10.1002/adsc.201900165
Advanced Synthesis & Catalysis
General procedure: cis-1,3-diaryl tetrahydroisoquinolines 9
In summary, visible light-absorbing anthraquinone
In a 10 mL crimp cap vial, amine 5 (1.0 eq, typically 0.14-
0.20 mmol) was dissolved in dry acetonitrile (0.15 M) and
1,5-diaminoanthraquinone (2 mol-%) was added. The vial
was sealed and O2 was bubbled through the reaction
mixture via cannula, then an O2-ballon was fitted to the
cannula. The reaction was stirred rapidly with irradiation
between two CFL lamps (2 × 18 W, light color 71,
2 × 2.73 cd, 450±50 nm) for the time indicated. Then, the
solvent was removed (the vial was directly fitted to the
evaporator) and the residue in the same vial was cooled to
0 °C.
Cyclization with TFSA: Trifluoromethanesulfonic acid
(100 eq) was slowly added. The vial was sealed and the
reaction mixture was heated to 120 °C for the time
indicated. The reaction mixture was poured into ice-water
and basified with 1 M NaOH. The mixture was extracted
with EtOAc (3 ×), dried over MgSO4 and concentrated.
The crude product was purified by column
chromatography.
catalysts C and D are highly active in the selective
photocatalytic aerobic oxidative dehydrogenation (a-
ODH) of primary and secondary benzylamines to N-
benzylidenebenzylamines. α-Branched dibenzylami-
nes undergo regioselective dehydrogenations to
branched aldimines that could be converted into cis-
1,3-diarylated tetrahydroisoquinolines by way of
diastereoselective super acid-mediated Pictet-Speng-
ler cyclizations.
a)
b)
Cyclization with TFA: Trifluoroacetic acid (100 eq) was
slowly added. The vial was sealed and the reaction mixture
was heated to 50 °C for the time indicated. The reaction
mixture was poured into ice-water and basified with 1 M
NaOH. The mixture was extracted with EE (3 ×), dried
over MgSO4 and concentrated. The crude product was
purified by column chromatography.
11
single crystal X-ray
structure of 9b
CCDC 1875510
dicationic cyclisation
transition structure
Figure 2. (a) Single crystal X-ray structure of compound
9b. (b) Proposed dicationic transition structure 11 in
TFSA-mediated cyclizations.
Acknowledgements
Experimental Section
MB acknowledges financial support of this work by the Deutsche
Forschungsgemeinschaft (DFG, grant BR 3748/2-1).
General procedure: imines 2
In a 10 mL crimp cap vial, benzylamine derivative 1 (0.60
mmol, 1.0 eq) was dissolved in dry MeCN-d3 (2.00 mL,
0.3 M) and 1,5-diaminoanthraquinone (C, 1.4 mg,
1 mol-%) and NMP (N-methylpyrrolidine-2-one, 58 μL,
1.0 eq., internal standard) were added. The vial was sealed
and O2 was briefly bubbled through the reaction mixture
via cannula, then an O2-balloon was fitted to the cannula.
The reaction mixture was stirred rapidly while being
irradiated between two CFL lamps (2 × 18 W, light
color 71, 2 × 2.73 cd, 450 ± 25 nm, temperature 30 °C),
and conver-sion was monitored by 1H-NMR (100 μL
aliquots of the reaction solution were taken and diluted
with MeCN-d3). After full conversion of substrate 1, the
yield of imine 2 was determined against the NMP standard.
Preparative procedure: The same procedure was followed
in MeCN solution, without NMP. After the optimal
reaction time as determined before, the reaction mixture
was evaporated and the crude product was dried in high
vacuum to afford the imine 2 as an oil of typically >90-
95% purity (red discoloration caused by the residual
catalyst). Labile imines 2 should be used in subsequent
reactions without purifi-cation.
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General procedure: α-benzyldibenzylamines 5
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1.0 eq) was dissolved in dry toluene (2.00 mL, 0.3 M) and
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balloon was fitted to the cannula. The reaction was stirred
rapidly while being irradiated between two CFL lamps
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reactor temperature 30 °C) for the time indicated. Then, the
reaction mixture was purged with N2 and cooled to 0 °C. A
solution of benzyl Grignard reagent in THF (3 eq) was
slowly added. The reaction mixture was stirred overnight
at ambient temperature. Saturated NH4Cl-solution (1 mL)
was added followed by NaHCO3-solution (3 mL). The
mixture was extracted with Et2O (3×), the combined
organic layer was dried over MgSO4, filtered and
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column chromatography.
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4
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