Pyrene, cyanation

The pyrene molecule is transferred by irradiation to its cation radical [29]. This reacts at the oil/water interface by nucleophilic attack from the cyanide ion. Typically, the cyanated product remains in the organic phase. 

OS 43] [R 14] [P 32] A yield of 28% of the cyanated pyrene was obtained in a first run by two-liquid layer (oil/water) flow, using a residence time of 210 s and room-temperature processing (0.2 pi min 300 W, > 300 nm wavelength) [29]. Using a three-liquid layer (water/oil/water) flow resulted in a yield of 73%. The content of non-reacted pyrene was 8%. Thus, to close the balance, about 20% by-products had to be formed, i.e. conversion was high. The nature of these by-products was not identified (Figure 4.66). 

Figure 4.66 Photoq anation of pyrene (PyH) to the corresponding cyanated product (PyCN) in dicyanobenzene (DCB). Left schematic of the flow inside the micro reactor and a microscope image of the chip micro channels. Right GC and mass spectra of samples from micro flow processing [29].

OS 43] [R 14] [P 32] Experiments rrm without NaCN did not yield the cyanated photoproduct ]29]. Therefore, NaCN and not 1,4-dicyanobenzene has to be considered as the source of the CN anion that is incorporated into the pyrene moiety. 

OS 43] [R 14] ]P 32]The cyanated pyrene product leaves the reaction channel completely via the organic phase ]29]. Since the reaction occurs with aqueous dissolved NaCN, one can draw the conclusion that complete extraction of the product was achieved. 

Kitamura and coworkers demonstrated the photocyanation of pyrene by manipulating stable organic/aqueous (oil/water) laminar flow inside the microchannel (polystyrol microchannel chip with a channel, 100 pm wide, 20 pm deep and 350 mm long) [46], This two-layer oil/water system gave only 28% of the desired cyanated pyrene in 210 s under irradiation by a 300 W high-pressure Hg lamp (Scheme 4.32). However, the yield was improved (73%) by using a water/oil/water three-layer flow system. The 2.5-fold increase in yield was attributed to the greater surface area-to-volume ratio in the three-layer system. 

The photocyanation of pyrene in a microchaimel through an oil-water interface was investigated by Ueno et al. [13]. The microchips employed are made of polystyrene by embossing with a silicone template. The phase transfer reaction proceeds in four steps as depicted in Figure 16.2. In the first step, a photoinduced electron transfer in the oil phase (polycarbonate) occurs from the aromatic hydrocarbon pyrene (DH) to the electron acceptor 1,4-dicyanobenzene (A). The cationic DH " radical is subsequently the target of the nucleophilic attack of the cyanide anion at the oil-water interface. The cyanated product DCN is insoluble in water and goes back into the oil phase. Experiments without a cyanide source (NaCN) in the aqueous phase show no reaction. Hence it can be excluded that the nudeophilic-substituted cyanide originates from the electron acceptor 1,4-dicyanobenzene. 

A different kind of nucleophilic aromatic substitution reaction, namely cyanation reactions, was described by Kitamura and coworkers [49]. Thqr investigated the photocyanation of pyrene by mixing an aqueous solution of NaCN and a propylene carbonate solution of pyrene and 1,4-dicyanobenzene in Y-shaped microfluidic chips made of polymers (Scheme 4.26). Since the reaction takes place at the oil-water interface, an increase in interfadal area was a major driver for employing microreactors. 

In a subsequent study, the authors investigated the electrochemical cyanation of pyrene in polymer microfluidic chips with integrated electrodes [50]. As in the photocyanation experiments [49], the reaction was carried out in an oil-water system. 1-Cyanopyrene was obtained as the sole product in quantities depending on flow rate and on the position of the electrodes inside the microfluidic chips. Moreover, the electrochemical cyanation of pyrene was also carried out by employing an acetonitrile solution of pyrene containing tetrabutylammonium perchlorate and an aqueous solution of NaCN. 1-Cyanopyrene was obtained in 61% yield and the amount of 1,3-dicyanopyrene could be successfully reduced from 14% obtained in macroscopic processes to 4% obtained in the microfluidic setup. 

K. Ueno, F. Kitagawa, N. Kitamura, One-step electrochemical cyanation reaction of pyrene in polymer microchannel-electrode chips. Bull. Chem. Soc. Jpn. 2004, 77, 1331-1338. 

Figure 9.1 Cyanation of pyrene in an electrochemical flow microreactor with a serial electrode configuration.

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