One‐Pot Synthesis of Chiral N‐Arylamines by Combining Biocatalytic Aminations with Buchwald–Hartwig N‐Arylation

Abstract The combination of biocatalysis and chemo‐catalysis increasingly offers chemists access to more diverse chemical architectures. Here, we describe the combination of a toolbox of chiral‐amine‐producing biocatalysts with a Buchwald–Hartwig cross‐coupling reaction, affording a variety of α‐chiral aniline derivatives. The use of a surfactant allowed reactions to be performed sequentially in the same flask, preventing the palladium catalyst from being inhibited by the high concentrations of ammonia, salts, or buffers present in the aqueous media in most cases. The methodology was further extended by combining with a dual‐enzyme biocatalytic hydrogen‐borrowing cascade in one pot to allow for the conversion of a racemic alcohol to a chiral aniline.


Materials and General Methods
General 1 H and 13 C NMR were recorded on a Bruker Avance 400 instrument (400 MHz for 1 H and 100 MHz for 13 C) in CDCl3 using residual protic solvent as an internal standard. Reported chemical shifts (δ) (in parts per million (ppm) are relative to the residual protic solvent signal (CHCl3 in CDCl3, 1 H = 7.26; CDCl3, 13 C = 77.0). GC analysis was performed on an Agilent 6850 GC (Agilent, Santa Clara, CA, USA) with a flame ionization detector (FID) and autosampler. HPLC was performed on an Agilent 1200 series system (Santa Clara, CA, USA) equipped with a G1379A degasser, G1312A binary pump, a G1367A well plate autosampler unit, a G1316A temperature-controlled column compartment and a G1315C diode array detector. Chromatograms were monitored at 265 nm. All solvent mixtures are given in (v/v) ratios. All chemicals were purchased from Sigma-Aldrich and were used as supplied.

General procedure for ChiAmDH catalysed reductive amination of ketones
The reductive amination of ketones 4 and 7 was performed in ammonium formate buffer (final conc. 1M, pH 9.0, 20 mL), containing NAD (1 mM), purified ChiAmDH (1 mg/mL) and cbFDH (0.25 mg/mL) and DMSO (5 % v/v). The substrate (50 mM) was added directly and the reactions were shaken in a bench-top shaker at 37 °C. After 48h, small aliquots of the reactions were quenched by the addition of 10M NaOH (100 µL), and extracted with EtOAc (2 x 350 µL). The combined organics were dried over anhydrous MgSO4. Conversion to amine and enantiomeric excess was analysed by GC-FID as detailed in the section "Analytics". The remainder of the reaction buffer was stored at 4 °C until use in the N-arylation reactions.

General procedure for ATA-117 catalysed amination of ketones
The asymmetric amination of ketone 4 was performed in potassium phosphate buffer (50 mM, pH 8) containing ATA-117 (1 mg/mL), lactate dehydrogenase (1 mg/mL), PLP (1 mM), NAD+ (1 mM) and DMSO (5 % v/v) and D-alanine (250 mM) glucose dehydrogenase (1 mg/mL) and glucose (100 mM). The substrate (50 mM) was added directly and the reactions were shaken in a bench-top shaker at 30 °C. After 48h, small aliquots of the reactions were quenched by the addition of 10M NaOH (100 µL), and extracted with EtOAc (2 x 350 µL). The combined organics were dried over anhydrous MgSO4. Conversion to amine and enantiomeric excess was analysed by GC-FID as detailed in the section "Analytics" The remainder of the reaction buffer was stored at 4 °C until use in the N-arylation reactions.

General procedure for the two step N-arylation
The reaction mixture from the transaminase reaction (1 mL) containing amine (R)-1 (50 µmol) was basified to pH 14 by addition of NaOH (100 µL) and extracted with toluene (3 x 2 mL). The combined organics were dried over anhydrous S6 MgSO4.. The toluene solution was charged with NaOtBu (10 mg, 100 µmol, 2 eq. ), CyJohnPhos (2 mg, 5 µmol, 0.1 eq.) and Pd(OAc)2 (1.5 mg, 7 µmol, 0.14 eq.). The solution was bubbled with nitrogen for 10 minutes followed by addition of the appropriate aryl bromide (60 µmol, 1.2 eq.). Reactions were refluxed for 20 hours and subsequently quenched by addition of water (5 mL). The organics were dried over anhydrous MgSO4. Conversion to and enantiomeric excess of the N-arylamine was determined as detailed in the section "Analytics". Conversions to the Narylamines 3a-c and 3f-g are given in table S1.

General procedure for S-IRED catalysed reduction of imine 8
The reduction of imine 8 was performed using potassium phosphate buffer (final conc. 100 mM, pH 8.0), containing NADP (1 mM), purified S-IRED (1 mg/mL) and DMSO (5 % v/v) The substrate (50 mM) was added directly and the reactions were shaken in a bench-top shaker at 30 °C. After 48h, small aliquots of the reactions were quenched by the addition of 10M NaOH (100 µL), and extracted with EtOAc (2 x 350 µL). The combined organics were dried over anhydrous MgSO4. Conversion to amine (S)-9 and enantiomeric excess was analysed by GC-FID as detailed in the section "Analytics" The remainder of the reaction buffer was stored at 4 °C until use in the arylation reactions.