Research Article: Highly Crystalline K‐Intercalated Polymeric Carbon Nitride for Visible‐Light Photocatalytic Alkenes and Alkynes Deuterations

Date Published: November 08, 2018

Publisher: John Wiley and Sons Inc.

Author(s): Chuntian Qiu, Yangsen Xu, Xin Fan, Dong Xu, Rika Tandiana, Xiang Ling, Yanan Jiang, Cuibo Liu, Lei Yu, Wei Chen, Chenliang Su.

http://doi.org/10.1002/advs.201801403

Abstract

In addition to the significance of photocatalytic hydrogen evolution, the utilization of the in situ generated H/D (deuterium) active species from water splitting for artificial photosynthesis of high value‐added chemicals is very attractive and promising. Herein, photocatalytic water splitting technology is utilized to generate D‐active species (i.e., Dad) that can be stabilized on anchored 2nd metal catalyst and are readily for tandem controllable deuterations of carbon–carbon multibonds to produce high value‐added D‐labeled chemicals/pharmaceuticals. A highly crystalline K cations intercalated polymeric carbon nitride (KPCN), rationally designed, and fabricated by a solid‐template induced growth, is served as an ultraefficient photocatalyst, which shows a greater than 18‐fold enhancement in the photocatalytic hydrogen evolution over the bulk PCN. The photocatalytic in situ generated D‐species by superior KPCN are utilized for selective deuteration of a variety of alkenes and alkynes by anchored 2nd catalyst, Pd nanoparticles, to produce the corresponding D‐labeled chemicals and pharmaceuticals with high yields and D‐incorporation. This work highlights the great potential of developing photocatalytic water splitting technology for artificial photosynthesis of value‐added chemicals instead of H2 evolution.

Partial Text

Deuterium‐labeled compounds had found their significant utility in synthetic mechanistic study, mass spectrometry‐based quantification and pharmaceutical industry over the past few years.1 The first approval of deuterated drug deutetrabenazine (SD‐809) by the US Food and Drug Administration in 2017 represented a new milestone in heavy drugs/chemicals, which triggers intense emotions in the development of useful heavy drugs as well as new‐generation deuteration strategies.2 Selective installation of deuterium atoms in the target organic/pharmaceutical compounds is therefore of high synthetic interests but still remains challenge. The state of art C–H/C–D exchange process is a powerful deuteration strategy but often suffers from harsh reaction conditions, limited scope (mainly for deuteration of sp2 or sp bonds), or nonselective multiposition labeling.3 Controllable deuteration of organic functional groups (halides, carbonyls, alkenes, etc.) provides a promising route for selective installation of deuterium atoms in the target organic compounds, which therefore have attracted growing attention.4 C=C or/and C≡C bonds are ubiquitous in many important fine chemicals and natural products. However, there are surprisingly few reports for selective deuteration of C=C or/and C≡C bonds with green deuterium source to uniquely install deuteriums in unactived sp3 bonds. One example developed by Stokes’ group uses expensive additives such as B2(OH)4 to engage and activate D2O for deuteriation of alkenes, where very limited deuteration examples were presented.5 Thus, developing a general approach combining new catalytic system with green and inexpensive deuterium source (D‐source) for selective deuteration of alkenes and alkynes with broad reaction scope is highly desired.

As illustrated in Figure1a, a newly formed KPCN was controllably synthesized via salt confinement growth followed by pyrolysis. Solid KBr was utilized as a solid template for the growth of KPCN because of its higher melting point (Tm = 730 °C) than the reaction temperature (580 °C). A high molar ratio of KBr/melamine (more than 4:3) provided confined spaces (the interspaces of KBr crystals), thus leading to the confined growth of KPCN within the KBr crystals (red dashed lines in Figure 1a) to give a crystalline structure. Additionally, the KBr salt could also act as a reactant which provides the K cations (about 9.73 wt%, inductively coupled plasma (ICP) analysis) inserted into the melon chains (the proposed structure inserted in Figure 1b) or layers for the formed joint melting phase between melamine and KBr.

In summary, we have pioneered a promising integrated photocatalytic system utilizing water splitting technology for selective deuteration of alkenes and alkynes over a crystalline KPCN semiconductor. The present KBr salt‐template‐confined growth process achieved an unprecedented high crystalline KPCN material with remarkably improved light harvesting capabilities and charge carrier separation; thus achieved a significantly enhanced activity over KPCN for visible light photocatalytic H2/D2 evolution and transfer deuteriation of alkenes/alkynes to furnish high value‐added heavy chemical/pharmaceuticals. We are currently working to better understand the catalytic mechanism and the atomically‐insight into the structure of KPCN and discover new‐generation catalysts that enable us to harness our strategy for widespread artificial photosynthesis of heavy chemicals/drugs.

Preparation of PCN and KPCN: In a typical synthesis, melamine (3.0 g, Alfa Aesar) was ground with KBr (2.0 g, Alfa Aesar) in 3 mL EtOH and 1 mL glycol in an agate mortar. After drying at 65 °C, the resultant mixture was heated to 550 °C for 3 h at a rate of 3.3 °C min−1 in a tube furnace (inner diameter is 5 cm) with open two ends in an air atmosphere. After it was cooled to room temperature, the bright yellow‐green product was washed with boiling deionized water several times and collected by filtration, followed by drying at 60 °C under vacuum. This sample is denoted as KPCN. The samples with different KBr/melamine ration were prepared under the same procedure. For comparison, 3.0 g melamine was directly heated to 550 °C for 3 h without the KBr, which was referred to as PCN.

The authors declare no conflict of interest.

 

Source:

http://doi.org/10.1002/advs.201801403

 

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