Hg lamps to enable the direct irradiation of molecules
with UV light. The intramolecular [2 + 2] photocycloaddition of Cookson’s
dione and the intermolecular [2 + 2] photocycloaddition of maleimide
with 1-hexyne were used as test reactions to examine the effectiveness
of this reactor. High productivities, equivalent to 210 g h–1, were obtained for the simple intramolecular reaction, demonstrating
the scalability of the reactor. The effects of flask size, reaction
mixture volume, and use of borosilicate or quartz glassware were also
investigated.
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