Catalytic acceptorless complete dehydrogenation of cycloalkanes

Abstract

The advancement of an effective hydrogen liberation technology from liquid organic hydrogen carriers, particularly cycloalkanes such as cyclohexane and methylcyclohexane, holds significance in realizing a hydrogen-centric society. However, the attainment of homogeneous catalytic acceptorless dehydrogenation characterized by elevated selectivity for thorough aromatization under mild conditions remains unrealized. In this study, a catalyst system, facilitated by a double hydrogen atom transfer processes, has been devised for the catalytic acceptorless dehydrogenation of inert cycloalkanes at ambient temperature under visible light irradiation. Through the synergistic utilization of tetrabutylammonium chloride and thiophosphoric acid hydrogen atom transfer catalysts, successful catalytic acceptorless dehydrogenation with comprehensive aromatization has been accomplished with potential liquid organic hydrogen carrier candidates and showcased high functional group tolerance.

Fig. 1. Catalytic acceptorless dehydrogenation of cyclohexane derivatives.

a Hydrogenation/dehydrogenation cycle for LOHC application. b Previous CAD reaction. c Current work: complete dehydrogenation.

Fig. 2. Proposed reaction mechanism.

Proposed ternary hybrid catalytic system for the complete dehydrogenation of inert cycloalkanes.

Fig. 3. Optimization of reaction conditions.

^aDetermined by GC analysis using decane as an internal standard. ^bPd(BF₄)₂•4MeCN was used. ^cNi(NTf₂)₂•xH₂O was used. ^dPC2 was used instead of PC1. ^ePC3 was used instead of PC1. ^fAfter 16 h, the reaction mixture was filtered through a short pad of silica gel. Adding another set of the catalysts, the reaction was continued for a further 16 h. ^gWithout PC.

Fig. 4. Substrate scope.

General procedure: 1 (0.20 mmol), PC1 (0.0020 mmol), Co-1 (0.0050 mmol), TBACl (0.010 mmol), TPA (0.0025 mmol), and pyridine (0.060 mmol) were reacted in benzene (4 mL) at room temperature under blue LED irradiation for 18 h. After filtration through a short pad of silica gel, this process was repeated. ^aThe reaction was conducted using Co-2 and 3-Ph-py in PhCl. ^b1.0 mmol scale.

Fig. 5. Application to continuous flow system.

For 2a and 2c, Co-1, py, and benzene were used. For 2b, Co-2, 3-Ph-py, and PhCl were used.

Fig. 6. Quantitative evaluation of hydrogen gas evolution by two-pot transfer hydrogenation.

Quantitative hydrogen gas detection by the dehydrogenation of 1d and hydrogenation of 6 using two-pot interconnected COware apparatus.

Fig. 7. Mechanistic insights.

a Transient absorption profiles at 400 nm in the presence or absence of alkane 1a or alkene 4a. b Control experiment from alkane 1a without TBACl. c Reactions from alkene intermediate 4a under various conditions. d Transient absorption profiles at 400 nm. e Results of global analysis of conditions (1), (2), and (3) of Figure S15. Decay associated spectra of TPA radical (red line) and chlorine-benzene π-complex (blue line) are shown. f Working hypothesis for the mechanism of dual HAT catalysis.